Chapter 2. Microstructural Evolution in Thin Films of ... · 2 Pontificia Universidade Catolica, Department of Materials Science and Metallurgy, Rio de Janiero, Brazil. 3 Harvard-MIT

Chapter 2. Microstructural Evolution in Thin Films

Chapter 2. Microstructural Evolution in Thin Films ofElectronic Materials

Academic and Research StaffProfessor Carl V. Thompson, Dr. Paul Evans, Professor Clifton G.John Melngailis, Professor Henry I. Smith

Visiting Scientists and Research AffiliatesChenson K. Chen,' Dr. Roberto R. DeAvillez,2 David J. Edell,3Smith,5 King-N. Tu5

Fonstad, Jr., Dr. En Ma, Dr.

Harold J. Frost, 4 David A.

Graduate StudentsSergio A. Ajuria, Geoffrey Burns, Jaeshin Cho, Lawrence A. Clevenger, Andrew D. Dubner,Jerrold A. Floro, James S. Im, Eva Jiran, Harold Kahn, Yachin Liu, Hai Longworth, Joyce E.Palmer, Hui Meng Quek, Jaesang Ro

Undergraduate StudentHoward Zolla

Technical and Support StaffCelia Slattery

2.1 Modeling ofMicrostructural Evolution inThin Films

Sponsors

Joint Services Electronics ProgramContract DAAL03-89-C-0001

National Science FoundationU.S. Air Force - Office of Scientific Research

Contract AFOSR 85-0154

Project StaffProfessor Carl V. Thompson, Harold J. Frost,Jerrold A. Floro, Yachin Liu

We are developing analytic models fornormal, secondary and epitaxial grain growth

in continuous thin films as well as particlecoarsening in discontinuous films. Theeffects of surface or interface energyanistropy play especially important roles inthese processes. We have developed com-puter models for film formation by crystalnucleation and growth to impingement undera variety of conditions. The topology andgeometry of grain structures have beenshown to strongly depend on the conditionsof film formation. We have also developedcomputer simulations for two-dimensionalnormal grain growth and secondary graingrowth.

We are developing analytic models and com-puter simulations in parallel with exper-imental studies in model systems. Computer

1 MIT Lincoln Laboratory.

2 Pontificia Universidade Catolica, Department of Materials Science and Metallurgy, Rio de Janiero, Brazil.

3 Harvard-MIT Health Sciences Program.

4 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire.

5 IBM, Thomas J. Watson Research Center.


simulations have allowed us to explainmicrostructrual features which are generallycharacteristic of thin films, including: (1)stagnation of normal grain growth, (2)development of lognormal distributions ofgrain sizes and (3) abnormal grain growthwhich leads texture development or epitaxy.Grain growth models also provide a theore-tical context for research on microstructureengineering using impurities, ionbombardment, substrate-surface-topographyand precipitates.

Publications

Thompson, C.V. "Coarsening of Particles ona Planar Substrate: Interface EnergyAnistropy and Application to GrainGrowth in Thin Films." Acta Metall.36:2929 (1988).

Thompson, C.V. "Observations of GrainGrowth in Thin Films." In MicrostructuralScience for Thin Film Metallizations inElectronic Applications, 115. Eds. J.Sanchez, D.A. Smith, and N. Delanerolle.Warrendale, Pennsylvania: the Minerals,Metals, and Materials Society, 1988.

2.2 Reliability andMicrostructures ofInterconnects

Sponsors

Joint Services Electronics ProgramContract DAAL03-89-C-0001

Semiconductor Research Corporation

Project Staff

Jaeshin Cho, Harold Kahn, Hai Longworth, Pro-fessor Carl V. Thompson

We are investigating the use of precipitatesto produce metallic films and interconnectswith engineered microstructures. We depositinitially layered alloy films (e.g., films depos-ited in a sandwich structure with a pure Wlayer between pure Al layers). When heated,precipitates form in the center of these films.These precipitates lead to a drag force whichimpedes grain boundary motion and

supresses normal grain growth. Eventually,at elevated temperatures, a minor fraction ofthe grains begin to break free of the precipi-tates and grow abnormally. This can ulti-mately lead to very large grains and alsochanges in the crystallographic texture offilms. With this technique, it is possible tocontrol the grain sizes, the distribution ofgrain sizes, and the distribution of grain ori-entations over very broad ranges. We havenow observed these effects in a number ofaluminum alloy systems including AI-Cu-Cr,Al-Ag-Cr, Al-Mn-Cr, AI-Mn, and AI-W.

