UHV CHEMICAL VAPOUR DEPOSITION OF UNDOPED AND IN-SITU DOPED POLYSILICON FILMS

The use of low pressure chemical vapour deposition (LPCVD) for the growth of thin films of polysilicon is widely used in integrated circuit fabrication processes. For several emerging technologies, chiefly large-area displays and multilayer memories, the crystalline quality of the as-deposited film is of paramount importance, as the polysilicon is the active semiconductor layer. It is important, in these applications, to lower the deposition temperature to avoid damage either to the substrate or circuit elements already present. The dependence of film morphology on the deposition rate, gas flow rate and reactor temperature is discussed. A reduction in pressure from typical LPCVD pressures (around 200 mTorr) causes the transition temperature to be lowered, allowing a crystalline film to be grown at relatively low temperatures. To achieve this, a special UHVCVD system has been constructed to investigate layer growth at pressures down to 0.2 mTorr. Details of work carried out on the growth of in-situ phosphorous doped polysilicon are discussed. Uniform phosphorous doped polysilicon films are grown without the need for wafer cages and/or complex gas injectors. At deposition temperatures in the region of 630°C and reactor pressures of less than 5 mTorr uniform doped polysilicon can be deposited. The resistivity and dopant concentrations can be controlled by altering the phosphine-to-silane gas flow ratio