Thermal Diffusivities of Bilayer Films Deposited on Substrates Characterized Simultaneously by Transient Reflecting Grating Technique

Abstract

The transient reflecting grating (TRG) technique has been applied to characterize the thermal diffusivities of thin films in trilayered structures. Based on the TRG behavior in the trilayered structures of opaque film/transparent film/substrate, a two-dimensional theoretical model describing the thermal diffusion processes in the trilayed structures is presented and calculated by the finite-element method. The normalized TRG signals decayed with time can be expressed as a triple-exponential function with different time-dependent coefficients. The influences of the thermal properties of the first and second layer films on the TRG signals are calculated and discussed separately. By fitting the theoretical decay curves to the experimental measured data of a series of trilayered structures (Au/ZnO/Si and Al/ZnO/Si with different film thicknesses), the thermal diffusivities of both films in the trilayered structures can be evaluated simultaneously. The results show that the thermal diffusivities of the thin films are strongly dependent on the thickness of the thin films and significantly lower than the corresponding bulk materials, and decay with the decreasing thickness of the films.