GB1355365A - Electron guns - Google Patents

Abstract

1355365 Electron guns; ion pumps AMERICAN OPTICAL CORP 15 June 1971 [15 June 1970] 28003/71 Heading H1D An electron gun for an electron microscope comprises a field emission cathode 45, Fig. 4, surrounded by a shield electrode 90 operated at or near cathode potential and adapted to protect the cathode from excessive discharge voltages and preferably also forming a magnetic shield and an enclosure which can be evacuated to a lower pressure than the rest of the apparatus. A titanium coated filament 91 operated either continuously or discontinuously evaporates titanium on to the electrode 90 to getter reactive gases, and their action is assisted by cooling the electrode via a beryllium oxide ring 102 which conducts heat to a tank 100 filled with liquid nitrogen. An electrode 95 at 500-3000V accelerates electrons from the cathode and intercepts part of the beam thereby forming an electron cloud which tends to ionize any stray gas molecules which are then attracted to electrode 90, so that a local getter-ion pump is established. In this way the pressure within electrode 90 may be reduced to about 10<SP>-10</SP> Torr, whilst the rest of the apparatus is kept at about 10<SP>-6</SP>-10<SP>-7</SP> Torr by the pump 135. The cathode 45 may be 1000Š in diameter and is attached to a mount 51 crimped between two electrodes (of which one, 52, is shown in Fig. 4) connected by respective leads 80 and 81 to a terminal 77. The electrodes plug into an insulating post 55 secured to a metal shaft 60 received in a support plate 65 and sealed to the end cap 62 by a bellows 61. Four screws 66 (only two shown in Fig. 4) allow the plate 65 to be tilted in order to displace the cathode 45 in X and Y directions; a knob 68 allows the shaft 60 and therefore the cathode 45 to be moved longitudinally in order to focus the beam. A first anode 75 is connected through a resistance 83 to electrode 95 which is connected to input terminal 79, and a second anode 85 is maintained at 20 kV relative to the cathode. A plate 110 contains four holes ranging in diameter from 25 to 250 microns and is movable transversely; if this plate is fully retracted the seal 114 is withdrawn to allow the chamber 41 to be exhausted through a vent 125. For isolating the chamber 41 from atmosphere while a specimen is being changed, a plate containing an aperture 131 for the beam may be slid across the end of a tube 107 and sealed to it by a ring 143<SP>1</SP>. The chamber 41 is sealed by a ring at 113 and hinged at 152 so that it may be opened to replace the cathode.