AD-A245 929

RL-TR-91-312Final Technical ReportNovember 1991

AN XPS STUDY OF THE FORMATION OFTHIN Pt AND Ir SILICIDE OVERLAYERS ONSi (100) 2xl SURFACES

University ot Cardiff

S.J. Morgan and R.H. Williams DTIEELECTEFB

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a3 ABSTRATad- 2WThe formation of thin (5-1009) Platinum and Iridium silicide films on the clean Si(100) 2xl surface has been studied using XPS. A detailed analysis of the Si 2p andPt/lr 4f core-level photoemission lineshapes and intensities has allowed thedetermination of the phase formation sequence, and the resulting change in Schottkybarrier height, with annealing. Progressively silicon rich silicides were formedwith increasing temperature, at temperatures greater than 6000C. This process waslimited by rapid surface segregation of silicon atoms from the substrate. Small.changes in relative Schottky barrier height were observed for different silicidephases, with the monosilicide phases giving the lowest Schottky barriers.

14. SUBJECT TERMS 1 NUMBER OF PAGES24

Platinum and Iridium sllicide films, schottky barriers i& PRICE CODE

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1 Introduction

To understand quantitatively the electrical properties of transition metal(TM) silicide and TM / silicon interfaces it is necessary to gain insightinto the chemical, electronic and structural properties of the interfaces.Silicide contacts formed using platinum and iridium are known to givethe highest Schottky barrier heights to n-type silicon, this property makessuch contacts useful in microelectronic and infra-red imaging technologies(1,2] and interesting also from a fundamental scientific viewpoint [3J.

Core-level photoemission spectroscopy, using X-ray and synchrotronradiation, is an established technique for monitoring Schottky barrier for-mation and chemical reactions [4]. An X-ray photoelectron spectroscopysystem, capable of resolving the Fermi level shifts (related to band bend-ing) from chemical core-level shifts (related to chemical reaction) of theSi 2p core-level, has been used to study platinum and iridium silicidephase formation on clean Si(100)2xl surfaces. This approach allowschanges in Schottky barrier height and silicide phase to be measuredconcurrently using one analytic technique.

Previous work on the formation of platinum ind iridium silicide thinfilms have shown that a range of silicide stoichiometries are possible [5-91.In the first instance, platinum shows metal rich Pt 2Si formation at lowtemperature followed by PtSi formation at higher temperature (400°C)[5,61. Iridium si!icide follows a similar reaction path with IrSi forming atroom temperature. For thin iridium films on silicon annealed at 300"Csome dispute exists about whether IrSil.6 or IrSij.75 is produced [7,81(IrSim.7s is reported to be a semiconducting compound (9J). At higherannealing temperatures, -1000°C, IrSi3 is reported to form [7]. TheSchottky barrier heights of TM sijicide / silicon systems change withchanging silicide stoichiometry [3]. The aim of' this work is to correlatechanges in chemical phabe of platinum and iridium silicides with changesin Schottky barrier height on clean Si(100)2xl surfaces. (Note that IrSi1 .5 ,IrSil 6, and IrSij.75 are equivalent to Ir.2Si 3 , Ir5Sis, and Ir3Si5 respectively).

Chemical core-level shifts have proved useful in establishing Schottkybarrier models ba,'I. on in interface bonding approach [10] the resultsof the data analysis are also discussed in this context.

The layout of the paper is as follows, Section 2 describes'the ex1f-ri-mental set-up used, the results are presented in Section 3 and discussedin relation to previous work, concluding remarks are made in Section 4.

