Electroplated in-Plane MEMS Electrospray Emitter
Authors: Wang Xiaochen , Gu Jing , Deng Weiwei , Cho Hyoung J. 
Published: February 2020

Electrospray is an atomization technique by which an electrically conductive liquid through a small capillary is charged with high voltage (kV) and ejected to a ground electrode. The size of droplets is dependent on multiple parameters including the liquid conductivity, flow rate, and evaporation rate of the liquid. Although electrospray has been studied widely, especially for its application in mass spectroscopy, it gained a renewed attention due to its scalability and direct-writing capability on various substrates, which promises cost-effective nanomanufacturing. Another interesting application area of the electrospray is space propulsion which takes advantage of the high specific impulse produced by electrospray operated in the ionic regime. In both cases, in order to scale up the throughput or increase the thrust power, utilization of multiple electrospray emitters is considered as an indispensable approach. However, even with identical nozzles, the electric field near each nozzle is not identical due to edge effects. This non-uniformity leads to an uneven electro-hydrodynamic pulling force on the liquid among the nozzles, yielding different flow rates from one emitter to another. The solution is to make the viscous pressure drop across each nozzle dominant over the electro-hydrodynamic pulling force. Therefore, embedded structures that can create high flow impedance are desirable to achieve uniform feeding of low flow rate of liquid to each emitter.
MEMS (microelectromechanical systems) fabrication techniques can address these known challenges. Using a batch fabrication method, many identical electrospray units can be made concurrently with high precision without resorting to the traditional machining techniques. At the same time, in-plane high flow resistance structures can be fabricated. We designed and fabricated in-plane metallic electrospray devices by photolithography and electroplating. The novelty of the proposed research lies in its embedded flow restriction structure, scalability, and ease of fabrication. We propose this in-plane design to increase the effective channel length and thus, flow impedance. In addition, the in-plane design allows us to integrate more sophisticated microfluidic features that further increase the flow impedance while preventing clogging. We embedded an array of microposts within a microchanel for such purpose.
The electrospray emitter was designed to have a 2.2 mm diameter reservoir and a 4.2 mm long channel section. The thickness of channel wall was 400 um and the width was 260 um. An array of microposts was embedded within the channel. The individual diameter of the micropost was 32 um and the gap between the microposts was 10 um. For fabrication, we spin-coated a layer of a thick positive PR(AZ40) on the Au/Ti seed layer on a Si wafer. The channel’s bottom wall was defined by photolithography. After Ni striking, the bottom layer was electroplated using a nickel sulfamate bath and a pulse plating method. The second layer of PR with around 75um thickness was then spin coated, baked, aligned and developed for the micropost and side wall patterns. Electroplating was done until the Ni was overplated, merged together and formed a top layer. The enclosed device with a spray hole was then immersed in acetone and cleaned ultrasonically to remove PR residues. Finally, the device was released from the wafer after etching Au/Ti layer. The design and fabrication strategy for in-plane MEMS electrospray emitter yielded a robust and scalable device that is promising for emission and propulsion.