Electrospray Deposition of Polyimide with Passive Material Focusing

Electrospray deposition (ESD) is a novel printing technology that uses a strong electric field to atomize a liquid suspension into a plume of electrically charged micro-droplets. Solvent evaporation within the plume leaves behind dry solute that is directed onto a substrate to build thin films with micro- and nano-scale structure. Silicon substrates were targeted using ESD to print films of polyimide, an important material in electronics manufacturing. Using electrostatic lensing, the material was passively focused to constrain the deposit area and increase the rate of film growth. Various processing conditions were investigated, including polyimide concentration, flow rate, spray time, and solution electrical conductivity. For low rates of polyimide delivery, the film thickness increased linearly for spray times up to 60 mins. During printing, the film roughness would increase and decrease cyclically, indicating a charge-induced pin-holing mechanism in the evolving film. The rate of film growth was suppressed at high rates of polyimide delivery due to the accumulation of electric charge on the substrate during printing.