Kinetics of WSi2 formation in the thin-film system W/PtSi/Si

Abstract

Alloying kinetics data on thin-film materials systems of interest in large-scale integration (LSI) technology are useful in the design of high-temperature processing schemes. In the present work, we have investigated the kinetics and mechanism of WSi2 formation in the thin-film system W/PtSi/Si, which is encountered in the Ohmic contact and Schottky barrier areas of silicon integrated circuits. In situ x-ray diffractometer measurements were carried out on various thin-film samples in the temperature range 690–840 °C. All the samples consisted of 2000-Å W film on 900-Å PtSi on 10-mils-thick n-type (100) Si doped with arsenic to a resistivity of 10 Ω cm. The amount of WSi2 formed was found to be equal to the amount of W consumed, indicating that an excess of Si atoms (diffusing species) is present at the reaction interface. The rate of transformation of W to WSi2 first increases with time and then it decreases. The data can be represented in terms of the generalized kinetic law: x = 1-exp[−K(T)tn], where x is the volume fraction of W transformed, K(T) is a temperature-dependent rate constant, t is time, and the index n is experimentally found to be equal to [inverted lazy s]1.35 in most cases of W/PtSi/Si samples annealed between 690 and 840 °C. A relatively high thermal activation energy of [inverted lazy s]4.4 eV/mole was deduced from an Arrhenius plot of half-time for transformation vs reciprocal of the annealing temperature. A comparison of the present results with those available on a W–Si thin-film system indicates that an intermediate layer of PtSi substantially accelerates the rate of formation of WSi2 and that PtSi may be also affecting the thermodynamic processes involved in WSi2 formation. It is suggested that the WSi2 nucleation process may involve, simultaneously, creation of W–Si bonds at the expense of Pt–Si bonds and restoration of the latter by out-diffusing silicon atoms. All the WSi2 nucleation sites at the W/PtSi interface are probably saturated before appreciable growth occurs.