Electron beam generating source
United States Patent [1 1 Suganuma [111 3,878,424 [451 Apr. 15, 1975 ELECTRON BEAM GENERATING SOURCE Inventor: Tadao Suganuma, Tokyo, Japan Nihon Denshi Kabushiki Kaisha, Tokyo, Japan Filed: July 17, 1973 Appl. No.: 380,100  
 Assignee:  
 [30] Foreign Application Priority Data July 20, 1972 Japan 47-72865 US. Cl 313/337; 343/82 BF; 250/310; 219/121 ED; 313/346; 313/336 Int. Cl H0lj l/20; l-lOlj 19/14 Field of Search 313/82, 337, 82 BF, 441, 313/446, 336, 346, 310, 311; 250/310; 219/121 EB References Cited UNITED STATES PATENTS 9/1950 Fay 313/337 X 3,005,123 10/1961 Griffiths 313/82 R 3,389,290 6/1968 Yoshid et a1 313/346 R 3,500,106 3/1970 Berchtold 313/337 X 3,571,590 3/1971 Katagiri et a1. 250/311 3,610,988 10/1971 Schmitz 313/82 R 3,763,388 10/1973 Benda 313/337 3,792,263 2/1974 Hashimoto et a1.... 250/31 1 Primary Examiner-Saxfield Chatmon, Jr. Attorney, Agent, or FirmWebb, Burden, Robinson &amp; Webb [5 7] ABSTRACT This invention relates to an electron beam generating source comprising a cathode and anode, the opposite and facing surfaces of the cathode and anode being flat and parallel so that the electrostatic field formed between the two electrodes is parallel and perpendicular to the surfaces of both electrodes, thereby preventing the electrons emitted from the electron emitter from being affected by lens effects, for example, spherical aberration.  
 4 Claims, 6 Drawing Figures I a l 5 7 HEATING sounce 0.0. 8 men v1 vomma:- SOURCE \8 1 ELECTRONBEAM GENERATING SOURCE This invention relates to an improved and novel electron beam generating source for use in apparatus such as transmission type electron microscopes or scanning type electron microscopes. More particularly. this invention relates to an electron beam generating source which is free from lens effects or aberrations.  
  The major operational criterion for an electron beam generating source used in an electron microscope or the like is the brightness of the electron beam produced by the source. Without sufficient brightness it is virtually impossible to obtain an extremely narrow, high density probe. These are two of the main prerequisites for ensuring a high resolution microscope image, especially in the case of an electron scanning microscope.  
 In sources ofthe type presently available; in other words, the conventional triode type guns consisting of a filament, Wehnelt electrode and anode, brightness is insufficient. This is because of lens effect caused by gun geometry with its concomitant spherical aberration, a factorwhich is especially pronounced in the case of electrostatic lenses. As a consequence, the diameter of the crossover image formed between the filament and the anode is large, resulting in low brightness. ltis therefore an object of this invention to provide anelect ron beam generating source designed so that the electron beam it produces is not affected by the above mentioned lens effect.  
  Another object of this invention is to provide an electron beam generating source having a degree of brightness sufficient to ensure a high resolution microscope image. I  
  Yet another object of this invention is to provide an electron beam generating source suitable for use in an electron scanning microscope.  
  Briefly. according to this invention a cathode and anode are provided with substantially flat and parallel opposite facing surfaces such that the electrostatic field formed therebetween is perpendicular to said surfaces. An optical axis is formed along a line perpendicular to both surfaces. An aperture is provided in the anode about the optical axis. A thermionic emitter is provided on the cathode at the optical axis. Preferably, the thermionic emitter is a coating of low work function on the cathode.  
  These and other objects and advantages of this invention will become apparent by reading the following detailed description in conjunction with the accompanying drawings of which:  
  FIG. I is a schematic view of one embodiment of the electron beam generating source according to this invention.  
  FIG. 2 is a plane view of the cathode 19 shown in FIG. 1.  
