A top assembly for a processing chamber having a back plate and a hub is provided. The back plate has a first portion and a second portion. The first portion is connected to the second portion through a central region of the back plate, wherein a gap is defined between opposing surfaces of the first and second portions outside the central region. The first portion includes an embedded heating element. The hub is affixed to a top surface of the second portion of the back plate over the central region. The hub has a top surface with a plurality of channel openings defined within a central region of the hub and a bottom surface having a central extension with a plurality of channels defined therethrough. The bottom surface includes an annular extension spaced apart from the central extension.