Characterization of evaporated gold-indium films on semiconductors

Thin Solid Films, 29 (1975) 145-154 © Elsevier Sequoia S.A., Lausanne---Printed in Switzerland

145

CHARACTERIZATION OF EVAPORATED GOLD-INDIUM FILMS Olq SEMICONDUCTORS

T. G. FINSTAD, T. ANDREASSEN AND T. OLSEN Institute of Physics, University of Oslo, Oslo 3 (Norway) (Received February 7, 1975; accepted February 17, 1975)

Evaporated Au-In films with total thicknesses in the range 200--6000 A have been studied by backscattering of 1.5 MeV He ÷ ions, SEM, X-ray diffraction and with the electron microprobe. Both Si and GaAs were used as substrates, and the atomic percentage of Au in the films was less than 30. A fast diffusion within the films was observed which resulted in the formation of the A u l n 2 phase at room temperature. The compositional structure and the annealing behaviour of the films have been studied in the temperature range 25°--600 °C. A model for the annealing behaviour of the films is presented. The Au-In films have been used for ohmic contacts to GaAs and a correlation between the resistance of such contacts and the alloying behaviour is attempted.

1. INTRODUCTION Au-In films have been used as metallization on various microelectronic devices and hence have technical importance. Such films have been reported to give low ohmic but mechanically unstable contacts to n-type GaAs 1, 2. Our aim is to study the alloying process of these contacts but we have found it advantageous to investigate the metal films and their annealing behaviour in some detail first. In this paper we report on Au-In films on Si and GaAs substrates in the temperature range 25°--600 °C. Rutherford backscattering analysis has been successfully used to investigate thin multilayer metal films on semiconductors and has contributed to the understanding of various metallization problems in microelectronics3. Backscattering experiments yield information on the distribution of mass as a function of depth and hence the depth distribution of the elements in the film and substrate. However, many metal-semiconductor systems and especially their annealing behaviour are too complicated to be satisfactorily investigated by the backscattering technique alone. Other methods yielding information on properties such as chemical composition and topography are also needed. We have investigated our films by backscattering analysis, X-ray diffraction, with the electron microprobe and by SEM as well as ordinary microscopy. In Section 2 we present the experimental procedure and the various analysing techniques. The experimental results are presented and discussed in Section 3.

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T. G. FINSTAD, T. ANDREASSEN, T. OLSEN

We report on the compositional structure of the as-evaporated films and of the annealed films on Si. Finally we present measurements of films on GaAs and some preliminary attempts to correlate the electrical resistivity of A u - I n contacts and the observed alloying behaviour. 2. EXPERIMENTAL The substrates used in the experiments were < 100> Si crystals and < 100>cut boat-grown GaAs crystals. In and Au with a rated purity of 99.99 ~,, were sequentially evaporated from separate filaments onto the substrates at a speed of 5-10 ~tg cm -2 s -1 and at a pressure of approximately 1 × 10 -5 Torr. The Si and GaAs substrates could be cooled during the evaporation. During each evaporation, control samples were produced that contained only one of the metals. Backscattering analysis of these samples gave us an estimate of the film thickness ~. The annealing o f the samples was performed in a quartz tube furnace in a flow o f 80 ~ N2-20 ~ H2. The time from the evaporation of the films to the analysis ranged from 1 h to several months. In the evaluation o f the films we used the backscattering technique. This method gives information on the depth distribution of the elements averaged over the area of the beam spot (,-~ 1 mm2). The method has been described in detail by several authors 4. In the present investigation we used backscattering o f 1.5 MeV He + ions through an angle o f approximately 150 °. The energy resolution was 15-20 keV and the ion current was typically 1 nA. In order to analyse the backscattering spectra rapidly and reliably we developed a computer program that calculates the expected backscattering spectrum from various film configurations. In this program and in other calculations we used the stopping power data from Ziegler and Chu 5. To identify different phases we used Debye-Scherrer X-ray diffraction, and to get information on the topography o f the films we used SEM, When this technique is used in the backscattering mode it also gives some information on the lateral distribution o f the elements over the surface. Further information on the lateral distribution o f the elements over the surface was gained by using an electron microprobe with a resolution of the order of 1 txm2. Application of the electron microprobe for investigating thin films is described by Beatty and Gerhard 6. 3.

