SE1851527A1

SE1851527A1 - Aperture device and analyser arrangement

Info

Publication number: SE1851527A1
Application number: SE1851527A
Authority: SE
Inventors: Anders Nilsson; Peter Amann
Original assignee: Scienta Omicron Ab
Priority date: 2018-12-07
Filing date: 2018-12-07
Publication date: 2020-06-08
Also published as: SE542903C2; SE1950445A1; EP3891495A2

Abstract

An aperture device (31) is described which is attachable to a lens system (13) comprising a first end (36), and a second end (37) at a distance from the first end (36). The aperture device comprises an end surface wall (40) with an end surface (S) and an aperture means (39) comprising at least one aperture (38), wherein the aperture device (31) is to be arranged with the end surface (S) facing a sample surface (Ss) which emits particles from a region which is elongated along a first direction (a). The lens system (13) is arranged to form a particle beam of charged particles, emitted from the sample surface (Ss). The aperture means (39) in the end surface (S) is elongated along a second direction (b), wherein the aperture device (31) is to be arranged with the second direction (b) essentially aligned along the first direction (a) in order to maximize the number of particles that enter the aperture means (39).

Description

1APERTURE DEVICE AND ANALYSER ARRANGEMENT TECHNICAL FIELD The present invention relates to an aperture device and an analyser arrangement for analysing e.g. theenergies, the start directions, the start positions and spin directions of charged partic|es emitted froma particle emitting sample. ln particular, the present invention relates to an aperture device and ananalyser arrangement for use in a photoelectron spectrometer of hemispherical deflector type. Morespecifically, the present invention relates to an aperture device and an analyser arrangement forphotoelectron spectroscopy at pressures as high as or higher than ambient pressure, i.e., at pressures up to a few bars.BACKGROUND ART Photoelectron spectroscopy (PES) x-ray photoelectron spectroscopy (XPS) is one of the most versatilemethods for the investigation of surfaces on the atomic scale the electronic and geometrical structureon surfaces and bulk. lt provides quantitative information about, e.g., the elemental composition andchemical specificity, e.g., oxidation state, of the surface. At the typical electron energies used in X-rayphotoelectron spectroscopy (XPS) (100 eV- 1000 1500 eV) and Hard X-ray PES (HAXPES) (2000 eV - 10keV) the interaction of the emitted electrons with atoms is strong. This means that the mean free pathof the electrons is only on the order of several monolayers, giving XPS exquisite surface sensitivity andseveral nanometres (nm) for HAXPES. However, photoelectrons are also strongly scattered by gasmolecules. This is a limitation when trying to increase the pressure. As many surface reactions, such as,e.g., catalyse, is performed at a relatively high pressure and temperature, it would be highly desirable to be able to perform PESXPS at high pressures and temperatures.

Ambient pressure x-ray photoelectron spectroscopy APXPS was pioneered by K. Siegbahn et al. in theearly 70s. The basic approach in most APXPS experiments of today is the use of a differential pumpingscheme, where the sample is arranged in a chamber or in an in situ measurement cell. The sample isplaced close to a differentially-pumped aperture. The pressure distribution in front of the aperture is not homogeneous and lower than the background pressure in the in situ cell/chamber.

According to common knowledge in the technical field, the sample has to be placed at a distance ofabout one to two aperture diameters from the aperture to ensure that the pressure at the samplesurface is close to the background pressure in the in situ cell. Thus, in order to minimize the interaction of the photoelectrons with the gas between the sample surface and the aperture the distance between 2the sample surface and the aperture should be kept to a minimum. Also, in order to enable a lowpressure in the analyser arrangement with a reasonable amount of vacuum pumping, the aperture into the analyser arrangement should be small.

For many applications it is desirable to be able to perform XPS at pressures of 1 bar and above.Examples of interesting reactions to be studied include hydrogenation reactions which takes place at10-30 bar and ammonia production which takes place at 100 bar. However, with such a high pressurethe mean free path of photoelectrons is very short. An example of an application in which a highpressure is desirable is the analysis of catalyst surfaces during catalysis. For such an application 1 barof, e.g., carbon monoxide is a suitable environment. The mean free path for 10 keV electrons in carbonmonoxide at 1 bar pressure is about 30 um. Thus, the distance between the sample surface and theaperture should preferably be on the order of 30 um to enable a reasonable part of the photoelectronsto pass into the aperture. Due to the extension of the end of the lens system, at which end theaperture is arranged, such a small distance between the aperture and the sample surface will limit thepossible angle of incidence of the x-rays on the sample surface. With a small angle of incidence, thearea of the sample surface illuminated by x-rays will be elongated and larger. This will result in a lowerintensity on the sample surface as the lens system and the detector arrangement will only acceptelectrons, which are emitted within a limited area perpendicular to the optical axis and within a limitedangular range. This will in turn lead to a lower flux of photoelectrons through the aperture, which inturn results in the need for longer measurement periods. An advantage of gracing incidence of the x-rays is that more x-rays are absorbed close to the surface of the sample. This enhances the surface sensitivity.

