Embodiments of the present disclosure relate to a showerhead assembly for use in a processing chamber. The showerhead assembly includes a porous insert disposed in a space defined between a gas distribution plate and a base plate to moderate the corrosive radicals resulting from plasma ignition to reduce particle issues and metal contamination in the chamber. The porous insert is a conductive material, such as metal, used to reduce the gap electrical field strength, or may be a dielectric material such as ceramic, polytetrafluoroethylene, polyamide-imide, or other materials with a low dielectric loss and high electrical field strength under conditions of high frequency and strong electric fields. As such, the electrical breakdown threshold is enhanced. The porous insert may reduce and/or eliminate showerhead backside plasma ignition and may include multiple concentric narrow rings that cover gas holes of the gas distribution plate.