GB1410262A - Field optical systems - Google Patents

Abstract

1410262 Electron guns; ion beam apparatus; ion pumps AMERICAN OPTICAL CORP 14 Feb 1973 [14 Feb 1972] 7346/73 Heading H1D Field emission apparatus comprises field emission tip 21, electrode system 23, 24 producing focusing and accelerating fields and field electrode 22 for extracting charged particles, the field electrode including two spaced apertures (e.g. in electrodes (35), (32) of Fig. 2, not shown), the proximal aperture exposed directly to the unfocused emission from the tip and being large enough to prevent any substantial proportion of the beam striking the proximal face of the field electrode, the distal aperture limiting the beam to the desired diameter. The gun may be in a scanning microscope and may produce negative or positive particles (e.g. electrons or ions). An ionizable gas is introduced from ion production. Electric potentials, aperture sizes, distances and spot sizes are disclosed, for example in relation to tip size. V 1 /V 0 Determines focal distance while impedance Z prevents the extraction voltage rising to the main accelerating voltage. Ve may be controlled independently of V 1 to produce focal length control as well as beam size. Annular shield 20 prevents h.v. discharges and condenses Ti sublimated from coil 19 so constituting a vacuum pump, and in conjunction with inherent ion pumping provides by electrodes 21 and 22, i.e. ionization by primary and secondary particles, attains a 10<SP>-9</SP> to 10<SP>-11</SP> torr vacuum. Coil 19 is on a depending flange which prevents Ti coating the tip, apertures or insulators. Deflection lens 16 synchronized to a scanning system for control of video receiver enables programmed operation. Detection may be by transmitted, secondary, reflected or absorbed electrons, photons, or X-rays. Electrode 23, 24 geometries are discussed, tip 21 is on insulator (26) which may be axially adjustable (e.g. by the mechanism as in patent Specification 1,355,365), extractor electrode (35), (32) has open-side walls permitting ionized particle and contaminant passage. Raised portions about the apertures trap contaminants, and insulator (31) improves h.v. protection. Regions (41a), (41b) are at different vacuum levels and differentially pumped. Open structure shield (33) reduces ion bombardment of tip 21, ions being ultimately trapped on shield 20. Coating the tip with Zr or other high work function material or altering the tip geometry ensures emission from a smaller source. Magnetic and/or mechanical and/or electrostatic trapping may be used as well as sublimation methods to inhibit tip damage.