GB2389450A

GB2389450A - Electron source

Info

Publication number: GB2389450A
Application number: GB0125961A
Authority: GB
Inventors: Stewart John Bean
Original assignee: YORK ELECTRON OPTICS Ltd
Current assignee: YORK ELECTRON OPTICS Ltd
Priority date: 2001-10-30
Filing date: 2001-10-30
Publication date: 2003-12-10
Also published as: GB0125961D0

Abstract

An electron optical column has a high brightness electron source that is able to operate with stable emission properties in both ultra high and high vacuum environments. The source comprises a cathode 1 operated such that it is subjected to potential applied between the cathode 1 and a positively biassed anode 20, which establishes a field at the cathode. The electron emitter 1 is shaped in the form of a truncated cone, with either a flat or curved emitting region 2 facing the extraction electrode 20 and subsequent optics. The emitter may comprise a single crystal of a rare-earth hexaboride, preferably lanthanum hexaboride. The cathode may be a thermionic emitter, heated by a heating assembly (see figure 1). The trajectory of emitted electrons is such that a virtual source 3 is established within the cathode 1.

Description

1 2389450

Electron source and electron column The invention relates to electron sources (also known as cathodes or emitters) for electron microscopes, cathode ray tubes, x-ray tubes' and related apparatus in which electrons from a source are accelerated to a desired final energy and then manipulated by various electron optical devices, for example lenses and deflectors, in order to be positioned upon a sample, target, or screen. There are presently two classes of designs for electron sources, as used in electron columns, as described by Reimer that provide means to introduce electrons into the subsequent optics at the correct energy and with useful combination of electron-optical properties. The first is referred to as ' field emission" and the second is referred to as 'thermionic".

The first of these two known technologies presently employed in electron optical columns, and forming an essential part of an electron microscope (1), uses field emission sources comprising nanometric scale cathodes to provide the highest

performance presently available.

Several limitations of this first known technology are revealed when high currents are required at the specimen in applications such as electron beam lithography and electron beam recording Firstly, with a very small field emission

source, and hence a small emitting area, the current density at the surface of the cathode is so high that the mutual interaction of electrons increases the energy spread in the beam. The energy spread in the beam may be as high as 2 eV under condition of high current density when high angular intensities are required. This leads to a reduction in performance of the electron optical column.

A second limitation of this first known technology is that at high total emission currents outgassing occurring at the first electrode, and other structures close to the cathode, leads to higher pressures locally than the sensitive surface chemistry of these first known cathodes can tolerate. It is known that even for the most robust of the field emission technologies, which is based upon zirconiated tungsten single

crystals, produces poor performance when the pressure in the source region rises above 5 x 10 -9 mbar.

A third limitation of this first known technology is that at low acceptance angles the virtual source size becomes dominated by diffraction effects and the benefits of the nanometric scale of the cathode are lost (2). These small angles are

nearly always in use in scanning electron microscopes when the highest resolution, and hence the low probe currents, is required.

The second known technology presently employed in electron optical columns (1), forming an essential part of an electron microscope, uses a thermionic cathode such as a tungsten hairpin or rare earth boride single crystal operating with a control electrode often referred to as the Wehnelt. The vacuum requirement in the source regions of these electron sources can be modest as these materials are stable in pressures as high as I x 10-6 mbar. In this second known arrangement the control electrode is held at a negative potential with respect to the cathode and so establishes an electrostatic field such that only those electrons released from the end of the

hairpin or the circular area on the truncated cone of the single crystal rare earth boride are able to accelerate towards the anode and hence to the subsequent optics. In this second known electron optical arrangement the emitted electrons follow trajectories that cross the electron optical axis between the control electrode and the grounded anode as they are accelerating towards the subsequent optics In this way a large real source of electrons is created that then has to be de-magnified by the subsequent optics prior to being positioned on the sample.

Several limitations of this second known technology are well understood.

Firstly, with this conventional arrangement the real source is approximately 15 um in diameter depending upon the size of the emitting area on the cathode. This large size reduces the effective brightness of the source, despite a high angular intensity, and forces the subsequent optics to be designed to be strongly de magn;.yang if a suitablj small image of the source is to be projected onto the specimen and good image resolution is to be formed. The consequence of the demagnification, associated with these known electron optical columns, is that the available current in small probe sizes is small.

