WO2009141655A2

WO2009141655A2 - Improved particle beam generator

Info

Publication number: WO2009141655A2
Application number: PCT/GB2009/050547
Authority: WO
Inventors: Derek Eastham
Original assignee: Nfab Limited
Priority date: 2008-05-22
Filing date: 2009-05-20
Publication date: 2009-11-26
Also published as: GB0809331D0; WO2009141655A3

Abstract

Embodiments of the invention provide a particle beam generator apparatus comprising: a source member; a first electrode; and a second electrode, the generator being operable to extract particles from the source member by means of a potential difference applied between the source member and the first electrode, the first electrode being arranged to focus the particles toward a focal point a distance F1 from the first electrode, the second electrode being arranged to focus the particles to a point a distance F1' from the first electrode, distance F1' being greater than distance F1 thereby to increase an effective focal length of the first electrode.

Description

IMPROVED PARTICLE BEAM GENERATOR

In an earlier patent application (PCT/GB2008/050050) the present applicants described a new type of sub-miniature scanning electron microscope which worked by directly imaging the field emission site of a nanotip.

All the aberrations in the system could be neglected if two conditions were met. Firstly, the microscope size and, importantly, the focal length need to be below about 100 microns. Secondly, the beam diameter in the instrument should be only a small fraction of the aperture diameter, since it is known that the aberrations in an electron microscope become small if the ratio of the beam diameter to the aperture diameter is small. This ratio may be referred to as the fill factor.

If the above two conditions are met then it is possible to design a microscope having a beam spot that is at least the same size as the emission site at low electron beam energies (around 500 eV). Under certain conditions it is also possible to produce a beam spot with a smaller area (or diameter) than the image site, for example by increasing the accelerating potential to achieve a beam energy of a sufficiently high value.

Reference to the size of the beam spot is intended to mean reference to the size of the cross-section of the beam at the focal point of the beam. This is typically measured in terms of the diameter of the beam at this location, and may be referred to as the 'beam diameter' at the focal point, or 'focussed beam spot diameter'. It is to be understood that the beam cross-section may not necessary be circular.

Since the microscope works by directly imaging the field emission site of the nanotip, the performance of the microscope, which may be measured by the diameter of the focussed beam spot, depends on the size of the field emission sites on the nanotip. Over the past 10 years it has become possible to make atomic sized emitters on the end of a nanotip. Thus, the size of the image of the field emission site and hence the resolution of the microscope may be made to be of atomic dimensions.

FIG. 1 shows a known nanotip 10 having a tip radius R of around 5nm. Other larger tip radii are also useful particularly when an atomically sharp nanotip is to be provided. Smaller tip radii are also useful in some embodiments. In some embodiments the nanotip 10 is formed from single crystal tungsten. Other materials are also useful.

A stable structure in the form of a tetrahedron of gold atoms 12 is formed on the nanotip as shown in FIG. 1 (b). The tetrahedron has a single atom at an apex located away from a main body portion 1 1 of the nanotip 10. This atom provides an electron source when an electric field of sufficiently high magnitude is generated between the nanotip 10 and an external electrode. The angle of emission of the electrons is small and can be as low as 6 degrees ensuring that an electron beam generated by the microscope has a relatively small cross-sectional area. This allows a relatively low value of the fill factor to be obtained.

This application is concerned with improvements in the optical configuration of the microscope. In particular, the application is concerned with improvements in the way in which particles such as electrons are accelerated and focussed.

In a first aspect of the invention there is provided particle beam generator apparatus comprising: a source member; a first electrode; and a second electrode, the generator being operable to extract particles from the source member by means of a potential difference applied between the source member and the first electrode, the first electrode being arranged to focus the particles toward a focal point a distance F1 from the first electrode, the second electrode being arranged to focus the particles to a point a distance F1 ' from the first electrode, distance F1 ' being greater than distance F1 thereby to increase an effective focal length of the first electrode.

Embodiments of the invention have the advantage that particles may be both focussed by the first electrode and accelerated in the region between the first and second electrodes. In some embodiments only one insulator is required in the region between the first and second electrodes thereby simplifying a manufacturing process of the generator. In some embodiments, no insulator is required in the region between the first and second electrodes. Preferably the first and second electrodes are provided in the form of plate members, each plate member being provided with an aperture therethrough, the apparatus being arranged whereby particles extracted from the source member pass through the apertures in the plate members. The electrodes may be other than plate members, such as ring members or any other kind of member.

