Exploring the Limits of Bottom-Up Gold Filling to Fabricate Diffraction Gratings

Gold deposition on rotating disk electrodes, Bi³⁺ adsorption on planar Au films and superconformal Au filling of trenches up to 45 microm deep are examined in Bi³⁺-containing Na₃Au(SO₃)₂ electrolytes with pH between 9.5 and 11.5. Higher pH is found to increase the potential-dependent rate of Bi³⁺ adsorption on planar Au surfaces, shortening the incubation period that precedes active Au deposition on planar surfaces and bottom-up filling in patterned features. Decreased contact angles between the Au seeded sidewalls and bottom-up growth front also suggest improved wetting. The bottom-up filling dynamic in trenches is, however, lost at pH 11.5. The impact of Au concentration, 80 mmol/L versus 160 mmol/L Na₃Au(SO₃)₂, on bottom-up filling is examined in trenches up to ≈ 210 microm deep with aspect ratio of depth/width ≈ 30. The microstructures of void-free, bottom-up filled trench arrays used as X-ray diffraction gratings are characterized by scanning electron microscopy (SEM) and Electron Backscatter Diffraction (EBSD), revealing marked spatial variation of the grain size and orientation within the filled features.