Effects of minor addition of N₂/O₂ impurities on silicon nanostructure formation behavior in hydrogen plasma process

We have developed various Si processing techniques using a relatively high-pressure hydrogen plasma (> 3 kPa). In the course of developments, we found that hydrogen plasma treatment with minor air addition led to a micrometer-scale cone structure on the Si surface. Since this obtained surface showed very low reflectance (0.5%), this hydrogen plasma technique is expected to apply to high-performance photoelectric devices. In this study, we investigated the impacts of minor N₂/O₂ gas addition (flow rate concentration < 1%) into hydrogen plasma on the obtained Si surface morphology after the plasma process. As a result, we revealed that a small amount of N₂ addition resulted in pillar Si nanostructure formation with rounded tips. On the other hand, O₂ addition led to minor roughening Si surface. In addition, when the N₂ and O₂ were simultaneously supplied to the process atmosphere, the high-aspect-ratio Si nanocone could be obtained. The optical emission spectra of the plasma implied that minor O₂ addition enhanced atomic hydrogen generation in the plasma. This result suggests that O₂ addition promotes the formation of sharp-tipped nanocone structure through enhancement of chemical etching by atomic hydrogen.