Origin of electrical signals for plasma etching endpoint detection

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 impedance was measured vs. time during etching, simultaneous with Langmuir probe measurements. At endpoint, a decrease in impedance coincided with increases in ion current and electron energy. The data, analyzed by a numerical model of the discharge, indicate that changes in electron emission yield were relatively insignificant or entirely absent. Thus the impedance change is not a surface effect but is, instead, predominantly or entirely a gas-phase phenomenon. [doi:10.1063/1.3662973]