Evaporation of water droplets on photoresist surfaces – An experimental study of contact line pinning and evaporation residues

We have systematically studied the interaction of ultrapure, de-ionized water droplets with chemically amplified, deep-ultraviolet photoresist layers during evaporation by means of experiments. The contact lines of the evaporating droplets undergo two pinning events. The footprint diameters during pinning D_1,2 scale with the initial droplet diameter D₀ approximately as D_1,2∼D₀ ^4/3. Evaporated droplets leave a residue behind, generally in the form of an ultrathin layer (order 1–10 nm) with a sub-micron thick mound in the center. We have systematically characterized the residue dimensions as a function of the initial droplet size, the photoresist composition and process conditions. Post-evaporation rinsing steps were found to be unable to completely remove a deposit, depending on how long after droplet evaporation they were performed. Our results indicate that the occurrence of so-called watermark defects might be related to deliquescence induced by ionic residues.