a which transmit the reactive gas provided by the holes 109 b in the backing metal electrode 109 the reactive gas is injected through ducts 111 connected to an external gas source 113 the pumping means are not illustrated the rf source 115 is connected to the electrode 109 as illustrated the design of the backing electrode 109 could have been a traditional u201c shower head u201d as electrode 3 in fig1 another option is the cascaded gas manifold design which is shown in fig1 in fig1 a microwave capacitive plasma reactor 40 is diagrammatically illustrated the illustration shows a possible design according to which a rather thick tailored layer generally referenced as 120 the thickness of which is designated as e 9 is used to compensate for the drastic non uniformity due to electromagnetic propagation the illustrated reactor 40 is a reactor for etching a rather small wafer the microwave comes from a coaxial wave guide 121 which expands gradually at 122 u201c trumpet u201d shaped to avoid reflection then the microwave reaches the process zone 13 where the wave should converge to the center of the reactor which is cylindrical for the dimensions the substrate 35 arranged on a flat counter electrode 126 has a diameter of about 10 cm and an 1 ghz wave is generated by the microwave generator 123 30 cm free space wave length the central thickness of the tailored layer 120 if made of quartz should be about the same as the space 13 of the free plasma itself it is presently proposed that the tailored layer.