c before or during release in the vacuum chamber the precursor is delivered to the vacuum chamber e g through a heat resistant porous body with a porosity of e g 30 to 50 the precursor can continuously be metered in small quantities through a tube which delivers it as a vapor or a mist to the porous body otherwise the precursor can be introduced in batch in an open reservoir which is placed in the chamber before this is put to vacuum this reservoir can be electrically heated prior to the start of the deposition process inside the chamber to form the vapor a bundle of alloyed tungsten filaments is placed as a cathode typically at about u2212 150 v dc bias voltage in front of the earthed porous body the body itself with the inlet tube for the precursor at the backside of the porous body may be mounted in the lower part of the chamber the filament bundle is convexly bent e g to a half circle in a vertical plane and with a length of 15 to 30 cm the current in the cathode filaments is preferably between 50 and 120 a ac the plasma current measured between filament bundle and earthed porous body can then typically amount to about 0 5 to 3 a preheating the precursor may offer the advantage that the current needed afterwards in the cathode filaments for generating the plasma could be.