ANALYSER ARRANGEMENT FOR PARTICLE SPECTROMETER

Abstract

The present invention relates to a method for determining at least one parameter related to charged particles emitted from a particle emitting sample, e.g. a parameter related to the energies, the start directions, the start positions or the spin of the particles. The method comprises the steps of guiding a beam of charged particles into an entrance of a measurement region by means of a lens system, and detecting positions of the particles indicative of said at least one parameter within the measurement region. Furthermore, the method comprises the steps of deflecting the particle beam at least twice in the same coordinate direction before entrance of the particle beam into the measurement region. Thereby, both the position and the direction of the particle beam at the entrance of the measurement region can be controlled in a way that to some extent eliminates the need for physical manipulation of the sample. This in turn allows the sample to be efficiently cooled such that the energy resolution in energy measurements can be improved.

Claims

1. A method for determining at least one parameter related to charged particles emitted from a particle emitting sample, comprising the steps of: forming a particle beam of said charged particles and transporting the particles between said particle emitting sample and an entrance of a measurement region by a lens system having a substantially straight optical axis; deflecting the particle beam in at least a first coordinate direction perpendicular to the optical axis of the lens system before entrance of the particle beam into the measurement region; deflecting the particle beam in the same at least first coordinate direction at least a second time before entrance of the particle beam into the measurement region; and detecting the positions of said charged particles in said measurement region, the positions being indicative of said at least one parameter.

2. The method according to claim 1, wherein the first deflection of the particle beam is effectuated by a first deflector, and the at least second deflection of the particle beam is effectuated by at least a second deflector arranged downstream of the first deflector at a distance therefrom along the optical axis of the lens system.

3. The method according to claim 1, wherein the particle beam is deflected at least twice also in a second coordinate direction perpendicular to the first coordinate direction and the optical axis of the lens system before entrance of the particle beam into the measurement region.

4. The method according to claim 1, wherein all deflections of the particle beam takes place within the lens system, meaning that at least one lens acts on the particles before the first deflection of the particle beam and at least one lens acts on the particles after the last deflection of the particle beam.

5. The method according to claim 1, wherein at least one deflection of the particle beam is effectuated by a deflector package comprising four electrodes arranged in a formation of essentially quadrupolar symmetry wherein the four electrodes form two electrode pairs serving as deflectors in a respective coordinate direction, further comprising the steps of: applying a first deflector voltage between one of the two electrode pairs of the deflector package; applying a second deflector voltage between the other electrode pair of the deflector package, and applying voltages of quadrupolar symmetry to the electrodes of the deflector package, superposed on said deflection voltages.

6. The method according to claim 1, further comprising the step of controlling the deflections of the particle beam such that a predetermined part of the angular distribution of the particles forming the particle beam passes the entrance of the measurement region.

7. The method according to claim 6, further comprising the step of controlling the deflections of the particle beam such that said predetermined part of the angular distribution of the particles passes the entrance of the measurement region in a direction being substantially parallel to the optical axis of the lens system.

8. The method according to claim 1, wherein said at least one parameter relates to at least one of: the energies of the charged particles; the start directions of the charged particles; the start positions of the charged particles, and the spin of the charged particles.

9. The method according to claim 1, wherein the step of detecting the positions of the charged particles involves detection of the positions in two dimensions, one of which is indicative of the energies of the particles and one of which is indicative of the start directions or the start positions of the particles.

10. An analyser arrangement for determining at least one parameter related to charged particles emitted from a particle emitting sample, comprising: a measurement region having an entrance allowing said particles to enter the measurement region; a lens system for forming a particle beam of said charged particles and transporting the particles between said particle emitting sample and said entrance of the measurement region, said lens system having a substantially straight optical axis; a deflector arrangement comprising a first deflector for deflecting the particle beam in at least a first coordinate direction perpendicular to the optical axis of the lens system before entrance of the particle beam into the measurement region, and a detector arrangement for detecting the positions of the charged particles in the measurement region, said positions being indicative of said at least one parameter, the deflector arrangement further comprising at least a second deflector operable to cause deflection of the particle beam in the same at least first coordinate direction (x, y) at least a second time before entrance of the particle beam into the measurement region.

11. The analyser arrangement according to claim 10, wherein the second deflector is arranged downstream the first deflector at a distance therefrom along the optical axis of the lens system.

