Sputtering of Insulators in the Electronic Stopping Region Citation Griffith, Joseph Edward (1979) Sputtering of Insulators in the Electronic Stopping Region. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/b5x2-gf26. https://resolver.caltech.edu/CaltechTHESIS:10292015-093835228 Abstract Using track detectors we have measured sputtering yields induced by MeV light ions incident on a uranium containing glass, UO 2 and UF 4 . No deviation from the behavior predicted by the Sigmund theory was detected in the glass or the UO 2 . The same was true for UF 4 bombarded with 4 He at 1 MeV and with 16 O and 20 Ne at 100 keV. In contrast to this, 4.75 MeV 19 F(+2) sputters uranium from UF 4 with a yield of 5.6 ± 1.0, which is about 3 orders of magnitude larger than expected from the Sigmund theory. The energy dependence of the yield indicates that it is generated by electronic rather than nuclear stopping processes. The yield depends on the charge state of the incident fluorine but not on the target temperature. We have also measured the energy spectrum of the uranium sputtered from the UF 4 . Ion explosions, thermal spikes, chemical rearrangement and induced desorption are considered as possible explanations for the anomalous yields. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: (Physics) Degree Grantor: California Institute of Technology Division: Physics, Mathematics and Astronomy Major Option: Physics Thesis Availability: Public (worldwide access) Research Advisor(s): Tombrello, Thomas A. Thesis Committee: Unknown, Unknown Defense Date: 7 May 1979 Record Number: CaltechTHESIS:10292015-093835228 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:10292015-093835228 DOI: 10.7907/b5x2-gf26 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 9257 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 29 Oct 2015 21:56 Last Modified: 27 Nov 2024 20:14 Thesis Files Preview PDF - Final Version See Usage Policy. 28MB Repository Staff Only: item control page

SPUTTERING OF INSULATORS IN THE ELECTRONIC STOPPING REGION Thesis by Joseph Edward Griffith In Partial Fulfillment of the Requirements for the Degree uf Doctor of Philosophy California Institute of Technology Pasadena, California 1979 (Submitted May 7, 1979) -ii- For our knowledge is imperfect, and our prophecy is imperfect; but when the perfect comes, the imperfect will pass away. When I was a child, I spoke like a child, I thought like a child, I reasoned like a child; when I became a man, I gave up childish ways. For now we see through a glass darkly, but then face to face. I Corinthians 13. 9- 12 -iii- ACKNOWLEDGMENTS Many have made important contributions to this effort, but I have space to name only a special few. Foremost among them is Tom Tombrello, whose sound advice and unwavering support made it possible to push on through month after month of initially disappointing results. Don Burnett facilitated the track work through generous donations of time and equipment. A number of crucial ideas arose out of discussions with enthusiastic colleagues. Jon Melvin, Ziggy Switkowski, Pete Haff, Jim Mayer, Marc-A. Nicholet and John Poate all made useful suggestions. Several students helped in the execution of the experiments. In performing the microprobe runs for me, Tim Benjamin and John Jones always gave more than was requested. Sharon Streight analyzed one of the sticking fraction runs for her senior thesis at Occidental College. Mari Weller graciously shared her expertise in helping with the Rutherford backscattering run. contribution was profound. Bob Weller's He participated fully in the sticking fraction, low energy yield and energy spectrum experiments, and I share all credit with him for these projects. Finally, I dedicate this dissertation to my parents and my sister, whose love shaped this work by shaping me. - ivABSTRACT Using track detectors we have measured sputtering yields induced by MeV light ions incident on a uranium containing glass, uo 2 and UF 4 • No deviation from the behavior predicted by the Sigmund theory was detected in the glass or the was true for UF and 20 4 uo 2 . The same 16 bombarded with 4 He at 1 MeV and with 0 Ne at 100 keV. In contrast to this, 4. 75 MeV sputters uranium from UF 4 19 F(+2) with a yield of 5. 6 :!: 1. 0, which is about 3 orders of magnitude larger than expected from the Sigmund theory. The energy dependence of the yield indicates that it is generated by electronic rather than nuclear stopping processes. The yield depends on the charge state of the incident fluorine but not on the target temperature . We have also measured the energy spectrum of the uranium sputtered from the UF . 4 Ion explosions, thermal spikes, chemical rearrangement and induced desorption are considered as possible explanations for the anomalous yields. -v- TABLE OF CONTENTS I. Introduction 1 II. Theory 7 III. Experiments 20 A. General Techniques 20 1. Hardware 20 2. Track Dectector Method 23 3. Charge Integration 27 4. Data Analysis 29 5. Errors 32 B. C. D. E. Sticking Fraction Experiments 34 1. Introduction 34 2. Data Analysis 37 3. The Experiments 38 Experiments with Glass Targets 41 I. Target Preparation 41 2. Results 42 Experiments with uo 2 45 Targets 1. Target Preparation 45 2. Results 46 Experiments on U F 48 I. 4 Target Preparation 48 2. Rutherford Backscattering Experiment 51 3. Low Energy Sputtering of UF 4 54 4. High Energy Sputtering of U F 59 5. Energy Spectrum Determination 4 63 -viTABLE OF CONTENTS (Cont. ) IV. Discussion 67 Appendix A - Standard Glasses Used for Neutron Irradiation 72 Appendix B - Selected Chemical and Physical Properties of Uranium Fluorides 75 References 77 Tables 81 Figures 114 -1- I. INTRODUCTION A nonrelativistic charged particle passing through a solid loses energy in two ways. It may collide with individual electrons and thus experience what we call electronic stopping. go nuclear stopping by colliding with nuclei. Or it may under- The relative importance of the two mechanisms depends on the energy of the incident particle. Furthermore, the mechanism that dominates determines how the solid will dissipate the deposited energy. At energies below a few tens of kilovolts nuclear stopping dominates. The incident particle collides with the nuclei via a screened Coulomb interaction, which results in very little electronic excitation. An atom struck by the particle is called a primary recoil. The primary recoil can itself be quite energetic, sometimes receiv- ing many hundreds of electron volts. It collides with other atoms, which in turn collide with still more. Eventually, the kinetic energy of the primary recoil will be shared by a multitude of low energy atoms called a cascade. If the cascade is generated close enough to the surface of the solid some of these low energy atoms may reach it and escape into space. has been sputtered. In that event, we say that the solid The sputtering yield, S, is defined as the number of atoms that escape per incident particle. here: We speak loosely the escaping objects may not be atoms but molecules or ions. In addition, we may want to restrict our attention to only one type of atom in the solid and thus describe a partial sputtering yield. A widely used theory of sputtering based on this picture has been developed from neutron transport theory by Sigmund and his coworkers. It will provide the bench mark against which we will -2compare the exotic forms of sputtering to be encountered later. At higher energies the electronic stopping power begins to dominate and eventually becomes orders of magnitude larger than the nuclear stopping power. Much of the deposited energy serves to ionize the constituent atoms, producing energetic free electrons . The fraction ionized directly by the incident particle . is called the primary ionization, J. For us, this will be the most important component of the electronic stopping power. Since electrons do not efficiently transfer momentum to nuclei, one might expect that little of the energy would appear as nuclear kinetic energy. In insulators, however, an extraordinary phenomenon sometimes appears. Consider an insulator and an etchant that is capable of slowly dissolving it. For instance, mica and concentrated hydrofluoric acid are a combination we will encounter many times in what follows. If a particle with a sufficiently large ionization rate, such as a fission fragment, penetrates the mica it will leave in its wake a badly damaged lattice. We know this because the hydrofluoric acid will preferentially attack the region around the flight path thousands of times faster than it dissolves the undamaged lattice . After a few minutes of etching, a hole with a volume of several cubic microns will have formed around the flight path. Under an optical micro- scope the hole appears as a tiny track, which is what we call it. The mechanism by which tracks form is still controversial. Experiments in which latent, unetched tracks are erased by high temperature annealing suggest that the atoms in the lattice have been violently jostled out of their original positions. Yet we said earlier -3- that little atomic movement is expected in the electronic stopping region. One attempt to resolve this dilemma is known as the ion explosion model, proposed by Fleischer, Price and Walker (1975). In it the ionized atoms remain charged long enough for Coulomb repulsion to propel them into interstitial positions. They suggest that in metals, which do not produce tracks, the electron refilling time is too short to allow the mechanism to take effect. Though we will discuss the model in more detail later, it will be useful mainly for heuristic purposes: precise calculations and reliable predictions are not yet possible. In spite of the model's limitations, Peter Haff was able to use it to make a prediction (Haff 1976). He reasoned that if the ions do acquire considerable kinetic energy from Coulomb repulsion, then they should produce the same effect that the primary recoils produce in the nuclear stopping region. That is the ion explosion should generate a cascade, which should in turn produce a sputtering yield that may be exceptionally large. Furthermore, the behavior of the yield would differ drastically from that produced by the Sigmund mechanism. Haff sputtering would exhibit a peak in the electronic stopping region rather than in the nuclear stopping region, would appear only in insulators. and it . At the time the prediction was made, no exceptionally high energy sputtering data for insulators existed, so we designed a series of experiments to test the hypothesi s. This dissertation is a description of those experiments . Experimental verification of the prediction might at first thought seem straightforward. It is not. One of the cardinal requirements for a correct sputtering measurement is that the sputtered surface -4be atomically clean. Unfortunately, even at 10 -9 torr many surfaces at room temperature will absorb a monolayer of gas from the residual atmosphere in which they sit. At low energies the sputtering action itself may be used to keep the surface clean if one employs a sufficiently intense beam. This trick does not work at the high energies necessary for testing the Haff hypothesis. At several MeV the beam carries so much heat that it will evaporate the target long before the sputtering rate becomes sufficient to clean the surface. As we shall see, that heat can be turned to our advantage if it is applied in a controlled manner to a restricted class of targets. Learning to do this properly required some trial and error; our initial attempts were not entirely successful. Nevertheless, the data from those early experiments are instructive and will be discussed in detail. Our first experiments used targets consisting of soda-lime glass that had been doped with enriched uranyl nitrate. siderations motivated this choice. Two con- The first is that soda-lime glass is a well known track detector, which will produce tracks when irradiated with heavy ions as light as fluorine. Since fluorine was the beam we wanted to use, the glass was a natural choice. The second factor involves the method by which we detect the sputtered atoms. As reported in the thesis by Ron Gregg (1977), mica track detectors can be used to measure sputtering yields for targets . . z3su . conta1n1ng By adding a small amount of we hoped to monitor its sputtering behavior. 235 U to the glass Unfortunately, the data from this target quickly told us that surface contamination was -5influencing the yield. Irradiations at relatively high beam currents revealed "sputtering yields" much smaller than anticipated by Haff. The complexity of the glass, however, made it difficult to interpret the results in an unambiguous way. For instance, electron micro- probe studies of the beam spot indicated that the glass was losing large amounts of sodium due to excessive heating of the target. We corrected for this by reducing the beam current, which caused the yield to drop to unmeasurable levels. The beam current dependent yields were a strong indication that the cooler target surface was also a dirtier surface. The contamination problem with the glass targets proved to be insurmountable, so we tried uranium oxide targets next. Though no 1 one had ever seen tracks in uranium oxide, it has a relatively high resistivity, which suggested that it might exhibit the desired phenomenon. There are indications that surface contamination also had some influence on the uranium oxide yields, but the data are good enough to show that no mechanism of the type envisioned by Haff operates in it. loophole existed. This result was by no means conclusive since a After performing the experiment we learned that even small deviations from stoichiometry in uranium oxide can cause the resistivity to plummet (Ishii et al. 1970). Since we did not have precise control over the composition of our target, we had no assurance that the resistivity was actually high enough to allow the Haff mechanism to operate. For 'this reason we switched to uranium tetrafluoride targets. Fluorine at 4. 75 MeV sputters UF 4 with a yield three orders of magnitude larger than the Sigmund theory predicts. Furthermore, -6the yield exhibits a peak in the electronic stopping region where the Sigmund theory says that it should be monotonically decreasing. behavior of this target is especially fortuitous because UF does not tightly bind gas molecules to its surface. 4 The apparently Consequently, gentle heating allowed us to overcome the contamination problem. The data show that the sputtering yield is independent of target temperature between 70°C and l 70°C for pressures in the 10- 9 torr range. This not only suggests that the target is free of contamina- tion but also that if thermal spikes are producing the high yield, then the temperature of the spikes must be very high. The thermal spike model is one of several alternative explanations for the anomalously high yield. We will discuss them along with the Haff model in the next section. Before proceeding to the theoretical part of this dissertation, In all of the sputtering experiments de- we make one final note. scribed here, the sputtered particles were collected on catcher foils. The sputtered particles impinge on the foil' s surface with energies in the few eV range. To calculate the sputtering yield from the ex- periment one must know how many of the sputtered particles bounced off the catcher foil. Theoretical calculations are not yet feasible, so we measured the sticking fraction. Within the context of this thesis, it is needed only as a catcher foil calibration. ment is, however, important in its own right, The measure- so we will present a few results that extend beyond the immediate goal of determining uranium sputtering yields. -7II. THEORY The collision cascade of the Sigmund and Haff models is one of several mechanisms that can eject atoms from a surface under bornbardment. Thermal spikes, stimulated desorption and chemical re- arrangement are also conceivably capable of producing sputtering. Therefore, the deviation of the UF sputtering yield from the 4 Sigmund prediction does not in itself confirm Haff' s hypothesis. Having found the enhanced yield, we had to design a set of experiments capable of discriminating among the various possibilities. To understand the strategies we adopted, one must know the essential characteristics of each candidate. Only the Haff model will be discussed in those characteristics. detail. In this section we will sketch At the end of this section we will present the standard sta- tistical mechanical arguments that are applied to surface coverage phenomena, so the reader can understand the technique used to keep the target surface clean. Since we have adopted the Sigmund theory as our bench mark, we now present the formulas necessary for calculating the sputtering yield in that theory. For derivations it is best to consult Sigmund's papers (1969 and 1972). with atomic number Z 1 We consider an incident particle and atomic mass A 1 atomic target with atomic number Zia has number density N (A - 3 ). impinging on a mon- and mass A:a. The target We denote the surface binding energy by U (eV), which is usually taken to be the sublimation energy of the bulk material. The interaction of two Thomas-Fermi atoms is characterized by a screening radius, -8- where a 0 is the Bohr radius, . 5292 A. It is convenient to define a reduced energy, aA2 E e = --------2 Z .1 Zs e (A1 +Aa) where e is the electronic charge and E the incident particle. is the · laboratory energy of In the Sigmund theory the sputtering yield is proportional to the nuclear stopping cross section, Sn(e) = JTdo where a is the cross section for a collision transferring energy T to the struck particle. A compact empirical fit to some results from the Lindhard theory of stopping has been given by Winterbon et al. ( 1970): S (e:) = 9 [ln(x+ r) - x/r] n 8 e: where r = (l+x2 )1 /a, x = (2>..'e: 4 / 3 )1 13 and>..'= 1.309. This is related to the nuclear stopping power, et al. ~~ In' by (Lindhard 1968) dEI = dx ~ Finally, Sigmund expresses the sputtering yield with the formula S(E) = . 042 A. -:a tJ I dEI a.(Ag /Ai) N dX n 1 cos If (2. 1) where If is the angle of the beam with respect to the target normal, and the dimensionless function a(A:a /A1 ) is given by Andersen and Bay (1975). The a. function is independent of e: only for e: < 1, -9where the electronic stopping is negligible (Sigmund _!! _!!. At energies corresponding to e > I 0, the behavior of a 19 71) . is poorly understood. A simple prescription for treating binary targets has been developed by Haff and Switkowski (1976). of two types of atoms, a that na + ~ = 1. Suppose the target consists and b, with abundances na and Then the sputtering yield of a, &-,b a such 8a'b, is expressed = n a (na S a + nbSb) where Sa and Sb are calculated from Eq. S - is similar. ab ~ 2. 