INFLUENCE OF THE DISCHARGE FREQUENCY (35 KHZ AND 13.56 MHZ) ON THE COMPOSITION OF PLASMA ENHANCED CHEMICAL VAPOR-DEPOSITION A-C-H FILMS

Amorphous carbon films have been deposited by plasma enhanced chemical vapor deposition from methane and hydrogen gas mixtures excited at two different frequencies, 35 kHz and 13.56 MHz. Large differences observed in the optical emission spectra of the discharge have been attributed to changes in the main dissociative excitation processes of the CH4 molecules. The deposition rate and film composition also depend on the discharge frequency. At 35 kHz, the high concentration of atomic hydrogen during the excitation process favors the etching of the amorphous carbon and extremely thin films are obtained. However, at 13.56 MHz, the deposition rate is much higher and the films present a high polymeric content, which has been related to one of the dissociative mechanism proposed at this frequency (i.e., CH4 + e --> CH2 + H-2 + e).