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My Research is on the failure, fracture and reliability of thin film materials for MEMS. In my Ph.D. dissertation research, I designed experiments and conducted mechanical and fracture property measurements of thin film materials, viz., polycrystalline silicon (polysilicon) and amorphous diamond-like carbon (ta-C) employed in general MEMS applications, and gold (Au) and platinum (Pt) employed in RF-MEMS because of their good electrical conductivity and reduced mechanical stiffness. Large part of my research focused on the mode I and mixed mode I/II fracture of the aforementioned brittle materials. Fracture strength measurements of ceramic materials are stochastic in nature and cannot be used as failure criteria. Fracture toughness on the other hand is a material property, which is independent of the specimen geometry and includes the effect of the micro/nano structure. I designed a microscopic analogue of conventional edge crack fracture experiments and performed fracture toughness measurements in both polysilicon and ta-C MEMS scale specimens to be used as failure and reliability criteria. On the other hand, Au and Pt are both used widely in MEMS/NEMS industry as structural materials as well as electrodes because of their high electrical and thermal conductivity. However, the performance of metallic MEMS devices depends strongly on the rate of loading. In particular, Au is known to exhibit rate dependent behavior. Until my research, there were no extensive studies on the rate dependence of thin films for MEMS applications. I developed a new method to perform tensile tests on metallic films at varying loading rates and obtained a full-field measurement of strain in these films. The method employs optical microscopy, but the resolution achieved is comparable to experiments conducted in an SEM or TEM. |