1. The expression for Their analysis applies only if the atoms leave the surface individually. If the sputtered particles appear as molecul es or molecular fragments, then the expression would need to be modi - fied . Sigmund's theory also makes precise predictions concerning the energy spectrum of the sputtered particles. The spectrum, which is common to all collision cascade models including Haff's, is distinctly different from those of the other mechanisms we will consider. peaks at one high energies. Fig. half the surface lloinding energy and falls as It E-:a at A typical example of such a spectrum is shown in I taken from R.A. Weller's thesis (1978). In our discussion of the Haff model, we will pre s ent an explicit functional form for the energ y spectrum. The crucial difference between the Sigmund and Haff models is in the mechanism that initiates the cascade. Recall that in the elec - tronic stopping region most of the energy lost by an incident ion i s -10deposited directly into the electrons. Many of these electrons are excited into continuum states, leaving behind ions in the lattices. This situation can generate an ion explosion if two conditions are fulfilled. First, enough ions must be produced so that most of them have an ionized neighbor. This establishes a strong electrostatic potential between the ions, which will, if given time, convert into kinetic energy. The second condition is that the free electrons in the target do not recombine with the ions so quickly that they do not have enough time to start moving. be about 10-14 seconds. The time required is believed to Apparently, this refilling time is controlled by the bulk resistivity of the material, since tracks form only in materials with high electrical resistivity (p ;;, 2000 0-cm) (Fleischer et al. 1975 ). Thus, we do not expect to see devia:tions from the Sigmund prediction due to this mechanism at high incident energies in metals and semiconductors. The method by which Haff made these notions precise is a derivative of the theoretical approach to sputtering taken by Thompson (Thompson 1968 and Haff 1976a). Thompson's paper. We begin with an equation from Let t (E, 9) be the flux of particles emerging from the sputtered surface with energy E to the surface normal. and angle 9 with respect If the incoming beam is perpendicular to the surface, then we have t(E,9) = Es q(Ee) d&. where d is the interatomic spacing and U is the surface binding energy. A detailed discussion of the factors in the integral can be -11found in Thompson 1 s paper. We merely need to know that the inte- gral expresses the energy deposited as primary recoils by the beam per unit time per unit volume. The factor fl, which is approximately O. 5, is the efficiency with which that energy is converted into dis Haff replaces ri times the integral with placement damage. f 6E/Ab where f is the incident flux, 6E is the average energy of a primary recoil and Ab is the mean free path to produce a primary recoil. Now ~(E,0}/f is the sputtering yield, cos 0 4rr S(E, 0) This expression explicitly displays the energy spectrum and the angular distribution of the sputtered particles produced by a collision cascade. If we integrate over S(0} d n = Ab ZU E, we get cos 0 4rr and integration over dO gives d 6E s = 8I Ab U So far, we have made no assumption concerning how transferred to a primary recoil. XE is The formula applies to both the Sigmund and Haff mechanisms, and it is the manner in which llE is computed that distinguishes the two approaches. Haff takes llE to be the kinetic energy produced by the Coulomb repulsion between two ions sitting side by side in the lattice. same mass, then each acquires 2! llE. If the two ions have the Haff also requires that ex- actly one pair is produced per lattice spacing, which implies that -12- A.b d = 2· Realistically, one expects the number of ions per lattice spacing to vary with J. If one allows this number to float with J, however, difficulties arise in attempting to calculate the average charge tin of each ion. S So we have = 81 UtiE (Z. 2) for a beam incident perpendicular to the surface. To calculate 6E we take -tiE (R(te)) = (tin e):a (I 1) (2. 3) d - R(te) where R(t ) is the distance between the ions when the electronic e refilling quenches the process. tin is fixed at d J (2. 4) =2= T where T is the average energy expended by the incident particle to produce a free electron. This formula for 6Ii is a manifestation of Haff' s demand that exactly two ions be formed per lattice spacing. Implicit in this assumption is the existence of a threshold: have J ~ T/d to trigger the explosion. we must It suggests that light jons may never have sufficient ionizing power to induce this type of sputtering, just as they are incapable of forming tracks in many insulators. Substituting Eq. 2. 3 and Eq. 2. 4 into Eq. 2 . 2 we get (2. 5) At this point it is instructive to estimate some of the number s predicted by these equation s . The functi onal dependence of f~ -13with respect to energy is shown in Fig. 2. But the magnitude of so we estimate it by setting J J is not well known, J will be discussed in greater detail below. dEI e Rl 300 eV/A., = dE/dxl e· For S MeV 19 F in- We take T = KZt, where K is cident on UF4 , dx the Bloch constant (Rl 10 eV) and Zt is the average atomic number of the target. For this case, length is 2. 3 A (Larson ~ al. for d. T = 256 eV. In UF 4 the U-F bond 1964), which is the value we will use Substituting these numbers into Eq. 2. 4 gives t:ii': 1.4 To estimate 6E we set R(t ) e = 00. which implies 1:E = 11 eV . One worries that this is rather low relative to the displacement energy of an atom iri the lattice. Haff suggests that the high degree of excitation in the neighborhood of the ion reduces the displacement energy considerably. Nevertheless, the primary recoil energies can- not be much greater than the lattice binding energies. One likely consequence of this is a significant distortion of the energy spectrum above a few electron volts; it would fall off more rapidly than our Furthermore, equations predict. sputtered particles would be pro- duced only by events occurring very close to the surface, and the behavior of those particles would be strongly influenced by the structure of the crystal lattice at the surface. Finally, taking U = 3 eV, which is the sublimation energy, we get S = 0. 5 at 5 MeV . This value should be considered a lower bound, because it.is possible -14- · that the excitation in the lattice also reduces the surface binding energy. A reduction in the surface binding energy not only increases the predicted yield but also shifts the peak in the energy spectrum. According 'to ;Eq. 2. 5, the dependence of energy is determined by J, which we take to be effective charge of the incident ion, and tion rate of a bare proton. Heckman et al. For Z p on the z is the Z2 J p . is the primary ioniza- we use an expression due to (1963 ): being the ion's atomic number and 13 Z J S = v/c. We display Reliable theoretical estimates for fluorine in Fig. 3. J p Z for are not available, since the energy regime of interest is well below that in which the Born approximation is valid. Experimentally determined ionization cross sections for protons and electrons in various gases are available from the work of Schram ~ !.!· (1965 and 1966) and De Heer et al. (1966). All of the ionization curves have roughly the same shape, so we have arbitrarily chosen the argon data as an illustration. The electron data have been scaled to the proton energy m equivalent to the electron's velocity: E _..£. E where E = p me e p = protron energy, m e Ee = electron energy, = electron mass. J p mp = proton mass and The data can be fit to a curve of the form = EAp ln(BEp ) . A least square analysis gives B = 4 . 5 x 10-2 /keV. compare the functional form to the data, and in Fig. Z2 J . p In Fig. 4 we 5 we show The Haff model predicts that the sputtering yield should -15- r follow if the inc.i dent ion strikes the surface in the equilibrium charge state. In practice, the ion usually enters the solid with a charge that may be two to four units below the equilibrium value. The path length needed to equilibrate the ion's charge is not known. But if it exceeds a few monolayers, the yield will be reduced because the ion explosions occurring close to the surface have the strongest influence on the yield. Furthermore, S should increase as the charge state of the incident beam increases. The ion explosion's dependence on the electronic component of the stopping power sharply differentiates it from the Sigmund mechThat contrast does not necessarily exist, however, between anism. the ion explosion and thermal spike models. Thermal spikes have been theoretically linked to both the nuclear stopping power and the electronic stopping power. The former mechanism has been most actively pursued by Kelly (1977), while the latter has been recently explored by Davies et al. ( 1979) in frozen gas targets. In essence, they argue that the energy of a primary recoil, whether it be an entire atom or an individual electron, eventually degrades to an energy distribution approximating thermal equilibrium in a small region around the incident ion's path. The spike is believed to reach quasi-equilibrium in about 10-11 seconds. The elevated tern- perature produced at the target surface, which could be as high as 0 10, 000 K, allows some of the atoms to evaporate. Unfortunately, severe problems arise when one attempts to theoretically estimate the sputtering yield. Little is known about the thermal conductivity in the excited region. Even less is known about the spatial distri- bution of the deposited energy, making it difficult to calculate the -16temperature at the surface. Due to these limitations we will make only a few qualitative statements on thermal spike behavior. At the core of most thermal spike models lies the assumption that the flux of sublimed atoms can be described by simple kinetic theory coupled with the Clausius-Clapeyron expression for the vapor 1 pressure. Thus, S is proportional to exp(U /kT)/T2, where T is the sum of the target temperature and the temperature increase due to the spike. U is the sublimation energy as before. The depen- dence on the initial target temperature can be very strong within a few hundred degrees of the melting point of the target. Furthermore, the sputtered atoms exhibit an energy distribution characteristic of 1 an equilibrated Maxwell-Boltzman gas: S(E) ex E2exp(- E/kT), where E This distribution has been is the energy of the sputtered atom. seen in the sputtering of gold at high temperatures by Chapman et al. (1972). The angular distribution would be proportional to cos e irrespective of the direction of the incident beam. Unfortunately, no precise statement can be made concerning the dependence of S on the stopping power. Another potential sputtering mechanism, which follows the electronic stopping power, is known as stimulated desorption. The mechanism we have in mirid is an analogue of the Frank-Condon process used to describe the dissociation of molecules. illustrates it. Fig. 6 An atom or molecule on the surface of the target is promoted from its ground state to an antibonding state or an excited bonding state. If the final energy is greater than the zero energy of the final state, then the potential 6f the final state will propel the -17particle out to infinity. The energy distribution of the desorbed particles will depend on the shapes of the initial wavefunction and the final potential curve. possible. Energies up to several electron volts ar e Since this is a purely surface phenomenon, it would be sensitive to the initial charge state of the incident ion. It would not, however, be sensitive to the mass of the incident particle: energetic electrons initiate this process with high efficiency (Menzel 1975). The final mechanism we consider is chemical sputtering. contingency could arise in the UF 4 Thi s targets, because uranium is known to form several different fluorides. Presumably, the new molecule would appear due to the disruption and rearrangement caused by the incident ion. After migrating to the surface, it would escape into space with low energy. Chemical effects could also en- hance other mechanisms if they tended to change the stoichiometry at the surface by preferentially removing either uranium or fluorine. The least ambiguous way to test for chemical sputtering is to look at the sputtered particles with a mass spectrometer to see if the yield is dominated by a single molecular species. In lieu of that, anomalous sputtering behavior in the nuclear stopping region. which normally would be adequately described by the Sigmund theory, is the most likely indicator of chemically induced erosion. In the next section we will describe the apparatus used to explore the mechanisms that produce sputtering in the electronic stopping region. As explained in the introduction, working in this energy regime poses some problems. In particular, we had to adopt a new -18approach to maintaining a clean surface. The technique used was ; ! motivated by certain general properties of the adsorption isotherms that describe the coverage of a surface equilibrated with a gas at pressure p and temperature T. As a concrete example, we will consider the Langmuir isotherm, which is applicable to localized adsorption of monatomic gases at up to one monolayer coverage. Derivations can be found in Hill (1960). Let 9 (p, T) be the fraction of the surface covered, where 0<9<1. For the Langmuir isotherm we have X(T)p 9(p, T) = 1 +x(T)p 0 where X(T) = q(T)e- U /kT eJJ (T)./kT (U <-0). µ 0 (T) is the chemical potential of the gas, and q(T) is the partition function for an atom in a three-dimensional harmonic oscillator well. exp(- U/kT) is common to all isotherms. The factor It arises from the require- ment that we have a consistent energy scale for both the atom at infinity and the atom in the potential well at the surface. Thus, when we calculate the partition function of the bound atom, we must add the surface binding energy U to the bound state energies so as to place zero energy at the top of the well. (tS; leV}, If U is small enough this factor supplies us with a powerful lever with which we can pry contaminant molecules from the surface. sures, At very low pres- 9 is approximately proportional to p exp(- U/kT). Under the right conditions we gain far more by heating the target than by improving the vacuum. Unfortunately, we cannot explicitly calculate the surface covera ge -19in our experiment, because we do not know the binding energies of the most common contaminants (H 2 and CO) to the targets we will use. In addition, we have none of the standard surface analysis tools, so we must fall back on more general arguments. nique we will adopt in the UF 4 The tech- experiments will be to measure the sputtering yield as a function of target temperature. tells us how the yield should behave. The isotherm At low temperature, the sur- face will probably be contaminated, so S will be low. As the tem- perature increases, the surface will become cleaner . so that S will rise until the surface contamination is negligible. For the clean surface, S should be constant with temperature (if the sputtering mechanism allows) until the temperature becomes high enough for thermal spike effects to set in. Fig. This behavior is demonstrated in 7 for several different pressures. with great care, because a pitfall exists. The argument must be used One of the contaminants could bind so tightly to the surface that it does not come off until the target begins to evaporate. To check for this we can perform some low energy sputtering experiments on the targets so that sputter cleaning can be used. The low energy runs will allow us to com- pare yields obtained from a heat cleaned target with those from a sputter cleaned target. If the results agree, then we can be certain that the heated target is clean. -20- III. A. EXPERIMENTS GENERAL TECHNIQUES The experiments we performed in our exploration of high energy sputtering fall into four types. Fig. 8 illustrates them schematically. We began with high energy yield measurements in which the sputtered uranium atoms were collected on cylindrical aluminum catcher foils. To check for target surface contamination, we performed some low energy yield measurements using the same catcher foil configuration. To calibrate the catcher foils, we measured sticking fractions using a double catcher arrangement in which some sputtered uranium atoms bounced off a primary foil onto a cylindrical secondary foil. Finally, we measured the energy distribution produced by our high energy mechanism with the apparatus developed by R.A. Weller (1978). In the next few sections we will discuss some general features of these experiments. We begin by describing the vacuum chambers and ac- celerators used in the irradiations. 1. Hardware We used the Caltech tandem accelerator to perform the high energy measurements. In most of the runs, we employed 19 F beams with energies from 1. 16 MeV to 28. 5 MeV and charge states ranging from +2 to +5. Stable beam currents between I nanoamp and 1 microamp were readily obtainable. used 4 In two additional runs, we He beams at energies from 0. 5 MeV to 2. 0 MeV. The en- ergies were determined by a 90° bending magnet coupled with an Alpha Scientific, Inc. Model 3193 NMR gaussmeter, which allowed us to set beam energies accurate to within a few kilovolts. The -21- beams for the low energy experiments were provided by a 150 kV duoplasmatron ion source. The beam energy is determined by the potential produced with a Deltaray Model Ll50-5-ARD power supply . . This power supply was calibrated by R. A. precision voltage divider. Weller in 1975 using a Because no calibrations have been performed since then, we do not claim to know the energy to better than 5%. Fortunately, the experiments performed with the 150 kV ion source do not require extremely accurate beam energies. The beam lines for both accelerators were kept at pressures between 10- 6 torr and 10- 7 torr with oil diffusion pumps. The liquid nitrogen cold traps above these pumps were filled a few hours before each run to minimize the hydrocarbon partial pressure. Even so, the vacuum was very dirty relative to the conditions that had to be maintained in the target chamber. To isolate the targets from this dirty vacuum, a liquid nitrogen cooled in-line cold trap 16" long and with 7I16 11 inner diameter was placed between the beam line and the ultrahigh vacuum (UHV) system containing the target. Further- more, a copper or gold seal straight-through valve was used to completely isolate the UHV chamber from the rest of the system except when beam was on target. With the straight-through valve closed, base pressures better than 10- 9 torr were usually obtained. Typical pressures during the runs were about 10- 8 torr, though this did depend somewhat on the type and intensity of the beam, which was the main gas load on the system. Several different target chamber configurations were used during the experiments. We defer until later discussion of the specific -22details. At this point we will discuss the components, which were used as a mix-and-match ensemble to construct the chambers. For a detailed discussion of our ultrahigh vacuum technique, the reader should consult the thesis by Ron Gregg ( 1977). The vacuum pumps were the heart of each system, and the chambers were designed around them so as to minimize the pumping impedance between pump and target. sublimation pumps. Two of the pumps were titanium The first was a small Varian model rated at 50 1/ s, while the second was a 500 1/ s Ion Equipment COV-500. The latter was supplementt!d with a 25 l/s ion pump, which is the second type of pump we used. In addition to the 25 1/ s pump, we had an 11 l/s Ultek D-I ion pump and a 60 l/s Ultek D-1 ion pump. employed the 60 1/ s pump in a unique manner, Melvin. tions, We suggested by Jon The central chamber of the pump was 'cleared of obstruc.so we could shoot the beam through it. Thus, we were able to place this pump between the target and the in-line cold trap. To be certain that the magnets in the pump would not steer the beam, we measured the field in the pump chamber with a Bell, Inc. 240 gaussmeter. Model The field did not exceed 30 gauss in the central region of the chamber. No problems have been encountered in steer- ing even light beams through the pump. Finally, molecular sieve sorption pumps were used to obtain pressures of about 1 micron of Hg, so the ion pumps could be started. The striking fraction runs were performed in a large chamber described in the Gregg thesis. done in standard 1-3/8" I. D. All of the other experiments were stainless steel crosses. All connec - tions were made with Con-Flat flanges and OFHC copper gaskets. -23All valves leading into the UHV section of the chambers had either OFHC copper or gold seals. Other commercially made components used included a bellows, windows and an electrical feedthrough. Any pieces built specifically for this project were constructed from the following materials: stainless steel, copper, Corning machinable glass ceraanic, alumina ceramic, mica, tantalum, tungsten, sapphire Be-Cu snap rings and chromel-constantan thermo- and aluminum. couples were also used. lime glass, U0 2 The targets themselves consisted of soda- and UF . 