An curvature measurement technique was used to characterize the stress evolution during reaction of a Ni film and a silicon substrate to form nickel silicide. Stress changes were measured at each stage of the silicide growth. When the nickel films were subjected to long-time isothermal annealing, stresses that developed during silicide formation gradually relaxed. Fitting the experimental results with a kinetic model provides insight into the volumetric strain and relaxation behavior of the reacting film and the reaction product.Singapore-MIT Alliance (SMA

Bernstein, R.

Li, Yi

Liew, K.P.

Thompson, Carl V.

Yeadon, Mark

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CHARACTERIZATION AND MODELING OF STRESS EVOLUTION DURINGNICKEL SILICIDES FORMATIONK. P. LIEW1, Y. LI2, M. YEADON3, R. BERNSTEIN4, C. V. THOMPSON41Singapore-MIT Alliance, Advanced Materials for Micro- and Nano- Systems2 Department of Materials Science, National University of Singapore3 Institute of Materials Research and Engineering, National University of Singapore4 Department of Materials Science and Engineering, Massachusetts Institute of TechnologyABSTRACTAn curvature measurement technique wasused to characterize the stress evolution duringreaction of a Ni film and a silicon substrate toform nickel silicide.  Stress changes weremeasured at each stage of the silicide growth.When the nickel films were subjected to long-time isothermal annealing, stresses thatdeveloped during silicide formation graduallyrelaxed. Fitting the experimental results with akinetic model provides insight into thevolumetric strain and relaxation behavior of thereacting film and the reaction product. 1. INTRODUCTIONThe stresses observed during silicideformation show an overall trend of compressivestress [1]. All formation of refractory metalsilicides causes compressive intrinsic stress.  Fortransition metal silicides, some literatures [2-4]show that the formation of Pd2Si, Pt2Si, Co2Siand CoSi2 causes compressive stress, while thereare some contradicting reports [5-6] showing thatthe formation of Ni2Si, Co2Si and Pt2Si causestensile stress. The reason for this inconsistency isnot fully understood though it is commonlyattributed to the different experimentalconditions. Thus, more experiments are neededfor each material system to confirm the type ofstress generated by the reactions. Theexperimental system used in this study is thebinary Ni and Si system. Koos et. al. [5]conducted the earliest experiment on the silicidesof evaporated nickel films. Tensile stress wasobserved by Koos et. al. during the formation ofNi2Si. This was opposite to the compressivestress observed by Tsai et. al. [9] during theformation of Ni2Si.A model for the stress evolution during theformation of silicides was also developed. Zhangand d’Heurle [7] proposed a model that took intoaccount the simultaneous stress formation due tothe reaction and the relaxation of stresses. Themodel also included an important parameter,which was the stress induced by the reaction atthe instant of reaction, which would help inunderstanding the origin of stress generatedduring the reaction. The model provided asatisfying qualitative picture of stress evolutionduring the formation of Pd2Si and TiSi2. Nowork was done to compare the stress changesduring nickel silicides formation and the modelbefore. Buad et. al. [8] tested his experimentaldata for Pd/Si system with the model. However,the intrinsic stress at the instant of silicideformation was not reported in his paper.In the present work, stress development andrelaxation during the formation nickel silicideswas studied using the curvature measurementtechnique. The experimental data was fitted withthe Zhang and d’Heurle’s model. 