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,D-2 scale with the initial droplet diameter D-0 approximately as D-1,D-2 similar to D-0(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.