Thermo-Harvested Autarkic Wireless Integrated Transmitter (THAWIT)

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For the internet of things in general and medical sensors in particular, low power wireless data transmitters are required. Up to date, most of such transmitters require batteries or inefficient radio frequency (RF) harvesting. In THAWIT, we want to investigate and demonstrate the first wireless data transmitter, which can be supplied by means of compact, integrated, thermo-electric harvesting. To allow supply-autarkic operation, we have to meet the following challenges. The power consumption of the transmitter must be massively reduced. Moreover, the power density of the thermoelectric devices must be increased. A major challenge is the capability for integration on silicon limiting the possible material compositions. We want to develop silicon-compatible harvesting elements with a leading-edge power density beyond 10 μW/mm2 at a moderate temperature difference of 7 K. To meet this ambitious goal, we rigorously optimise the geometry and material compositions. A high thermoelectric leg height is necessary for high output power density, affecting the choice of the photoresist and deposition parameters of the thermoelectric material. A new process for the p-type thermoelectric leg is researched to reduce the thermal conductivity. In addition, we develop a silicon-compatible micro-supercapacitor serving as energy buffer. To enable compatibility with the fabrication technology and equipment at IFW, we realize it by a high-performance all-solid-state interdigital in-plane micro-supercapacitor. It is based on photo-lithographically patterned sputtered electrodes and a solid electrolyte. Considering energy saving losses and component areas of 2 mm2, the CMOS transmitter must operate with a dc power of 10 μW or less. To enable such an ultra-low dc power, we study direct modulated oscillators with aggressive duty-cycling of around 0.1 %, ultra-fast ramp-up and miniaturized leakage in the sleep phases. In addition to impedance-matched antennas requiring buffer amplifiers, we consider also inductive antennas which can be directly incorporated in the LC-resonator of the oscillator. A textile-compatible woven or printed antenna is designed for the demonstrator. The demonstrator shall operate according to the medical implant communication service (MICS). The final goal is to show data transmission around 400 MHz with a data rate of 1-10 kb/s for distances of up to 1 meter. THAWIT combines the complementary competences of Gabi Schierning, IFW, in the area of thermoelectric devices, and Frank Ellinger, TUD regarding low power integrated RF circuits.