Si films

Thin Sohd Films, 68 (1980) 373-379 © Elsevier Sequoia S.A., Lausanne---Printed m the Netherlands

373

THE STRUCTURE OF GOLD SILICIDE IN THIN Au/Si FILMS* H. L. GAIGHER AND N. G. VAN DER BERG

Department of Physws, Universtty of Pretoria, Pretoria (South Afrwa ) (Received September 10, 1979; accepted October 13, 1979)

Vacuum-deposited Au/Si double layers were heat treated to the eutectic temperature and were examined by transmission electron diffraction. Electron diffraction patterns ascribed to crystalline silicides were observed. Some of the patterns and associated d spacings were in satisfactory agreement with a primitive cubic unit cell with a = 6.75 A as proposed previously. Other diffraction patterns were consistent with the assumption of an orthorhombic unit cell with a = 6.8, b = 7.5 and c = 9.56 A.

1. INTRODUCTION The interaction of metal films with silicon is of considerable importance in semiconductor technology. Of particular interest is the formation of metal silicides which are widely used for Schottky barriers and ohmic contacts in devices. The characteristics ofsilicide formation for different silicon-metal systems (e.g. Si-Pt, SiPd, Si-Ni etc.) have been the subject of many investigations1. For the case of Si-Au, however, the formation and structure of possible silicide phases have not yet been unambiguously established. Under equilibrium conditions the Si-Au system is a simple eutectic with no intermediate phases. However, on rapid cooling from the liquid state, usually followed by annealing, X-ray diffraction revealed non-equilibrium phases. Five different structures have been proposed for these phases. Complex f.c.c, structures with lattice parameters 7.84 and 19.50 A respectively were found by Anantharaman et al. 2 Predecki et a l ) and Krutenat et al. 4 reported a "/-brass-type structure (b.c.c.) with lattice parameter a = 9.60 A. A g-phase with an orthorhombic structure with a = 7.82, b = 5.55 and c = 11.16 A was suggested by Andersen et al. 5 Suryanarayana and Anantharaman6 proposed a cubic ~-Mn-type structure with a = 6.75 A. For the case of thin gold films on silicon substrates the existence of gold silicide was inferred from a splitting of the 92 eV Auger electron spectroscopy peak of silicon (refs. 7-10). In a low energy electron diffraction (LEED) study of gold films evaporated onto Si(111) substrates three different gold silicide LEED patterns were observed, depending on the annealing parameters I o. The silicides corresponding to the different patterns have either a different crystal structure or a different * This work forms part of a project of the Institute of Microstructures

374

H . L . GAIGHER, N. G. VAN DER BERG

orientation. The lattice spacings calculated from such patterns (9.35, 7.35 A etc.) are different from those obtained from the X-ray diffraction studies. It is the purpose of this investigation to study the structure of possible gold silicides in thin Au/Si couples directly by transmission electron diffraction (TED). 2. EXPERIMENTAL Gold (99.99%) and silicon were consecutively vacuum deposited to thicknesses of about 400 A onto cleaved sodium chloride crystals, held at room temperature, by electron gun evaporation in a vacuum of 10- 7-10- 6 Torr. The silicon was obtained from p-type wafers of resistivity 5-10 f~ cm. Deposition rates were usually about 1 A s- i. The bilayers were floated off in water, were mounted on microscope grids and were heat treated (a) directly in the heating stage of the electron microscope or (b) in stainless steel capsules in a vacuum of 10 -5 Tort. The use of the heating stage allowed continuous observation of structural changes as heating progressed but the exact temperature at any stage was uncertain because of beam heating effects etc. A more reliable measure of the temperature at which structural changes occurred was obtained by placing the specimen in a stainless steel capsule heated by a molybdenum heater block, the temperature being monitored by a thermocouple inside the capsule. The heating rate was roughly 20 °C rain- 1 and the initial cooling rate about 55 °C min- 1. The films were examined by transmission electron microscopy (TEM) and TED in a Hitachi HU-11B electron microscope. Most d spacings were calculated from diffraction patterns that were obtained using the lower (high resolution) diffraction facility of the microscope, i.e. the specimen was placed just above the projector lens, which was switched off. Thallium chloride was used for calibration purposes. 3. RESULTS AND DISCUSSION