4 Owing to the fragility of some of the targets, the system was never baked at temperatures above 200°C. Consequently, pump down time for the UHV systems was about one week. 2. Track Detector Method All of the targets described here contained 235 u. Uranium con- taining targets were employed, so we could exploit the phenomenon mentioned in the introduction: a fission fragment from 235 u can produce a track in mica, which is observable under an optical microscope after the mica has been etched. sputtered 235 u In practice, we collected on high purity aluminum, pressed against a clean mica surface. which was then tightly On a second mica we placed a standard glass containing a known concentration of 235 u. After irradiating them together with neutrons, we etched both micas and determined the fission track densities. amount of 235 Thus, we could compare the u on the catcher with that in the standard. We performed the neutron irradiations at the UCLA Nuclear Energy Laboratory in their R- 1 reactor. Two ports of the reactor -24were used. For neutron fluences Cif 1016 /cm:a - 1016 /cm:a, we placed the samples in the center vertical access hole. column was used when fluences of Ia13 /cm:a - Iol 4 /cm:a The thermal were desired. The energy spectrum of the neutrons has been determined by the reactor staff. In the vertical hole, 56% epithermal, the neutron flux is approximately 44% thermal and 0. 4% fast, while in the thermal column we have 99% thermal and tion lasted 1-2 hours. 1% epithermal. A typical irradia- The samples could not be handled for about a week after each irradiation due to the activity induced in the micas. The mica used was muscovite supplied by the Perfection Mica Co. in 1-1/2" X 2 11 rectangles. A clean surface was prepared by cleaving the micas with a scalpel so that the surfaces exposed to the catcher foil touched nothing but that catcher. The catcher was sandwiched between the two halves of the cleaved mica. wiches were stacked in a small lucite holder. The sand- A sandwich containing the standard glasses was placed on top of the stack, which was then clamped down with a piece of lucite and two nylon screws. Two standard glasses spaced a few centimeters apart were used for the irradiations in the center vertical hole. This precaution was taken because the neutron flux spatial gradient is small in a region of the hole only 5 11 in length. To be certain that we were always in that region, the track densities produced by the two identical glasses were compared after each irradiation. In no irradiation did we find any evidence to indicate that the micas had been exposed to an inhomogeneous flux. Two types of standard glasses were used in the experiments. FoT irradiations in the thermal column, we used a glass containing -25- o. 10% by weight uo3 from a pre-war source. made by Corning Glass Works. This was custom The concentration of uranium oxide in the glass was determined by weighing the initial ingredients and assuming that no losses occurred during melting. For irradiations in the center vertical hole, we used Standard Reference Material No. 235 612 (NBS-612) from the National Bureau of Standards. u The concentration of the glass was determined by isotope dilution. The calculations used to infer neutron fluences from the standards have been reproduced in Appendix A, In this appendix we also pre- sent the results from an irradiation in which the two types of glass were exposed to the same neutron flux. Since the NBS-612 glass is by far the more thoroughly studied of the two, we wanted to see how the 0, 10% glass compared with it. for irradia.tion in the same manner. Both glasses were prepared After being cut with a diamond saw, they were polished with 1 micron alumina powder. then rinsed in dilute nitric acid followed by methanol. than one 235 They were Since less U atom in 106 fissioned in each irradiation, they could be reused indefinitely. The nitric acid and methanol rinses were occasionally repeated to maintain cleanliness. Approximately one week after each run, the micas from both the catchers and the standards were etched in 48% hydrofluoric acid. A typical etch was performed at 22°c for about 20 minutes. Since the appearance of the tracks is rather insensitive to small variations in the etching conditions, no special effort was made to keep conditions uniform. thos(~ We will assume that the etch reveals the tracks with 100~ efficiency (Fleischer .!:,.! al. 1975). After rinsing and dry- ing, the micas were taped to a microscope slide for observation. -26The microscope used for scanning is an exquisite instrument built by Leitz Wetzlar (serial number 646120). It has stage microm- eters, which allow one to reproducibly position the mica along both axes to within a few tens of microns. A reference point was es- tablished on each mica by scratching a small cross on the surface with a scalpel. To determine track densities, we must be able to count the tracks within a square of known size. The boundaries of the sq.uare were determined by a grid on an eyepiece reticle. For each objective lens of the microscope, the area covered by the grid was determined with a Unitron objective micrometer with 10 micron spacing between the lines. The objective micrometer was also used to check the accuracy of the stage micrometer along the x-axis of the stage (see Fig. 9). Since the track densities encountered were never more than a few times 10 6 /cm2 , transmitted light was used to count them. With the exception of two sticking fraction runs, all tracks for this thesis were counted by the author. Though every track counted was a fission fragment track in mica, there were differences in the track populations encountered. Tracks from a catcher foil were all pro- duced by uranium atoms sitting at the surface of the mica. Thus, the tracks were all of about the same length (- 10 microns). The tracks from the standard glass, however, were produced by uranium atoms distributed throughout a glass matrix . lengths varied from about 10 microns to zero . Consequently, the ir As we note in Appen- dix A, this difference influences the precision with which the t wo uranium surface concentrations can be compared. -273. Charge Integration The track detectors allow us to determine the number of uranium atoms that leave the target during an experiment. In a sputtering yield measurement, we must also count the beam particles entering the target. We accomplish this by using a current integrator to monitor the flow of electrons into the target induced by the positively charged beam. For an accurate measurement, one must know that all electrbns entering or leaving the target do so through the integrator. Great care was required, since beam currents down to I nanoamp were employed. Several mechanisms can lead to incorrectly determined currents. First of all, when the beam strikes the target, it can knock several electrons out into space. The same happens when the beam strikes collimators upstream, which sprays excess electrons onto the target. Furthermore, the ion pumps produce free electrons that can get t o the target. Heated surfaces can boil off electrons that might reach the target. Finally, if the target is not properly insulated, leakage through the insulator can produce erroneous readings. In Fig. 10 we show the chamber configuration used for the UF yield measurements. 4 The yield experiments on glass and differed in that we did not use the heater and thermocouple. that two current meters are shown. uo 2 No te The "house" meter was used to help focus and steer the beam through the . 12" hole in the t a ntalum collimator. Once the beam had been threaded through the hole ont o the target, we maximized the target current while minimizing the collimator current. A typical collimator current during the run was a few tenths of a n anoamp. A +300 V b i as was maintained on the -28collimator to keep electrons from spraying onto the target and to suck up any electrons drifting down from the ion pump. A t<tntalum disk with a 1/2" hole was placed 2. 5" down stream from the collimator to catch any electrons that might escape its attractive field. To protect the target from electrons in the 11 l/s ion pump, a strong horseshoe magnet was used, though experience indicated that it was not necessary. The target-catcher foil combination was designed to act as a Faraday cage. They were electrically connected outside the chamber so that electrons lost from the target would be collected by the catcher. Both were kept at +300 V bias. The thermocouple and heater were insulated from the target with alumina ceramic and sapphire. With bias on target the leakage currents to the target were negligible. The heater proved to be troublesome at high tern- peratures, since the target bias attracted electrons boiling off the hot tungsten wire. The current integrator used, a Brookhaven Instruments Corporation Model 1000, was capable of balancing out this current at target temperatures up to 200°c, but the integrator had to be reset every time the target temperature was changed, To be certain that the integration was working properly, some tests were performed with 2. S nanoamps of fluorine on target. The beam was fluctuating with an amplitude of about O. 2 nanoamps, so we could not detect changes in the current smaller than 0, 1 nanoamps. Doubling the bias voltages and turning off the ion pumps did not affect the current reading. A strong magnet had no influence on the current except between the target and collimator, where there seemed to be an increase of about 0. 1 nanoamps. Removing the bias -29from the target doubled the current reading. Even at the lowest beam currents, we believe that the current integration was good to about s%. One problem remains. Any beam striking the catcher foil will be detected as beam on target. catclier foil, Should beam particles be lost to the the measured sputtering yield would be too low. aiming error of I I 16 11 will cause the problem to appear. An To guard against this, the chamber was carefully aligned using a telescope mounted at the tandem's switching magnet. chamber was rigidly clamped in place. After alignment the At the end of each irradia- tion, the beam spot was carefully examined and found to be perfectly round. UF 4. 4 Nevertheless, there are some indications that a few of the runs were affected. We will discuss them later. Data Analysis As we stated earlier, the track detectors allow us to determine the number of atoms sputtered from the target during bombardment. In this section, we will describe the calculations that allow us to make that determination. The arguments we will present apply to both the yield and sticking fraction experiments. Recall that in these experiments the catcher (or secondary) foils are wrapped about the target in a cylindrical geometry. We will imagine the thin cylinder to be a segment of the hemisphere generated by rotating the cylinder a bout the target normal. Let R be the radius of the hemisphere, and let 0 be the angle relative to the target normal (see Fig. 9). In effect, 9 denotes the direction of a sputtered particle as it leaves the target. We will describe the angular distribution of the sputtered -30particles with the functional form f(8) = A cosB 0 . To determine the total number of particles, we integrate f(8) over the hemisphere: J 1T /2 Total = (A COSB 8)(2rrR2 sin 8 d8) 0 (3. 1) :a = 21TR A B+ 1 In the yield experiments, R = 1. 43 cm or 2rrR:a = 12. 85 cm:a; in the sticking fraction experiments, R = 1. 27 cm or 2rrR:a = 10. 13 cm:a. Of course, this analysis does not apply when the sputtered particle distribution is not symmetric about the target normal. In general, we can exploit this symmetry only when the incident beam is perpendicular to the target. The data appear in raw form as a map of the number of tracks counted at a given magnification versus position along a data band on the mica detector (see Fig. 9). Before we can fit the numbers to the form A COSB 8, we must convert the positions along the x-axis into angles. Since we know the radius, we only need to know the position corresponding to some given angle, for finding this point are available. say 8 = In the first, o. Two methods we determine the e = 0 be the midIn the second, we fit the data to A COSB e left and right hand boundaries of the band and let point between them. using a series of zero points spaced . 01 cm apart and choose the zero point resulting in the smallest one we adopted. x 2 • The latter method was the For data bands 4. 5 cm long, the average discrepancy -31between the two methods was 0. I cm. The parameters A and B were determined by applying a weighted least squares analysis (Bevington 1969) to the expression ln N( 9) = ln A + B ln cos 9 where N(9) is the number of tracks counted at angle e. counts follow a Poisson distribution with variance N. In the weighted The track least squares routine, we must use the variance of ln N, which is in this case l/N. A computer program based on Bevington' s treat- ment of the weighted least squares analysis was used to compute A and B along with the standard deviations of A the program generated the x2 for the fit. and B. In addition, We will present results from this program when we discuss the specific experiments. At this point, A To use A is merely the number of tracks at e = O. we must convert it into a density of uranium atoms. The first step in the conversion is to compute the area covered by these tracks. That comes from the area covered by the reticle grid and from the number of frames scanned at each value of x on the mica. Now we have the track density, which we can convert into a 235 u density using the neutron fluence from the standard glas s es as explained in Appendix A. dividing the the targ et. 23 Finally, we obtain the uranium density by SU density by the atomic abundance of the 23 SU in In most ca s es the isotopic composition of our targets had been altered, so we depended on our supplier, Oak Ridge National Laboratory, to give us the relevant numbers . Having conve:i;ted A into the number dens ity of uranium atoms at 9 = 0, we can now apply Eq. 3. 1 to obtain the total number of uranium atoms sputter e d . -32From the charge integration we know the number of incident atoms, so we divide this into the total number of uranium atoms to give us the partial yield S(U). A small correction to this number remains to be made; we will discuss it at the end of the section on errors. 5. Errors From time to time in the previous sections and in Appendix A we have described errors that can enter into our measurements. effort will be made to recount them here. the reader of the most serious one, No We will, however, remind which is the 20% uncertainty in the neutron fluence due to poor knowledge of the range of a fission fragment in glass. In the neutron fluence determination we can claim far greater precision than accuracy. The precision of this measurement is determined by two factors. The first is the number of tracks counted on the mica for the standard glass. In most cases we counted between one and two thousand tracks giving us a standard deviation of 3% or less, observer, The other factor is counting errors by the a problem much more difficult to quantify. Spot checks by the author indicated that track counts were reproducible to within about 2%. The source of error we want to concentrate on in this section is of an entirely different nature, Recall that the radii of our catcher and secondary foils are less than 1. 5 cm. Consequently, significant errors could appear if the beam spot is a s much as O. I cm off center. Misalignments of this magnitude turned out to be unavoidable, so we wrote a computer program to numerically study the behavior of the data when such displacements occur . The geometry of the -33- analysis is illustrated in Fig. 11. We simulated a beam spot with a rectangular grid of 25 equally weighted source points. Each source point produced an imaginary flux of sputtered particles with angular distribution cosB0 q>. In all cases quoted here, B 0 L = W (see Fig. For a given point on the catcher foil, 11 ) . equals 1 and the response produced by a source point is proportional to (cos B0 The definitions of R and a cp)(cos a) Ra can be determined from the figure. A two-dimensional Simpson's rule routine was used to sum the contributions from the entire grid. The program performed this calculation for 21 equally spaced points on the catcher foil and then fit those points to the functional form A COSB e using a least squares routine . .. In this way we were able to study the behavior of A - and B as a function of beam spot size and displacement vector D. By specifying D we could displace the beam spot along all three axes. In Table 1 we exhibit some of the results from the program. The numbers have been normalized to give A/(B+l) = 1. 0 for a O. 5mm square beam spot that is exactly centered. All cases cor- respond to the same amount of material leaving the beam spot. The most important conclusion to be drawn from the results is that while B is sometimes sensitive to alignment errors, A/(B+l) is not. In p'articular, note the behavior with respect to displacements along the z-axis (D z < 0 implies the target is too far from the catcher). have reason to believe that the target in the UF 4 We runs was displaced about Imm from center in the negative z direction. Thus a sputtered -34angular distribution with B0 with B = 0. 75. = 1 would be detected as a di,stribution Indeed, the measured angular distributions tended to fall between B = 0 . 7 and B = 0. 8. Even with this error, however, the calculated yield will be wrong by only I if, . This correction is s o small we will not include it in the data analysis . In Figs. 12 and 13 we show a pair of angular distributions computed by the program. At the end of the last section, we alluded to a correction that would be made to the data. That correction arises from the follow- ing consideration. Imagine a sputtered uranium atom approaching the catcher foil. It most likely has an energy in the low eV range (Weller 1978). We ask, "What is the probability that it will stick to the catcher's surface? 11 the sputtering yield. We must know the answer to calculate Since no reliable theory describing this situa- tion exists, we initiated a program to measure the sticking proba~ bility. We will describe it in the next section. B. STICKING FRACTION EXPERIMENTS 1. Introduction Our sticking fraction (or trapping probability) measurements were initially motivated by the work of Close and Yarwood ( 1967), who studied the trapping of low energy noble gas atoms on tungsten surfaces. Their data indicated that the trapping efficiencies fell drastically at energies below a few hundred electron volts. Though we susJ_)ected that this behavior was peculiar to the noble gases, their paper undermined our belief that low energy uranium atoms stuck to aluminum oxide surfaces with unit probability. were reinforced by the papers of Hurkmans et al. (1976 , Our fears 1976a, -351977) and Overbo sch ~ .!!