SE1650967A1 - Electron spectrometer with a displaceable lens - Google Patents

Abstract

18 AB STRACT A charged particle spectrometer of hemispherical analyzer type for analyzing aparticle emitting sample (1 1), the spectrometer comprising at least a firstmechanism (26; 26') configured to move at least at least a part of the lens (12) Withrespect to the aXis between the sample spot and the analyser entrance in a coordinate direction synchronously With a deflection of the particle beam. (Pig. 2)

Description

ELECTRON SPECTROMETER The present invention relates to electron spectrometers in general, and in particular to a novel means and method for operating in angular mode.

Background of the Invention In a photo-electron spectrometer of hemispherical analyzer type, a centralcomponent is the measurement region in which the energies of the electrons areanalysed. The measurement region is formed by two concentric hemispheres,mounted on a base plate, and with an electrostatic field applied between them. Theelectrons enter the measurement region through an entrance and electrons enteringthe region between the hemispheres with a direction close to perpendicular to thebase plate are deflected by the electrostatic field, and those electrons having akinetic energy within a certain range defined by the deflecting field will reach adetector arrangement after having travelled through a half circle. In a typicalinstrument, the electrons are transported from their source (typically a sample thatemits electrons after excitation with photons, electrons or other particles) to theentrance of the hemispheres by an electrostatic lens system comprising a plurality of lenses having a common and substantially straight optical aXis.

The lens system and the detector arrangement will only accept electrons which areemitted within a limited area perpendicular to the lens aXis and within a limitedangular range. Furthermore, the source has to be positioned within a narrow rangein the z-direction to achieve the best properties (in terms of sensitivity andresolution). This necessitates mounting the sample on a manipulator allowing both translations and rotations in all coordinate directions, i.e. six degrees of freedom.

In many applications of for example Angle Resolved Photoelectron Spectroscopy(ARPES) a complete measurement requires full detection of a solid angle with atotal cone opening of 30 degrees from a well aligned sample. Depending on sampleand excitation energy/ kinetic energi the required angular range may vary. Theangle resolution requirements also varies with application but typically range from 1 degree down to better than 0.1 degrees. In energy resolution the desired span is from 0.5 eV down to 0.5 meV depending on application. In order to achieve a highresolution measurement the analyser arrangement must have suff1cient angularand energy resolution, but since the hemispherical analyser arrangement onlyaccepts electrons emitted Within a limited angular range perpendicular to the lensaXis, the sample manipulator must have very high precision movements andrepeatability. The manipulator is needed to accurately rotate and tilt the sample to build up the complete 30 degree solid angle data set.

HoWever, in recent years the illumination systems have reached a much higherlevel of spatial resolution Which means that extremely minute crystallites can beobserved. Thereby the manipulation, i.e. rotation of the sample becomes very difficult.

One Way of eliminating the sample manipulation is to provide a second deflectorinside the lens and close to the first deflector in order to bring the electron beam atthe entrance to the measurement region in alignment With the optical axis of the lens.

Spectrometers provided With such deflectors inside the lens have been sold by VG Scienta AB.

Despite the fact that this system eliminated the need for sample manipulation, it still suffers from some distortion in the recorded images.

Summary of the Invention In order to improve the quality of the recorded images, the present inventor has deviseda novel device Which also eliminates the need for sample manipulation and in addition provides less distortion.

Thereby there is provided a charged particle spectrometer of hemispherical analyzertype for analyzing a particle emitting sample. The spectrometer comprises ameasurement 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 andsaid entrance of the measurement region, said lens system having a substantiallystraight optical aXis; a deflector arrangement in the lens comprising a deflectorconfigured to deflect the particle beam in at least one coordinate direction (X, y)perpendicular to the optical axis of the lens system before entrance of the particlebeam into the measurement region, a detector arrangement for detecting thepositions of the charged particles in the measurement region, wherein the detectorarrangement is configured to determine the positions of the charged particles intwo dimensions, one of which is indicative of the energies of the particles and one of which is indicative of the start directions or start positions of the particles.

