Printhead fabrication methods, printhead substrate assembly 32 fabrication methods, and printheads are described. According to one aspect, a printhead fabrication method includes providing a substantially planar upper surface 34 of a substrate assembly 32 which comprises circuitry 13, wherein the providing comprises, using a fill material 26, filling a plurality of valleys 15 of the substrate assembly 32 which are defined by a plurality of circuitry protrusions 14 of the circuitry 13. The method may also include, after the providing the substantially planar upper surface 34 of the substrate assembly 32, providing a plurality of printhead structures 50 over the substantially planar upper surface 34 of the substrate assembly 32 to form a printhead.