Thin-film characterization with a dual-channel dispersion-encoded imaging low-coherence interferometry approach

Fabrication of thin-film structures sets high demands on quality, precision and reliability of the manufacturing process. Appropriate thin-film characterization should deliver nanometer-accurate film thickness and 3D topographical resolution, as well as the ability to characterize mm-sized surface areas in an in-line manner. This work presents a dispersion-encoded low-coherence interferometer in a Mach-Zehnder configuration which is operated in a dual-channel mode. The primary channel utilizes a dispersive element to provide a controlled phase variation of the interference signal in the spectral domain. This phase variation is traced and used as measure for film parameters. The signal detection is performed by an imaging spectrometer to allow the scan-free data acquisition in one lateral domain. The second channel utilizes the back-reflected light from the sample's substrate material. This enables the in-system evaluation of substrate parameters to improve the accuracy of the measurement. The experimental setup was established and evaluated on industrial-grade indium-tin-oxide coated PET-foil substrates. From the gathered data it could be shown that a thickness resolution of the film thickness is in the order of 5 nm and can be achieved with a lateral spatial resolution of 4 mm. The advantage over other approaches is that signal processing is fast and spatially resolved data is gathered in a scan-free approach.