Effect of phosphorus doping on stress in silicon and polycrystalline silicon

An investigation of the polysilicon stress properties as a function of film thicknesses and phosphorus doping showed that as-deposited films are moderately compressive, and become less compressive with increasing film thickness. High temperature PBr3 diffusion in silicon produces wafer bending corresponding to a tensile stress in wafer. Following a PBr3 diffusion, polysilicon films, however, become less compressive. Subsequent oxidation introduces an additional compressive stress component of the order of 2−3×109 dyne/cm2 for oxidation temperatures between ∼900−1000 °C. The thermal expansion coefficients were similar for doped and undoped films (α∼2.9 ppm/°C) and slightly less than for 〈100〉 silicon, while the doped films were found to be less stiff than undoped ones but both were less stiff than 〈100〉 silicon. The observed changes in polysilicon stress due to film thickness and phosphorus doping have been interpreted in terms of a grain growth model wherein those factors which lead to enhanced grain growth also result in reduced compressive stresses.