The inventive idea is to displace (i.e. move from one position to a slightly differentposition, incrementally) at least a part of the lens with respect to the axis betweenthe sample spot and the analyser entrance in at least a first coordinate directionand then to subject the particle beam to one single deflection inside the lenssystem. The displacement is made synchronously with the deflection of the particlebeam, whereby the trajectories of said charged particles will enter themeasurement region. The particle beam will thus enter the lens “off-axis”, which causes the beam to be focused at a different point.

The term “nominal position” of the lens or lens axis should be taken to mean asituation where a particle beam running along a horizontal line from the samplespot to be studied follows the lens axis and is focused on the entrance slit at a point coinciding with the lens aXis.

In particular it should be noted that beams having start directions deviating fromthe horizontal that would be focused above the entrance slit before themeasurement region in the nominal position of the lens, can be made to be focused at a point below the slit if the displacement is made in an appropriate manner.

Therefore, it will suffice with one single deflection stage inside the lens in order tobring the beam back to horizontal, i.e. aligned, or at least parallel, with a nominal optical axis.

There are several possible Ways of achieving this effect, e.g. tilting the lens, bendingthe lens at some point along its length, or moving the entire lens in the coordinate direction in question.

In one embodiment the lens is suspended in a multidirectional pivot point at thatend of the lens that is adjacent to the entrance of the measurement region such that the lens can be tilted around the pivot point in said coordinate direction (x, y).

There is also provided at least a first tilting mechanism configured to tilt the lens in said coordinate direction synchronously With a deflection of the particle beam.

In one embodiment of the spectrometer the mechanism for tilting the lenscomprises a motor, an actuator rod connected to the motor, and a spring loaded device arranged to keep the lens in contact With the tilting mechanism.

Preferably, the spectrometer comprises a further tilting mechanism arranged atright angles to the first tilting mechanism, configured to tilt the lens in a secondcoordinate direction (x, y) synchronously With a deflection of the particle beam,Whereby the spring loaded device is arranged symmetrically opposite the first and second tilting mechanisms at an angular distance of about 135°.

In another embodiment the entire lens is suspended in a mechanism that allows it be moved in a desired coordinate direction.

In still another embodiment the lens is subdivided in a plurality of lens elements,but at least two lens elements, Which are connected in a manner such that the lens can be bent at the position Where the elements are joined.

All of the above embodiments achieve the same result to enable the particle beam to be realigned by using one single deflector unit.

In a second aspect the invention provides a method for determining at least oneparameter 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 ofa measurement region by means of a lens system having a substantially straightoptical axis, said lens being suspended in a multidirectional pivot point at that endof the lens that is adjacent to the entrance of the measurement region such that thelens can be tilted in said coordinate direction (X, y); deflecting the particle beam inat least a first coordinate direction (x, y) perpendicular to the optical axis of thelens system before entrance of the particle beam into the measurement region,detecting the positions of said charged particles in said measurement region, thepositions being indicative of said at least one parameter, detecting the positions ofthe charged particles involves detection of the positions in two dimensions, one ofwhich is indicative of the energies of the particles and one of which is indicative of the start directions or start positions of the particles.

In one embodiment the lens is tilted in said coordinate direction synchronouslywith the deflection of the particle beam, whereby the trajectories of said charged particles will enter the measurement region.

In another embodiment the entire lens is moved, and in a further embodiment the lens is bent.

Further scope of applicability of the present invention Will become apparent from thedetailed description given hereinafter and the accompanying dravvings which are givenby way of illustration only, and thus not to be considered limiting on the present invention, and wherein Fig. 1 schematically illustrates a part of an electron spectrometer embodying a novel feature for operation in angular mode; Fig. 2 shows the same apparatus as in Fig. 1 wherein the lens has been slightly tilted in accordance with the novel feature; Fig. 3 illustrates a multi-directional hinge; Fig. 4 schematically illustrates a manipulator system for tilting the lens in at least one coordinate direction; Fig. 5 schematically illustrates the control system; Fig. 6 a-b is a comparison between the particle beams in a lens With and Without deflector, respectively; Fig. 7 a-b is a comparison between the particle beams in a lens With deflector but no tilt and a lens With a deflector With tilt, respectively; Fig. 8 illustrates double deflection of light using prisms; and Fig. 9 illustrates tilting a lens and a single deflection using a prism.

