A reactor for plasma-enhanced chemical vapor deposition having a showerhead electrode facing the wafer being CVD deposited, the showerhead having a large number of jetting holes for jetting processing gas towards the wafer. Two deep grooves are formed around the area of the showerhead containing the jetting holes. The grooves are formed from opposite sides of the showerhead and are radially offset from each other, thereby forming a thin wall between the grooves in the body of the showerhead. The thin wall acts as a thermal choke, thus reducing the heat flow to the support of the showerhead and also rendering the temperature distribution more uniform across the face of the showerhead. The thin wall further acts as a mechanical bellows to accommodate differential thermal expansion between the showerhead and its support.