the invention relates to apparatus for producing a microwave plasma for the treatment of substrates , and in particular for the plasma polymerization of monomers for coating substrates . the apparatus consists of a reaction chamber with a carrier for conveying the substrates and a way for maintaining an atmosphere of ionizable gases and monomers . the apparatus also has a first and at least one second wave - guide structure which are arranged at opposite acute setting angles to the surface of the substrate carrier and are each connected at one end to a microwave transmitter by way of a hollow conductor . thus , the treatment intensities , i . e . the rates of deposition , are superposed , and this leads to greater uniformity in the properties of the product . the apparatus may also include angling the wave - guide structures toward each other and staggering crossbar structures in the wave - guides for further improvement .