Deep reactive ion etching (DRIE)

Deep receive ion etching (DRIE) include two process: Bosch process and Cryogenic process (Reference 1).

Bosch process is the major DRIE method, which alternatively switches between SF6 (etching) and C4F8 (polymer passivation) to form deep hole/trench with high aspect ratio.

Source: Purdue University
Source: Alcatel
Passivation process

The image of the deep etching.

Source: Prof. N Cheung, U.C. Berkeley