  FIG. 3 is a sectional view of an alternate electron emitter.  
  FIGS. 4, 5, and 6 are schematic illustrations of other embodiments of this invention.  
 &#39; Referring to FIG. I, an electron gun chamber 1 is part of a vacuum chamber comprising an electron microscope column 2 associated with the electron gun chamber 1. An insulator is mounted in the gun chamber to support the electrical components of the gun. The insulator is filled with insulating pitch 4. A high voltage cable 5, part of which is buried in the solidified insulating pitch and lead wires 6 and 7 constitute part of the electrical conducting path of the electrons emitted by the gun. The external end of the lead wire 6 is connected to a heating source 8. The external end of the lead wire 7 is connected to the heating source 8 and also to a dc. high voltage source 9. The internal ends of lead wires 6 and 7 are connected to rods 10 and 11 respectively. Conductive plate springs 12 and 13 are secured to&#39;an insulating member 14 by screws 15 and 16 respectively. One end of the plate springs 12 and 13 are in contact with the rods 10 and 11 respectively, while the other end of the said plate springs are connected to one end of a cathode heater plate 17 and a cathode 19 respectively. The cathode heater plate 17 is made of tantalum. for instance, or any other element having a suitably high melting point such as tungsten or molybdenum and has a width of-approximately 3 to 7 mm for example. A base plate 18 of the cathode is connected to the other end of the cathode heater plate 17. Thus, a closed circuit connecting the two ends of the cathode heater plate 17 and the heating source 8 is constituted.  
  FIG. 2 shows the underside plane view of the cathode 19 described in FIG. 1. The cathode base plate 18 is provided with an elongated cut-out&#34; into which the cathode heater plate 17 is fitted so that the surface of the plate lies flush with the lower surface of the base plate 18. A narrow gap 20 serves to insulate the cathode heater plate l7 from the base plate18. A coating 21 of oxide (e.g. barium-strontium-calcium) having a low work function is placed at the position where it is desired that electrons be emitted as explained herein. The oxide coating is roughly microns in diameter and about 50 microns thick.  
  Returning to FIG. 1, an anode 22 is located below the base plate 18. The anode 22 is maintained at ground potential. Moreover, the upper surface of said anode is flat and lies parallel to the lower surface of the base plate 18. The anode is provided with central aperture 23, for example 2mm in diameter, through which the electron beam passes.  
  In the above-described embodiment, since the lower surface of the base plate, which is supplied with negative high voltage by the dc. high voltage source 9, is flat and the upper surface of the anode, which is grounded, is flat and lies parallel to the lower surface of said base plate 18, the electrostatic field between the base plate and the anode is parallel and perpendicular to both surfaces. Also, since the cathode heater plate 17 is supplied with heating current by the heating source 8, the oxide coating 21 is maintained at a high temperature (e.g. l000-1500 C). The oxide coating is positioned directly above the anode aperture along the optical axis. Since the work function of the oxide coating is low, thermionic electrons 24 are emitted from the oxide only. These electrons are accelerated by the electrostatic field, existing between the cathode l9 and the anode 22, along the optical axis 25 without crossing over. As a result, the accelerated electrons pass through the anode aperture 23 adjacent to a magnetic field lens (not shown) and the beam is spot focused to the desired diameter.  
  Since the electron beam generating source according to this invention is free from an electrostatic lens effect as understood from the above disclosure, the brightness of the electron beam generating source is not reduced by spherical aberration and the obtainable brightness is high.  
  In the above-described embodiment, the oxide utilized as an electron emitter is simply coated on the surface of the cathode heater plate.  
  FIG. 3 show a sectional view of another electron emitter according to this invention in which the center of the cathode heater plate 17 is electronically perforated with a hole of predetermined diameter. Oxide is then applied to the hole to form a packing of oxide 26, the under side of which flush with the cathode heater plate 17.  
