• Silicon
• SEM
• Nanostrures
• Thermoelectric devices

The focus of this thesis is the fabrication of a thermoelectric device (TEG) with high efficiency, consisting of silicon nanostructures. The efficiency of a thermoelectric generator is related to the figure of merit, Z : maximizing Z is equivalent to choosing a material with a high thermal conductivity , a high Seebeck factor and a thermal conductivity as small as possible. The choice of silicon, is mainly due to its properties: it is a bio-sustainable material and is present in abundance on Earth; in addition, the its chemical, physical and structural characteristics are well known and this makes it much used in the nanotechnology and nano-electronics industry. However, silicon, compared to other materials used for thermoelectric applications, shows a problem: it has a high thermal conductivity , equal to 148 W/(m K), mainly due to the effect of phonons. Several works, present in the literature, have shown that, by reducing up to a nanoscale, the conductivity of the up to a few W/(m K). In this way the free path of phonons becomes comparable with the size of the thermoelectric device and therefore the propagation of lattice vibrations is limited by surface scattering, resulting in a sharp reduction in thermal conductivity. The subject of the thesis was focused on the development of planar nanostructures, carried out by high resolution electron beam lithography (HR-EBL) and plasma-assisted chemical etching techniques (RIE). The device was made on a silicon on insulator (SOI) 260 nm substrate and, using lithography techniques, have been realized two different layers: the first for the layout of the mask, the heart of the device, and a second for the gold contacts, useful for the measurements. The various process steps were made up of different phases:
• Substrate preparation and resist deposition (PMMA);
• Exposure: the substrate, through the layout of the mask, is subjected to 2 the action of electronic beam radiation, in order to sensitize of the resist covering the areas of the substrate;
• Development of the resist: removal from the substrate, by chemical solvents, of the portions of resist that have become more soluble;
• Deposition of a thin film of aluminium by evaporation: the metal it covers the substrate areas left without resistance;
• Lift-off: elimination, with very powerful chemical solvents, of the remaining parts of un polymerized resist, so that only the metal circuit paths exactly complementary to those of the drawing imprinted on the mask.
After these phases, through a dry etching process and an etching through BHF, we proceeded with the creation and suspension of the nanomembranes of the device.In parallel with the manufacture of the device, simulations were carried out with COMSOL Multiphysics, to evaluate the heat transmission resulting from the application of a temperature difference on the structure of interest. Specifically, a thermal simulation has been implemented and a multiphysical simulation, which both led to a reduction in temperature limited mainly to the ends of nanostructures. The final part of the work focused on the measurement of the device, in order to check that it is working property. In detail, the following were carried out: a four-contact electrical measuring, to characterize the resistance of the central metal and a 3 omega measuring, to estimate a value of thermal conductivity