Experiments were performed demonstrating that ion irradiation enhances diffusion-induced grain boundary migration (DIGM) in polycrystalline Au/Cu bilayers. Here, a model is presented relating film-averaged Cu composition in Au with treatment time, grain size and film thickness. Application of this model to the experimental results indicates that irradiation enhances DIGM by increasing the grain boundary velocity. Effects of DIGM and irradiation-induced grain growth on the temperature dependence of ion mixing in bilayers are discussed

Alexander, D. E.

Baldo, P. M.

Gao, Y.

Rehn, L. E.

UNT Digital Library

The effect of DIGM and irradiation-induced grain growth on interdiffusion in bilayer ion-beam mixing experiments

THE EFFECT OF DIGM AND IRRADIATION-INDUCED GRAIN GROWTHON INTERDIFFUSION IN BILAYER ION-BEAM MIXING EXPERIMENTS*Dale E. Alexander, L.E. Rehn, Peter M. Baldo and Y. Gao**Materials Science DivisionArgonne National LaboratoryArgonne, IL 60439AN[a'MSD/CP--78183DE93 005553November 1992oontractoroi the U.S.GovernmentundercontractINo. W-Zl-10g-ENG.38. Accordingly.the U.S.IGo_nrr_nt r_ lt noN_dmtve,royally-f,l_Ikm_ to_¢_h o, rqxtxtuc,rhoix*_heaformIoi thlzcontrbution,or allowm_ to doso,foriu.s.Gov_nrn_I qX_. IWill be presented at the Fall Meeting of the Materials Research Society: Symposium A,Boston, MA, November 30-December 4, 1992.* Work supported by the U.S. Department of Energy, BES-Materials Sciences, undercontract #W-31-109-ENG-38.** Visiting scholar from the General Research Institute for Non-Ferrous Metals, Beijing,PRC.MASTER• &, ". ,,¢, ,.)_ '"'_. _E_,'.)L;L_'_'_t:','.v, i6 _;",._Li!. ..• THE EFFECT OF DIGM AND IRRADIATION-INDUCED GRAIN GROWTH ONIN'I_RDIFFUSION IN BILAYER ION-BEAM MIXING EXPERIMENTS*• DALE E. ALEXANDER, L. E. REHN, PETER M. BALDO and Y. GAO**Materials Science Division, Argonne National Laboratory, Argonne, IL 60439ABSTRACTExperiments were performed demonstrating that ion irradiation enhances diffusion-induced grain boundary migration (DIGM) in polycrystalline Au/Cu bilayers. Here, a model ispresented relating film-averaged Cu composition in Au with treatment time, grain size and filmthickness. Application of this model to the experimental results indicates that irradiationenhances DIGM by increasing the grain boundary velocity. The effects of DIGM andirradiation-induced grain growth on the temperature dependence of ion mixing in bilayers arediscussed.INTRODUCTIONIon-beam mixing experiments employing bilayer geometries are commonly used to studymass transport resulting from irradiation. Typically, RBS profiles are fit to solutions of thediffusion equation to extract an effective diffusion coefficient during irradiation. The dependenceof this diffusion coefficient on ion properties (mass, energy, energy deposition) and targetproperties (density, thermochemistry) has been used to probe the fundamentals of ion-solidinteractions.Bilayer experiments examining the temperature dependence of ion mixing have generallyshown the existence of two regimes [1]. In addition to a temperature-independent regime, atemperature-dependent regime typically begins near room temperature. The kinetic processesgoverning mixing in this thermally assisted regime are not well-understood. Attempts tounderstand this dependence have invoked some form of radiation-enhanced diffusion [2] orcascade effects [3]. Only limited attention has been paid to the role that the thin film _,microstructure may have on interdiffusion in this temperature dependent regime [4].Diffusion-induced grain boundary migration (DIGM) is a well-documented, highly -_efficient mode of interdiffusion observed during thermal annealing of a variety of polycrystalline ,-diffusion couples [5]. Diffusion of a solute in a grain boundary can induce the boundary tomigrate, forming an alloyed zone in its wake. DIGM occurs at relatively low temperatures and .,,,...may therefore impact on the interpretation of ion-beam mixing in the thermally-assisted regime.This paper summarizes experiments examining the effect of ion irradiation on DIGM inAu/Cu bilayers. Additionally, the influence of film microstructure on the Arrenhius behavior ofion mixing is addressed.