Vacuum/volume 41/numbers 4 -6 /pages 1128 to 1131/1990 0042-207X/9053.00 + .00 Printed in Great Britain Pergamon Press plc

Thickness dependent migration of Au films G Pet6, T Lohner and L Pogbny, Central Research Institute for Physics, H- 1525 Budapest, Hungary

and

T Andersson, Chalmers University of Technology, Physics Department, S-41296 Gdteborg, Sweden

The influence of the annealing of a A u - S i O 2 - S i layer with different thickness of Au is investigated by Rutherford Backscattering and Scanning Electron Microscopy. The heat treatment induces gold migration which strongly depends on the thickness of Au. There is a lack of this migration at 25 nm region of Au.

5.9rim 1. Introduction Si02---~ --~ I--

The study of the metal-sil icon-dioxide contact is important l ~ I~ [ ^ . . [ ~ due to the central role of the interface in MOS-devices and integrated technology. Clean Si surfaces react with gold at low temperatures when a critical film thickness is reached. Hiraki et ~ ,Born

al have shown that the gold/silicon system forms an alloyed 9 80o _ : ) . ~ ~ . . metallic phase as observed by splitting in the silicon valence =~ ~ 6oo , , ~ , : . ~.

band Auger spectrum 1. This intermixing at the surface film ~ g,oo* +:'. growth has been found to be strongly influenced by an initial ,~ o | \ surface oxide 2. The oxide reduces the reaction with the silicon. ~ 200t However the splitting of the silicon Auger peak has also been ~= 1 "+~"~ ' S; ' shown onto an oxidized surface when the gold film and oxide v:o ' z~o ' 28o :: CHANNEL NUMBER

layer are approximately equal in magnitude 3. The gold interacts , ;f with SiOx because of the free silicon bonds 4. In SiO2 very few 110 111 1.2

bonds are free which in turn gives a poor adhesion between gold and substrate due to the weak interaction 6.

Au-SiO2 structure has earlier been investigated with 50 nm thick gold layers on 20-200 nm thick SIO2. After heat treat- ments within 500-700°C the SlOE degraded leading to silicon- gold reactions 7,s. In the present investigation we analyze the Au-S iO2-S i system as a function of the thickness of the gold layer.

2. Expermental

Silicon dioxide, 10 nm of device quality was grown by thermal oxidation at 1100°C on silicon (111) wafers. Following this the substrates were loaded in a vacuum system giving 10-4pa. Gold films were deposited onto the oxide by thermal evapora- tion from a W-spiral while the substrate was at room tempera- ture. The thickness of the gold layers ranged from 6 nm to 130 nm. Thickness control was performed during the evapora- tion process by quartz crystal and was in addition measured after the evaporation by Rutherford Backscattering. The A u - SiO2-Si structures were later annealed during 15 min at 520°C a n d 10 - 4 Pa in another vacuum system.

3. Results and discussions

The RBS spectra for both as-evaporated and annealed samples are presented in Figures 1, 2 and 3 with the gold thickness as a

• AS EVAPORATED + AFTER ANNEAUNG

{ 520"C, 15rain)

~o :

ZJ

440 460

13000

~zooo

1000

177 118 i;" ENERGY OF BACKSCATTERED ~He + PARTICLES [MeVI

Figure 1. Rutherford Backscattering (RBS) spectrum of Au-SiO2-Si layers at 5.9 nm thickness of gold film, before and after annealing.

parameter, 5.9, 25 and 130 nm, respectively. The spectra exhibit two prominent features: the peak and the edge. The peak is due to the scattering from the gold atoms at the surface layer while the lower energy edge corresponds to the Si atoms in the substrate and within the SiO2 layer. These different Si atoms could not be resolved because the SiO 2 layer was very thin (10 nm). It is evident from the curves that spectra of annealed samples are substantially modified as compared to those from the as-evaporated.

The spectra from the 5.9 nm thick layer are characteristic for the thickness range 5.9 to 19 nm. Here annealing results in a substantial broadening of the gold peak and an energy shift which is corresponding to backscattering from silicon atoms moved close to the surface.

For thicker films, 25 nm, the shape of the Au signal and the Si-edge are practically unchanged after the heat treatment.

For the thickest films the Au signal changes dramatically in shape after thermal treatment as can be seen for the 130 nm film. Here the Au signal decreases considerably and overlaps the Si spectrum. In addition the Si spectrum gives evidence for backscat- tering from silicon atoms situating on the surface of the sample.

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G Pet6 et al. Thickness dependent migration of Au films

J

800- ~ .

~ 6ooi

g 400-

200-

i 240

lOnm

.+

2~o ' 28o C~A~E~ NUMBER

, #

• AS EVAPORATED

+ AFTER ANNEALING

( 520"C, 15 rain )

420

10000

8000 .:~

6000 °÷

"+ 4000 +

2000 •+

440 460

110 111 1.2 117 118 119 ENERGY OF BACKSCATTERED 4He+ PARTICLES [MeV]

Figure 2. RBS data for Au-SiO2-Si at 25 nm thick gold film with and without heat treatment.

g 6000 ~ 5 0 0 0

N

t " °" 4000 10nm • AS EVAPORATE0

~°®l I " A~T~R ANNEAL,NG " ~ . j ' .

