Multilayer metal stack heater

A silicon photonic integrated circuit has a waveguide 112 on its top surface with a heater element 110 on the waveguide. The heater element has a first metal layer on the waveguide and a second metal layer, with a thickness less than 300nm and a different composition to the first metal layer, on the first metal layer. The first metal layer may be a barrier layer. The second metal layer may have a sheet resistance lower by a factor of three or a conductivity greater by a factor of three than that of the first metal layer. The first metal layer may be composed of titanium tungsten and the second layer may be composed of gold. The second metal layer may have a thickness less than 100, 50 or 30nm. The heater element may have a third metal layer on the second metal layer with a different composition than the second metal layer. The third metal layer may be composed of titanium tungsten. The heater element may have contact pads 115 at, and electrically connected to, each end where one contact pad which is not on the waveguide has an upper surface suitable for forming a wire bond.