Effects of gas distribution on polysilicon etch rate uniformity for a low pressure, high density plasma

The geometry of low pressure inductively coupled plasma sources is usually considered to be a main factor in determining both the plasma and processing uniformity over large area wafers. We demonstrate experimentally that the gas flow distribution can have a major impact on both the plasma density and etch rate uniformity at low pressures where one might expect diffusion to make gas flow distribution less important. 150 mm polysilicon on oxide wafers were etched in SF6/Ar (1:1) plasmas between 6 and 20 mTorr. Using a single gas inlet produced polysilicon etch rates that varied by 30% along the gas flow direction, but were highly uniform perpendicular to the gas flow direction. A gas distribution ring, on the other hand, produced highly uniform etch rates with variations less than 4% overall using the same source. Langmuir probe measurements of the ion saturation current spatial profiles in argon and SF6 discharges demonstrated significant gradients across the reactor for a single gas inlet with the largest current density near the inlet. (C) 1998 American Vacuum Society. [S0734-211X(98)06502-0].