Measured cross sections and ion energies for a CHF3 discharge

Abstract

Trifluoromethane (CHF3) is used in semiconductor plasma processing chambers to achieve high-etch selectivity of an oxide layer over a silicon substrate. Such surface etching is governed by the ion and molecule fluxes near the surface, the concentrations of which are dependent upon species interactions in and their transport through the plasma. In order to assist in the interpretation of ion flux measurements and to provide fundamental data required for plasma modeling, we report the first total cross sections for significant ion-molecule reactions occurring in CHF3 discharges. The reactions studied include collision-induced dissociation for CF3+ on CHF3, dissociative charge transfer for CF3+ and F+ on CHF3, and electron detachment from F− on CHF3. Collision energies range from a few to a few hundred electron volts. In addition, ion-flux energy distributions and relative ion intensities have been measured and are presented for dc townsend discharges with E/N values ranging from 5×10−18 to 25×10−18 V m2 [5 to 25 kTd]. The townsend discharge results are qualitatively interpreted using the cross-section measurements.