IN-SITU PHOSPHORUS-DOPED VLPCVD POLYSILICON LAYERS FOR POLYSILICON THIN-FILM TRANSISTORS

Polysilicon devices on glass substrates for large-area applications, s uch as poly-Si thin-film transistors in active-matrix displays, need a complete low-temperature process, especially to fabricate the drain a nd source polysilicon layers as well as the active channel layer. For this purpose, we have developed a very low pressure chemical vapour de position process allowing in-situ phosphorous doping. By varying total pressure and phosphine/silane ratio, we control the doping concentrat ion level over a large range (10(18) to 5 x 10(20) cm-3). Depending on deposition conditions, films are first amorphous or partially crystal lised. The films are then fully crystallised by a 12 h in-situ vacuum annealing at 600-degrees-C. They are physically and electrically chara cterised. It is observed that in the 30 to 90 pascal pressure range, t he dopant activation rate, electrical carrier mobility, and conductivi ty of the layers are optimised whatever the doping level. First runs o f low temperature processed TFTs involving in-situ highly doped source and drain layers have given promising results.