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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 800 nm were deposited at with a pressure of 100 mTorr. For annealing investigations, films were annealed in a nitrogen atmosphere for 2 hours at temperatures between 600 and . The implantation doses of boron and phosphorus varied between and . This corresponds to a doping concentration of and . 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 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.

The effect of high-temperature annealing on Young's modulus Eand the intrinsic stress U of thin films made of LPcvo-polysilicon was investigated. The films were annealed for 2 hours in a nitrogen atmosphere at temperatures between 600 "C and 11 00 "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 a/ (1 990 Technical Digest, lEEE Solid-state Sensor and Actuator Workshop, Hilton Head Island, SC, USA p 70). LpcvD-polysilicon was produced with a SiH4 flow rate of 70 sccm and a total pressure of 100 mTorr at 620 "C. The film thickness was 460 nm. For the as deposited films the method of membrane deflection yields a Young's modulus of 151 f 6 GPa and an intrinsic stress of -350 f 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 "C. Young's modulus seems to increase a little with increasing annealing temperature up to 162 f a GPa at 1100 "C. The values for E and U 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.

A computerized measuring system is presented which permits determination of the elastic properties of thin films. Membrane theory yields a definite relationship between the deflection of a thin membrane and an external load. From analysis of this function, both Young's modulus and intrinsic stress can be determined independently of each other. The applicability of the measuring method will be demonstrated by investigations carried out on rectangular membranes of Dolvcrvstalline silicon and silicon nitride of various sizes.

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