A plasma source includes a first electrode and a second electrode having respective surfaces, and an insulator that is between and in contact with the electrodes. The electrode surfaces and the insulator surface substantially define a plasma cavity. The insulator surface defines one or more grooves configured to prevent deposition of material in a contiguous form on the insulator surface. A method of generating a plasma includes introducing one or more gases into a plasma cavity defined by a first electrode, a surface of an insulator that is in contact with the first electrode, and a second electrode that faces the first electrode. The insulator surface defines one or more grooves where portions of the insulator surface are not exposed to a central region of the cavity. The method further includes providing RF energy across the first and second electrodes to generate the plasma within the cavity.