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
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
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
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)