IEEE Transactions on Nuclear Science, Vol. NS-28, No. 2, April 1981
The Use of Ion Accelerators and Synchrotron Radiationto Study the Interaction of Helium with Metals
S. E. Donnelly*, J. C. Rife , J-M Gilles*, A. A. Lucas*
*Institute for Research in Interface Science (I.R.I.S.)FNDP - Namur, Belgium
tNational Bureau of Standards, Washington, DC
Summary
A technique is described which examines theproperties of helium trapped in bubbles in implantedmetals. Helium implanted materials are characterizedusing resonant elastic proton backscattering andTransmission Electron Microscopy (T.E.M.). Spectros-copic measurements using synchrotron radiation in thefar vacuum vutraviolet are then performed to examinethe density sensitive optical absorption resulting fromthe 1S - 2P transition in the implanted helium. Experi-mental data for helium implanted aluminum thin films arepresented which indicate atomic densities in small(50A diameter) bubbles of the order of 1023 atoms cm 3.
Introduction
Helium atoms in both the cladding of fissionreactors (by transmutation under neutron bombardment)and in the first wall of the proposed controlledthermonuclear reactor (C.T.R.) (a-particles from the D-T reaction) play an important role in the formation ofmicro-cavities. In the former case these cavitiescontain at most trace amounts of helium (underpressur-ized) and are called voids, whereas in the lattercase, they contain large numbers of helium atoms andmay be overpressurized. The terms over and under-pressurized refer to an equilibrium bubble whosepressure is in equilibrium with its surface energy ysuch that:
P r (1)
where r is the bubble radius. Both voids and bubblesare undesirable as they result in swelling of theirradiated material' (voids and bubbles) and blister-ing and flaking2 (bubbles). In conjunction with thepossibly more important effects of sputtering andarcing, blistering and flaking may result in an erosionof the first wall of the C.T.R. and a contamina-tion of the plasma with unwanted "high Z" impurities.
The distinction made between bubbles and voidsindicates that an important parameter in the study ofmicrocavities is the pressure or density of gas whichthey contain. This is particulary true for bubbleswhere knowledge of helium pressure prior to blisterformation would enable the correct model of blisterformation to be elucidated. (The choice is generallyconsidered to be between a "Gas Pressure Driven" and a
"Lateral Stress Driven" model)3. Despite the varioustechniques which have been brought to bear on thehelium/metal system, however, to date there is nonewhich has been capable of measuring this importantparameter.
This paper will describe some recent experimentscarried out at I.R.I.S.-Namur, Belgium and the SURF IISynchrotron Radiation Facility, National Bureau ofStandards, Washington, DC in which, for the first time,a spectroscopic technique has been brought to bear on
the measurement of helium density in microcavities.The technique consists of measuring the density
sensitive absorption, due to the 1S - 2P transition, byhelium atoms in specimens whose gas content and bubbledistributions have been characterized by proton back-scattering and transmission electron microscopy respec-tively. Although-measurements are currently underwayon bulk materials, this paper will deal with the experi-
mentally simpler case of thin films. In view of itsoptical transparency in the wavelength region ofinterest, aluminium has been used for these measure-ments. The technique is currently being extended toother materials with suitable optical properties and,by measuring the same electronic transition usingElectron Energy Loss Spectroscopy, to materials notsuitable for optical measurements.
Experimental
Aluminum films %2000A in thickness are prepared byevaporation in UHV of 99.999% pure aluminium in asy-stem containing a microbalance permitting an accuratemeasurement of mass thickness. The aluminium is deposi-ted on a previously evaporated NaCl substrate so thatthe films can be subsequently floated off and caught onstandard 3 mm diameter copper T.E.M. grids which have a0.6 mm diameter central hole. T.E.M. indicates a meangrain size of the order of 500A.
