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

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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