· (1977) in which the trapping of alkali metals on hot tungsten surfaces was experimentally explored. Though the behavior of the alkali metal atoms was different from that of the noble gases, they too demonstrated that trapping probabilities could be much less than one at energies relevant to our own experiments. To remedy the situation we undertook the series of sticking fraction experiments illustrated in Figs. 8 and 14. aim of this effort was to calibrate our catcher foils, Since the primary the criteria for a correct experiment were different from those usually demanded in a surface scattering experiment. For instance, our catchers are exposed to a flux of uranium with the energy distribution shown in Fig. 1. Consequently, we used an unanalyzed beam of sputtered particles rather than the customary monoenergetic beam. Further- more, during a sputtering experiment the catcher foil surfaces are contaminated with gases from the vacuum, so initially at least we made no special effort to clean our primary foils. To have done so might have produced misleading results. Without the extraordinary sensitivity afforded us by our track technique, the experiment would have been much more difficult for the following reason. If the sticking probability is close to 1, which was the case, we must severely limit the fluence to which the primary foil may be exposed. foil exceeds, If the accumulation of uranium on the primary say, O. l monolayers, then the probability that an in- coming uranium will strike a uranium rather than a substrate atom becomes unacceptably large. Therefore, we always limited the uranium fluence on the primary to less than 5 X 1013 /cm2 • This implies that the uranium surface density on the secondary was very -36small. Perhaps, even 0. 1 monolayers on the primary foil is too much. We must consider the possibility that the uranium atoms reaching the secondary do not do so by merely bouncing off the primary. Conceivably, they could be atoms that stuck to the primary but were subsequently resputtered by incoming uranium atoms or even argon atoms backscattering from the uranium foil. Fortunately, two simple tests allow us to check for this possibility. In the first, we perform a normal sticking fraction run using a sputtered uranium beam. u foil to produce the Then we immediately repeat the run using the same catcher foils but with a beam. 23 5 23 8 u foil to produce the sputtered Our t:r:ack detectors are blind to 238 u, so this second run will have no influence on the final result unless the incident beam resputters 235 u already on the primary. The second test is conceptually similar to the first, but in this one we measure the sticking fraction as a function of the fluence on the primary. 235 u Suppose that the incoming uranium atoms are resputtering uranium from the primary with an effective yield S. The number of uranium atoms resputtered will be {fluence of U atoms ){average surface coverage) { S) • The average surface coverage during the run is 1 /2 of the final coverage and is proportional to the fluence of uranium atoms. Thus, t he fraction not sticking to the primary will be proportional to the fluence of uranium atoms on the primary if all of the atoms reaching the secondary do so through resputtering. Similar arguments can be made if backscattered argon is causing the resputtering. We will -37present results from both tests later. 2. Data Analysis We will quote our results in terms of the fraction f of uranium atoms that do not stick to the primary foil. To see how we deter- mine f, consider N atoms incident on the primary. ( 1 - f) N of these stick, while fN atoms strike an imaginary hemisphere with radius equal to that of the secondary foil. ( 1 - £) fN stick to the hemisphere. Of these fN atoms, Since f will turn out to be small, we will not worry about the fate of those atoms not sticking to the secondary. Now we take the ratio of the number on the hemi- sphere to that on the primary: atoms on hemisphere atoms on primary = (1 - f) fN = ( 1 - f) N f . From our analysis leading to Eq. 3. 1 1 we know how to calcu- late the number of atoms on the hemisphere. We must now deter- mine the number of atoms on the primary. The sputtered uranium beam incident on the primary is collimated by a hole 0. 4 cm in diameter. The resulting beam spot produces a uniform track density on its associated mica except at the rim of the spot, where the track density gradually falls to zero over a distance of about O. 1 cm.. To find the area of the spot, we use the stage micrometer of the microscope to determine a set of about 20 points on the rim where the track density has fallen to approximately 1 /2 of the value in the interior of the spot. A typical error in determining the radial position of such a point is about . 01 cm, to 10~ in the area. which translated into errors of up In practice, we expect to do much b¢tter than -38- IO)t, since randomly fluctuating errors about the 1/2-density radius should tend to cancel. To calculate the area of the spot, we choose a point close to the center and compute the distances r(9) to the 20 points. We then numerically evaluate the integral 2ir ~ f 3 r (9) d9 . 0 The value obtained is stable against changes in the position of the center point. With the area in hand, we can determine the total number of uranium atoms in the spot by counting a few thousand tracks in the interior. This, along with the number of uranium atoms on the hemisphere, immediately gives us the sticking fraction. In addition to the error in the spot area, one other possible source of error is worthy of note. 23 5 u Due to the very high density of in the primary spot, we must irradiate it with neutrons in the reactor's thermal column rather than in the center vertical hole, where the secondary goes. Consequently, we compare the primary against the O. IO)t glass and the secondary against NBS-612. In Appendix A we show that the two standards agree very well. Fur- thermore, since we eventually consider only the ratio of the neutron fluences, the uncertainty in the fission fragment range does not plague us. Thus, the only errors from the neutron fluence deter- minations are the usual 3% errors due to counting statistics. 3. The Experiments The sticking fraction experiments were performed in the chamber shown in Fig. 15. Two schematic views of the configura- tion inside the chamber are shown in Fig. 14. The heater shown in -39the. figure was used in only two of the runs; in all of the others the primary foil sat at room temperature. Surface contamination prob - lems motivated our use of the heater. As we explained at the end of the theory section, even with base pressures in the 10- 9 torr range one expects to find a monolayer of gas adsorbed on most surfaces. Since heating can desorb much of this gas, we wanted to see if heating the primary foil to 150°c would affect the sticking The heat was provided by pas sing about 1-1 /2 amps through fraction. a thin tantalum foil. We measured the. temperature with a chromel- constantan thermocouple connected to an Intersil ICL7106 millivolt In lieu of a reference thermocouple, we measured the room meter. temperature with a mercury thermometer and took that to be the temperature corresponding to O. 0 mV. The temperature reading was accurate to within z c. 0 In each run we could perform two independent measurements by mounting two primary-secondary ass~mblies (we call them cages) on a cylinder 2. 5" in diameter. The sputtered uranium atoms were 235 produced by irradiating an enriched uranium foil (93% u) with · argon. The uranium foils were cleaned with concentrated nitric acid and rinsed with acetone shortly before loading them into the UHV chamber. No sputter cleaning was performed. By using two uranium foils along with shields we could irradiate the cages independently. During the runs the argon beam was the main gas load on the systern: typical pressures were a few times 10- 8 torr. Base pressures in the chamber before each run were usually a few times 10- 9 torr. In all cases the secondary was high purity aluminum foil supplied by Ventron Alfa. This same foil was used for the primary -40in five runs. Due to the high purity of the foil, we did not attempt to clean it before loading it into the cages. cleaved mica was used as a primary. In one run, freshly Two runs employed primaries consisting of aluminum evaporated onto a mica substrate, and two runs employed similar primaries consisting of evaporated gold. The evaporations were performed in a bell jar pumped with a liquid nitrogen chilled baffle and an oil diffusion pump. Typical pressures were about 10- 6 torr. The results from these experiments are shown in Table 2. The precision achieved in our measurement of f, the fraction not sticking, can be inferred from the scatter in the four Al foil runs using 80 keV argon: 2. 42 + . 33%, which corresponds to a 14% The fluctuations in B are larger than expected and difficult error. to explain:. though they may be due to warping of the primary foils. Note that f primary. does not scale with the uranium surface density on the Furthermore, · the run in which the primary was also ir- radiated with sputtered 238 u did not produce an anomalous result. Thus, we may conclude that the uranium residing on the secondary foil did not get there through re- sputtering. for f lie in the 2'% to 3'f, range. All but two of the values This may indicate that surface contamination controls the sticking probability. The mechanisms producing the two excursions out of this range are not understood. The low value of £ for the heated gold may be due to the high mass of the substrate atoms. The low value resulting from the 40 keV argon run is unexplained. Three representative angular distributions are displayed in Figs. 17 and 18. 16, -41- Much of our difficulty in interpreting these numbers arises from the absence of an adequate theoretical description of the interaction. Several reasonably successful theories for surface scatter- ing exist. but they apply only when the incident atom is lighter than the surface atoms. In that case. the interaction with the surface can be described in terms of a single binary collision. The mass ratio with which we are dealing makes the picture much more complicated. Unless some sort of collective action occurs. the heavy uranium atom must undergo multiple scattering just to turn around. C. EXPERIMENTS WITH GLASS TARGETS 1. Target Preparation Of the track detectors usable in a UHV system. common soda- lime glass has one of the lowest thresholds for track registration. In addition. it is a system that is easy to manipulate; through simple procedures. we can dope the glass with controlled quantities of uranium. These glasses are, however. complicated. and many aspects of their behavior are poorly understood. finally proved to be overwhelming . These complexities But in struggling with them, we discovered some features of the targets' behavior under bombardment that are intriguing. To produce the target. we selected a Gold Seal microscope slide and ground it into a fine powder with pestle. a sapphire mortar a nd A uranyl nitrate solution was prepared by dissolving 45 m g of enriched uranium (93.32% by weight 235 u) in 0.5 ml of 70%HN0 . 3 An unknown fraction of the uranium had oxidized, which introduc ed a n error of no more than 10% into the uranium concentr.ation determination; -42100 µl of this solution was injected into . 75 g of the glass, which was then stirred and baked to drive off the water. The mixture was 0 sealed in a platinum capsule and baked at 1500 C for 14 hours to insure complete mixing. During this time a small hole developed in the capsule allowing about a third of the glass to ooze out and be lost. 0 After quenching, the glass was annealed at 600 C for over 8 hours before opening the capsule. One large piece was obtained, which w a s cut and polished with alumina to form a target surface with dimensions 0. 5 cm by 0. 3 cm. If we assume that no losses occurred during baking, then the glass is about 1.1% 235 u by weight or ap- proximately 0. 3% atomic. After the glass was finished, Tim Benjamin and John Jones per- formed some electron microprobe analyses of the glass. They em- ployed a 15 keV electron beam with a current of 5 nanoamps. For a given spot, the duration of each irradiation was 200 seconds. At this energy, the microprobe samples the top 2-3 microns of the glass. At first, beam spots 30 microns in diameter were chosen, but the data indicated that the electron beam was driving out over 90% of the sodium oxide. alleviated the problem. Expanding the beam spot size to 50 microns In Table 3 we show the results from one of the runs. 2. Results The sputtering yield experiments were performed in the chamber shown in Figs. 19 and 2.0. In Figs. 21 and 22 we show the catcher foil holder, which allowed us to perform three sputtering runs without breaking vacuum. The catcher foil shown in the picture wa s for -43an incident beam normal to the target surface ('f our glass experiments, we set 45°, so 9 =o 0 t = 45 0 . = 0 0 ). In most of The holder was also rotated still corresponded to the center of the catcher foil. The holes in the catcher foil were shifted accordingly. Since the target could not be moved, all three of the runs were performed on the same beam spot. At the end of each set of runs, the target was polished to expose a fresh surface. The initial runs were per- formed with fluorine beams at energies ranging from 2 MeV to 20 MeV and at beam currents of about 20 particle nanoamps. cases the beam power exceeded 3 watts/cm2 • In some Since the target was a good thermal insulator, it could not efficiently dissipate the heat generated. The heating produced some dramatic effects. The beam spot became powdery, and a raised annulus around the spot indicated that the glass had flowed. As we can see from Table 3, the micro- probe studies reveal preferential loss of sodium, which means that it is evaporating or diffusing away from the beam spot. The angular distributions of the sputtered uranium atoms do not, however, indicate that the uranium evaporated. In Figs. 23 and 24 we show angular distributions from two of the high current runs. In all cases, 8 = 0° corresponds to the target normal. Note that these distributions peak at about 45° relative to the normal, which is 90° with respect to the beam. duce such a distribution. Evaporation could not pro- A further surprise is the sharpness of the angular distribution produced by such a rough surface. For com- parison, we display in Fig. 25 an angular distribution produced by a slightly oxidized pre-war uranium foil irradiated at If' = 45°. The -44surface of the foil was not nearly as rough as that of the glass after irradiation, but the angular distribution produced by it is similar. Finally, in Fig. 26 we show the angular distribution from a 10 MeV run on the glass target. nanoamps. In this case the beam current was only 5 Note that the peak is not as sharp. Other experimenters have also observed such shifts in the angular distributions of sputtered particles produced by irradiation at oblique incidence. But the magnitude of the forward shift tends to decrease as the beam energy increases (Oechsner 1975). Since the targets were irradiated at \jr the total sputtering yield. = 45°, we cannot compute Nevertheless, we can compare the uranium densities at the peaks of the various angular distributions. are shown in Table 4. glass with the 23 5 u. 235 The data In the table we compare the results from the u yield from the uranium foil, which contains . 72% Total sputtering yields for uranium foils will be quoted in the next section, and we will see that no anomalously high yields were detected. Thus, we may conclude that the yields from the glass tar- gets are not exceptional either. Clearly, the only parameter strongly influencing the yields from the glass is the beam current. In fact, some irradiations of the glass targets were performed at normal incidence with beam currents below 5 nanoamps. catcher foils . No uranium above background was detected on the Thermal effects provide the only mechanism through which the beam current can control the yield. If the uranium atoms were being sputtered by thermal spikes, then high currents would enhance the yield by increasing the target temperature. Yet, we -45have argued that the shape of the angular distributions precludes this. On the other hand, if surface contamination were suppressing the sputtering, then high currents could increase the yield by keeping the surface clean. In section II we pointed out that high tempera- tures can desorb contaminant gases from a target surface. case, In this such cleansing action would be aided by the flux of sodium evaporating from the surface. Therefore, we conclude that the glass surface was probably contaminated during our sputtering runs. D. EXPERIMENTS WITH 1. Target Preparation uo2 TARGETS The most convenient alternative to the glass targets was uo 2 . As a binary compound, it escapes many of the complications inherent in the glass. Furthermore, its high melting point allowed us to use intense beams without evaporating its constituents. not a known track detector. Of course, it is Nevertheless, at 400°K it has an elec- trical resistivity of about 4 X 10 4 0-cm (Ishii et al. 1970), which is over an order of magnitude greater than the generally accepted minimum resistivity for track registration. Unfortunately, Ishii and his co- workers demonstrated that even tiny deviations from stoichiometry can cause the resistivity to drop by orders of magnitude. Whereas we were not equipped to monitor the uranium-oxygen ratio to the required precision, the results would provide a good test of the ion explosion idea only if the anticipated anomaly were seen. We can explain away the ion explosion's failure to appear by invoking a nonstoichiometric uranium-oxygen ratio. In all of the runs, the target consisted of an oxide layer on a -46cold rolled uranium foil. A picture of the surface of such a foil can be found in the Gregg thesis ( 1977}. Before preparing the oxide layer we always cleaned the foil by etching it in concentrated nitric acid and then rinsing it in distilled water and acetone. The manner in which we prepared the oxide changed as we learned more about the system. Practically all of what we know about the oxidation of uranium comes from the work of Flint, Polling and Charlesby ( 1954}. ·In several runs with a natural uranium target, oxide layer to be as thin as possible, we wanted the so we loaded it into the UHV system and began pumping within 30 minutes of removing it from the rinse. Since no color change occurred, the oxide layer was less than Q 500 A thick (Flint et al. 1954}. In several other early runs we simply let an enriched foil sit out in air at room temperature for a few days. The pale brown color it developed indicates that the oxide layer was about 500 A thick. An oxide layer formed in this manner does not protect the uranium from further oxidation, which means that the layer is not continuous. A continuous protective layer will form, however, if the oxidation occurs at elevated temperatures. Based on Flint's results, we decided to produce 700 A thick layers by heating the foil in air at I00°C for 20 minutes. layer was a beautiful deep blue. on the samples, The resulting oxide We performed no surface analysis so we cannot guarantee the purity or the stiochiom- etry of the targets. Even if the uranium-oxygen ratio had been ex- actly what we desired, it probably would have changed under bornbardment. 2. Results All of the runs were performed using the chamber and catcher -47foil holder shown in Figs . 