Paper PS2-TuM10
Control of SiO₂/Si Interface States during Plasma Etching Processes

Tuesday, October 16, 2007, 11:00 am, Room 607

Plasma processes are indispensable in the fabrication of MOS LSI devices. During plasma processes, however, serious problems can occur, such as charge-build up damage and UV photon irradiation damage. In particular, UV irradiation from the plasma causes drastic degradation of sub-50nm MOS LSI device characteristics, because the penetration depth of UV photons into dielectric films is more than 10nm. We have previously reported that UV photons from plasma effectively generate E’ centers (Si dangling bond in SiO2 film) in SiO2 films. However, the relationship between UV irradiation from plasma and interface states’ (Pb centers, Si dangling bond at SiO2/Si interface) densities must be basically understood during the plasma process, because the generation of interface states directly degrades the electronic properties of MOS LSI devices. We focus on SiO2/Si interface state density trends during conventional continuous wave (CW) plasma and pulse-time-modulated (TM) plasma etching processes. In order to evaluate the generation of interface state density, 5nm thick thermally grown SiO2 films were formed on Si substrates. The SiO2 films were then irradiated using Ar inductively coupled plasma. After that, we evaluated the Pb centers using electron spin resonance spectroscopy. Before the plasma irradiation, the density of the Pb centers was less than 1x10¹⁰ cm^-2 spins. After the Ar CW plasma irradiation, and without any substrate RF bias power, Pb centers drastically increased. That is, the Pb centers increased drastically even after irradiation using conventional Ar CW plasma and elimination of ion bombardment. We also investigated the dependence of ion bombardment energy on the density of Pb centers in conventional CW plasma. However, even when 100W of RF power was applied during the plasma process, the ESR spectrum did not change. This result indicates that Pb centers are mainly generated by irradiation with UV photons during plasma processes. Therefore, we investigated the effects of TM plasma on eliminating Pb centers. During the TM plasma irradiation, the UV photon irradiation was drastically reduced during the plasma’s "off" period. After the TM plasma irradiation, the Pb centers densities reduced dramatically to less than 1x10¹⁰ cm^-2 spins, compared to those observed after CW plasma irradiation. We found that TM plasma was the most promising candidate for the elimination of Pb centers during the plasma etching processes.