Electron irradiation and adhesion at silicon-gold interfaces prepared under ultra high vacuum conditions

Surface Science 169 (1986) L355-L361 North-Holland, Amsterdam

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SURFACE SCIENCE LETTERS ELECTRON IRRADIATION AND ADHESION AT SILICON-GOLD INTERFACES PREPARED UNDER ULTRA HIGH VACUUM CONDITIONS H. D A L L A P O R T A and A. C R O S UA C N R S 783, Dbpartement the Physique, Facultb des Sciences de Luminy, Case. 901, 13288 Marseille Cedex 9, France

Received 7 October 1985; accepted for publication 5 December 1985

Silicon-gold interfaces have been grown and exposed to electron beam irradiation under ultra high vacuum conditions. The adhesion of the gold film is good when Au is evaporated on an atomically clean Si(111) surface. In contrast, the presence of a thin SiO2 native oxide between Au and Si decreases strongly the adhesion of the metallic layer. Electron irradiation of the clean Si-Au interface provokes the agglomeration of the metallic layer. Direct determination of the sample temperature shows that thermal effects induced by the electron beam cannot be neglected.

During past years evidence has been accumulating that the adhesion of thin deposited metal films to their substrates can be enhanced when exposed to various radiations such as energetic heavy ions [1,2], 1 0 - 4 0 keV electrons [3,4] and v a c u u m UV ( 1 0 - 2 0 eV) p h o t o n s [5]. The mechanism causing the better adhesion is poorly understood and is generally i,elieved to be associated with electronic processes at the interface rather than atomic intermixing. In particular secondary excitations may play a great role in strengthening the interface bonds by changing the local electron distribution and the atomic configuration. In view of the growing interest in these p h e n o m e n a and their important potential applications, we have investigated the adhesion of gold film on silicon substrate. It has been reported that the adhesion of these two materials is enhanced by electron irradiation [3]. In the experiments reported in this letter, both the cleaning of the Si substrate, the gold deposition and the electron irradiation have been carried out under ultra high v a c u u m ( U H V ) conditions. For thin enough gold layers the surface techniques allow one to gather information on the electronic and structural properties of the interface. The properties of the S i - A u junction are well k n o w n [6-.9] so that a precise characterization of the b e a m effects m a y be expected. Experiments have been made in an U H V c h a m b e r (base pressure < 10 -9 Torr), with facilities for in situ Auger (AES), electron energy loss (EELS) 0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

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spectroscopies and high energy electron irradiations. The (111) surface of n-type polished silicon wafers was cleaned by repetitive short heat treatments (2-3 rain, 1373 K) until AES analysis did not reveal any trace of contaminants. Gold of 99.99% purity was then evaporated in a vacuum of 10 -9 Torr and the coverage 0 defined as the ratio between the number of deposited Au atoms and the number of Si surface atoms (7.8 × 1014 a t o m s / c m 2) was calibrated with AES spectra and a quartz microbalance. The gold-silicon interface was irradiation in situ with a high voltage electron gun. A differential pumping between the electron gun and the main chamber allows to keep the vacuum below 5 x 10 -9 Torr during the electron irradiation. The energy density on the sample was changed by varying the accelerating voltage between 10 and 50 kV and the electron current between 1 and 40 ~A. The size of the beam spot was estimated from the luminescence spot on the surface and could be varied between 0.5 and 2 mm. These sizes were confirmed later by scanning microscopy analysis of the surface. An infra-red pyrometer using a lead sulfide detector was used to determine the temperature of the sample during irradiation. The adhesion was estimated by the peeling test with scotch tape or by rubbing the surface with a dry cotton bud (Q-tip). With this crude test we have observed strong differences in adhesion of gold layers depending on the preparation mode. When the silicon wafer is cleaned ex situ by conventional procedure, the surface is always covered by a very thin oxide layer (5-10 A). A gold layer ( ~ 200 .&) deposited onto this surface in a vacuum of 10 s Torr is easily removed by scotch tape or by abrasion with cotton bud. If the Au layer is deposited in a vacuum of 10 -9 Torr, on the surface covered by the native oxide, the film is also removed by cotton abrasion. However, if under UHV conditions ( P < 10 9 Torr), the silicon surface is in situ annealed at 900-1000°C, no contaminant (oxygen, carbon) is left at the surface (impurity concentration < 1%) and the gold layer deposited on this surface is now impossible to detach with scotch tape or by abrasion with cotton bud. After irradiation by the electron beam (40 kV, 40 ffA), the Au layer is still impossible to detach by the peel test. We shall present in the following the characterization of the S i - A u interface formation and the effect of the beam irradiation. The evolution of the Auger spectra during the Au deposition on a clean Si surface is shown in fig. 1. For the clean Si surface (curve A), we observe the Si(LVV) peak at 92 eV. Curve B corresponds to the deposition of 10 gold monolayers on the substrate maintained at room temperature. We note the appearance of the Au(NVV) Auger transition at 68 eV and the splitting of the Si(LVV) transition into two peaks at 90 and 94 eV. This phenomenon has been extensively described elsewhere [6-9]. In S i - A u interface is a well known example of a diffusive interface and its formation at room temperature can be summarized as follows: The first monolayer grows by a layer-by-layer mode ( F r a n k - V a n der

