Lifetimes of thin films in ion beam experiments

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LIFETIMES OF THIN FILMS IN ION BEAM EXPERIMENTS* D. R A M S A Y

Department qf Phys'ics, Statllbrd Univers'iO', Stanlhrd, ('al!ybrnio 94305, U.S..4.

Prolonging the life of stripper toils would greatly facilitate long term heavy ion beam experinlents. The average lifetinle of a 200/k carbon foil in a 3 MeV argon beam (3 # A ) is reported to be about 20 min. We have investigated the behavior of thin ( - 2 0 0 AI foils in a 1 MeV :z beam (50/~A). Various materials and preparation techniques were used in an attempt to determine the m e c h a n i s m of foil failure. Differences were observed m both lifetimes and modes of failure.

The short lifetime of carbon stripper foils used in electrostatic accelerators is a serious experimental limitationS). In attempt to overcome this handicap we have investigated the behavior of thin ( ~ 200 ~) foils in a 1 M e V ~-beam ( 5 . 0 # A beam current). The results of the work to date suggest that other materials may be better as stripper foils. The 3 MeV Model KN positive ion accelerator at Stanford University was used to provide a 5 . 0 # A alpha beam at 8 0 0 - 9 0 0 keV. It was found that these conditions closely paralleled actual stripper foil behavior. The 5 # g / c m 2 carbon stripper foils followed the pattern of thickening and breakage reported in the literature2). The foils were mounted on 0.5 mm thick tantalum frames with a 12.5 m m hole. Visual observation of the foil at all times was possible through a plexiglass window (fig. 1). The actual current through the target was monitored. The change in foil thickness was measured using elastic scattering from the target material. Initially carbon foils of about 5 Hg/cm 2 supported by 25/,tg/cm 2 of F o r m v a r were exposed to the beam to obtain a standard against which experimental variations could be measured. A 200 A foil deposited by arcing carbon rods in vacuum onto a teepol*-coated glass slide (fig. 2) exposed to the beam for only a few seconds, showed a dark beam spot area almost instantly. (It is not related to the plastic supporting medium because it appears on carbon foils which are entirely self-supporting). Supported in part by the National Science Foundation. Formvar (polyvinyl formal) 33% oxygen, 59"/,, carbon, 7.8% hydrogen by weight. + Teepol (sodium secondary alkyl sulphate) liquid detergent sold by G. H. Wood and Co. Ltd., Toronto M82 5M8, Canada.

In the beginning we sometimes observed a pin point ''hot spot" on the foil in the area exposed to the beam. It was difficult at first to determine whether this was due to a pinhole in the foil or a •

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Fig. 2. Carbon evaporator. 11. S T R I P P E R

FOILS

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D. RAMSAY

MINUTES dust particle. It was found to be a dust particle or 0 25 50 75 IOO more often a fragment of carbon debris that is floatI I I ing on the surface of the water and is picked up 6with the foil when mounting. We tested a large Pl/C area carbon foil and by turning it to an acute angle could easily see the instant high temperature created when the beam penetrated a thicker area. As reported previously the foil thickens and final~5 ly ruptures. Generally these tears were in the beam J spot area for foils that had a supporting backing. J ~ Fig. 3 presents a composite graph of these carbon / I± foils, showing the change of thickness in /ag/cm 2 versus integrated beam current. Breakage of the 5 IO 15 20 25 foils occurred at different times but the rate of BEAM CURRENT : l , O O 0 / z COULOMBS) thickening was constant and we were to see this shape many times in the course of this investiga- Fig. 4. Thickness change of platinum/carbon foils vs integrated ~-beam current of 1 MeV. Solid line: self-supporting foil; dotted tion. It did not vary, in the case of carbon, whether line: foil supported by Formvar. there was a supporting film or not. The slope remains the same even though the foils ranged in initial thickness from 4 to 1 2 / l g / c m < The current layers onto carbon foils with no appreciable change through the foil was about 6.5/~A for a beam in lifetime4). We ran a series of tests in which current of 5.0/tA. The arrows indicate the breakage carbon and platinum were deposited simultaneously points of three of the trial foils. The extremes resulting in a thin foil which was an alloy of the ranged from 6 4 m i n (16000/aC) to 127 rain two elements. Films not supported with Formvar showed the familiar carbon slope shown as the (32 000 /IC). The average thickening was ~ 5 0 % . The literature has shown that this increase is not upper solid line of fig. 4. However, films supported due to the cracking of residual diffusion pump oil with 25/.~g/cm 2 of Formvar (the lower broken line) in the beam line3). But we ran a 25/ag/cm 2 self- showed a markedly steeper slope. They thickened supporting boron foil to check our own system. The more rapidly and consequently broke much sooner. beam spot temperature was about 900°C for It has been suggested that mass transport of 90 rain. We saw no surface discolorations or thick- carbon atoms 5) caused by the electron current in ening. In fact the foil got 10% thinner and carried a these foils may be a factor in the thickening. Tantavery low target current of 2/~A compared to carbon lum in carbon has a diffusion factor 100 times less which had about 6 / I A passing through it. Other than carbon alone and is not as susceptible to this experimenters have tried evaporating thin metallic electromigration. Tantalum carbide is also a highly M I NUTE S