2 Experimental

The experiments were conducted in an oil diffusion pumped Vacuum Gen-erators MarklI Escalab. The UHV system was comprised of three cham-bers, an analysis chamber, a preparation chamber with a UHV load-lock,and an evaporation chamber. Samples, evaporation sources, and quartz

crystal film thickness monitors were mounted on portable stubs that couldbe introduced into the system via the rapid entry load-lock. All thechambers had a base pressure of < 2 x 10- 1 nib for the experiments.The surface science instruments utilised in the analysis chamber were aMg Ka X-ray source, and a three channel hemispherical analyser witha resolution of 2% of the pass energy. The total instrumental resolutionusing a 5eV analyser pass energy was 0.7eV, The preparation chamberhoused a rear-view LEED system for monitoring surface reconstructions,and a railway track for transferring the samples, evaporation sources, andfilm thickness monitors. The evaporation chamber contained a stage formounting the evaporation sources, a manipulator for holding the sampleholders and quartz crystal film thickness monitor.,

The Si(100) samples were cleaned by annealing at 850°C for 20 rnins,samples thus treated produced sharp two domain 2x1 LEED patternsand broad XPS scans showed no trace of oxygen or carbon contamina-tion. The low annealing temperature ensured that problems with shallowsurface doping by contaminants did not occur [11].

Platinum evaporation sources were constructed using a tungsten fila-ment loaded with 99.9'o pure platinum wire. After thorough outgassingslow evaporztions were performed at pressures no higher than 1 x 10-mb.Iridium depositions were more difficult, 99.9% pure iridium wire (0.25mmdia.) filaments were used that gave low evaporation rates by sublimationwhen resistively heated. This method tended to have somnwhat higheroutgassing rates than platinum during evaporation, the maximum pres-sures being

material used in this work the band bending extends for about a microninto the substrate. The inelastic mean free path of the Si 2p photoelec-trons excited using Mg Ka X-rays is about 25A [131. Therefore, as thephotoelectron escape depth is much smaller than the depletion width,the binding energy of the Si 2p core-level peak can be used to monitorsubstrate band bending j4j. The Fermi level of the clean p-Si(100)2xlsurface is reported to be pinned at 0.35 eV above the valence band max-imum [14]; contacts formed with the high work function metals platinumand iridium cause the Fermi level position to be pinned closer to thevalence band maximum-thus reducing the ban-d bending. Changes inband bending at the silicon surface caused by the presence of a platinumor iridium overlayer will cause a lowering of the Si 2p binding energy.Also, changing the chemical environment of the silicon atonts causes achange in the electronic potential surrounding such atoms, which in turngive rise to chemical C. e-level shifts. Si 2p XPS spectra measured for bulksilicides have shown that the silicide core-level cheaical shifts occur at ahigher binding energy than the bulk silicon [15], therefore, for the analysisof data utilising p-type silicon surfaces it is easy to differentiate betweenFermi level and chemical shifts. (For n-type samples, of course, the Fermilevel and chemical shifts are ii. the same direction-which makes th,.' dataanalysis more difficult.)

The intensities of the core-level photoemission peaks can be used todetermine the chemical state of a compound. The intensity of a core-level peak is dependent on the number of atoms being illuminated bythe X-ray beam, the escap, depth of the photoelectrons concerned, andthe photoemission cross-section. Given that the photoemission signalcomes solely from the com,;o':d, it is easily shown that the relativeconcentration of atoms present in a single phase of the compound is givenby ".' -

r I2pA4/X4! eqn()J4t A2pX2p

where I is the intensity of the core-level, A is the electron escapedepth of the cor.:-level photoelectrons, and X is the photoemission cross-section. Data tables for photoemission cross-sections were used in thedata analysis (161, and photoelectron escape depths were estimated us-ing the formasmi devised by Seah and Dench [131; these parameters aregiven in Table 3.1. Equation 1 can be used to determine the chemicalstate of thin silicid,.- films from XPS data, provided stoichiometric com-pound forriation hi-s occurred and given sufficient resolution to resolvechemicahi shifbcd core level components. Given these coakditions, the rvalue determined using the intensities of reacted core-level components ofa silicide, can be used to determine the phase of a re'.Lcted species using:

MSi, eqn(2)

3

In this work, the intensity of the reacted Si 2p and metal 4f core-levelpeaks were used .o determine the chemical phase of the silicides formed.

