Fabrication of Very-High-Aspect-Ratio Micro Metal Posts and Gratings by Photoelectrochemical Etching and Electroplating

A high-yield fabrication process for dense arrays of very-high-aspect-ratio (VHAR) freestanding metal posts and gratings is developed. Silicon molds of regularly arranged through-holes or trenches are first fabricated by photoelectrochemical etching. By studying the etching parameters, including geometry constraint, current density and potential, electrolyte concentration, and etching time, we succeed to produce dense arrays of VHAR holes ( depth = 610 μm; diameter = 5 μm; pitch = 14 μm) and trenches (depth = 320 μm ; width = 4 μm; pitch = 8 μm) with yields higher than 99% on 2-cm² processing areas. The VHAR molds are then filled with metals using a new bottom-up electroplating technique, which features an intermittent vacuum degassing to remove the air and hydrogen bubbles from such deep and narrow voids during the plating. Zinc and nickel are successfully electroplated in high quality, and the freestanding metal structures are obtained by removing the silicon molds by XeF₂ etching. Obtained are maximum aspect ratios of 120 : 1 for posts (height = 600 μm; diameter = 5 μm; pitch = 14 μm) and over 60 : 1 for gratings (height = 250 μm; width = 4 μm; pitch = 8 μm) with yields higher than 99% on ~ 0.5-1-cm² samples.