Development Of A System For Thermal Property Measurements Based On Wavelength-Tunable Time-Domain Thermo Reflectance

Thermal properties,such as thermal conductivity and interface thermal resistance,are the focus of attention in the fields of electronic industry,energy harvesting and conversion,and so on.For instance,decrease of interface thermal resistance in micro-/nano-electronic devices can facilitate heat dissipation and extend the lifetime of devices.As another example,thermoelectric materials,with great potential for recovery and utilization of waste heat,must have low thermal conductivity,so accurate measurement of thermal conductivity is the basis of developing high-efficiency thermoelectric materials.Time-domain thermoreflectance(TDTR)technique based on pulsed laser,as an effective method for measuring thermal properties of materials,has been widely used in the study of thermal properties,such as thermal conductivity of various materials and interface thermal resistance in heterogeneous materials.In a traditional TDTR system,the probe is usually a laser beam from a commercial Ti: sapphire laser at wavelength near 800 nm and Al is chosen as the corresponding metal thermal transducer for a high signal-to-noise ratio due to its high thermoreflectance signal near 800 nm.However,ease of oxidation and low melting temperature of Al impede the application of this “800 nm + Al” combination for thermal property measurements in special experimental conditions(such as high temperature).In addition,with popularity of novel laser sources covering different wavelengths,extended adaptability of TDTR to different laser wavelengths is beneficial for people to adopt this method for thermal property measurements more flexibly.In view of the limitations of common TDTR systems,this dissertation plans to improve the present system,by combining theoretical calculations and experiments,in the selection of metal thermal transducers and probe wavelengths.First,for commonly-available metals,the analysis of electronic band structure and the Drude model are used to quantify the derivative of the reflectance versus the temperature d R/d T at different wavelengths,to find the wavelength corresponding to the maximum of this derivative.Then,an optical parametric amplifier(OPA)for tuning the probe wavelength is introduced into a TDTR system,so as to realize dual selection of metal thermal transducers and probe wavelengths.In this dissertation,Au and Cu are selected as the metal thermal transducers for research.The thermoreflectance spectra of these metals are analyzed by theoretical calculations,and the wavelength at which the reflectance is most sensitive to temperature change is identified.Based on the theoretical analysis,the probe wavelengths within the appropriate range are selected for TDTR experiments.The experimental results are compared with theoretical calculation results to verify the feasibility of wavelength-tunable TDTR.This dissertation elucidates systematically the mechanism of optical response of metal thermal transducers to temperature change,and provides a theoretical guideline for optimizing the selection of metal thermal transducers and probe wavelengths in TDTR.This work is beneficial for improving the adaptability of this method to diverse experimental environments,new types of lasers,and different materials.