After checking for low pinhole transmissivity, thefilms are mounted in a small turbo-pumped U.H.V. chamberon the end of a beam line of the 3 MV Van de Graaffaccelerator at the LARN Laboratory, Namur. Implantationis carried out by means of a 5keV ion gun with thespecimen temperature held at any required value be-tween -150°C and 500°C. Gas content as a function ofion dose (the dose is calibrated using a small Faradaycup) is measured by rotating the specimen 90° into theproton beam from the accelerator. Backscattering mea-surements are performed at 1500 and 2.2 MeV, at whichenergy there is an elastic resonance for proton scat-tering off helium5 (2He + 'p + 'Li* + PHe + |p) whichgives rise to a cross section approximately 400 timesgreater than the Rutherford value. The combination ofthis resonance with a beam diameter of lOQPm, so thatonly the self-supporting part of each film is sampled,results in a sensitivity sufficient to measure %0.1 at% He in resonable measuring times. Post implantationannealing of specimens, if required, is subsequentlyperformed whereupon the bubble distributions arerecorded using a Philips EM 300 transmission electron'microscope. The self-supporting aluminium films aresufficiently thin to allow microscopic examinationwithout the necessity of thinning, so that micrographsobtained are projections of the bubble distributionsacross the whole film thickness. In some cases speci-mens are also inspected by scanning electron microscopyto examine the film deformation that occurs for high
6dose bombardmentsThe specimens are then transferred to the 2.2 m
grazing incidence monochromator on a beam line at theSURF II Synchrotron Radiation Facility at the NationalBureau of Standards, Washington DC.7 The experi-mental set up is illustrated schematically in Figure1 and some details will be briefly discussed here. More
complete descriptions can be found elsewhere. 8 9
Monochromatized radiation in the wavelengthrange 400 - 700A is normally incident on the specimenand transmission through the film is measured using a
channeltron detector.The resolution of the system as defined by the
entrance and exit slits to the monochromator is normally2A. The measured transmitted flux is normalized to theincident flux (as measured on the photodiode beammonitor) and differential data are obtained by sequen-tial measurements on implanted and non-implanted but
Fig. 1: Experimental set up for spectroscopic measure-ments.
otherwise identical specimens. The effective absorp-tion coefficient of the helium can then be obtained as:
1 Alneff t IAl+ He
where t is the film thickness and where IAl and I l+He'the normalized transmitted intensities are given Dy:
IAl = exp(-Alt) non-implanted
and
IAl+He exp(-PA1+Het) implanted
with pAl and pAl+He the respective absorption coef-ficien s.
Results and Discussion
Helium Trapping: Before dealing with the opticalmeasurements it is interesting to look at the trappingbehavior of the helium as measured by proton back-scattering. Figure 2 illustVates the trapping curvefor implantation of 5 keV He ions into a 53.4 (+ 0.1)
ig (1979 A) self-supporting Al film at room tempera-ture. The atomic concentrations measured representthe ratios of the total quantity of helium and thetotal quantity of aluminium seen by the proton beam.Consideration of the calculated range profile forhelium10 in aluminum at 5 keV indicates that the peakvalues may be up to a factor of three greater than thesequoted average values.
The slope of the initial rapidly rising portionof the retention curve indicates that even at low
doses the helium retention is fairly low for roomtemperature implantations (%19%). The fact thatvacancies are mobile in aluminum at room temperaturemay contribute to this low trapping behaviour butcannot totally account for it as data on implantationsat -150°C (not shown) indicate an increase in trappingefficiency to only 24%. [This low trapping behavior iscurrently under investigation as a function of energy,
temperature and dose rate and will be reported in duecourse]. At a dose of about 1.75 x 1017 ions cm 2the curve begins to saturate although a slow increasein gas content with dose is still observed. For im-plant doses up to this "knee" in the trapping curve,T.E.M. observations reveal resolvable bubbles first ofall in the grain boundaries at a dose of -u3 x 1016
(2)
Fig.02: Trapping curve for He normally incident on1979A Al film at room temperature.