Detailed Description of Preferred Embodiments Fig. 1 illustrates schematically a part of an electron spectrometer embodying theinvention, namely the sample 10, the electron lens 12 having an optical axis 13, a pairof deflectors 14a, 14b, an entrance slit 16a to the measurement region of ahemispherical analyser M (only indicated With a broken line), a hinge mechanism 18suspending the lens 12 in a multi-directional pivot point, via a beam 20 rigidlyattached to the body of the lens 12. The hinge 18 is attached to the base plate 22 of the hemispherical analyser M. Inside the measurement region there is a second slit 16b.

The novelty of the apparatus resides in a preferred embodiment in a tilting mechanism24. This mechanism in a first embodiment comprises a motor 26, preferably an electric motor, preferably a stepper motor.

The motor is controlled by a control unit CU that also controls the voltages onthe deflectors 14a, 14b, the control being schematically indicated With broken lines, and Will be described further beloW.

The motor 26 is configured to actuate a pushing member 27, capable ofmovement in a vertical direction. The pushing member 27 is suitably anactuator rod to the upper end of which is attached a support plate 28 on which the lens 12 rests.

In Fig. 2 the lens 12 has been slightly tilted from the horizontal, i.e. the entranceregion to the lens 12 has been moved a small distance (maximum about 5 mm)from the horizontal. This is clearly seen as the optical axis of the lens deviates 13' from its nominal position 13.

The hinge mechanism 18 will now be described briefly with reference to Fig. 3.Such hinge mechanisms are provided on most electron spectrometers accordingto prior art and used for adjustment purposes, and form no part of the invention per se.

The hinge mechanism comprises a beam member 20 rigidly mounted (e.g.welded or bolted) to the lens body 12. The beam protrudes out from theproximal end of the lens body 20. At the end of the protruding portion of thebeam 20 the beam has a through-hole 31. The through-hole has a widerdiameter at the top than at the bottom, see the magnified encircled portion, i.e.there is a small step 32 at the lowermost part of the hole. Note that thedimensions are not to scale. In the hole there is a sleeve member 34. Thus, dueto the step 32 there will be a small circumferential gap G between sleeve 34 andthe inner circumference of the through-hole 31. Resting on the periphery of thesleeve 34 there is a spring member 36, suitably a cup spring. A bolt or screw 38is anchored in the base plate 22, and when tightened the screw and spring willexert a strong downward force which ascertains electrical contact. The bottom side of the beam 20 at the hole is slightly concave (not shown).

This construction enables slight movement of the lens 12 in all directions.

Fig. 4a shows the device for enabling tilting of the lens in at least one direction.

Apart from one vertically oriented mechanism 24', which can move the lens 12in the X direction, there can also be provided a horizontally arrangedmechanism 24” for moving the lens 12 in the Y direction. There is also provideda spring loaded support device 35. It comprises a support plate 36, a guide rod37 attached to the frame-work (not shown) so as to be slidable, and a spring 38exerting a pressing force on the support plate 36. This device 25 keeps the lens 12 in contact with the tilting mechanisms 24', 24”.

In operation the control unit 28 will perform a number of actions such asdefining the energy E and the angle ®X by setting the energy, setting the lensvoltages, setting voltages on the deflectors 14a, 14b. The motor will be energizedsuch that the lens is tilted incrementally to a defined extent T which can befractions of millimeters per increment, and where the maximum tilt T is a few mm, i.e. about maximum i 10 mm, as shown in Fig. 2.

When these actions have been performed an exposure is carried out whereupon the procedure is repeated for a new set of values for energy E and the angle (EX.

Thus, an image (2D) is built by a stepwise procedure where a plurality of exposures are carried out by the detector.

This procedure of setting the motor increments in relation to the deflectorvoltages will be referred to as the tilting mechanism (i.e. motor and actuator rod) being operated synchronously with the deflection of the beam.

