Thermoelectric Enhancement of Thin Si Films by means of Block Copolymer Driven Nanostructuration

Thermoelectricity explains the conversion of heat into electricity and vice versa, and a plethora of applications are available nowadays for thermoelectric systems in the fields of cooling, power generation and sensing. Traditionally, the most used thermoelectric materials have been based on chalcogenides, as the main reason that has driven the material choice has been the maximization of the thermoelectric figure of merit (ZT). This figure of merit indicates the performance capabilities of a material as a thermoelectric, and depends on its Seebeck coefficient and its electrical and thermal conductivities. Nevertheless, these widely used materials are based on alloys of scarce, expensive, and toxic elements (Bi, Te, etc.) that are difficult to integrate within the semiconductor industry. On the contrary, silicon (Si) is a poor thermoelectric material when in its bulk form but it has been shown to improve around 100 times its thermoelectric performance when it is nanostructured in two- or one-dimensional structures, then becoming a competitive alternative [1]. This improvement is mainly justified by the effective reduction of the phononic thermal conductance in the nanostructured material.<br/>In this contribution, we present the study of suspended Si ultra-thin films as thermoelectric material, which are much easier to integrate into semiconductor technologies. Although the enhancement in thermoelectric performance of thin films is modest when compared to one-dimensional structures, this is compensated and enhanced by introducing surface nanostructuration with the aim to reach thermoelectric figures of merit similar to the best reported values in the literature for Si structures. These structures are based on rough surface Si nanowires (NWs), whose large-scale fabrication remains unsolved as the synthesis of high density and uniform Si NW arrays with homogeneous nanoscopic surface morphology is a challenging process. Here, we present a cost-effective and scalable approach for the fabrication of the membranes by block copolymer (BCP) nanopatterning. BCP based technologies present manufacturing advantages, as it is a technology easy to scale up for high-volume manufacturing, very cost-effective and capable of achieving sub-10 nm resolution [2].<br/>Nanostructured Si membranes are fabricated on the device layer of a Silicon on Insulator (SOI) wafer with ultrathin device layer (30 – 50 nm). A thin film BCP is self-assembled perpendicularly oriented to the Si surface. The BCP used for surface nanostructuration is polystyrene-block-polymethylmethacrylate (PS-b-PMMA) with cylindrical or lamellar morphology and a period between 28 and 80 nm. After self-assembly, the PMMA block is selectively removed, and PS features are transferred into the Si underneath by reactive ion etching (RIE). As a result, we obtain a nanostructured surface with a hexagonal distribution of holes when a BCP with cylindrical morphology is used as a mask, and a fingerprint-type nanostructuration when a lamellar BCP is used as a mask. The period of such structures is controlled by properly blending BCPs of different molecular weights and the depth and the shape of the walls are tuned by the RIE conditions. The final structures are doped by spin-on dopant to achieve an optimal doping concentration, 10¹⁹-10²⁰ cm^-3, for thermoelectric applications. This method has been chosen because it’s a low cost, wafer scale compatible and non-destructive process, and the characterization of the doping values obtained has been done with the 4-probe method on the thin films.<br/>We will study the effect of surface nanostructuration on the thermoelectric performance. The thermal conductivity of the membranes will be evaluated using the <i>3w Völklein</i> approach on an appropriate test structure, in order to find the most optimal nanostructuration conditions for thermoelectricity.<br/> <br/>[1] D. Dávila, et al. Nano Energy 2012, 1 (6), 812<br/>[2] C. Pinto-Gómez et al. Polymers 2020, 12(10), 2432