A further limitation, of this second known technology, is the mutual interaction of electrons in the real source. The current passing through the real source at the first crossover is usually 30 to 50 uA, and seldom less than 1 uA, and this causes the energy spread of the beam in the direction of the optical axis to be increased This increased energy spread then increases the diameter of the probe on the specimen and this reduces the performance, usefulness, and productivity of the electron optical column. This is a particular problem if the final beam energy is approximately one thousand electron volts, or less, as the energy of the electrons as

they pass through the real source is substantially less than the final beam energy. The adverse effect of the mutual interaction of electrons will always be worse towards lower beam energies This limitation is a significant contributing reason for the poor performance of this second known technology when these columns are operated at low beam energies.

Another limitation of this second known technology is that at low beam energies, for example one thousand electron volts, the field gradient established at the

cathode by the anode is so small that space charge limitation begins to reduce the emission of the cathode. In order to attempt to reduce this limitation manufacturers of electron optical columns may introduce complex mechanical means to very the gap between the controlling electrode and the anode. Even with these mechanisms in place the performance at low beam energies is reduced.

In its broadest aspect, it is the object of the present invention to provide an electron optical column that features an electron source that provides the source brightness associated with field emission sources of the first known technology whilst

operating in the high vacuum environment typical of those source designs of the second known type.

Accordingly, this invention provides an electron optical column that includes a cathode that is heated and emits electrons and provides stable operation at pressures as high as 1 x 10 - 6 mbar. E!ectrnns from the cathode are drawn towards an Anode that is held at a positive potential with respect to the cathode As a result of this action the trajectories of the emitted electrons appear to emanate from a small virtual source within the thermionic cathode despite the macroscopic size of the emitter. The thermionic cathode is typically a single crystal of a rare earth bonde oriented along the (100) axis. The crystal may in the form of a truncated cone with a polished circular disc of diameter from 5 to 100 um or it may have a radius terminating the cone again with a radius in the range from 5 to 100 um.

Owing to the large size of the cathode, the potential applied to the first electrode, the anode, establishes a relatively low electric fields on the cathode so that

the opportunity for a discharge to develop is reduced even at pressures that are normally used to operate crystals of this nature. This pressure range can be from the ultra high vacuum range unto, at least, 1 x 10-6 mbar. A further benefit of the large

size of the cathode is that the density of electrons close to the cathode surface is reduced (for a given emission current) and so the mutual interaction of electrons in the vacuum close to the emitter surface is eliminated An energy spread of 0 8eV has been measured for a lanthanum hexaboride cathode operating in a thermionic electron source. (3) It is to be expected that lower values can be anticipated for the present invention. The angular intensities of upto 1000 uA/sr can be expected from the present invention can be deduced from the existing literature (2) . These are values that are equivalent to the known performance of some of the field emission technologies.

A preferred embodiment of the invention will now be described, in a nonlimiting manner, with reference to the accompanying drawings in Figures 1, 2, 3, and 4: Figure 1 shows a cross section through an electron optical column comprising the invention Figure 2 shows in more detail the arrangement of the cathode and anode In Figure 3 is shown an alternative source module.

In Figure 4 is shown a Further alternative source module In Figure 1 is shown an electron column conning part of an electron microscope or related equipment. The electron column comprises a thermionic cathode 1, attached to a heater assembly I I which in tern is attached to two pins 14. The pins 14 are conventionally brazed or otherwise attached to the insulating member 15 The pins are used to make electrical contact to the external circuit, not shown, that provides the heating current. This cathode assembly 1,11,14,15, is held in an emitter carrier 13.

The emitter carrier 13 provides mechanical support and protection for the cathode.

The emitter carrier also ensures cylindrical symmetry of the equipotentials about the electron optical axis to avoid astigmatism being introduced into the source image. A small voltage may be applied to the emitter carrier if required.

The electron optical axis 50 of the column is also shown. The electron optical column is held in vacuum enclosures and vacuum pumps, not shown, and is connected to appropriate electrical supplies, which for clarity, are also not shown. The mechanical means necessary to hold the electron optical elements in place is also not shown for the purposes of clarity. The cathode is held at a negative potential chosen to

s impart the required energy when the electrons interact with the specimen The specimen 40 is conventionally, but not necessarily, held at ground potential.

