Optimization of the automated spray layer-by-layer technique for thin film deposition

Abstract

The operational parameters of the automated Spray-LbL technique for thin film deposition have been investigated in order to-identify their effects on film thickness and roughness. We use the automated Spray-LbL system developed at MIT by the Hammond lab to build 25 bilayer films of poly (ally amine hydrochloride) (PAH) and poly (acrylic acid) (PAA). Each of the 10 operational parameters of this system are explored individually to isolate each parameter's effect on film thickness and roughness. The parameter effects are analyzed for apparent trends to determine the parameters best suited for adjusting film thickness and roughness. The optimal parameters for thickness adjustment are polyelectrolyte solution concentration, polyelectrolyte spray time, spraying distance, air pressure and polyelectrolyte charge density. These parameters are independent of the type of species used to construct the film, and thus the trends should apply to any species used to construct thin films. The effect of each of the 10 operational parameters is examined in detail. While researching the parameter effects, polyelectrolyte interdiffusion in the films was observed. This interdiffusion is investigated using both the conventional dipped LbL and Spray-LbL deposition techniques. Interdiffusion is shown to be dependent on 3 factors, the charge density of the polyelectrolytes, the molecular weight of the polyelectrolytes, and the contact time between the polyelectrolyte solutions and the surface of the film. Interdiffusion is observed when the PAH is partially charged, the polyelectrolytes chains have a low molecular weight, and the contact time is sufficiently long enough to allow for interdiffusion. The significantly reduced contact time during the automated Spray-LbL process not only speeds up the film deposition time, but also significantly hinders the interdiffusion of PAH resulting in much thinner films than what is possible from dipping. Finally, the uniformity of the films produced using the automated Spray-LbL system is investigated. Films deposited on substrates greater than 1 in diameter area exhibit more than 20% variance in thickness. Adjustments were made to the setup of the system in an effort to expand this area of film thickness uniformity. However, it is determined that the design of this automated Spray-LbL system limits the film uniformity to an area of 1 in diameter.