The interaction of Ni-Pt alloy with silicon

Thin Solid Films, 89 (1982) 381-385 METALLURGICAL AND PROTECTIVE COATINGS

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THE INTERACTION OF Ni-Pt ALLOY WITH SILICON* F. NAVA AND S. MANTOVANI Istituto di Fisica, Universitd, Via Campi 213/A, Modena (Italy) G. PIGNATEL AND G. QUEIROLO Dipartimento di Fisica, SGS, Castelletto di Settimo Milanese, Milan (Italy) G. CELOTTI Consiglio Nazionale delle Ricerche, Istituto LAMEL, Via Castagnoli 1, Bologna (Italy) (Received August 11,1981; accepted September 23, 1981)

Diffusion and segregation effects during the formation of silicides in the interaction of an Ni-Pt alloy with Si(100) and Si(111) substrates were investigated by means of 4He + megaelectronvolt Rutherford backscattering spectrometry, Auger electron spectroscopy coupled with Ar ÷ ion sputtering and glancing angle Xray diffraction as functions of the heat treatment (150-850 °C). The Ni-Pt alloy film was deposited onto silicon by codeposition using a system of electron guns. After low temperature annealing of the sample the nickel segregates and NiSi is formed at the original silicon-alloy interface. Further annealing promotes the interdiffusion of platinum into the NiSi. The barrier heights measured on n-type silicon confirm these observations. At low temperatures or after short annealing times the barrier height is 0.70 eV. Further annealing increases this value until saturatipn is reached at, for Pt45Ni55 alloy, 0.77 eV. At high temperatures the contact degrades.

1. INTRODUCTION The interactions of metals with silicon have been widely investigated both for applications in electronic devices and to obtain a better understanding of the physical mechanisms responsible for the formation of compounds under nonequilibrium conditions 1-4. When a metal film is deposited onto silicon it has been found that near-noble metals such as platinum, palladium and nickel react at low temperatures (below 300 °C) while refractory metals such as titanium, vanadium and tungsten require higher temperatures to form compounds. Moreover, it has been shown that a uniform alloy of a near-noble metal and a refractory metal deposited onto silicon reacts to form initially a silicide of the near-noble metal ~'6. The nearnoble metal is segregated from the alloy during the heat treatment. Subsequent heating to high temperatures promotes the formation of the refractory metal silicide. Phase separation occurs and no ternaries have been detected7. An alloy film of two refractory metals, namely Ti-W, deposited onto silicon forms a ternary after heat treatment at 700 °C a. No phase separation has been observed. Two films of near* Paper presented at the Fifth International Thin Films Congress, Herzlia-on-Sea, Israel, September 21-25, 1981. 0040-6090/82/0000-0000/~02.75

© Elsevier Sequoia/Printed in The Netherlands

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noble metals deposited sequentially onto silicon form a ternary after heat treatment at high temperatures 9-12. The reaction paths are determined by the order of deposition of the two metal layers. Phase separation has been observed only in alloys of one near-noble and one refractory metal. This feature has been explained by considering the great difference in the reaction temperatures of the two metals. From the metallurgical point of view it is important to determine whether phase separation also occurs with an alloy of two metals which react with silicon at about the same temperature 2. N i - P t is a suitable alloy for such an investigation. In addition to the usual elemental analysis techniques such as Auger electron spectroscopy (AES), X-ray diffraction and megaelectronvolt Rutherford backscattering spectrometry (RBS), electrical properties can be used to investigate the processes occurring at the silicon interface. Nickel and platinum have barrier heights on n-type silicon of 0.67 eV and 0.87 eV respectively; these are sufficient to allow a correlation between the barrier height and the composition at the interface of the silicide and the semiconductor. Our purpose in the work reported here was to investigate the interaction with silicon of an N i - P t alloy deposited onto Si(100) and Si(111 ) substrates. 2.

EXPERIMENTAL DETAILS

Single-crystal silicon wafers cut normal to the (100) or (111 ) axis were cleaned with organic solvents and then dipped into hydrofluoric acid just before they were loaded into the evaporation chamber. The alloy was prepared by the simultaneous deposition of the two elements from two electron guns. The silicon wafers were n type of resistivity 5-10 f~ cm. The annealing was performed in a 10- 7 Torr vacuum furnace. The analyses were performed by RBS, AES and glancing angle X-ray diffraction and by measuring Schottky barrier heights. 3.

RESULTS AND DISCUSSION

Figure 1 shows the barrier height measured in a sample with an Ni55Pt45 alloy. Figure l(a) refers to isothermal annealing at two different temperatures. In the asdeposited sample the barrier height is about 0.66 eV. After the sample has been annealed for a few minutes the barrier height reaches 0.7 eV. After further annealing for 20 min at 450 °C and for 200 min at 400 °C the barrier height starts to increase and reaches a maximum of 0.77 eV after 2000 rain. The ideality factor n is of the order of 1.02. Isochronal annealing (for 60 min) has similar effects (Fig. l(b)). In the asdeposited sample the barrier height is 0.66 eV and an increase is observed only after annealing at 350 °C. The maximum value of 0.77 eV is reached at temperatures of around 550 °C. Annealing at temperatures higher than 600 °C produces a degradation of the characteristics of the diode (n = 1.3-1.5) and a strong decrease in the apparent value of the barrier height. In Fig. l(b) we also report the results obtained with a capacitance-voltage (CV) technique. The same trend is observed. In fact, when the annealing temperature is increased the barrier height first increases, reaching a maximum at around 500 °C, and then decreases. There is a systematic difference of 50-60 meV from the values