RESULTS AND DISCUSSION

Figure 1 shows a typical backscattering spectrum for the case in which In is evaporated prior to the evaporation of Au. In this particular case we evaporated 110 gg cm -2 (570/~) of Au upon 300 gg cm -2 (4100 A) of In. F r o m the backscattering spectrum it may be deduced that Au as well as In is present on the surface and that the ratio between the numbers o f Au and In atoms is 1:2. X-ray diffraction investigations on similar samples identified the phase Auln2. The intensity ratio between the X-ray lines indicated some orientation o f the crystallites which had a size o f at least 500 A. By comparing Fig. 1 with a calculated spectrum

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Fig. 1. Energyspectrumfor 1.5 MeV 4He ÷ ions backscatteredfrom a sample prepared by evaporating a layer of 570 ,~ Au on top of 4100 ,~ In on a Si substrate. The energies for backscatteringfrom Au and In at the surface are indicated by EAuand El, respectively. The spectrum is interpreted as backscattering from an even Auln2 layer on top of an In film which varies in areal density (ttg cm -2) over the surface. Backscattered particles from Au in Auln2 have energies between E1 and Ehu and from In in the same layer between E2 and E~n.The inset shows the film schematicallyand the wavy boundary between Si and In indicates the varying density of the In film. from an Auln2 film containing the same amount of Au as was evaporated onto the substrate, we find that all the gold is contained within the Auln2 film. This means that the counts below the energy marked E1 in Fig. 1 are attributable to Si and In. The relatively steep decrease in the spectrum at El indicates that the Auln2 layer is relatively even in thickness. The shape o f the backscattering spectrum below E2 in Fig. 1 can be explained by assuming that there is interdiffusion of In and Si or that there is an In layer behind the Auln2 layer with an areal density (~tg cm -2) that varies over the size o f the beam spot. Backscattering from samples in which the metal film had been etched off showed no signs o f diffusion, and backscattering from samples which had been implanted with Ar ÷ as a marker prior to evaporation o f Au and In showed that the etch did not attack the Si substrate. Thus we conclude that there is an In film o f varying thickness under an Auln2 film o f rather even thickness. Electron microprobe measurements have indicated that the period in the variations in the areal density o f the In film over the surface is less than 1-2 ~tm. The period in this variation might be a function o f the Au:In mass ratio and the evaporation conditions. It should be noted that the In films were o f even thickness prior to the evaporation of Au, as can be seen from the slope o f the low energy side o f the backscattering spectrum from the pure In film (Fig. 2). Figure 3 shows a map o f the various amounts o f Au and In which were evaporated in the cases when the phase Auln2 was identified by X-ray measurements. Auln2 was found whenever the ratio between the Au and In atoms was less than 1:2. This was also the case when the evaporation order was reversed.

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T, G. FINSTAD~ T. ANDREASSEN, T. OLSEN

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Furthermore, the phase A u l n 2 was identified for substrates held at - 5 0 ° C during the evaporation. Therefore the rapid interdiffusion of Au and In in the