J. Knudsen et al. ”A versatile instrument for ambient pressure x-ray photoelectron spectroscopy: TheLund cell approach", Surface Science 646 (2016) 160-169, describes an alternative ambient pressure cell which provides a gas flow at the sample.

Gas flow at the sample is also described in Surface Science Reports 73 (2018).

SUMMARY OF THE INVENTION An object of the present invention is to provide an aperture device and an analyser arrangement which at least alleviates the problems with the prior art.

An object of the present invention is to provide an aperture device and an analyser arrangement withwhich the aperture may be arranged close to a sample surface while still enabling a high flux of photoelectrons through the aperture device. 3At least one of these objects is fulfilled with an aperture device, or an analyser arrangement according to the independent claims.

Further advantages are achieved with the features of the dependent claims.

According to a first aspect of the present invention an aperture device attachable to a lens system isprovided. The aperture device comprises a first end, and a second end at a distance from the first end.The aperture device comprises an end surface wall with an end surface and an aperture meanscomprising at least one aperture, wherein the aperture device is to be arranged with the end surfacefacing a sample surface of a particle emitting sample which emits particles from a region which iselongated along a first direction. The lens system is arranged to form a particle beam of charged particles,emitted from the sample surface and entering the lens system through the aperture means at the firstend and to transport the charged particles to the second end, when the aperture device is attached tothe first end of the lens system and the sample surface is arranged facing the at least one aperture. Theaperture device is characterized in that the aperture means in the end surface is elongated along asecond direction, wherein the aperture device is to be arranged with the second direction essentiallyaligned along the first direction in order to maximize the number of particles that enter the aperture meanS.

By arranging an elongated aperture means aligned with the particle emitting elongated region of thesample surface of the particle emitting sample it is possible to collect a larger part of the photoelectrons emitted from the sample surface of the particle emitting sample. ln case the analyser is a hemispherical analyser with an entrance slit, it is advantageous to have an elongated aperture oriented in the same direction as the elongation of the slit.

By the aperture means being elongated is meant that the extension in the second direction, from oneedge to the most distant opposite edge, is larger than the extension in a direction perpendicular to the second direction, from one edge of the aperture means to the most distant opposite edge.

The aperture means may comprise at least two apertures in the end surface wall, wherein the aperturesare arranged at different positions along the second direction. lf the aperture is constituted by only twoapertures the extension of the aperture means in the second direction is the distance from the edge ofthe first aperture being most distant from the second aperture to the edge of the second aperture beingmost distant from the first aperture. The extension of the aperture means in the direction perpendicularto the second direction is equal to the extension of one of the apertures in said direction, more preciselyof the aperture that has the largest extension in said direction. ln case both apertures are circular and have the same diameter, the extension in the second direction is equal to the centre-to-centre distance 4between the apertures plus one diameter. The extension in the direction perpendicular to the second direction is equal to the diameter of the aperture.

The aperture means may comprise at least two apertures in the end surface wall, wherein the aperturesare arranged at different positions along the second direction. By arranging a number of apertures in the end surface, electrons may be collected from larger area of the particle emitting sample.

The apertures may be arranged along a line in the end surface. Such an arrangement of the apertures isfavourable in that it is easy to fabricate two apertures as each one of the apertures may be fabricatedwith the methods according to the prior art. ln the prior art it is established practice to fabricate single apeftUfeS.

As an alternative to having two or more apertures, the aperture may be elongated. An elongatedaperture is elongated along the second direction. The shape of the elongated aperture can vary. lt is of course also possible that the aperture device comprises a number of elongated apertures.

The aperture means may comprise a plurality of apertures in the end surface wall, wherein the aperturesare distributed in the end surface along the second direction as well as in the direction perpendicular tothe second direction. Such an arrangement may be favourable for example in the case where theapertures are small in relation to the region on the sample which emits particles. The diameter of theaperture determines the minimum achievable distance between the aperture and the sample surface ifa high pressure is to be maintained at the sample surface. According to established theories a distancebetween the sample surface and the aperture being twice the diameter of the aperture enables apressure at the sample surface of 99 % of the pressure at a very large distance from the aperture, whena vacuum is present on the opposite side of the aperture. At a distance being equal to the diameter ofthe aperture the pressure is 95 % of the pressure at a large distance from the aperture. Thus, the possibledistance between the aperture and the sample surface is approximately equal to the diameter of theaperture to maintain a reasonable pressure at the sample surface. For a very small distance between thesample surface and the aperture, a very small diameter of the aperture is necessary. Thus, if the regionon the sample which emits particles is large in relation to the apertures a number of apertures may be necessary to collect electrons from a large part of said region.

