GB754101A - Improvements in or relating to cleaning of surfaces by subjecting the surfaces to electronic bombardment - Google Patents

Abstract

754,101. Coating by vapour deposition. EDWARDS & CO. (LONDON), Ltd., W. March 31, 1952 [March 30, 1951], No. 7468/51. Class 82(2). [Also in Groups XX and XL(a)] Apparatus for subjecting surfaces, e.g. optical surfaces such as lenses prior to deposition of thin films, to electronic bombardment, e.g. for cleaning purposes, comprises a vacuum chamber 1, an electron gun 6 adapted to produce a stationary divergent beam of electrons, and a preferably dome-shaped work-holder 4 for removably supporting a plurality of objects 11 such as lenses in such manner that the surface to be bombarded of each of the objects can be positioned so as to be substantially normal to the beam at the same distance from the centre of divergence of the beam so that each of the surfaces is subjected to a substantially equal and uniform bombardment by the beam. Thin films are afterwards applied to the objects 11 by a metal vapour (magnesium fluoride is also referred to) crucible 10 which can be positioned on the axis of the chamber by attachment to an arm operated by a rotating member outside the vacuum chamber. In a modification using a rotatable work-holder the electron gun is off-set from the axis of rotation of a disc-like work-holder, the arrangement being such that the surfaces of the objects on the work-holder are moved successively through the beam substantially normal thereto and at an equal distance from the centre of divergence thereof. The crucible 10 can then be positioned on the axis of rotation of the work-holder. The temperature of the crucible can be raised by conventional resistance heating of refractory metal foils or by filament heating. Alternatively the evaporator may be connected to the positive side of the H.T. supply and heated by electron bombardment or the method described in Specification 754,102 can be used. The electron gun 6 is mounted on the platform 5 and comprises a spiral thoriated tungsten filament 7, a focussing electrode 8 and an accelerating anode 9. The focussing electrode 8 comprises a circular flat box, the central portion of the top of which is apertured and shaped like the frustum of a cone, and the anode 9 comprises a disc having a central apertured portion also resembling the frustum of a cone but of less vertical height than the central portion of the electrode 8. The accelerating anode 9 is so designed that the current passing to it is negligible, e.g. 10 per cent, in comparison with that bombarding the work-holder. In one method of operation the anode potential and the workholder potential are the same, as also are the potentials of the focussing electrode and the filament. The divergency of the beam is altered by altering the potential of the accelerating anode, e.g. by using a potential divider connected across the source of H.T. supply. The position of the electron gun with respect to the workholder and the potential of the accelerating anode of the gun also affect the divergency of the beam. Potentials, currents and dimensions are given. The gas pressure is preferably kept below 3 x 10À4 mms. of mercury by vacuum pump exhaustion through tube 3.