A study of surface diffusion of metals in tungsten for NEMS applications

Diffusion of 14 different ~20 nm thick metals into 100 nm thick tungsten films at 700°C for up to 90 minutes was studied using atomic force microscopy and scanning electron microscopy. W-W yielded a restructured surface even at this relatively low (700°C) temperature (melting temperature of W is 3422°C). Al, Cr and Pt films completely diffused and even reduced (20-50 nm) the original W surface height in certain cases. The Al-W alloy had low contact resistance but slightly higher surface roughness (5 nm rms compared to 2 nm rms). Cu and Mo completely diffused into W but they did not reduce W´s original thickness. Fe, Ta, Au, Ni, Ag, Co, Ti and NiCrFe increased the apparent Metal-W thickness. We conclude that Al, Cr, and Pt can be used as etch-less sacrificial layers with desirable electrical contact properties.