This work presents a new plasma treatment for the adhesion enhancement of a plasma deposited silica layer to polycarbonate. After initial oxygen plasma treatment of the polycarbonate surface, a thin hydrogenated amorphous carbon layer is deposited before the silica layer. This interlayer is found to increase the number of Si-O-C bonds at the interface. The treatment leads to a significant adhesion enhancement of the silica layers and to increased resistance to temperature changes and moisture.