Electron diffraction patterns from the as-deposited films showed rings due to gold only (Fig. l(a)). The silicon was amorphous as was evidenced by the broad diffuse rings which could be seen in the electron microscope but are barely perceptible on the photographic prints. TEM micrographs showed that the gold possessed a well-crystallized grain structure with grain sizes of about 150-1400 A (Fig. l(b)). The diffraction patterns indicated that the {111} planes of the gold crystallites were preferentially parallel to the film plane. This preferred (111) orientation was deduced from the low intensity of the (111), (200) and (311) diffraction rings (Fig. l(a)) as well as the appearance of arcs, instead of complete rings, if the specimen was tilted. On heating the bilayers, growth of the gold crystallites accompanied by further development of [111] texture became evident at about 150 °C. On further heating crystallization of the silicon took place at about 180 °C, which resulted in sharp silicon rings appearing (Fig. l(c)). Note the nearly complete absence of the (111) and (200) gold rings in Fig. l(c) as a result of the preferred (111) orientation. Crystallization of the silicon often occurred with typical dendritic morphology (Fig. l(d)) accompanied by agglomeration of the gold. The relatively low crystallization temperature and the morphology of crystallization for silicon and germanium on various metal films have been previously investigated by

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Herd e t al. 1 ~ Crystallization and agglomeration continued as heating progressed. Since silicon has been found to dissolve and diffuse rapidly into thin gold films at relatively low temperatures (about 150°C) 12, the agglomerated phase was most probably gold plus silicon rather than pure gold. At temperatures approaching the eutectic temperature (about 370 °C) melting of the agglomerated phase set in, the melt spreading in liquid-like fashion across the surface of the polycrystalline silicon film. At this stage the diffraction pattern showed, in addition to sharp silib,on rings, broad diffuse tings approximately in the positions of the (111) and (220) gold rings (Fig. l(e)). On cooling abrupt crystallization occurred whereupon diffraction spots that were due to neither gold nor silicon appeared (Figs. 2 and 3). T E M micrographs (Fig. l(f)) showed the presence of a thin surface layer on the residual silicon film. The spots originated from this layer of presumably gold silicide, as is subsequently argued.

Fig. 1. (a) Electrondiffractionpattern from as-depositedAu/Sifilmsshowinggold tings only.(b) TEM micrograph of as-depositedAu/Si film.(¢) Diffractionpattern after crystallizationof silicon at about 180°C. (d) TEM mierographof Au/Sifilmsoon after the start of siliconcrystallization.(e) Broad diffuse ring due to the liqmd phase at about 370 °C. The sharp nngs arc due to silicon.(f) TEM micrograph showinga layerof gold sillcideon polycrystalhnesiliconfilm.(Magmficatlons,(b) 60000 x, (d) 19800 x, (f) 98600 x .) Careful consideration was given to the possibility that diffraction patterns such as those in Figs. 2 and 3 might be due to double diffraction resulting from thin gold single crystals epitaxially aligned on silicon single crystals. Several factors ruled out this possibility. (a) Consideration of the double diffraction to be expected from the alignment of relatively low index planes (e.g. (111) Si I[(111) Au etc.) did not yield the observed patterns. (b) An explanation in terms of double diffraction would require a primary pattern due to either gold or silicon with at least all low order reflections very strong, which was not observed. (c) As far as could be judged from the

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H . L . GAIGHER, N. G. VAN DER BERG

diffraction p a t t e r n s the r e s i d u a l silicon was n o t a single crystal b u t was polycrystalline. These diffraction p a t t e r n s are therefore a s s u m e d to be d u e to the f o r m a t i o n o f silicides.

Fig. 2. Examples of selected area diffraction patterns that are consistent with a primitive cubic lattice (a = 6.75 A). The camera constant was about 76.7 A ram. The zone axes are (a) [100], (b) [110] and (c) [111]. The rmgs in (b) are due to silicon.

