Oxide growth on silicides in oxygen plasma

Applied Surface Science 36 (1989) 185-195 North-Holland, Amsterdam

OXIDE GROWTH ON SiUCIDES

185

IN O X Y G E N PLASMA

A. C L I M E N T *, J.P. E N A R D , B. L A V E R N H E , J. P E R R I E R E Groupe of Physique des Solides de FENS, Universitd Paris VII, Tour 23, 2 Place Jussieg 75251 Paris Cedex 05, France

A. S T R A B O N I , B. V U I L L E R M O Z a n d D. L E V Y * * Centre National d'Etudes des T~ldcommunicatio,t¢, CNS, B.P. 98, Chemin du Vieux Chine, 38243 Meylan Cedex, France

Received 2 June 1988; accepted for publication 5 July 1988

We have shown the possibility of growing thick oxide films cn refractory metal silicides by plasma oxidation in the 500-900 °C temperature range. Thin layer~ of Si-rich silicides TiSix and WSiy (x, y > 2) deposited onto Si or SiO2 by cosputtering have been oxidized in and RF plasma at floating potential. The oxide growth rate, composition and thickness, and the depth distribution of cations were determined by the complementary ase of nuclear reaction analysis and RBS. We have found that silicon in excess in the films diffuses through the silieide towards the surface to form a SiO2 passivating overlayer. The Si oxide growth rate is diffusion limited, while for long treatment oxidation of the silicide itself occurs with the formation of an oxide mixture (metal oxide and SiO2).

1. I n t r o d u c t i o n P l a s m a c x i d a t i o n is a low t e m p e r a t u r e t e c h n i q u e used to g r o w dielectric films o n metal o r s e m i c o n d u c t o r surfaces [1]. U s i n g this t e c h n i q u e to g r o w SiO 2 metal free films o n the sihcides will have the a d v a n t a g e o f lowering the t e m p e r a t u r e with respect to the t h e r m a l o x i d a t i o n at high t e m p e r a t u r e . H o w ever, as p l a s m a o x i d a t i o n can be c a r d e d o u t at - 600 ° C a n d even lower, the f o r m a t i o n o f fresh ~ihcide layers at the i n n e r Si-silicide interface, that preserves the silicide integrity d u r i n g oxidation, m a y n o t take place. I n o r d e r to o v e r c o m e this possible p r o b l e m , a n e w a p p r o a c h is to start f r o m a silJcide o r m e t a l - S i m i x t u r e richer in Si t h a n the stoichiometric value in o r d e r to allow for the following t r a n s f o r m a t i o n d u r i n g p l a s m a oxidation: MSi x --, MSi 2 + Si02, * Permanent address: Departamento de Fisica Aplicada C-XII, Universidad Aut6noma Madrid, Cantoblanco, E-28049 Madrid, Spain. * * Permanent address: Bull, Rue J. Jaur~s 78340, Les Clayes/Bois, France. 0 1 6 9 - 4 3 3 2 / 8 9 / $ 0 3 . 5 0 © Elsevier Science P u b l i s h e r s B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

186

,4. Climent et al. / Oxide growth on silicides in oxygen plasma

by means of SiO 2 formation at' the outer surface from the excess Si in the MSi x (x > 2) mixture. This new way of approaching silicide oxidation can have the advantages of the ~ndependence of the nature of the substrate over which the silicide to be oxidized is deposited. In this work we have thus checked the possibility of growing metal free SiO2 over TiSi,, and WSiy by plasma oxi:laticn. The experiments were carried out at floating potential in order to avoid the formation of oxide mixture (M and Si oxide) observed during plasma ano~ization of silicides [2,3] (i.e. during oxide growth assisted by an electric field).

