Abstract: A vacuum chamber contains a crucible (4) and an electron beam source (5) for evaporating material in the crucible. A substrate holder (6) holds substrates (7) above the crucible (4) with a process space therebetween. A magnetron cathode (11, 12) is located in each of two compartments (9, 10) located on either side of the process space. An aperture (21, 22) connects each compartment to the process space; each cathode (11, 12) carries a target (13, 14) facing away from the respective aperture (21, 22). The cathodes are connected to a medium frequency RF power supply (16), and process gas is supplied to the compartments by lines (17, 18).