It is difficult for wet etching to achieve good anisotropy in fabrication; therefore dry etching is adopted to solve this problem.
Dry etching includes physical sputtering, plasma etching and reactive ion etching. The following figure presents their difference.
Due to the balanced performances, reactive Ion Etching (RIE) is commonly adopted in nano fabrication, where etching gas is introduced in plasma chamber to generate radicals for chemical reaction and ions for ion bombardment. It is critical that the by-products formed by chemical reactions must be volatile enough that they can be evacuated out from the vacuum chamber. For example in copper etching, the product, CuCl2, is not volatile enough; thus the RIE for copper is quite difficult for practical application.
The above table summarizes the typical gases for RIE of difference materials. The following figure shows the process conditions effects.
References:
1) Dr. Lynn Fuller’s presentation from internet, Rochester Institute of Technology
2) E. Chen’ s presentation, CIMS, Harvard University