Bottom-Up Gold Filling in New Geometries and Yet Higher Aspect Ratio Gratings for Hard X-Ray Interferometry

An extreme bottom-up filling variant of superconformal Au electrodeposition yielding void-free filling of recessed features is demonstrated with diffraction gratings composed of a two-dimensional patterned "chessboard" array of square vias of aspect ratio (depth/width) ≈ 23 as well as one-dimensional arrays of trenches having aspect ratios exceeding 50 and 65. Deposition on planar and patterned substrates is examined in several near-neutral x mol·L^-1 Na₃Au(SO₃)₂ + 0.64 mol·L^-1 Na₂SO₃ electrolytes (x = [0.08, 0.16, 0.32]) containing ≈ 50 micromol·L^-1 Bi³⁺ additive. The electrolytes are similar to those used in earlier work, although the upper bound on Au(SO₃)₂ concentration is twofold greater than previously described. Filling results are complemented by associated current and deposition charge transients whose features, particularly with well controlled pH, exhibit repeatable behaviors and timescales for incubation of passive deposition followed by bottom-up, void-free filling. While incompletely filled features can exhibit substantial via-to-via variation in fill height, self-passivation that follows complete bottom-up filling results in highly uniform filling profiles across the substrates. Visibility measurements capture the quality and uniformity of the as-formed wafer scale gratings. X-ray phase contrast imaging demonstrates their potential for imaging applications.