  FIG. 4 shows a schematic illustration of another embodiment of this invention in which a conventional triode gun is utilized for the purpose of heating only a narrow portion of the base plate 18 by bombarding it with electrons. In the figure, 27 is a tungsten filament which is supplied with heating current by a source 28. A Wehnelt electrode 29 is connected to the filament 27 by a bias source 30 so that the potential of the Wehnelt electrode 29 is kept lower than that of filament 27. A d.c. high voltage source 31 is connected between the filament 27 and the base plate 18.  
  By passing current through the filament 27, the resultant outflow of thermionic electrons 32 are converged by the electrostatic field formed below the filament 27 and strike up against the center of the base plate 18. Since the degree of convergence is high, the irradiated area of the plate is very small (in the order of 100 microns). Accordingly, the area of the plate which is heated and which by continuous electron bombardment is maintained at around 2500 C is also in the order of 100 microns, said small area emitting electrons which are free from lens effect. Thus, the outflow of electrons from the plate 18 are accelerated towards the anode 22 along and in parallel with the optical axis of the electron beam generating source.  
  FIG. shows a schematic illustration of yet another embodiment of this invention in which a needle 33 is arranged with its apex touching the upper surface of the base plate 18. By passing current through the needle 33 from the source 34, heat is generated at the needle apex since the electrical resistance at that point is large. Consequently, the base plate at the point of contact is also heated and proceeds to emit the thermionic electrons 24 which are drawn towards the anode 22 along and in parallel with the optical axis 25.  
  H6. 6 shows an embodiment of the invention in which the cathode 19 is cylindrical in shape so as to accomodate a heater 35 which is connected to an energy source 36. The underside of the base plate 18 coinciding with the optical axis is thinly coated with a layer of oxide having a low work function. The diameter of the oxide layer is approximately 100 microns. A sup- 4 the supporting electrode 37 are connected by a dc high voltage source 9. Thus, by passing current through the heater 35, the cathode 19 is heated and thermionic electrons 24 are emitted from the oxide and drawn towards the anode 22 along the optical axis 25.  
  lt will be appreciated from the heretofore described embodiments that the electron beam generating source according to this invention is effective in overcoming lens effect, and by so doing, permits the source brightness to be sufficiently high to ensure an electron probe having the required high density and small cross sectional area to provide the degree of resolution necessary for effective electron scanning microscopy.  
 What is claimed is:  
  1. A thermionic narrow electron beam generating source comprising a cathode and anode arranged in a vacuum chamber, the opposite and facing surfaces of said cathode and anode being substantially flat and parallel and a source for applying high voltage between said cathode and anode such that an electrostatic field is formed between said two electrodes which is parallel and perpendicular to said surfaces, said cathode comprising a plate having a centrally located thermionic electron point emitting means, and said anode having an aperture therein about an optical axis perpendicular to each electrode and extending through the emitting means and means for supplying heat to a small area of the cathode plate at the optical axis whereby the electron beam generated thereby is substantially free of lens effects.  
  2. An electron beam generating source as described in claim 1 in which said heating means consists of a filament and a Wehnelt electrode which serve to bombard the surface of said plate not facing the anode. i  
  3. An electron beam generating source as described in claim 1 in which said heating means comprises an electron current carrying needle whose apex touches the surface of said plate not facing the anode.  
 4. A thermionic narrow electron beam generating source comprising a cathode and anode arranged in a vacuum chamber, the opposite and facing surfaces of said cathode and anode being substantially flat and parallel and a source for applying high voltage between said cathode and anode such that an electrostatic field is formed between said two electrodes which is parallel and perpendicular to said surfaces, said cathode comprising a heating plate which is flush with the surface of the cathode and having a centrally located thermionic electron point emitting means comprising a material of low work function coated on the surface of the&#39; heating plate facing the anode, and said anode having an aperture therein about an optical axis perpendicular to each electrode and extending through the emitting means and means for supplying the said heating plate with an electrical current whereby the electron beam generated thereby is substantially free of lens effects.