Uniform Metal-Assisted Chemical Etching for Ultra-High-Aspect-Ratio Microstructures on Silicon

Recently, uniform metal-assisted chemical etching (UMaCE) has been demonstrated as an effective wet etch method for fabrication of deep trenches and holes on silicon (Si). However, attempts to achieve higher aspect ratio by UMaCE was not successful because etching in random directions was observed. In this paper, the etching uniformity in UMaCE is systematically studied at different etching solution composition and catalyst configuration. The surface chemistry evolution of Si during etching is characterized by X-ray photoelectron spectroscopy, water contact angle, and electrical impedance spectroscopy. Based on the data, the reaction kinetics is analyzed, which shows that an electropolishing-type charge transport and a higher amount of oxide species on Si surface help mitigate the random etching behavior and effectively promote the aspect ratio of the etching with uniformity. Under the rationalized condition, uniform trenches with width of $2~\mu \text{m}$ and aspect ratio of 120:1 is successfully fabricated by UMaCE. [2018-0212]