We are also developing new techniqueswhich allow statistical characterization offailure of contacts and interconnects forsilicon-based integrated cricuit technology.We are using these techniques to correlatefailure rates and mechanisms with micro-structures of interconnect lines and contactdiffusion barriers. We are investigating tech-niques for controlling microstructures inorder to improve contact and interconnectreliability, especially under conditions whichcan lead to electromigration.

We have recently shown that interconnectlines with bimodally distributed grain sizeshave drastically reduced reliabilities. Also,for lines with monomodally distributed(uniform) grain sizes, increasing the grainsize (relative to line width) results in anincrease of both the median time toelectromigration-induced failure and thelognormal standard deviation in the time tofailure, see figures 1-3. The net result, inlarge populations of lines with monomodalgrain size distributions, is little or no changein the time to the first failure. We haveexplained these results in terms of a "failureunit model" in which grain boundaries aretaken to be the individual units which areresponsible for the reliability of a line. Thesuccessful application of this model indicatesthe importance of the properties of individualgrain boundaries in controlling interconnectreliability. We are now investigating the sta-tistical reliability of lines with single, con-trolled grain boundaries in order tounderstand in detail the type of microstruc-tural features which limit interconnect reli-ability.

22 RLE Progress Report Number 132


1000

(d) bamboo strutureannealed and patterned

e (c) large groin structurex_ ann 20 min

LL. (a) an-deposded5

(b) bimodal groin size distributionannealed 30 min at 450"C

Cumulative Failure (s)

Figure 1. Distribution of failure times for2.2-pm-wide, AI-2%Cu-0.3%Cr lines: (a) as-deposited(MTTF = 9.8 h, DTTF = 0.197), (b) after annealing for30 min. at 450'C to develop a bimodal grain size dis-tribution (MTTF = 1.08 h, DTTF = 0.223), (c) afterannealing for 20 min. at 550*C to obtain mono-modallydistributed large grains (MTTF = 26.3 h, DTTF =0.465), and (d) after annealing for 20 min at 550*Cbefore patterning, to obtain bamboo microstructures(MTTF = 35.0 h, DTTF = 0.580).

(a) { >

(b)

(d)

Figure 2. Sketches of the microstructures of the linesdescribed in Figure 1, based on transmission electronmicroscopy.

Figure 3. Median time to failure and deviation in thetime to failure vs. linewidth/grain size ratio forAI-2%Cu-0.3%Cr lines with monomodally distributedgrain sizes.

Publication

Cho, J., and C.V. Thompson. "The Grain SizeDependence of Electromigration InducedFailures in Narrow Interconnects." Appl.Phys. Lett. 54:2577 (1989).

2.3 Epitaxial Grain GrowthSponsors

AT & TU.S. Air Force - Office of Scientific Research

Contract AFOSR 85-0154

Project Staff

Jerrold A. Floro, Professor Carl V. Thompson, Pro-fessor Henry I. Smith

In the past year, we demonstrated that graingrowth in polycrystalline films on singlecrystal substrates can lead to epitaxial films.This new approach to obtainingheteroepitaxial films can lead to ultrathinfilms with reduced defect densities comparedto films deposited using conventional tech-niques. In epitaxial grain growth, ultrathinpolycrystalline films are deposited on singlecrystal substrates. When these polycry-stalline films are heated to elevated temper-atures, epitaxial grains with low


film-substrate interface energies grow andconsume misoriented grains. Because theinitial polycrystalline films are deposited atlow temperatures, fully continuous ultrathinfilms can be obtained. Conventional Volmer-Weber epitaxy which is carried out at higherdeposition temperatures can not be used toobtain equivalently thin epitaxial films. Wehave developed kinetic analyses for epitaxialgrain growth and are testing these analysesthrough experiments on model systems,including Au and Ag films on mica and NaCI.The ultimate goal of our research is todevelop a general understanding of whenand how epitaxial grain growth can be usedas a route to low-defect-density ultra thinheteroepitaxial films in film/substrate systemswith large lattice mismatches.

2.4 Heteroepitaxy in LatticeMismatched Systems

Sponsor

U.S. Air Force - Office of Scientific ResearchContract AFOSR 85-0154

Project Staff

Joyce E. Palmer, Jerrold A. Floro, Geoffrey Burns,Professor Carl V. Thompson, Professor Clifton G.Fonstad

Heteroepitaxial growth of films with poorlattice matching with single crystal substratesoften leads to films with high bulk as well asinterface defect densities. When atom byatom or layer growth occurs, bulk defects aregenerally generated during strainaccomodation well after film nucleation andthe early stages of film growth. Alternatively,strain accomodation can occur through for-mation of low energy interfaces during com-petitive growth of grains or nuclei whichinitially have a variety of orientations. Weare investigating these post-nucleationepitaxial processes in continuous and discon-tinuous films. Model systems includeGaAs-on-silicon and epitaxial metals onalkali halide crystals.