ions cm . Increasing the dose results in resolv-able bubbles in the grains as well as the grainboundaries until at %l x 1017 ions cm-2 a fairlyuniform population of bubbles with diameters 40 - 60Ain diameter is observed. (The mean bubble diameterincreases with increasing dose). Beyond the knee inthe trapping curve larger bubbles (> 100A) begin toappear, presumably by agglomeration of some of thesmaller bubbles and the diameters and number of theselarge irregular shaped bubbles increase with furtherdose increases. A high density of smaller (%50A)bubbles is, however, always present even at the highestdoses. Finally for doses in excess of about 3 x 1017ions cm 2 bubbles with diameters ranging from 050A orless up to %1000A are observed. Bombardment to dosesbeyond 5 x 1017 ions cm 2 results in the formation ofbubbles so large that their surfaces intersect thesurface of the film resulting in pinholing and even-tual destruction of the film. Beyond the knee in thetrapping curve, therefore, little additionalhelium is retained in the aluminium but bubble growthis induced by supplying vacancies to the bubbles.
Large bubbles can also be produced by annealing ofspecimens containing 1.5 at % helium or more to about4000C. In this case, however, the population of small(%100A) bubbles is observed to diminish and the largerbubbles are faceted whilst the helium content of thefilm is reduced to %30% of its pre-anneal value. Theselarge faceted bubbles would not be expected to behighly pressurized as during the anneal they have beenable to acquire thermally generated vacancies.
Spectroscopic Measurements: The helium 11S -21Ptransition occurs at 584.3A (21.2eV) in a rarefied gasso that the wavelength region of interest is fromabout 500 to 600A although measurements have also beenmade on an autoionizing transition which occurs atabout 200A4. Model calculations relevant to bubbles,of mechanisms which could be expected to change theform and position of the optical absorption resultingfrom this lS-2P transition have been calculated byOktaka and Lucas.11 However, none of the mechanismscalculated in this paper were sufficient to account forthe shift and broadening measured by Surko et al. byreflectivity on liquid helium1 2 or the even greatershift observed in our measurements. We have, there-fore, developed a model for high density helium, basedon the repulsive Pauli interaction between the elec-trons in the atom excited by the radiation and thesurrounding ground state helium atoms. Both the ls andthe 2p electrons feel this repulsion but the effect is
1821
A
I
1- 1 - - 1 -1-
-r I
I- I
I II-1 IgratingfromSIJRF
i .
much greater for the 2p electron which is located at agreater average distance from its own nucleus. Theeffect of this repulsion is an increase in all energylevels in the excited atom but this increase is mar-kedly greater for the 2p level, resulting in an in-crease in the energy (blue shift) for the lS-2P tran-sition. Details of a first order perturbation theorycalculation of this effect will be given elsewhere4and are not reproduced here but Figure 3 shows thevariation of peak wavelength for the absorption as afunction of helium atomic density calculated using thismodel. The data of Surko et al. on liquid helium areused as a calibration point for this curve. The widthof the absorption is also a function of helium densitybut calculation of this is more complex and has not yetbeen attempted. It should be pointed out that thismodel calculates only gas/gas effects and is thereforevalid in a regime where these dominate. For smallbubbles or agglomerates less than say 20A in diameter,gas/substrate interactions may begin to play a role andalthough some such effects have already been calculatedwe have, as yet, no all-encompassing theory of line-shape broadening and shift for very small cavities.
anrt
*4
570Z
0
I-
£0
550
4
os.0 4 8 12 16 20
He DENSITY (He cm-3)x 022
Fig. 3: Calculated variation in wavelength for IS-2Ptransition in helium as a function of density.