2. E x p e ~ m e n t ~ This films, of TiSi x ( - 100 nm) and WSiy ( ~ 200 nm) were deposited by cosputtefing on Si(100) single crystals and on SiO 2 thermally grown on Si (25 or 100 nm thick SiO 2 for the TiSix deposit and 100 nm thick SiO 2 for the WSiy deposit). The composition of the metal-sificon mixtures, MSix, was deliberately enriched in Si (x, y > 2), al,lowing that the Si atoms in excess could form the SiO 2 overlayer during the plasma treatment. The plasma oxidations were carried out in a R F (13.56 MHz) system from C N E T [4]. Oxygen plasmas were obtained in the i to 6 × 10 -2 mb pressure range and at a R F power of 300-400 W in a reactor which was a quartz tube continuously pumped during the experiments. The inner diameter of the tube being 50 ram, the average R F power density was 0.15-0.20 W / r a m 2. The quartz tube was located in a sliding furnace ~hose temperature was regulated up to 950°C. Experiments were performe~l in the 500--900°C temperature range. A bare SI substrate was included in ;all the oxidation runs m order to compare the oxide growth on Si and on silicides. The oxygen.-, content of the samples before and after plasma oxidation was determ_;ned using the 160(d, p)lTO* nuclear re.~tction [5]. Absolute values were obtained by comparison with reference targets known within 3%. The composition and thickness of the as-deposited MSix films were determined by Rutherford backscattering spectrometry (RBS). The composition of the remaJrfing MSi x, compound after plasma oxidation as well as that of the oxide formed during the treatment were also determined by RBS. The precise depth distribution of the various constituents of the films were determined using the simulation program R U M P [6] for RBS spectrum interpretation. A typical example of such an analysis is given in fig. 1 which represents the experimental RBS spectrum recorded on a TiSi x film deposited on SiO2 (100 nm) and oxidized in plasma. The theoretical surface position of each element is indicated, and the oxygen incorporated during the treatment is clearly evidenced, well separated from the oxygen of the SiO 2 layer between the silicide and the substrate. The continuous line shows the calculated spectrum

1g7

A. Climent et al. / Oxide growth on silicides in oxygen plasma

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Fig. l. Experimental RBS spectrum of a TiSix film deposited on SiO2 (100 ran) fitted with the RUMP simulation program (continuous line).

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Fig. 2. Assumed composition versus thickne~s, d, (in units of 10 Is atoms/cm 2) of the sample giving the simulation of the RBS spectra in fig. 1.

188

d. Climent e; al. / Oxide growth on silicldes in oxygen plasma

obtained by using the simulation program R U M P assuming the composition profile presented in fig. 2 This profile shows the formation of SiO 2 at the external surface, overlying inner layers with mi:
3. Resul~s and discussien The composition of the as-deposited MSi~ .mixture was determined by RBS :o be TiSi2.4s with an overall a m o u n t of - Y90 × 10 is a t o m s / c m 2 a n d WSi2.55 with an overall a m o u n t of - 1000 × 1015 a t o m s / c m 2. These values gave the best fit to the experimental RBS spe.ctra when simulated with the mentioned R U M P program. During plasma oxidation of the sflicides in the 5 0 0 - 9 0 0 ° C temperature range, thick oxide films are formed. In order to distinguish between the effects associated with the presence of the oxygen plasma a n d the pure thermal oxide growth, a complete set of samples were subject to thermal treatments without plasma u n d e r the same conditions of oxygen pressure, duration a n d temperature of the treatment. In table 1 we present the a m o u n t s of oxygen atoms incorporated during such treatments and those incorporated when the oxygen plasma is present. T h e a m o u n t of oxygen atoms incorporated in the :amples during plasma oxidation was found to be d e p e n d e n t on the plasma conditions: oxygen pressure, power supplied by the R F generator a n d oxygen flow rate. D u r i n g these treatments the silicide interacts with the ambient oxygen leading to the oxide formation at the external surface. In addition, interactions can also Table 1 Amount of oxygen atoms incorporated (× 101S atoms/cm2)

Si TiSix/Si TiSi~/SiO2 (25 nm) TiSix/SiO2 (100 nm) WSiy/Si WSiy/SiOz (100 nra)

700°C4h Thermal 3 7 6 1 -

Plasma 294 226 210 240 44.7 555

900°C4h Thermal 8 13 9

Plasma 309 369 477 478 384 377

Net amount of incorporated oxygen atoms in the different substrates measured by the nuclear reaction 160(d,p)~70*. Thi~ table shows clearly that the thermal effect in the incorporation of oxygen is negligible when no plasma is present.

