US3728570A

US3728570A - Electron probe forming system

Info

Publication number: US3728570A
Application number: US00199899A
Authority: US
Inventors: K Smith; D Swann
Original assignee: Department Of Eng University O
Current assignee: University of Cambridge
Priority date: 1968-07-07
Filing date: 1971-11-18
Publication date: 1973-04-17
Anticipated expiration: 1990-04-17
Also published as: GB1291221A; DE2000889A1

Abstract

An electron gun comprising an electron source and an anode which together can produce a beam of electrons and which are both disposed substantially within a lens which serves to help focus the electron beam.

Description

United States Patent Smith et a1.

[ Apr. 17, 1973 ELECTRON PROBE FORMING SYSTEM Inventors: Kenneth Charels Arthur Smith; David John Swann, both of c/o The Department of Engineering, University of Cambridge, Trumpington Street, Cambridge, England Filed: Nov. 18, 1971 Appl. No.: 199,899

Related U.S. Application Data Continuation of Ser. No. 866,317, Oct. 14, 1969, abandoned.

Foreign Application Priority Data [56] References Cited UNITED STATES PATENTS 2,897,396 7/1959 Von Ardenne ..313/336 X 3,358,174 12/1967 Glenn, Jr ..313/336 3,374,386 3/1968 Charbonnier et al. .....313/336 X 2,944,172 7/1960 Opitz et a1. ..313/7 3,532,923 10/1970 Vogcl ..313/336 X 3,312,856 4/1967 Latferty et al.... ..313/346 3,462,635 8/1969 Broers ..313/336 X 2,272,353 2/1942 Ruska ..313/336 X FOREIGN PATENTS OR APPLICATIONS 1,012,040 12/1965 Great Britain 250/495 Primary Examiner-Herman Karl Saalback Assistant Examiner Saxfie1d Chatmon, Jr.

Attorney-John H. 0. Clarke 57 ABSTRACT An electron gun comprising an electron source and an anode which together can produce a beam of electrons and which are both disposed substantially within a lens which serves to help focus the electron beam.

6 Claims, 4 Drawing Figures L 01 l// 1 N/ TROGEN TRA P l D/FFUS/ON :1

PUMP 20 ORBION TYPE VACUUM PUMP PATENTED APR 1 7 I975 SHEET 1 [1F 2 ORB-ION TYPE VACUUM PUMP L /OU/D N/ TROGEN TRA P ELECTRON PROBE FORMING SYSTE This application is a continuation of application, Ser. No. 866,3 l7 filed Oct. 14, 1969, now abandoned.

This invention relates to an electron gun suitable for use in an electron probe forming system of high brightness such as used in scanning electron microscopes.

Known electron guns comprise an electron source and an anode with a hole in it through which the electrons pass after being accelerated towards the anode by a voltage applied between the source and the anode. The electrons passing through the anode form a beam which may be focussed to form a probe by a separate demagnifying lens system through which the beam passes after leaving the gun, and/or by an electrode in the gun between the source and the anode. The source is usually of the thermionic type and gives an effective source size of the order of 50 microns in diameter, and

the probe formed may have a diameter of the order of 100 A at its focus, representing a demagnification by the lens system by a factor of the order of 5,000.

According to the present invention we propose an electron gun comprising an electron source and an anode which together can produce a beam of electrons and which are both disposed substantially within a lens which serves to help focus the electron beam.

The arrangement of the lens around the electron source and anode allows the lens to be operated at short focal lengths of the order of only a few millimetres which in turn is advantageous in helping to keep optical aberrations small. This advantage is particularly important where the electron source is of the field emission type. Such a'source can give effective source size of the order of as little as 100 A and the lens then need operate at only one-to-one magnification, and under these conditions aberrations can be particularly large with lenses not having short focallengths. The lens is preferably a highly asymmetric magnetic lens.

According to a further feature of the invention the electron source is of the field emission type and is characterised in that it is made of lanthanum hexaboride. Preferably it is operated in a pulsed mode, and is disposed within a housing formed by the anode, this housing being adapted for connection to a vacuum pump so that the emitter can be operated under highly evacuated conditions.

These characteristics all serve to extend the working life of the source. Ideally, the space around the source should be an ultra high vacuum in order to give the longest possible working life, but it is found that this condition can be relaxed slightly in order to aboid the necessity of having to use unduly complex pumping systems, while at the same time retaining an acceptable working life for the source. Where the electron gun is used in an enclosed probe forming system, the remainder of the system may communicate with the electron gun through the hole in the anode in which case the whole system needs to be evacuated. The remainder of the system need not be evacuated to the same degree as the electron gun, however, and the hole in the-anode can be made small enough to support the required differential pressure across the hole.

