Low-cost x-ray mask based on micropattern sputtered lead film for x-ray lithography

Abstract

In this work, a low-cost x-ray micromask is developed by sputtering a lead (Pb) film on a Mylar sheet substrate through microshadow masks and the x-ray mask is experimented for patterning an SU-8 negative photoresist on a stainless steel substrate. In addition, the required Pb layer thickness as a function of SU-8 photoresist thickness is studied and compared to that of conventional gold x-ray mask. The Pb layer and SU-8 photoresist thicknesses are varied from 8 to 19 μm and from 150 to 350 μm, respectively. Sputtering is selected for Pb thick film deposition due to its high sputtering yield. The Pb mask is used for x-ray lithography of SU-8 photoresist with 125 μm wide microchannel patterns, designing for microfluidic chip fabrication. The x-ray source for x-ray lithography is produced by synchrotron radiation at Siam Photon Laboratory, Thailand. For 180 μm thick SU-8 photoresist, Pb film thickness of around 8 μm is required to block x ray at a dose of 4200 mJ/cm3. This is less than twice the thickness required for a gold absorbing layer, which is about 5 μm. A similar relationship is seen for different SU-8 photoresist thicknesses. In addition, a steep sidewall angle of ∼89.5° for SU-8 microchannel pattern is obtained. The results demonstrate that the Pb based x-ray mask offers high lithographic quality at a very low cost. Therefore, it is highly promising for commercial applications.