Electrochemical Micromachining on Porous Nickel for Arrays of Electrospray Ion Emitters

The emission of molecular ions from ionic liquid ion sources (ILIS) poses a number of advantages in applications from materials science to space propulsion. However, practical implementation of these field emitters requires their grouping into dense arrays of emitters to increase current throughput. A process to micromachine ILIS on porous metals is presented using a mixture of photolithographic and electrochemical etching techniques. It is found that emitter uniformity and pore integrity are achieved with pulsed voltage etching in a regime controlled by the thickness of the layer composed of products from the electrochemical dissolution process. Postetching in a presaturated solution is then applied to obtain a smooth rounded shape of porous metal emitters, which is adequate for ILIS operation. A prototype implementation is demonstrated in porous nickel with a mean pore size of 5 μm achieving uniform triangular arrays of 480 ILIS in a 1-cm² area mounted on a silicon frame. Each emitter is about 150-μm tall with a radius of curvature of about 15 μm at the tip.