Oxide Breakdown due to Charge Accumulation during Plasma Etching

Charge accumulation on an insulating gate oxide may cause electrostatic breakdown. Exposing a wafer containing MOS devices to a plasma discharge during patterning of poly gate structures may, under certain conditions, cause damage to the device due to such a charge accumulation. At 13.56 MHz the charge that can be transferred to the wafer is so small that it is unlikely that the gate will reach a level where electrical breakdown will occur. If, however, the frequency is low or a dc supply is used, breakdown of the oxides may appear. Samples subjected to a low frequency or dc plasma also exhibit an increase in the density of states at the oxide semiconductor interface. A similar distribution and density of states was also found if a current from a constant current source was passed through the oxide. This current level was of the same value as the current through the samples in the plasma. We thus conclude that charge accumulation is a major reason for electrical breakdown failure to occur for devices plasma etched at low frequencies.