We have investigated the time dependent reliability of a-C:F films, and found that SiO 2 or SiN cover layers on a-C:F films peeled off partially after about a week when the a-C:F film was grown from C4F8. and annealed in N2 or grown from C4F8+CH4. The interfaces between a-C:F and SiN have a mixing layer indicating a radical reaction at the interface. An a-C:F film grown from C4F8+CH4 has greater water-absorption than a film grown from C4F8 Thus, hydrolysis and radical formation after annealing may cause degradation of the interface. We found that hydrogen annealing effectively decreases the dielectric constant and suppresses such time-dependent degradation.