In Fig. 4b an alternative embodiment is illustrated.

It comprises a ball joint 29 (spherical bearing), i.e. a ball, suitably of metalalthough other materials may be usable, mounted (enclosed) in a socketattached to the lens body. Using a rigid rod 27' actuated by a motor 26' asshown restricts this embodiment to movement in one coordinate direction (X direction).

Now the actual control of the synchronous operation Will be briefly described.

Fig. 5 schematically illustrates the control.

A control unit CU, schematically indicated by a box drawn With broken lines,comprises memory units for storing data, and digital to analog converters DACfor the lens voltages and for the motor drive, respectively, comprising aprocessor P configured to retrieve data from memory, said data being convertedto analog signals for setting voltages to deflectors and for actuating a motor in the tilting mechanism synchronously With the voltage settings Thus, the setting of parameters is done by providing data from tables DTab(8)memory of the control unit CU. Corresponding tables MTab are provided for theincremental operation of the motor. There are provided a plurality of DAC(Digital to Analog Converters), one for each deflector plate 1-8 in the element 04 (octopolar configuaration).

In the same manner there is a DAC for the motor drive.

The tables DTab(8) and MTab, respectively, contains voltage valuescorresponding to every start angle ®X for the electrons that are to be scanned.

Thus, the tables contains values Which are a function of said start angles (EX.

As already indicated above, a complete scan cycle comprises a) setting voltagesfor the deflection for a given start angle ®X and b) a voltage (Set value (V))corresponding to a desired movement of the lens, by running the motor and inresponse thereto movement of the rod 27 (same element as in Fig. 1), and c) repeating a) and b) for all angles (EX, from e.g. -5° to +5°.

Coupled to the motor is a potentiometer PM that Will yield a voltage (Actualvalue) in response to the rotation of the motor axis, and when the Actual value = Set value the PID will cause the motor to stop, and an exposure is made.

Now the operation of a system incorporating the novel tilt mechanism will be described.

Fig. 6a schematically shows an electron beam emanating from an emittingsample spot and how the beam is affected by a simplified lens. Each dotrepresents a point on the lens through which an electron beam passes and isrefracted, i.e. the spots where the beam changes direction. In this simplifiedillustration a single lens element with focus on the slit at the right end is shown.As can be clearly seen in this simplified figure, electrons running in a straightline, i.e. at a start angle of O° will be focused on the slit and enter themeasurement region in a straight line, whereas electrons having a start angle > O° (e.g. 15°) will be focused on a different spot.

In Fig. 6b a deflector has been introduced. As can be seen the deflection resultsin the electrons taking a direction that deviates from the horizontal and misses the slit.

Fig. 7a, which is the same as Fig. 6b, is to be compared with Fig. 7b wherein thelens is tilted (un-tilted position in broken lines) and the deflector is activated (electron beam shown by solid lines).

The broken beam lines in Fig. 7b (as a continuation of the solid electron beamlines) show the electron beam if the deflector is not activated. It is important torecognize that the tilting of the lens must be suff1cient that the focus withoutdeflection hits the slit member at a point below the slit. When the deflector isactivated it is redirected to a horizontal pathway so as to hit exactly on the slit and enters the measurement region. 11 It is important to recognize that the tilting of the lens is a compromise. What onewould wish to achieve is to move the entire lens vertically. This is certainlypossible, but would however be more complicated since the lens is bulky (800mm long) and arranged inside a vacuum Chamber. Instead a very slight tilting ofthe lens achieves the same effect since the tilt angle is so small that it can bedisregarded and for all practical purposes it is equivalent to a verticaltranslational movement of the lens. It would be equally possible to move thesample, i.e. a relative movement of sample / lens but again, the sample is attached to a very bulky structure, and moving it is complicated.

A mechanism for moving the entire lens is shown in Fig. 10 and described further below.

A further possibility would be to bend the lens. In practice the lens is made upfrom a plurality of segments, and it would be possible to actually cause a slightbend at a joint between two segments. Such bending would of course for allpractical purposes be equivalent to a tilting as disclosed herein. Such bending is shown in Fig. 11 and described further below.