Figure 4 illustrates the optical absorptionspectra and the T.E.M. micrographs for three specimenscontaining different types of bubble distribution. Alsoindicated is the position and width of the absorptionmeasured in liquid helium. Figure 4a shows data for a
specimen containing only very small observable bubblesS20A which quite probably represent the large diametertail on a bubble size distribution which peaks at muchlower (non-resolvable) diameters. As such a bubbledistribution is normally concomitant with low heliumconcentrations, rendering the optical measurementsdifficult, the quanity of helium in this specimen hasbeen augmented by implantation with 3.8 x 1016 ions cm 2on both faces of the film. The absorption in this case
has been dramatically blue shifted by about 80A com-
pared with the line resonance. Unfortunately, as dis-cussed above, it is currently not possible to interpretdata resulting from such small bubbles. This spectrumdoes, however, serve to indicate the absorption peakresulting from small (and probably submicroscopic)bubbles. Such a component, if present in the spectrum
of a specimen containing larger observable bubbleswould indicate the presence of a submicroscopic bubbledistribution co-existent with that observed by T.E.M.In such a case, this component could be subtracted fromthe spectrum to yield the absorption data correspondingto only the observable bubble population.
I0x
0
q67 ~ C , 8584 A
5 _
4 ,_
2
0480 520 560 600 640 0
WAVELENGTH (A)
Fig. 4: Absorption spectra and micrographs for threespecimens a) implanted to a dose of 3.8 x 1016 ions cm-2on both faces; b) implanted to a dose of 1 x 1017 ionscm 2; and c) implanted to a dose of 5.5 x 1017 ions cM-2and post annealed to 450°C.
Figure 4b illustrates the spectrum and micro-graph for a specimen implanted to a dose of Xl x 1017ions cm 2 and containing 1.45 at % helium. The bubblepopulation in this specimen is fairly uniform consist-of (two dimensionally) a homogeneous distribution ofbubbles with diameters in the range 40 - 60A. Theabsorption spectrum consists of a single peak centeredon 550A with a FWHM of 36A and by comparison withFigure 4 does not exhibit a peak structure which wouldindicate a substantial submicroscopic component to thebubble distribution. Using the theoretical curve ofFigure 4 to convert from absorption peak position tohelium density in the bubbles yields a value of
1822
I I I I I I I I
;- bubbles -I_IIII I
Is 1 I I I .1 I I I
I
just less than 1.2 x 1023 atoms cm3 . The significanceof this value will be discussed below.
Finally, Figure 4c shows data for a specimeninitially implanted to a total concentration of 2.7 at% He and subsequently annealed to a maximum temperatureof 450C for one hour. Evident in this specimen aremany faceted bubbles generally several hundred Angstromunits in diameter as well as smaller bubbles (<1o0A)and a small number of large irregular blister typefeatures (<1OOOA) some of which have intersected a filmsurface and burst. Bubbles have been able to growduring the anneal by the acquisition of thermallygenerated mobile vacancies. The faceted bubblesstrongly resemble voids frequently observed afterannealing of neutron irradiated materials1 and wouldnot be expected to contain high pressure helium. Thedashed curve on Figure 5c indicates the absorptionspectrum prior to annealing which consists of a broadstructure appearing to contain several badly resolvedpeaks. As pre-anneal T.E.M. examination of thisspecimen revealed a large range of bubble sizes9 (up toseveral hundred angstroms) but with the bubble sizedistribution peaked at %50A, this structure isbelieved to reflect the large variation in gas densityresulting from this broad size distribution. Onannealing, the high density of smaller bubbles isreduced and the number of large bubbles (which becamefaceted) is increased. The loss of helium concomitantwith this indicates that the small bubbles eitherdiffuse or dissociate during the anneal.
The spectrum after the anneal exhibits a sharppeak at exactly (within the resolution of the system)584A - the position of the line resonance - as well assome structure extending to lower wavelengths. Thissharp peak is presumably due to absorption in therelatively low pressure helium in the large bubbles(<100 atmospheres) whilst the rest of the structure isdue to the population of smaller bubbles still present.
