Micro Thermoelectric Devices: From Thermal Management to Powering the Internet of Things

Micro-thermoelectric devices (µTEDs) hold significant potential for various applications in the biomedical field, powering internet-of-things devices, and thermal management. To enable their use in such applications, robust packaging is crucial, allowing direct thermal contact between the µTEDs and the target heat sink and source. However, conventional packaging techniques designed for larger modules are unsuitable for µTEDs due to excessive thermal resistance between the capping material and the device, which hampers performance. In this study, we present the fabrication of µTEDs using optimized geometry and contact resistance, coupled with a novel packaging technique that seamlessly integrates with on-chip systems. Our fabrication process involves the creation of micro thermoelectric coolers (μTECs) with vertically free-standing leg pairs, eliminating the need for a top plate. To meet real-world requirements, we embed these free-standing devices to form a flat surface at the top level, ensuring direct thermal contact with the heat sink. This embedding not only enhances mechanical stability but also provides chemical stability. We employ photoresist as the filling material due to its low thermal conductivity and excellent processability. By characterizing the μTECs with and without photoresist using a thermoreflectance thermal imaging technique, we analyze the devices' cooling efficiency and the impact of the photoresist matrix on their performance. The fabrication process of the μTEC involves a combination of photolithographic patterning and electrochemical deposition of Bi2(TexSe1-x)3 and Te, which serve as the n-type and p-type thermoelectric materials, respectively. Through optimization of the geometry and contact resistance of the μTECs, we achieved a maximum cooling effect of approximately 10.8 K at an applied electrical current of 235 mA. Furthermore, our reliability studies demonstrated a rapid response time of 700 µs and over 100 million cooling cycles without device failure. We also conducted finite element analysis to investigate the impact of geometry and contact resistance on the cooling power density and net cooling temperature, offering valuable guidelines for fabricating thermoelectric coolers with optimal performance. In the case of embedded µTECs, the photoresist matrix acts as a thermal shortcut that results in additional thermal loss, leading to a slight reduction of the maximum cooling from 10.8 K to 9.6 K. This decrease can be attributed to thermal loss through the polymer. Nevertheless, the embedded devices exhibit a rapid cooling response time of 721 µs and demonstrate only a slight reduction in cycling reliability, with a lifespan of 85 million cycles. These embedded, optimized, stable, and easily scalable µTEDs present exciting opportunities for widespread applications in the biomedical field, powering internet-of-things devices, and localized heat management.<br/> <br/>References: 1. Zhang, Q., Deng, K., Wilkens, L., Nielsch, K. et al. Micro-thermoelectric devices. Nat. Electron. 5, 333– 347 (2022). 2. Dutt, A. S., Deng, K., Li, G., Pulumati, N., Ramos, D. L., Barati, V., Garcia, J., Perez, N., Nielsch, K., Schierning, G., Reith, H., Adv. Electronic Mater. 8, 2101042 (2022). 3. Li, G., Garcia Fernandez, J., Lara Ramos, D.A., Nielsch, K. et al. Integrated microthermoelectric coolers with rapid response time and high device reliability. Nat. Electron. 1, 555–561 (2018)