The invention will now be described by way of example with reference to the accomapnaying drawings where:

FIG. 1 shows a schematic cross-sectional view of an electron probe forming system having an electron gun according to the invention;

FIG. 2 shows the electron gun of FIG. 1 to a larger scale;

FIG. 3 shows a similar view to that of FIG. 2; and

FIG. 4 shows a modified form of the electron gun of FIGS. 2 and 3.

The electron probe forming system of FIG. 1 includes an electron gun comprising a field emission electron source 1 having a finely pointed tip of lanthanum hexaboride. This source 1 is carried at the end of a tubular support member 2 which projects through a base plate 3 and which can be translated and tilted relative to this base plate 3 by adjustment means 4 which connects the support member 2 to the base plate 3. A tubular anode 5 is disposed around the source 1 and support member 2 and is supported in position at its end nearest the base plate 3 by an annular member 6 which is connected to the base plate 3. The other end of the anode 5 is closed and has a central hole 7 through it at a point adjacent the tip of the source 1. The end of the anode nearer the source 1 has a reduced cross-section so that it can be received within the wide-bore pole piece 8 of a conventional asymmetrical magnetic lens 9. The other pole piece 10 has a bore 11 through it and is disposed adjacent the closed end of the anode 5 with the bore 11 in alignment with the hole 7 and the tip of the source 1 so that a beam of electrons emitted from the source 1 can pass through the hole 7 and bore 11 to form a probe as described below.

In the overall system of FIG. 1 the magnetic lens 9 is supported between a housing element 12 which is connected to the member 6, and a housing element 13 which defines a specimen chamber 14. A flexible bellows 15 is provided between the support member 2 and the base plate 3 to provide an air-tight seal between these members, and the joints between the base plate 3, member 6 and anode 5 are made air-tight so that, if it were not for the hole 7, these members would define an air-tight chamber containing the source 1. The joints between the magnetic lens 9 and the housings 12 and 13, and the member 6 and the housing 12 are also made air-tight so that a further air-tight chamber including the specimen chamber 14 and the interior of the magnetic lens 9, is provided around the anode 5. A pump of the orb-ion type (not shown) is connected to the chamber within the anode 5, and a diffusion pump 20, and perhaps a further backing-up pump, are connected to the outer chamber around the chamber 5. By these means both chambers can beevacuated, and provided the hole 7 is sufficiently small a high or ultra-high vacuum can be produced in the inner. chamber while a normal vacuum is produced in the outer chamber, a high differential pressure being established between the chambers across the hole 7. This arrangement therefore satisfies the high vacuum conditions required for satisfactory operation of field emission type sources such as the source 1, without having to maintain the whole of the probe forming system under these high vacuum conditions. A heating filament may also be provided in the inner chamber to heat the anode 5 and support member 2, so as to achieve a degree of outgassing and thus a higher vacuum in the inner chamber.

Operation of the above system in producing an electron probe can best be described with reference to FIGS. 2 and 3. An accelerating voltage is applied between the source 1 and the anode 5 which causes electrons emitted from the tip of the source 1 to be accelerated towards the anode 5. The trajectories of such electrons are shown by the broken lines in the drawings. The trajectories of all of these electrons are affected by the magnetic lens 9 and some electrons move so as to pass through the hole 7 in the anode 5 and bore 10 in the pole piece 10, and come to a focus 16 in a plane 17. This focussed beam of electrons comprises the electron probe and in the case of a scanning microscope, for example, the plane 17 could be the plane of the specimen, deflecting means being provided to scan the probe across the specimen. The hole 7 in the anode 5 provides the electron optical limiting aperture of the system, but in alternative arrangements a limiting aperture may be placed in any suitable position after the anode. In FIG. 4 a limiting aperture 19 in the form of a disc with a hole in it is provided in the end wall of the anode.

In the arrangement of FIG. 2 a large proportion of the electrons emitted from the source 1 strike the anodeS thereby releasing gas which raises the pressure within the anode. A method of avoiding this is showin in FIG. 3. The geometry of the

pole pieces

8 and 10, and the strength of the magnetic field produced by them are arranged so that electrons emitted from the source 1 are brought to a first focus at the hole 7 and are then brought to a second focus 16 in the plane 17.

An electron optical beam limiting aperture (not shown) is situated betweeen the hole 7 and the plane 17, as in this case the hole 7 no longer serves as a limiting aperture. By thus bringing the beam to a focus in the plane of the hole 7 the majority of the electrons are made to pass thorugh the hole 7 and bombardment of the anode by electrons is substantially eliminated. Further, since the cross-section of the beam at the first focus is small, the size of the hole 7 may be reduced which makes it easier to maintain a high vacuum within the anode 5.

