TIN AS A PHOSPHORUS OUTDIFFUSION BARRIER LAYER FOR WSIX DOPED-POLYSILICON STRUCTURES

Phosphorus-doped amorphous or polycrystalline silicon can yield a conf ormal, low resistance, thermally-stable plug for the high-aspect-ratio , sub-half-micron contact-holes found in current development prototype s of future 64 and 256 Mega-bit DRAMs. When directly contacted to a si licide layer, however, such as WSi(x) found in polycide gate or bit li ne metallization/contact structures, the outdiffusion of phosphorus fr om the doped-silicon layer into the silicide can occur, resulting in a n increase in resistance. The characteristics of both the doped-silico n and WSi(x) layers influence the outdiffusion. The grain size of the doped silicon appears to control diffusion at the WSi(x)/doped-silicon interface while the transition of WSi(x) from an as-deposited amorpho us to a post-annealed polycrystalline state appears to help cause unif orm phosphorus diffusion throughout the silicide film. The results of phosphorus pre-doping of the silicide to reduce the effects of outdiff usion are dependent upon the relative material volumes and interfacial areas of the layers. Due to the effectiveness of the TiN barrier laye r/Ti contact layer structure used in Al-based contacts, Ti and TiN wer e evaluated on their ability to prevent phosphorus outdiffusion. Ti re acts easily with doped silicon and to some extent with WSi(x), thereby allowing phosphorus to outdiffuse through the TiSi(x) into the overly ing WSi(x). TiN, however, is very effective in preventing phosphorus o utdiffusion and preserving polycide interface smoothness. A WSi(x)/TiN /Ti metallization layer on an in situ-doped (ISD) silicon layer with I SD silicon-plugged contact-holes yields contact resistances comparable to P+-implanted or non-implanted WSi(x) layers on similar ISD layers/ plugs for contact sizes greater than approximately 0.5 mum but for con tacts of 0.4 mum or below the trend in contact resistance is lowest fo r the polycide with TiN barrier/Ti contact interlayers. A 20 nm-thick TiN film retains its barrier characteristics even after a 4-hour 850-d egrees-C anneal and is applicable to the silicide-on-doped-silicon str uctures of future DRAM and other ULSI devices.