In-situ full-wafer metrology via coupled white light and monochromatic stroboscopic illumination

Abstract

This work presents a novel optical setup to provide scalable in-situ metrology during spin coating. Stroboscopic white light imaging provides high resolution color videos of the process, at a temporal resolution matching the spin speed, where thin film interference colors are observed. Monochromatic specular reflection intensity data from the center of rotation provides a thickness profile at this point. By developing a color to thickness relationship in-situ with the combination of these techniques and leveraging the large-area data provided by color imaging, the thickness at any point on the wafer is reconstructed via a mapping procedure with minimal a-priori information. Experiments are carried out on full 3″ diameter wafers spun with pure xylene or pure butyl acetate, and the thickness profile at all points on the wafer can be determined. Differences in the topology of these solvents whilst drying are linked back to the solvent properties. The color to thickness mapping procedure is shown to have less than 5 % error in determined thickness values between 2μm and 100nm. The possible length scale resolved by the imaging is fully discussed as a function of radius, spin speed, strobe pulse duration and hardware used. The studies in this work achieved a minimum lateral resolution of 315μm when observing a full wafer, which is sufficiently detailed to properly reconstruct thickness variations caused by common spin-coating defects such as comets. The large area and scalable nature of this metrology technique lends itself to applications in semiconductor manufacturing where substrates of 300 mm are standard.