Characterization Of Heat Transport In Nano-films By Femtosecond Laser Pump-Probe Method

As the reduction in size and the increase in speed of microelectronic devices, heat management at nano-scale is a critical issue.It is urgent and important to study the thermal transport behavior in specific space and time domain of micro/nano devices and their components. This thesis examines the characterization of thermal transport in nanometers using femtosecond laser pump-probe method. The theory for interpreting the measurement results is developed using a linear time-invariant systems, account-ing for pulse accumulation effects.The first work of this thesis was optimized and improved the optical system of single-color of femtosecond laser pump-probe.method, which can improve the sig-nal-to-noise in a certain degree. The signal quality and measurement accuracy was higher than the former's measurements by using this single wavelength measurement system.The experimental system was used to study the nonequilibrium heat transfer in nano metal films with different thickness. Exploring parabolic two-step model (PTS) to fit the experimental data. During the fitting process, we considered the proportional relationship with the change of electron and phonon temperature, which affect the reflectivity. The experimental results show that the electron-phonon coupling factors are affected by the film thickness and grain diameter, and in the thickness-range from tens to hundreds of nanometers, the thermal conductivity increases with the increasing thicknesses of the film. In addition, the experimental result verifies that the reflectivity of probe laser is affected by electron and phonon temperature at the same time.The second work of this thesis was established a two-color femtosecond pump-probe thermoreflectance system, which was based on the single-color pump-probe method. This experimental system used the second-harmonic generator to double the frequency of the pump pulses. Before pump and probe beams radiating the detector, a color filter which has a transmission of10-9at the wavelength around double frequency of pump beam was used to isolate the scattered pump beam. As results, it can avoid the interference of pump signal and achieve accurate and efficient measurements. In addi-tion, this two-color experimental system can be explored for the study the characteri-zation of thermal transport over time scales from femtoseconds to nanoseconds.The two-color pump-probe experimental system measured the different thickness metal/semiconductor samples with two layers nano-structure, including: Al/Si, Ni/Si and Au/Si samples; besides that, it also measured Al/HfO2/Si samples with three lay-ers nano-structure. The sampling theorem was used to analysis the frequency domain signal, establish the relationship between the thermoreflectance signals and modula-tion frequency, and finally got the heat transfer model of multilayer structure. By comparing between the experimental data and heat transfer model, the interface ther-mal resistance and thermal conductivity were obtained. The results show that the or-der of interface thermal resistances between the metals and semiconductor were at10-8m2K/W., the thermal conductivity of silicon was close to the bulk conductivity. The value of interface thermal resistances, including the three of Al/HfO2, HfO2and HfO2/Si were quite equal to the value of the SiO2/Si.