The effects of postannealing treatment in ambient forming gas (10%normalH2:90%normalN2) on low- k SiOC(H) films deposited by plasma-enhanced chemical vapor deposition were investigated. The use of SiOC(H) films has certain advantages due to the presence of alkyl groups in the film, which result in improved hardness properties, compared with previously reported low- k materials. Metal-oxide-semiconductor capacitance–voltage measurements at 100 kHz indicated that the relative dielectric constant ( k -value) of the as-grown film was approximately 2.4. When rapid thermal annealing (RTA) temperatures of up to 500°C were used, the Si–O–C bonds were nearly maintained when the annealing was conducted using an ambient of forming gas, whereas they substantially decreased in an ambient of normalN2 gas. The decrease in Si–O–C bond content results in an increase in k -value. In the film that underwent an RTA treatment at 600°C , the alkyl groups of the film were released and the Si–O network was enhanced in both ambient gases, which has a critical effect on the change in k -value.