* Work funded by the U. S. Department of Energy, BES-DMS, Contract #W-31-109-Eng-38.** Visiting scholar from the General Research Institute for Non-Ferrous Metals, Beijing, PRC' EXPERIMENTAL• Bilayers were prepared by e-beam evaporation of Au onto single-crystal MgO substratesfollowed by an overlayer of Cu. Polycrystalline Au films were produced with a grain sizegreater than or equal to the film thickness. As demonstrated previously [6], this microstructureis well-suited for analyzing DIGM kinetics. In order to produce the Au films, annealingtreatments at temperatures between 675-725 K were performed to facilitate grain growth afterroom temperature deposition on MgO, and to harden the films against further thermally-drivengrain growth at subsequent treatment temperatures (<_550K).Samples were irradiated with 1.5 MeV Kr ions at temperatures ranging between 300-550K. Dose rates varied between (0.5-1.0)x1012 cm -2 s-1 and doses were typically lx1015 cm -2.However, for _ne sample, a maximum dose of 6x1015 cm -2 was studied. Table I details theexperimental parameters examined.1.5 or 3.0 MeV He Rutherford backscattering spectrometry (RBS) was used to profilethe Cu present in the Au films before, and after, irradiation and annealing treatments. Directevidence of DIGM resulting from ion irradiation was obtained from detection of Cu alloyedzones in Au using small probe X-ray energy dispersive spectroscopy (XEDS) performed on across-section TEM sample [7]. A Hitachi 2000 transmission electron microscope with a fieldemission gun, operated in analytical mode at 200 kV, was used for analysis. Small electron-beam probes, approximately 20 ]k in diameter, were employed.RESULTS AND DISCUSSIONResults of RBS analysis showed that nearly uniform distributions of Cu were present inthe Au films consistent with DIGM having alloyed a certain volume fraction of the Au layerduring irradiation or annealing. Small probe )(EDS performed on an irradiated cross-sectionTEM sample showed these DIGM alloyed zones to have compositions between 14-20 at.% Cu[7], similar to those measured in a thermal study by Pan and Balluffi [6]. Values of film-averaged Cu composition, C, obtained from analysis of RBS spectra showed variations between3-34 at.% Cu in irradiated areas and 2-27 at.% Cu in annealed-only areas. In ali cases,irradiation enhanced the amount of DIGM relative to annealed-on!y areas.A simple model of DIGM is able to account for the observed variations in film averagedcompositions, C, extracted from RBS spectra. These compositions may be related to themeasured experimental parameters of treatment time, t, film thickness, !, and average grain ,,diameter, L, as follows. In a time, t, a grain boundary perpendicular to the f'dm surface migratesin a direction parallel to the surface with an average velocity, v, forming behind it asubstitutionally alloyed zone with Cu composition, C'. In a simple two-dimensional picture ofthe process, the f'dm-averaged composition is then given by-- 2 C'vtC = (1)LIf the boundary motion is described by _L_,average diffusivity, DDIGM, then the velocity becomes,• v=DDmM/g, and eqn.(1) may be expressed as,- tC = 2 C' DDIGM g---_ • (2)Note that DDIGM describes boundary migration and represents the average area per unit timeswept out by grain boundaries during DIGM.The functional dependence described by eqn.(2) is displayed in Fig. 1 for the dataobtained at 500 K. Remarkable lineafity is exhibited by both the irradiated and annealed-onlydata in support of the simple model of DIGM outlined above. The irradiation enhancement ofDIGM is clearly evidenced by the approximately factor of three greater slope in the irradiatedversus annealed-only linear-fits. The underlying mechanism for this enhancement is presentlyuncertain. However, from examination of eqn.