H. Dallaporta, A. Cros / Silicon-goM interfaces-

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Energy(eV) Fig. 1. Evolution of Auger spectra (primary beam energy: 2 kV, sample current 1 /xA). (A) Clean silicon surface. (B) After evaporation of 10 gold monolayers. We note the apparition of the Au(NVV) transition at 68 eV and the splitting of the Si(LVV) one. (C) After 30 rain irradiation, 40 kV, 20 /~A (measured temperature = 200°C). The intensity ratio of the Au(NVV) peak to the Si(LVV) one is decreased. It corresponds to some possible diffusion of Si atoms towards the surface and to the beginning of the agglomeration process (suggested by the unsplit Si(LVV) line). (D) After 30 rain irradiation, 40 kV, 40 /.tA (measured temperature = 400°C). Longer exposures do not change the spectrum which is identical to the one obtained after thermal annealing at 400°C. It corresponds to metal crystallites on top of an Au monolayer which covers the Si(111) surface.

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Merwe type) and one observes the intermixing of Si and Au atoms and the formation of a gold-silicon alloy. For increasing coverages, the Si concentration in the gold film decreases. An interesting peculiarity is the presence at the surface of segregated Si atoms. They are imbedded in a gold matrix and are no longer in the sp a hybridization state characteristic of Si atoms in crystalline silicon. There is hybridization between the Au 5d valence states and the silicon 3p states [10]. This appears in the Auger spectra in the splitting of the Si(LVV) peak at 90 and 94 eV (fig. 1, curve B). The interface has then be irradiated with the electron beam (spot size -- 2 mm) at a constant accelerating voltage (40 kV). For low current intensity ( < 10 ffA), there is no modification of the AES spectrum even with prolonged exposure (10 h). The first evolution (fig. 1, curve C) is observed after irradiation at 20 ffA during 30 min evidenced by a slight increase of the Si(LVV) peak intensity, We have measured simultaneously an increase (T = 200°C) of the sample temperature. Longer exposures do not change the AES spectrum. The splitting of the Si(LVV) transition is no longer seen. This means that the signal mostly originates from Si atoms in a covalent hybridization state. We also observe that the Au(NVV) intensity has decreased. These two points suggest that the metal film begins to agglomerate and uncovers the Si substrate which produces an unsplit Si(LVV) peak. This tendency is reinforced after irradiation with a 40/xA current (fig. 1, curve D) corresponding to a measured temperature of 400°C. The crystallites cover a small portion of the surface which explains the strong decrease (increase) of the gold (silicon) signal. The Auger spectra are similar to the one obtained after a thermal annealing at 400°C of the Au-Si interface [7]. Extensive studies have shown in this case that the crystallites lie on a gold monolayer (fig. 2). It is interesting to note that the Si(LVV) peak of curves C and D is

