Applications of CCD-Thermoreflectance to Thin Film Thermoelectric Characterisation

Abstract

In recent years the microelectronics industry has been calling out for alternative thermal cooling approaches due to increased heat fluxes in microscale electronic and photonic devices. One such approach is to use thin film thermoelectric devices (TECs) due to their reliable solid-state design, low energy consumption and high speed. This thesis is centred around the use of CCD - thermoreflectance imaging (CCD-TR) as a characterisation tool in the development of thin film thermoelectric devices. Various new methodologies for CCD TR were developed including the introduction of new correction factors for shutter time and DC offsets that eliminate errors in the standard approach. This includes a new method to indirectly measure the Seebeck coefficient of a thermoelectric device if thermal imaging through CCD-TR is taken of the topmost contact. To demonstrate this, CCD-TR imaging is carried out on a TEC designed for integration in a photonic package. Annealing steps were carried out on this device, which by CCD-TR imaging was shown to increase its Seebeck coefficient and its predicted heat pumping capacity. CCD-TR was adapted for CW laser induced heating and demonstrated as a viable method to measure the thermal conductivity of thin films and also describe the dewetting temperatures of metallic gold thin films on a quartz substrate. In general, this technique allows a known heat flux to be applied at the microscale. A full thermal characterisation of a thin film electrodeposited TEC material is demonstrated using CCD-TR. This characterisation yielded a record breaking 720 Wcm-2 cooling heat flux for a thin film thermoelectric device. This heat flux comes from the ability of the TEC to be functionally thinned while minimising its contact resistance through cleaning steps during fabrication. This demonstrates that relatively simple techniques such as electrodeposition can rival more complex and demanding MBE and MOCVD grown superlattices