Thermal expansion coefficient of polycrystalline silicon and silicon dioxide thin films at high temperatures

The rapid growth of microelectromechanical systems (MEMS) industry has introduced a need for the characterization of thin film properties at all temperatures encountered during fabrication and application of the devices. A technique was developed to use MEMS test structures for the determination of the difference in thermal expansion coefficients (α) between poly-Si and SiO2 thin films at high temperatures. The test structure consists of multilayered cantilever beams, fabricated using standard photolithography techniques. An apparatus was developed to measure the thermally induced curvature of beams at high temperatures using imaging techniques. The curvatures measured were compared to the numerical model for multilayered beam curvature. The model accounts for the variation in thermomechanical properties with temperature. The beams were designed so that the values of Young’s moduli had negligible effect on beam curvature; therefore, values from literature were used for ESi and ESiO2 without introducing significant error in curvature analysis. Applying this approximation, the difference in thermal expansion coefficients between αSi and αSiO2 was found to increase from 2.9×10−6 to 5.8×10−6 °C−1 between room temperature and 900 °C. These results suggest that the α for poly-Si thin films may be significantly higher than values for bulk, crystalline Si.