The present invention provides systems, methods and apparatus for heating substrates in a processing chamber to temperatures up to at least 700.degree. C. In accordance with an embodiment of the invention a heater assembly with an inner core of high thermal conductivity is encased in a shell of lower thermal conductivity, creating a nearly isothermal interface between the core and shell. The inner core is brazed to the shell, promoting thermal transfer, and acts as a thermal short between opposing surfaces of the shell. The heater assembly is designed to minimize thermal stresses arising from the difference in the thermal expansion coefficients of the various components of the multi-layered heater assembly. In one embodiment of the invention, two independently-powered heating elements are arranged concentrically to each other to create a dual zone heater. A thermal gap in the inner core between the inner and outer heating elements de-couples the zones and provides a more controllable temperature profile at the surface of the heater, including excellent temperature uniformity. In one embodiment, an RF isolator is placed between a heater and a support shaft, allowing the heater to be powered as an electrode in a plasma process.