Passivation mechanisms in cryogenic SF6/O2 etching process

Passivation mechanisms of Si trenches involved in SF6/O-2 cryogenic plasma etching were investigated in order to better control the process and avoid defects. Trench sidewalls and profiles were ex situ characterized geometrically by SEM and chemically by spatially resolved XPS experiments. These measurements reveal that the passivating layer is removed during the increase of the wafer chuck temperature leading to a very clean surface of the sidewalls after processing. Nearly no SiO2 formation on the sidewalls was observed after the very low temperature etching (-100degreesC). A two-step process was defined to rebuild the passivating layer after its destruction and continue the trench etching. The necessary conditions for properly rebuilding the passivating layer give precious information about its chemical composition. These experiments clearly show that sulfur is not a necessary element to form an efficient passivating layer.