In-Situ Multi-Scale RGB Imaging Studies of Spin Coating Using 3 Wavelength Laser or Broadband White Stroboscopic Illumination

Spin coating remains a valuable technique for thin film fabrication due to the wide range of materials that can be processed, and its inherent speed and reproducibility. The microscopic morphology of these films is of critical importance to the associated performance in their applications, which vary from sensors to photovoltaics and electronics. Wafer-scale metrology is also significant for the characterization and detection of defects and nonuniformity, especially in semiconductor manufacturing.<i> </i>Inherently, spin coating provides a challenging platform to directly observe dynamic topological and morphological information on, especially with direct methods such as imaging, due to high angular velocities and processing times of a few seconds. In addition, ex-situ measurements are often needed to unambiguously interpret in-situ optical data due to fringe order ambiguities.<br/>In previous work [1] we have demonstrated how broadband illumination can provide full-wafer thickness reconstruction of spun coat solvent topology through the development of an in-situ colour-to-thickness relationship. Here, we present a multi wavelength laser illuminated microscopy technique on spun coat films to produce colour videos of the process. This configuration also takes advantage of a standard colour camera's Bayer filter to provide facile and inexpensive multi wavelength metrology. This allows us to produce colour images using simultaneous 406, 520 and 642nm wavelength illumination. The crosstalk in the camera has been removed and thus the resultant images can be considered 3 wavelength snapshots of the process, that can be interpreted to reveal spatially resolved thicknesses at each time point. Our investigation of the proposed technique involves the observation of a variety of model systems, at different objective lens magnifications, from simple pure solvent and single polymer systems to polymer blends and more complex systems. Theoretical analysis shows that extraction of instantaneous phase differences between colour bands can unambiguously provide thickness values across the whole FOV, without the need for additional ex-situ techniques.<br/>This work represents a landmark because the technique can provide unambiguous and spatially resolved thickness measurements without the need for time consuming and expensive ex-situ techniques. It has far-reaching implications across numerous thin film materials, especially where the final structure is critical, by providing a tool to investigate morphological developments in situ. Increased understanding of the development of these structures is key to unlocking the full potential of materials processed by spin coating.<br/><br/>[1] Atkinson, Jack Benjamin Philip and Howse, Jonathan, In-Situ Full-Wafer Metrology Via Coupled White Light and Monochromatic Stroboscopic Illumination. Available at SSRN: <u>https://ssrn.com/abstract=4782503</u> or <u>http://dx.doi.org/10.2139/ssrn.4782503</u>