Origin of Electrical Signals for Plasma Etching Endpoint Detection

To investigate the electrical changes observed at plasma etching endpoints, experiments were performed in an inductively coupled, rf-biased reactor, during CF4/Ar plasma etches of silicon dioxide films on silicon substrates. The rf bias current, voltage, and impedance were measured vs. time during etching. Simultaneously, a Langmuir probe measured the electron and ion densities, ion current density, and electron energy distribution function (EEDF). At endpoint, we detected changes in the Langmuir probe parameters caused by changes in the flux and density of oxygencontaining etch products. Measured changes in ion current density, EEDF, and voltage, when input into a numerical model of the plasma and sheaths, fully explain the change in discharge impedance measured at endpoint. Changes in the yield of electrons emitted from the wafer surface were not significant. The understanding provided by this work should enable electrical endpoint detection to be used with greater reliability and confidence.