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

Abstract

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 D1,2 scale with the initial droplet diameter D0 approximately as D1,2∼D04/3. 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.