2. EXPERIMENTSThe nickel film was deposited on HFcleaned Si (100) substrate using magnetronsputtering at the vacuum level of 5x10-7 torr.Sputtered sample was then annealed in-situ inFSM thin film stress gauge in which curvaturechanges during reaction was measured. Theannealing ambient was forming gas consisting of96% N2 and 4% H2. The force per unit width(F/W) in the film could be calculated from themeasured curvature changes using the Stoneyequation [10]:( )ossff KKtEtWF −−==2)1(6 νσwhere σf is the stress in the film, tf is the filmthickness, F is the force in the film, W is thewidth of the sample, Es is the Young’s modulusof the substrate, νs is the poisson ratio of thesubstrate, ts is the substrate thickness,  K is themeasured curvature, and Ko is the initialcurvature.The first experiment involved annealing thesample at a constant heating rate of 5oC/minfrom room temperature to 600oC. In the second-40-2002040600 200 400 600temp (oC)F/W (N/m)1st heating2nd heatingexperiment, the sample was annealedisothermally at temperature ranging from 230oCto 250oC for 4 hours. After the stressmeasurement, the samples were analyzed usingXRD, SEM and TEM to identify phases andstructural changes in the film. Figure 1 Plot of the total force per unit widthverse temperature for 30 nm Ni/Si annealed at5oC/min to 600oC3. RESULTS AND DISCUSSIONS(A) Characterization of total force per unitwidth changes during silicidation processFig. 1 shows the changes in F/W in nickelfilm on Si as a function of temperature duringthe heating cycle with a constant ramping rate.The curve for the first heating cycle shows amaxima at 210oC and two minimas at 317oC and388oC respectively. The results agree with theresult Tsai et. al. reported [9]. The two minimaswere proven to be related to the formation ofNi2Si and NiSi at 317oC and 388oC respectively.Tsai et. al. [9] also reported that no nickelsilicide was formed after the film was heated to234oC. Thus the first maxima can only beattributed to morphological changes in nickelfilm. From the literature, Knauth et. al. [11]reported that nickel film experienced graingrowth after being annealed at the temperaturerange between 150-300oC. In the present work,four point probe measurement showed a drop insheet resistance of the film from 11.6 to 9mohmcm after being annealed from roomtemperature to 200oC. It is known that the graingrowth in thin film will give a drop in the sheetresistance of the film [12]. Therefore, the initialincrement of tensile stress should be due to graingrowth in nickel film.  During the second heating cycle, the forcesin the film changed linearly with increasingtemperature. The features observed in the curvefor first heating cycle were absent, indicatingthat the film had fully transformed into NiSi, asshown by Tsai et. al. To understand the origin of the forcechanges in the film, we need to look at thevolume changes caused by the reaction, whichthe literature [1] had extensively discussed. InTable 1, the volume changes during theformation of Ni2Si are listed. Two sets ofnumbers are presented in this table. The first ofthese is the fraction of the final volume to theinitial volume of the reactants (nickel plussilicon), which shows a considerable reduction.The second number shows the relative volumeexpansion undergone by silicon upon alloyingwith nickel to form Ni2Si. This volume change isquite huge and can be used to explain thecompressive stress caused by Ni2Si formation. The analysis for the formation of NiSi ismore complicated as two reaction interfaces areinvolved. Firstly, one Ni is released from Ni2Si,which causes 35% volume decrease in the Ni2Silayer. Following that, the released Ni atomwould diffuse into Si to form NiSi, which causes11% volume increment at the NiSi/Si interface.The net result of these two steps would lead to anincrease in free volume in the Ni2Si and in turnshould generate a tensile stress. However, this isin contradiction with what is shown in Fig. 1.This result shows that the current understandingon the volumetric changes that causes stresses atreaction interface is not sufficient in explainingreaction that involves multi-interface. Relaxationprocess at different interface may need to betaken into account. Table 1 Volume changes during the formation ofNi2SiMolar volume ofreactantsVolume fraction ofreaction product toreactantsNi Si Ni2Si/ (Ni+Si)Ni2Si/(Si)13.18 12 0.78 1.62                                                                                                                                             -40-20020400 50 100 150 200time (min)F/W (N/m)230 250         240Figure 2 Plot of total force per unit width versetime for 30 nm Ni/Si annealed at 230-250oC for240 minFigure 3 XRD spectrum of nickel film annealedat 250oC for 240 minFig. 