Improved design of an all-Si based thermoelectric microgenerator

Abstract

Environmental energy harvesting to power Internet of Things (IoT) systems can be achieved through thermoelectric microgenerators (μTEGs) potentially eliminating the need for batteries or extending their operational life. μTEGs are good candidates due to their scalability and their adaptability to different thermal gradients and energy densities. However, the commonly used thermoelectric materials with good thermoelectric properties (e.g. Bi2Te3) are not compatible with down-sizing the generators to the microscale by using MEMS technology. Conversely, materials traditionally used in microelectronics (e.g. Si) have poor thermoelectric performance limiting the efficiency of thermal- to- electrical conversion due to their high thermal conductivity. The key to deal with these issues lies on silicon micromachining and nanostructuring yielding to a significant thermal conductivity reduction of the functional silicon material (nanostructuring) and the improved thermal management of the silicon-based device (micromachining). After having worked with the architectural development of the unitary μ-thermocouple, this work reports on an improved compact design of series connected silicon-based μ-thermocouples to enhance the generated power. Each thermocouple features a planar architecture with a suspended microplatform surrounded by a bulk Si rim. Bottom-up silicon nanowires are integrated as the thermoelectric active material which captures a fraction of the internally available temperature difference turning the heat flow into electricity and hence into useful power. A thin film layer of W closes the thermoelectric circuit in a uni-leg configuration. In order to multiply the output, the improved design consists of 10 μ-thermocouples arranged in series in an area of 50 mm2. For the purpose of this work, each thermocouple can be measured individually. They harvest about 3 nW when the heat source available is at 125°C. These values are low as expected for microdevices into which a heat exchanger is not integrated, the resulting bad thermal contact to the ambient avoiding an effective cooling by natural convection. In any case, the generated power is increased when connecting electrically the different μ-thermocouples in series reaching power densities of 60 nW/cm2. The fabricated all-silicon based microgenerator provides a promising energy harvester for advanced IoT systems operating in low-grade waste heat environments.