Plasma CVD method and apparatus are described. The apparatus comprises a vacuum chamber in which two pairs of electrodes are provided. A high frequency voltage is applied to one of the pairs in order to produce a plasma from a reactive gas in the chamber. A substrate to be coated is located between the other of the pairs. A relatively low frequency voltage is applied to the other pair of electrodes. By virtue of the low frequency voltage, the substrate is exposed to the bombardment of ions of the plasma during deposition. The bombardment functions to remove relatively soft portions of the depositing material.