Alternating SiCl4/O-2 passivation steps with SF6 etch steps for silicon deep etching

Deep etching of silicon has been investigated in an inductively coupled plasma etch reactor using short SiCl4/O-2 plasma steps to passivate the sidewalls of the etched structures. A study was first carried out to define the appropriate parameters to create, at a substrate temperature of -20 degrees C, a passivation layer by SiCl4/O-2 plasma that resists lateral chemical etching in SF6 plasma. The most efficient passivation layer was obtained for a SiCl4/O-2 gas flow ratio of 2:1, a pressure of 1 Pa and a source power of 1000 W. Ex situ analyses on a film deposited with these parameters show that it is very rich in oxygen. Silicon etching processes that alternate SF6 plasma etch steps with SiCl4/O-2 plasma passivation steps were then developed. Preliminary tests in pulsed-mode conditions have enabled etch rates greater than 2 mu m min(-1) with selectivities higher than 220. These results show that it is possible to develop a silicon deep etching process at substrate temperatures around -20 degrees C that uses low SiCl4 and O-2 gas flows instead of conventional fluorocarbon gases for sidewall protection.