Development of method for measuring three dimensional thermal properties of Si/NiFe/AIN thin film using a angle femtosecond thermoreflectance technique

本研究的目的是开发一种非接触和非破坏性的方法来测量垂直于薄膜表面的热扩散率，这需要使用飞秒热反射技术获得足够高的时间分辨率的温度响应。利用飞秒脉冲激光的热反射技术，我们研制了一种能够测量几十到几百ps范围内温度响应的仪器。通过对铝和钼试样的测量，验证了仪器的有效性和测量原理。在本研究中，通过对光学系统和检测系统的改进和优化，获得了热反射信号，足以分析垂直于薄膜表面的热扩散率，而不受衬底的影响。用最小二乘法拟合观测到的热反射信号，用排除衬底影响的单层模型的温度变化公式确定热扩散系数值。该方法也适用于体样品，并与薄膜样品的结果进行了比较。由于较高的时间分辨率，可以在较短的时间范围内进行测量，因此我们可以讨论脉冲激光加热后物质中电子和声子的行为。通过XPS测量证实了样品表面存在天然氧化层。利用椭偏光谱法测量了样品表面的光学常数和结构。同时给出了天然氧化层对热传递现象的影响。研究发现，脉冲加热后100ps内的快速能量输运具有足够的时间分辨率和信噪比，热扩散系数为κ = 1。5 × 10^<-5> m^2 s^<-1>。Mo体的热扩散系数为κ = 2.1 × 10^<-5>m^2s^<-1>。因此，Mo的热扩散系数在薄膜和体之间的差异似乎很小。然而，这些值与文献中给出的Mo块体的参考值κ = 5.43 x 10^<-5>m^2 s^<-1>不同。这种差异的原因可能是由于弹道电子散射和伴随有限时间滞后的电子-声子能量输运，这可能在飞秒脉冲加热后的皮秒时间范围内的测量中被检测到。脉冲加热在样品表面产生的弹性波也会影响热扩散系数。少

The purpose of this study is to develop a noncontact and nondestructive method for measuring thermal diffusivity perpendicular to the thin film surface, which requires obtaining temperature response with sufficiently higher time-resolution by using a femtosecond thermoreflectance technique. We have developed the instrument that is able to measure temperature response in the range from several tens of ps to several hundreds of ps by the thermoreflectance technique with a femtosecond pulse laser. The validity of the instrument and the principle of the measurement have been confirmed through measurements of Al and Mo bulk samples. In this study, thermoreflectance signal, which is sufficient to analyze the thermal diffusivity perpendicular to the thin film surface without any influence from substrate, is obtained by both improvement and optimization of the optical and the detection system. The thermal diffusivity value is determined by a least square method fitting the observed thermorefle … More ctance signal with a temperature variation formulae derived from a single layer model excluding effects of the substrate. This technique is also applied to bulk samples and the results are compared with those of thin films. Since the measurement in shorter time regime is possible due to the higher time-resolution, we are allowed to discuss behavior of electron and phonon in the substance just after the pulse laser heating. Existence of natural oxide layer on the sample surface is confirmed by XPS measurement. Measurement of optical constants and structure analyzes of the sample surface are performed by spectroscopic ellipsometry. The influence of the natural oxide layer on the thermal transport, phenomenon has also been given. It is found that rapid energy transportation within 100ps after the pulse heating is obtained with sufficient time-resolution and a signal-to-noise ratio giving the thermal diffusivity value of κ = 1. 5 x 10^<-5> m^2 s^<-1>. The thermal diffusivity of Mo bulk was obtained to be κ = 2.1 x 10^<-5>m^2s^<-1>. Thus, the difference of the thermal diffusivities of Mo between film and bulk appears to be small. However, these values differ from a reference value of κ = 5.43 x 10^<-5>m^2 s^<-1> given in the literature for the Mo bulk. The reason of this difference may be due to ballistic electron scattering and an electron-phonon energy transportation accompanying a finite time lag, which are likely detected in our measurements within picosecond time regime after the femtosecond pulse heating. An elastic wave generated on the sample surface by pulse heating may also affect the thermal diffusivity. Less