Growth of hydrogenated amorphous carbon films in pulsed d.c. methane discharges

We report the preparation of hydrogenated amorphous carbon (a-C:H) films from asymmetrical bipolar pulsed d.c. methane discharges. The films were deposited at 10 Pa of pressure on c-Si substrates placed onto an electrode powered by a pulsed d.c. generator. The asymmetrical bipolar pulsed d.c. voltage waveform consisted of a fixed positive pulse amplitude of 40 V followed by a variable negative pulse whose peak amplitude was varied from −400 up to −1400 V. Pulse frequencies of 100, 125, 150 and 200 kHz were used at a constant positive pulse time of 2 μs. In addition, a series of a-C:H samples were grown from r.f. capacitive discharges at bias voltages from −200 to −800 V. Pulsed d.c. a-C:H films 1-μm-thick were deposited at growth rates up to 60 nm/min and with internal compressive stress values between 1 and 1.5 GPa. Fourier transform-infrared and Raman analyses revealed the diamond-like character of the films. The effects of pulse parameters on the growth and structural properties of the films are discussed and compared to those of films obtained by conventional r.f. plasma-enhanced chemical vapour deposition.