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slightly broader than the corresponding peak observed for pure crystalline silicon. The broadening may be attributed to the interfacial Si atoms nearest neighbours to Au atoms and exhibiting the Si(p)-Au(d) hybridization. Their contribution is much smaller than that of the underlying, covalently bonded Si atoms and thus only appears as a broadening of the Si(LVV) line. The presence of crystallites on the surface has been confirmed by scanning electron microscopy (fig. 3). The electron irradiated spot is clearly seen in the center of the picture. It corresponds to a zone where the density of crystallites is small and their mean size is typically 3-4/~m. In contrast, the crystallite size (1 t~m) decreases and their density increases in the non-irradiated area. The latter, although not exposed to the electron beam, was nevertheless heated due to the diffusion heat and to the thermalization by the sample holder. The temperature measured by the infra-red (IR) pyrometer was averaged on a spot defined size of ~-2 m m and was found to be nearly constant on the whole surface even on a non-irradiated area. To make sure that the luminescence of the sample under the electron beam would not influence the pyrometer reading, we have measured the temperature at the back of the Si wafer and found the same value. The fact that the crystallites have larger mean size and smaller density in the irradiated area can be ascribed to the existence of a temperature gradient which is not detected by the pyrometer averages measurement. Under the electron beam, the temperature is higher and it is known that a higher temperature gives a smaller number of larger crystallites in agreement with the theory of nucleation and growth of thin films [11]. The agglomeration of the metallic film does not depend on its thickness. It is a general phenomenon which has been observed for very thin Au films (2 monolayers) and for much thicker ones (several hundreds ~ngstr6ms). The deposition of thin films allows to probe directly the interface with the Auger spectroscopy since the escape length of electrons with 60-90 eV kinetic energies is in the 7 ,~ range. The Auger Si(LVV) and Au(NVV) transitions probe directly the valence band and are sensitive to the local environment of the atoms. Beside the modification due to the structural changes (fig. 1), no modification specific to electronic effects has been evidenced in these transitions. Finally, it must be noted that the electron doses producing the agglomeration are similar to the ones known to induce the adhesion enhancement [3], typically 1017 cm -2. Within the sensitivity of AES and EELS, our results indicate that the most dramatic changes induced by the electr,~,n beam concern the morphology of the metallic film. In conclusion, our results can be summarized as follows: (1) The adhesion of a 200 ,~ thick gold layer is considerably enhanced when Au is evaporated on an atomically clean S i ( l l l ) surface, prepared under U H V conditions. In order to remove the film, one needs to destroy it mechanically

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Fig. 3. Scanning electron microscopyshowing the irradiated surface.

by scrubbing with a hard blade. On the contrary, a similar gold layer evaporated on a Si surface cleaned ex situ, i.e. covered by a few ~ngstr~Sms of oxide, is easily removed by the peeling test. (2) Electron beam irradiation of the Si-Au interface prepared under UHV conditions does not change the adhesion of the gold layer (within the sensitiv-

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ity o f the peel test) a n d p r o v o k e s its a g g l o m e r a t i o n . D i r e c t d e t e r m i n a t i o n of the s a m p l e t e m p e r a t u r e shows that t h e r m a l effects a s s o c i a t e d to the e l e c t r o n i r r a d i a t i o n c a n n o t b e n e g l e c t e d a n d are r e s p o n s i b l e for the a g g l o m e r a t i o n . (3) Surface t e c h n i q u e s w h i c h directly p r o b e the m e t a l - s i l i c o n i n t e r f a c e a n d are sensitive to the local e n v i r o n m e n t of the a t o m h a v e n o t e v i d e n c e d specific m o d i f i c a t i o n of the local e l e c t r o n d i s t r i b u t i o n s . W e t h a n k J. M a r f a i n g , S. N i t s c h e a n d the C e n t r e de R e c h e r c h e s sur les M ~ c a n i s m e s de la C r o i s s a n c e C r i s t a l l i n e in L u m i n y for the s c a n n i n g e l e c t r o n m i c r o s c o p y analysis.

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