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

The rapid growth of microelectromechanical systems (MEMS) industry has intr oduced a need for the characterization of thin film properties at all tempe ratures encountered during fabrication and application of the devices. A te chnique was developed to use MEMS test structures for the determination of the difference in thermal expansion coefficients (alpha) between poly-Si an d SiO2 thin films at high temperatures. The test structure consists of mult ilayered cantilever beams, fabricated using standard photolithography techn iques. An apparatus was developed to measure the thermally induced curvatur e of beams at high temperatures using imaging techniques. The curvatures me asured 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 h ad negligible effect on beam curvature; therefore, values from literature w ere used for E-Si and E-SiO2 without introducing significant error in curva ture analysis. Applying this approximation, the difference in thermal expan sion coefficients between alpha(Si) and alpha(SiO2) was found to increase f rom 2.9x10(-6) to 5.8x10(-6) degrees C-1 between room temperature and 900 d egrees C. These results suggest that the alpha for poly-Si thin films may b e significantly higher than values for bulk, crystalline Si. (C) 2000 Ameri can Institute of Physics. [S0021- 8979(00)04109-8].