The fact that the source 1 operates by field emission means that it can possess a very small effective source size, i.e. one having a diameter of the order of 100 A. Therefore the magnetic lens 9 need not demagnify the source size but can operate at one-to-one magnification to produce probes with a focus at the specimen surface of the order of lOO A in diameter. Under these conditions optical aberrations may be troublesome and thus the magnetic lens 9 is operated at as short a focal length as possibly to reduce such aberrations. The arrangement according to the invention with the source 1 with the pole pieces of the magnetic lens 9 allows the lens 9 to operate at the shortest possible focal lengths of the order of only a few millimetres.

In alternative probe forming systems to that described above, one or more additional lens may be provided for the electron beam to pass through after leaving the electron gun and before being focussed to form the final probe. In one system an additional pole piece similar to pole piece 10 may be provided adjacent to the latter so that the two form a lens between them. This lens would add greater flexibility of focussing to the system. This additional lens may be excited by the coil of lens 9 or by a separate coil. In other systems the relative positions of the source 1 and the hole 7, the geometry of the lens 9 and the strength of its magnetic field may all be adjusted to produce a convergent or divergent electron beam which is matched to the lenses that form the final probe. The advantage of the lens 9 in having a short focal length would still apply to such systems. In one such system the electron gun would operate as shown in FIGS. 2 or 3 and would operate under strongly magnifying conditions i.e. the focus formed at the plane 17- would be situated a considerable distance from the magnetic lens 9. A second lens would demagnify the focus to produce the final probe. Such a two lens system would facilitate beam alignment and the provision of scanning coils or other apparatus between the lenses.

According to a further feature of the invention the electron gun could be provided with a further electrode in the form of a grid or cathode 18 around the tip of the source 1 as shown in FIG. 4. By means of this grid 18 the beam of electrons emitted from the source 1 can be pulse by applying voltage pulses to the grid. These voltage pulses may be pulses which are negativegoing with respect to the source 1 and which stop the emission of electrons when applied to the grid.

The object of pulsing the electron beam as described above is simply to extend the working life of the field emission type source 1. This feature and the use of lanthanum hexaboride or similar refractory material for the tip of the source, and the high. vacuum within the anode all serve to extend the working life of the source 1. The total effect of these features in improving the working life of the source is such that the vacuum condition within the anode can be relaxed slightly while still giving an acceptable working life. Thus the requirement for an ultra high vacuum can be dropped and instead a vacuum produced within the anode such that only a moderate differential pressure, roughly one order of magnitude, is produced across the hole 7 in the anode 5. a

The grid voltage may also be used to obtain a variation in the effective accelerating voltage of the electron gun. In the absence of a grid the potential to which the anode can be raised is restricted by the fact that an excessive potential would produce such a field at the tip as to cause excessive emission, leading to rapid erosion of the tip. By the insertion of the gird we can control the field immediately around the emitter so that it is constant over a wide range of anode voltages. Thus nearly constant field emission operating conditions may be maintained over a range of anode voltages of, for example, 2 to 10 kilovolts if the grid is varied over the range 3 to 2 kilovolts. The precise voltages will depend upon the geometry of the electrode arrangement. In all cases, however, the accelerating voltage is of the order of only a few kilovolts and where higher beam energies are required the electron gun could be followed by an orthodox accelerating electrode system.

In the description above, we have simply referred to the source 1 as being of the field emission type, but it should be understood that this embraces both continuous and pulsed cold field emission and continuous and mionic emitter operating in the Schottky mode. With this latter source the brightness of the probe may not be as great as with the field emission type of source but vacuum conditions would be less stringent.

What we claim is:

1. An electron beam forming system comprising an electron source, said source being a field emission tip, said tip being made of lanthanum hexaboride, an anode, said anode being of pot-like shape defining an open end and a closed end and enclosing said tip, said anode having therein a hole centrally disposed in the closed end thereof and spaced from said tip, an electromagnetic lens surrounding and enclosing said tip and anode, said lens having first and second axially spaced pole-pieces with mutually aligned respective first and second apertures therein, said anode protruding through said first aperture in said first pole piece with the hole in the closed end of said anode aligned with said second aperture in said second pole-piece, said tip being on the opposite side of said hole from said second aperture, and said lens being disposed to focus into a beam electrons emitted from said tip and passing through said hole and said last-mentioned aperture, a specimen chamber defined on the opposite side of said second aperture from said anode and tip, relatively lower vacuum pumping means connected to said specimen chamber and relatively higher vacuum pumping means connected to the interior of said anode.

2. The system set forth in claim 1 including a control electrode, said control electrode being disposed around said tip and shielding said tip from said anode.

3. The system of claim 2 including means for applying voltage pulses to said control electrode in a manner such as to intermittently stop the emission of electrons from said tip.

