ANALYSIS AND MODELING OF IN-SITU BORON-DOPED POLYSILICON DEPOSITION BY LPCVD

The low pressure chemical vapor deposition of in situ boron-doped polysilicon films from a mixture of SiH4, and BCl3 was investigated in a tubular horizontal hot-wall reactor. Only when isothermal conditions and no distributed injection scheme are used, this process presents the following features: (i) the presence of the boron doping gas enhances the deposition rate of polysilicon; and (ii) a sharp drop in the polysilicon growth rate and boron concentration along the load is generally observed which may be detrimental to the reactor yield. Based on a combined approach of modeling and experimental results, an understanding of the complex physicochemical phenomena involved is presented. In particular, the accelerating effect of boron and the slowing down effect of hydrochloric acid, one of the products of chemical reactions, are demonstrated. The global model developed, based on a continuously stirred tank reactor approach for mass balances, has proven to be a valuable tool for determining the effects of process parameters, and specially the influence of the X(BCl3)/X(SiH4), ratio on deposition rate. An adequate solution for solving the practical problem of depositing boron-doped polysilicon on a large number of wafers with a satisfying uniformity is proposed, which requires a reactant injection scheme and temperature ramping.