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obtained from the current-voltage (I-V) technique. A satisfactory explanation for this has not yet been found. Moreover, these differences cannot be interpreted on the basis of the parallel diode model in which patches of NiSi (barrier height, 0.66 eV) and PtSi (barrier height, 0.87 eV) are assumed to occur. According to this model the ratio of the area covered by NiSi to that covered by PtSi is 0.05 from 1-V measurements and 0.5 from C - V measurements. In the literature there are many cases where 1-V and C - V techniques give different results 13. Figure 2 shows the RBS spectra of the sample as deposited and after annealing at 400 °C and at 700 °C for 30 min. The arrows indicate the positions of the elements i

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at the surface. The profile of the as-deposited sample (Fig. 2(a)) shows a sharp interface between the silicon and the N i - P t alloy. The composition of the alloy as measured by comparing the relative areas under the platinum and the nickel signals is NissPt45. The thickness of the metal film is 1450/~. After annealing of the sample, nickel is segregated at the silicon-alloy interface. The lower energy part of the platinum signal increases in height confirming the segregation of nickel. The c o m p o u n d at the interface, identified by the heights of the silicon signal, is NiSi. Further annealing increases the quantities of NiSi and PtSi formed. Later, at 450 °C platinum starts to diffuse into the NiSi. The reaction proceeds until annealing at 700 °C produces complete intermixing of nickel and platinum. X-ray diffraction confirms the results from RBS. At 400 °C PtSi and NiSi are detected.

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INTERACTIONOF N i - P t WITH Si

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RBS is not sufficiently sensitive to the presence of small quantities of platinum at the silicon surface. The conventional AES depth-profiling technique was used and Fig. 3 shows the results. After annealing of the sample at 400 °C for 20 min (Fig. 3(a)), the presence of NiSi at the silicon surface, as a result of segregation, is clear. After 400 min of annealing a tail of platinum is present in the NiSi layer (Fig. 3(b)). Further annealing for 5000 min increases the quantity of platinum in the NiSi (Fig. 3(c)). The barrier height increases when platinum is present in the NiSi layer. The metallurgy of the system is quite complex. NiSi forms first and nickel is segregated from the N i - P t alloy. A platinum-rich layer is left. After further annealing of the sample the reaction leads to a phase separation with NiSi near the silicon surface and PtSi above it. The barrier height is affected by the NiSi and becomes 0.7 eV. Annealing for longer periods or at higher temperatures promotes the diffusion of platinum into NiSi and an increase in barrier height to 0.77 eV. It is not clear why NiSi forms first. 4. CONCLUSION In conclusion we have shown that an alloy of N i - P t deposited onto silicon reacts by forming NiSi first. The segregation of nickel increases the concentration of platinum in the alloy. At higher temperatures or after longer annealing times platinum starts to diffuse through the NiSi towards the NiSi-Si interface. The electrical measurements substantially confirm these observations. At low temperatures or after short annealing periods the barrier height is substantially controlled by NiSi. At high temperatures platinum reaches the NiSi-Si interface and the barrier height increases until it reaches a saturation value. ACKNOWLEDGMENTS We appreciate the help and the encouragement given by Professor G. Ottaviani of the Physical Institute of Modena. We acknowledge the assistance of the IBM Central Services Facility (J. Cuomo) for sample preparation. REFERENCES 1 K.N. Tu and J. W. Mayer, in J. M. Poate, K. N. Tu and J. W. Mayer (eds.), Thin Films--Reactions and Diffusions, Silicide Formation, Wiley, New York, 1978, Chap. 10. 2 G. Ottaviani, J. Vac. Sci. Technol., 16 (5) (1979) 1112. 3 S.P. Murarka, J. Vac. Sci. Technol., 17 (4) (1980) 775. 4 G. Ottaviani and J. W. Mayer, in M. J. Howes and D. V. Morgan (eds.), Reliability and Degradation: Devices, Circuits and Systems, Wiley, New York, 198t, Chap. 2. 5 J.W. Mayer, S. S. Lau, M.-A. Nicolet and R. S. Nowicki, J. Electrochem. Soc., 123 (I) (1976) 120. 6 J.O. Olowolafe, K . N . TuandJ. Ang~lello, J. AppLPhys.,50(lO)(1979)6316. 7 G. Ottaviani, K. N. Tu, J. W. Mayer and B. Y. Tsaur, Appl. Phys. Lett., 36 (4) (1980) 331. 8 J.M. Harris, S. S. Lau, M.-A. Nicolet and R. S. Nowicki, J. Electrochem. Soc., 123 (1) (1976) 120. 9 T.G. Finstad, Thin Solid Films, 51 (1978)411. l0 T.G. Finstad and M.-A. Nicolet, J. Appl. Phys., 50 (1) (1979) 303. 11 S. Thomas and L. E. Terry, J. Appl. Phys., 47(1) (1976) 302. 12 L.E. Terry and J. Saltich, Appl. Phys. Lett., 28 (4) (1976) 229. 13 I. Ohdomari and K. N. Tu, J. Appl. Phys., 51 (7) (1980) 3735.