Investigation of the electrostatic focusing of beams of electrosprayed nanodroplets for microfabrication applications

Abstract

This article presents a focused electrospray beam source and discusses its potential for microfabrication. Its main elements are an electrospray emitter electrode (a point source of charged nanodroplets), an extractor electrode, a skimmer electrode, and an Einzel electrostatic lens. The focusing parameters of the source are calculated by integrating the equations of motion of the charged droplets in the axisymmetric electrostatic field generated by the electrodes. The results of the model are validated with a laboratory source replica by characterizing the sputtered region produced by the focused beam on a silicon target and comparing it with the image obtained with the model. In the experiments, the size of the focused beam at the image plane is at least 20 times smaller than that of the unfocused beam, despite the presence of aberrations that have a negative effect on the ability to concentrate the beam. In a well aligned source, the sputtered area is close to a disk, and spherical and chromatic aberrations are the most significant nonidealities making the size of the image larger than the ideal one. When the emitter is deliberately misaligned, spherical and chromatic aberrations continue increasing the size of the image, while astigmatism distorts its circular shape. All aberrations are reduced by increasing the strength of the focusing electrostatic field while maintaining the net acceleration potential of the beam. The focusing column increases the particle density of the beam and advances the development of electrosprayed nanodroplet beams as a tool for surface engineering.