A New Technique for Enhancing Sensitivity of the 2ω Method by Applying a Bismuth Film Thermoreflectance Sensor on Top of the Metal Film–Dielectric Substrate Sample

Abstract

In this paper, a new technique for enhancing the sensitivity of the 2ω method is discussed. In the conventional 2ω method, the metal film is periodically Joule-heated and the ac temperature response at the surface of the metal film is measured by thermoreflectance to determine the thermal resistance of a thin film deposited on a dielectric substrate and also the interfacial thermal resistance between the layers. The problem with the conventional 2ω method is that it is very hard to obtain good sensitivity of the ac temperature measurement owing to the very low thermoreflectance coefficient of the metals. The sensitivity can be enhanced by just applying a Bi film thermoreflectance sensor on top of the metal film–dielectric substrate sample. However, the applied Bi film produces another thermal resistance and also interfacial thermal resistance between the Bi film layer and the metal film layer, which might affect the result of the measurement. Then we tried to solve the thermal conduction equation of the thermal system. In this case, only the metal film layer is Joule-heated and the ac temperature response at the surface of the Bi film layer is measured by thermoreflectance. Thus, we obtained exactly the same approximate equation as that of the thermal system for the metal film–dielectric substrate sample, showing that the thermal resistance of the Bi film and also the interfacial thermal resistance between the Bi film and the metal film are unobservable. We successfully determined the interfacial thermal resistance between the Au film and the Al2O3 substrate with much improved precision compared with the conventional 2ω method by measuring the Bi film–Au film–Al2O3 substrate sample.