A. Climent et aL / Oxide growth on sificides in oxygen plasma

189

occur at the other interface leading to interdiffusion between the silicide and the substrate. This last p h e n o m e n o n was found to increase with temperature. In this paper we will emphasise on the results obtained at 4 × 10 -2 mb, 300 W and 700 o C. For these conditions acceptable oxide growth rates are observed while interdiffusion with the substrate is not the dominant phenomenon. 3.1. T i s i f i c i d e

Fig. 3 shows the amount of oxygen atoms incorporated as a function of time for TiSx films on Si and SiO 2 (25 and 100 n m thick) samples oxidized in plasma under the preceding conditions. For the purpose of comparison, the curve corresponding to the oxidation of bare Si under the same conditions is also presented. Two regimes can be distinguished in the plasma oxidation of TiSi x. In the first regime (for times lower than 5 h) the behaviour is quite similar to that observed for Si and there is little dependence on the nature of the substrate. In the second regime (times greater than 5 h), the oxide growth kinetics are greatly accelerated compared to the oxidation of bare Si, leading to rate laws of the type: d ~ t " with n > l . Moreover differences in the behaviours appear following the nature of the silicide substrate. i

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0 500 t I min. Fig. 3. Total contents of oxygen atoms (in units of 1015 O atoms/cm 2) of the TiSi~ layer as measured by the nuclear reaction 160(d.p)nO * versus treatment time. Resuits on bare Si are also shown for comparison.The square of the ne~ amount of incorporated oxygen atoms in TiSix (,,) (mean value of the three different substrates) is given to show the fit d ~ - d 2 ffi Bt. Experimental conditions: 700 o C, 300 W. 100

300

190

A. Climent el a~ / ,gxide g~'owth on silicides in oxygen plasma

AS it will be seen later, the first regime in the oxide growth kinetics corresponds to the formation of a pure SiO 2 overlayer. In order to elucidate the kinetics of the oxidation process and to compare it to the growth on bare Si, we have tried to fit these result following the classical procedure adopted for the study of thermally grown SiO 2 [71. We have found that a direct fit of the data presented in fig. 3 in the form: d ~ - d~ = B t ,

with B = 279 x (10 ~s O atc.ms/cm2) 2 min - I , is possible and gives also a good agreement. Here d i is the measured oxygen atoms content of the film prior to oxidation. Due to the dispersion of the experimental data, it is not possible to give a precise law for the growth of the SiO 2 overlayer during the plasma oxidation of TiSi x. However, despite this point, it seems reasonable to conclude that the oxide rate law is c c t l / 2. This means that the limiting step in the process is the diffusion of the migrating species, and, taking into account the similarities between the TiSi x and bai'.: Si plasma oxidation, this migrating species would be the same. Following this analysis, the thickness of oxide can be expressed by. d~

( D t ) 1/2,

where D is the diffusion coefficient of the mobile species. Assuming that D is exponentially d e p e n d e n t on T according to D = D O exp( - E a / k T ) , we can estimate the activation energy E~. In fact in the 500-800 ° C range a n d for a treatment time of 4 h, a value of Ea = 0.6 eV was obtained. Rt~S analysis was carried out on these samples and typical spectra recorded on TiSi~ films deposited on Si a n d SiO 2 (100 nm) and oxidized during 0.5 a n d 9 h are given in figs. 4 and 5. Examination of these spectra lead to the following conclusions: - The oxygen contribution superimposed on the Si signal indicates progressive oxygen incorporatio:., and these atoms form an oxide film which, in the first regime (figs. 4a and 5 , ) does not affect the silicide since the shape of the Ti peak remains unaltered. - This su, face oxide is a pure SiO 2 layer since Si is always present in the surface while the Ti peak is shifted towards lower energies indicating that the silicide is buried below the SiO 2 layer. - Since the Ti yield increases, a change in the stoichiometry is observed (TiSi~,, with x ' < x ) due to the loss of Si atoms in excess which diffuse through the silJcide towards the external surface to form the SiO 2 overlayer. As the duration of treatment increases, the second regime is reached (figs. 4b a n d 5b) and the surface oxide continuously grows, but the Ti peak spreads. -

191

A. Climent et al. / Oxide growth on silicides in oxygen plasma

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192

A. Climent et aL / Oxide growth 9n silicides in oxygen pla.,,'ma

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400

A. Climent et al. / Oxide growth on silicides in oxygen plasma

193

This indicates interaction of the silicide with the substrate and incorporation of Ti in the oxide. Thus going back to fig. 3, the analysis of RBS spectra shows that the second regime corresponds to the growth of an oxide mixture (SiO 2 and TiO2) while the first regime is associated with the formation of pure SiO2, i.e. Si atoms initially in the silicide in the substrate are incorporated into the oxide. 3.2. W silicide