12. The analyser arrangement according to claim 10, wherein the deflector arrangement is operable to cause deflection of the particle beam at least twice also in a second coordinate direction perpendicular to the first coordinate direction and the optical axis of the lens system before entrance of the particle beam into the measurement region.

13. The analyser arrangement according to claim 12, wherein the deflector arrangement comprises at least one deflector package comprising four electrodes arranged in a formation of essentially quadrupolar symmetry wherein the four electrodes of the deflector package form two electrode pairs serving as deflectors in a respective coordinate direction of said first and second coordinate directions.

14. The analyser arrangement according to claim 13, further comprising a control unit configured to apply individual voltages to each of the electrodes.

15. The analyser arrangement according to claim 10, wherein the deflector arrangement and the lens system are arranged such that at least one lens element of the lens system is positioned upstream of all deflectors of the deflector arrangement and at least one other lens element of the lens system is positioned downstream of all deflectors of the deflector arrangement.

16. The analyser arrangement according to claim 10, wherein all deflectors of the deflector arrangement are arranged within the same lens element of the lens arrangement.

17. The analyser arrangement according to claim 10, wherein the deflector arrangement form an integral part of the lens system.

18. The analyser arrangement according to claim 10, further comprising a control unit operable to cause the deflector arrangement to deflect the particle beam such that a predetermined part of the angular distribution of the particles forming the particle beam passes the entrance of the measurement region.

19. The analyser arrangement according to claim 18, wherein the control unit is operable to cause the deflector arrangement to deflect the particle beam such that said predetermined part of the angular distribution of the particles passes the entrance of the measurement region in a direction being substantially parallel to the optical axis of the lens system.

20. The analyser arrangement according to claim 10, wherein the detector arrangement is configured to determine the positions of the charged particles in two dimensions, one of which is indicative of the energies of the particles and one of which is indicative of the start directions or the start positions of the particles.

21. A particle spectrometer for analysing a particle emitting sample, wherein the particle spectrometer comprises an analyser arrangement according to claim 10.

22. The particle spectrometer according to claim 21, wherein the particle spectrometer comprises a photo-electron spectrometer of hemispherical deflector type.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The present invention will become more fully understood from the detailed description provided hereinafter and the accompanying drawings which are given by way of illustration only. In the different drawings, same reference numerals correspond to the same element.

[0051] FIG. 1 illustrate a photo-electron spectrometer of hemispherical deflector type according to prior art.

[0052] FIG. 2 illustrates particle trajectories through the lens system of the photo-electron spectrometer shown in FIG. 1.

[0053] FIG. 3 illustrates an aperture slit and an entrance slit of the measurement region of the photo-electron spectrometer shown in FIG. 1.

[0054] FIG. 4 illustrates a photo-electron spectrometer of hemispherical deflector type according to an exemplary embodiment of the invention.

[0055] FIGS. 5A and 5B are end views of two deflector packages of an analyser arrangement according to an exemplary embodiment of the invention.

[0056] FIG. 6 illustrates parts of an analyser arrangement according to an exemplary embodiment of the invention.

[0057] FIG. 7 illustrates an exemplary way in which deflector voltages can be applied to electrodes of the deflector packages shown in FIGS. 5A and 5B.

[0058] FIGS. 8A and 8B illustrate particle trajectories through the lens system of an analyser arrangement according to the invention, without and with applied deflector potentials.

[0059] FIGS. 9A to 9C illustrate how selected parts of the angular distribution of emitted particles can be deflected in accordance with the principles of the invention.

DETAILED DESCRIPTION

[0060] FIG. 4 illustrates a particle spectrometer 30 according to an exemplary embodiment of the invention. Besides the differences described hereinafter, the components and functionality of the particle spectrometer 30 are identical to the components and functionality of the photo-electron spectrometer 1 of hemispherical deflector type according to prior art, described in the background portion with reference to FIGS. 1 to 3. Elements shown in FIG. 4 that correspond to elements in FIGS. 1 to 3 are provided with the same reference numerals and further descriptions thereof are omitted.

[0061] The particle spectrometer 30 is hence a photo-electron spectrometer of hemispherical deflector type comprising an analyser arrangement adapted for analysis of energies and start directions or start positions of charged particles emitted from a particle emitting sample 11.

[0062] As seen in FIG. 4, the analyser arrangement includes a deflector arrangement 31 comprising a first deflector package 29 and a second deflector package 29. Each of the first and the second deflector packages is devised and configured in accordance with the single deflector package 29 of FIG. 1, described in the background portion.

