SURFACE A N D INTERFACE ANALYSIS, VOL. 24, 416-418 (1996)

Effect of Ion Beam Mixing and Compound Formation on Sputter Depth Profile of a Ta/Si Multilayer Thin Film

M. G. Stepanova Keldysh Institute for Applied Mathematics R.A.S., Miusskaya p1.4,125047, Moscow, Russia

Surface composition changes of a Ta/Si multilayer thin film during 3 keV Ar+ ion sputtering are studied theoreti- cally using a model based on the linear transport theory in the diffusion approximation. The influence of collisional mixing, preferential sputtering, dynamic relaxation and silicide formation on the shape of the sputter depth profile is considered. It is shown that the silicide formation at Ta/Si and Si/Ta interfaces can be responsible for the abrupt changes of slope of sputter depth profiles found experimentally for the Ta/Si film.

INTRODUCTION

Ion beam-induced compositional changes are known to be a crucial factor affecting the accuracy of sputter depth promng of A/B interfaces and multilayer films.'-6 Ion beam mixing leads to an interface broadening and preferential sputtering additionally changes the sample's surface composition. As a result, experimental Auger electron spectroscopy (AES) or x-ray electron spectroscopy (XPS) sputter depth profiles of A/B interfaces differ from real composition-depth profiles in the samples. Theoretical approaches which describe the interface broadening have been reviewed many times.'-6 There also are a number of dynamic Monte-Carlo simulations reported in the literature which readily reproduce the effect of collisional mixing and preferential sputtering on sputter depth profiles for A/B interfaces.'-''

However, in certain cases experimental sputter depth profiles exhibit changes of slope which cannot be explained by collisional mixing and preferential sputter- ing alone. Figure 1 shows an XPS sputter depth profile for a Ta/Si multilayer film from Ref. 11 as an example. Clearly seen are abrupt changes of the slope in the

To 80 -

- t 10- To L 6 0 -

.I -

5 0 -

5 4 0 - Si f 30-

10 -

- : 2 0 - 0

-0 0 20 30 40 50 6 0 7 0 80

Sputter time ( m i d - Figure 1. Concentration XPS sputter depth profile of the Ta/Si multilayer thin film obtained by 3 keV Ar+ ions at 45" incidence. Reprinted with permission from Fig. 5 in Ref. 11.

CCC 0142-2421/96/060416-03 0 1996 by John Wiley & Sons, Ltd.

increasing regions of the silicon profile and in the decreasing regions of the tantalum profile. This effect was attributed to TaSi, formation at Ta/Si and Si/Ta interface~.~.'' Sharp changes of the slope of sputter depth profiles were observed also in other multilayer films, e.g. in Ni/Cr/Si,'2 Ni/CrNi,13 Mo/S~/MO.'~*'~

Although ion beam-induced chemical effects in multi- component targets are widely discussed in the liter- ature,'6-2' the relation of the observed changes of the slope with compound formation is not yet completely clear. In this work we propose a model which allows the effects of collisional mixing, preferential sputtering, dynamic relaxation and silicide formation to be studied on sputter depth profiles of A/B interfaces and we apply the model to explain the asymmetric shape of the sputter profile for the Ta/Si multilayer film. At the same time, the qualitative results discussed below are of general character and can be applied for a wide number of layered films.

THE MODEL

The model used in this work is based on the linear transport theory in the diffusion appr~ximation. '~-~~ The dependence of the concentration Ci(x, t ) of com- ponent i on depth x and sputter time t in a binary film is defined by numerical solution of

a a at ax - CAX, t ) = -d(x)JO Yic N-'CXO, t) + V(t) - C ~ ( X , t)

where i = Ta or Si. The first term on the right-hand side of Eqn (1)

accounts for sputtering from the targets surface: YF is the sputtering yield of component i ; f, is the density of the ion current: N is the targets atomic density near the surface; and d(x) is the delta-function. In the second term, V(t) is the rate of surface recession due to sputter- ing.

Collisional mixing and dynamic rela~ation,~ are included in the model through the coefficients R(x, t )

Receiued 22 December 1995 Accepted 8 February 1996

SPUTTER DEPTH PROFILE OF A MULTILAYER FILM 417

and &x, t). In our previous paper" it has been shown that

with i # j . The quantity Vriyx) in Eqn (2) and the quantity D p 4 x ) in Eqn (3) are familiar ones for the dif- fusion approximation in the linear transport

(x) accounts for the directed part of recoil flux due to collisional mixing, and D p ( x ) is the effective diffusion coefficient also associated with mixing. The dependencies V p ( x ) and D p ( x ) were obtained by optional numerical solution of master equations for Ta and Si recoil collision cascade^^^*^^ in a model homogeneous target TaSi, . The numerical pro- cedure has been described in detail in Ref. 25. Note that performing the calculations for the TaSi, target does not lead to serious limitations in the applicability of the resulting functions e ( x ) and Drix(x), the latter being normalized to the target's composition by definiti~n.'~

The relaxation parameter R , in Eqns (2) and (3) depends on the 'genuine' diffusion coefficients of com- ponents, D, and on their atomic volumes R: R,= Ri DJRj D j . Inclusion of the relaxation parameter R, is a distinctive feature of the which assumes a diffusional mechanism of stress relaxation as discussed in Ref. 25. Elastic stresses created in the target by col- lisional mixing relax through the directed part of diffu- sion fluxes enhanced by the stresses, and preferentially due to the component which has a greater product Ri Dp25,26 The special case of R, = 1 corresponds to the well-known homogeneous (or stoichiometric) relaxation which is used in the linear transport theory.24 In this paper Rij is treated as a phenomenological parameter.