In some embodiments the apparatus is arranged such that a diameter of a particle beam extracted from the source member and focussed by the first and second electrodes is less than a diameter of the apertures formed in the first and second electrodes.

This has the advantage that an aberration of the particle beam may be reduced. In some embodiments a chromatic aberration of the particle beam may be reduced. Alternatively or in addition in some embodiments a spherical aberration of the particle beam may be reduced.

It is to be understood that embodiments of the invention provide a simplified particle beam generator requiring only two electrode lenses compared with embodiments of a particle beam generator disclosed in PCT/GB2008/050050 by the present applicants, which have four electrode lenses.

Embodiments of particle beam generator apparatus disclosed in PCT/GB2008/050050 and in Journal of Applied Physics 105 147029 (2009) are arranged to image directly an emission site of a source of particles such as electrons and have little or no aberrations. This is believed to be at least in part because the particle beam has a diameter less than that of the aperture through which the beam passes, such that the effects of electron diffraction may be ignored.

Embodiments of the present invention also have little or no aberrations and this is also believed to be at least in part because the particle beam has a diameter less than that of the aperture through which the beam passes, such that the effects of electron diffraction may be ignored.

It is to be understood that embodiments of the present invention in the form of scanning electron microscopes may be fabricated more cheaply than convention large scale scanning electron microscopes employing magnetic lenses and the like. The diameter of the particle beam extracted from the source member and focussed by the first and second electrodes may be substantially equal to or less than one half of the diameter of the apertures formed in the first and second electrodes.

The diameter of the particle beam extracted from the source member and focussed by the first and second electrodes may be substantially equal to or less than one tenth of the diameter of the apertures formed in the first and second electrodes.

Preferably the source member is provided with a free end having at least one atomically sharp element provided thereon.

The source member may comprise a primary tip element and a secondary tip element, the secondary tip element being supported by the primary tip element, the secondary tip element providing said atomically sharp point and being arranged in use to emit particles.

Preferably the secondary tip element comprises a substantially tetrahedral arrangement of atoms.

The secondary tip element may comprise one selected from amongst gold, platinum and iridium or a mixture thereof.

The particles may comprise at least one selected from amongst electrons, ions and protons.

The particles may consist essentially of at least one selected from amongst electrons, ions and protons.

Four or more electrodes may be provided, respective electrodes being arranged whereby alternate electrodes provide one of either a focusing effect or a defocusing effect and the other electrodes provide the other of either the focusing effect or defocusing effect.

In a second aspect of the invention there is provided a method of generating a beam of particles comprising: extracting particles from a source member by means of a potential difference applied between the source member and a first electrode; focussing the particles by means of the first electrode toward a focal point a distance F1 from the first electrode; focussing the particles by means of a second electrode spaced apart from the first electrode downstream of a flux of the particles to a point a distance F1 ' from the first electrode, distance F1 ' being greater than distance F1 thereby to increase an effective focal length of the first electrode.

Preferably the method further comprises the step of applying respective potentials to the source member, first electrode and second electrode.

Preferably the first and second electrodes are each provided with an aperture therethrough.

Preferably the method further comprises the step of extracting particles from the source member and passing the particles through the apertures in the first and second electrodes.

Preferably a diameter of a particle beam extracted from the source member and passing through the apertures in the first and second electrodes is less than a diameter of the apertures.

Preferably the diameter of the particle beam extracted from the source member and passing through the first and second electrodes is substantially equal to or less than one half of the diameter of the apertures formed in the first and second electrodes.

The diameter of the particle beam extracted from the source member and passing through the first and second electrodes may be substantially equal to or less than one tenth of the diameter of the apertures formed in the first and second electrodes.

The method may comprise providing at least four electrodes and applying respective potentials to the electrodes thereby to cause alternately a focusing and defocusing effect on an electron beam propagating through the respective electrodes. For a better understanding of the present invention, and to show how it may be carried into effect, reference will now be made by way of example to the accompanying drawings, in which:

FIGURE 1 is a schematic diagram of (a) a primary tip element and (b) a primary tip element having a secondary tip element thereon in the form of a tetrahedral arrangement of gold atoms;

FIGURE 2 is a schematic illustration of an optical arrangement of an electron beam generator according to a previous patent application by the present applicants;

FIGURE 3 is a schematic illustration of an electron optical arrangement according to an embodiment of the present invention;

FIGURE 4 shows (a) an embodiment of the invention having a pair of lenses in the form of perforated electrodes and (b), (c) the structure of first electrodes according to further embodiments of the invention; and

FIGURE 5 shows a computer simulation of electron trajectories in one embodiment of the invention.

