Thermophysical analysis of thin films by lock-in thermography

Abstract

A method for the determination of the thermophysical properties of thin films is introduced. The procedure is based on the contact-free thermal analysis of free standing thin films by means of a lock-in thermography system. The thermophysical properties are deduced from the thermal diffusion length and the temperature amplitude of a thermal wave propagating in the sample excited by a laser heat source. An infrared camera images the wave and the thermal diffusion length Λa of the amplitude and the thermal diffusion length Λp of the phase are measured. Thermal losses influence both Λa and Λp. Their geometric mean ΛaΛp is, however, not effected by thermal losses. In turn 1∕Λa2−1∕Λp2 is determined by the thermal losses and does not depend on the lock-in frequency, as Λa and Λp do. These theoretical predictions are experimentally verified in this work. The measured values of Λa and Λp yield the in-plane thermal diffusivity and a damping factor. The latter quantifies the thermal losses to the ambient. The use of a vacuum chamber and a temperature calibration are not required. If, however, the camera is calibrated, the in-plane thermal conductivity and the volumetric heat capacity are obtained from the temperature amplitude of the thermal wave. The measurement accuracy is 10% for the thermal diffusivity, thermal conductivity, and volumetric specific heat. The thermophysical properties measured for thin films of copper, nickel silver, and polyimide agree with literature values.