Study of multi-step X-ray exposures to fabricate microstructure

Compared with single-step exposure of X-ray light source of high-power synchrotron radiation, lithographic process using multi-step exposures with a conformal mask has some advantages such as a thinner absorption layer, better heat dissipation, and shorter developing time. In this study, a conformal mask with an aurum (Au) absorber is fabricated using traditional ultraviolet (UV) lithography and Au electroplating process. For X-ray exposure, an analytical model of the absorption dosage (exposure dosage) along the exposed depth of x is developed. It can estimate accurately the time and dosage required for exposure. Based on the calculated dosage, 2-mm-thick PMMA (Polymethylmethacrylate) template with high-aspect ratio of 20 is fabricated by this multi-step exposures and developing process. With this template, nickel-cobalt (Ni–Co) alloy electroplating process is applied to fabricate a metallic mold with high hardness. For high-aspect-ratio microstructures, hardness and internal stress of the metallic mold by electroplating is a key factor to the quality. Experimentally, it is found that changing the ratio of Ni sulfamate to Co sulfamate can affect hardness and internal stress of the deposit. Though the hardness of the electroplated alloy is improved due to Co sulfamate, internal stress is induced at the same time. Thus, it is necessary to add a stress reducer to reduce internal stress. The currently result shows that the hardness of the mold of about 550 Hv with about 25wt.% of Co and zero internal stress can be made successfully after adding a stress reducer. With this electroplating technique, the 2-mm-thick and 100-μm-wide microstructure with high hardness is fabricated.