3.2 Platinum Silicide Formation

Two types of platinum / silicon experiments were performed, onie witha thick 100A platinum overlayer and the other concentrating on thin5-20A overlayers. XPS spectra were recorded for a range of anneaiin3temperatures. The curve fitted data parameters are shown in Table 5.2

After deposition of .90A of platinum onto the clean Si(100)2x1 sur-face the photoemission features associated with the silicon substrate areattenuated (see Figures 1 and 2), so no information on Fermi level pinningcan be obtained for this experiment. The Pt 4f core-level XPS spectra arerepresentative of polycrystallire platinum, no silicon, oxygen or carbonfeatures were observed indicating pure platinum evaporations. The pureplatinum 4fI emission is centred at 70.80 eV binding energy. After an-nealing at 350OC photoemission from the Si 2p core level is observed, andthe predominant Pt 4f7 emission shifts + 1.70eV to higher binding energy.However, photoemission from from unreacted platinum is still observedat 70.80eV. Also, the emission from the Si 2p core-level is displaced fromthe value expected for elemental Si 2p by + 1.05eV-clearly major atomicinterdiffusion has occurred.

The half width half maximum (HWHM) of the Pt 4fh remains un-changed after annealing-indicating that the emission centred at 72.50eVis from single phase silicide. Comparing the relative intensities of the re-acted Si 2p and Pt 4f core-levels using eqn 1 yields an r value of 1.0,indicating the formation of PtSi. At this stage of the annealing proce-dure the silicide layer is composed of PtSi together with a small quantityof unreacted platinum /

Further annealing produces no further changes until the temperaturereaches -650 0C when emission from the elemental Si 2p binding energyappears a't 98.90eV, this becomes more pronounced at 750C. Also, thePt 4f1 shows a rapid decrease in intensity. Clearly, outdiffusion of silicon2atoms onto the surface has occurred that attenuates the signal from thesilicide layer. Thus, in this experiment, any further silicide reactions areimpossible to measure using XPS.

For the thin film experiments 5-20A films were deposited onto cleanSi(100)2xl surfaces that we'te subsequently annealed to promote plat-inum / silicon reaction.

Typical Si 2p and Pt 4f core-level XPS spectra are shown in Figures 3and 4. The clean Si 2p emission shows an slightly asymmetric peak withthe maximum intensity centred at 98.90eV. (The asymmetry of the Si 2pphotoemission peak caused by the spin orbit splitting Df 0.61eV that isnot fully resolved.)

After deposition of 5A of platinum the Pt 4f spectrum shows a single

4

component centred at 70 90eV. The Si 2p spectrum shows large rhangeswith a chemically sitifted component centred at 99.40eV and a shift inthe bindir,s energy of the bulk Si 2p core-level. The Si 2p core-levelphotoemission from the silicon substrate shows a shift of 0.25 t.-V to higherbinding ener!y, indicating a Fermi level shift of -0.25eV with tespect tothe clean S; 100)2xl Fermi level pinning position. The relative intensityratio of the Si 2p and Pt 4f components show that i't 2Si formation occursfor Pt / Si(100) interface formed at room temperature.

Annealing this structure at 3500C promo tes further chemical shifts.The Pt 4f emission is now ct.ntred at 72.50eV (the same as PtSi- deter-mined in the thick film experiment) and the Si 2p spectrum exhibits anew component centred at 99.90eV. The relative intensities of the reactedcore-level components of 1.0, and the magnitude of the binding energiesof the Si 2p and Pt 4f core-levels indicate the presence of PtSi. A furth,:rdecrease in the binding eitergy of the bulk Si 2p core-level componentalso occurs, this is consistent with further Schottky barrier lowering ofthe order of - 0.05eV.

Further annealing at higher temperatures produces a large increasein the elemental ',i 2p peak intensity and a rapid decrease in the Pt 4fpeak intensity indicating that silicon outdiffusion has started at a lowertemperature than for the thick film experiment.