CHARACTERIZATION OF GOLD--INDIUM FILMS

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films seems not to be the result of any heating during the evaporation. The formation of a laterally inhomogeneous In film was observed, regardless of film thickness, whenever the ratio between Au and In atoms was less than 1:2, but there was more variation in the In thickness as the AuIn2 film became thicker. Evaporated films with a ratio of Au to In atoms close to 1:2 showed poor adherence to the substrates and could often be blown off. We are at present investigating films in which the ratio of Au to In atoms is larger than 1:2. Thomas 7 has observed a rapid diffusion in Au-In films at room temperature. Using Auger electron spectroscopy he showed that within 10 min the surface of an Au-In (2:98) film was enriched to 15~ Au. Powell et al. s have reported large areas of the phase AuIn2 in diffusion studies on the interface between solid Au and solid In at 140°-150°C. Their measurements indicated that In was the diffusing component. We now turn to the measurements of annealed Au-In films on Si and suggest a model for this system. Figure 4 shows a series of backscattering spectra from a 570 A Au film that was evap9rated on top of an In film 4100 A thick on a Si substrate and then annealed at various temperatures for 15 min. Different annealing times in the range 3~-20 min had very little effect on the shape of the spectrum. Annealing at temperatures below the melting point of In (155.4°C) produced no significant change in the spectra compared with the room temperature spectrum. Above the melting point a decrease in the Au yield at the surface can be observed. The shapes of the spectra for annealing temperatures up to 450 °C suggest that a fraction of the scattering from the AuIn2 film seen at room temperatures is superimposed on a smooth curve. This can be explained by the following model. According to the phase diagram for the Au-In system9 part of the Auln2 film will dissolve above the melting point for In. This partial melting of the film will take place preferentially along grain boundaries, leaving areas of the original Auln2 film in a solution of In and Au which is poorer in Au than Auln2. Upon cooling, Auln 2 precipitates from the melt without, however, re-forming the original film. It is reasonable to assume that this precipitation takes place relatively homogeneously throughout the melt. From the phase diagram one would expect more dissolution~ from the Auln2 film as the annealing temperature is raised. This is reflected in the backscattering spectra by the decreasing Au yield at the surface, which can be observed in Fig. 4. X-ray measurements have identified the presence of Auln2 after annealing, as was also observed by Paola 1. Further support for our model has been gained by SEM investigations. Figure 5 shows a SEM picture of one sample after heating to 450 °C. One can see some squares of brighter colour than the background. The brighter colour indicates that the squares contain more of the heavier element Au than the background. We interpret this picture as showing platelets of Auln2 which were part of the original continuous film observed at room temperature. Electron probe measurements of the sample were consistent with this interpretation. It may be seen from the backscattering spectrum at 450°C that there is

150

T . G . FINSTAD, T. ANDREASSEN, T. OLSEN

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Fig. 4. Energy spectra for 1.5 MeV 4He + ions backscattered from a sample prepared by evaporating a layer of 570 A Au on top of 4100 A In on a Si substrate. The different spectra were taken after annealing at the listed temperatures for 15 min.

Fig. 5. Scanning electron microscope picture of a sample prepared by evaporating 360 A Au on top of 2700 .~ In on Si and then annealed at 450 °C for 2 min. The microscope was run in the backscattering mode. The picture is interpreted as platelets of AuIn2 in a mixture which on the average is poorer in Au than A u I n 2.

CHARACTERIZATION OF GOLD--INDIUM FILMS

151

no Au left at the surface but, from the step at energies higher than would be expected from scattering by In at the surface, there appears to be some just inside the surface. This may be due to oxidation of the film during annealing. In is known to oxidize easily1°. This thin oxide layer is not seen in the SEM pictures. Some oxidation is also indicated by the fact that the leading edge from In gets rounded off. This tendency is more marked at higher temperatures (500 ° and 600 °C) and then there is also a visible change in the films. We now consider the Au-In films on GaAs. These films seem to behave similarly to films on Si substrates at temperatures up to 350 °C. On unannealed samples we observed an AuIn2 layer on top of an In layer with a varying areal density. This is reflected in Fig. 6 which shows a series of backscattering spectra from a 380 A Au film evaporated on top of a 2900 A In film and annealed at various temperatures for 10 min. I

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The backscattering spectrum from an unannealed sample is shown at the bottom of Fig. 6. One can see the same characteristic steps at the leading edges from the Au and in portions of the spectrum as were seen in Fig. 1. The only difference is that for GaAs substrates the counts from the substrate are at higher energies because of the heavier mass of the substrate atoms. In Fig. 6 the counts

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T . G . FINSTAD, T. ANDREASSEN, T. OLSEN