2000 , .,,,,o,~_:

200 250 300 350 400 450 CHANNEL NUMBER

11o ;.2 ii~ 116 i;8 E N E R G Y OF B A C K S C A T T E R E D 4He+ PARTICLES [ M e V ]

Figure 3. Rutherford Backscattering spectrum for Au-SiO2-Si at 130 nm thickness of Au before and after heat treatment.

The modifications of the spectra of annealed samples de- scribed above could be explained either by in-depth penetration of gold into both SiO2 and Si or by the development of islands on the SiO2 surface via lateral migrations as was detected for Pb films deposited on a Si substrate 5.

For identification of these processes a complementary inves- tigation of the surface morphology and the surface chemical composition were carried out by SEM and EDAX. SEM pictures taken on the as-evaporated and annealed samples with the thickness of 5.9 nm is shown in Figures 4(a) and (b). The corresponding RBS spectrum is given by Figure 1. The heat treatment changes the morphology also for the 130 nm thick film but at 25 nm thickness the morphology of the Au-SiO2-Si system is unchanged in as-evaporated compared to annealed sample (cf Figure 5).

The processes caused by the heat treatment are different for the 5.9 nm and 130 nm thicknesses. At 5.9 nm a network for- mation of the Au layer is evident while at 130 nm thickness island formation and grains growing take place as it is shown in Figure 6.

The gold content of the sample with the 130 nm thick gold film was measured by EDAX. The lateral compositional profile clearly shows that the Au signal corresponds to regions where

Figure 4. The morphology of Au-SiO2-Si with (a) and without (b) annealing. The other parameters are same as Figure I.

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G Petd et al: Thickness dependent migration of Au films

Figure 5. The morphology of Au-SiO2-Si with (a) and without (b) annealing. The other parameters are same as Figure 2.

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Figure 6. The morphology of Au-SiO2-Si with (a) and Without (b) annealing. The other parameters are same as Figure 3.

G Pet5 et al: Thickness dependent migration of Au films

the Si signal is attenuated and vice versa. This finding clearly demonstrates the non-uniform distribution of gold on the sur- face as gold was not detectable outside the islands.

4. Conclusions

For a gold film evapoated onto the SiO2-Si substrate, there is a thickness dependent migration process, induced by annealing. This redistribution process is a lateral migration of Au on the SlOe surface. The lateral migration of gold is strongly depen- dent on the thickness of the gold film and there is a critical thickness of about 25 nm where a lack of the migration was found.

Two different films were formed during the annealing due to the lateral surface migration. For the thinner layers (d ~ 6 - 19 nm) a network was formed and for a thicker layer (d ~ 130 nm) islands growing with large crystal grains are characteristic.

In the thinnest layer the network is formed by the heat treatment, because the layer is not continuous and annealing induces an energetically favourable state of the discontinuous layer which is the network.

At the 25 nm thickness region the Au film is continuous policrystalline layer. As the grain sizes are comparable with the thickness of the film the large ratio of the grains boundaries terminates on the surface. In this case the annealing induces grooving at the intersection of the boundaries and the surface, simultanously with the grain growth. As the grooving is much faster it can prohibit the grain growth 9 explaining the un- changed morphology.

In thicker films the grain growth is favourable as the smaller ratio of the boundaries terminates the surface, and islands with large grains will be formed. Our opinion is that the similar effect was demonstrated earlier for 50 nm thi~k gold film de- posited onto fused quartz and annealed at 700°C 7'8.

5. Summary

The Au-SiO2 interaction by 520°C 15 min annealing is mainly gold migration on the SiO2 surface. This migration depends on the thickness of the gold film and there is a thickness range where the migration does not take place at all. The effect may be explained by the thermal induced grooving.

Acknowledgement

The support of OTKA grant 1794 is kindly appreciated.

References

A Hiraki, Surface Sci Rep, 3, 357 (1983). 2 L Braichovich, C H Graner, P R Streath, C Y Su, P V Chye, I Lindau and W E Spicer, Phys Rev, (B)20, 5131 (1979). 3 T G Anderson, J Appl Phys, 52, 7212 (1981). 4 D W Morgan, M H Howes and C J Madams, J Electrochem Soc, 123, 295 (1976). 5 C J Madams, D V Morgan and M J Howes, J Appl Phys, 45, 5088 (1974). 6R S Bauer, R Z Bachrach and Billson, Appl Phys Lett, 37, 1006 (1980). 7 E I Alessandrini, D R Campbell and K N Tu, J Appl Phys, 45, 4888 (1974). 8 E I Alessandrini, D R Campbell and K N Tu, Vacuum, 13, 55 (1976). 9 W W Mullins, J Appl Phys, 28, 333 (1957).

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