Thermal gas effusion from hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films, deposited onto negatively biased substrates in a 13.56-MHz hydrocarbon glow discharge system, have been investigated by mass spectroscopic thermal effusion measurements. Depending on the bias voltage U sub B as the most important deposition parameter, hydrocarbons and/or H2 molecules are desorbed at threshold temperatures between 300 and 600 degrees C. The threshold temperature increases with increasing bias voltage while the mass of the desorbed molecules decreases. a-C:H films deposited at low bias release H2, CH4, and higher hydrocarbons whereas from hard a-C:H films deposited at U sub B above 500 V, only H2, is released. Using double-layer a-C:H/a-C:D films, it is shown that H2 and CH4 molecules are formed in the volume of the film followed by molecular diffusion through the a-C:H network. For high-bias voltages (U sub B above 500 V), the reduced pore size of the strongly crosslinked a-C:H network is shown to prevent diffusion of hydroca rbon molecules, while the films are still permeable for hydrogen. (IAF)