Plasma Etching of Tungsten Polycide Structures Using  NF 3 ‐ Mixed Halocarbon Etchants

Abstract

The feasibility of utilizing NF3-mixed halocarbon etchants to anisotropically etch tungsten polycide structures has been investigated. Chemical vapor deposited tungsten silicide and low-pressure chemical vapor deposited polysilicon have been etched using various mixtures of CF3C1, CF2C12, and CF3Br with NF~. Anisotropy, undercutting, and notching were evaluated using cross-sectional scanning electron microscopy. Etch rates and profiles were controlled by utilizing halocarbon-rich (>50%) binary halocarbon etchants. The etch rate data indicate that the volatility of the tungsten halide formed during etching influences the silicide etch rate and profile while the polymer forming ability of the halocarbon controls the etch characteristics of the polysilicon. BackgroundRefractory metals, such as tungsten and molybdenum, and their silicides have attracted considerable attention as materials for high-density, high-speed integrated circuits (ICs). Disilicides have been shown to be structurally compatible gate electrodes for SiQ (1). However, the electrical properties of the silicide/oxide interface are considered to be unacceptable for actual device applications (42, 43). Doped-polysilicon gates provide excellent interfacial characteristics but exhibit an unacceptably high resistance as linewidths shrink below 1 ~m. Silicide/polysilicon (polycide) structures exhibit low resistances (due to the silicide) while retaining the interfacial characteristics of the poly-Si. The silicides of Mo, W, Ta, and Ti have been considered as gate materials due to their rather good compatibility with IC processing requirements (2).Critical to the fabrication of useful silicide or polycide gates is the ability to obtain anisotropic etch profiles. Initial work focused on the use of fluorinated plasmas, based on CF4 (31, 43), SF6 (15, 28), and NF3 (11-15), primarily for etching Mo and MoSi2 (8-10). Chlorinated and mixed fluoride/chloride etchants were also reported (15-18). More recently, emphasis has been placed on the utilization of tungsten and WSi 2 in device fabrication (19-42), particularly in conjunction with polycide structures (31-35). Again, fluoride-(21-31) and chloride-based (36-42) etchants were utilized for patterning these materials. Plasmas containing CF4, SF6, CI~, and BC13 have been used for this purpose. Missing from the literature is a comprehensive study of the use of NF3 as a perspective etchant.Pure NF3 plasmas selectively but isotropically etch silicon and polysilicon over SiO~ (50:1 and 15:1, respectively) at rates greater than 1 ~/min (3-7). The addition of halofluorocarbons to NF3 (at >50%) provides sufficient sidewall passivation to give directional-to-anisotropic profiles (3-5). NF3 etching of NbSi, Mo, MoSi2, and polysilicon has also been reported (11-15). Data for NF3-etched WSi2 is limited. Etch rates on the order of 4000/~Jmin for WSi~ on GaAs (parallel plate RIE system), with etch selectivities of 100:1 and 25:1 to GaAs and SIO2, respectively, have.been re-) unless CC License in place (see abstract). ecsdl.or...