Improvement of filling bismuth for x ray absorption gratings through the enhancement of wettability
Filling materials with high x ray linear absorption coefficients in high aspect ratio (HAR) structures is a key process for the fabrication of absorption gratings used in x ray differential phase contrast imaging. Bismuth has been chosen as an effective filling material in micro casting technology, because of its low cost both in price and facility use. However, repellence on structure surfaces against molten bismuth leads to an obstacle in terms of completely filling bismuth into the small aperture and HAR microstructure formed by photo assisted electrochemical etching in 5 inch silicon wafers. We propose and implement a novel method of surface modification to completely fill bismuth into these structures with periods of 3 mm and 42 mm, respectively, and as deep as 150 mm. The modified surface with a Bi2O3 layer covering the structure surface, including the side walls, induces an enhanced bismuth filling ratio. The superiority of the method is demonstrated by micrographs which show filled microstructures compared to the previously used method, where only a layer of 100 nm SiO2 was covered. Furthermore, we have observed that the improved micro casting makes the absorption gratings clean surfaces, and no post treatment is needed.