Integrated circuit (IC) package substrates having high density interconnects with a sputter seed layer containing a copper alloy, as well as related structures, devices, and methods, are disclosed herein. For example, in some embodiments, a package substrate may include a first dielectric layer, a sputter seed layer disposed on the first dielectric layer, wherein the seed layer includes a copper alloy, a patterned conductive layer disposed on the seed layer, and a second dielectric layer over the patterned conductive layer.