19 through 22.. section are for beams incident at = o0 (normal incidence). t results are displayed in Table 5. All results quoted in this The In Table 6 we show some results from a Sigmund theory calculation using the expression for binary sputtering from Haff and Switkowski (19 76 ). One should bear in mind that we have stretched the Sigmund theory to energies far higher than intended by its author . a For instance, the behavior of the function has been explored to values of e no higher than 10 (Sigmund et al. 1971), while in this calculation we encountered e values greater than 103 • Lacking an explicit expression for a, we some- what arbitrarily set it equal to O. 1. For the binding energy U we chose 5. 4 eV, the sublimation energy of uranium metal. In view of the uncertainties involved, the agreement between the Sigmund numbers and the thick oxide yields is astonishing. the numbers fluctuate considerably, parent. Though several general trends are ap- First of all, the yields tend to decrease as the oxide thickness increases. In fact, the yields from the thinnest layers are just a factor of two below those expected for pure uranium from the Sigmund theory. We suspect that the collision cascades in the thin oxide layers were depleted in oxygen, which would account for this behavior. Furthermore, there seems to be no dependence on the heating from the beam: the largest yield (. 04) appeared in the run with the lowest beam power (. 14 watts/cma ). Finally, the yield decrea1:1es with increasing energy as expected from the Sigmund theory. The yields show no sign of exotic effects from the ion explosion mechanism. -48E. EXPERlMENTS ON UF 4 In contrast to the glass and oxide targets, thing but trouble, which gave us no- our uranium tetrafluoride targets proved t o be ideally suited to studying the exotic sputtering processes for which we had been searching. The reward for this choice was two-fold. This compound 1 s extraordinary properties allowed us to overcome the usual technical difficulties associated with sputtering yield measurements. In addition, the target displayed an unexpected richness and subtlety in its behavior under ion bombardment. The rather extensive experiments described in the next few sections only begin to reveal the sputtering phenomenon that we have discovered, of the mechanism involved still eludes us. and the identity Nevertheless, not all of our experiments were immediately directed toward uncovering that identity. To be certain that our numbers were not misleading us, some technical issues had to be resolved. partly because UF 4 These issues arose is an unfamiliar substance--especially in this context. To help acquaint the reader with UF fluorides, we have assembled some information on their physical and 4 and the other uranium chemical properties in Appendix B. I. Target Preparation In vacuum, UF 4 melts without decomposing at 1309°K (Rand and Kuba s chewski 1963 ). It also vapor:iizes nondestructively (Hilden- brand 1976), which opens up the possibility o f utilizing it as a thin film. By evaporating a few thousand angstroms of UF highly polished copper backing, 4 onto a we immediately solve several problems. Being an excellent thermal and electrical insulator, it would cause -49serious complications due to heating and charging if irradiated in bulk form. The close proximity of a conducting substrate mitigates both problems. Furthermore, by evaporating onto a polished surface we achieve a smooth UF sully the UF 4 itself. 4 surface without having to polish or otherwise The evaporation can, however., introduce prob- lems of its own. When considering the use of evaporated targets for sputtering, we must keep in mind the experiments of Andersen and Bay (1972) with evaporated Cu targets. They found that gaseous contaminants incorporated into the target during deposition can suppress the measured yield by over a factor of two. At 5 x 10- 7 torr, the pressure at which their evaporations were performed, the flux of molecules striking the surface exceeds 1014 /cm-. - .s. To reduce the contamina- tion to a few percent, one must deposit nearly 10 monolayers every second. They obtained acceptable results from 10, 000 A layers, which were deposited in 5-10 minutes. They also discovered that the evaporated films tended to expel the foreign moelcules when exposed to UHV conditions--even at room temperature. In performing our own evaporations, we strove to reproduce the conditions achieved by Andersen and Bay. We obtained anhydrous UF in powdered form from two sup.., 4 pliers. A batch from Ventron Alfa was of unknown isotopic content. We used it for a Rutherford backscattering experiment to be described later. Oak Ridge National Laboratory supplied a highly enriched batch (93. 08 atomic 'f, 235 u) for use in all of the sputtering experiments. For evaporation, the powder was loaded into a tantalum boat consisting of a sealed tube with a small hole in the middle. The boat could - 50be heated to nearly white heat by passing up to 100 amps through it. Of course, separate boats were maintained for the two batches to avoid diluting the enriched uranium. The evaporations were performed in a Veeco VE- 775 vacuum system incorporating an oil diffusion pump with a liquid nitrogen chilled baffle. The vacuum chamber was equipped with a Sloan crystal oscillator for monitoring the mass density of UF 4 deposited and with a shutter for controlling the duration of the evaporation. The pressure in the chamber was measured with an ion gauge. After loading the boat and the copper target into the chamber, we allowed it to pump down overnight with the baffle at room temperature. During this time, the tantalum boat was heated in the vacuum with a 40 amp current to help outgas the UF , 4 which is slightly hygroscopic. On the following day the baffle was chilled. which made the pressure fall to about 5 X 10- 7 torr within a few hours. The current through the boat was gradually increased until it could be maintained at 80 amps without significantly degrading the vacuum. tion by opening the shutter. We began evapora- In less than 2 minutes we would deposit 0 about 2, 000 A of UF ceeded 10- 6 torr. 4 onto the copper at pressures that rarely ex- We terminated the evaporations by quickly closing the shutter. Assuming that the gas incident on the target stuck with unit efficiency, we calculated that most of the evaporations produced films with less than 10% contamination. One unusually poor run resulted in an estimated contamination of 42~, but this film did not produce results at variance with the other targets . This is probably owing -51to our handling of the targets after evaporation. Immediately after removing the target from the bell jar, we loaded it into the UHV system and started pumping. The targets' exposure to atmospheric pressure always lasted less than an hour . Once the chamber pres s ure was below 10- 7 torr, heating tapes were used to elevate the target 0 temperature to about 200 C. It was baked for a week at this tern- perature as the pressure fell to below 2 x 10- 9 torr. We believe that the target cleaned itseli by outgassing during the baking, and the consistency of the results obtained bears this out. 2. Rutherford Backscattering Experiment To be certain that our evaporated films were really UF , 4 we measured the uranium to fluorine ratio in one of our films using backscattering. For these runs we did not evaporate onto a copper backing, because the copper would have hidden our fluorine signal. Instead, we evaporated 22µg/cm 2 ing carbon foil. 4 We u~ed energy for the analysis. UF 4 onto a 9µ.g/cm 2 self support- He at 1. 8 MeV and 2. 0 MeV incident Applying Bragg's rule, we estimate from Ziegler's tables ( 1977) that a 2. 0 MeV passing through 22 µ.g/cm2 of UF . 4 4 He ion loses about 5 keV Since this is small relative to the incident energy, we will calculate all scattering cross sections at the incident energy. To calculate the uranium-fluorine ratio from the data, we must I . know the differential scattering cross section in the lal;>, :~ 4 238 lab For He scattering from U at 2 MeV, electronic screening causes the cross section to deviate from the Rutherford value by about z<f,. Consequently, we will calculate the cross sections in the -52The computation was performed with an HP-67 Lindhard theory. programmable calculator. For derivations the reader should consult Lindhard et al. ( 1968) and Marion (1970). Now we must define some nota- ti on. Consider an ion with mass A 1 and atomic number Z 1 on an atom with mass A.;a and atomic number Z:;i. center of mass scattering angle, and let angle. incident Let 9 be the 'It be the lab scattering In Section ll we defined the parameters a (screening length), e (reduced energy) and A. 1 (fitting parameter). Let Tl= e sin(9/2). Now we apply a fit to the Lindhard scaling function due to Weissmann and Sigmund ( 19 73 ): f(Tl) =A.' '1"11/3[1 + (2).').a/sTlsfe] -3j:a • In the center of mass frame we have dcr ( e) dw I cm To convert this into the lab frame, where x = A 1 I Aa. dcr('-lt)I dw We may now a pply the formula =[dcr(sin-1(xsin\jr)+\jr)I lab we note that dw 2 ][(xcos + l-x.asin cm 1-xasina'-lt To complete the analysis we will need one more result. a] The final e nergy Ef of the scattered ion is related to its initial energy E 0 by the expression Ef = KE0 , where -53- K('f) = '¥ = 135° and 150°. Our experiments were performed at dcr (I\!) we show the computed values for dw In Table 7 l and KE0 for these angles. lab An ORT EC surface barrier detector (Model No. BA-17- 50-100; Serial No. 17-7441) was used to detect the backscattered alphas. detector's bias was +60 volts. The A collimator with a 1/4" diameter hole was mounted on the face of the detector; its distance from the target was 4. 31". We calibrated the detector with a 212 Pb source, which emits alphas at 6. 051 MeV, 6. 090 MeV and 8. 785 MeV. The full width at half maximum of the 6. 051 MeV and 8. 785 MeV peaks We used the same foil for all four runs. was 19 keV. nanoamps of 4 He(+l) on target, With 5 each run lasted a little less than 20 minutes. The results from the runs are shown in Table 8 . a sample spectrum in Fig. 27. we used the expression · F atoms U atoms = We exhibit To obtain the fluorine-uranium ratio, I counts/~~ I F counts I ~a . U w F U We have a z<f, statist~cal uncertainty in the fluorine counts and, perhaps, l~ uncertainties from the background subtractions and screened cross section calculations. Thus, the variation from the expected value of 4. 0 is within experimental error. Since the ratio seems to decrea s e as the distance of closest approach decreases, we may be getting a small systematic error from inelastic channels in the -54scattering from fluorine. 3. Low Energy Sputtering of U F 4 Low energy sputtering yield measurements allow us to answer two important questions relating to the high energy experiments. first is technical; the second is fundamental. The We turn to the tech- nical issue now, reserving the fundamental one for the end of this section. As we noted in Section I, high energy beams cannot be used to sputter clean a target surface due to the heat they deposit in the substrate. Low current sputter cleaning is ineffective, even at 10- 9 torr, because it cannot remove contaminant molecules from the sur face as rapidly as they arrive at the surface from the residual atmosphere in the chamber. Consequently, we depend on careful control · of the surface temperature to maintain the cleanliness necessary for an accurate yield measurement. Our arguments in Section II indicate that this approach works only if U F molecules to its surface too strongly. 4 does not bind gas We have no a priori knowledge of the bond strengths involved, but we can test the effectiveness of our technique with low energy sputtering. the test is simple. The strategy adopted for First, we perform a sputtering run with low beam current at 100 keV on a target that has been processed in the same way that the targets for the high energy runs are processed. After this run, we increase the beam current to sputter clean the target, removing a few monolayers in a few minutes. Imme:dia tely after the cleaning is completed, we perform a second sputtering run. If the measured yields from the two runs agree, then we may be -55assured that the targets used in the high energy experiments were clean. The chamber used in these runs is shown in :fig. 28. similar to the configuration used for the high energy runs. It is The dif- ferences are due to its being mounted on the sputtering leg of the 150 kV ion source. In particular, the 50 l/s sublimator was added to help counteract the heavy gas load from the high current beams. The current integration technique was described in Section III. A . 3 (see Fig. 10). The target was identical to that used in the high energy measurements. The target is shown in Figs. 29 and 30. the polished surface of the Cu block. The UF 4 film is on (Note that in the photograph the reflection of the support post can be seen. } The beam current is collected from the stainless steel support post with a feedthrough wire not visible in either figure. Embedded in the copper is a chromel-constantan thermocouple. The thermocouple junction sits in a small cavity directly behind the point where the beam hits. As with the sticking fraction experiments, the temperature was monitored with an Intersil millivolt meter. The tungsten heater is a coil sand- wiched between two sapphire disks. Both the thermocouple and the heater are electrically insulated from the copper. Recall that the catcher foil holder shown in Figs. 21 and 22 has a spring at one end. When hooked to the feedthrough, lected from the catcher foil also. it allows current to be col- The holder mounts onto a bellows. which allows us to fully retract the catcher foil during the cleaning runs. The tungsten heater could be used to raise the target tem- o perature to about 200 C. Above that temperature two undesirable -56- effects set in. Electrons from the hot wire spoiled the current inte- gration, and the UF 4 film tended to peel from the copper surface at temperatures above 250°C. UF 4 test. To be certain that sublimation of the is not a problem at these temperatures, we performed a simple We exposed a catcher foil to the target when it was at tempera- tures ranging from ZI0°C to 240°c . The exposure lasted one hour, which is over ten times longer than the duration of a typical sputtering run. We found no uranium above background on the catcher fo i l. . In all of the low energy runs, The thickness of the UF 0 the target temperature was 163 C . 4 film was about 2000 A. Using a computer program dubbed "LSS-10" (Johnson and Gibbons 1969) we computed the projected range of 100 keV 16 0 in UF . 4 The oxygen Q penetrates about 1500 A into the film with a standard deviation in the 0 projected range of 640 A. Though using lower energies gives greater sputtering yields, we chose 100 keV as the energy for all of the low energy runs, because lower energies would allow 't he incident beam particles to accumulate too close to the surface. The runs were performed in two sets, each beginning with a fresh target. By the end of each set, the accumulated dose from the irradiations was about 2. Z X 1016 ions/cm:a. Now we detail the history of the two sets of runs. In Table 9 we list the runs in chronological order and display the relevant numbers. In the first set we intended to use an 16 0 beam but discovered later that the magnet calibration was wrong by one mass unit. The problem was corrected before we ran the second set. Fortunately, the beam type is not a crucial element in the test. The very first run (1-1) was performed under the conditions used in -57the high energy experiments. Note that very little material was removed from the surface during this run. After completing it we performed the first cleaning run, which lasted for about 7 minutes. With the intense beam in the chamber, the pressure rose to 7. 5 X 10- 8 torr. Assuming that this was predominately Na, we estimate that the flux of molecules striking the surface was 3 cm2 -s. x 1013 / Given a uranium yield of . 15 ·and a fluorine yield of . 60, we were removing about 8 x 1013 atoms/cm2 -s from the target, so we were fulfilling the conditions necessary for producing a clean surface. When the cleaning run was complete, we chopped the beam, moved the catcher foil into position and immediately started the yield measurement. At high beam current the yield measurement lasted only 2 seconds. The elapsed time between the cessation of the cleaning run and the end of the data run was 22 seconds. not have been quick enough, This may since it is comparable to the time re- quired to reform a monolayer of gas on the surface- -if the sticking fraction of the molecules is close to one. possibility below. We will comment on this All of the subsequent runs were at high beam currents, and they were handled in a similar manner. The yields determined from the first set of runs demonstrated that our heated target produces results in good agreement with a target cleaned in a more traditional manner (see Table 9). But because of the uncertainty about surface contamination accumulating betweerA the cleaning and data runs, we performed the second set . 16 using 0 and 20 Ne. The strategy here was to check the effective- ness of our sputter cleaning technique by determining the ratio of the yie ld s f rom t h e 16 0 , . d'ia t•ions. and ZON e irra The Sigmund theory -58produces very accurate estimates for such ratios. If the theoretical and experimental numbers agree, then we may be confident that our technique is producing accurate numbers. for the 16 0 From Table 9 we see that beam we found S(U) = O. 17 and for the found S(U) = O. 26. 20 Ne beam we Thus, the experimental ratio is 1. 53. Sigmund theory we obtain a ratio of 1. 46. From the Since the experimental ratio is subject to an uncertainty of 10-20%, the agreement is better than we had a right to expect. We may now use the experimental numbers to estimate the effective surface binding energy encountered by uranium and fluorine atoms leaving the UF 4 surface. so-called inelastic a function, In the computation we will use the which was calculated with electronic stopping taken into consideration (Andersen and Bay 1975). With a binding energy of 2.5 eV, the Sigmund theory numbers can be brought into agreement with the yields determined using the beams. Recall that the sublimation energy of UF 4 16 0 and 20 Ne is 3.2 eV (see Appendix B). We now turn our argument around and claim that these results demonstrate that the Sigmund theory provides an excellent descripti on of the low energy sputtering mechanism in UF , 4 which brings up the fundamental point we mentioned at the beginning. In the next section we will examine some sputtering yields in the electronic stopping region that definitely do not fit into the Sigmund picture of sputtering. Since we believe that the anomalous sputtering is due to electronic energy loss mechanisms, we expect that sputtering of UF 4 will be- have normally at energies where the electronic stopping is not d o minant. And so it does. -594. High Energy Sputtering of UF 4 According to the Sigmund theory a 4. 75 MeV the surface of a UF 7 x 10- 3 4 19 F ion striking target should sputter uranium with a yield of (we chose a = O. 1 and U = 2. 5 eV). In Table 10 we display our experimentally determined sputtering yields. The experimental yield corresponding to the conditions just described is about three orders of magnitude larger than the Sigmund theory prediction. Clearly, some new mechanism is involved. To help reveal its identity, we explored the variation in the yield as a function of several variables: the energy of the incident ion, the charge state of the incident ion, the mass of the incident ion, the target temperature, the pressure in the vacuum chamber and the number density of ions implanted into the target. The experiments were performed in the chamber shown in Figs . 