Apparatus and methods for plasma etching are disclosed. In one embodiment, an apparatus for etching a plurality of features on a wafer comprises a chamber, a feature plate disposed in the chamber for holding the wafer, a gas channel configured to receive a plasma source gas, an anode disposed above the feature plate, a cathode disposed below the feature plate, a radio frequency power source configured to provide a radio frequency voltage between the anode and the cathode so as to generate plasma ions from the plasma source gas, a pump configured to remove gases and etch particulates from the chamber, and a clamp configured to clamp the wafer against the feature plate. The clamp includes at least one measurement hole for passing a portion of the plasma ions to measure a DC bias of the feature plate.