Patent GB-1313145-A (GB)
Title: Apparatus for keeping the path of a working beam free of particles free of impurity
IPC: H01J29/84
Filed: 1970-07-30
Abstract: 1313145 Electron beam machining apparatus K-H STEIGERWALD 30 July 1970 [6 Aug 1969] 36865/70 Heading H1D An electron beam processing machine (e.g. milling, boring, cutting, welding, heating, annealing) including apparatus for keeping the path of the working beam free of impurity particles, comprising a screening device 6 associated with the working beam for catching impurities (e.g. occluded gases, sputtered or evaporated material), an ionization device 30 acting in the region of the working beam path to ionize particles with a greater ionization probability than that with which the working beam ionizes the impurity particles and a deflecting device 22, 24 producing a deflecting field on ionized impurity particles, which move at thermal speeds, to remove from the beam path on to the screening device. Ionization may be by U.V., or electrons for example, (e.g. 1 amp. cm.<SP>-2</SP> slow electrons) and deflection may be electrostatic, electromagnetic or magnetostatic. Specific deflecting voltages are disclosed, e.g. a few hundred volts, a 0À1 MeV working beam being substantially undeflected. Further screens 18, 20 may be included, screen openings being slightly larger than beam diameter, the deflecting electrodes and screens are preferably replaceable, may be arcuate and curve round the beam. Further replaceable plates 26, 28 of opposite polarity to plates 22, 24 to reduce the effect on the beam may be included. Ionizer 30 is a U.V. radiator and auxiliary electron source 36 may be included, located away from source 2 within the deflectors, the electrons being accelerated by plate 26 or separate accelerating electrodes. To avoid change of screen 6 an adjacent interchangeable screen may be provided. Fig. 2 (not shown) includes auxiliary electron cathode (48), screen (54) reducing interference with the deflecting field, auxiliary electron ionizer (34) in magnetic focusing coil 8 acting as a deflector extending the slow electron tracks by spiralling and increases the probability of ionization, or spiralling may be by a further magnetic device. Fig. 3 (not shown) is an ionizer with auxy. control electrode (56) and anode (58) of sieve or lattice form, or electrode (56) may be slotted, the latter and screen (54) more negative than cathode (48) and preferably regulatable, anode (58) more positive and controllable in potential to determine quantity and focusing. Fig. 4 (not shown) includes a focusing device formed as an electrostatic cylinder lens including control electrode (78) slightly more negative than cathode (76) and preferably regulatable, positive anode (80) and negative deflector (82) with slots parallel to cathode (76), and positive deflecting electrode (84), a multiplicity of such sources being included in circumferential electrodes (Fig. 5, not shown). Fig. 6 (not shown) includes cathode (88), annular auxiliary anode (92) focusing ring electrodes (94), (96), potential being instanced and Wehnelt control electrode (not shown) may be included to adjust beam current. Additional magnetic device ring magnet (98) effects additional focusing and if sufficiently large may produce spiralling. Figs. 7, 8 (not shown) includes cathode (106), grid (108) of wire mesh or spiral, surrounded by an envelope of perforated or imperforate sheet or wire mesh, or wire spiral (14) potentials being disclosed, electrons oscillating between grid (108) and spiral (114), ionized heavy impurity particles being at least partly deposited on screens located in axial regions outside of the ionization and deflecting device. When the beam passes through an intermediate partially evacuating chamber, deflecting and ionization devices may be provided therein or outside, the relatively high pressure allowing cold cathode, e.g. point or high current arc discharge, a ring magnet as in Fig. 7 being optional. Intermediate chamber (120) (Fig. 10) (not shown) includes deflectors (22), (24), the ionizer being formed by workpiece (16) and electrode (122), the arc being between projection (126) and the workpiece, the screening devices not being shown. In all embodiments, the acceleration distance, e.g. between electrodes (2), (6), lie outside the operative region of deflection. More than two ionizers and deflectors may be included.