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1 October 2005 Low-stress ultra-thick SU-8 UV photolithography process for MEMS
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Patterning thick SU-8 with conventional photolithography facilities is important for fabricating various MEMS structures. However, the fabrication of thick SU-8 MEMS has experienced severe problems such as cracks, distortions, or delaminations during the fabrication process and/or postservices, due to the large internal stress generated during the photolithography process. In this work, an in-depth finite element analysis (FEA) is performed to investigate the causes and effects of the internal stresses. Analytical results show that the post-exposure bake (PEB) temperature is the main factor in developing the resulted internal stress. Under the guidance of analytical results, an optimized UV photolithography process for the fabrication of ultra-thick low-stress SU-8 patterns is developed with conventional (simple) equipment. A low PEB temperature of 55°C reduces the internal stresses by more than 70% compared to those fabricated with the recommended procedure. Experimental results indicate that cracks, distortions, and delaminations are eliminated from the fabricated SU-8 structures using the newly developed procedure. In addition, the patterned SU-8 has a Young's modulus of 2.5 GPa and an ultimate strength of 50 MPa, which is about 50% higher than previous reported values.
©(2005) Society of Photo-Optical Instrumentation Engineers (SPIE)
Bo Li, Miao Liu, and Quanfang Chen "Low-stress ultra-thick SU-8 UV photolithography process for MEMS," Journal of Micro/Nanolithography, MEMS, and MOEMS 4(4), 043008 (1 October 2005).
Published: 1 October 2005

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