1 9 and 2 0. and 30. The target was the one illustrated in Figs. 29 We have already described the details of the target and how it was prepared. used was about 2000 Recall that the thickness of the UF A. 4 films A 1 MeV 19 F ion will in most cas e s 0 penetrate the film and stop after traveling over 5000 A into the copper substrate. Furthermore, the total dose implanted into the beam spot was usually very small. Consequently, we do not believe that any significant amount of fluorine from the beam accumu lated in the UF 4 film. The catcher foil holder shown in Figs. 21 and 22 could be used for three runs without breaking vacuum . The target was immobile, so the same beam spot was used for all three irradiations . A fresh target was prepared for -60each new catcher foil. After most of the irradiations, the beam spot was not visible when the target was removed from the chamber. One hour exposure to air, however, always caused a perfectly round spot to appear. well aligned. This led us to believe that the chamber was Nevertheless, over the course of seven sets of runs we noticed that one catcher foil position was producing results consistently low by about 20~, which would occur if the beam were hitting the catcher foil. We threw out the seven data points involved and realigned the chamber. The anomaly disappeared. These seven points were the only ones discarded. To demonstrate the quality of the angular distributions obtained, we display a representative one in Fig. 31. It was probably flattened by target misalignment (see Section Ill. A). We will discuss the temperature and pressure dependence of the yield first, because it bears on the accuracy of the data. results are shown in Table 11. 0. 3 watts/cm2 The Beam powers ranging from to 0. 03 watts/cm2 were employed for the 19 MeV and 9. 5 MeV runs. We expect no significant heating from the beam, since the copper 1 s conductivity suppresses hot spots. In fact, no variation in the yield with beam power was observed. We can see that the yield is stable from 70°C to 170°C. The 19 MeV 0 points at 40°C and 21 o c suggest the behavior illustrated in Fig. 7, but the variation is not large enough to be conclusive. additional 19 MeV runs are tabulated in Table 12. Three These were not included with the others, because of the rather extreme conditions under whic h they were execut ed. The beam power for these was -612 over 3 watts I cm Note that the pressures are much higher than • quoted for the other runs. at 1. 5 x 10- 7 The low yield from the run conducted torr indicates that pressures this high will result in surface contamination -- even at 205°C. We believe that the same is true for the run performed at 37°C. But even with these large excursions from our usual conditions, the yield was stable to within a factor of 2. One additional comparison between Tables 11 and 12 is fruitful. Note that in the runs displayed in Table 12 the amount of material removed from the surface is two orders of magnitude greater than in the other 19 MeV runs. In addition, the 19 MeV runs in Table 11 vary in the amount of material removed by a factor of 4 but show no significant variation in the yield. In other words, we can discern no dose dependence in the yield. Now that we have established that our data are not being influenced by temperature, pressure or dose, we may turn to the variables concerning more fundamental aspects of the sputtering process. The most striking feature of the yield is its energy dependence, which is displayed in Fig. 32 along with an electronic stopping power curve from Northcliffe and Schilling (1970). the energy dependence Unfortunately, cannot be decoupled froin an apparent incident charge state effect, which we show for the 4. 75 MeV and 9. 5 MeV points. So far, the charge state effect is not firmly established, but we have reason to expect it to occur if the ion explosion is producing the sputtering (see Section II). At a given energy the tandem accelerator does not allow much latitude in -62choosing the incident charge state. In particular, the machine does not permit us to easily irradiate the target with ions whose charge is close to the equilibrium value (see Fig. 3 ). An additional complication arises from our ignorance of the behavior of the charge state as the ion penetrates the target surface. The distance necessary to equilibrate the charge is not known. Further- more, the incident ion ejects 5- 10 electrons out into space when it strikes the surface. We do not know what effect this has on the ion or the sputtering mechanism. In any event, the dependence of the sputtering mechanism on the electronic stopping power (or some component of the electronic stopping power such as the primary ionization rate) is evident. At energies above 9. 5 MeV the squar e of the primary ionization rate provides a better fit to the data than the electronic stopping power (see Fig. 2), but it also does not decrease as steeply as the data. Our final test involved the dependence of the yield on the mass of the projectile. 2 MeV and 1 MeV. We irradiated the target with 4 He(+l) at 4 MeV, If the sputtering behaves as predicted by the Haff model, we expect the yield to be the fluorine yield reduced by the ratio of the effective charges to the fourth power: = S F (U)(~He))4 Z (F) At 1 MeV this expression predicts a yield of 8 x 10- 2 • estimate of the yield from the Sigmund theory, with U = 2 . 5 eV, gives us S(U) = 3 x 10 - 4 at 1 MeV. An a = 0. 1 and We established -63the 4 He fluence for the experiment using the Haff prediction rather than the Sigmund prediction, and the track density produced by the catcher foil was just measurable. about one. The signal-to-noise ratio was From this we obtained a crude estimate for the yield at 1 MeV, which was S(U) = 2 x 10- 4 • The data were not good enough to allow us to determine the energy dependence of the yield, but the magnitude of the results suggests that the 4 He induced sputtering is mediated by the Sigmund mechanism. 5. Energy Spectrum Determination Time-of-flight experiments have proved to be powerful probes of the mechanisms that produce sputtering (Thompson 1968 and Weller and Tombrello 1978). Since the high energy yield 'measure- ments described in the previous section are incapable of discriminating between cascade and thermal spike processes, we adapted the apparatus designed by Weller (1978) to our high energy experiment in an attempt to decide the issue. Though the energy spectrum determination was successful, we will see that the results were inconclusive. Characteristically, the UF But the experiment was not in vain. 4 behaved in a surprising manner. In both of the runs performed, we used a 4. 74 MeV 19 F(+2) 0 beam transported into the North 1 o leg of the Caltech tandem. The chamber used is shown in Fig. 33. Fortunately, only minor modifications of Weller's apparatus were required, so we refer the reader to his thesis for most of the details. Beam chopping was provided w i th a set of plates between the tandem tank and the -6490° analyzing magnet. As before, the collector wheel rotated at 500 rev/sec, and the radius of the collector was 5. 08 cm. The pressure in the motor chamber during the runs was always below 10- 6 torr. Since long runs resulting in high fluences were required, we redesigned the UF UF 4 target. 4 The target surface consisted of a film about 5000 A thick that was evaporated onto a polished copper backing. The film was evaporated and baked under the same conditions used for the other UF 4 targets (see Section III. E. 3 ). The target assembly was mounted on a bellows with 2 cm travel. By moving the target during the run, we limited the fluence on any spot of the film to less than 5 x 1016 /cm2 • Since the design pre - eluded an internal heater and thermocouple, we heated the target externally with heating tapes and estimated the temperature with a surface thermometer. We performed the first run at a target 0 temperature of 170 C and the second run at a temperature of ° 7 5 C. In both runs the chamber pres sure was about 1 0- 8 with the beam on target. torr There were two very important dif- ferences between the two runs. In the first run we biased the target at +300 V to suppress electrons, while in the second the target was not biased. Furthermore, in the first run we employed a collector wheel with two slots, while in the second the wheel had only oo.e slot. The analysis of the data was performed in the manner described by Weller (1978 ). In the first run the distance " between target and wheel was 74. 8 cm, so the relationship between E and z is -65= E 8 8 0 . 35 e V / z2 (Run I) • E is the energy of the sputtered particles. z is a dimensionless parameter related to the travel time t required for a sputtered particle to reach the wheel: For the second run ( 2 • so x 1 o- 6 I s e c )t • z = ~ = 87.2 cm, so E = 1196. 4 eV / z 2 (Run II) • For the arrival time spectra we plot the track density on the wheel (arbitrarily normalized) versus z. Figs. 34 and 35. We display these in For the energy spectra we plot the track density (again arbitrarily normalized) times z 3 versus the sputtered particle energy E (see Figs. 36 and 37). The difference between the two sets of data is striking- especially when we compare the arrival time spectra. that the difference is due to the target bias. We believe Any sputtered particles that were positively charged would be accelerated by the +300 V bias, which would cause the yield to pile up at low z. Recall that in the first run we employed a wheel with two slots, so we see two identical spectra in the plot. If we assume that the particles in the two sharp peaks were accelerated by the bias, we obtain a charged fraction of 4 7'!>. This is extraordinarily high given that Sigmund sputtering rarely produces charged fractions of more than a few percent. It is not surprising, however, in view -66of the ionizing power of the incident beam. We believe that the true energy spectrum of the sputtered uranium is represented by the yield from the second run. that the energy spectrum has a kink at about 5 eV. Note This means that the data at energies above 5 eV are probably not reliable, since the enhancement at high energies may be caused by extremely low energy particles from the beam pulse during the previous revolution of the wheel. On the other hand, the data at energies below 5 eV are of high quality. fits to the data below 5 eV. In Figs. 38 and 39 we show two Remember that we are attempting to discriminate between thermal and cascade phenomena. Con- sequently, the first fit takes the Maxwell-Boltzmann form l 1. 7 x 107 Ez exp(-E/. 62 eV) 0 corresponding to a temperature of 7000 K. The second fit is a three parameter fit to the functional form expected from a cascade (see Weller 1978 and Section II): 3.1x1d3 E (E + 1. 2 e V )6 . l • We will comment on the significance of these fits in the next section. -67IV. DISCUSSION With the data in hand, we may again consider the relative merits of the sputtering models presented in Section II. Our goal was to amass sufficient evidence to allow us to unambiguously determine the identity of the mechanism producing the high yields in UF • 4 We begin by assessing the extent to which we have attained that goal. Having done so, we will see that some defi- ciencies remain, so we will propose a few additional experiment s to remedy them. Several of the contenders may be quickly eliminated. From our earlier discussions we know that the Sigmund theory cannot describe the sputtering behavior of UF region, 4 in the electronic stopping though it works well at lower energies. The success of the Sigmund theory at low energies indicates that chemical rearrangement is not producing the anomalous yields. If chemical effects were influencing the high energy yields, we would see some manifestation of them in the violent disturbances induced by the low energy bombardment. One slightly worrisome point c on- cerning chemical sputtering remains to be tested, however. The anomalous yield was seen only when bombarding a fluoride targ e t with fluorine. We will comme nt on this later. low yields obtaine d from the high energy 4 Finally, the very H e i rradiations dem- onstrate that induced desorption is not important h e r e . a mechanism been available , it. the 4 Had such He would surely have trigg e r e d -68Only the thermal spike and ion explosion mechanisms remain serious candidates, and the data do not allow us to choose between them. The energy dependence of the yield from UF either model, and so can the mass dependence. 4 can be fit to From the energy spectrum data, one might argue that the thermal spike mechanism is favored, since the temperature obtained is reasonable while the parameters obtained from the cascade fit are not. Yet, this is not a cogent argument, because we know that the energy of the primary recoils produced by an ion explosion should be unusually low. In addition, the high degree of ionization at the surface could significantly lower the surface binding energy. Therefore, we expect the energy spectrum produced by such a weak cascade to deviate from the shape illustrated in Fig. 1. Moreover, the charge s.tate dependence of the yield tends to undermine the argument in favor of the thermal spike picture. Though one might aver that the unequilibrated charge state influences the rate of heat deposition at the surface, it is difficult to imagine that the region along the ion's path could support the induced temperature gradients long enough to influence the evaporation rate from the spike. Clearly, more experimental work is needed. Before proposing some new experiments to resolve the ion explosion versus thermal spike issue, we must turn our attentior.. to a few loose ends remaining from the experiments just complet~d. To finally put our worries about chemical effects to rest, experiments are needed. two The first is a low energy yield deter- mination with a fluorine beam on UF ; the second is a high energy 4 -69yield determination with a neon beam on UF • 4 The mass dependence of the yield also needs further study: experiments . 12 35 employing C and Cl beams would be informative. Finally, the statistics for the charge state effect need to be improved with a few additional runs. We will probably perform these in an improved chamber with tighter tolerances for the target position and with a larger catcher foil radius. This would allow us to determine the angular distributions with greater accuracy. The experiment with the highest priority will only require a trivial modification of the current apparatus. glass and Recall that the uo2 irradiations performed at 45° incidence produced angular distributions that peaked at 90° relative to the beam direction. Though this was probably due to the Sigmund mechanism, we suspect that the ion explosion would behave similarly but for a different reason. In the ion explosion the coulomb repulsion is likely to propel the primary recoils radially outward from the incident ion's path. Such an effect could not come from a therma l spike, because the equilibration process occurs so slowly that the spike will not remember the direction from which the ion came. So we propose to irradiate a UF incident at 45°. 4 target with high energy 19 F If the angular distribution deviates from a cosine peaked at the target normal, we may be assured that thermal processes are not at work. The success of our Rutherford backscattering technique suggests one last experiment on UF • 4 We could sputter a target similar to the self supporting foil used for the uranium-fluorine ratio -70determination. By monitoring the ratio, we could experimentally determine the fluorine sputtering yield relative to the uranium yield. If the ratio changes, we will know that the UF 4 is not leaving the surface as a complete molecule. Naturally, one wonders if the mechanism operating in UF high energies will appear in other insulators. 4 at Biersack and Santner (1976) have sputtered KCl and found yields that follow the electronic stopping power. But, unlike the UF 4 sputtering, the KCl yields exhibit an exponential dependence on the target temperature, which indicates that alkali halide sputtering is mediated by a different mechanism. Brown~ aL (1978) have studied the sputtering of ice by light ions with energies in the MeV range. They found very high yields, which seem to behave in a manner similar to that of the UF • 4 Unfortunately, the extraordinary dif- ficulties encountered in working with such a target have precluded a comprehensive set of experiments. sputtering experiment on Si0 2 Surprisingly, a low energy performed by Ahn et al. (1975) has uncovered an effect that may be related to our high energy m e chanism. They found that simultaneous bombardment with 1 keV Xe and 3. 5 k e V e lect rons produced a substantially higher yield than with Xe alone. Though they attempted to explain the enhance d yield with an induced desorption model, one should not e xclude the possib:Uity that the electronic excitation induced by the electron beam is trigge ring some compone nt of the mechanism found at hi g h ene rgies. In particular, the electron b e am may be reduc i n g the surface binding energ y. A similar effect has been invoke d by -71Thompson and Johar ( 1979) to explain anomalies found in sputtering by polyatomic ions. They suggest that the disruption produced by the nuclear stopping power decreases the binding energy. As these clues accumulate we begin to realize that the primitive theories presented in Section II do not encompass all of the phenomena associated with high energy sputtering. For instance, none of them allow for binding energies that may be influenced by the dynamics of the sputtering mechanism. Whether minor mod- ifications of one of the current models will result in an adequate description is not yet clear. But we suspect that substantial advances in our ability to describe disordered systems will be required before a comprehensive theory will emerge. -72Appendix A - Standard Glasses Used for Neutron Irradiation Two techniques can be employed when using a uranium containing standard glass to determine neutron fluences. We can etch the latent fission fragment tracks in the glass itself, or we can etch latent tracks in a piece of mica that was firmly pressed against the glass during the irradiation. have adopted the latter approach, glass . In our experiments, we since it allows us to reuse the Ron Gregg has compared results from the two approaches and found them to be in agreement (Gregg 1977 ). With either technique , one must know the range of a fission fragment in the glass matrix. this number is quite large. first, Unfortunately, the uncertainty in Two values have been quoted. r = 2.24 x 10- 3 g/cm2 , The is due to Haines, and it is dis- cussed in detail by Gregg (1977). The second, r =2.91x10- 3 g/cm2 , was quoted by Carpenter ( 1972) in his measurement of the U concentration in NBS-612 with the track technique. Using this rang e , Carpenter obtained a value for the U concentration that deviated less than 2% from the values obtained with mass spectrometric analyses. Though this result makes the Carpenter number rathe r attractive, we will adopt the value used in the Gregg thesis: r = 2.24 x 10- 3 g/cm2 • We will apply it to both standard glass es , even though their compositions differ slightly (see Table 14 ). Even if the true range of the fission fragment were accurately known, a further problem would arise due to difficulties in counting etch pits from fragments that penetrate only a micron or less into the mica. The threshold for d e t e cting very shallow pits varies among -73observers, and a given observer will not necessarily detect the Thus, we can measure the neutron flux with an pits consistently. accuracy no better than about 20%. Given the range of a fission fragment, we can calculate the effective number of in the glass. 235U/ cm.2 235 u atoms per square centimeter available The appropriate formula is 23 U) (6. 