Embodiments of the present invention pertain to methods of forming features on a substrate using a self-aligned triple patterning (SATP) process. A stack of layers is patterned near the optical resolution of a photolithography system using a high-resolution photomask. The heterogeneous stacks are selectively etched to undercut a hard mask layer beneath overlying cores. A dielectric layer, which is flowable during formation, is deposited and fills the undercut regions as well as the regions between the heterogeneous stacks. The dielectric layer is anisotropically etched and a conformal spacer is deposited on and between the cores. The spacer is anisotropically etched to leave two spacers between each core. The cores are stripped and the spacers are used together with the remaining hard mask features to pattern the substrate at triple the density of the original pattern.