The nature of the trapped hole annealing process

Abstract

Post-irradiation positive-bias annealing and negative-bias reverse annealing of trapped positive charge in MOS oxides were studied as a function of temperature. Below 125 degrees C, only a slight increase in the positive annealing response was observed, consistent with a tunneling process to a trap level below the Si valence band edge. A much large increase in the trapped-hole annealing rate observed above this temperature is discussed in terms of two possible mechanisms: (1) tunneling to an excited state above the Si conduction band and (2) thermal detrapping of holes to the oxide valence band. The annealing of positive oxide-trapped charge appears to proceed at least partly by a compensation process called negative-bias reverse annealing. This effect is only slightly increased at elevated temperatures in hardened oxides, where it is significant even at room temperature. However, in a soft oxide, which did not exhibit significant negative bias reverse annealing at room temperature, the effect was observed at elevated temperatures. Two unexpected results related to the interface trap density are discussed: the late-time saturation value of the radiation-induced interface trap density increases with annealing temperature if positive bias is maintained, and the interface trap density oscillates with alternating bias at elevated temperatures. >