Effects of N2 and O2 plasma treatments of quartz surfaces exposed to H2 plasmas

Abstract

This paper presents a study of methods for reducing the erosion of SiO2 in a high-power density (10–40 W/cm3), purely inductive H2/Ar plasma, using a toroidal transformer-coupled plasma source operated at 0.5 Torr. Quartz samples were exposed to plasma densities of 1–3 × 1013 cm−3 and H atom temperatures of 4000–8000 K [electron densities and H translational temperatures were measured by Stark and Doppler broadening of H Balmer-β (Hβ) emission at 486.1 nm]. Laser interferometry was employed to monitor time-resolved temperatures of the quartz substrate. Etching rates were measured by stylus profilometry, and roughness was quantified by atomic force microscopy (AFM). For 5 min discontinuous H2/Ar plasma exposure (0.5 Torr, 16 W/cm3, 1 min plasma-on, 9 min plasma-off per cycle with five cycles), the etching rate during the plasma-on time was 224 nm/min. This was much higher than the 16 nm/min rate observed for a continuous 1 h H2/Ar plasma exposure. This trend was ascribed to the higher substrate temperatures reached with continuous plasma operation and a negative dependence of the etching rate on temperature, described by an activation energy of roughly −5 kcal/mol. When exposure to 1 min H2/Ar plasmas was alternated with 1 min O2/Ar plasma treatments and 12 min plasma-off periods, the etching rate was reduced to near-zero and the extent of surface roughness was reduced by at least fivefold. N2/Ar plasma treatments were less effective in reducing the etching rate (to 57 nm/min), while the roughness to the surface caused by exposure to the H2/Ar 1 min plasmas was nearly eliminated. A mechanism for the erosion process is proposed, involving the penetration of H atoms below the surface where they insert into Si–O–Si linkages to form SiH and SiOH groups. This opening of the SiO2 network allows easier penetration of H, further bond breakage, and crack propagation that eventually leads to the shedding of small silica particles. Periodic exposure to O atoms hydroxalizes these subsurface regions and subsequently reforms Si–O–Si linkages, accompanied by the formation of H2O that presumably diffuses to the surface and desorbs.