The apertures may be essentially circular. A circular shape is easy to manufacture. Furthermore, mosttheories regarding flow of gas are based on circular apertures. Thus, established theories for the pressure at the sample surface may be used when the apertures are circular.

The ratio between the diameter of an aperture and the distance to an adjacent aperture is at least 2, and preferably at least 3. This ratio is derived from theoretical calculations. A smaller distance than two 5times the diameter of an aperture introduces, according to theoretical calculations, so-called cross talkbetween the apertures. ln case established theories are not necessary to use, it is, of course possible tohave a shorter distance between the apertures. The diameter of an aperture is not limited by theinvention. However, if a small distance between the aperture and the sample surface is desirable thediameter of an aperture may be is less than 200 um, preferably less than 100 um, and most preferred,less than 50 um. The diameter of the aperture determines the minimum achievable distance betweenthe aperture and the sample surface if the pressure at the sample surface shall be equal to the pressure far away from the aperture, i.e., at a distance of 10 times the diameter.

The diameter of each one of the apertures may have an increasing diameter from the end surfacetowards the lens system. This is favourable in that the aperture in this case is defined by the opening inthe surface facing the sample surface, i.e., an increasing size of the aperture in the direction away fromthe sample surface does not affect the effective aperture size. An increasing diameter contributes to thepressure decreasing more rapidly on the inside of the aperture device in which a low pressure is to be maintained.

The apertures may have been formed by laser ablation. The laser ablation is preferably performed fromthe side of the end surface wall facing away from the sample surface as this produces the desired conicalshape of the aperture. Laser ablation is also advantageous in that the edges of the aperture becomesragged. According to experiments, ragged edges seem to be favoura ble in that they make it more difficultfor gas molecules to pass. This is advantageous in that a large pressure difference is to be maintained between the opposite sides of the end surface wall.

The end surface wall may have a thickness of no more than 200 um, preferably no more than 800 um,and most preferred no more than 30 um. A thin end surface wall is favourable in that the problems witha high pressure in the apertures compared to the interior of the lens system are decreased by makingthe distance with high pressure shorter. ln other words the pressure on the inside of the aperturedecreases very rapidly with an increasing distance from the aperture. However, the pressure in theaperture provides a large resistance to the electrons. Thus, by making the end surface wall thinner the probability of electron scattering is reduced.

According to a second aspect of the present invention an analyser arrangement is provided fordetermining at least one parameter related to charged particles emitted from a particle emitting sample.The analyser arrangement comprises a measurement region comprising an entrance allowing at least apart of said particles to enter the measurement region, a lens system comprising a first end and a secondend arranged at the entrance of the measurement region at a distance from the first end. The lens system is arranged to form a particle beam from charged particles, emitted from a sample surface of a 6particle emitting sample, which enter at the first end and to transport the charged particles to the secondend. The analyser arrangement also comprises an aperture device according to the first aspect of theinvention and any of the features described with reference to the first aspect, attached to the first endof the lens system. The aperture device may or may not be an active part of the lens system. By active ismeant that the surfaces of the aperture device are included in the formation of the electrical fields responsible for the lens effect.

The arrangement is primarily an electron spectrometer. ln the following, preferred embodiments of the invention will be described with reference to theappended drawings, in which corresponding features in the different drawings will be denoted with the same reference numeral. The drawings are not drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows an analyser arrangement according to an embodiment of the present invention.

Fig. 2 shows in more detail the aperture device and the sample of Fig. 1 according to an embodiment of the present invention.

Fig. 3 is a view from the sample towards the aperture device in Fig. 2.

Fig. 4 is a view from the sample towards the aperture device according to an alternative embodiment of the present invention.

Fig. 5 is a view from the sample towards the aperture device according to an alternative embodiment of the present invention.

Fig. 6 is a view from the sample towards the aperture device according to an alternative embodiment of the present invention.

Fig. 7 shows in larger detail in cross section a part of the end surface wall with two apertures as shown in Fig. 2.

Fig. 8 shows in detail from below the aperture means in Fig. 7.

