The objective of the research was to create a repeatable procedure to print patterns of polymer films in spots of uniform thickness and precise location on the surface of silicon nitride photonic chips. The motivating use-case is chemical sensors based on waveguide enhanced Raman spectroscopy (WERS). By covering sensor elements on the photonic chip surface with a polymer film that absorbs a target analyte species, the limit of detection can be reduced and selective sensor response to the target species can be enhanced. Both uniformity and thickness of the polymer layer is important in this application. The target analyte must diffuse through the polymer film to the sensor surface, so too great a thickness results in slow sensor response. The polymer film also acts as a cladding for the waveguide, so nonuniform coverage results in optical power loss and no recoverable Raman spectra. Currently, piezo-electric printing is used to print at similar scale volumes. However piezo-electric printing is expensive, slow, and proven unreliable with these polymer solutions. The semi-contact printing method works by transferring a nanoliter-scale volume droplet of polymer solution from the tip of a nozzle to a precise location on the WERS chip surface. This is achieved by using a computer controlled XYZ positioning system to maintain a small gap that pins liquid between the nozzle tip and WERS chip surface at the desired print location. The nozzle is then raised and liquid-surface attractive forces keep the droplet on the substrate as solvent evaporates. The research reported here focuses on minimizing the "coffee ring effect", which results in nonuniform polymer film thickness due to evaporation driven liquid flow from the center to the edges of the pinned droplet contact line. Three approaches were investigated to create uniform polymer films: (1) reducing evaporation rate using solvents having lower vapor pressure, (2) introducing opposing surface tension driven flow using binary solvent mixtures having different surface tension, and (3) eliminating droplet contact line pinning by chemical modification of the WERS chip surface prior to printing. An optimal printing method was identified that produces uniform polymer films that completely coat the WERS sensor element. Experiments are underway to obtain Raman spectra from these coated sensors to test performance and durability.
