Knowledge of thermophysical properties, such as thermal conductivity and thermal diffusivity of thin films with sizes down to micro and nanometers is extremely important from the standpoint of heat transfer in the manufacturing process and technological applications of thin films. Theoretically and experimentally, it has been demonstrated that thermal conductivity of thin films could differ significantly from bulk values because of the different microstructure and energy transfer mechanism in thin films.

  We carry out combined experimental and theoretical studies on thermophysical property measurements of thin films. Two techniques have been developed for investigating the thermal properties: the photoacoustic technique (PA) (link) and the femtosecond thermal reflectance technique (fs-TR). The PA technique is suitable for measuring bulk materials as well as thin films with thickness down to 1 micrometer, while the fs-TR can measure thin film as thin as a few tens of nanometers.

  Both methods are non-contact, and require minimum sample preparation. We have provided measurement services to industries, academic institutions, and government laboratories. For more information, contact Prof. Xu.

Theory of PA Measurement of Thermal Properties

    When a heating source, normally a laser beam, is periodically irradiated on the sample surface, the temperature of air adjacent to the sample surface also varies periodically. The temperature variation in air causes a pressure variation, which can be sensed as an acoustic signal. In the photoacoustic measurement, the acoustic response in air is measured. Since the acoustic response, both in amplitude and phase, is related to the thermal properties of the sample, the unknown sample properties can be determined from the acoustic measurement.

    We have developed a generic expression of the PA effect for a multilayer material. This expression takes thermal and optical properties, geometry of a multilayer structure, as well as the thermal contact resistances between layers into consideration. Also the PA signal caused by the displacement of the sample surface is worked out. Based on the general expression of the PA effect derived, a computer code is developed, which uses the least-square method to fit the experimental results and find the unknown properties.

    The experimental set-up for the photoacousitc measurement is shown below:

    The following figure shows the measured phase shift of SiO2 thin films on a Si substrate. The thickness of SiO2 is 484.5 nm. The thermal conductivity of the SiO2 film is found to be 1.52 W/m. K by least-square fitting. The deviation between the measurement and fitting is 0.38 degree.

Femtosecond Thermal Reflectance Measurement of Thermal Properties (to be added)