Sidewall surface chemistry in directional etching processes

A prerequisite of successful microstructure fabrication in electronic materials using plasma-based etching methods is the ability to maximize the ratio of ion-enhanced etching reactions relative to spontaneous etching reactions. To produce vertical etching profiles, the rate of the etching reaction in line-of-sight of the plasma has to be large, whereas the lateral etching rate should vanish. We present a review of the approaches that have been used for silicon, aluminum, SiO2 and polymeric materials to suppress etching reactions at microstructure sidewalls. These approaches include the judicious choice of the primary etching gas, adding certain gases to the main etching gas, lowering the substrate temperature, mask material redeposition, or alternate etch and deposition cycles. Our knowledge of the sidewall chemistry resulting from these approaches, e.g. the production of sidewall passivation layers, and experimental methods that have been employed to study these, are reviewed. The impact of sidewall chemistry on the etching profiles of the final microstructure is also discussed.