Measurements of the presheath in an electron cyclotron resonance etching device

The first direct measurement of a collisional Bohm presheath from plasma potential measurements is given. By measuring the presheath thickness in front of a grounded wafer stage, a determination of the collision mean free path for ions in an electron cyclotron resonance etching tool has been made. Presheaths were measured in N₂ and CF₄ plasma using an emissive probe. The presheath thickness in N₂ was found to be linearly dependent on the mean free path. Measurements of CF₄ plasmas, for which the collision cross sections are unknown, have shown results similar to those found for nitrogen. This result has enabled an extrapolation to be made of the effective cross section for collisions in plasmas created from CF₄.