022 x 10 =zr [(wt. fraction 238 • 03 t 235 iso ope fraction U]. )] [ · The factor of i arises from geometrical considerations: only -;} of the fission fragments produced within r of the surface travel in a direction allowing them to penetrate the surface. each fission event produces two fragments. Remember that In Table 13 we display the relevant numbers for NBS-612 and the 0. 10% Corning glass. The numbers for the Corning glass are based on a uo fraction of l 0- 3 • With the 235 3 weight We used a molecular weight of 286. 03. u surface density in hand, we can calculate the number density of tracks from a given neutron fluence. tracks/cm2 a (neutrons/cm2 ) ( 235 u/cm2 ) a is the thermal neutron fission cross section, which we take to be 582 x 10- 24 cm2 • a( = 235 In Table 13 we also quote the reciprocal of u / cm2 ), which gives us the number of neutrons per track. The NBS-612 glass has been thoroughly studied by the National Bureau of Standards. In addition to the Carpenter measurement, two independent isotope dilution studies produced -74numbers differing by only O. 1% (Carpenter 1972 ). have been performed on the Corning glass. No such studie 1:1 To check the accuracy of the O. 10% glass, we irradiated it together with the NBS-612 glass in the UCLA Reactor thermal column. Owing to the low track density from the NBS glass, its mica was etched for 80 minutes in 48% HF rather than the normal 20 minutes for the 0.10% glass. We do not believe that this introduced a significant error into the measurement. The track densities obtained were NBS- 612: 1. 94 ± • 07 x 1 04 I cm2 Corning 0. 10%: 1. 31:i:.04 x 106 /cm2 • The ratio of these densities is 1. 48 ± • 10 x 10-a . From Table 13 we predict a ratio of 1. 49 x 10- 2 , so we can accept the numbers quoted by Corni ng. -75Appendix B - Selected Chemical and Physical Properties of Uranium Fluorides Six fluorides of uranium are known: UF . , 4 5 UF 5 and UF 6 • UF , UF , UF . , 3 4 4 25 We will discuss UF , 3 most of our effort directed toward UF . 4 UF 4 and UF , with 6 Unless explicitly noted, all information quoted here is from Katz and Rabinowitch ( 1 951) or Rand ap.d Kubaschewski (1963 ), 0 25 C we use the notation H 298 . For the heat of formation at Energies will be expressed in calories or kilocalories; the reader should note that 1 eV /atom corresponds to 23. 07 kcal/mole. A list of important chemical reactions involving uranium fluorides has been compiled by Steunenberg and Vogel (1961). UF 3 This compound forms deep red-violet crystals with an orthorhombic unit cell. 2UF 4 At 1000°C it may be prepared with the reaction + H 2 ;::: 2 UF 3 + 2HF. proportionates: 4UF 3 ;::: 3UF At this temperature it also dis4 negligible rate below 900°c. + U. Both reactions proceed at a Its preparation from UF 4 requires extremely pure UF , which may be obtained by sublimating it in 4 high vacuum. UF H 298 = -345 :l: 10 kcal/mole. 4 UF 4 is an insulator with ionic bonds (Ellis 1976 ). brilliant green crystals with a monoclinic unit cell. molecular volume is 77. 5 A3 , It forms The and the U-F bond distances in the -760 0 lattice are 2. 249 A - 2. 318 A (Larson ~ al. 1964 ). formed with the reaction U0 -450 ;I: 5 kcal/mole. 800°C . 2 It may be + 4HF ~ UF 4 + 2H 2 0. It is stable in air to zoo 0 H = 298 c and in o 2 to It can hydrate to UF (2.5H 0) with a heat of -9 :i: 2 4 2 kcal/mole, but the water may be expelled by heating in vacuum at 0 200 C for 24 hours. UF 0 4 melts at 1309 K. p(torr) Its vapor pressure obeys the expression = 760 exp(-AG/RT) where = AG(cal/mole) and R = 1. 986 cal/°K. 0 200 C p = 7 x 10 -21 75, 100 - 90. 3T + 13. ST log 10 T 0 At 100 C p = 7 x 10 -30 torr, and at torr. The electrical resistivity of sintered UF measured by Reynolds and Middleton ( 1952 ). 4 pellets has been They quote a value of 108 0- cm, but the purity of the sample is not known. thermal conductivity of fused UF 4 at 6o 0 The c is 5 x 10- 3 cal/sec-cm- °K = 2 x 10- 2 watt/cm- °K (Steunenberg and Vogel The thermal diffusion coefficient is 8 X 10- 3 crn2 /sec. 1961). UF 6 may be prepared with the reaction UF 4 + F 2 ~· UF 6 , which proceeds slowly at temperatures below 250°C. -511.5 kcal/mole. UF 6 H 298 = melts at 337.2°K and sublimes at 330°K. -77REFERENCES Ahn, J., Perleberg, C.R., Wilcox, D.L., Coburn, J.W. and Winters, H.F., 1975, J. Appl. Phys. 46, 4581. Andersen, H. H. and Bay, H. , 1972, Rad. Eff. Q, 67. 1975, J. Appl. Phys. 46, 2416. Bevington, P.R., 1969, Data Reduction and Data Analysis for the Physical Sciences (McGraw-Hill, New York), pp. 92-118. Biersack, J.P. and Santner, E., 1976, Nucl. Instr. Meth. 132, 229. Brown, W.L., Lanzerotti, L.J., Poate, J.M. and Augustyniak, W.M., 1978, Phys. Rev. Lett. 40, 1027. Carpenter, B.S., 1972, Anal. Chem. 44, 600. Chapman, G. E., Farmery, B. W., Thompson, M. W. and Wilson, I.H., 1972, Rad. Eff. Q, 121. Close, K.J. and Yarwood, J., 1967, Brit. J. Appl. Phys. ~. 1593. Davies, J.A., L'Ecuyer, J., Matsunami, N., Ollerhead, R. and B~ttiger, J., 1979, to be published. De Heer, F.J., Schutten, J. and Mous.tafa, H., 1966, Physica g, 1766. Ellis, W.P., 1976, Surface Sci. ~. 37. Fleischer, R., Price, P. and Walker, R., 1975, Nuclear Tracks in Solids (University of California Press, Berkeley), pp. 3105. Flint, O., Polling, J.J. and Charlesby, A., 1954, Acta Met. ~. 698. Gregg, R., 1977, Ph.D. Thesis, California Institute of Technology. -78Haff, P.K., 1976, Appl. Phys. Lett. 29, 473. l 976a, "Notes and Comments on Sputtering", Caltech preprint BAP- 7, unpublished. Haff, P. K. and Switkowski, Z. E. , 1976, Appl. Phys. Lett. 2 9, 549. Heckman, H.H., Hubbard , E.L. and Simon, W.G., Rev. 1963, Phys. 129, 1240. Hildenbrand, D. L., 1976, Los Alamos Scientific Laboratory Technical Report No. PYU-4822 . Hill, T. L., 196 O, An Introduction to Statistical Thermodynamics (Addison-Wesley, Reading, Massachusetts), pp. 124-140. Hurkmans, A., Overbosch, E.G., Olander, D.R. and Los, J., 1976, Surface Sci. 54, 154. Hurkmans, A., Overbosch, E.G. and Los, J., Sci. 1976a, Surface 2.2., 488. 1977, Surface Sci. 62, 621. Ishii, T., Naito, K. and Oshima, K . , 1970, J. Nucl. Mat. 2.§_, 288. Johnson, W.S. and Gibbons, J.F., 1969, Projected Range Statistics in Semiconductors, dist. by Stanford University Bookstore. Katz, J. J. and Rabinowitch, E., 1951, The Chemistry of Uranium (McGraw-Hill, New York), pp. 349-449. Kelly, R., 1977, Rad. Eff. ~, 91. Larson, A. C., Roof, R. B. and Cramer, D. T., 1964, Acta Cry st. ..!2., 5 55. Lindhard, J., Nielsen, V. and Scharff, M., 1968, Mat. Fys. Medd. Dan. Vid. Selsk. 36, No. 10. -79Marion, J.B., 1970, Classical Dynamics of Particles and Systems (Academic Press, New York), pp. 291-312. Menzel, D., 1975, Surface Sci. 47, 370. Northcliffe, L.C .. and Schilling, R.F . , 1970, Nuclear Data Tables A7, 233. Oechsner, H., 1975, Appl. Phys. ~, 185. Overbosch, E.G., Hurkmans, A. and Los, J., ~, 1977, Surface Sci. 417. Rand, M. H. and Kubaschewski, 0., 1963, The Thermochemical Properties of Uranium Compounds (Oliver and Boyd , Edinburgh), pp. 13-18. Reynolds, D. C. and Middleton, A. E., 1952, AEC Report No. BMI-259. Schram, B.L., DeHeer, F.J., Van der Wiel, M.J. and Kistemaker, J., 1965, Physica ll• 94. Schram, B.L., Moustafa, H.R., Schutten, J. and DeHeer, F.J., 1966, Physica 32, 734. Sigmund, P., 1969, Phys . Rev. 184, 383. 1972, Rev. Roum. Phys . ..!1.• 823, 969 and 1079. Sigmund, P., Matthies, M.T. and Phillips, D.L., Eff . 1971, Rad. .!l• 39. Steunenberg, R.K. and Vogel, R.C., 1961, in Stoller, S.M. and Richards, R. B. (eds.) Reactor Handbook: Volume II Fuel Reprocessing (Interscience, New York), pp. 251-254. Thompson, D.A. and Johar, S.S., 1979, Appl. Phys . Lett. 34, 342. Thompson, M.W., 1968, Phil. Mag. Weissmann, R. and Sigmund, P. 18, 377. 1973, Rad. Eff • .!.2.• 7. -80- Weller, R. A., 1978, Ph.D. Thesis, California Institute of Technology. Weller, R.A. and Tombrello, T.A., 1978, Rad. Eff. 37, 83. Winterbon, K.B., Sigmund, P. and Sanders, J.B., Fys. Medd. Dan. Vid. Selsk. lZ,, No. Ziegler, J.F., 1977, Helium: 1970, Mat. 14. Stopping Power and Ranges in All Elemental Matter (Pergamon Press, New York). -81- Table 1 Results from the program to assess the errors induced by target misalignment. The geometry used for the cal- culation is shown in Fig. 11. The calculated distribution, initially produced by a flux with angular dependence cos Bocp, was fit to the form A cos Be and normalized so that a per- fect measurement would give B = 1 and A = 2. simulated beam spot was used. The parameter r 2 is a A square measure of the goodness of fit; r 2 = 1 implies a perfect fit (see Section III. A. 5 ). Input Parameters: Foil radius = 1. 43 cm Bo = 1 II II 11 o.o II II 11 11 0. 5 1. 0 2.0 " " 2.0 3.0 " " II 11 II 11 11 1. 0 4.0 8.0 11 " 11 II 11 II 11 11 11 0.0 11 0.0 1. 0 11 11 11 11 11 " " " A 1.000 0. 998 0.990 o. 998 o. 972 0.891 0.990 0.962 0.918 1.062 1. 009 1. 021 0.972 0.990 1.000 0.998 0.978 1. 000 B +1 0.993 0.993 0. 994 0.971 0.932 0.830 2.0 -2.0 -1. 0 0.0 1. 0 2.0 " " 11 0.570 0.750 0.993 1. 355 2. 194 0.5 0.0 o.o o. 0 " 1.000 (mm) B (mm) (mm) D L x z D y (mm) D TABLE 1 0.997 1. 000 1. 000 1. 000 1.000 1.000 0.994 0.979 0.925 0.998 0.964 1. 000 1.000 1. 000 1. 000 r~ I 00 I N -83Table 2 Results from sticking fraction experiments. In 2A we show numbers of immediate interest, while in 2B we show the numbers on which the calculations are based. f is the fraction not sticking to the primary surface. B and the secondary track density at 9 = o0 were taken from a program fitting the track counts to the form A cos B 9 (see Section III. B ). II II 23 23 8 II 23 Mica - 1/25/77 10 *Also irradiated with sputtered II 152 Au Film - 4/25/78 8 9 Al Film - 5/10/77 7 If Al Film - 5/10/77 6 23 23 Al Foil - 8/28/77 5 Au Film - 4/25/78 80 23 Al Foil - 11 /28/77* 4 II 40 23 Al Foil - 11/28/77 3 152 II 23 Al Foil - 7 /29/77 2 u. 80 23 Al Foil - 1/25/77 (OC) Ar Beam .Energy (KeV) Temperature l Primary TABLE 2A 3.22 3. 79 3.60 1. 28 1. 22 .958 2. 14 2.22 1. 76 4.32 U Fluence on Primary (1013 /cm2 ) 235 . 86 . 71 . 70 1. 05 . 52 . 52 . 74 . 74 . 69 . 88 B 3.02 1. 27 2.87 2. 17 2.55 I. 62 2.30 2.05 2.48 2.83 f (~) I I ~ 00 1. 02 1. 98 . 20 1. 93 10 . 590 1. 14 . 19 1. 32 9 1.41 1. 88 1. 88 1. 47 1. 27 . 590 1. 47 1.49 1. 59 1. 59 . 915 1. 41 1. 27 . 591 1. 18 1. 18 . 702 1. 02 Standard Track Densities (106 /cm2 ) O. 10'.t NBS-612 2.29 .18 1. 25 . 645 . 20 .956 7 8 . 503 . 19 .918 6 . 293 . 21 ,334 . 928 . 17 1. 48 4 5 . 971 . 20 . 477 2. 78 .18 .22 ·Secondary Track 0 Densi~ at 9 = o ( 1a6 /cma) 1. 54 . 727 2. 59 Primary S~ot Area (cm ) 3 2 1 Primary Track Density ( 106 I cm2 ) TABLE 2B I \J1 I ():) -86Table 3 Results from electron microprobe studies of the uranium doped soda-lime glass target. Note the decrease in the sodium content in the irradiated spot Section III. C ). (see 75. 0 78.6 11. 2 6.9 Edge of Beam Spot Center of Beam Spot 73. 7 14.2 Unirradiated Glass 2 Si0 2. I 3.0 2. I I. 9 2.5 3.3 AI o 2 3 MgO Composition (weight percent) Na o 2 TABLE 3 8. I 7. 3 7. 2 Cao I I ~ 00 -88- Table 4 Relative sputtering yield data for The glass target was 0. 3 atomic '% foil target was 0. 72 atomic if, 235 u. 19 235 F incident at 45°. u, while the uranium The 235 u density per incident ion is quoted at the peaks of the angular distributions, which occur at approximately 90° with respect to the beam direction. The standard glass used was NBS-612. In part A we show numbers of immediate interest, in part B we show the numbers from which those in 4A were calculated (see Section W. C ). 9 7 22 -3 100 8 2 1 10 II II II 5 6 7 U Foil 1 5 10 II 4 8 5 20 10 II 3 4 1 7 20 14 II 2 6 (10- /cm ) u Density/Incident lo~ 5 2 22 235 20 Beam Current (particle nanoamps) Glass Beam Energy (MeV) 1 Target TABLE 4A I 00 I -.D 2. 0 1. 5 2.5 1. 5 1. 4 fl II 16. 1. 6 II 16. fl II II II 3.3 1. 5 6.3 2 3 4 5 6 7 8 II 1. 8 1. 6 4.4 1 17. 2.8 Track Density at Peak (104 /cm2 ) Standard Glass Track Density ( 105 I cm2 ) 19 .F Fluence/la1 4 TABLE 4B I '°0 I -91Table S Experimental sputtering yields for at normal incidence. 19 F incident on uo 2 In SA we show numbers of immediate interest; in SB we show the numbers from which those in SA were calculated. The uranium yield is defined as the number of uranium atoms leaving the target per incident ion. In runs 1 through 3 we employed a pre-war foil* (0. 72% z3 sU atomic); in all other runs we employed a foil enriched to 99. 7'f, (atomic) 23 Su. The parameter B and the track density were determined from the program used to fit the track counts to the form A cos B 0 (see Section Ill. A). The standard glass used was NBS-612 (see Section Ill. D). Errors: Track density at e= o 0 B Standard glass track density :I: 3'f, *The adjective "pre-war" indicates that the isotopic content of the uranium has not been altered. The term alludes to World War Two during which the plant at Oak Ridge was built. 2 9 3 3 2 3 2 0.9 1. 0 0.4 0.4 0.5 0.4 0.5 0.4 25 25 23 33 10 25 13 9 5 10 2 10 15 20 20 30 500 II 700 II II II II II 4 5 6 7 8 9 10 11 9 8 0.7 15 20 II 10 3 1. 2 40 40 Uranium Yield/10- 3 10 0.5 5 B 2 Beam Current (particle nanoamps) II <500 Beam Energy (MeV) 2 1 (1) Oxide Thickness TABLE SA '° I I N 9.51 5.85 2.07 6.3 II 6.6 6.3 II II II II 4 5 6 7 8 9 10 11 . 765 1. 10 .385 1. 42 . 562 II .268 II 3 11 l. 47 . 810 l. 47 II . 810 II I. 61 II . 53 7 63. 2 I. 41 . 565 32. Standard Glass Track Density ( 106 I cm2 ) Track Density at 9 - = 0° (105 /c~a) 1 19 F Fluence/1013 TABLE SB I I \,/.) -.!) -94Table 6 Sigmund theory yields for uranium sputtered from U0 by 19 F. We have taken a= 0.1 and U = 5.4 eV. 2 We have used the Haff and Switkowski formula for binary sputtering (see Section II and III. D). -95TABLE 6 Energy (MeV) Uranium Yield 2 1 x 10-:a 5 6 x 10- 3 10 4 x 10- 3 15 3 x 10- 3 20 2 x 10- 3 30 2 x 10- 3 -96Table 7 Calculated values for the laboratory differential scattering cross section and for the final energy in the laboratory (KE;, ) of the incident 4 He atom. A screened coulomb potential from the Lindhard theory was used (see Section III.E.2). 1. 878 1. 11 . 900 150 2.0 I. 48 1.888 1. 35 . 963 135 2.0 1. 52 1. 691 I. 37 . 810 150 1. 8 1. 24 I. 82 (10- 23 cma /Sr) 1. 700 dcr/dw Ilab 1. 66 2 .867 135 1. 8 (10- 26 cm /Sr) · dcr /dill llab 4H e on 238U KE0 (MeV) KEo (MeV) Angle (degrees) Eo (MeV) 4tteon 19 F TABLE 7 '° I I -..) -98- Table 8 Results from Rutherford backscattering analysis of UF 4 with a 4 He beam. (see Section III. E. 2 ). For UF 4 we expect F /U = 4. 0 67087 2408 2834 2606 150 135 150 1. 8 2.0 2.0 75196 79686 65268 2359 135 1. 8 Uranium Counts Fluorine Counts Angle (degrees) Eo (MeV) TABLE 8 3.88 3.91 3.98 3.98 F/U I '°'° I -100Table 9 Results from low energy sputtering yield measurements. The first run (I- 1) was performed under the con- ditions used in the high energy experi ments: cleaning preceded it. no sputter All of the others were immediately preceded by cleaning runs . The elapsed time between the cessation of the cleaning run and the completion of the yield determination never exceeded 22 seconds. In calculating the number of monolayers removed, we assumed that we have 5. 5 x 1014 molecules/cm2 • The uranium yield is the number of uranium atoms ejected per incident ion. standard glass used was NBS-612. the track density at The The parameter B and e = o were determined by the program 0 that fits track counts to the form A cos Be (see Section III. E. 3 and Appendix A). Errors: Track Density at 0 = B Standard Glass Track Density Neutron Fluence o0 :l: 2~ :l: 5~ 4200 11 11 II-2 11 25. 11 II-1 Cleaning 4.0 Cleaning I-3 4200 4500 4500 11 20Ne 11 4200 3.0 4200 11 II Cleaning 160 . 03 4500 II II I-2 . 05 5. 2 . 03 3.3 . 03 3.0 4500 H . 04 Monolayers Removed 7. 5 30 Beam Current (particle nanoamps 8 /cm ) Cleaning 14NH Beam 1. 5 Pressure -a (10 torr) I-1 Run TABLE 9A 0.26 o. 17 0. 14 -- 0. 16 0.21 Uranium Yield I ...... 0 ...... -.92 -. 78 Cleaning I-2 Cleaning 1-3 Il-2 Cleaning Il-1 Cleaning . 78 . 91 3.41 1. 02 . 60 I-1 1. 05 111. 1. 05 111. 1. 05 111. 1. 05 4. 79 3.30 2.61 3. 12 = 111 . Track Density at e o30 5 (10 /cm ) Fluence ( 1<>1 4 particles I cm3 ) Run B TABLE 9B II 4.88 II ti 4.93 Standard Glass Track Density (105 /cm3 ) I 0 N ,_. -103Table 10 Yields of uranium sputtered from UF 19 F. 4 by high energy The averages and standard deviations were calculated directly from the uranium yield column. In calculating the number of monolayers removed, we assumed that we have 5. 5 x 1014 molecules I cm2 and that four fluorine atoms leave the surface for every uranium atom sputtered. glass used was NBS-612. density at The standard The parameter B and the track e = 0° were determined from the program used to fit the track counts to the form A cos Be (see Section III. E. 4 and Fig. 32). Errors: Track Density at 8 = 00 ;I: 2"' B :!: 51> Standard Glass Track Density :!: 8~ Neutron Fluence :201> ti ti 5 ti II 28.5 II II II II ti ti ti ti 4 II 14 15 16 17 18 19 20 21 19.0 II 13 ti II II ti ti II II II 11 3 II 9.50 II II II II II 4. 75 II " 2.38 II . 697 2.44 2. 60 6. 53 4.46 5. 73 8. 17 6. 10 5.91 5. 14 4.68 6. 19 5.33 6.96 2. 71 2.56 2.45 1. 86 2.04 2.82 1. 78 2 1. 19 12 10 11 1 2 3 4 5 6 7 8 9 Uranium Yield Charge State Energy (MeV) TABLE lOA 1.8 7. 0 2. 4 5. 5 7. 1 5. 6 . 70 2.5 Average Yield 0. 38 0. 60 1. 5 1. 0 0.11 Standard Deviation I ,;.. 0 ..... 14 15 16 17 18 19 20 21 13 1 2 3 4 5 6 7 8 9 10 11 12 4.2 1. 9 8.0 5.8 2. 1 2.6 6. 6 4. 7 5.3 2.3 4.8 6.3 5. 6 7. 1 6.3 2.9 2. 7 1. 7 1. 6 4. 7 4.7 . 71 .95 . 77 . 85 . 73 .84 . 74 . 82 . 79 . 88 . 63 . 83 . 83 • 72 . 81 . 81 . 70 . 81 • 72 • 72 • 72 Monolayers Removed/ 10-a B 3.92 3.69 7.33 10. 5 3.64 5.20 6. 94 4.28 10. 1 4.47 5.00 6.81 5.25 7.64 11. 1 5. 14 4.82 3.32 2.60 5.03 5.07 Track Density at e = o0 p 06 I cm2 ) TABLE lOB 4.92 9.33 4.92 5.90 9.40 9.91 5.20 4. 93 5.90 9.91 5. 26 5.20 4.93 5.20 9.29 8. 78 8.78 9. 40 9.33 5.26 5.Z6 Standard Glass Track Density ( 106 /cm2 ) I I 0 1..11 -106Table 11 Temperature and pressure dependence of the uranium yields produced by bombarding UF and 19 MeV. 0 4 with 19 F at 9. 5 MeV Note that these yields are stable between 0 70 C and 170 C. 0 All of the runs, except those at 40 C and 210°c, are included in Table 10 (see Section III.E.4}. 5 5 8 30 8 8 4 40 70 19.0 19.0 8 4 8 8 II II 120 11 170 210 II II 19. 0 II 19.0 19.0 II II 150 160 170 II II 9.50 9.50 Pressure (10- 9 torr) 3 4 (OC) Target Temperature 70 Energy (MeV) TABLE 11 3.6 2,82 2. 71 2. 04 2.56 2.45 1. 86 1. 4 6. 19 5.33 4.68 5.91 5. 14 Uranium Yield 3.6 2.8 2.4 2.3 1. 4 5.4 5. 5 Average Uranium Yield I 0 -.J I ..... -108Table 12 Results from three runs in which UF with 19 MeV 19 F(+4 ). 4 was bombarded We show these yields to demonstrate the stability of the measurement against large increases in pressure and in the fluorine dos e . taken from Table 10, is 2. 4 :i: O. 38. The expected yield, The track densities in the data bands were too high to allow a complete scan, so only a few hundred tracks were counted at the peak of each angular distribution. The yields quoted are subject to an uncertainty of perhaps :SO~ (see Section III. E. 4). 15. 3.0 3.0 164 37 Pressure (10- 8 torr) 205 (OC) Target Temperature TABLE 12 6.5 14 . 6.2 Monolayers Removed 1. 4 3.0 1. 3 Uranium Yield I "' I .... 0 -110Table 13 Numbers used to predict neutron fluences from the standard glasses. The Corning glass was used for fluences of about 1014 neutrons I cm2 , and NBS- 612 was used for fluences of about 1016 neutrons I cm2 • 235 In calculating the u surface densities, we used a fission fragment range of 2. 24 x 10- 3 g/cm2 , which is subject to a large uncertainty (see Appendix A). 7.20 x 10- 3 2. 392 x 10- 3 37.38 + 0.08 x 10- 6 NBS-612 Isotope Fraction of 235u 8. 32 x 10-4 Weight Fraction of Uranium Corning 0. 10% Glass TABLE 13 1. 01 x 108 6. 78 x 109 2. 53 x 1a11 Neutrons per Track 1. 70 x 1013 235U/cm2 I ..... I -- -112- Table 14 Standard glass compositions by weight (see Appendix A). 