Apparatus according to an electron microscope disclosed in a previous application by the present applicants is shown in FIG. 2. In this apparatus electrons 3 emitted by a nanotip 1 having an energy corresponding to the potential difference between the nanotip and a first electrode 2 having an aperture provided therein were first focussed by the first electrode 2 (also referred to as an entrance lens) and subsequently accelerated in an accelerator section labelled Ace. The accelerator section Ace is provided with an accelerator electrode 4. A potential difference is established between the accelerator electrode 4 and the first electrode 2 to cause electrons emitted by the nanotip 1 to increase in energy as they pass through the accelerator section Ace. An einzel lens 6 downstream from the accelerator section Ace was used to focus the electrons to a focal point 5 of the einzel lens 6 thereby forming a 'beam spot' on any object located at the focal point 5.

FIG. 3 shows an optical arrangement of an electron microscope according to an embodiment of the present invention in which electrons emitted by a nanotip 10 are drawn towards an entrance lens 20 and focussed by the entrance lens 20. The entrance lens 20 is provided by a first electrode 22 (FIG. 4) in the form of a perforated conducting plate.

The electrons are then accelerated towards an exit lens 40, which is provided by a second electrode 42 (FIG. 3). The exit lens 40 acts as a concave lens whereas the first lens 20 acts as convex lens. That is, the entrance lens 20 (or 'first lens') causes the electrons to converge more strongly towards a notional point being the focal point of the first lens 20 whilst the exit lens 40 causes a divergence of a path of the electrons whereby the electrons converge less strongly.

It is noted that the lens characteristics are typically not adjustable independently - the equipotentials are almost the same in both but the second lens is weaker because the energy of electrons passing through the exit lens 40 is higher than that of electrons passing through the entrance lens 20.

The exit lens 40 (or 'second lens') also acts in an opposite direction to the first in that a trajectory of electrons passing through the second lens is adjusted whereby an angle between the electron trajectory and optic axis of the arrangement is decreased. In other words rays are bent outwardly by the second lens but inwardly by the entrance lens such that an effective focal length of the two lenses is greater than the actual focal length of the entrance lens 20 in the absence of the exit lens 40.

As shown in FIG. 3 the effective focal point of the entrance lens 20 is a point 50 a distance F from the entrance lens 20.

It is to be understood that the effectiveness of the second electrode 42 as a lens is reduced for a given applied potential due to the increased energy of the electrons following their acceleration in the region between the first and second electrodes 22, 42.

A focal point of the electron beam is formed beyond the second electrode 42 at a position indicated at 50 in FIG. 3 and FIG. 4.

The configuration of the optical arrangement of FIG. 3 and FIG. 4 allows a more simple electron microscope structure to be formed. In some embodiments a structure having only one insulator may be formed, the insulator being sandwiched between the first and second electrodes. In some embodiments no material is provided between the first and second electrodes. The gap may be evacuated. Alternatively the gap may be gas-filled, for example air-filled.

In some alternative embodiments the first and second electrodes are supported by support members that are electrically isolated from one another. In some embodiments the support members are individually movable with respect to one another thereby to allow precise alignment of the electrodes. One or both of the support members may comprise one or more nanopositioning elements.

In one embodiment the nanotip 10 is positioned around 100nm from the first electrode 22. In some embodiments a different distance may be used. In some embodiments this distance is of the order of 1 to 10 microns or more.

The first electrode is formed from a conducting plate element around 1 μm in thickness and has an aperture of diameter d2. In some embodiments d2 is around 0.5 μm but can be up to 5 μm. A potential V_p is applied to the first electrode, V_p being typically around - 1000 V. In some embodiments a potential V, of around -1020 V is applied to the nanotip 10 with the nanotip 10 arranged to be around 100nm from the first electrode.

However the values of the potentials applied to the first electrode 22 and the nanotip 10 may be varied thereby to vary the energy of the electron beam incident on a specimen exposed to the electron beam.