2.5 Kinetics of Thin FilmSilicide Formation

Sponsor

International Business Machines Corporation

Project Staff

Lawrence A. Clevenger, Dr. En Ma, Dr. Roberto R.DeAvillez, Professor Carl V. Thompson, King-N. Tu

There is considerable current interest in theuse of refractory metals or refractory metalsilicides as interconnects, as gate materials inMOS devices and for low contact resistancediffusion barriers at metal-silicon contacts inintegrated circuits. One method of silicideformation is through reaction of metallic thinfilms with silicon substrates or polycrystallinesilicon films. This application raises funda-mental questions about the rate and productsof thin film metal-silicon reactions. There arefour critical parameters in analysis and mod-eling of these reactions; interdiffusivities, freeenergy changes, surface energies and inter-face reaction constants. Of these, the firsttwo parameters are fairly well understoodand can be predicted. The purpose of thisproject is to develop a better understandingand predictive capability for the last twoparameters. Surface energies are beingdetermined through silicide precipitationexperiments and the kinetics of thin filmsreactions are being studied through thermal,TEM, and X-ray analysis of reactions inmultilayer thin films.

We have found that in Pt/amorphous-Si,Ni/o-Si, V/L-Si and Ti/a-Si multilayer films,an amorphous silicide is the first phase toform. These phases are thermodynamicallystable only if crystalline silicide formation iskinetically suppressed. We have alsodetected evidence for nucleation at the earlystages of the formation of crystalline silicides.These results suggest that phase selection inthin film reactions is governed by nucleationbarriers.

We have also observed explosive reactions inmultilayer metal/o-Si films. These reactionscan propagate in a room temperature ambientat velocities over 20 meters per second. Thisself-rapid-thermal-annealing process resultsin homogeneous films composed of thestable high temperature crystalline silicide.


2.6 Coarsening of Particles ona Planar Substrate

Sponsor

National Science Foundation

Project Staff

Yachin Liu, Professor Carl V. Thompson

Very small particles on a planar substrate canexchange material by atomic diffusion of theparticle constituent on the substrate surface.This generally leads to an increase in theaverage particle size and spacing and canalso lead to the development of restrictedcrystallographic orientations. This processcan be very important in the early stages ofthe formation of a thin film. We have devel-oped a theory to describe the evolution ofparticle sizes and orientations and are testingthis theory by experimentally characterizingparticle coarsening in model systems.

2.7 Thin Film Zone MeltingRecrystallization of Silicon

Sponsor

International Business Machines Corporation

Project Staff

Paul Evans, James S. Im, Chenson K. Chen, Pro-fessor Carl V. Thompson

Techniques for producing device-qualitysingle-crystal films of semiconductors oninsulator (SOI) are of interest for multilayeror multimaterial integrated circuits, displaydevices and low-cost, high-efficiency solarcells. Such films can be obtained throughdirectional solidification of confined thinfilms (zone melting recrystallization, ZMR).While there are analogies to bulk crystalgrowth, in ZMR there are also phenomenaand mechanisms unique to thin-filmsolidification of radiatively heated silicon.Direct observation of dynamic and staticliquid-solid interfaces complements theore-tical modeling of solidification. We arestudying these phenomena in order todevelop means of controlling and optimizingthin film growth by ZMR.


We are also using ZMR to prepare thin filmbicrystals in order to study the electronicproperties of grain boundaries in silicon. Wewill correlate electronic properties with struc-tural features, as revealed using high resol-ution electron microscopy.

2.8 Capillary Instabilities inThin Solid Films

Project StaffEva Jiran, Professor Carl V. Thompson

Very thin metallic and semiconductor films(<200A) are being used in an increasingvariety of applications. Most solid films areused on substrates with which they would, inequilibrium, form non-zero contact angles.Therefore, even solid films tend to becomediscontinuous or bead in order to reducetheir total film/substrate interface energy.This phenomena occurs in both continuousand patterned films. The rate of solid statebeading is a strong function of the dimen-sions of a film or line as well as the micro-structure of the film or line. For example, thebeading rate rapidly increases withdecreasing film thickness. We are exper-imentally characterizing the kinetics ofbeading of thin films of gold on SiO 2 . Filmpatterning allows independent study of bothhole formation and hole growth.

2.9 Focused Ion Beam InducedDeposition

SponsorInternational Business Machines Corporation

Project Staff

Jaesang Ro, Andrew D. Dubner,Melngailis, Professor Carl V. Thompson

Dr. John

It is now possible to produce ion beams withdiameters as small as 500 A. This permitsuse of focused ion beams for high spatialresolution implantation, sputtering and depo-sition. In principal, the latter can be used inintegrated circuit mask repair or high resol-ution direct writing of interconnects. We areinvestigating the mechanisms of ion-beam-


induced chemical vapor deposition frommetal-bearing gases.