14% n<f> NBS-612 Na 0 2 17% 2 73"1 Si0 Corning o. 10% Glass Component TABLE 14 12% 6% Cao 2% 41' Al 0 2 3 I v.> I -114Figure 1 The energy spectrum, S(E), of 235 U sputtered from a uranium metal target (reprinted from the thesis by R. A. Weller (1978)). The vertical scale is arbitrary. The smooth curve is an empirical fit to the data, while the broken curve has the form 6. 5 E/(E + 5. 4)3 (see Section II). -115- '' -·-c: '' '' '' '' '' \ \ '\ \ Cf) \ ::::> >,._ 0,._ \ \ 10-3 i,._ 5 0 w - ( /) \ \ '\ ' \ \ ' \ '\\ \,\\' . 5 ', \ 2 \ \ \ 2 100 5 E (eV) Figure 1 . 1000 -116Figure 2 The primary ionization rate squared for fluorine (Z = 9). It expresses the energy dependence of the sputtering yield in the Haff model. For an explanation of its origin, 3, 4 and 5 and the text (Section II). see Figs. -11 70 --------'""T'"""------~----------r--------r---------r---------r----------,r--------r---------, N 00 ...... 0 • Ol n ...... (!) N ...... :::r 0 llJ a: a: :J 0 en UJ ~"' a: L a: z > _. => I- a: ........ ow .L D ..... 1-1 I- a: "-;' G N i:...... a: w a:> z z D c)W '"""" >- a: a: L i:...... ex: (!) CL 0 . :::r 0 N 0 ,____________________________________________ o·e o·z. o·s o·s o·fl o·£ o·z o·s (SlINn AHHH1IB8l::f) G**r 0 . . ~.__--------------------------~--------oC> Figure 2 O"l o·o -118Figure 3 The equilibrated charge of fluorine passing through a solid as predicted by Heckman, Hubbard and Simon ( 1963 ): z = z [I - 10-~(137f3/Zo.56 )] (see Section II). -119- .-------------------------------------------------~--------------...-----------------.~ N . a:> . <O -. ::f' UJ z ....... a: 0 :::> ~""' _J -=> LL. ~ a: LL. '> 0 ~ UJ 0 -~ _...._,, a: a: ::c >- u c..!J a: UJ a:> :z UJ UJ > ....... 0 1-- u w LL. LL. w . <O 0 . ::f' 0 N . C> . C> L-------.....L.------1---------..L..-------1...---------L-------.....L.---------1-=::::===*--------10 o·s o·e O"l o·s o·s o·h Z 3A I 1J3.:J::J3 Figure 3 o·E o·t o·o .. 120 .. Figure 4 A fit to the ionization cross section data for 40 Ar from the work of Schram et,!!. (1965 and 1966) and DeHeer et al. (1966). Data from ionization by protons is used at low energies, and data from ionization by electrons is used at high energies. The original electron energies have been multiplied by m /m p to produce a consistent energy scale. J A p = E p e The fit is of the form ln(BE ) p where B = 4. 5 X 10-a /keV (see Section II). -121--~~-.-~~---.-~~~r--~~-.--~~-....~~~~~~-r-~~-...~~----.~ N . co -. co . ~ · _. LLJ I- a: a: ~;--... -:::::> z 0 :E a: ......... > 1-t I- a: N ow • :E z -~ '-"' D G a: 1-t >a: a: UJ coz 0 UJ L 1-t a: !l... CD 0 . ~ 0 N 0 . 0 '--~~..._~~~~~---~~~..._~~-L~~~4--~-~~~~--'-~~--'0 o·s o·e O'l o·g o·s o·h o·£ <SlINn A8~81I88~) r Fig ure 4 o·t o·o -122Figure 5 Primary ionization rate for fluorine, Z~ J p (see Section II). which is equal to -123- ..... (lO 0 . II . co N ...... =". .... UJ I- cr: a: ":!-. ...... :::::> z ~ D ....., a: .......... I- > cc UJ 0 N z .....L ......, .... cb 1-1 D ~ >a::: >a: cr: UJ (lO -4'..· - c:iUJ ~ aQ_: . co 0 N 0 . . C> .__~~_._~~_......~~~--~~--~~--~~~---~~---~~~----~~~o o·s o·e O"l 0·9 o·s o·~ o·£ (SlINn A8~81I88~) r Fig ur e 5 o·~ 0·1 o·o -124- Figure 6 Schematic of the Frank-Condon process as it applies to induced desorption. An atom bound to the surface is promoted from its ground state to one of the two excited states. If the energies of the excited atoms are greater than the energies of the states at infinity, they will escape with the energy spectra shown to the left (see Section II). -125- >(!) er w z w I --i------EMISSION PROBABILITY EXCITED BONDING STATE 1 I I >- er I z I (!) w w .EMISSION PROBABILITY I I I I -- ANTI BONDING STATE 1 ~ ~ a:: w z w _J <l: 1-- I I I I I GROUND STATE WAVEFUNCTION z w l- o a... DISTANCE FROM SURFACE Figure 6 -126Figure 7 Behavior of measured sputtering yield for three chamber pressures: P 1 , Pr;i, P 3 • For a given pressure, the target becomes cleaner as the temperature increases. The tempera- ture region corresponding to the flat plateau produces accurate yields. If the temperature rises too high, an exponential in- crease in the measured yield sets in as the target begins to sublime (see Section II). -127- 0 _J w >0 w 0:: ::::> Cf) <( w ~ TARGET TEMPERATURE Figure 7 -128Figure 8 Schematic drawings of the three major types of experiments performed for this dissertation. In all of them 235 u is sputtered from a target and collected on high purity aluminum (see Section III. A). -129- SPUTTERING YIELD EXPERIMENT: HIGH AND LOW ENERGY CATCHER FOi L TARGET BEAM / STICKING FRACTION EXPERIMENT URANIUM TARGET BEAM SECONDARY FOIL ENERGY SPECTRUM EXPERIMENT BEAM TARGET SPUTTERED U MOTOR Figur e 8 -130Figure 9 The relationship between the catcher foil geometry and the configuration of the data bands on the mica. foil holder is shown in Figs. 21 and 22. The catcher The partitions on the holder allow three separate irradiations for one foil, which results in three data bands on the mica (cross-hatched areas). The gaps at the centers of the bands are due to the square holes through which the beam passes. The cross at the upper left corner of the mica serves as a reference point for setting the stage micrometers. In scanning the mica, the coordinates of the frames scanned are recorded. tually, the x-coordinate translates into the angle Section III. A). Even- e (see -131- +~™ y ~~ ~~ () =0 Figure 9 x -132Figure 10 Schematic chamber configuration use.d for the UF measurements. In the glass and U0 2 did not have a heater or thermocouple. 4 yield experiments, the target The functions performed by the various parts of the chamber are described in the text. A picture of the chamber is shown in Fig . 20 (see Section III. A) . ~· t-zj 0 ~ - Ii i: (JQ ION BEAM HOUSE CURRENT MET ER 300 v I; HOLE ) 60 1/s PUMP TANTALUM COLLIMATOR 11 WITH 0.12 DIAMETER Doov + BROOKH AVEN CURREN T IN TEGRATOR \ \ HEATER POWER SUPPLY INTERSIL _ MILLIVOLT METER PUMP ION 11 i./s PERMANENT MAGNET THERMOCOUPLE CATCHER FOIL WITH 11 1/4 SQUARE HOLE'\ I TARGET TANTALUM 11 WITH 1/2 DIAMETER HOLE I w I ....... w -134Figure 11 Geometry for the computer program used to assess the error induced by target misalignment. We simulated the beam spot with a rectangular array of source points (shown below) each of which emitted particles with the distribution cosB0 cp. The vector :f5 expresses the displacement of the beam spot from the correct position (see Section III. A). -135- CATCHER FOIL FOIL AXIS BEAM SPOT y L y xi w t-----+----+----+-----t SOURCE ~ < t-----+----+-----+------t / SIMULATED BEAM SPOT Fig ur e 11 POINTS -136Figure 12 Results from the computer program used to assess the error induced by target displacement. The crosses represent points calculated by integrating the response from the source points. The continuous line is a fit of the form A cosB 9. The parameter A was renormalized to 1 in the graph. A square beam spot was used (see Section III. A). Input parameters: Foil radius Ro = I. 43 cm Spot width L = O. 20 cm B0 = 1. 0 .... Displacement D Output parameters: B = (0. 0, 0. 0, O. 0) cm = 0. 993 A/(B + 1) = 1. 000 -137- C) Kl.. 0 (11 ........ C) t--t a_ o' .. a: tC) LL.l ::c t(11 . ....... CJ I CJ I Q) . ('f") CJ I C) . lJ1 C) "--~~~_.__~~~~~~~~....._~~~-'"~~~~"--~~~~ a·o s·o ~·o 3SNQdS38 032Il~W8QN Fig ur e 12 I -138Figure 13 Results from the computer program to assess the error induced by target displacement. The crosses represent points calculated by integrating the response from the source points. The continuous line is a fit of the form A COBB e. eter A was renormalized to 1 in the graph. The param- A square beam spot was used (see Section III. A). Input parameters: Foil radius Ro = 1. 43 cm Spot width L B0 = 0. 20 cm = 1.0 Displacement D = (0. 0, 1. 0, O. 0) cm Output parameters: B = . 932 A/(B + 1) = 1. 021 -139- <D (Y") • 0 . Ln N 0 . (Y") ....... 0 ........ CL + o' . a: IC> L.L.J :J: I- . (Y") ....... C> I Kl• C> I . <D ('I') C> I C> Ln • C> l·t o·t a·o s·o ~·o AllSN30 ~J~8 l 03Zil~W8QN Figure 1 3 l·o o·o I -140Figure 14 Schematic drawing of the experimental apparatus for the sticking fraction experiments. The top figure displays the arrangement inside the cylinder used to hold the cages. Each cage contained a primary- secondary assembly like the one below, though the heater was included in only two of the runs. By using two uranium foils and two shields, we could irradiate the cages separately (see Section III. B). -141- CAGE URANIUM FOIL ARGON BEAM CAGE ALUMINUM SECONDARY FOIL / 0.4 cm DIAMETER HOLE R = l .27cm EVAPORATED FILM ON MICA () ~ TANTALUM FOi L HE AT I NG E LE ME NT--~~~-=-----+--+----..........._. MICA WIRE TO HEATER POWER SUPPLY WIRE TO HEATER POWER SUPPLY THERMOCOUPLE Figure 14 -142Figure 15 UHV chamber used for the sticking fraction experiments (see Section III. B). ..... 1-ij V1 ...... Ii C1l s:: O'Q DIFFUSION PUMP OIL I ...... ,,.. ------; BEAM FROM 150 kV ION SOURCE COLD TRAP COV-500 PUMP 0 D or: ELECTRICAL FEEDTHROUGH VALVE TO SOR PT ION PUMP FARADAY CUP WITH WINDOW TARGET MAN I PULATOR I w if>. I .- -144Figure 16 Angular distribution of sputtered 235 u scattering from an aluminum foil with an oxidized surface at Z3°C. At the peak, each point represents about 770 tracks, which implies a statistical error of :I: 4%. B = O. 69 :i:. 02. The fit is of the form AcosB. The uncertainty in A is ;I: 1% (see .Section III. B). -145- -. (Y) C> + ........ a... o' . a: C> I - w I I- . (Y) ........ C> I IJ') . 0J C> I <D . (Y) C> I . C> Ln '--~~~_._~~~~...___~~~____.....~~~~_..._~~~-----''--~~_;;;:-.i 0·1 a·o A1ISN30 s·o ti"O l"O ~J~~l 032Il~W~~N Fig ur e 16 o·o 0 I -146~ Figure 17 Angular distribution of sputtered 235 u scattering from an evaporated aluminum film with an oxidized surface at 23°C. At the peak, each point represents about 90 t racks, which implies a statistical error of :!: 11%. form AcosB 0 • B = O. 52 :!: • 05 . :!: 3if, (see Section Ill. B ). The fit is of the The uncertainty in A is -147- Cl'> . ('t') 0 If) C'\I 0 (") . ....... 0 ........ a_ + o' •CI 1-- 0 w :r: ....._ + + ('t') . _.. 0 I If) . C'\I 0 I . (X) ('t') Cl I . Cl l{) 0 l.I 0·1 a·o g·o ~·o AlISN30 ~J~81 032Il~W8QN Fig ure 17 z·o o·o I -148Figure 18 Angular distribution of sputtered 235 u scattering from an evaporated aluminum film with an oxidized surface at 152°C. At the peak, each point represents about 120 tracks, which implies a statistical error of %.91'. form AcosBe. B = 1. 05 :l: • 07. :t. 31' (see Section III. B ). The fit is of the The uncertainty in A is -149- ('I') ....... C> _., a.. o". a: 0 1-- w I r-- -. ('I') 0 I . ~ 0 I . <X> (r') C> I 0 lJ') 0 z·1 0·1 e·o s·o ~·o Al!SN30 ~J~~l 03Zil~W8~N Figure 18 z·o o·o I -150Figure 19 Chamber used for high energy sputtering yield . experiments (see Section III. C ). ITj '° ..... (!) 'i ~ ~- TRAP COLD BEAM 11 VALVE 1 UHV CRADLE 60 1/s ION PUMP I rit<UUG H: :;.;'7 • . LLU• I Y2" VALVE TO SORPTION PUMP I I I ..... U'1 ION PUM P ';" II Us -152Figure 20 Photograph of chamber shown in Fig . 19. The Bennington Flag was a gift from the manag e ment of a local Armenian restaurant on July 4, 1976 (se e Se c tion III. C ). -153- Figure 20 -154- Figure 21 Photograph of catcher foil holder used in the sputtering yield determinations. The ceramic piece on top insulates the holder from the bellows. The spring is used to electri- cally connect the catcher foil to the outside (see Section III. C). -155- Figure 22 -lSbFigure 22 Photograph of catcher foil holder with catcher foil. The three square holes allow the beam to pass through. The scale reads in centimeters (see Section III. C ). -157- 0 Figure 22 -158Figure 23 Angular distribution produced by on the glass target at 45°. and Section III. C ). 20 MeV 19 F incident This is run +1 (see Table 4 -159- C> + + CD . ('r') + C> + + + + . + + + C> + + + .... ('r') + + + C> + ........ a_ + + C> ........... + + . a: w C> ........ :r: ........ + + + ('r') + + 0 + I + .... . U1 ('\I C> I . <D ('r') C> I . C> LI) '--~~~__._~~~~...._~~~---'...._~~~__._~~~~......_~~~----i 0·1 e·o Al1SN30 s·o ti"O l"D ~J~8l 032Il~WY~N Figure 23 o·o 0 I -160Figure 24 Angular distribution produced by 8 MeV 19F incident on the glass target at 45°. Section III. C ). This is run ;fs (see Table 4 and -161- C> + + . CD (T) + C> + + + + + + C> + + + + + (T) . ..-1 C> + + + ~ a_ + o' • er: I- + + C> + + + I- + (T) . ..-1 + + + + C> I . Ln N C> I CD (r) • C> I . @ .___~~~--~~~~--~~~---~~~~---~~~--~~~~~ o·t e·o g·o ~·o Al1SN30 ~J~8l 032Il~W8~N Fig ur e 24 ~·o w ::r: 0 I -162Figure 25 Angular distribution produced by 10 MeV 19 F incident on a pre-war uranium foil at 45°. Table 4 and Section III. C ). This run 1f8 (see -1630 . IJ') 0 + CD + + . (T') + 0 + + + ~ + + + + + . 0 + -. (T') + + + C> ......, + CL + o'. a: + o.,_ + w :c .,__ + + + -. + ('r') + + 0 I + Lil . C\I 0 I CD (T') 0 • I @ ....__~~~--~~~~---~~~--~~~~---~~~--.~~~~- 0·1 e·o s·o ti·o A1ISN30 ~J~81 032Il~W8QN F ·i.gure 25 ~·o . 0 I -164Figure 26 Angular distribution produced by 10 MeV on the glass target at 45°. and Section III. C ). 19 F incident This is run 4f4 (see Table 4 -1650 t.n • C> + + + . ~ + + C> + + + + + + + . ~ C> + -. + (T) + + C> + + ........ + CL + o . a:' o.,_ + w :J: ...._ + + + + (T) + • + + C> I . t.n N 9 . CD (T) C> I C> t.n .__________________________,..___________________________. o·t a·o s·o ~·o ~·o AlISN30 ~J~8l 032Il~W~ON Figure 26 0I • -166Figure 27 Energy spectrum of 1. 8 M e V UF 4 target. 4 He scattered from a The large peak on the right corresponds to the uranium; the peak on the left corresponds to the fluorin e (see Section 11!.E.2). -167q "' " )( .....II a: t- 1.J..J J: t- '-" " )( I " ""* ~ "I " "I " "I :I " " " * •" ! I )( )( . )(~. ><" *"<11oc 111:)()( "" " )( llC . )( •" """• "x " " )( :i I "I~ (:"••1x " :i •• )( )( •" "xi ! "* )( "" """ • * llC • .. . • " "*I X )(X b '2:, X ~ ~ 0131.1- Figu re 27 ~ 0 "'! 0 b -168Figure 28 Chamber used for low energy yield measurements with the 150 kV ion source. Current integration was performed in the manner illustrated in Fig. 10 (see Section Ill. E. 3 ). ~ · 1-rj N 00 ~ ~ (IQ BEAM ...... ""?"' CHAMBER WITH 500 l/s PUMP >' ELECTR ICAL FEEDTHROUGHS , ~ I PUMP '° I 11 !/s ION CATCHER FOIL &t==ii=flMANIPULATOR 50 !/s SUBLIMATOR -170Figure 29 Photograph of the target assembly for UF The UF 4 4 sputtering. film is on the shiny front surface of the copper block (note that the reflection of the support post can be seen). Fig. 30 is a schematic drawing of this target (sec Section III. E. 3 ). -171- Figure 29 -172Figure 30 Schematic drawing of the target for shown in Fig. 29. UF 4 sputtering The thermocouple and tungsten heater are electrically insulated from the copper block. Charge is collected from the target with a connection to the support post, which is not visible in the drawing (see Section III. E. 3 ). -173- EVAPORATED UF4 FILM \_~ \.. MACHINABLE GLASS BEAM SAPPHIRE TUNGSTEN HEATER ~ HOOK FOR CATCHER FOIL~ SPRING MACHINABLE GLASS UHV YWZ 2/'\i)//;}f.2!~HROUGH Figure 3 0 -174Figure 31 Angular distribution of UF 4 film with 4. 75 MeV 19 235 U atoms sputtered from a F(+3 ). This run produced a uranium yield of 8. 2, the highest we ever observed. · cos • f i. t is 81 The e, w h.ich was prob a bl y n a ttene d somewh a t b y target misalignment. The scatte r in the data is typical of the results from the UF 4 targets (see Section III. E. 4). -175- a:> . (\") 0 lJ') N • 0 -. ('r) 0 CL o' . a: Ot-- W :x: t-- -. ('r) 0 I ~ 0 . I . ~ 0 I ~ a . ~·t 0·1 e·o g·o ~·o A11SN30 ~J~81 03Zll~W~ON F i.g ur e 1 1 ~·o o·o I -176- Figure 32 Sputtering yield of uranium produced by on UF . 4 19 F incident The electronic stopping curve was calculated from the tables of Northcliffe and Schilling (1970) with the Bragg rule. The numbers used are shown in Table 10, The numbers beside the data points indicate the charge state of the incident beam. The error bars correspond to the standard deviations of the measured yields in those cases for which more than one run was performed. The points without error bars represent single runs (see Section III. E. 4 ). N VJ (p 'i i::: ()Q .... t"Ij 19 F on UF4 •2 0 2 3 0 • •2 0.2 k3 I ~ :t /1 sr 0.4 • .. 0.6 ••4 I·4 0.8 1.0 1.2 1.4 Fluorine Energy [MeV /a mu] • 1.6 ••5 1.8 EI ectronic Stopping Power 0 100 200 ........ "'O "'O wl >< L...J >Q) oc:t 300 ,......, i400 9~~---.-~~--~~....-~---,.--~-.-~~-.-~~~~~.--~-.-~~-. Uranium Sputtering Yield for I I I ' I. I I -.J -.J .... -178Figure 33 UHV chamber used in the energy spectrum cxperiments. The target was mounted on the bellows, and a sliding contact allowed the beam current to be collected from the electrical feedthrough. The 11 1 Is ion pump was used in Run II only (see Section III. E. 5 ). ..... c l%j VJ vJ ro '"i ()'Q BEAM ....... 7" MOTOR AND WHEEL HOUSING COLD TRAP PUMP 111/s ION BELLOWS ELECTRICAL FEEDTHROUGH PUMP 60 1/s ION UHV VALVE WINDOW PUMP SORPTION '°• -....I I -180Figure 34 Arrival time spectrum for the U F with +300 V bias on the t a rg e t. which 4 target ir radiated The wheel h ad two s l ots , produce two identical spectra. If we assume that the counts in the two p eaks co rr e spond to atoms accele r ated by the target potential, 4 7'f,. th en we ob t ain a charged fraction of The yield is expressed in arbitrary units . The energy spectrum produced by this ar Li val time spectr um is shown in Fig. 38 (see Section III. E. 5 ). -181C? f2 + + C? ~ + + C? + + + + + + + + + + + + ,.... ..... ...,, ~ C? ~ C? ~ + + x: a: Iu ~ (/) Q ~ + ::::> + + + + C? g + ~ ..... I- t + + + + + + Q .....J •N ~ > ..... iq + lE + ~ + ~ Q LL.. ::::> f'i + c a: UJ + UJ II- C? Kl + + + + + ::::> a.. (/) C? ~ + + + + + C? Lil + + + + q ~ + + + + q Lil + f 0"0€ + o·sz + + o·oz + o·s1 0131,1. Figure 3<1 + 0·01 q + o·s o·o 0 -182Figure 3 5 Arrival time spectrum for the UF with no bias on the target. slot. 4 target irradiate d The wheel used had only one The yield is expre s s c d in a rbHr a ry units. The corresponding energy spectrum is djspl a y e cl in Fig . (see Section III. E. 5 ). 3<) -183C> ~ + + 9 ~ + + 9 fZI + + + 9 tR + + 9 + ""' ..... ...... ~ + ....., + %:'. 9 ~ + ~ I- u + 1.1.J Q.. 9 + (J') 1.1.J ~ + %:'. ...... I- + _J a: > ...... C> 'N ~ + a: a: a: + 9 + :;j' ~ LL. :=) + 0 1.1.J + a: 1.1.J II- 9 IQ + + :=) Q.. (J') + + 9 + fi3 + + + + 9 !!! + + + + q + ~ + + + + q + Lil + ~ +'.fl' o·st s·zt o·ot s·L 013IA Figure 3 5 o·s + s·z q o·o 0 -184Figure 36 Energy spectrum for Run I in which the target was biased at +300 V. The shoulder corresponding to the energies above 10 eV was produced by the accelerated particles (see Section III.E. 5) . .. 1as .. SPUTTERED U ENtRGY SPECTRUM •+ + + + ++ + +•+ + + + + + D - + uj 10-3 + >- + + + 10~ + 10-s 10-a.____..__._._............ ............ ............ .......... 101 102 10 3 ~...___,___..__._~ 10-1 100 ...___.__.__.._~ ENERGY <EV) Fig ur e 36 ..____.___._~.........., -186Figure 37 Energy spectrum from Run II in which the target was not biased. reliable. The points at energies above 5 eV are not We believe that the yield was enhanced by very low energy particles produced by the beam pulse from the previous cycle of the wheel (s ee Section III. E. 5 ). -187- SPUTTERED U ENERGY SPECTRUM + + + 10-1 10-2 + + + + + + + + + + + + + +++ •+ + + + + 10-" + + + + 10-s + + + Figu1'<! 37 -188Figure 38 A two parameter fit to the data from Run II using the Maxwell-Boltzmann distribution: 1. 7 x 107 l Ez exp(-E/. 62 eV) , which corresponds to 7000°K (see Section Ill. E. 5 ). -189- FIT T~ SPUTTERED U ENERGY SPECTRUM THERMAL + + 10-1 + 0 _J w >- + ......... 10-3'--~~--~__._~..__._.._~~~~~~~-'---''--...__~_,_ 10-1 10° ENERGY <EV) Fig ure 38 10 1 -190Figure 39 A three parameter fit to the data from Run II using the collision cascade distribution: E 8 3.1x10 (E (see Section III. E. 5 ). + 1.2 ev) 6 · 1 -191------~· FIT TO SPUTTERED U ENERGY SPECTRUM CASCADE 10-1 Cl _J UJ >- 10-2 ........ 101 10~--~~---~----------.-....._._....._~~---~------'-~-..&. 10-1 100 ENERGY <EV) Figure 39