[0063] With simultaneous reference to FIGS. 5A and 5B illustrating end views of the first deflector package 29 and the second deflector package 29, respectively, this means that each of the first and the second deflector package comprises four electrodes 33A-33D, 33A-33D, each of which covers an azimuthal angle close to 90 deg. The two oppositely arranged electrodes in each deflector package form an electrode pair 33A/33C, 33B/33D, 33A/33C, 33B/33D operable to generate an electrical field between them by application of a deflector voltage, V.sub.x, V.sub.y, and so operable to deflect the charged particles passing between the electrodes of the deflector package in one coordinate direction. Each such electrode pair hence forms a deflector for deflecting the charged particles in one coordinate direction.

[0064] Given a three dimensional Cartesian coordinate system with its z-axis along the optical axis 15 of the lens system 13, and with the hemispheres 5 symmetrical with respect to the (y, z) plane, one electrode pair 33A/33C, 33A/33C of each deflector package 29, 29 is arranged to deflect the charged particles in the x-direction, and the other electrode pair 33B/33D, 33B/33D of each deflector package 29, 29 is arranged to deflect the charged particles in the y-direction. An electrode pair arranged to deflect the charged particles in the x-direction will hereinafter sometimes be referred to as an x-deflector, and an electrode pair arranged to deflect the charged particles in the y-direction will hereinafter sometimes be referred to as the y-deflector.

[0065] As illustrated in FIG. 6, showing a more detailed view of parts of the analyser arrangement, the deflector voltages applied to the electrodes 33A-33D, 33A-33D of the deflector packages 29, 29 are controlled by a control unit 35. The same control unit 35 may also be configured to control the lens voltages applied to a plurality of concentric electrodes constituting the lenses L1-L3 of the lens system 13.

[0066] The sign and the magnitude of the deflector voltage, V.sub.x, V.sub.y, applied between each electrode pair 33A/33C, 33B/33D, 33A/33C, 33B/33D of the deflector arrangement 31 may be independently controlled by the control unit 35. As illustrated in FIGS. 5A and 5B, the deflector voltage of the x-deflector 33A/33C in the first deflector package 29 is denoted V.sub.x1, and the deflector voltage of the x-deflector 33A/33C in the second deflector package 29 is denoted V.sub.x2. Likewise, the deflector voltages of the y-deflectors 33B/33D, 33B/33D in the first and second deflector packages are denoted V.sub.y1 and V.sub.y2, respectively.

[0067] Thus, the deflector electrodes 33A-33D, 33A-33D in each deflector package 29, 29 are so arranged that a voltage, V.sub.x, applied between one pair of opposite electrodes 33A/33C, 33A/33C provides deflection only in the x-direction and a voltage, V.sub.y, between the orthogonal pair 33B/33D, 33B/33D provides deflection in the y-direction only. Then, any required deflection (x, y) can be achieved through the application of a combination of voltages for deflection in the x- and y-directions. By a proper combination of the voltages applied to the deflector electrodes, it is then for any combination of entrance angles of the charged particles into the lens system 13 possible to achieve simultaneously that the exit direction from the deflector region, i.e. the direction of the charged particles when having passed the last deflector of the deflector arrangement 31, is parallel to the lens axis 15, and that the exit occurs along this axis. This means that the trajectory for this particular direction is substantially unchanged by the part of the lens system 13 that is situated after the deflector arrangement 31 (i.e. downstream the deflector arrangement from the particles' point of view).

[0068] As illustrated in FIG. 7, voltages V.sub.q of quadrupolar symmetry can be superposed on the deflector voltages V.sub.x, V.sub.y applied to the electrodes of the deflector packages. Although FIG. 7 only shows the first deflector package 29 it should be understood that voltages V.sub.q of quadrupolar symmetry can be superposed on the deflector voltages in any or both of the first 29 and second 29 deflector packages. These superposed voltages, V.sub.q, are also controlled by the control unit 35 to achieve focusing in one plane and defocusing in the orthogonal plane, thereby reducing distortions in the angular map.

