A support 55 comprises a dielectric 60 covering a primary electrode 70, the dielectric 60 having a surface 75 adapted to receive a substrate 25 and a conduit 160 that extends through the dielectric 60. The thickness of a portion of the dielectric 60 between an edge 195 of the primary electrode 70 and a surface 180 of the conduit 160 is sufficiently large to reduce the incidence of plasma formation in the conduit 160 when the primary electrode 70 is charged by an RF voltage to form a plasma of gas in the chamber 30 during processing of the substrate 25.