3.3 Iridium Silicide Formation

Owing to the difficulties with iridium deposition r -ly thin fim studieswere able to be conducted. The experimental procedure was identical tothe thin film platinum / silicon experiments. The Si 2p and Ir 4f core-lev?!spectra are shown in Figures 5 and 6, and the cuive fitting parametersare given in Table 3.3. The pressure during iridium deposition rose tolxlO-°mb which made o;xygen and carbon contamination a problem.

After a 7A iridium deposition onto the clean Si(100)2xl surface majorchanges are observed in the Si 2p core-level lineshape. A Fermi level shiftof the bulk Si 2 p component from 98.90eV to 98.70eV to lower bindingcz.ergy is observed, and a chemical core level shift at 99.20eV bindinger rgy is seen. The Ir 4f? peak is situated at 60.50eV binding energy,the i-me as the tabulated value of the Ir 4fI binding energy [17] (aftr.,2correction for the difference in analyser work functior,). However, in thisanalysis there is no bulk Ir 4f, .,- compare with to determine ,;rhether

2there is any broadening of the peak. Howe.,er, the HWHM of 0.60eVis the same to that of the pure Pt 4f, peak and remains unchangedafter annealing-inplying that the iridium atoms are situated in a singlephase system. The intensity ratio of the reacted Si 2p component andthe Ir 4fi peak indicate IrSi formation in agreement with previous studies(71, reinforcing the assertion that the chemical environment of the iridiumatoms is a single phase.

Annealing at temperatures between 300°C and 5000C produced anIr 4f 7 shift of +0.30eV to 60.80eV binding energy. A Si 2p reacted com-ponent is seen at 99.30eV in conjunction with a Fermi level shift of thebulk Si 2p of +0.05eV. The intensity ratio of the Si 2p chemical shiftand the Ir 4f I peak gives an r value of 1.6 that has an estimated uncer-2tainty in r of ±20%. This observation is in agreement with reference [7],however, given the uncertainty in r (and the contamination problem) thepresence of IrSil.75 (or a mixture of the two phases) cannot be commentedupon. The +0.05eV Fermi level shift is consistent with an increase in theformation of the Schottky barrier height after IrSil.6 formation.

Previous studies [71 have reported that IrSi 3 forms at annealing tem-peratures of -1000°C. However, in this study no further chemical shiftsof the core-levels were observed after annealing 5-2JA iridium films onthe Si(100)2xl surface at temperatures between 6000C and 10000C. Theonly change that was observed was a rapid increase in the Si 2p signal(with a corresponding decrease in the Ir 4f intensity) caused by siliconsegregation to the surface-which attenuates the photoemission featuresassociated with reactions in the silicide layer.

3.4 Discussion

The phase formation seqc.vnces of thin film platinum and iridium sili-cide on Si(100) substrates :xhibit similar trends, with both systems showincreasing silicon content with annealing.

The rosults for platinum silicide formation are largely in agreementwith other studies [5,6,151, however, a UPS / Auger study by Matz etat. inferred [18] the existence of an intermixed silicide-like phase for theformation of the platinum / silicon interface e.t room temperature, asopposed to the single phase'Pt 2Si in this work. The disparity betweenstudies of low temperature platinum / interface interface formation hasbeen nc.ed by Rossi [15] who attributed any variations in results as beingcaused by the uncontrolled kinetics of the interface formation.

The observation that thin platinum films deposited on Si(100)2xl sur-faces form PtSi implies that thick platinum films (-.100A) form a bilayerstructure, comprising of a. reacted silicide interfacial layer and an unre-acted platinum overlayer. The ultimate stoichiometry of a reacted inter-facial layer cannot be commented upon as XPS spectra cannot provideinformation on interfaces which are buried at a depth much greater thanthe photoelectron mean free paths. Unfortunately, because of the absenceof any signal from the silicon subsi rate nothing can be inferred about theFermi level pinning position at the interface.

The phase formation sequence of iridium silicides has also been thesubject of some dispute [7,81. The silicide formed at room temperatureis generally agreed to be the monosilicide IrSi-for thin films, as foundin this study. Thicker iridium films (-I00A) give a bilayer structure

6

composed of an IrSi interfacial layer and an unreacted iridium overlayer[7], which is somewhat similar to the behaviour of the room temperatureplatinum / silicon interface.

