Robust Bottom-Up Gold Filling of Deep Trenches and Gratings

Abstract

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 μm deep and 305 μm 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 μmol∙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 μm—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” averages 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.