Comparison of diamond-like carbon films deposited from 40 kHz and 13.56 MHz rf plasmas

R.f. plasma discharges are a standard method for depositing diamond-like carbon (DLC) films. This work looks at the effect of the r.f. driving frequency by studying films deposited at both 40 kHz and 13.56 MHz. Films were grown on a 10 cm diameter driven electrode from CH4-H-2 and C2H2-H-2 gas mixtures onto 75 x 25 mm(2) soda glass substrates at process pressures in the range 6-80 Pa (0.05-0.6 Torr). Film growth was monitored by in situ ellipsometry (at 675 mm) which allows measurement of the refractive index, extinction coefficient and thickness. The refractive index has been shown previously to be a reliable measure of film quality as determined by the hardness, scratch resistance and wear resistance. Films are assessed as a function of deposition pressure and r.f frequency. The him quality is seen to improve as the pressure decreases and as the r.f. power increases for both driving frequencies. It is necessary to deposit 13.56 MHz films at low pressure, typically less than 12 Pa (0.1 Ton). Similar quality films may be deposited at 40 kHz at pressures up to 80 Pa (0.6 Torr).