A study of thick PMMA film etching by the method of combined deep x-ray lithography and thermal development

For mass-production of three-dimensional fine structures, photolithography using deep X-ray with very short wavelength from a synchrotron radiation (SR) light source is a good method. After SR irradiation, the X-ray exposure mask pattern is engraved into the surface of the polymethylmethacrylate (PMMA) thick film. Then additional heat treatment, that is thermal development, etches the depth of the engraved area deeper. This sequential process is a direct and dry etching method. Maximum depth which is reached by the thermal development corresponds to the existent depth of the small-size molecules which can scatter easily to the air. Especially, SR light, compared to long wavelength light, can make the molecules small in the complete volume irradiated, not only near the surface but also near the bottom. The etching rate of thermal development was related to the size of the molecules and to the composition of the film which were estimated from molecular weight (M.W.) and density measurement, respectively. The etching rate accelerated under higher temperature of 135°C and lower pressure of 0.1 kPa. A microchannel with 15-μm-depth and 50-μm-width was formed on the surface of a thick PMMA film with a thickness of 20 μm.