[0069] FIGS. 8A and 8B are diagrams showing the projections in the (y,z) plane of some trajectories through the lens system 13 for different start directions of charged particles from the particle emitting sample 11 (located at z=0 and with a small extension around y=0 in the illustrated coordinate systems) without and with deflector potentials V.sub.x, V.sub.y, applied to the first and second deflector packages 29, 29 during angular operational mode of the lens system 13. The vertical axis of the diagrams shows the y-coordinate of the previously discussed three-dimensional coordinate system, and the horizontal axis shows the distance from the sample in the z-direction of the same coordinate system, i.e. the distance from the sample along the optical axis 15 of the lens system 13. The axes are drawn in arbitrary units and to different scales. Trajectories illustrated by continuous lines are trajectories of particles emitted from the sample 11 at a take-off angle of 0 degrees with respect to the optical axis 15 of the lens system while trajectories illustrated by dashed lines and dash-dotted lines illustrate corresponding trajectories for take-off angles of 4 and 8 degrees, respectively.

[0070] FIG. 8A illustrates trajectories when the lens system 13 is operated in an angular mode, without deflector voltages applied to the deflector packages 29, 29. Particles emitted on the lens axis 15 will be guided through the lens system under influence of the different lenses L1-L3 (see FIGS. 4 and 6) to the centre of the plane 26 of the entrance slit 8. Particles emitted with other angles (.sub.x, .sub.y) to the lens axis will be focused to other defined positions on the entrance slit plane 26.

[0071] FIG. 8B illustrates trajectories when the lens system 13 is operated in an angular mode, with deflector voltages V.sub.x, V.sub.y applied. In this exemplary embodiment, the deflector voltages applied to the electrodes 33A-33D, 33A-33D of the deflector packages 29, 29 are controlled such that a part of the angular distribution of the particles, namely the part comprising particles emitted with a take-off angle of 8 degrees to the lens axis 15, is guided to the centre of the plane of the entrance slit 8, where it enters the measurement region 3 in the direction of the lens axis 15. Particles emitted with other angles (.sub.x, .sub.y) to the lens axis will be focused to other defined positions on the entrance slit plane.

[0072] In this exemplary embodiment, the first deflector package 29 bends the particle trajectories downwards, while the second deflector package 29 bends in the opposite direction in such a way that the chosen trajectory gently approaches the lens axis 15. Trajectories starting with other directions will leave the lens system at positions which are all displaced by substantially the same amount, keeping the dispersion substantially the same as without deflection.

[0073] FIGS. 9A-9C also illustrate how selected parts A, B of the angular distribution of emitted particles can be deflected such that the selected part enters the entrance 8 of the measurement region 3 in a direction being substantially parallel to the optical axis 15 of the lens arrangement using the inventive concept described herein, no matter the take-off angle .sub.x, .sub.y from the sample 11. FIGS. 9A and 9B illustrate the angular distribution of the particle beam, denoted by reference numeral 39, and FIG. 9C show these angular distributions mapped onto the hemisphere entrance plane 26 after deflection of the particle beam.

[0074] FIGS. 9A and 9C together illustrate a desired deflection of a part A of the angular distribution of the particle beam, and FIGS. 9B and 9C together illustrate a desired deflection of a part B of the angular distribution of the particle beam, which parts A and B comprise particles selected to be analysed in the measurement region 3 with respect to e.g. their energies, start directions, start positions or spin. In accordance with the example illustrated in FIG. 8B, two deflections in a single coordinate direction (the y-direction) is sufficient to make any selected part A of the angular distribution within the strip limited by the dashed vertical lines in FIG. 9A enter the measurement region in a direction being substantially parallel with the lens axis 15, while two deflections in each of the two coordinate directions perpendicular to the lens axis 15 (i.e. the x- and y-directions) are required to make any selected part B of the angular distribution within the strip limited by the dashed vertical lines in FIG. 9B enter the measurement region in a direction being substantially parallel with the lens axis. Any selected part of the angular distribution between the dashed vertical lines in FIG. 9B, with its centre at (.sub.x, .sub.y), can be made to enter the measurement region if the voltage V.sub.x is set to make trajectories with the fixed start direction .sub.x0 exit with x=0 and dx/dz=0, while the voltage V.sub.y is varied to make successive directions .sub.y exit with y=0 and dy/dz=0.