Weiss et al. [8] reported the formation of Ir 2Si and IrSi 3 for coevapo-rated iridium-silicon films on SiO 2 substrates, the SiO 2 does not supplysilicon for the silicide reaction allowing the stoichiometry of the silicide tobe controlled by the relative concentrations of silicon and iridium atomsin the coevaporated layer. However, for the clean Ir / Si(100) exper-iments performed in this work there is an unlimited supply of siliconatoms available for reaction from the substrate, which prevents metal-rich silicide formation. The formation of thin film IrSi3 was not observedbecause of rapid segregation of silicon atoms onto the surface at tempera-tures greater than -650°C, this problem was not mentioned by Wittmeret al. [7] who reported IrSi3 formation at 1000°C. It may be possibleto circumvent this problem using rapid thermal annealing to form IrSi3before a significant quantity of silicon has diffused to the surface.

The Schottky barrier trends of platinum and iridium silicide are alsosimilar with the monosilicide phases producing the largest band bendingchange. Relating the observed band bending changes to absolute Schot-tky barrier heights is problematic because of the uncertainty in the initialFermi level pinning position. However, the technique is accurate enoughto compare relative Schottky barrier heights. These results show thatthe lowest barriers to p-type (equivalent to the highest barriers on n-typematerial) Si(100)2x1 were produced by the monosilicide PtSi and IrSiphases.

4 Conclusions

XPS has been used to monitor the phase formation sequence and Schot-tky barrier formation of platinum and iridium silicides on Si(100)2xl sur-faces. The platinum phase formation sequence was found to be Pt 2 Si atroom temperature with PtSi formation occurring above 3500C. For irid-ium overlayers, IrSi was found to form at room temperature. Annealingat 300*C-6000C produced an IrSij.s compound-although the presence ofan IrSij.7 s compound could not be entirely ruled out. Attempts to pro-duce further silicon-rich silicides by high temperature annealing were un-successful because of problems with surface segregation of silicon atoms.Band bending shifts of the substrate Si 2p core-level, during silicide for-mation, show that the lowest barriers on p-type silicon (highest on n-typematerial) were produced by the monosilicide phases.

7

5 Acknowledgements

S. Morgan would like to thank Rome Laboratory for financial support.

8

6 References

1. S. P. Murarka, J. Vac. Sci. Technol., 17(1980)775.

2. F. D. Shepherd and J. M. Mooney, FElectro Optical Imaging SystemsIntegration, Proc. SPIE., 762(1987)35.

3. C. Calandra, 0. Bisi, and G. Ottaviani, S-trf. Sci. Reports.,4(1985)271.

4. R. H. Williams, C. P. Srivastava, and and 1. T. McGovern,Rep. Prog. Phys., 43(1980)1357.

5. G. Rossi, 1. Abbati, L. Braicovich, 1. Lindau, and W. E. Spicer,Phys. Rev., B25(1982)3627.

6. S. Mantovani, F. Nova, C. Nobili, M. Conti, and G. Pignatel,J. Appi. Phys. Lett., 44(1984)326.

7. M. Wittmer, P. Gelhafen, and K. N. Tu, Phys. Rev.,B35( 1987)9073.

8. B. Z. Weiss, K. N. Tn, and D. A. Smith, J. Appi. Phys.,53(1982)3342.

9. S. Petrsson, J. A. Reimer, M. H. Brodsky, D. R. Cambell,F. d'Heurle, B. Karlssonk, and P. A. Tove, J. Appi. Phys.,53(1982)3342.

10. K. Hirose, 1. Ohdarmi, and M. Uda, Phys. Rev., B37( 1988)6929.

11. R. T. Tung, K. K. Ng,J. M. Gibson, an~l A. F. J. Levi,Phys. Rev., B33(1986)7077.

12. D. Briggs and M. P. Seah, in Practical Surface Analysis,John Wiley & Sons.

13. M. P. Seah and W. A. Dench, Surface and Interface Analysis,

1(1979)2.

