Formation of carbon coatings employing plasma torch from argon-acetylene gas mixture

Abstract

Amorphous carbon films were formed on Si (111) wafers from argon-acetylene gas mixture at atmospheric pressure by direct current (DC) plasma torch discharge. The Ar/C2H2 gas volume ratio varied from 12 to 100, and the distance between plasma torch nozzle exit and the samples was 0.005, 0.01 and 0.02 m. SEM revealed carbon coatings thickness in the range of 20-270 &mgr;m, and variation of the growth rate from 0.067 &mgr;m/s to 1.5 &mgr;m/s. Growth rate of the coatings increases decreasing Ar/C2H2 gas ratio and the distance. The Raman spectra of carbon films indicate the upward shift of the D (~1360 cm-1) and G (~1600 cm-1) peaks, compared to typical diamond-like carbon (DLC). a-C:H coatings deposited at higher Ar/C2H2 gas ratio (60 and 100) and distance d greater than or equal to 0.01 m contain high sp3 bond fraction and are attributed to DLC films. However Raman spectra shape and ID/IG ratio demonstrate existence of diamond phase mixed with glassy carbon phase. Films produced at lower Ar/C2H2 ratios are graphite-like carbon (GLC). The Fourier transform infrared (FTIR) spectroscopy has shown that film transparency increases decreasing acetylene gas content. Reflectance of the films depends on Ar/C2H2 gas ratio and distance, and varies from 60% up to 90%. The IR spectra showed clear evidence of C=C and C=O bonds in GLC films and presence of sp3 CH2 symmetric (2850 cm-1) and antisymmetric (2920 cm-1) modes in DLC coatings.