from the substrate and the counts from In overlap due to the uneven thickness of the In film. The backscattering spectra from identically evaporated A u - I n films on GaAs and Si reveal no significant differences up to 350 °C. The differences between the spectra in Fig. 4 and Fig. 6 up to 350 °C can be correlated with the different Au:In ratios in the films and perhaps to slightly different processing conditions. Relatively drastic changes that are unique to films on GaAs substrates are seen in the backscattering spectra after annealing at 450"=500°C. These changes also depend upon annealing time. It is seen that the counts from the metal film and the substrate overlap completely. Such changes in the backscattering spectra might be due to some kind of "bailing" of the A u - I n films, but in this case it seems more probable that there is some mixing of the metal film and the substrate. We tried to etch away the metal film to see if there had been some diffusion into the substrate, but etches that attacked the metal films on Si failed to attack similar films on GaAs substrates. Furthermore, only a small change in the backscattering spectra could be observed after etching. This strongly indicates some kind of mixing o f the substrate and the film. In addition, there was a pronounced difference in the colour of the films on Si and on GaAs above the 450 °C annealing temperature. The work of other investigators on related systems also suggests that some kind of mixing takes place. Sinha and Poatell have shown that Ga atoms diffuse through an Au film 700 A thick during heating for 2 h in air at temperatures as low as 250 °C. They also observed diffusion distances of Au into GaAs of the order of 1500 A for the same conditions. Gyulai e t al. 12 have shown that atoms from an Au film penetrate deeply into GaAs at 400°-500 °C. We have tried to correlate the contact resistance of evaporated A u - I n contacts to GaAs with the backscattering spectra observed for different annealing temperatures. Figure 7 shows measurements of the specific contact resistance as a function of annealing temperature for a contact that was prepared by evaporating 250 A Au on top of 1600 A In. The specific contact resistance drops by four orders of magnitude at around 400°-500 °C. It rises again with annealing above 600 °C. Such U-shaped curves have been observed by others 1' 2 It seems that the drop in the contact resistance is correlated with the beginning of mixing of the semiconductor and the metal film. The rise seen in the curve in Fig. 7 may be explained by the severe oxidation o f the metal film during our processing procedures. In some cases, with thinner films and less oxidizing annealing conditions, we have observed that the whole metal film is consumed by the substrate leaving no metal on the surface. This may result in an increased contact resistance. Finally, the rise might also be caused by damaging effects of the alloying, as observed by Gyulai e t al. 12 The exact shape of the curve of contact resistance v e r s u s annealing temperature has been shown to depend upon heating rate, holding time, cooling rate and the ratio of Au to In 2. We are at present investigating this alloying of the metal film and the substrate in more detail, for this system and for the similar system of Au-Sn films on GaAs.

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CHARACTERIZATION OF G O L D - I N D I U M FILMS I

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Fig. 7. The mean specific contact resistance of a pair of A u - l n contacts after annealing at different temperatures for 2 min. The contacts were made by evaporating 250 A of A u on top of 1600 A In on a < 100>-cut G a A s wafer with a rated electron concentration n = 8 x 1017 cm -3 and a resistivity p = 3 . 2 x 10 -3 ~ cm. ACKNOWLEDGMENTS

We wish to thank our colleagues in the Electronics Group for assistance and helpful discussions, especially Mr. T. Henriksen, who participated greatly in the initial phases of this project. We also thank members of the staff at the Central Institute for Industrial Research, Dr. D. Ruzicka for supplying the GaAs crystals, Mrs. T. U Rolfsen for collecting the X-ray data and assisting us in their interpretation, and Mr. J. A. Horst for help with the electron microprobe. Thanks are also due to Mrs. E. Jensen of the Electron Microscopy Laboratory. Finally we acknowledge the cooperation of our Van de Graaff crew as well as financial support by NAVF. REFERENCES 1 C . R . Paola, Solid State Electron., 13 (1970) 1189. 2 S. Knight and C. Paola, in B. Schwarz (ed.), Ohmic Contacts to Semiconductors, Electrochem. Soc., New York, 1969, p. 102. 3 S . T . Picraux, E. P. EerNisse and F. L. Vook (eds.), Application oflon Beams to Metals, Plenum Press, New York, 1974. 4 W . K . Chu, J. W. Mayer, M.-A. Nicolet, T. M. Buck, G. Amsel and F. Eisen, Thin Solid Films, 17 (1973) 1. 5 J . F . Ziegler and W. K. Chu, Thin Solid Films, 19 (1973) 281. 6 R . P . Beatty and J. Gerhard, in B. Schwarz (ed.), Ohmic Contacts to Semiconductors, Electrochem. Soc., New York, 1969, p. 324.

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S. Thomas, Appl. Phys. Lett., 24 (1974) 11. G.W. Powell and J. D. Braun, Trans. AIME, 230 (1964) 694. F . A . Shunk, Constitution of Binary Alloys, 2nd Suppl., McGraw-Hill, New York, 1969, p. 72. G. Brady, Materials Handbook, McGraw-Hill, New York, 1944, p. 308. A . K . Sinha and J. M. Poate, Appl. Phys. Lett., 23 (1973) 666.

J. Gyulai, J. W. Mayer, V. Rodriguez, A. Y. C. Yuand H. J, Gopen, J. AppL Phys., 42(1971) 3578.