Abstract: Apparatus for use in treating semiconductor wafers or other work pieces by an active ion technique or by chemical vapour deposition, comprising a "vacuum" chamber having means for connection to an evacuating pump and to a source of the selected gas, an induction coil surrounding part of the chamber and connected to an alternating current supply to create a plasma within the chamber containing ionised elements of the gas, a support for the work piece within the chamber, and means for creating an electric field between the support and part of the chamber where the plasma exists.