EP0287774A2 - Cathode thermionique en épingle à cheveux - Google Patents

Cathode thermionique en épingle à cheveux Download PDF

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Publication number
EP0287774A2
EP0287774A2 EP88102556A EP88102556A EP0287774A2 EP 0287774 A2 EP0287774 A2 EP 0287774A2 EP 88102556 A EP88102556 A EP 88102556A EP 88102556 A EP88102556 A EP 88102556A EP 0287774 A2 EP0287774 A2 EP 0287774A2
Authority
EP
European Patent Office
Prior art keywords
temperature
apex
wire
cathode
legs
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP88102556A
Other languages
German (de)
English (en)
Other versions
EP0287774A3 (fr
Inventor
Otto Dr. Dipl.-Ing. Winkler
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
OC Oerlikon Balzers AG
Original Assignee
Balzers AG
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Balzers AG filed Critical Balzers AG
Publication of EP0287774A2 publication Critical patent/EP0287774A2/fr
Publication of EP0287774A3 publication Critical patent/EP0287774A3/fr
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/13—Solid thermionic cathodes
    • H01J1/15—Cathodes heated directly by an electric current
    • H01J1/16—Cathodes heated directly by an electric current characterised by the shape

Definitions

  • Hairpin cathodes made from wires of refractory metals, especially tungsten, are now commonly used as standard electron sources, e.g. used in electron microscopes and other electron optical instruments.
  • the tungsten hairpin cathodes that are used today in electron microscopes are made of pure or thoriated tungsten wire with a diameter of 0.12 - 0.14 mm.
  • the inner bending radius at the apex is usually 0.05 - 0.1 mm.
  • This hairpin is connected at both ends of its legs by spot welding to the heating current leads in the cathode base. Both legs should have the same length so that the emitting apex is equidistant from the cold ends of the hairpin and therefore reaches the highest temperature during operation. The greatest material removal should therefore also take place at this point take place by evaporation and the service life can be determined by the temperature and wire thickness at this point.
  • the temperature sink at the apex is hardly measurable pyrometrically. It is only a few degrees.
  • a prerequisite for the same height of the temperature maxima to the left and right of the apex is that the heat balance in the two legs is exactly symmetrical. If this is not the case, an asymmetrical temperature distribution is created, as shown by the dashed line. This asymmetry intensifies over time, because not only does the increase in resistance due to evaporation on the one hand increase more and more, but also the degradation on the other hand decreases as the temperature at the apex is kept constant. As the dash-dotted line in FIG. 1 shows, the temperature difference will increase until a leg melts away catastrophically.
  • the object of the present invention is to extend the life of hairpin cathodes by reducing the temperature drop at the apex and thus the tendency to destabilize the temperature distribution, and reducing the evaporation losses in this area by suitable measures.
  • thermionic hairpin cathode made of a high-melting metal wire, which is characterized in that the temperature gradient near the apex is increased by increased heat dissipation along the two legs without reducing the cross-sectional area of the wire.
  • the object of the invention is achieved in that the heat radiation at a distance from the apex, which corresponds to 10% to 50% of the leg length, is locally increased by enlarging the surface without appreciably reducing the cross-sectional area of the wire.
  • the wire is deformed in the regions of the two legs adjoining the apex (without a substantial change in the cross-sectional area) of the wire in such a way that it obtains a semicircular profile, and at the same time the flat sides of the two sides become as possible are a short distance from each other.
  • the temperature gradient starting from the apex along the leg becomes much steeper.
  • a temperature distribution as shown in FIG. 2 then results.
  • the solid curve shows the original temperature distribution under ideal conditions, and the dashed line shows the distribution after enlarging the surface at a point on the legs that is approximately 2 mm from the apex.
  • the total length of the legs was 8 mm.
  • the local enlargement of the radiating surface is achieved in that tungsten wire spirals 3 of approximately 0.6 mm in length and 0.4 mm in diameter are pushed on at a distance of approximately 2 mm from the apex. To achieve a firm fit, they are pressed a little flat. After the cathode has heated up, they then connect to the wire core by diffusion welding and thus obtain the necessary good thermal contact.
  • Figure 6 shows the same cathode at the end of its life after 48 hours of operation at a peak temperature of 2900 K. With this excessive temperature, the test time should be shortened. It can be seen that it was possible to move the point 5 of the highest temperature close to the apex and thereby increase the service life many times over. The lifespan achieved would have been 6 to 7 times at the usual cathode temperature of approx. 2750 K, i.e. 300-350 hours instead of 20-50 hours, provided that the temperature or emission of the cathode is kept constant.
  • the local increase in radiation is achieved by pressing the tungsten wire flat. Care must be taken to ensure that the minimum thickness of the flat-pressed area 4 is not fallen below, since otherwise there is a risk that the percentage cross-sectional reduction there per hour will be greater than at the apex and the temperature gradient will gradually disappear due to excessive local resistance increase.
  • the flat pressed area In order to achieve a sufficient surface enlargement, the flat pressed area must be longer than the wire spiral in the first example.
  • a suitable dimensioning is e.g. an embossing of approx. 1.5 mm length with 0.4 mm width. As in the previous example, this reflects a local surface enlargement of approx. 0.7 mm2.
  • FIGS. 8-11 relate to an embodiment in which the apex region of the hairpin cathode was deformed in a die at a temperature of 300-400 ° C. in such a way that the two legs are given a semicircular profile 6, as shown in FIG.
  • the embossing is made to a length of 0.3-0.5mm.
  • the flat sides touch each other initially and would form a short circuit if the legs were not subsequently spread slightly, so that a wedge-shaped gap 7 of 0-30 ⁇ m in width is created.
  • the opposing surfaces can neither radiate appreciably from this narrow gap, nor can considerable amounts of material evaporate to the outside.
  • the radiation and evaporation losses of this cathode section are reduced by approximately 25% in this way.
  • Embossing the legs has another important advantage.
  • An approximately hemispherical cathode end 8 is then formed at the apex of the hairpin.
  • a cone or a pyramid 9 can be ground on this hemisphere, as shown in broken lines in FIGS. 8 and 9, so that a tip cathode with a long service life is produced. It even contributes to an increase in the service life if the relatively large accumulation of material at the tip, resulting from the embossing, with its large radiation losses, is reduced to the permissible level in this way and the temperature gradient near the tip is thereby increased.
  • FIG. 11 shows the geometry of such a cathode, which is provided with additional cooling spirals as in FIG. 5, but without a ground point, after 50 hours of operation at 2900 K.
  • the lifespan is not yet over after this time and it would be significantly extended if the asymmetry of the embossed leg areas were suppressed by grinding a tip and increasing the temperature gradient.
  • a suitable solution consists in either deliberately designing the leg length to be different, or in arranging the leg areas with increased radiation at different distances from the apex or in different surface sizes. So that the geometry and current direction remain assigned to one another, the connection points of the current leads on the cathode base would then have to be marked accordingly or made unmistakable.