14. D. Haneman, Rep. Prog. Phys., 50(1987)1045.

15. G. Rossi, Susrf. Sci. Reports, 7(1981)1.

16. J. J. Ych and L. Lindau, Atomic Data and Nuclear Data Tables,31(1985)1.

17. J. C. Fuggle and N. Mirtenson, J. Elect. Spect., 21(1980)275.

18. R. Matz, R. 3.. Purtell, Y. Yokota, G. W. Rublof, and P. S. Ho,J. Vac. Sci. Technol., A2( 1984)253.

9

19. J. Bardeen, Phys. Rev., 71(1947)717.

20. V. Heine, Phys. Rev., 138(1965)1689.

21. W. E. Spicer, 1. Lindau, P. Skeath, and C. Y. Yu,J. Vac. Sci. Technol., 17(1980)1019.

22. W. M6nch, Phys. Rev. Lett., 50(1987)1260.

10

Table I Data Analysis Inelastic MEP's and Photoemission

Cross-sectons.

Core- Kinetic Inelastic Cross-level Energy mfp 1 section 2

(ev) (A) (Mb)

Si 2p 1154.7 24.5 0.02

Pt 4f 1182.8 24.8 0.40

Ir 4f 1193.1 24.9 0.35

1. Calculated using the formalism In reference [ 131.

2. From reference [16]

Table 2 Curve-fitting Results for Platinum / Silicon

React ion Core- Chemical Fermi SIl~cideTemperature Level Shift (eV) Shift (eV) phase

Room Si 2p +0.50 -0.25 Pt SiTemperature Pt 4f +1.00-2

350k Si 2p +1.00 -0.30 PtsPt 4f +1.70-

So0CSi 2p +1.00 -PtSlPt 4f +1.70 + segr-

_________egat ion

>600'C Si 2p segreg- segre-Pt 4f tion g ato n]

Pt U7- Binding Energy = 70.80eV2

pSI(100)2xl 2P3 binding Energy =98.90eV

Table 3 Curve-fitting Results Iridium / Silicon

Reaction Core- Chemical Fermi SulicideTemperature Level Shift (eV) Shift (eV) phase

Room Si 2p +0,30 -0.20 IrSiTemperature Ir 4f 0.00

350 0C Si 2p +0.40 -0.15

Ir 4f +0.30 -r~ 1.6

5o00C Si 2p +0.40 -0.15 I~jIr 4f +0.30 -1.6

>600 0 Ir 2p segrega- - segreg-

Pt 4f tion at ion

p-Si(100)2xl 2p3 Binding Energy = 98.90eV2

Ir 4f7 Binding Energy =60.50eV

21

Pt/Si(l00)2xl

750C Anneal

-W-c2 650C Anneal

L,.

"+" 500C Anneal

350C Anneal

94 96 98 100 102 104

Binding Energy (eV)

13

Pt/Si(1 OO)2x1

650C Anneal

------------ -

4 14

Pt/Si(1 00)2x 1

750C Anneal

-~~ 550C Anneal

350C Annaal

4)

RT Depositijon . % %

- --- - --- -

t

94 96 98 100 102 104

Binding Energy (eV)

15

Pt/Si(l OO)2xl

~750C Anneal

/550C Anneal

4-

*35im*Em.1..dII*.~d/350C Anneal

-.. Room Teimperature

i ... i. ...... ... .,,--- ----

65 67 69 71 73 75 77 79 81 83 85

Binding Energy (eV)

16

lr/Si(1 00)2x 1

550C Anneal

5Ca Annea0l

94 ~ ~ ~ I 969 0 0 0

Bidn Eeg

L.. - S----------------17

Ir/Si(1 OO)2x1

850C Anneal----------------

550C Anneal

Room Temperature

54 56 58 60 62 64 66 68 70 72 74 76 8

Binding Energy (eV)

18

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