Enhanced Reduction of Thermal Conductivity in Amorphous Silicon Nitride-Containing Phononic Crystals Fabricated Using Directed Self-Assembly of Block Copolymers

Abstract

Studies have demonstrated that the thermal conductivity (κ) of crystalline semiconductor materials can be reduced by phonon scattering in periodic nanostructures formed using templates fabricated from self-assembled block copolymers (BCPs). Compared to crystalline materials, the heat transport mechanisms in amorphous inorganic materials differ significantly and have been explored far less extensively. However, thermal management of amorphous inorganic solids is crucial for a broad range of semiconductor devices. Here we present the fabrication of freestanding amorphous silicon nitride (SiN x ) membranes for studying κ in an amorphous solid. To form a periodic nanostructure, directed self-assembly of cylinder-forming BCPs is used to pattern in the SiN x highly ordered, hexagonally close packed nanopores with pitch and neck width down to 37.5 and 12 nm, respectively. The κ of the nanoporous SiN x membranes is 60% smaller than the classically predicted value based on just the membrane porosity. In comparison, holes with much larger neck widths and pitches patterned by e-beam lithography lead to only a slight reduction in κ, which is closer to the classical porosity-based prediction. These results demonstrate that κ of amorphous SiN x can be reduced by introducing periodic nanostructures that behave as a phononic crystal, where the relationship between the smallest dimension of the nanostructure and the length scale of the mean-free paths of the dominant, heat-carrying phonons is critical. Additionally, changing the orientation of the hexagonal array of nanopores relative to the primary direction of heat flow has a smaller impact on amorphous SiN x than was previously observed in silicon.

Ultimate suppression of thermal transport in amorphous silicon nitride by phononic nanostructure

“…Al pads with thickness of 130 nm are deposited on the center and the edge of the suspended bridge structure for the thermal transport measurements. Detailed information on experimental fabrication processes is discussed in the "Fabrication of PnCs” section and in our recent report ( 27 ).…”

Akhiezer mechanism dominates relaxation of propagons in amorphous material at room temperature

“…However, recent theoretical works of Larkin et al [17] and Zhou et al [18] have shown that propagons in amorphous silicon (a-Si) can have long MFPs that range from tens of nanometers to several micrometers, and thus propagons could contribute to about 40% of the total κ of the bulk [17,18]. Experimental works favor their theoretical analysis, in which 50% of κ reduction has been reported in the cross-plane direction of a-Si thin films [19] and in-plane directions of suspended nanostructures [20][21][22]. These findings strongly suggest that propagons in fact are able to contribute to a relatively large part of the total κ of amorphous, and thus should be considered when analyzing the heat conduction, despite their small density of states.…”

Fabrication of Nanodevices Through Block Copolymer Self-Assembly

“…Fortunately, the self-assembly of BCPs provides a simple, convenient, and lowcost strategy to construct photonic nanodevices with a variety of complex architectures. Currently, two specific routes have been broadly adopted: BCP co-assembly (Hoheisel et al, 2015;Sarkar and Alexandridis, 2015;Stefik et al, 2015) and BCP templating (Kim et al, 2016;Hulkkonen et al, 2018;Zhou et al, 2020).…”