In a plasma CVD process in which a substrate (10) bias voltage is applied for ion bombardment control during coating, the substrate voltage (US) is produced independently of the coating plasma (20) and is varied during coating. Also claimed is equipment for carrying out the above process, including a substrate voltage generator (13) which is controlled separately from the plasma generating system (15-19). Also claimed is a novel multilayer structure consisting of alternate individual hard material and carbon or silicon layers.
