Model-based coherence scanning interferometry for reflectivity and film measurement

Coherence scanning interferometry (CSI) is a widely used surface topography measurement technique known for its subnanometer resolution, non-contact capabilities, and 3D surface profiling. It is commonly employed in research, process development, quality control, and in-line metrology across various industries, including optics, automotive, data storage, MEMS, and flat-panel displays. Recent advancements in image processing have extended CSI’s capabilities to include reflectivity spectrum imaging for flat samples and thin film thickness measurement. However, several challenges remain in these applications that hinder their full potential. In reflectivity spectrum imaging, Fourier magnitude is often used to represent the measured values related to a flat sample’s reflectivity. However, this approach overlooks the effects of numerical aperture-induced spatial coherence, which can impact the accuracy of the measurements. For thin film thickness measurement, the 1-D model-based technique is a widely accepted method, but the impact of inaccuracies in the prior information—such as refractive index, numerical aperture, and light source spectrum—on measurement accuracy remains insufficiently explored. This gap in understanding limits the precision of model-based CSI methods, especially when dealing with complex thin film structures.