A Novel Method for the Measurement of Thermal Expansion of Thin Film using Laser Scanning Confocal Microscopy

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Abstract

For the fabrication of Micro Electro-Mechanical System (MEMS), film fabrication process combined with photolithography is thought to be one of the fundamental techniques. However, the physical properties of films and membranes strongly depend on their processing routes. For designing MEMS as engineering products, the control of physical properties such as the coefficient of thermal expansion (CTE) and hardness are important. However the CTE measurement of thin films is very difficult because the magnitude of displacement to be measured is about 1⁄10000 of the thickness of the specimen, i.e., of the order of nano-meter for specimens with micron-meter thickness. In this study the measurement of CTE of several ten micron meter-thick metal thin film based on an optical interference was attempted using a Laser Scanning Confocal Microscope (LSCM). The specimen was set between two alumina single crystal plates as optical flats. He-Ne laser is reflected from the bottom surface of the upper optical flat (which is transparent) and the top surface of the lower optical flat, and these two rays interfere either constructively or destructively, resulting in the formation of interference fringes. It was shown that by counting the number of fringes observed using LSCM, the expansion of a specimen as thin as a few-tenth micron meter can be detected.

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