The expression for the equilibrium pressure of abubble (equation (1) above) yields for a 50A diameterbubble and a surface energy of 1000 ergs cm 2 a pres-sure of %8,000 atmospheres. Following Evans13 andusing the experimental data of Bridgmannl 4 to convertfrom pressure to density an atomic density of4.5 x 1022 atoms cm-3 is obtained. For the specimenillustrated in Figure 4b containing bubbles 40 - 60Ain diameter our spectroscopic measurements indicate adensity of 1.18 x 1023 atoms cm-3 implying that thebubbles are considerably overpressurized. On amacroscopic level this number seems very large. On amicroscopic level, however, experimental data such asthat of Caspers et al.15 on helium trapping in molyb-denum indicate that a monovacancy may trap up to aboutsix helium atoms. A helium density of 1.18 x 1023atoms cm3 is equivalent to an average of just lessthan two helium atoms per vacancy in our bubbles.
Conclusions
This paper has reported a technique, developedover the last two years which uses synchrotron radia-tion as a primary tool and proton backscattering as asecondary tool to look at the state of helium im-planted into metal films. Measurements indicate thatsmall helium bubbles in aluminium are highly over-pressurized supporting the "Gas Pressure Driven" modelof blister formation.
and the technical assistance of I. Morciaux, M. Renierand the SURF II staff was also greatly appreciated.
This collaborative project has been made possibleby the award of NATO research grant Number 1970 andthe support of the Belgian Ministry for Science Policy.
References
1. See Review by D.I.R. Norris Rad, Effects 14(1972) p. 1 and 15 (1972) p.l. Also RadiationInduced Voids in Metals, J. W. Corbett and L. C.Ianniello, USASymp. Series. 26 CONF - 710601(1972).
2. S. K. Das and M. Kaminsky, Advan. Chem. Ser. 158,(Am. Chem. Soc., Washington, DC 1976) p. 112also M. Kaminsky, S. K. Das, Proceedings of 4thConference on the Scientific and IndustrialApplications of Small Accelerators, (Denton, TX(1976) p. 238.
3.
4.
J. H. Evans, J. Nuc. Mater. 76 and 77 (1978) 228.
J. C. Rife, S. E. Donnelly, J. J. Ritsko, A. A.Lucas, J-M Gilles, to be published.
5. G. Freier, E. Lampi, W. Sleator, J. H. Williams,Phys. Rev. 75 (9) (1949) 1365.
6. S. E. Donnelly, G. Debras, J-M Gilles, A. A.Lucas, Rad. Eff. Letters 50 (2) (1980) 57.
7. For a description of SURF II, see R. P. MaddenNucl. Instrum. Meth. 172 (1980) 297 and papers byA. Parr and G. Rakowsky, this conference.
8. J. Rife and J. Osantowski, Nucl. Instrum. Meth.172 (1980) 297.
9. S. E. Donnelly, J. C. Rife, J-M Gilles, A. A.Lucas, J. Nuc. Mater. 94 and 95, to be published.
10. J. Ziegler. The Stopping and Ranges of Ionsin Matter, Vol. 4 (Pergamon, New York, 1977).
11. K. Ohtaka and A. A. Lucas, Phys. Rev. B 18 (9)(1978( 4643.
12. C. M1. Surko, G. J. Dick, F. Reif, W. C. Walker,Phys. Rev. Letters 23 (15) (1969) 842.
13. J. H. Evans, J. Nuc. Mater. 61 (1) 1976.
14. P. W. Bridgman. Collected Experimental PapersVol. III, (Harvard Univeristy Press, Cambridge,MA, 1964).
15. A. van Veen, L. M. Caspers. Proceedings of theConsultants Symposium on Inert Gases in Metalsand Ionic Solids, (Harwell, U.K., 1979).Published by HMSO.
Acknowledgements
We should like to thank R. Andrew, F. Bodart andD. Ederer for much useful discussion and R. P. Maddenand G. Deconninck for making available the facilitiesat SURF and LARN respectively. The thin filmswereprepared by P. Rietveld in the laboratories of H.Eschbach and I. Mitchell - all of whom we acknowledge