The subsequent optics is adapted to receive electrons from the cathode 1 and anode 20 and manipulate the beam in a manner appropriate to the intended application of the electron optical column. The subsequent optics 30 will therefore vary across different applications, and these variations do not form part of the present invention, but would typically comprise a single lens or a series of lenses and associated beam steering means, beam defining means, astigmatism correction means, and beam deflection means. These electron optical means may be either electrostatic or magnetic or a combination of the two. The electron- optical axis may deviate Tom linear in order to meet a requirement of the overall column eg a monochromator but is shown as linear in Figure 1 for convenience One such example is revealed in US patent number 6,239,430.

In Figure 2 is shown in more detailed view of the cathode and anode section in order to appreciate the electron-optical arrangement according to the present invention. The cathode I is heated, by conventional means not shown, to a temperature sufficient to release electrons from its surface. The cathode is held in a vacuum enclosure equipped with vacuum pumps sufficient to achieve a pressure of 5 x 10 - 6 mbar or lower. The anode 20 is held at a positive potential relative to the cathode 1 so that lines of equal potential are established between the cathode and the anode Two such equi-ptentials 5a and Sb are shown to illustrate the electron optical situation. Electrons emitted from the conical surfaces 6 are attracted towards the anode as shown by the example trajectory 7 and the anode 20 captures these electrons. Electrons emitted from the face 2 on the cathode and facing the optical axis 50 are also attracted to the anode and some will pass through the anode as shown by trajectories 4a and 4b. In Figure 2 the form 2 of the cathode 1, in the region of the optical axis, is shown as a polished flat circular area orthogonal to the optical axis but a spherical form could also be employed It is within the scope of the invention that the shape of area 2 may be in the forth of a polygon when viewed from the electron optical axis 50.

For those electrons emitted from the face 2 on the cathode, a virtual source 3 is formed within the cathode It is this virtual source that forms the object for the subsequent optics

In Figure 3 is shown an alternative source module in which the emitter carrier 60 has flange with a least one mounting face 61 and said flange 60 may contain features that allow different mounting or locating means to be used appropriate to the application. In Figure 4 is shown a further alternative source module in which the emitter carrier 70, with its reference mounting face 71, is now fitted with an insulating item 76 which can be fabricated from a number of materials such as zirconia or alumina conventionally used to provide electrical isolation in vacuum applications. In turn this insulating item 76 is attached to an anode 73 with a reference face 72.

Further advantages of the invention will be obvious from the foregoing.

References l Reimer, L. (1985) Scanning Electron Microscopy, Springer Series in Optical Sciences, Vol 45. ISBN number 3-540-13530-8 2 Tuggle, D. W. and Swanson, L W. J Vac Sci. Technol. B. Vol 3, No 1, Jan/Feb 1985 3 Kimball Physic Inc. Technical Bulletin # 1,aB6 - 05B, 1991

Claims

- 7 - CLAIMS

1. An electron source assembly comprising a cathode that has stable electron emission properties at pressures above 5 x 1()- mbar and up to 10-6 mbar. and an electrode that is dispersed between the cathode and subsequent optics and is positively biased NVitll respect to the cathocle.

2. An electrode source as.schly according to claim 1, wherein the cathode comprises a rare eartl, 1lexahoride.

3. An electrode source assenhly according to claim 2, wherein the cathode comprises lanthauin lexaboride.

4. An electrode source assembly according to claim 1 2 or 3, wherein the cathode is formed as an axially symmetric truncated cone.

5. An electrode source assembly according to claim 4, wherein the diameter of the circle truncating the cone is in the range 5 up to 100 um.

6. An electrode source assembly according to claim 1, 2 or 3, wherein 5 talc cathode is fondled ax a conical shale terminated by a curved surface.

7. An electrode source assembly according to claim 6, wherein the radius of said curved suriacc is in tle range 5 urn to 10() um.

8. In electrode source assembly according to any of the prcccding claims, wherein the cathode is surrounded by a member having an aperture :0 through which the cathode protrudes.

9. An electrode source assembly according to any D1 the prcccding claims. including Koreans tar locating, the callodc

10. An clectrcde source assembly substantially as hereinbclorc descriLcd w ith refctcnce to l igures I and 2 ol the at coman>irg drawings.

5

11. An electrotic source asseshly substaiially as hereinbetorc descriLcd with reference lo Figures I and 2' as modified by Figure 3 or 4, of the accompanying cira\vings.