Performance of SU-8 Membrane Suitable for Deep X-Ray Grayscale Lithography

In combination with tapered-trench-etching of Si and SU-8 photoresist, a grayscale mask for deep X-ray lithography was fabricated and passed a 10-times-exposure test. The performance of the X-ray grayscale mask was evaluated using the TERAS synchrotron radiation facility at the National Institute of Advanced Industrial Science and Technology (AIST). Although the SU-8 before photo-curing has been evaluated as a negative-tone photoresist for ultraviolet (UV) and X-ray lithographies, the characteristic of the SU-8 after photo-curing has not been investigated. A polymethyl methacrylate (PMMA) sheet was irradiated by a synchrotron radiation through an X-ray mask, and relationships between the dose energy and exposure depth, and between the dose energy and dimensional transition, were investigated. Using such a technique, the shape of a 26-mu m-high Si absorber was transformed into the shape of a PMMA microneedle with a height of 76 mu m, and done with a high contrast. Although during the fabrication process of the X-ray mask a 100-mu m-pattern-pitch (by design) was enlarged to 120 mu m. However, with an increase in an integrated dose energy this number decreased to 99 mu m. These results show that the X-ray grayscale mask has many practical applications. In this paper, the author reports on the evaluation results of SU-8 when used as a membrane material for an X-ray mask.