Thus, in generic terms one can say that at least a part of the lens is displaced (or moved) in a desired coordinate direction.

In Fig. 7b it is clearly seen that tilting and deflecting in a synchronous mannerwill cause the electrons to enter the slit essentially along the horizontal axis. Thevery small deviation of the lens from the horizontal due to the tilting isnegligible. The lens is in the order of 800 mm long and the maximum deflection at the lens opening is 5 mm, in a normal case 1-2 mm.

An analogy from optics of how the system works can be to imagine an imagebeing focused by a single lens on a screen on a given spot. If the lens is movedin one direction the light will enter the lens off-center, and as a consequence the image will also move on the screen. In order to bring the image back to the 12 center one could place a prism between the lens and the screen. The prism“deflects” the light in a parallel manner, which is exactly what the deflector does to the electron beam.

A situation similar to the prior art using two deflections by using two prisms P1,P2 is shown in Fig. 8. Thus, light from a light source LS having a start angle ofabout 15°, and focused by a lens L will be refracted a first time P1 so as to point to a point below the slit S and a second time P2 to enter the slit S.

In Fig. 9 a situation analogous to the invention, i.e. moving the lens and performing one deflection.

Fig. 9a shows a light emitted from a light source LS is shown. The light isfocused by a lens L and refracted by a prism P2. As can be seen the light passes the slit S at an angle.

Fig. 9b shows a situation where the lens L has been moved downwards a smalldistance (e. g. 2 mm) as indicated by the arrow. Thus, the light will be focused -differently since now the optical axis is displaced too. Therefore the prism P2 ifpositioned properly will refract (deflect) the light to align it to be parallel with the optical axis.

This is completely analogous to the situation in Fig. 7b.

In Fig. 10 one of the alternative mechanisms for displacing the optical axis isschematically shown, namely a mechanism for displacing the entire lens vertically.

Thus, the lens 12 in the shown embodiment is suspended by two supportstructures, e.g. rods 27 like in the embodiment shown in Fig. 1, via a supportplate 28. Of course a ball joint type suspension 29 would be equally applicable in this embodiment. The rods are actuated by motors 26. 13 Fig. 10 a shows the lens 12 in a “nominal position”. In Fig. 10b the motors 26have withdrawn the rods 27 such that the lens 12 has been moved slightly in the vertical direction, a distance D, as illustrated by the arrow A.

Fig. 11 schenatically illustrates an embodiment wherein the lens is subdividedin two segments via a joint 1 10 at some point along the lens 12. The ends of thelens must be pivotally suspended. Thus, this embodiment allows the segmentsto be moved at the joint 110 with respect to the optical axis. Fig. 11a is the“nominal” position and Fig. 11b shows the bent situation (slightly exaggerated), i.e. the part of the lens 12 at the joint 110 has been displaced a distance D”.

The mechanism enabling this movement can be the same or similar to what is shown in Fig. 10, although ball joints may be preferable in this embodiment.

Claims (15)

:

1. A charged particle spectrometer of hemispherical analyzer type for analyzing aparticle emitting sample (11), the spectrometer comprising: a measurement region (M) having an entrance allowing said particles to enterthe measurement region (M); a lens system (12) for forming a particle beam of said charged particles andtransporting the particles between said particle emitting sample and said entranceof the measurement region, said lens system having a substantially straight opticalaxis (13); a deflector arrangement (14a, 14b) in the lens configured to deflect the particlebeam in at least one coordinate direction (X, y) perpendicular to the optical axis(13) of the lens system before entrance of the particle beam into the measurementregion (M), a detector arrangement (9) for detecting the positions of the charged particles inthe measurement region,characterised by the detector arrangement (9) being configured to determine the positions of thecharged particles in two dimensions, one of which is indicative of the energies ofthe particles and one of which is indicative of the start directions or start positionsof the particles, and at least a first mechanism (26; 26') configured to displace at least a part of thelens (12) with respect to the axis between the sample spot and the analyserentrance in at least a first coordinate direction synchronously with a deflection of the particle beam.