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refractory compound, so we next tested tantalum carbide thin films. Although two foils not supported by plastic exceeded 120 min, two others supported by Formvar were no different than carbon. They all thickened and failed in a similar manner and when plotted conformed to the earlier data (fig. 5). Another candidate was boron carbide. It has a stable rhombohedral crystal structure with a hardness exceeded only by diamond and it would appear that if one of the modes of failure is due to shrinkage of the film caused by radiation damage 3'6) then it would be resistant. The results of eight tests showed the data were very consistent. We deposited films with barium iodide as a substrate which is also rhombohedral and films with teepol as a release agent. We ran films with Formvar and without Formvar and nothing changed. There were some minor variations. Films on barium iodide supported by the plastic backing lasted longer than their carbon counterparts, several exceeding 30 000 #C. The target current was less by one-half but they thickened linearly and increased by roughly 25% before breaking (fig. 6). By reactively evaporating silicon and carbon we would easily produce a silicon carbide film although the exact composition was not definite. It could show some variation in behavior. The points for each thickness reading are shown in fig. 7. The slight decrease in the number of counts from elastic scattering during the first two or three minutes of a run we assumed to be from the Formvar burning off. This particular film went on for another twenty minutes, and at 29 000 #C it broke. At that time, it was 58% thicker than when it started. All the foils

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in our first series of experiments had one c o m m o n denominator and that was the supporting plastic, in our case Formvar. We ran another set of three silicon/carbide foils, this time entirely self-supporting. Fig. 8 shows that the thickening slope is gone. These foils lasted four hours, did not thicken at all but did break at 55 0 0 0 # C as compared to the carbon foils which broke at 18 0 0 0 # C . The beam spot area was still intact but the foil tore loose around one third of its perimeter where it joins the target frame and was no longer mechanically stable. We tested three silicon/carbide foils without Formvat, three silicon/carbide foils with Formvar and one with polyethylene. All the supported foils thickened. The m a x i m u m thickening of the unsupported foils was 8%. The mechanism of breaking was very different from the tears in the beam area :seen with carbon foils. The tears started at the edge MI~J

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I1.

STRIPPER

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D. RAMSAY

of the foil and either tore loose from the target frame or ran in toward the beam spot area across the edge of it and back to the frame. We were able to get an approximate analysis of 20% silicon in these films. However, until further foils are fabricated, tested and analyzed, we cannot say what the exact structure of these films was. The experiments imply that the thickening of at least the silicon/carbide foils was due to the Formvar. It is interesting that of all the materials tested only the silicon/carbide showed virtually no thickening when the Formvar was not used. The work leaves many questions unanswered but the results strongly suggest that a better material than pure carbon does exist for use as solid strippers.

References l) A. E. Livingston, H. G. Berry and G. E. Thomas, Nucl. Instr. and Meth. 148 (1978) 125. 2) j.L. Yntema, IEEE Trans. Nucl. Sci. NS-23 no. 2 (April 1976). 3) T. W. Conlon, D. B. Gayther and L. G. Sandals, Prog. Report, April 1974 - March 1975, Nucl. Physics Div. A.E.R.E. Harwell. 4) K. Takimoto, R. lnada, J. Schimiza, E. Takada, M. Fukada and J. Muto, Ann. Report 1977, Department of Physics, Kyoto University, Kyoto, Japan. s) D. Ramsay, Proc. 5th Ann. Conf. INTDS, Los Alamos, N.M. (1976); LA-6850-C. 6) V, Koslowsky and J, R. Parsons, Prog. Report July 1976 Sept. 1976, Chemistry and Materials Div. Nucl. Labs, Atom Energy of Canada, Chalk River, Canada.