4. An electron beam forming system comprising an electron source, said source being a field emission tip, said tip being made of lanthanum hexaboride, an anode, said anode, being of pot-like shape defining an open end and a closed end and enclosing said tip, said anode having therein a hole centrally disposed in the closed end thereof and spaced from said tip, an electromagnetic lens surrounding and enclosing said tip and anode, said lens having first and second axially spaced pole-pieces with mutually aligned respective first and second apertures therein, said anode protruding through said first aperture in said first pole piece with the hole in the closed end of said anode aligned with said second aperture in said second pole-piece, said tip being on the opposite side of said hole from said second aperture, and said lens being disposed to focus into a beam electrons emitted from said tip and passing through said hole and said last-mentioned aperture.

5. An electron beam forming system comprising an electron source, said source being a field emission tip, an anode of pot-like shape defining a substantially air tight chamber having a closed end enclosing said tip and having a hole centrally disposed therein and spaced from said tip, a control electrode disposed around said tip and shielding said tip from said anode, an electromagnetic lens surrounding and enclosing said tip and anode, said lens having first and second axially spaced pole-pieces with mutually aligned respective first and second apertures therein, said anode protrudin throu h said first aperture in said first pole piece wfih the ole m the closed end of said anode aligned with the second aperture in said second pole piece, said tip being on the opposite side of said hole from said second aperture, and said lens being disposed to focus into a beam electrons emitted from said tip and passing through said hole and said last-mentioned aperture.

6. An electron beam forming system comprising an electron source, said source being a field emission tip, an anode of pot-like shape defining a substantially air tight chamber having a closed end enclosing said tip and having a hole centrally disposed therein and spaced from said tip, an electromagnetic lens surrounding and enclosing said tip and anode, said lens having first and second axially spaced pole-pieces with mutually aligned respective first and second apertures therein, said anode protruding through said first aperture in said first pole piece with the hole in the closed end of said anode aligned with the second aperture in said second pole piece, said tip being on the opposite side of said hole from said second aperture, and said lens being disposed to focus into a beam electrons emitted from said tip and passing through said hole and said lastmentioned aperture, structure defining a specimen chamber, said specimen chamber lying on the opposite side of at least said second aperture from said anode, relatively lower vacuum pumping means connected to said specimen chamber and relatively higher vacuum pumping means connected to the chamber defined by the interior of said anode.

Claims

1. An electron beam forming system comprising an electron source, said source being a field emission tip, said tip being made of lanthanum hexaboride, an anode, said anode being of potlike shape defining an open end and a closed end and enclosing said tip, said anode having therein a hole centrally disposed in the closed end thereof and spaced from said tip, an electromagnetic lens surrounding and enclosing said tip and anode, said lens having first and second axially spaced polepieces with mutually aligned respective first and second apertures therein, said anode protruding through said first aperture in said first pole piece with the hole in the closed end of said anode aligned with said second aperture in said second pole-piece, said tip being on the opposite side of said hole from said second aperture, and said lens being disposed to focus into a beam electrons emitted from said tip and passing through said hole and said last-mentioned aperture, a specimen chamber defined on the opposite side of said second aperture from said anodE and tip, relatively lower vacuum pumping means connected to said specimen chamber and relatively higher vacuum pumping means connected to the interior of said anode.

5. An electron beam forming system comprising an electron source, said source being a field emission tip, an anode of pot-like shape defining a substantially air tight chamber having a closed end enclosing said tip and having a hole centrally disposed therein and spaced from said tip, a control electrode disposed around said tip and shielding said tip from said anode, an electromagnetic lens surrounding and enclosing said tip and anode, said lens having first and second axially spaced pole-pieces with mutually aligned respective first and second apertures therein, said anode protruding through said first aperture in said first pole piece with the hole in the closed end of said anode aligned with the second aperture in said second pole piece, said tip being on the opposite side of said hole from said second aperture, and said lens being disposed to focus into a beam electrons emitted from said tip and passing through said hole and said last-mentioned aperture.

6. An electron beam forming system comprising an electron source, said source being a field emission tip, an anode of pot-like shape defining a substantially air tight chamber having a closed end enclosing said tip and having a hole centrally disposed therein and spaced from said tip, an electromagnetic lens surrounding and enclosing said tip and anode, said lens having first and second axially spaced pole-pieces with mutually aligned respective first and second apertures therein, said anode protruding through said first aperture in said first pole piece with the hole in the closed end of said anode aligned with the second aperture in said second pole piece, said tip being on the opposite side of said hole from said second aperture, and said lens being disposed to focus into a beam electrons emitted from said tip and passing through said hole and said last-mentioned aperture, structure defining a specimen chamber, said specimen chamber lying on the opposite side of at least said second aperture from said anode, relatively lower vacuum pumping means connected to said specimen chamber and relatively higher vacuum pumping means connected to the chamber defined by the interior of said anode.