Single-crystalline films of silicon with a thickness of 200 nm (silicon-on-insulator, Soitec) served as the thermoelectric material. A layer of SiO2 (500 nm) formed on the top silicon by plasma-enhanced chemical vapor deposition served as masks for doping. Patterning relied on photolithography and a combination of reactive ion etching (CF4 + O2) and wet etching (buffered oxide etchant) to define openings in the SiO2 for solid-state diffusion doping. The p- and n-type regions resulted from doping with boron (1000°C, 14.5 min) and phosphorus (1000°C, 5.5 min), respectively. Each dopant type required its own SiO2 mask. A photolithographically patterned layer of photoresist (S1805, MicroChem) defined regions of via holes (3 μm diameter, 50 μm pitch), formed through silicon by reactive ion etching (SF6). These holes also enabled etching of the buried oxide layer by immersing the wafer in 49% hydrofluoric acid for 30 min. Thoroughly rinsing the sample with deionized water prepared it for transfer printing. A flat slab of polydimethylsiloxane (Sylgard 184, 1:4) enabled retrieval of the photoresist/Si film and delivery onto a bilayer of polyamic acid (precursor to form 4-μm PI, PI 2545, HD MicroSystems) and poly(methylmethacrylate) (PMMA; 200 nm, MicroChem) spin coated sequentially on a silicon handling wafer. Immersion in acetone dissolved the photoresist. Baking in a vacuum oven completed the curing of the PI. Last, photolithography and reactive ion etching (SF6) patterned the silicon into isolated p- and n-type serpentine-like layouts.

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