Robust Bottom-Up Gold Filling of Deep Trenches and Gratings

This work extends an extreme variant of superconformal Au electrodeposition to deeper device architectures while exploring factors that constrain its function and the robustness of void-free processing. The unconventional bottom-up process is used to fill diffraction gratings with trenches 94 microm deep and 305 microm deep, with aspect ratios (height/width) of just below 20 and 15, respectively, in near-neutral 0.16 mol·L^-1 Na₃Au(SO₃)₂ + 0.64 mol·L^-1 Na₂SO₃ electrolyte containing 50 micromol·L^-1 Bi³⁺. Although the aspect ratios are modest compared to previously demonstrated void-free filling beyond AR = 60, the deepest trenches filled exceed those in previous work by 100 microm - a nearly 50 % increase in depth. Processes that substantially accelerate the start of bottom-up deposition demonstrate a linkage between transport and void-free filling. Final profiles are highly uniform across 65 mm square gratings because of self-passivation inherent in the process. Electron microscopy and electron backscatter diffraction confirm the fully dense Au and void-free filling suggested by the electrochemical measurements. X-ray transmission "fringe visibility" average more than 80 % at 50 kV X-ray tube voltage across the deeper gratings and 70 % at 40 kV across the shallower gratings, also consistent with uniformly dense, void-free fill across the gratings.