AbstractsEngineering

This work presents a comprehensive framework for the fabrication and further optimization of microscale thermoelectric generators, with electrochemical deposition applied as the primary synthesis method for the constituent materials. The pulsed galvanostatic deposition of n-type Bi2Te3 and p-type Sb2Te3 from nitric acid based electrolytes is investigated and optimized with respect to the composition, morphology and thermoelectric properties of the resulting films. The optimized plating protocols can be used for the deposition of near-stoichiometric materials with excellent morphology and a thickness of at least 50 µm for Bi2Te3 and 10 µm for Sb2Te3. For the measurement of the thermoelectric material properties, specialized, non-destructive characterization procedures are developed and evaluated, which allow the determination of the layer properties in the presence of an optimized seed layer. This allows for an efficient in-line characterization of the deposited materials, with an accuracy of ~10% for the electrical resistivity and carrier concentration and ~20% for the determination of the Seebeck coefficient. As the quality of the as-deposited electroplated layers is not sufficient for the integration in highly efficient thermoelectric devices, a post deposition annealing in a saturated tellurium atmosphere is applied in order to further improve the stoichiometry and crystallinity of the materials. Due to the corresponding increase in carrier mobility and reduced carrier concentration, the annealing procedure leads to a significant improvement in the thermoelectric properties for both n- and p-type materials, with an estimated maximum ZT value at room temperature of ~0.4 for Bi2Te3 and ~0.6 for Sb2Te3. The material choices and fundamental concepts of the targeted device fabrication process are first established and optimized based on a reduced process for the manufacture of thermoelectric unicouples. The chip layout created for this purpose contains all structures necessary for the evaluation and optimization of the deposition process, the material properties and compatibilities, as well as the final unicouples. Based on this platform, the ideal photoresists, contact materials and processing conditions are identified and transferred to the final, full device fabrication process. This process introduces two novel technological approaches, which are the use of two simultaneously processed layers of a dry film photoresist as a template and the use of a commercial automated dispenser as a structuring tool for the successive deposition of the n- and p-type thermoelectric material. The resulting fabrication process can, based on the technologies introduced here, largely be conducted outside a cleanroom environment and is therefore significantly less complex and expensive compared to standard procedures. The finally fabricated proof-of-principle microscale devices contain seven thermocouples with a leg diameter of 300 µm and a leg height of 50 µm, gold as the lower and copper as the upper contact material, Bi2Te3 as n- and… Advisors/Committee Members: Woias, Peter (advisor).