Vertical Self-Defined Thermoelectric Legs for Use in ThinFilm Micro Thermo Electric Generators (uTEG)

Abstract

Micro thermoelectric generators (μTEGs) are solid-state devices that directly convert thermal power into electrical power through the Seebeck effect, a solid-state transduction mechanism. Through this effect, μTEGs can harvest power from temperature gradients available in their operating environment. They are capable of providing a robust and long-term power solution for remote sensing and internet of things (IoT) applications where there exists high servicing costs, harsh environments, or the need for long-term device operation. In particular, thin-film based μTEGs are desirable due to ease of process integration, high throughput, material quality, and reproducibility. However, thickness constraints inherent to thin-film processes limit their potential usage. In conventional TEGs, the thickness of the thermoelectric film itself determines the separation distance between the hot and cold terminals. A very thin thermoelectric film thus creates a thermal short. This reduces the temperature difference across the device, limiting power output. The focus of this dissertation is to remove this thermal limitation in thin-film generators. This is accomplished through a new μTEG design that decouples the height of the thermoelectric elements from the film thickness. Central to the implementation of the proposed design is the creation of thermoelectric (TE) films deposited over the sidewalls of high-aspect vertical columns. In this way, the height of the columns, and not the thickness of the TE film, sets the separation distance between the hot and cold ends of the thermocouples. In this thesis, performance of this new μTEG design is analyzed. Bi2Te3 and Sb2Te3 thermoelectric films compatible with the proposed design are developed and integrated into functional μTEGs. The impact of column material, thermocouple height, and fill factor on μTEG performance are also presented. The thermoelectric films used in this design are industry standard Bi2Te3 and Sb2Te3. The crystal structure of these films grown on vertical surfaces was found to differ significantly from that grown on standard planar substrates. Potential causes for this difference and impact on μTEG performance are investigated. Additionally, factors that impact Bi2Te3 and Sb2Te3 film growth are studied. These factors include surface topology, substrate material, deposition temperature, and the presence of a seed layer. Key components required for the successful fabrication of μTEGs utilizing high-aspect column designs were developed. These include the creation of thermally insulating high-aspect columns, contact formation between N & P thermoelectric elements, and die attachment. The fabricated μTEGs have thermocouple heights of 20 μm using 2 μm thick films and a fill factor of 17.5%. The measured power output of the fabricated generators is 4-5 μW/K2/cm2. These μTEGs use thermoelectric films grown over sidewall surfaces. The power factors of the sidewall films were 0.85 and 1.36 mW/K2m for the N and P type films, respectively. Sidewall film performance was poorer in comparison to N and P type thermoelectric films grown under similar conditions on planar surfaces. These planar films had power factors of 3.63 and 1.30 mW/K2m for the N and P type materials.