[0075] Once again with reference to FIG. 6, the first 29 and the second 29 deflector packages are arranged concentrically around the optical axis 15 of the lens system 13, separated by some distance, such that the charged particles pass between the electrode pairs 33A/33C, 33B/33D, 33A/33C, 33B/33D of the deflector packages on their way between the particle emitting sample 11 and the entrance 8 of the measurement region 3. For different applications the number of lens elements in the lens arrangement 13 and/or the length of the complete lens arrangement 13 (including the integrated deflector arrangement 31) can vary substantially as different applications may require different combinations of individual lens elements L1, L2, L3. Preferably, none of the deflectors of the deflector arrangement 31 should be located closer to the end of the lens element L2 within which it is arranged than approximately one lens element radius from an end of said lens element L2. Furthermore, the distance between the first 29 and the second 29 deflector packages should preferably be at least the radius of the lens element L2 within which the deflector packages are arranged. Thus, when the first 29 and second 29 deflector packages are arranged within the same lens element L2 having a certain lens element radius, the first deflector package 29 is preferably located at a distance of at least one lens element radius from the front of the lens element L2, and the second deflector package 29 is preferably located at a distance of at least one lens element radius from both the first deflector package 29 and the end of the lens element L2. This is to avoid electrostatic potential cross talk between the first and second deflector package as well as in order to give the charged particle some time to change its direction before entering the next deflector.

[0076] Furthermore, the deflectors 33A/33C, 33B/33D, 33A/33C, 33B/33D of the deflector arrangement 31 are preferably so situated with respect to the lens elements L1-L3 of the lens arrangement 13 that the region containing the deflector electrodes and their separation is substantially free from electrical fields other than those generated by the deflector electrodes themselves. To this end, as illustrated in FIGS. 5A and 5B, the deflector electrodes 33A-33D, 33A-33D are preferably arranged inside a cylindrical tube 41, 41 with their electrical potentials referred to the potential of this tube. Thus, in the preferred embodiment in which the deflector arrangement 31 comprises two deflector packages 29, 29, each comprising four electrodes 33A-33D, 33A-33D, the electrodes of each deflector package form cylindrical sectors with a four-fold rotational symmetry so as to form a substantially cylindrically shaped deflector package, which cylindrical deflector package is arranged within an outer cylindrical tube 41, 41.

[0077] Although integrated in the lens system 13 in the exemplary embodiment illustrated in the drawings, it should be appreciated that the deflectors 33A/33C, 33B/33D, 33A/33C, 33B/33D of the deflector arrangement 31 may be arranged in other ways in relation to the lens system 13 and the individual lens elements L1-L3 thereof. For example, the deflector arrangement 31 and all its deflectors might be placed in an upstream position between the sample 11 and the front of the lens system 13 or in a downstream position between the exit of the lens system 13 and the entrance slit 8 of the hemispheres 5. Such arrangements may be advantageous in some circumstances, insofar that they further decouple the deflection and lens actions. For instance, for a system that is entirely dedicated to observation of one single direction at a time (e. g. a dedicated spin detection system), an upstream position of the deflector arrangement 31 might allow a larger angular range than the integrated solution. The increased distance between the sample 11 and lens arrangement 13 would, however, result in an unfavourable reduction of the angular acceptance for normal applications. With a downstream position of the deflector arrangement 31, the increased distance between the last active lens element L3 and the entrance slit 8 of the measurement region 3 would reduce the flexibility in dispersion and energy range.

[0078] Therefore, in a preferred embodiment of the invention, the deflectors 33A/33C, 33B/33D, 33A/33C, 33B/33D of the deflector arrangement 31 are arranged in relation to the individual lens elements L1-L3 of the lens arrangement 13 such that at least one lens acts on the particle beam before the first deflection thereof, and at least one lens acts on the particle beam after the last deflection thereof. Also, all deflectors of the deflector arrangement 31 are preferably arranged within the same lens element L2 of the lens system 13, meaning that all deflectors of the deflector arrangement are surrounded by the same electrical potential. This is advantageous in that it facilitates control of the deflector voltages and lens voltages required to make a desired part of the angular distribution of the particle beam pass the entrance 8 of the measurement region 3 in parallel with the lens axis 15.

[0079] As discussed above, in a preferred design, the deflector electrodes are shaped as cylindrical sectors packaged inside two deflector packages 29, 29 with a four-fold rotational symmetry, and the two deflector packages are identical both in cross section and length. However, it should be understood that neither of these features is essential for the operation of the analyser arrangement. Planar or otherwise shaped electrodes are conceivable and might have advantages e. g. to reduce distortions of the angular patterns. Arrangements with 8 (or 4 n) poles in at least one of the packages are also possible. Reflection symmetry with respect to the (x, z) and (y, z) planes is highly desirable from the practical point of view, but not strictly necessary.

[0080] It should also be understood that the invention is not limited to the embodiments described above, but can be varied within the scope of the subsequent claims.