Landscapes

  • Solid Thermionic Cathode (AREA)
  • Electron Sources, Ion Sources (AREA)
EP88102556A 1987-04-24 1988-02-22 Cathode thermionique en épingle à cheveux Withdrawn EP0287774A3 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
CH158287 1987-04-24
CH1582/87 1987-04-24

Publications (2)

Publication Number Publication Date
EP0287774A2 true EP0287774A2 (fr) 1988-10-26
EP0287774A3 EP0287774A3 (fr) 1990-03-07

Family

ID=4213784

Family Applications (1)

Application Number Title Priority Date Filing Date
EP88102556A Withdrawn EP0287774A3 (fr) 1987-04-24 1988-02-22 Cathode thermionique en épingle à cheveux

Country Status (3)

Country Link
US (1) US4899078A (fr)
EP (1) EP0287774A3 (fr)
JP (1) JPS63308853A (fr)

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1622184B1 (fr) * 2004-07-28 2011-05-18 ICT Integrated Circuit Testing Gesellschaft für Halbleiterprüftechnik mbH Emetteur pource source d'ions et procédé pour sa fabrication
US7544523B2 (en) * 2005-12-23 2009-06-09 Fei Company Method of fabricating nanodevices

Family Cites Families (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1144418B (de) * 1961-07-20 1963-02-28 Siemens Planiawerke A G Fuer K Verfahren zur Herstellung einer Kontaktschicht auf einem silizium-haltigen Werkstoff
US3356887A (en) * 1965-07-30 1967-12-05 Frederick C W Heil Fe cathode redesign
NL7101954A (fr) * 1971-02-13 1972-08-15
US3817592A (en) * 1972-09-29 1974-06-18 Linfield Res Inst Method for reproducibly fabricating and using stable thermal-field emission cathodes
CA1141420A (fr) * 1980-06-20 1983-02-15 Stephen Lhotsky Filament, procede d'affutage electrolytique et appareil permettant la mise en oeuvre du procede

Also Published As

Publication number Publication date
US4899078A (en) 1990-02-06
EP0287774A3 (fr) 1990-03-07
JPS63308853A (ja) 1988-12-16

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