Phosphorus-doped amorphous or polycrystalline silicon can yield a conformal, low resistance, thermallystable plug for the high-aspect-ratio, sub-half-micron contactholes found in current development prototypes of future 64 and 256 Mega-bit DRAMs. When directly contacted to a silicide layer, however, such as WSix found in polycide gate or bit line metallization/contact structures, the outdiffusion of phosphorus from the doped-silicon layer into the silicide can occur, resulting in an increase in resistance. The characteristics of both the doped-silicon and WSix layers influence the outdiffusion. The grain size of the doped silicon appears to control diffusion at the WSix/doped-silicon interface while the transition of WSix from an as-deposited amorphous to a post-annealed polycrystalline state appears to help cause uniform phosphorus diffusion throughout the silicide film. The results of phosphorus pre-doping of the silicide to reduce the effects of outdiffusion are dependent upon the relative material volumes and interfacial areas of the layers. Due to the effectiveness of the TiN barrier layer/Ti contact layer structure used in Al-based contacts, Ti and TiN were evaluated on their ability to prevent phosphorus outdiffusion. Ti reacts easily with doped silicon and to some extent with WSix, thereby allowing phosphorus to outdiffuse through the TiSix into the overlying WSix. TiN, however, is very effective in preventing phosphorus outdiffusion and preserving polycide interface smoothness. A WSix/TiN/Ti metallization layer on an in situ-doped (ISD) silicon layer with ISD silicon-plugged contactholes yields contact resistances comparable to P+-implanted or non-implanted WSix layers on similar ISD layers/plugs for contact sizes greater than approximately 0.5 µm but for contacts of 0.4 µm or below the trend in contact resistance is lowest for the polycide with TiN barrier/Ti contact interlayers. A 20 nm-thick TiN film retains its barrier characteristics even after a 4-hour 850 anneal and is applicable to the silicide-on-doped-silicon structures of future DRAM and other ULSI devices.