High aspect ratio Si micro-holes formed by wet etching using Pt needles

Si micro-holes with a high aspect ratio can be formed by use of catalytic activity of Pt needles, i.e., contact of Pt needles with Si wafers immersed in an HF plus H2O2 solution. In the case of p-Si with similar to 10 Omega cm resistivity, the shape of the Pt needle is accurately transferred to the Si substrate, and the aspect ratio of 8 is achieved. High-resolution TEM observations of the sidewall of the Si micro-holes show that they are formed through formation of a nanocrystalline Si layer followed by its dissolution. By comparing micro-holes formed in n-Si and in p-Si, it becomes obvious that diffusion of injected holes, which form the nanocrystalline Si layer, affects the shape of micro-holes. In the case of n-Si, injected holes diffuse to the Si/solution interface due to the Si band-bending resulting from the difference between the redox potential of the solution and the Si Fermi level, and cause an Si dissolution reaction at non-contact regions with the Pt needle.