Laser interferometric calibration for real-time video color interpretation of thin fluid layers during spin coating

Using color video observation in conjunction with careful thickness calibration with laser interferometry it is possible to map the thickness profile evolution with time for fluid flowing radially outward on a rotating silicon wafer during spin coating. During the latest stages of a transparent fluid thinning on a spinning substrate, the fluid reaches thickness values such that interference effects cause the film to display distinct colors that correlate directly to thickness. These colors cycle through the visible spectrum in a predictable and meaningful way. The present work demonstrates that these colors can be easily captured using standard video equipment and that the corresponding thickness values can be calibrated using simultaneous normal-incidence laser interferometry at the center of the substrate. The combination of these two techniques allows for observation of the spinning wafer to understand fluid thickness profile evolution without being forced to know the exact illumination conditions or refractive index dispersion relationship for the fluid being examined. This technique provides meaningful time-resolved thickness profiles for fluids in the later stage of spin deposition and has potentially wider application in engineering process control and coating characterization.