Study on fabrication of sub-micron structures for MEMS using deep X-ray lithography

It is necessary for practical MEMS with high performance to be fabricated microstructures with sub-micron widths and gaps (lines and spaces). In sub-micron deep X-ray lithography, one of the most crucial considerations is the fabrication of an X-ray mask with thick X-ray absorbers having sub-micron width. An X-ray mask, composed of 1 μm-thick Au with a 0.6 μm line width and a 0.2 μm space as absorbers, 2 μm-thick SiC with 240 MPa of tensile stress as a membrane and 625 μm-thick Si as a frame, was fabricated. A procedure for the fabrication process of the X-ray mask that obtains the smallest deformation of the X-ray absorbers, was adopted. On the other hand, in order to reduce the influence of Fresnel diffraction on lithography accuracy, a PMMA resist was polymerized without residual stress, which had been the main cause of a warp in the substrate, by controlling of the polymerization process. As a result, a submicron PMMA structure with 0.23 μm-minimum width and 15 μm height was fabricated by deep X-ray lithography