Patent Number: 041918886
Section: description

DETAILED DESCRIPTION OF THE INNVENTION Referring first to FIG. 1 which shows the prior art three-electrode system, a neutral highly ionized plasma 10 upstream of the electrodes is contained within a discharge chamber (not shown) and held at +V.sub.1 volts, usually several hundred to several thousand volts. The three electrodes downstream of the plasma source each have apertures in axial alignment and include in the following order a screen grid 11, an accel grid 12 and a decel grid 13. The screen grid 11 is also held at +V.sub.1 volts, but the accel grid 12 is held at -V.sub.2 volts, also several hundred to several thousand volts. Thus, an electric field is set up by the screen grid 11 and the accel grid 12 which extracts an ion beam 14 from the plasma boundary 15 and focuses it through the accel grid hole. The third or decel grid 13 is held at zero volts or ground potential and minimizes the beam spread or divergence after the ion beam passes through the accel grid hole. The accel grid has a relatively small aperture in the range of less than 50% to approximately 10% the size of the aperture of the screen grid. As previously mentioned, the small aperture size of the accel grid minimizes the emission of the un-ionized atoms from the discharge plasma. Now with reference to FIG. 2 wherein the same reference numerals as used in FIG. 1 designate identical parts, the present invention utilizes the same three-electrode system as before except that the accel grid 16 has a relatively large drilled accel grid hole, on the order of 2.083 mm diameter in a preferred working embodiment. Thin foils 17 and 18 are secured to both faces of the accel grid 16 so as to cover the large, drilled hole. In a preferred embodiment, the foil used was 0.0127 mm thick tantalum spot welded on a 0.79 mm thick accel electrode. The ion beam 14 etches only through the thin foils 17 and 18 so that the etching time is much shorter than for a solid plate. For the example described, the holes were etched in approximately 11/2 hours with a total extraction voltage of one thousand volts (V.sub.1 =+500 volts, V.sub.2 =-500 volts) at a 2 mA/cm.sup.2 ion current density (argon ions). It will, of course, be appreciated that the foregoing example represents but one combination of extraction voltage and current density. A larger aperture will be generated by lower voltage and/or higher current density, and a smaller aperture generated by a higher voltage and/or lower current density. As can be clearly seen in FIG. 2, with the accel grid sputter site 19 being the doughnut-shaped volume defined by the interior surfaces of the foils 17 and 18 and the barrel of the drilled hole, the foils 17 and 18 act as a sputter shield, thereby preventing much of the emission of sputtered grid accel material that would normally occur with the prior art three-electrode system shown in FIG. 1. If the electrode system shown in FIG. 2 is disassembled and reassembled, the run-in time before direct accel impingement disappears is much less than that for the solid plate SHAG electrode 12 shown in FIG. 1 because only the foils 17 and 18 have to be etched. A simple modification of the inventon shown in FIG. 2 is to fabricate the accel grid with only the foil 17 on the downstream face. This modification has little effect on the ion optics and shortens even further the milling time required to generate the accel grid aperture. An alternative fabrication technique employing plating and etching processes can be used to make the accel grid. Specifically, a thin layer of nickel is first plated over a solid copper accel electrode. The thin nickel plate is ion beam etched to provide the small aperture on the upstream side of the accel grid. Using the ion beam-etched nickel as a mask, the copper is chemically etched to form the interior volume of the grid and expose the nickel on the downstream surface of the accel grid. The nickel plate is then etched by the ion beam to form the downstream small aperture. The nickel plate forms the foil attached to the faces of the accel grid, and the copper forms the body of the grid with the large hole apertures.