Pushing the Limits of Bottom-Up Gold Filling for X-ray Grating Interferometry

The application of superconformal Bi-catalyzed Au electrodeposition for void-free filling of recessed trenches in X-ray gratings used in phase contrast imaging is detailed. Filling of trenches with aspect ratio (height divided by width) up to 55 is demonstrated. Uniform bottom-up filling of patterned trench arrays across 100 mm (4 inch) diameter Si wafer is accomplished using a well-defined hydrodynamic flow field across the wafer surface. The filling process and microstructure are examined by scanning electron microscopy of cross-sectioned specimens. The importance of process optimization, from the design of potential (current) programming to electrolyte pH and concentrations of Na₃Au(SO₃)₂ and Bi³⁺, for the filling of high aspect ratio trenches is demonstrated. X-ray phase contrast imaging is used to assay the quality and uniformity of the as-formed gratings as well as demonstrate their application to imaging biological tissue. Successful void-free Au filling of micrometer range pitch, high aspect ratio trenches in Si gratings promises to advance X-ray grating interferometry and its application to X-ray phase contrast imaging.