Integrated circuit package substrates with high-density interconnect architecture for scaling high-density routing, as well as related structures, devices, and methods, are generally presented. More specifically, integrated circuit package substrates with fan out routing based on a high-density interconnect layer that may include pillars and vias, and integrated cavities for die attachment are presented. Additionally, integrated circuit package substrates with self-aligned pillars and vias formed on the high-density interconnect layer as well as related methods are presented.