Thermal conductivity measurements of thin films using a novel 3 omega method

For most micro- and nanoelectronic devices based on thin films applied for effective heat dissipation and thermoelectric devices for energy harvesting, thermal management is a critical subject for their device performance and reliability. This thesis focuses on the investigation of the cross- and in-plane thermal conductivities of both high- and low-thermal conductive thin film materials. Aluminum nitride (AlN), with its high thermal conductivity, has been studied, as it is a promising candidate for effective heat conductors in microelectronic devices. Copper iodide (CuI) has also been investigated in this thesis, because of its great interest in novel energy harvesting applications with low thermal conductivity and outstanding thermoelectric properties. Thermal conductivities of thin films tend to be substantially different from those of their bulk counterparts, which is generally caused by oxygen impurities, dislocations, and grain boundary scattering, all of which can reduce the thermal conductivity of the films. These effects also influence cross- and in-plane heat conduction differently, so that the thermal conductivities of the thin films are generally anisotropic in these two directions. Therefore, experimental work and theoretical analysis have been conducted to understand the effects of crystallinity, grain sizes, and interfacial structures of AlN and CuI films on their thermal conductivities as a function of film thickness. An improved differential multi-heater 3ω method was established and used to study the thickness-dependency of cross- and in-plane thermal conductivities of CuI and AlN thin films sputtered on p-type doped silicon substrates with film thicknesses varied between 70 - 400 nm and 100 – 1000 nm, respectively. Furthermore, our newly proposed 3ω Microscopy method, which combines the advantages of both the conventional 3ω method and atomic force microscopy (AFM) technology, was applied to quantitatively measure the local thermal conductivities of CuI and AlN thin films, with a spatial resolution in sub-micrometer range. Results revealed that both the cross- and in-plane thermal conductivities of the CuI and AlN thin films were significantly smaller than those of their bulk counterparts. The cross- and in-plane thermal conductivities were strongly dependent on the film thickness. Both the X-ray diffraction and 3ω Microscopy results indicated that the grain size of thin films significantly affected their thermal conductivity due to the scattering effects from the grain boundaries. Finally, the 3ω Microscopy has been proven to provide additional experimental findings, which cannot be identified or detected using conventional thermal characterization methods such as the standard 3ω technique. Its good spatially-resolved resolution for quantitative local thermal characterization, its nondestructive characteristic and without a need for sample preparation, make the 3ω Microscopy a promising thermal characterization method.