Removal of carbon films by oxidation in narrow gaps: Thermo-oxidation and plasma-assisted studies

The removal of hard amorphous hydrogenated carbon (a-C: H) films from narrow gaps simulating the macro-brush structures present in controlled fusion devices has been investigated. Films with thickness of 50-150 nm were generated through plasma assisted chemical vapor deposition (PACVD) in glow discharges of CH4/He on Si and stainless steel plates. The deposited plates were then arranged to form sandwich structures building narrow gaps and were subject to erosion by exposure to O2/He plasmas and to thermal oxidation by O2 and by a NO2/N2 (1:1) mixture. In the plasma etching experiments, the deposited layers were only partially removed by the plasma at the side wall gap surfaces, but were efficiently removed at the bottom of the gap. In the thermo-oxidation experiments, the deposited films were effectively and homogeneously removed with oxigen at 670 K and with the NO2/N2 mixture at T > 570 K.