Annealing of radiation-induced positive charge in MOS devices with aluminum and polysilicon gate contacts

Abstract

This paper discusses the removal of radiation-induced positive charge from MOS structures by low temperature thermal anneals. Results are presented for structures in which the gate oxide is covered either by an aluminum or by a polysilicon contact during the anneal. The anneals were performed in forming gas, nitrogen and hydrogen ambients. The presence of aluminum over the gate oxide is found to play an important role in the annealing of radiation-induced positive charge in these structures . While a 400‡C anneal is sufficient to remove this charge from capacit or structures with aluminum gates, it leaves a small amount of residual charge (about 6xl0¹⁰}cm²}) in structures with polysilicon gates. Anneals at temperatures in excess of 550‡C are required to remove this charge completely from the polysilicon-gated MOS devices. However when a thin layer of aluminum is present over the polysilicon contact during the anneal the charge can be removed easily at 400‡C. The results in capacitor structures are consistent with those found in polysilicon gate MOSFET’s with similar coverage over the gate oxide.