Study on silicon removal property and surface smoothing phenomenon by moderate-pressure microwave hydrogen plasma

Abstract

The Si removal property achieved by a localized microwave hydrogen plasma was investigated. The relationship between plasma generation parameters (input power, H2 gas flow rate, and initial surface roughness) and the removal property (etching rate and surface roughness) was revealed. Stress relief and removal of the affected Si layer by H2 plasma etching were also demonstrated. Moreover, this H2 plasma etching technique presents no etching-rate dependence on the crystal plane orientation. Therefore, even though a multi-crystalline Si sample was etched, a mirror surface could be revealed. In addition, a 100-μm-thick Si sample could be thinned to 5.7 μm by H2 plasma etching without breaking the sample. The surface roughness of the etched Si sample depends on the etching rate. A critical etching rate exists, which determines whether the etched Si surface becomes rough or smooth. When the etching rate is significantly lower than the critical etching rate, large surface roughness is generated owing to the formation of {111} platelet defects induced by H atoms in Si crystal. By contrast, when the etching rate is higher than the critical value, the roughness formation is suppressed, and the Si surface becomes smooth despite the high H flux supplied to the Si surface. The suppression of roughness formation is discussed by comparing the etching rate with the diffusion rate of H atoms into Si. It is important to maintain the Si etching rate higher than the H diffusion rate to obtain a smooth etched Si surface.