A method of plasma treating an article in a housing having a chamber in which the article such as a wafer can be treated with plasma. The housing includes at least one inlet port connected to an interior of the chamber through which process gas can be supplied to the chamber. A radiofrequency energy source is arranged to pass radiofrequency energy into the chamber and induce plasma in the interior of the chamber by activating, with an electric field induced by the radiofrequency energy source, process gas supplied to the chamber through the inlet port. A dielectric window having an inner surface thereof forms part of an inner wall of the chamber. Radiofrequency energy passes from the radiofrequency energy source to the interior of the chamber through the dielectric window. The dielectric window has a thickness which varies at different points along the inner surface thereof such that the thickness is largest at a central portion of the dielectric window. The dielectric window is effective to decrease the induced electric field in the interior of the chamber near the central portion of the dielectric window.