(2), the increased slope (=2C'DDIGM) indicatesthat irradiation may increase the amount of Cu alloyed in the wake of the boundary and/orincrease the boundary migration velocity. Plan-view XEDS data combined with cross-sectionTEM results [7] indicate that the alloyed zone composition is nearly the same for both irradiatedand annealed-only treatments (C'=20 at.%). Therefore the results indicate that irradiationenhances the grain boundary velocity.25 I I I I I I I ......20-- El Irradiated __, _ Annealed _./1--1 rn _10lr,.) :-0 .... ! I I I I I I0 1 2 3 4_ xlO_ 3 sgL A 2Fig. 1. Variation in RBS measured uniform Cu composition, C, in Au at T=500 K as afunction of treatment time, t, film thickness, l , and Au grain diameter, L, derivedusing simple description of DIGM [eqn. (2)]. Lines are least-squares linear fits.• Having demonstrated that DIGM can occur during irradiation, it is reasonable to considerits role in ion-beam mixing experiments of the type normally performed on bilayer films. Theefficient interdiffusion provided by DIGM at relatively low temperatures can help explain thetemperature dependence of ion mixing. Bilayer experiments typically use evaporated films withgrain sizes much less than the film thickness. An extrapolation of the DIGM results presentedabove to small grain sizes is complicated by the fact that irradiation induces grain growth in fine-grained films independent of DIGM (i.e. without chemical interdiffusion) [8]. Unlike thethermally-hardened Au films used in the DIGM studies, a strong driving force exists forirradiation-induced grain growth in fine-grained metal and semiconductor films even at LN/temperatures. Therefore, an extrapolation from the large grained DIGM results is notstraightforward.In a temperature regime where grain boundary diffusion is operative, the occurrence ofirradiation-induced grain growth can be.expected to enhance interdiffusion in a manner similar toDIGM. This issue has been previously addressed by those interested in extracting grainboundary diffusion coefficients from annealing experiments in which boundary migration hasoccurred [9]. Although originally derived for the interpretation of them_al studies, the analysismay be applied to irradiation-induced grain growth. Glaeser and Evans [9] have shown, in thelimit that the grain boundary migration distance, vt, is much greater than the boundary width, _5,that the depth dependence of the solute concentration is, 0vt 1L ' (3)where 5Dgb is the grain boundary diffusivity, Co is the solute surface concentration and the otherparameters are as described previously. Since irradiation-induced grain growth occursessentially at all temperatures (v>0), eqn.(3) indicates that under the appropriate experimentalconditions (dose rate, temperature, etc.) modest amounts of grain boundary diffusion may result .'.in significant increases in mixing between bilayers. Analogous to DIGM this process may help _.-explain observations of enhanced ion mixing at moderate temperatures. Note that eqn.(3)reduces to that derived for DIGM (eqn.(1)) in the limit that grain boundary diffusion is infinitelyfast and assuming Co=2C'.As an example, Fig. 2 shows concentration profiles in Ni obtained using eqn.