Same-Polarity Biasing to Tune Thickness of Self-Limiting Electrospray Deposition Coatings

A characteristic of self-limiting electrospray deposition (SLED), an electrostatically driven method of spraying objects, is that over time substrates develop a charged film on their surface which repels oncoming spray towards uncoated portions of the substrate. This phenomenon leads to uniform coatings of self-limited thickness on the substrate. However, if the substrate were to have a repelling electric field on it from the beginning of the spray, it would repel oncoming spray as if a charged film had developed on the substrate, effectively acting as if the substrate had already been sprayed for some time. We sprayed silicon wafer samples held at varying levels of applied voltage with SLED polystyrene coatings and used microscopic reflectometry to measure the resulting densified thicknesses. Compared to a sample sprayed for the same duration with no applied voltage, all the samples with applied voltage had a lower thickness deposited on them. In addition, we identified that the resulting thicknesses on the samples decrease as the amount of applied voltage increases linearly up to the point where virtually no coating is deposited on a sample. In this way, it should be possible to dial in a desired self-limited thickness for a given target coating thickness.