A method is provided for processing a substrate including providing a processing gas comprising an organosilicon compound comprising a phenyl group to the processing chamber, and reacting the processing gas to deposit a low k silicon carbide barrier layer useful as a barrier layer in damascene or dual damascene applications with low k dielectric materials. A method is provided for depositing a silicon carbide cap layer that has substantially no phenyl groups attached to silicon atoms from a processing gas comprising an oxygen-free organosilicon compound on a low k silicon carbide barrier layer.