Charges and defects in SiO2/Si systems after exposure to microwave plasmas

Abstract

SiO2/Si systems with the SiO2 films produced by plasma-enhanced chemical vapor deposition (PECVD) usually have a high density of interface traps (Dit) as compared to those with the SiO2 films grown thermally because in the former the systems have been subjected to plasma radiation. To simulate the radiation effects due to microwave plasmas during the deposition of SiO2 films by PECVD, we used high-quality thermally grown SiO2 films to form SiO2/Si systems and studied the changes of the behavior of such systems after being exposed to microwave plasmas at various device temperatures. The results show that for SiO2/Si systems without metallic electrodes, the density of electron-trapped charges, Qot, in the SiO2 bulk and Dit are higher after plasma radiation. For SiO2/Si systems with aluminum electrodes on the oxide and the Si surfaces, the effects of plasma radiation on the values of Qot and Dit are less, indicating that aluminum electrodes act as protective layers partly screening the plasma radiation, but the penetration of vacuum ultraviolet light from the plasma to the SiO2 bulk produces electrons and holes in SiO2 bulk, which react with oxygen vacancies in the SiO–Si (nonbridging) bonds forming positive trapped charge in the SiO2 bulk and at the SiO2/Si interface. The value of Dit increases with increasing SiO2 film thickness and decreases as the device temperature is increased from 25 to 200 °C during radiation. A further increase in device temperature beyond 200 °C leads to a reverse trend, implying that the defect generation and annihilation processes take place simultaneously during plasma radiation. Annealing in a forming gas at 400 °C removes most of the radiation-induced defects. It should be noted that under a PECVD situation the radiation damage could be much worse particularly at the beginning of the PECVD process whereas the SiO2 film is still very thin.