US3795837A

US3795837A - Electron guns

Info

Publication number: US3795837A
Application number: US00237800A
Authority: US
Inventors: T Chang
Original assignee: Cambridge Scientific Instruments Ltd
Current assignee: Cambridge Scientific Instruments Ltd
Priority date: 1971-03-25
Filing date: 1972-03-24
Publication date: 1974-03-05
Anticipated expiration: 1991-03-05
Also published as: GB1384243A; DE2212424A1

Abstract

In an electron gun for use in electron beam instruments such as electron microscopes and microanalysers the contamination that builds up on the control electrode (grid) is removed periodically and preferably automatically with the instrument out of use, by electron bombardment from a heater and possibly from the cathode itself, the control electrode being made some hundreds of volts positive with respect to the heater during this period. The heater may be one already present for heating the cathode. The bombardment may be in two thirty-second pulses applied immediately after the instrument is switched off.

Description

llnited States Schaedler et al. 316/28 Meat [1 [111 3,795,837

Qhang Mar. 5, 19741- ELECTRON- GUNS t Prima Examiner-Ro Lake Phil h Y k [75] Inventor N Y C ang or town Assistant Examiner-James B. Mullins i Attorney, Agent, or Firm-Scrivener, Parker, Scriv- [73] Assignee: Cambridge Scientific Instruments I n r nd Cl rke Limited, Cambridge, England v [22] Filed: Mar. 24, 1972 [57] ABSTRAKIT [2]] Appl. No.: 237,800 In an electron gun for use in electron beam instruments such as electron microscopes and microanalysers the contamination that builds up on the control [30] Foreign Pnomy Dam electrode (grid) is removed periodically and prefera- Mar. 25, 1971 Great Britain 7,966/71 automatically with the instrument out of use, electron bombardment from a heater and possibly U-S Clo the cathode itself, the control electrode g [51] 11 11. Cl.f made some hundreds of vohs i iv i respect to Fleld 0 Semc the heater during period. The heater y be one already present for heating the cathode. The bom- [56] References C'ted bardment may be in two thirty-second pulses applied UNITED STATES PATENTS immediately after the instrument is switched off.

2,774,645 12/1956 Batchelor 316/28 X 2,783J 5 Claims, 1 Drawing Figure ELECTRON cons This invention relates to electron guns used for generating beams of electrons in apparatus such as electron microscopes, X-ray micro-analysers and electron beam machining equipment. Such guns generally comprise an electron-emitting cathode and a control electrode in the form of a so-called grid or Wehnelt electrode, followed by one or more anodes to form the beam. The normal thermionic directly or indirectly heated cathode has limited emissive power for a given source size and considerable effort has been directed to developing sources of electrons of greater emitting power but small size. Attempts to increase the brightness of atungsten hairpin type of directly heated thermionic cathode result in a short life and have the further drawback of producing an undesirable spread of electron energies.

Lafferty has proposed some years ago the use of Ianthanum hexaboride or other rare earth borides as the electron emitting material, but the high reactive nature of these materials at the elevated temperature give rise to severe practical problems. Broers has proposed an electron gun using lanthanum hexaboride emitter by making a cathode in the form of a sharp-pointed rod of lanthanum hexaboride carried on a cooled mounting and indirectly heated by a solenoid-like heating coil around it. However, in addition to requiring a much higher vacuum than the remainder of the electron beam column, which can be troublesome in apparatus that has to be frequently demounted for other purposes, this heated lanthanum hexaboride cathode causes contamination of the control electrode by particles and decomposition products emitted from the cathode. Consequently although the life of a cathode itself may be many times that of a tungsten hairpin thermionic cathode, the electron gun structure may still require demounting at time intervals shorter than the life time of the cathode for decontamination. Attempts have been made to overcome the problem by the insertion of a protective cap between the cathode and the control electrode but with limited. success.

The aim of the invention is to reduce this problem of contamination and increase the interval between downtimes. According to the invention it is now proposed that, with the instrument out of normal use, the control electrode is heated by electron beam bombardment from the heater coil achieved by a positive potential applied to the control electrode with respect to the heater coil. The effect of this is to force off the control electrode any particles or decomposition products that may have built up on it arid to return them to the cathode. The potential applied to the control electrode may be of the order of four or five hundred volts positive with respect to the heater coil.

It will be understood that this cleaning operation can be performed at any time, and without'demountingthe parts or letting down the vacuum. Where the heating is done by the existing cathode heater coil no modification at all is required to the gun and the only modification required to the instrument is the provision of the necessary switching to apply the potential and perhaps additional heating current to the coil if required. According to a further feature of the invention it may be arranged that the cleaning operation automatically comes into action after use of the instrument. For example when the instrument is switched off at the end of a days work the positive potential on the control electrode and the current to the heating coil are automatically applied, either for an indefinite period or preferably for a limited period controlled by a timer. In a preferred arrangement the cleaning is carried out in two or more successive pulses, each having a duration of, for example, 30 seconds, and separated by an interval of only a few seconds.

