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22 January 2005 Temperature rise of the silicon mask-PMMA resist assembly during LIGA exposure
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Abstract
Deep X-ray lithography on PMMA resist is used in the LIGA process. The resist is exposed to synchrotron X-rays through a patterned mask and then is developed in a liquid developer to make high aspect ratio microstructures. This work addresses the thermal analysis and temperature rise of the mask-resist assembly during exposure at the Advanced Light Source (ALS) synchrotron. The concern is that the thermal expansion will lower the accuracy of the lithography. We have developed a three-dimensional finite-element model of the mask and resist assembly. We employed the LIGA exposure-development software LEX-D and the commercial software ABAQUS to calculate heat transfer of the assembly during exposure. The calculations of assembly maximum temperature have been compared with temperature measurements conducted at ALS. The temperature rise in the silicon mask and the mask holder comes directly from the X-ray absorption, but forced convection of nitrogen jets carry away a significant portion of heat energy from the mask surface, while natural convection plays a negligible role. The temperature rise in PMMA resist is mainly from heat conducted from the silicon substrate backward to the resist and from the mask plate through inner cavity air forward to the resist, while the X-ray absorption is only secondary. Therefore, reduction of heat flow conducted from both substrate and cavity air to the resist is essential. An improved water-cooling block is expected to carry away most heat energy along the main heat conductive path, leaving the resist at a favorable working temperature.
© (2005) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Aili Ting "Temperature rise of the silicon mask-PMMA resist assembly during LIGA exposure", Proc. SPIE 5715, Micromachining and Microfabrication Process Technology X, (22 January 2005); https://doi.org/10.1117/12.590909
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