Effect of post-nitride-passivation processing on the long-term stability of polysilicon integrated circuit resistors

Abstract

Results from 24 h experiments show that heat treatments of nitride-passivated polysilicon resistors at temperatures below that of the contact anneal, 450 °C, have a decisive influence on the stability of the resistors. In stress tests at 150 °C, it is shown that for heat treatments at 230 °C, the amount of loosely bound hydrogen in the grain boundaries and, consequently, the resistance drift is reduced by more than a factor of 2. For heat treatments above 270 °C, the drift is larger due to a reoccupation of released dangling-bond sites in the temperature range 230–270 °C during cool down. This is also the temperature range in which the largest reduction in the resistance drift is found. The heat treatments did not affect the mechanisms involved in the drift. There are no indications that hydrogen is released from the nitride at the highest temperatures, the reoccupation being independent of temperature. Possible mechanisms are discussed and the results are evaluated in terms of a previously introduced model for the time-dependent loss of loosely bound hydrogen.