Origin of electrical signals for plasma etching end point detection: Comparison of end point signals and electron density

Electrical signals are used for end point detection in plasma etching, but the origin of the electrical changes observed at end point is not well understood. As an etch breaks through one layer and exposes an underlayer, the fluxes and densities of etch products and reactants in the gas phase will change. The resulting perturbation in gas composition may alter the plasma electron density, which in turn may affect the electrical signals. Alternatively, changes in substrate electrical properties or surface properties, such as work function or emitted electron yield, may be involved. To investigate these effects, experiments were performed in a radio-frequency (rf)-biased, inductively coupled reactor, during CF₄/Ar plasma etching of silicon dioxide films on silicon substrates. A complete set of electrical parameters, for the bias as well as the inductive source, was measured and compared. The most useful end point signal was found to be the fundamental rf bias impedance, which decreases when the oxide is removed. A simultaneous increase in plasma electron density was measured by a wave cutoff probe. Analytical sheath models indicate that the measured change in electron density accounts for nearly all of the impedance decrease. The change in electron density can in turn be explained by the effects of etch products or reactants on gas composition. In contrast, electrons emitted from the wafer surface play at most a minor role in the changes in electron density and impedance observed at end point.