Characterization of Thin Films and Layered Structures by Pulsed Photothermal Radiometry

摘要

Pulsed photothermal radiometry relies on the use of a short optical pulse to quickly heat up a sample, and the detection of the transient thermal radiation from the sample surface. Similar to pulsed photoacoustic sensing techniques, PPTR has applications in spectroscopy, coating thickness measurement and powder aggregation detection, thin-film thickness or thermal diffusivity measurements, pigment characterization and fiber composite strength characterization. This reprint examines how the PPTR signal shape can be analysed to determine quantitatively the thickness of subsurface airgaps in layered structures. The effect of moisture uptake in polymer films on the PPTR signal is demonstrated. The shape and amplitude of the PPTR signal is a function of the optical absorption lengths at the excitation and observation wavelengths, excitation pulse duration, thermal diffusivity, heat capacity and IR emissivity of the surface layer, and subsurface structure. The PPTR signal shape has been calculated analytically for the case of a semi-infinite uniform solid or a uniform film. For the case of negligible heat diffusion during the excitation pulse, as is true for our experimental conditions, the amplitude of the PPTR signal is fully determined by the heat capacity, the optical absorption lengths and the emissivity of the top layer. (jhd)