Reaction pathways in remote plasma nitridation of ultrathin SiO2 films

Abstract

Low-temperature nitridation of 3 nm ${\mathrm{SiO}}_{\mathrm{2}}$ films using ${\mathrm{He/N}}_{\mathrm{2}}$ and ${\mathrm{N}}_{\mathrm{2}}$ 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 °C and 30 W rf power. Nitridation using an upstream ${\mathrm{He/N}}_{\mathrm{2}}$ remote plasma at 0.1 Torr incorporates nitrogen at the top surface of the ${\mathrm{SiO}}_{\mathrm{2}}$ 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 ${\mathrm{N–Si}}_{\mathrm{3}}$ local bonding configuration, irrespective of the spatial distribution of N atoms. Slightly more nitrogen is incorporated using a downstream ${\mathrm{He/N}}_{\mathrm{2}}$ plasma at each process pressure. By comparison, nitridation of ${\mathrm{SiO}}_{\mathrm{2}}$ films using a ${\mathrm{N}}_{\mathrm{2}}$ remote plasma at 0.1 Torr is very slow. Optical emission spectroscopy indicates that He dilution enhances the generation of ${\mathrm{N}}_2^{+}{\mathrm{(B }}^2{\Sigma }_u^{+})$ species by altering the plasma electron energy distribution and by providing an additional kinetic pathway (Penning ionization). Changing the ${\mathrm{He/N}}_{\mathrm{2}}$ remote plasma configuration from upstream to downstream (at 0.1 and 0.3 Torr) also enhances ${\mathrm{N}}_2^{+}{\mathrm{(B }}^2{\Sigma }_u^{+})$ generation. For upstream ${\mathrm{He/N}}_{\mathrm{2}}$ remote plasmas, the intensity of ${\mathrm{N}}_{\mathrm{2}}$ first positive emission from ${\mathrm{N}}_{\mathrm{2}}{\mathrm{(B }}^3{\Pi }_g)$ states increases with pressure, whereas the ${\mathrm{N}}_2^{+}$ first negative emission from ${\mathrm{N}}_2^{+}{\mathrm{(B }}^2{\Sigma }_u^{+})$ states decreases. We infer from these observations that ${\mathrm{N}}_2^{+}$ species are primarily responsible for top surface nitridation at 0.1 Torr, and that neutral species $[{\mathrm{N}}_{\mathrm{2}}{\mathrm{(A }}^3{\Sigma }_u^{+})$ metastables and N atoms] are associated with sub-surface nitrogen incorporation.