Honeycomb Support Structures to Stabilize PMMA Mold during LIGA Processing of High Aspect Ratio Gratings for XPCI Systems

Abstract

X-ray phase contrast imaging (XPCI) allows for medical and non-destructive testing of soft tissue [1]. High aspect ratio (HAR) gratings (100:1) are difficult to achieve and are a key element of XPCI gold gratings PMMA lithography, molding, and electroforming (LIGA) process [2]. Development of uniform, continuous HAR gold gratings over a large area is difficult as they easily tilt and delaminate. One technique for stabilizing the mold during development is to incorporate relief structures into mold, providing more surface area at the bottom, which results in segmented gratings [2]. In the current work, honeycomb supports are used to stabilize the PMMA mold during development and plating to realize continuous gratings centimeters in lateral length. Our goal is to fabricate micron size HAR gratings ~10 cm in lateral length that are ideal for XPCI applications enabling larger field of view for the XPCI imaging. Honeycomb structures were patterned on a 100 mm substrate. A sheet of PMMA is then bonded and fly cut to desired height. An x-ray exposure then patterns the PMMA mold prior to the fabrication of the top honeycomb structure. The gratings are then developed and electroplated having structural support on both sides. The use of bottom and top honeycomb arrays provide unique mechanical support to the fragile PMMA mold during development and electroplating of the HAR grating. Momose, A. and W. Yashiro, X-ray Phase Measurements with Talbot Interferometry and Its Applications. International Conference on Advanced Phase Measurement Methods in Optics an Imaging, 2010. 1236: p. 195-199. Mohr, J., et al., High Aspect Ratio Gratings for X-Ray Phase Contrast Imaging. International Workshop on X-Ray and Neutron Phase Imaging with Gratings, 2012. 1466: p. 41-50. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Figure 1: Schematic of mold stabilization design (left), honeycomb pattern (right) Figure 1