Dependencies of the Optical Properties of DLC Films on the Third Gas Addition

초록

The optical properties of diamond like carbon (DLC) films strongly depend on the reactant gas mixture composition. Oxygen has been shown to enhance both the growth and the quality of DLC films. The extension of the range of diamond formation from a higher hydrocarbon concentration in the gas mixtures and the improvement in the film quality are two major breakthroughs which were due principally to an increase in the concentration of hydrocarbon radicals in the plasma and the preferential etching of non-diamond components in the coatings by oxygen and hydrogen (1). Availability of higher amount of atomic nitrogen during film growth leads to the change of sp3/sp2 ratios in DLC films which in turn leads to the change of the physical properties of the films (2). In this work DLC films were synthesized using the rf-plasma CVD technique with the addition of small amounts of nitrogen and oxygen to the gas mixture of CH4 and H2. The pressure, the rf-power, and the CH4:H2 ratio were 430 mtorr, 80W, and 1:1, respectively, in the deposition process. Three remarkable changes in the FTIR spectra (Fig.1) are observed as the nitrogen concentration increases. The first observable changes in the spectra are the increase of the N-H stretching band in the 3300~3600 cm-1 range and the decrease of the sp1 C-H stretching band at 2900 cm-1. The second change in the spectra is the increase of the absorption band in the 2200~2300 cm-1 range. The third changes in the spectra are the increase of the absorption band at 1650 cm-1 and the absorption band in the 1500~1580 cm-1 range. The transmittances of the DLC films deposited with and without the addition of nitrogen or oxygen were compared each other in the visible light and ultraviolet light regions (Fig.2). FTIR analyses were performed to see the bond structures. The transmittance of the DLC film measured by UV/VIS spectrometry increased with increasing the nitrogen content from 6.3% to 17.4%. The transmittance of the DLC film in the UV