Quantitative temperature sensing and thin film thermal conductivity measurement by non-contact scanning thermal microscopy

Abstract

Scanning Thermal Microscopy (SThM) is a powerful technique that can measure samples’ thermal conductivity, temperature, Seebeck coefficient, and topography at the same time with excellent spatial resolution and sensitivity. It has a lot of potential applications in material science, nanoelectronics design, and even biological field. The current state-of-the-art SThM focuses on the contact mode that has inaccuracy due to surface artifacts and suffers from probe damage. Therefore, this thesis reports the investigation on the development, validation, and application of a quantitative non-contact SThM under ambient conditions for temperature sensing and high thermal conductivity 2D sample characterization. This thesis is divided into six chapters. The first chapter introduced the history and background of the SThM development and states the motivation of this thesis. The second chapter describes the experimental, numerical, and analytical methods for a proof-of-concept demonstration. The experiments involve a passive probe that measures sample temperature and an active that measures sample thermal conductivity in the diffusive and the transition regimes. The analytical models of the active and passive probes and a multilayer sample are also introduced. A cutting procedure forces the 3-Dimensional Finite Element Model (3DFEM) to take the transition heat transfer between the probe and the sample into account and a useful correlation of the probe-sample air gap thermal resistance is developed by the validated 3DFEM. Then the discussion continues to the calibration of the thermal exchange parameters, which are important for the quantitative and accurate temperature and thermal conductivity results. The probe-sample air gap thermal resistance correlation together with the analytical models forms the basis of the active and passive calibration techniques for temperature sensing. For film thermal conductivity measurement, the thermal exchange radii are found by fitting the 3DFEM heat flux profile on the sample surface. Chapter 3 reports the validated results and demonstrates the experimental setup having a 700 nm spatial resolution and 0.01 K temperature resolution of the temperature sensing and a film thermal conductivity of up to ~240 W/(m·K) and down to 0.2 W/(m·K) with thickness from 46.6 nm to 240 nm can be measured with the setup. Extended on the validated 3DFEM, Chapter 4 discusses the definitions and methods to evaluate the sensitivity and spatial resolution of commercial thermoresistive probes. The sensitivity and spatial resolution of the Wollaston wire probe were also documented in detail when measuring film thermal conductivity. Generally, a probe with a more focused heat source will have a higher spatial resolution and a probe with a more spreading heat source will have a better sensitivity. The existence of the cantilever may significantly affect the overall performance. The DS probe was found to have the best sensitivity and the Nanowire probe had the finest spatial resolution when they are measuring thermal conductivities. The Wollaston wire probe had the highest spatial resolution and the DS probe is most sensitive to the sample temperature change. The investigation of the probe performances inspires the study of modified probe geometry for the KNT and the Wollaston wire probe and the optimized Wollaston wire probe has a 2.4 μm diameter. Chapter 5 summarizes this thesis and highlights important conclusions. The unaddressed challenges of parasite effect of temperature sensing and further probe optimization and fabrication are emphasized as guidance for future research.