Film thickness profiles over topography in spin coating

A nonintrusive, in situ technique is presented for measuring film thickness profiles during spin coating. Stroboscopic interferograms are created using a pulsed laser synchronized to capture a particular feature on the substrate. Profiles measured using a commercial photoresist spun over features of width 10, 50,100, and 500 µm are compared to model predictions based on lubrication theory. Dry profiles from stylus measurements are compared to a simple uniform shrink model for drying that assumes flow and drying are sequential. Spin curves (film thickness vs. time) are used to estimate the time in the spin at which evaporation replaces flow as the dominant solvent loss mechanism. Despite the simplicity of the treatment of drying, the model predicts film profiles well except for dried profiles over the smallest features. Finally, a simple physical explanation of the observed asymmetry in dried profiles over surface topography follows from this approximate model.