2. Spectrometer according to claim 1, wherein the lens is suspended in a multidirectional pivot point at that end of the lens that is adjacent to the entranceof the measurement region such that the lens can be tilted around the pivot pointin said coordinate direction (X, y), and wherein the mechanism for moving at least the entrance region of the lens system (13) is a tilting mechanism.

3. Spectrometer according to claim 2, wherein the mechanism for tilting the lens comprises a motor, an actuator rod connected to the motor, and a spring loaded device (35, 36, 37, 38) arranged to keep the lens in contact With the tiltingmechanism (26; 26', 26").

4. Spectrometer according to claim 2, comprising a further tilting mechanism (26”)arranged at right angles to the first tilting mechanism (26; 26'), configured to tiltthe lens in a second coordinate direction (x, y) synchronously With a deflection ofthe particle beam, Whereby the spring loaded device (35.) is arranged symmetricallyopposite the first and second tilting mechanisms at an angular distance of about 135°.

5. Spectrometer according to any preceding claim, further comprising a control unit(CU), comprising a processor (P) configured to retrieve data from memory, said databeing converted to analog signals for setting voltages to deflectors and for actuating a motor in the tilting mechanism synchronously With the voltage settings.

6. Spectrometer according to claim 5, Wherein the data is provided as tables(DTab(8), MTab), one set of tables for each deflector plate (1-8) in the deflectorarrangement, and one table for the motor Wherein a specific voltage setting correlates to a specific motor setting to provide a specific tilting of the lens.

7. Spectrometer according to claim 1, Wherein the Wherein the mechanism formoving at least a part of the lens (12) With respect to the axis between the samplespot and the analyser entrance in at least a first coordinate direction is a mechanism that moves the entire lens (12) in said coordinate direction (x, y).

8. Spectrometer according to claim 1, Wherein the Wherein the mechanism formoving at least a part of the lens (12) With respect to the axis between the samplespot and the analyser entrance in at least a first coordinate direction is a mechanism that bends the lens (12) in said coordinate direction (x, y).

9. Spectrometer according any of claims 1, 2, 7 or 8, Wherein the mechanism comprises a ball joint (29) connecting the actuator rod (27') to the lens body. 16

10. A method for Operating a charged particle spectrometer of hemisphericalanalyzer type in angular mode, comprising moving at least a part of the lens (12) With respect to the axis between thesample spot and the analyser entrance in at least a first coordinate direction in adesired coordinate direction synchronously with the deflection of the particle beam;and deflecting an electron beam once inside a lens system of said spectrometer by means of a deflector arrangement.

1 1. A method for determining at least one parameter related to charged particlesemitted from a particle emitting sample (11), comprising the steps of: forming a particle beam of said charged particles and transporting the particlesbetween said particle emitting sample (11) and an entrance (8) of a measurementregion (3) by means of a lens system (13) having a substantially straight opticalaxis (13); deflecting the particle beam in at least a first coordinate direction (X, y)perpendicular to the optical axis of the lens system before entrance of the particlebeam into the measurement region, detecting the positions of said charged particles in said measurement region, the positions being indicative of said at least one parameter, characterised by the detecting the positions of the charged particles involving detection of thepositions in two dimensions, one of which is indicative of the energies of theparticles and one of which is indicative of the start directions or start positions ofthe particles, and displacing at least at least a part of the lens (12) with respect to the axisbetween the sample spot and the analyser entrance in said coordinate directionsynchronously with the deflection of the particle beam, whereby the trajectories of said charged particles will enter the measurement region.

12. The method according to claim 1 1, wherein the lens is suspended in a multidirectional pivot point at that end of the lens that is adjacent to the entrance 17 of the measurement region, and comprising tilting the 1ens in said coordinate direction (x, y).

13. The method according to c1aim 1 1, Wherein the entire 1ens is moved.

14. The method according to c1aim 1 1, Wherein the 1ens is bent at some point.

15. The method according to c1aim 11, Wherein the displacing is incrementa1.