Fig. 3. Examples of selected area diffraction patterns that are consistent with an orthorhombic unit cell (a = 6.8, b = 7.5 and c = 9.56 A). The camera constant was about 76.7 A ram. Faint rings in (d) are due to silicon. The zone axes are (a) [010], (b) [I10], (c) [011], (d) [1117, (e) [1]1"] and (f) ['[32]. Fig. 2. shows e x a m p l e s o f diffraction p a t t e r n s t h a t are, within e x p e r i m e n t a l error, consistent with a p r i m i t i v e cubic unit cell (possibly a [~-Mn-type structure) with a = 6.75 A, as was p r o p o s e d b y S u r y a n a r a y a n a a n d A n a n t h a r a m a n 6. Diffraction p a t t e r n s with [112] a n d [122] zone axes were also observed. S o m e o f the d spacings c a l c u l a t e d from these p a t t e r n s a r e listed in T a b l e I a n d t h e y generally c o r r e s p o n d closely with those expected for the p r i m i t i v e cubic structure o f gold

STRUCTURE OF GOLD SILICIDE IN THIN

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silicide. It is furthermore noteworthy that the (300), (221), (310) and (311) retteetions were invariably relatively strong, as would be expected for the 13-Mn-type structure 6. The [1301] patterns occurred most frequently, suggesting a preference to orient (001) parallel to the plane of the film. TABLE I LATTICE SPACINGS d CONSISTENT WITH A PRIMITIVE CUBIC UNIT CELL

(a .-- 6.75 ]k)6

hkl

d, present w o r k d for cubic unit cell 6

100

110

111

200

210

211

220

300 221

310

6.67 6.75

4.75 4.78

3 86 3.90

3 33 3.38

2.99 3.02

2.76 2.76

2.40 2 39

2.24 2.25

2.11 2.13

Many diffraction patterns (Fig. 3) and the d spacings associated were, however, not consistent with any of the structures previously proposed. It is easily verified, with reference to Table II, that these diffraction patterns are consistent with an orthorhombic unit cell with a = 6.8, b = 7.5 and c = 9.56 A. Diffraction patterns corresponding to zone axes []-31], [0~1], [121], IT20], [121], [1130] and [0." ~ were also observed, in addition to those of Fig. 3. Specific areas of the specimen were analysed by performing tilting experiments. In this way groups of diffraction patterns (e.g. Figs. 3(b), 3(c) and 3(e)) all belonging to the same specimen area were obtained, which greatly facilitated the three-dimensional reconstruction of the reciprocal lattice. There was no evidence for systematic extinctions and the diffraction patterns were consequently indexed on the basis of a primitive unit cell. T A B L E II EXAMPLES OF CALCULATED AND OBSERVED d SPACINGS AND INTERPLANAR ANGLES

¢~ FOR AN

ORTHORHOMmC UNIT CELL WITH a = 6.8, b = 7 5 AND C = 9.56 A

hkl 001 010 100 011 101 110 002 111 012 102 112 200 201 121 210 103 310

d~l(A )

dob,(A )

htkll 1

h2k212

~c,l(°)

~ob,'(°)

5.90 5.54 5.04 4 78 4.46 4.03 3.91 3.47 3.40 3.20 3.11 3.10 2.89 2.17

9.56 7 50 6.80 5.95 5.55 5.09 4 80 4.49 3.98 3.88 3.45 3.38 3.19 3.10 3.11 2.87 2.16

100 10I 10I 001 011 011 101 li0 001 111 111 20I 1]'2

111 111 210 101 112 101 110 0]'-f 111 310 20-f 1]-2 "f]'-I

49.05 74.70 42.08 54.58 35.76 69.04 52.87 58.10 62.21 36.94 62.53 43.65 73.81

49.6 75.2 42.0 54.3 36.5 70.1 53.3 57.0 62.0 35.8 62.9 43.9 74 0

" Oob, was obtained from direct angle measurements on the diffraction patterns.

