Elastic properties and microstructure of LPCVD polysilicon films

This paper investigates the influence of film thickness, high-temperature annealing and doping by ion implantation on Young's modulus and the residual stress of LPCVD polysilicon. Films with thicknesses between 100 nm and 300 nm were deposited at 620 degrees C with a pressure of 100 mTorr. For annealing investigations, films were annealed in a nitrogen atmosphere for 2 hours at temperatures between 600 and 1100 degrees C. The implantation doses of boron and phosphorus varied between 1 x 10(11) cm(-2) and 2 x 10(16) cm(-2). This corresponds to a doping concentration of 3 x 10(15) cm(-3) and 7 x 10(20) cm(-3). Young's modulus and the residual stress were determined by load-deflection measurements with suspended membranes and by the use of ultrasonic surface waves. The microstructure of the film and grain size was studied by TEM analyses and texture variations were investigated by x-ray deflection. Although Young's modulus was found to be very stable, it showed a small dependence on film thickness and annealing temperature. It varied between 151 GPa and 166 GPa. The residual stress could be strongly influenced by film thickness (-420 MPa to -295 MPa), annealing temperature (-350 MPa to -20 MPa) and ion implantation (-560 MPa to +30 MPa). The as-deposited film always showed compressive stress, a pronounced (110) texture and a grain size of around 55 nm. Strong correlations between the variations of the elastic properties and the variations in the film thickness, annealing temperature, grain size, mass density and refractive index were found. These correlations and the observed microstructure are used to develop a model for the origin of the compressive stress and for the mechanism of stress variation. A theoretical value for Young's modulus of textured polySi was calculated and corresponds well with the measured values.