Low cost X-ray lithographic mask based on microsputtered lead film on Transparencies

In this work, a low-cost X-ray micromask is developed by sputtering lead (Pb) film on Transparencies (Mylar) sheet substrate and the X-ray mask is experimented for patterning SU-8 negative photoresist on stainless steel substrate. Pb layer is sputtered on the substrate through Ni-electroplated microshadow masks with varying thicknesses from 8 to 19 μm. Sputtering is a suitable process for Pb thick film deposition due to its high sputtering yield. The tested pattern is 100 μm-wide microchannels, which is designed for microfluidic chip fabrication. Next, 150-350 μm thick SU-8 photoresist is spun on the stainless substrate. The Pb mask is then used for X-ray lithography by X-ray source produced by synchrotron radiation at Siam photon laboratory, Thailand. X-ray exposure is conducted with different X-ray doses and intensity. According to the experimental results, the lead film thickness of more than 10 μm is the required to block the X-ray for 200 μm thick SU-8 photoresist that requires X-ray dose of 4,200 mJ/cm³ produced at beam line current of 70 mA. This is almost twice of the thickness required for the gold absorbing layer, which is about 6 μm. Although Pb requires higher thickness than gold does, the proposed Pb based X-ray mask will be much more practical for commercial applications than gold X-ray mask due to much lower material cost and simpler fabrication process.