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ta. L. GAIGHER, N. G. VAN DER BERG

The [010] pattern was, for zero tilt, the most frequently observed and it indicates a preference to orient with [010] perpendicular to the specimen plane. Good single-crystal patterns were usually obtained from an area at least equal to the smallest selected area used (about 2 ~tm diameter). On increasing the selected area the spots spread into arcs, indicating azimuthal rotation. The whole area depicted in Fig. l(f) is, as far as the silicide is concerned, a single crystal. The silicide layer is, however, not continuous but contains a network of fine cracks (Fig. l(f)). These cracks probably result from shrinkage of the silicide during cooling. No significant difference in the appearance of the silicide layer could be detected for the two different structures involved. Two rectangular diffraction patterns (Fig. 4) that were not compatible with any of the proposed structures were observed. The lateral periodicities corresponding to these patterns were 7.6 and 4.8 A (Fig. 4(a)) and 12.6 and 4.9 A (Fig. 4(b)) respectively. Speculation as to the origin of these patterns is not warranted at this stage because of their lack of reproducibility and the complete uncertainty about the arrangement of atoms in the unit cell.

O t

Fig. 4. Unidentified diffractaon patterns. The n n g s in (b) are due to silicon.

Several studies 3' 6 on bulk Au-Si alloys have suggested a composition for the silicide phase close to that of the eutectic, i.e. 17.9 at.% Si. For example a composition of AusSi, i.e. 17 at.% Si, was reported for the cubic 13-Mn-type structure 6. However, the orthorhombic structure suggested by Andersen e t al. s contained about 25 at.% Si. For the case of thin films, where the thickness of the silicide is at the very most a few hundred ~ngstr6ms, a reliable determination of the composition is difficult. Green and Bauer 1°, using results obtained from Auger spectra, have estimated the composition of the observed silicides to be about 17 at.% Si. In the present study attempts to determine the composition of the silicide phases by means of electron microprobe analysis failed because of (a) the thinness of the silicide layers, (b) the very localized occurrence of the different phases and (c) the presence of unreacted silicon. The phases that form on cooling from the eutectic depend not only on the composition of the alloy but also on heating and cooling rates, annealing conditions and impurities. For example, small amounts of copper originating from the copper grids on which the samples were heat treated could affect the formation and

STRUCTURE OF GOLD SILICIDE IN THIN

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379

structure of the silicide phases. Further investigations of the influence of these factors on silicide formation are in progress. 4. CONCLUSIONS

Vacuum-deposited Au/Si thin film couples were heated to the eutectic temperature (about 370 °C). On cooling, at least two crystalline silicide phases were observed by TED. One of the phases is consistent with the primitive cubic structure (a = 6.67 A) proposed by Suryanarayana and Anantharaman6. An orthorhombic structure with a = 6.8, b = 7.5 and c = 9.56 A is suggested for the other phase. ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support of the University of Pretoria and the Council for Scientific and Industrial Research. Professor J. H. van der Merwe, Head of the Department of Physics, is thanked for his continual encouragement. REFERENCES

1 2 3 4 5 6 7 8 9 10 11 12

J.W. MayerandK. N. Tu, J. Vac. Sci. Technol.,ll(1974)86. T R Anantharaman, H. L. Luo and W. Klement, Jr., Nature (London), 210 (1966) 1040. P. Predecki, B. C Gtessen and N. J. Grant, Trans. Metall. Soc. AIME, 233 (1965) 1438. R.C. Krutenat, J.K. TienandD. E. Fornwalt, Metall. Trans.,2(1971) 1479. G A. Andersen, J.L. Bestel, A A Johnson and B. Post, Mater. Scl. Eng., 7 (1971) 83 C. Suryanarayana and T. R. Anantharaman, Mater. Sct. Eng, 13 (1974) 73. T. Narusawa, S. Komiya and A. Hlraki, Appl. Phys. Lett., 22 (1973) 389. M.T. ThomasandD. L. Styns, Phys StatusSoli&B, 57(1973)K83 K. Nakashima, M. Iwami and A. Hiraki, Thin Sohd Fdms, 25 (1975) 423. A. K Green and E. Bauer, J Appl Phys., 47 (1976) 1284 S R. Herd, P. C h a u d h a n a n d M H. Brodsky, J. Non-Cryst Sohds, 7(1972)309 A. Hlraki, M. A. Nicolet and J. W. Mayer, Appl. Phys. Lett., 18 (1971) 178