3.1 INTRODUCTION

Titanium silicide is one of the first few silicides considered for application in ultra-large-scale integrated circuits (ULSI) owing to its low resistivity, good thermal stability and compatibility with Si processes. There are two ways of using titanium silicide in Si-based devices: polycide gate electrode/interconnect and contact/interconnect in self-aligned silicide (SALICIDE) process. This chapter will first describe the methods and mechanisms of titanium silicide formation and then address the relevant materials issues, integration concerns and scaling limits for technological implementation in deep-sub-micrometre device applications. Various schemes for extending the practical limit of titanium silicide will also be briefly mentioned.

3.2 FORMATION OF TITANIUM SILICIDES

There are several forms of titanium silicides with different crystal structures and materials properties. Among them titanium disilicide is of great interest to Si microelectronics as it offers the lowest resistivity and exhibits good thermal integrity during post-silicidation processing. Titanium disilicide can be formed by annealing of (a) co-evaporated or co-sputtered TiSi _x film ( x ~ 2) deposited over Si; or (b) reacting a Ti thin film with either single crystalline Si or polysilicon at elevated temperatures. The first method is used in the so-called polycide technology in which an amorphous titanium silicide alloy (TiSi _x, x ~ 2) is co-deposited on top of the doped polysilicon. The composite structure is then annealed at 700 800 C to form the low resistivity titanium disilicide phase. This approach is presently used for gate metallisation in DRAM process.

The...