(3) andsuperimposing interdiffusion anticipated from cascade mixing at the interface (via acomplimentary error function profile). For grain boundary diffusion, Ni self-diffusion values -,_were used assuming irradiation temperatures of 450 and 500 K [10]. Grain size was taken asL=200/_ and the boundary migration velocity was assumed to be 0.1 /_ s-1 as determinedpreviously from Ar irradiation studies of Ni films [8]. An irradiation time of 600 s wasdetermined assuming typical experimental parameters of dose (1015 cm 2) and dose rate(1.7x1012 cm-2s-1). Cascade mixing was simulated using Dmix=10 16 cm 2 s1 [4]. The• resulting solute composition profiles show penetrating tails, which are particularly pronouncedfor T=500 K (see Fig. 2). Attempts to "force fit" simple one-dimensional solutions of the" diffusion equation to such profiles are expected to result in erroneously large ion-mixingdiffusion coefficients.0.5- l I I I"_ 04-_ --"_ " 450 KO_" Fig. 2. Calculated solute profiles inE 0.3- - Ni resulting from combinedO500 K ion-inducedgrain growth and_ grain boundary diffusion.m 0.2- "-" Dose =1015 cm -2 and t=600s.O.l-tm0 i I "--i0 200 400 600 800oDepth (A)Supporting evidence of the influence of film microstructure on ion mixing is found incomparison of two experiments investigating the temperature dependence of mass transportduring ion irradiation in Ni. Akano et al. [4] studied Ar ion-mixing of Pd into fine-grained(~100 ,/_ diameter) Ni. Muller et al. [11] examined the broadening of an isotopic marker insingle-crystal nickel resulting from 300 keV Ni irradiation. Both experiments employed nearlythe same damage rate (~10 -2 dpa s-l). The Arrenhius behavior of the ion irradiation diffusion ..,.,coefficient obtained form these experiments is shown in Fig. 3. Both sets of data exhibit twotemperature regimes of mixing with each having about the same magnitude of mixing in thetemperature-independent regime. However, in the fine-grained films, the enhancement indiffusivity occurs at a noticeably lower temperature than in the single-crystal Ni. Substantialgrain growth from 100-600/_ was also observed. Although Akano et al. [4] attribute thisenhancement to grain boundary diffusion it appears that the Glaeser and Evans description(eqn.(3)) would be more appropriate for understanding the observed interdiffusion behavior inthese fine-grained films. In the single-crystal Ni, an enhancement of diffusion is not observeduntil substantially higher temperatures, at which point conventional radiation-enhanced diffusionbegins." _ I I I I .,-,, 10"14 D Single XL Ni [ -'-:. Q I[] Q • Small Grained Ni-Pd10 15emil7,.. _ •tr==[] []10 "16- • -Oi i I I1.0 2.0 3.01/T (xl0 "3 K"])Fig. 3. Comparison of measured diffusion coefficients resulting from irradiation in single-crystal Ni [11] and fine-grained Ni-Pd [4].CONCLUSIONSExperiments have demonstrated that DIGM occurs during ion-mixing of polycrystaUineAu/Cu bilayers. DIGM, and more generally irradiation-induced grain growth coupled withboundary diffusion, can substantially enhance interdiffusion during irradiation. Thesemicrostructural phenomena in polycrystalline films may provide a key to understanding ion-mixing in the thermally-assisted regime.REFERENCES -.,.[1] L.E. Rehn and P.R. Okamoto, Nucl. Instrum. Methods B39, 104-113 (1989).[2] Y. -T Cheng, Phys. Rev. B 40, 7403-7405 (1989).[3] F. Rossi and M. Nastasi, J. Appl. Phys. 69, 1310-1319 (1991).[4] U.G. Akano, D.A. Thompson, W.W. Smeltzer and J.A. Davies, J. Mater. Res. 3, 1063-1071 (1988).[5] C.A. Handwerker in Diffusion Phenomena in Thin Films and Microelectronic Materialsedited by D. Gupta and P.S. Ho (Noyes Publications, Park Ridge, NJ, 1988).[6] J.D. Pan and R.W. Balluffi, Acta. Metall. 30, 861-870 (1982). "_[7] Y. Gao, D.E. Alexander and L.E. Rehn in these proceedings.[8] J.C. Liu and J.W. Mayer, Nucl. Instrum. Methods B19/20, 538-542 (1987).[9] A.M. Glaeser and J.W. Evans, Acta. Metall. 34, 1545-1552 (1986).[10] I. Kaur and W. Gust, Handbook of Grain and Interphase Boundary Diffusion Data(Ziegler Press, Stuttgart, Germany, 1989).[11] A. Muller, V. Naundorf and M.-P. Macht, J. Appl. Phys. 64, 3445-3455 (1988).

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The effect of DIGM and irradiation-induced grain growth on interdiffusion in bilayer ion-beam mixing experiments

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