Fabrication and characterization of freestanding phononic thermocrystal membranes via block-copolymer directed-self assembly

Block-copolymer (BCP) directed self-assembly (DSA) is a valuable technique that enables formation of defect-free, single crystal nanostructures over large areas. This is accomplished via a chemical template patterned via ebeam lithography. For perpendicularly oriented cylinder-forming BCPs, which spontaneously form a polycrystalline hexagonally close-packed lattice, controlling the orientation of the DSA pattern enables direct control of the in-plane lattice orientation, total pattern area, and number of periods. For BCPs such as PS-PMMA, the nanostructures can be transferred into an inorganic substrate, such as Si, and integrated into fabrication process flows for complex devices. Here, we present a methodology for integrating cylinder forming PS-PMMA DSA with a novel fabrication process to produce freestanding nanoporous Si membranes that scatter heat-carrying phonons. By controlling the orientation of the hexagonal lattice, a line-of-sight heat transport pathway can be opened or closed, which combined with nanometer scale of the BCP pores, provides the needed device length-scales to probe heat-carrying phonons. In this work, we report fabrication of and measurements on such membranes as a function of porosity, neck size, self-assembled vs DSA pores, and total pattern area.