Solution-processed thin polymer films combine favourable optoelectronic properties with low weight, flexibility and environmental friendliness, and are now manufactured at increasing scale. Scalable methods exhibit different drying kinetics, which govern final film thickness and morphology and thus device performance, there is a critical need for in-situ imaging metrology that can track dynamics and directly observe morphology across processes; colour imaging is currently underutilised in these applications.
First, we coupled in-situ broadband RGB reflectance imaging with a centre-point laser reflectance probe during spin coating to establish a calibration-free, colour-to-thickness relationship for dynamic solvent films. By mapping this relationship across a 3" wafer using a bespoke algorithm that resolves cyclical colour changes, we reconstructed transient solvent profiles with 5% accuracy and sufficient spatial resolution to detect common defects.
Next, we upgraded the imaging system with a multi-bandpass dichroic beam splitter and dual image projection to create a six-channel multispectral camera. Narrower detection bands and higher spectral resolution extended the upper thickness limit and enhanced model-based thickness reconstructions. We further demonstrated, for the first time, spectral reconstructions of thin-film interference using six-channel data, accurately recovering both the film’s reflectance spectrum and thickness with fewer assumptions than RGB-model methods.
Finally, simultaneous red, green and blue laser illumination is used to produce trichromatic reflectance images, exhibiting strong interference fringes on much thicker films. The much lower linewidth makes these interference colours distinct from previous methods. A newly developed fringe-sequencing interpretation unambiguously retrieves dynamic film thickness, capabilities unattainable with single-wavelength probes. Demonstrated on blade-coated polymer films, this approach promises broad application in real-time metrology of dynamic thin films and interferometric measurements.
This work overcomes the traditional limitations of RGB imaging for thin-film metrology, enabling the widespread use of low-cost, high-resolution, spatially resolved and high-frame rate RGB cameras across scalable coating processes.