2 shows the force changes during theformation of Ni2Si while 30 nm sputtered nickelfilm on Si (100) substrate was heatedisothermally at 230oC, 240oC and 250oC.Compressive stress in the film increased at thebeginning of the isothermal hold. After 20-50minutes of annealing, the value of thecompressive stress relaxed.Fig. 3 shows the XRD spectrum for the filmafter annealing for 240 minutes at 230oC. Thespectrum showed that nickel and Ni2Si were theonly two phases in the film after the annealing.The morphology of the film after the annealingwas investigated using SEM and TEM. Fig. 4shows SEM micrograph of the sample annealedat 230oC. Discontinuities appeared on the filmsafter extensive annealing. The cross section ofthe sample observed using TEM (as shown inFig. 5) showed that two phases coexisted afterthe nickel film was annealed at 230oC for240minutes. The upper layer was un-reactednickel while the bottom layer was Ni2Si. The un-reacted nickel film became discontinuous afterbeing annealed for 240 minutes. Thediscontinuities could be a result of agglomerationof the un-reacted nickel film. The nickel filmcould agglomerate, as it became thinner whilereaction proceeded. As the nickel film becamediscontinuous, the stress in the nickel film couldbe negligible. The forces change observed inFig.1 could be predominated by the forcesinduced by the formation of Ni2Si. Figure 4 SEM image for 30 nm nickel filmannealed at 230oC for 240 minFigure 5 TEM micrograph for nickel filmannealed at 230oC for 240 minSiNi2SiNi1.E+031.E+041.E+0540 42 44 46 48 502 thetaintensity (a. u.)Ni (111)Ni2Si (013) Ni2Si (020)1 µm0 50 100 150 200 250-30-20-100102030 F (exp) Fs Fm Fm+FsF/W (N/m)time (min)(B) Modeling the stress evolution during thereactionResult shown in Fig. 2 was fitted withZhang and d’Heurle’s model. Major assumptionsin the model are as follow:(1) stress builds up through layer silicidegrowth,(2) stress relaxes in growing silicide layerthrough Coble creepIn this model, two forces are considered. Theyare forces due to compound formation, Fs andforces in the un-reacted film film, Fm. The totalforces measured during silicidation, F is the sumof these two. These two forces are brieflyexplained as follow: (i) Forces due to compound formation, Fs:The silicide is divided into n sublayers ofthickness ∆l, each with an instantaneous stressσis. σis is equal to the intrinsic stress induced bythe reaction, σin subtracted by the relaxed stress,σrel. The overall force in the growing layer, Fs atany time t is the sum of forces over all layers, asgiven by the following expression. By assumingcoble creep, and diffusion controlled growth, Fsis given by the following expression:−= ∫ ττσ tdlkltFtlins exp''exp)()(02    (1)where k is the parabolic constant and τ is therelaxation time for the silicide.(ii) Forces in the un-reacted film, Fm:As the reaction proceeds, forces in theremaining un-reacted film will decrease not onlyas a function of its specific relaxation but alsothe decreasing film thickness. At zero time, whenthe reaction just begins, the metal film is under auniform stress, σm. The force in the film will beequal to σmβlo. Here β is the ratio of the metalthickness to the corresponding silicide thickness,βlo is the initial thickness of the metal film. Atthe temperature of reaction the stress in the metalfilm relaxes with a relaxation time τm. The forcein the film decreases as a function of the filmconsumption rate as well. Combining these twoeffects, Fm is given by[ ]2/1)(exp ktltF ommm −−= βτσ (2)  Finally, the total force developed during thesilicidation process is given by this expression:ms FFF += (3)Figure 6 shows the fitting curves for nickelfilm annealed at 250oC. It illustrates how theforce in the unreacted metal films, Fm and theforce in the silicide Fs evolve during theannealing cycle. Data obtained from the fitting of theexperimental result with the model is shown inTable 2.  