Micrometer Resolved Photothermal Displacement Inspection of Optical Coatings

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Abstract
Two-dimensional c.w. photothermal surface displacement (PTD) scans with high spatial resolution provide a new quality for thin-film characterization. This is demonstrated for optical single-layer films of ZrO2 and MgF2 on various substrates. Inhomogeneities of the films were detected with a lateral resolution < 2 µm. Variation of the modulated frequency was employed for depth-profiling. The resolving power has been investigated experimentally and a model for signal generation by absorbing inhomogeneities is presented. A calculation of the photothermal thin-film response for thermally thick coatings is carried out to obtain a better understanding of the PTD images with respect to film absorption and thermophysical sample properties.