Numerical solution of Eqn (l), with account for Eqns (2) and (3), gave the dependencies of the surface concen- trations C,,(O, t) and Csi(O, t) on the sputter time t of a multilayer film. The sputtering conditions were similar to that in Ref. 11: 3 keV Ar' ions incident at 43" with respect to the surface normal. The density of ion current was 3.5 x 1013 ions cm-, s-'. The model sample com- prised five double Si/Ta layers on an Si substrate. Each double layer was 20 nm thick, with the ratio of elemen- tal layer thicknesses dTddsi = 0.7, which is close to the sample's characteristics in Ref. 11.

RESULTS A N D DISCUSSION

theory;22-24 p i x

Figure 2 shows the theoretical concentration profile obtained in the simplest approximation. Accounted for are collisional mixing, homogeneous relaxation (RTasi = 1) and preferential sputtering. The sputtering yields Y& and Y$ were computed for pure tantalum and silicon, by solution of master equations for collision cascades discussed in previous paper^.'^.'^ The surface binding energies were equal to the sublimation energies of the pure elements: U,, = 8.08 eV and Usi = 4.64 eV. This gave the yields Y;, = 1.3 and Ygi = 2.7, under the sputtering conditions specified above. During the calcu- lations of the sputter depth profile, the atomic volumes of the components, RTa and Rsi , were equal to 0.02 nm3,

' 20 ' 4b ' 60 ' 8 ' 1bO' 120' Sputter time t ( m i n l

Figure 2. Theoretical concentration profiles C,,(O, t ) and C,,(O, t ) for the five-layer Ta/Si film bombarded by 3 keV Ar+ ions at 43" incidence.

which is close to the mean atomic volumes of pure tan- talum and silicon (0.018 nm3 and 0.021 nm3, respectively). The atomic volumes affect the rate V(t) in Eqn (1) through the relation

The extent of interface broadening obtained theoreti- cally approaches the experimental one. However, the profile in Fig. 2 is only of slightly asymmetric shape. It is best seen by comparing the profile's slope at its begin- ning (t < 10 min) and at the end (t > 120 min). Such profle asymmetry is a consequence of collisional mixing. This effect is adequately reproduced by Monte- Carlo simulati~ns.~-l~

Figure 3 demonstrates the effect of non- stoichiometric relaxation on the shape of the concentra- tion profile. The dependencies C,,(O, t) and Csi(O, t) shown in Fig. 3 were obtained at RTai = 0.2. In Refs 25-28 it has been shown that the non-stoichiometric relaxation entails segregation effects in the altered layer of ion-bombarded targets. From Fig. 3 it can be seen that in our case the non-stoichiometric relaxation makes the profile's asymmetry more pronounced. But the theoretical profile is still lacking the changes of slope observed experimentally in Ta/Si films.

Figure 4 shows the concentration profile obtained by taking into account the chemical effects near Ta/Si and Si/Ta interfaces. It was assumed that tantalum silicide is formed at time t at depth x in the target, provided that

59% < CSi(X, t) < 95% (5 ) The silicide formation was supposed to change the com- ponent sputtering yields Y:, and Ygi. As a result, the

Figure 3. Theoretical concentration profiles C,,(O, t ) and C,,(O, t ) for the Ta/Si multilayer film obtained by taking into account the non-stoichiometric relaxation R,,,, = 0.2.

418

i ' io ' do ' 60 ' ao ' 160' 120' 140' 160' 180'260

Sput ter time t ( m l n l

4 1 i 0 * 1'0 ' 2'0 ' j0 ' do ' 5'0 60 70

Sput ter t i m e t ( m i n )

Figure 4. Theoretical concentration profiles C,,(O, t ) and C,,(O, t ) (a) and their fragments (b) for the Ta/Si multilayer film, obtained by taking into account the silicide formation near the interfaces.

magnitude of the sputtering yields was jumping at regular intervals in the course of the film's sputtering, when the boundaries of the regions defined by Eqn (5 ) were reached the surface. Owing to the effect of prefer- ential sputtering, the change of Y;, and Y:i affected the

surface composition and the shape of the sputter depth profile. In our calculations, sputtering yields in the regions of the silicide formation of YFa = 1.3 and Ygi = 1.9 were chosen phenomenologically to fit the shape of the experimental sputter profile in Fig. 1. Atomic volumes OTa and RSi in Eqn (4) also were supposed to change with the silicide formation, becoming equal to 0.015 nm3 which corresponds to the mean atomic volume in TaSi, . The relaxation parameter RTai = 0.5 was used. Figure 4 shows that the resulting concentra- tion profile is of nearly the same shape as obtained experimentally.

CONCLUSIONS

M. G. STEPANOVA

REFERENCES

The results of calculations show that ion beam mixing and particularly dynamic relaxation can result in an asymmetric shape of sputter profiles for Sifra and Ta/Si interfaces, but they do not give rise to the abrupt changes of profile slope observed experimentally. However, owing to collisional mixing wide regions of interpenetration of the components arise where tanta- lum silicide is formed. Formation of the silicide changes the sputtering yields of components, which affects the composition profile through the effect of preferential sputtering. The sequence 'ion bombardment + interface broadening + silicide formation + change of sputtering yields + effect on surface composition' reproduced by the model calculations allows a theoretical sputter depth profile of nearly the same shape as observed experimentally, to be obtained.

Acknowledgements

This work has been partially supported by the Russian Foundation for Basic Research, Grant No. 93-01-01687.

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