Reaction pathways in remote plasma nitridation of ultrathin SiO2 films

Low-temperature nitridation of 3 nm SiO2 films using He/N-2 and N-2 remote radio frequency (rf) plasmas was investigated. On-line Auger electron spectroscopy and angle-resolved x-ray photoelectron spectroscopy (ARXPS) were employed to determine the concentration, spatial distribution, and local chemical bonding of nitrogen in the resultant films. Experiments were performed using a substrate temperature of 300 degreesC and 30 W rf power. Nitridation using an upstream He/N-2 remote plasma at 0.1 Torr incorporates nitrogen at the top surface of the SiO2 film. In contrast, a lower concentration of nitrogen distributed throughout the film is obtained when the process pressure is increased to 0.3 Torr. ARXPS indicates a N-Si-3 local bonding configuration, irrespective of the spatial distribution of N atoms. Slightly more nitrogen is incorporated using a downstream He/N-2 plasma at each process pressure. By comparison, nitridation of SiO2 films using a N-2 remote plasma at 0.1 Torr is very slow. Optical emission spectroscopy indicates that He dilution enhances the generation of N-2(+)(B (2)Sigma (+)(u)) species by altering the plasma electron energy distribution and by providing an additional kinetic pathway (Penning ionization). Changing the He/N-2 remote plasma configuration from upstream to downstream (at 0.1 and 0.3 Torr) also enhances N-2(+)(B (2)Sigma (+)(u)) generation. For upstream He/N-2 remote plasmas, the intensity of N-2 first positive emission from N-2(B (3)Pi (g)) states increases with pressure, whereas the N-2(+) first negative emission from N-2(+)(B (2)Sigma (+)(u)) states decreases. We infer from these observations that N-2(+) species are primarily responsible for top surface nitridation at 0.1 Torr, and that neutral species [N-2(A (3)Sigma (+)(u)) metastables and N atoms] are associated with sub-surface nitrogen incorporation. (C) 2002 American Institute of Physics.