2.10 Protective Coatings forIntegrated Circuits in an InVitro Environment

Sponsor

National Institutes of Health

Project Staff

David J. Edell, Professor Carl V. Thompson

We are investigating the use of variouscoating materials to prevent Na diffusion intointegrated circuits to be used in biomedicalapplications. We are correlating processingconditions, microstructural characteristicsand diffusion barrier properties to developstandard methodologies for deposition andcharacterization of protective coatings.

2.11 Publications

Ajuria, S.A. Photon Enhanced Grain Growth.S.M. thesis. Dept. of Mater. Sci. and Eng.,MIT, 1989.

Atwater, H.A., and C.V. Thompson. "TheRole of Point Defects in Ion-Bombardment-Enhanced and Dopant-Enhanced Grain Growth in Silicon ThinFilms." J. Nucl. Instrum. Meth. Phys. Res.B 39:64 (1989).

Blauner, P.G., J.S. Ro, Y. Butt, C.V.Thompson, and J. Melngailis. "DirectWriting of Metallic Submicron FeaturesUsing Focused Ion Beams." MaterialsResearch Society Symposium Proceedings129: 483 (1989).

Clevenger, L.A., C.V. Thompson, and K.-N.Tu. "Controlled and Explosive Reactionsin Nickel/Silicon Multilayer Thin Films."In Materials Research Society Interna-tional Meeting on Advance Materials10:431 (1989).

Clevenger, L.A., C.V. Thompson, and R.R. deAvillez. "Kinetics and Thermodynamics ofAmorphous Silicide Formation in Nickel/-

Amorphous-Silicon Multilayer ThinFilms." Proceedings of the Spring 1989MRS Meeting, Symposium D, San Diego,California. Forthcoming.

Clevenger, L.A., C.V. Thompson, A.J. Judas,and J.L. Olson. "Metastable Reactionsand Explosive Silicidation in Nickel/-Amorphous-Silicon Multilayer ThinFilms." In Selected Topics in ElectronicMaterials, 23. Ed. B.R. Appleton et al.Materials Research Society, 1989.

Clevenger, L.A. Controlled and ExplosiveSilicidation of Metal/Amorphous-SiliconMultilayer Thin Films. Ph.D. diss. Dept. ofMater. Sci. and Eng. MIT, 1989.

Coffey, K.R., L.A. Clevenger, K. Barmak, D.A.Rudman, and C.V. Thompson. "Exper-imental Evidence for Nucleation DuringThin Film Reactions." Appl. Phys. Lett.55:852 (1989).

DeAvillez, R.R., L.A. Clevenger, and C.V.Thompson. "Relaxation Phenomena inEvaporated Amorphous Silicon Films." J.Mater. Res. 4:1057 (1989).

Im, J.S. Experimental and Theoretical Inves-tigation of Interface MorphologiesObserved in Directional Solidification ofThin Si Films. Ph.D. diss. Dept. of Mater.Sci., and Eng., MIT, 1989.

Palmer, J.E., G. Burns, C.G. Fonstad, andC.V. Thompson. "The Effect of As4 Over-pressure on Initial Growth of GalliumArsenide on Silicon by MBE." Appl. Phys.Lett. 55:990 (1989).

Palmer, J.E. Evolution of Microstructure inUltra-Thin Films of GaAs and CaF 2 onSingle Crystal Silicon. Ph.D. diss. Dept.of Electr. Eng. and Comput. Sci., MIT,1989.

Phillips, J.M., J.E. Palmer, N.E. Hecker andC.V. Thompson. "The Effect of Annealingon the Structure of Epitaxial CaF 2 Filmson Si(100)." Proceedings of the Spring1989 Materials Research Society Meeting,Symposium D, San Diego, California,April 1989. Forthcoming.


Quek, H. M. An Investigation ofGraphoepitaxy in Thin Au and Bi Films.S.M. thesis. Dept. of Mater. Sci. and Eng.,MIT, 1989.


Spaepen, F., and C.V. Thompson. "Calorime-tric Studies of Reactions in Thin Films andMultilayers." Appl. Surf. Sci. 38:1 (1989).

Professor Carl V. Thompson checks a zone meter that was constructed in his laboratory.

Principal Research Scientist Dr. John Melngailis uses focused ion beams for patterned deposition from

adsorbed gas molecules and for patterned implantation or lithography.


Documents

Chapter 2. Microstructural Evolution in Thin Films of ... · 2 Pontificia Universidade Catolica, Department of Materials Science and Metallurgy, Rio de Janiero, Brazil. 3 Harvard-MIT