Relaxation time, τ of Ni2Si is found tofollow the Arrhenius behaviour. Activationenergy calculated from this data is 1.81±0.29 eV(shown in Figure 7). This is close to theactivation energy for Ni diffusion through Ni2Si(1.71eV) [13]. Hence Ni diffusion could be thelimiting phenomenon controlling the relaxationof this compound. This verifies the assumptionof Coble creep in this model.Figure 6 Force changes (experimental andtheoretical) in nickel film (30 nm) annealed at250oC for 240 min.Table 2 Curve fitting result for 30 nm nickel filmannealed at different temperaturesTemperature(oC)σin(GPa)τ(min)230 -2.86 79240 -1.5 28250 -1.65 16From Table 2, we also find the compressivestress caused by the reaction, σin to be 2.1±0.54GPa. The stress changes in the plane of the filmis related to the volumetric strain in the film bythis equation:3)1(∆−−=∆ γσEwhere ∆ is the volumetric strain, E/(1-γ) is thebiaxial modulus of Ni2Si. Taking E of Ni2Si fromthe literature as 161 GPa and assuming that γ is0.33, when the stress is 2.1GPa, the volumetricstrain is 1.29%. This value is much smaller than62% as shown in Table 1. Such a largedifference shows that the current way ofestimating the volumetric strain caused byreaction is not correct, or it is also possible thatthere is a huge stress generated before it can bemeasured by the current experimental method. Figure 7 Arrhenius plot of relaxation time, τ ofNi2Si 4. CONCLUSIONSFormation of Ni2Si causes a compressiveintrinsic stress. This agrees with the expectationof a volumetric expansion at the Ni2Si/Siinterface. Forces caused by the formation of NiSiare difficult to understand due to the presence oftwo reaction interfaces. The force changes duringthe formation of Ni2Si were well fit using amodel proposed by Zhang and d’Heurle. Therelaxation data obtained from the fitting dataindicates an activation energy close to theactivation for the diffusion of nickel throughNi2Si. Thus, Coble creep is a plausiblemechanism controlling the relaxation of stressesdeveloped during the formation of Ni2Si. Thevolumetric strain caused by the reactioncalculated from the fitting result is very lowcompared to the molar volume change during thereaction. The reason for this is not yetunderstood.REFERENCE1. F. M. d’Heurle, O. Thomas, Defect andDiffusion Forum, v 129-130, 137-150(1996)2. S. Yanagisawa, T. Fukuyama, J.Electrochem. Soc, 127, 1150 (1980)3. J. T. Pan, I. Blech, J. Appl. Phys. 55, 2874(1984)4. V. Svilan, J. M. E. Harper, C. Cabral, L.Clevenger, Mater. Res. Soc. Symp. Proc.,356, 167 (1995)5. V. Koos, U. Beck, H. G. Neumann, Phys.Stat. Sol. (a) 37, 194 (1976)6. J. Angilello, J. E. Baglin, F. M. d’Heurle, C.S. Petersson, A. Segmüller, in Thin FilmInterfaces and interactions, edited by J. E.Baglin, J. Poate (The ElectrochemicalSociety, Pennington, NJ, 1980) p3697. S.L. Zhang, F.M. d’Heurle, Thin Solid Films213, 34-39 (1992)8. P. P. Buad, F. M. d’Heurle, S.Chevacharoenkul, E. A. Irene, J. Vac, Sci.Technol, B 11 (2), 304-310 (1993)9. C. J. Tsai, K. H. Yu, Thin Solid Films 350,91-95 (1999)10. R. W. Hoffman, in G. Hass, R. E. Thun(Eds.), Physics of Thin Films, vol 3,Academic Press, New York, 1966, p. 211.11. P. Knauth, A. Charai, P. Gas, Grain Growth,28 (3), 325-330 (1992)12. P. S. Lee, D. Mangelinck, K. L. Pey, J.Ding, J. Y. Dai, C. S. Ho, A. See,Microelectronic Engineering, 51-52, 583-594 (2000)13. P. Gas, F. M. d’Heurle, Mater. Res. Soc.Symp. Proc., 402, 39-49 (1996)22.533.544.522 22.2 22.4 22.6 22.8 23 23.21/kT (1/eV)ln τ (min)Ea=1.81 eV

Characterization and Modeling of Stress Evolution During Nickel Silicides Formation

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Characterization and Modeling of Stress Evolution During Nickel Silicides Formation

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