The invention will now be further described by way of example with reference to the accompanying FIG- URE which illustrates diagrammatically an electron gun equipped with switching means for carrying out the invention.

The gun is of the kind proposed by Broers and described in British Pat. No. 1,150,516 (and the corresponding US. Pat. No. 3,462,635 and German Pat. No. 1,589,974) comprising an elongated cathode member 1 with a pointed end 2, made from a rare earth boride, preferably lanthanum hexaboride, and heated indirectly by a heating coil 3 wound around it but out of mechanical contact with it. The heat is imparted to the cathode member partly by radiation from the coil and partly by electron bombardment of the cathode by electrons from the coil, produced by an applied potential difference indicated diagrammatically at 4. The coil is supplied from a direct current source 5 of a few volts. The electrons emitted from the pointed end of the cathode are formed into a beam by a control electrode or so-called grid 6 which is a few hundred or thousand volts negative with respect to the electrode, and then accelerated by an anode 7 which is several thousand or tens of thousand volts positive with respect to the cathode. In practice it is usual for the anode to be at earth potential and for the control electrode and cathode to be at a substantial negative potential (indicated diagrammatically at 8) with respect to earth. The control electrode 6 is negative with respect to the cathode l, as indicated diagrammatically at 9.

The whole gun assembly is in an enclosure (not shown) at high vacuum.

It is found that, during prolonged use of the instrument in which thegun forms the source of an electron beam, contamination builds up on the control electrode 6. This contamination is found to be made up of lanthanum (but little boron) from the cathode l and tungsten from the heater coil 3, also some extraneous material.

In accordance with the present invention provision is made for disconnecting the control electrode from its normal potential and connecting it instead to the positive side of a source 10 of some hundreds of volts, for example between 400 and 500 volts, of which the negative side is connected to the heater coil; at the same time one end of the heater coil, for example the positive end where the coil is supplied from a direct current source 5, is electrically connected directly to the cathode, cutting out the potential difference 4. Thus when the switching operation according to the invention is performed, the control electrode 6 is made some hundred of volts positive with respect to both the heater coil and the cathode. Consequently it is bombarded with electrons emitted from the heater and cathode, and these clean off the contaminating particles. In practice we find that the voltage should be at least volts for a worthwhile effect, and not more than 1,000 volts. The switching can be provided by two-way switches shown diagrammatically at 11 and 12, which are operated only when the apparatus is out of normal use, and when the high negative potential 8 has been disconnected. In the drawing they are shown in their rest positions, i.e., those positions they occupy when the instrument is in normal use. At the same time a further two-way switch 13 connects the heater coil to a higher voltage supply 14 than the supply which feeds it during normal use. Consequently the coil runs at a higher temperature than during normal use.

The switches ll, 12 and 13 are controlled automatically by a timer 15 which is set in operation by the action of switching off the main high tension and heater coil supply of the instrument (electron microscope, micro-analyser or other equipment) in which the gun is employed, after a period of use. In the example under consideration the timer changes over the switches l1, l2 and 13 for a period of 30 seconds and then, after .a gap of a few seconds, for a further period of 30 seconds, and then switches itself off, returning the switches to their normal positions. It will be understood that the number, magnitude and duration of the switchon pulses may be altered to suit a particular gun.

We claim:

1. In an electron gun comprising a heater, a cathode, a control electrode and at least one anode the improvement which comprises means for automatically removing contamination from said control electrode after each operation of said gun, said means comprises first switching means operative automatically on switchingoff of said gun and acting to energise said heater and second switching means acting on switching-off of said gun to apply a positive potential to said control electrode with respect to said cathode, and timing means acting to de-activate said first and second switching means after a limited period of time.

2. In an electron gun, the automatic contaminationremoval means set forth in claim 1 wherein said timing means are operative to activate said first and second switching means for a second limited period of time following said first period after an interval.

3. In an electron gun the contamination-removal means set forth in claim 1 wherein said first switching means are operative to apply a potential to said control electrode which is positive with respect to said heater.

4. In an electron gun the contamination-removal means set forth in claim 1 wherein said potential is of the order of 400 volts.

5. In an electron gun the contamination-removal means set forth in claim 1 wherein said heater is a heater present for use in normal operation of said gun and wherein said first switching means are operative to energise said heater at a higher potential than that used in normal operation of said gun.

Claims

1. In an electron gun comprising a heater, a cathode, a control electrode and at least one anode the improvement which comprises means for automatically removing contamination from said control electrode after each operation of said gun, said means comprises first switching means operative automatically on switching-off of said gun and acting to energise said heater and second switching means acting on switching-off of said gun to apply a positive potential to said control electrode with respect to said cathode, and timing means acting to de-activate said first and second switching means after a limited period of time.

2. In an electron gun, the automatic contamination-removal means set forth in claim 1 wherein said timing means are operative to activate said first and second switching means for a second limited period of time following said first period after an interval.