The energy of the electron beam can also be varied by adjusting the position of the nanotip 10 and the voltages on both the nanotip 10 and the first electrode 22. The first electrode 22 may also be referred to as an extraction electrode 22.

The second electrode 42 is also in the form of a conducting plate element and has a thickness of around 1 μm. It is typically positioned around 6μm from the first electrode

22. In some embodiments the second electrode is at earth potential and has an aperture formed therein of the same diameter as an aperture formed in the first electrode. The apertures formed in the first and second electrodes are arranged to be formed in a central portion of each electrode. In some embodiments the first and second electrodes are substantially circular in shape. In some embodiments the size of the aperture formed in the second electrode is different to that of the aperture formed in the first electrode. Changing the size of this aperture allows a strength of an effect of the electrode on the electron beam to be changed.

An electron beam formed by extraction of electrons from the nanotip 10 is focussed by the entrance lens and accelerated in the region between the first and second electrodes 22, 42 following a trajectory illustrated in FIG. 3.

It should be noted that the dimensions provided above are only a guide since other combinations of dimensions will produce substantially the same effect as long as the overall geometry of the arrangement is preserved, and the focal length d3 is less than about 50 μm.

It is important to stress that in some embodiments the field at the tip may be adjusted by both changing the tip voltage (the potential difference between the first electrode and the tip, V_p-V,) and the distance between the nanotip 10 and the first electrode 22 so that the field emission current does not exceed 1 nA. This is because in some cases the tip may be damaged if a current in excess of this value is drawn from the nanotip 10.

As the focal length of the microscope is increased it may be necessary in some embodiments to correct for aberrations. This can be achieved by altering the profile of the aperture as shown in the examples of FIG. 4(b), and (c). If an aperture having a profile according to that shown in FIG. 4(b) or FIG. 4(c) is employed it will be possible to correct for spherical aberrations. Such aberrations are the principle cause of an increase in the final beam spot size in all scanning electron microscopes (SEMs).

FIG. 5 shows a computer simulation of electron trajectories for an electron microscope optical arrangement where the nanotip 10 (which is provided at a potential of around -1020 volts) is positioned around 100 nm from the first electrode 22 (also referred to as an extractor plate) which is provided at a potential of around -1000 volts. The dimensions of the elements of the arrangement are substantially as stated above.

The simulation uses a point source with a half angle of emission of around 6°. The beam is focussed to a spot having a diameter of around 1 .5 A at a distance of around 10 μm beyond the second aperture 42. In other words, the beam spot diameter including the effects of aberrations is 1 .5 A. An atomic emitter would give a spot size of V2 x 1 .5 A.

Since the depth of field is large, chromatic aberrations are negligible and aberrations and other effects due to the use of non circular apertures are also small. It should be noted that in the designs discussed above the fill factor is small provided an atomic emitter is used.

In some embodiments of the invention a series of stages of substantially planar, substantially parallel electrodes are provided in a stacked arrangement such that a higher energy electron beam can be obtained. The electrodes are arranged whereby an electron beam path is provided through the series of stages along a direction substantially perpendicular to a plane of each electrode. By adjusting a gradient of the electric field between one electrode and an adjacent electrode such that the gradient between one electrode and an adjacent electrode is higher than that between the one electrode and another adjacent electrode on an opposite side of the one electrode an alternately focusing/defocusing effect can be obtained in respect of an electron beam passing through the stacked arrangement. The alternately focusing/defocusing effect may be obtained without increasing an overall diameter of the electron beam.

By suitable choice of electric potential applied to respective electrodes of a stack a demagnified image of the source of electrons can be obtained.

In one embodiment of the invention, a first potential is applied to the source of electrons being a nanotip, and second, third and fourth potentials are applied to a series of three electrodes placed in succession adjacent the source. In one embodiment, the first potential is -2020V the second potential is -2000V, the third potential is -1200V and the fourth potential is OV. Other potentials are also useful. The final electrode downstream from the source is provided at ground (earth) potential.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

CLAIMS:

1. Particle beam generator apparatus comprising: a source member; a first electrode; and a second electrode, the generator being operable to extract particles from the source member by means of a potential difference applied between the source member and the first electrode, the first electrode being arranged to focus the particles toward a focal point a distance F1 from the first electrode, the second electrode being arranged to focus the particles to a point a distance F1 ' from the first electrode, distance F1 ' being greater than distance F1 thereby to increase an effective focal length of the first electrode.

