Origin of Electrical Signals for Plasma Etching Endpoint Detection
Mark A. Sobolewski
Electrical signals are used for endpoint detection in plasma etching, but the origin of the electrical changes observed at endpoint is not known. They may be caused by changes in the gas-phase densities of etch products and reactants or by changes in substrate surface properties such as photoemitted or ion-induced electron yield. To investigate these effects, experiments were performed in an inductively coupled, rf-biased reactor, during CF4/Ar etches of SiO2 films on Si wafers. The rf bias current, voltage, and impedance were measured vs. time during etching, simultaneous with Langmuir probe measurements. At endpoint, a decrease in rf bias impedance coincides with an increase in ion current density and a change in the electron energy distribution function (EEDF). The measured changes in ion current and EEDF, when input into a numerical model of the plasma and its sheaths, explain the measured change in discharge impedance, within the uncertainties of the measurements. Changes in the yield of electrons emitted from the wafer surface were not significant. The electrical endpoint signals are thus essentially gas-phase phenomena, presumably caused by changes in the flux and density of oxygen-containing etch products.
Applied Physics Letters
CF4, discharge, EEDF, endpoint, etch, fluorocarbon, ion current, plasma, process monitoring