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Electrostatic Focusing of Electrospray Beams


The electrospray ionization, also known as electro-hydrodynamic ionization, produces a beam of charged nanodroplets. These beams of nanodroplets can be used in material deposition, sputtering, and milling processes, all of which are currently done by beams of atomic and cluster ions. In electrospray atomization, a uniform beam of submicron-sized charged droplets is sprayed in vacuum in a strong electric field. The charged droplets are accelerated by the electric field induced by the electrodes. The space charge of the beam causes the droplets to repel each other and the beam to open up. In this dissertation, the development of an electrostatic column for electrospray beam focusing is presented. Coaxial electrodes were designed to extract and focus the beam. The beam is skimmed to agree with the paraxial assumption used in the projectile tracing. Fundamentals of the charged particle tracing under an electric field is discussed in detail and numerical simulations provide the projectile trajectory and image location where the beam is focused and energy density is maximized. These focused beams are used to sputter a semiconductor target. The performance of the focusing apparatus can be characterized based on the size and features of the impacted area. The error sources in lens system result in an increased image size appearing in the form of slightly sputtered area. These aberration figures are analyzed for different field strength to determine the size of imperfections in the apparatus and to be possibly corrected in next stages of development. With the exception of spherical and chromatic aberrations, other sources of errors can be eliminated or at least minimized if the misalignments between elements including electrospray source and electrodes are minimized. An innovative process is presented for fabricating the focusing lens using microfabrication techniques. This approach guaranties high precision and minimum misalignment between elements. Plasma activated bonding is used to permanently attach the silicon electrodes and glass standoffs. In addition to precision fabrication of apertures, the advantage of this approach over conventional machining is eliminating post fabrication assembly. Despite higher cost and difficulty of process, the microfabricated lens assembly successfully provides the voltage and accuracy requirements for a high resolution focusing lens.

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