2. Apparatus as claimed in claim 1 arranged to apply respective potentials to the source member, first electrode and second electrode.

3. Apparatus as claimed in claim 1 or claim 2 wherein the first and second electrodes are each provided with an aperture therethrough, the apparatus being arranged whereby particles extracted from the source member pass through the apertures in the first and second electrodes.

4. Apparatus as claimed in claim 2 wherein a diameter of a particle beam extracted from the source member and focussed by the first and second electrodes is less than a diameter of the apertures formed in the first and second electrodes.

5. Apparatus as claimed in claim 4 wherein the diameter of the particle beam extracted from the source member and focussed by the first and second electrodes is substantially equal to or less than one half of the diameter of the apertures formed in the first and second electrodes.

6. Apparatus as claimed in claim 4 or claim 5 wherein the diameter of the particle beam extracted from the source member and focussed by the first and second electrodes is substantially equal to or less than one tenth of the diameter of the apertures formed in the first and second electrodes.

7. Apparatus as claimed in any preceding claim wherein one or both of the first and second electrodes are provided in the form of one selected from amongst a plate member, a ring member and an annular ring member.

8. Apparatus as claimed in any preceding claim wherein the source member is provided with a free end having at least one atomically sharp element provided thereon, the apparatus being arranged to extract particles from the atomically sharp element.

9. Apparatus as claimed in any preceding claim wherein the source member comprises a primary tip element and a secondary tip element, the secondary tip element being supported by the primary tip element, the secondary tip element providing said atomically sharp point and being arranged in use to emit particles.

10. Apparatus as claimed in claim 9 wherein the secondary tip element comprises a substantially tetrahedral arrangement of atoms.

1 1 . Apparatus as claimed in claim 9 or claim 10 wherein the secondary tip element comprises a noble metal.

12. Apparatus as claimed in any one of claims 9 to 1 1 wherein the secondary tip element comprises one selected from amongst gold, platinum and iridium or a mixture thereof.

13. Apparatus as claimed in any preceding claim wherein the particles comprise at least one selected from amongst electrons, ions and protons.

14. Apparatus as claimed in any preceding claim wherein the particles consist essentially of at least one selected from amongst electrons, ions and protons.

15. Apparatus as claimed in any preceding claim wherein four or more electrodes are provided, respective electrodes being arranged whereby alternate electrodes provide one of either a focusing effect or a defocusing effect and the other electrodes provide the other of either the focusing effect or defocusing effect.

16. A method of generating a beam of particles comprising: extracting particles from a source member by means of a potential difference applied between the source member and a first electrode; focussing the particles by means of the first electrode toward a focal point a distance F1 from the first electrode; focussing the particles by means of a second electrode spaced apart from the first electrode downstream of a flux of the particles to a point a distance F1 ' from the first electrode, distance F1 ' being greater than distance F1 thereby to increase an effective focal length of the first electrode.

17. A method as claimed in claim 16 comprising the step of applying respective potentials to the source member, first electrode and second electrode.

18. A method as claimed in claim 16 or claim 17 wherein the first and second electrodes are each provided with an aperture therethrough.

19. A method as claimed in claim 18 comprising the step of extracting particles from the source member and passing the particles through the apertures in the first and second electrodes.

20. A method as claimed in 19 whereby a diameter of a particle beam extracted from the source member and passing through the apertures in the first and second electrodes is less than a diameter of the apertures.

21 . A method as claimed in claim 20 whereby the diameter of the particle beam extracted from the source member and passing through the first and second electrodes is substantially equal to or less than one half of the diameter of the apertures formed in the first and second electrodes.

22. A method as claimed in claim 20 or 21 whereby the diameter of the particle beam extracted from the source member and passing through the first and second electrodes is substantially equal to or less than one tenth of the diameter of the apertures formed in the first and second electrodes.

23. A method as claimed in any one of claims 16 to 22 comprising providing at least four electrodes and applying respective potentials to the electrodes thereby to cause alternately a focusing and defocusing effect on an electron beam propagating through the respective electrodes.

24. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.

25. A method substantially as hereinbefore described with reference to the accompanying drawings.