INFLUENCE OF THE DOPING GAS ON THE AXIAL UNIFORMITY OF THE GROWTH-RATE AND THE ELECTRICAL-PROPERTIES OF LPCVD IN-SITU DOPED POLYSILICON LAYERS

We report in-situ doping of polysilicon grown on glass substrates by a LPCVD process. The growth rate, the resistivity and the dopant concentration of boron in-situ doped polysilicon layers are studied as a function of the deposition pressure and the dopant gas to silane mole ratio. A dependence of the axial uniformity on pressure and B2H6/SiH4 mole ratio is put forward, and this effect appears to be very strong especially at high pressure. It is explained by a lowering of the diborane concentration in the gas mixture along the load, because of a different threshold for the thermal decomposition of diborane and silane. It is also put forward that a critical concentration of boron exists above which the growth rate is increasing. Improvements of the horizontal homogeneity are obtained by varying the total gas now rate.