The Effects of N 2 and Ar as the Ambient Gas During Rapid Thermal Annealing of Tungsten Silicide
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THE EFFECTS OF N2 AND Ar AS THE AMBIENT GAS DURING RAPID THERMAL ANNEALING OF TUNGSTEN SILICIDE. Paul Martin Smith and Michael 0. Thompson Department of Materials Science, Cornell University, Ithaca, NY 14853 ABSTRACT Rapid thermal annealing of amorphous tungsten silicide thin films (WO. 62 Si 0 .3 8) on Si at 1100'C in Ar and N2 ambient gases was studied. All films annealed in N2 exhibited good adhesion and remained smooth with no large grain growth. Films annealed in Ar showed grain nucleation and growth from the film perimeter until a critical stress was achieved, followed by stress-assisted grain growth. The resulting growth of large columnar grains caused the films to delaminate. Removal of the surface oxide/nitride layer from the silicide prior to annealing resulted in uniform nucleation and growth over the entire film during annealing in Ar, with no stress-assisted grain growth or film delamination. No difference, however, was observed during annealing in N2 . These results suggest that a surface film produced during annealing in N2 slows the nucleation and growth and consequently enhances film adhesion. INTRODUCTION As semiconductor device dimensions continue to be reduced, the processing complexity increases and metallization systems must tolerate higher processing temperatures while continuing to provide suitably low resistivities. The refractory metal silicides, possessing both high temperature stability and low resistivity, show considerable promise for future devices.' One of the most widely studied refractory silicides is tungsten disilicide (WSi 2 ), which can be deposited by chemical vapor techniques, codeposition, or by reaction of W with a Si substrate. As well as acting as a metallization layer, the silicide also forms a stable diffusion barrier between Si and Al at moderate temperatures. 2 Also, W can be selectively deposited on bare silicon while nucleation and precipitation are prevented on 3 Si0 2 surfaces. Further, WSi2 is stable enough to allow oxidation of Si from the substrate through the silicide.' Several problems, however, are associated with the WSi 2 system. Oxygen impurities at the W-Si interface are known to significantly inhibit the silicidation, resulting in an increase of the reaction temperature from 600°C to approximately 6 10000C.5, Additionally, WSi 2 is not a diffusion barrier to Si at high temperatures, as evidenced by the oxidation of Si through the silicide. Thus, undesirable silicide7 reactions with other metallization layers may occur during high temperature processing. In the W-Si equilibrium phase diagram, two compounds are observed: a complex W5Si3 (tetragonal) phase and a silicon-rich WSi 2 (cubic) phase.' The thin film reaction sequence0 for W deposited on Si is still uncertain, although the final phase appears to be WSi2.9,1 Multilayer depositions of Si and W with a capping layer of Si react to form only WSi 2 at temperatures near 600'C.1" This low reaction temperature is thought to result from oxygen-free interfaces. Tungsten deposited directly on a chemically o
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