As the case of plasma oxidation of Ti sificide samples, the kinetics of incorporation of oxygen atoms during plasma oxidation of WSiy shows two well-defined regimes (see fig. 6). The initial regime is slower and :,eems to follow a varia?ion similar to that of TiSi x or even bare Si. Nevertheless, as it shows this behaviour during a rathe]r short period of treatment, it is not possible to assign a particular law of oxide growth. In fact we obtain acceptable fits of the experimental data by a linear-parabolic law, a pure parabolic law ( d 2 - d . Z , = B t ) or even by a logarithmic law of the type d = d o log(1 + t / ~ ) . RBS analysis of these samples shows that the oxide grown in the first regime is pure SiO 2. A typical example (plasma oxidation during 2 h) is shown in fig. 7. The incorporation of oxygen atoms is clearly seen, and the pure SiO 2 surface layer is deduced from the shift towards lower energies of the W contribution, while the Si signal still remMns at its surface position. The increase in height of the W contribu60n and the corresponding decrease of the ,

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, 100

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500 , tlmin.

Fig. 6. Total contents of oxygen atoms (in units of 1015 O atoms/era 2) of the WSiy layer as measured by the nuclear reaction t60(d,p)lTo* versus treatment time. Experimental couditions: 700 o C, 300 W.

A Climent et al. / Oxide growth on silicides in oxygen plasma

194

Energy o.s 1.2xi0

(MeV)

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4. Conclm~om T o summarize this work we must note that we have shown that the plasma oyddation at floating potential of silicon-rich titanium a n d tungsten silicide can b e schematically described by a two-step process.

A. Climent et al. / Oxide growth on silieides in oxygen plasma

195

(i) First, it appears that the oxygen atoms incorpo, i t e d during the plasma treatment form a pure passivating SiO 2 overlayer. These SiO a metal free films o n the WSiy are formed by Si atoms in exce,.:s in the silicide until the W S i : composition is reached. T h e nature of the substrate (Si or SiO2) does not seem to have any effect. F o r the TiSi X films o n SiO 2, the behaviour is similar to the Wgiy films, b u t the final composition of the silicide lies between TiSi a n d TiSi 2. Moreover, when the substrate is pure Si, atoms coming from the substrate can take part in the formation of the SiO 2 overlayer. (ii) Further oxidation yields a high rate of oxide growth, and atomic d e p t h distributions show the presence of Ti or W cations in the oxide. In this last regime the silicide itself is oxidized leading to the formation of a n oxide mixture. In the first regime the oxide growth kinetics can be described by the same d 2 = B t law indicating that the limiting process in the diffusion of the mobile species. More work is needed to determine the nature of the migrating species (oxygen a n d / o r silicon) as well as the precise microscopic transport mechanism during oxide growth. At the present time, taking into account the presence of atomic oxygen in the plasma, one may expect some different mechanisms from that taking place during pure thermal oxide growth.

Ae~nowle4gmenis This work has been supported by the C N E T (convention C N S / C O N V / ll/RPT/MIM), b y the N A T O (fellowship to A. Climent), and by the C N R S (Greco No. 86).

References [1] J.R. Ligenza, J. Appl. Phys. 36 (1965) 2703. [2] E. Navarro, A. Climent, J.M. Martinez-Duart, B. Pellote, J. Perriere and J. Seijka, in: Proc. E-MRS, Strasbourg, 1986, p. 59. [3] J. Perriere, J. Siejka, A. Climent, E. Navarro and J.M. Martinez-DuarL J. Appl. Phys. 61 (1987) 2656. {4] CNET Patent No. 8318299 (1933). [5] G. Ams¢l, J.P. Nadai, E. d'Artemare, D. David, E. Girard and J. Mouiin, Nucl. Instr. Methods 92 (1971) 481. [6] L.R. Doolittle, Nucl. Instr. Methods B 9 (1985) 344. [7] B.F, Deal and A.S. Grove, J. Appl. Phys. 36 (1965) 3770.