VARIATIONS IN YOUNGS MODULUS AND INTRINSIC STRESS OF LPCVD-POLYSILICON DUE TO HIGH-TEMPERATURE ANNEALING

The effect of high-temperature annealing on Young's modulus E and the intrinsic stress sigma of thin films made of LPCVD-polysilicon was investigated. The films were annealed for 2 hours in a nitrogen atmosphere at temperatures between 600 degrees C and 1100 degrees C. Then Young's modulus and the intrinsic stress were determined by the membrane deflection method. An extended analytical theory for the membrane deflection was developed and the results correspond well with FEM analysis of Pan J Y et al (1990 Technical Digest, IEEE Solid-State Sensor and Actuator Workshop, Hilton Head Island, SC, USA p 70). LPCVD-polysilicon was produced with a SiH4 flow rate of 70 seem and a total pressure of 100 mTorr at 620 degrees C. The film thickness was 460 nm. For the as deposited films the method of membrane deflection yields a Young's modulus of 151 +/- 6 GPa and an intrinsic stress of -350 +/- 12 MPa. After annealing at temperatures higher than the deposition temperature the compressive stress started to decrease with increasing annealing temperature. It relaxed nearly completely after annealing at 1100 degrees C. Young's modulus seems to increase a little with increasing annealing temperature up to 162 +/- 8 GPa at 1100 degrees C. The values for E and sigma obtained with the membrane deflection method were compared with the values obtained by the method of ultrasonic surface waves. The method of ultrasonic surface waves yields systematically higher values for E. The discrepancy can be explained by the uncertainty of Poisson's ratio of polysilicon.