Now, let’s continue to learn about the Bosch technique.
In the sidewall and bottom passivation step, C4F8 (octafluorobutane) gas is introduced, which dissociates into various reactive species in the plasma environment, particularly CFx (x=2, 3, etc.), that can polymerize to form a polymer.
This polymer deposits a very thin passivation film on the sidewalls and bottom, protecting these areas from subsequent etching steps, thus maintaining the steepness of the sidewalls and the precision of the structure.
The deposition of this polymer layer is crucial for achieving high aspect ratio and steep sidewall silicon structure etching, ensuring that the etching process is anisotropic, meaning etching primarily occurs in the vertical direction rather than laterally.
