Thermomechanical properties of amorphous silicon and nonstoichiometric silicon oxide films

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Abstract
The thermomechanical properties of glow-discharge-deposited silicon and silicon oxide films were measured between room temperature and 100 °C as a function of composition and substrate temperature during deposition. A cantilevered beam measurement technique, allowing the simultaneous measurement of the linear thermal expansion coefficient of thin films and their mechanical compliance, was used and verified. Hydrogenated amorphous silicon films, deposited at 250 °C with a density of 2.0 g/cm3 and 20 at. % of hydrogen, exhibit a linear thermal expansion coefficient of about 4.4×10−6/°C and a biaxial elastic modulus of 150 GPa. The expansion coefficient of silicon oxide films deposited at 250 °C shows a systematic dependence on the fabrication conditions and ranges from about 10−5/°C to negative values. Strong correlations between the hydrogen concentration of the films, the film density, and thermal expansion coefficient were observed and are discussed. The biaxial elastic modulus of the oxide films is not strongly dependent on the gas ratio and is about 40 GPa, substantially smaller than the values obtained for thermally grown oxides. These differences are attributed to the effect of network terminations by hydrogen and –OH groups. Films of fixed composition (nitrous oxide/silane=12), deposited at substrate temperatures below 200 °C, exhibit an apparent negative coefficient of thermal expansion when first heated. However, this contraction has been determined to be due to the expulsion of water vapor, causing these films to densify. The true thermal expansion coefficients, measured after annealing at 100 °C for extended periods, are positive and do not change within experimental error for this fixed composition over the range of deposition temperatures.

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