DETAILED DESCRIPTION A photoelectron spectrometer 1 of the hemispherical type, in which an aperture device according to anembodiment of the present invention may be implemented, is illustrated Fig. 1. Thus, Fig. 1 also illustrates an analyser arrangement according to an embodiment of the present invention. ln the 7 photoelectron spectrometer 1 of the hemispherical type, a central component is the measurementregion 3 in which the energies of the electrons are analysed. The measurement region 3 is formed bytwo hemispheres 5, mounted on a base plate 7, and with an electrostatic field applied between them.The electrons enter the measurement region 3 through an entrance 8 and electrons entering theregion between the hemispheres 5 with a direction close to perpendicular to the base plate 7 aredeflected by the electrostatic field, and those electrons having a kinetic energy within a certain rangedefined by the deflecting field will reach a detector arrangement 9 after having travelled through a halfcircle. ln a typical instrument, the electrons are transported from their source (typically a sample 33with a sample surface Ss (Fig. 2) that emits electrons after excitation with photons, electrons or otherparticles) to the entrance 8 of the hemispheres by an electrostatic lens system 13. The lens system 13comprises an optical axis 15, a first end 36, and a second end 37 at a distance along the optical axis 15from the first end 36. The lens system 13 is arranged to form a particle beam of charged particles,emitted from the sample surface Ss of the particle emitting sample 33, which enter the lens system 13at the first end 36 and to transport the charged particles to the second end 37. The lens system 13 alsocomprises a plurality of lenses L1-L3 having a common and substantially straight optical axis 15. Thephotoelectrons from the sample surface Ss enters the electrostatic lens system 13 through an aperture device 31 arranged at the first end of the lens system 13.

For the following description, a Cartesian coordinate system with its z-axis along the optical axis 15 ofthe lens system 13 (in most cases an axis of rotational symmetry) will be used, and with the hemispheres symmetrical with respect to the (y, z) plane.

The lens system 13 and the detector arrangement 9 will only accept electrons, which are emittedwithin a limited area perpendicular to the optical axis 15 and within a limited angular range.

Furthermore, in order to be able to position the sample in relation to the aperture means 39, thesample is mounted on a manipulator 17 allowing both translations and rotations in all coordinate directions, i.e. six degrees of freedom.

Fig. 2 shows in larger detail the aperture device 31 which is attachable to the lens system 13. Theaperture device 31 comprises an end surface wall 40 with an end surface S and an aperture means 39comprising two apertures 38. The aperture device 31 is attached to the first end 36 of the lens system13. The aperture device 31 is arranged with the end surface S facing a sample surface Ss of a particleemitting sample 33, which emits particles from a region (Fig. 2), and which is elongated along a firstdirection a. Fig 3 is a view towards the end surface of the aperture device 31 in Fig. 2. The aperturemeans 39 in the end surface S is elongated along a second direction b. The elongated region 11, depicted by the dashed line, is the region of the sample surface Ss of the particle emitting sample 33, which is 8exposed to x-rays is. The region 11 which is illuminated with x-rays corresponds to the region on thesample surface Ss from which electrons are emitted. lt is to be noted that said region 11 which is exposedto x-rays is on the sample surface Ss and not on the end surface S of the aperture device 39, as may beerroneously construed from Fig. 3. Thus, the aperture device 31 is arranged with the second direction bessentially aligned along the first direction a in order to maximize the number of charged particles thatenter the aperture means 39, for this given aperture means 39. lt is possible to further increase thenumber of charged particles that enter the aperture means 39 by altering the shape of the aperturemeans 39. ln the embodiment shown in Fig. 3 the apertures 38 are circular and have a diameter D. Theapertures 38 are arranged at a distance x from each other. The ratio between the distance x betweenthe apertures 38 and the diameter D of the apertures should be at least 2 and preferably at least 3. Thisminimum ratio has been determined to make the so called cross-talk between the apertures so smallthat it might be ignored according to established theories. The absence of cross-talk here means thatthe pressure distribution at the sample surface below each aperture is the same as it would be with onlyone circular aperture in the aperture device 31. The diameter D of each one of the apertures 38 is lessthan 200 um, preferably less than 100 um, and most preferred, less than 50 um. The diameter D of theaperture should be small to allow the aperture to be placed close to the sample while maintaining asufficiently high pressure at the sample. According to presently established theories the distance xbetween the aperture 38 and the sample surface Ss should be kept at twice the diameter D of theaperture to achieve a sufficiently high pressure at the sample surface Ss. The pressure drops at thesample surface Ss, when the sample surface Ss is arranged closer to the aperture 38 than twice thediameter D of the aperture 38. The pressure drop is predictable. Thus, a predictable pressure isachievable for distances d between the sample surface and the aperture 38 being as small as equal tothe diameter D of the aperture 38. To achieve a predetermined pressure at the sample surface Ss whenthe aperture 38 is arranged at a distance d equal to the diameter D of the aperture 38 from the samplesurface Ss, the pressure, at a distance d of twice the diameter D, has to be higher than the desired pfeSSUfe.

The apertures 38 in the embodiment of Fig. 3 are circular. lt is, however, possible to have other shapes on the apertures. Circular apertures are easy to manufacture.

Fig. 4 is a view from the sample towards the aperture device 31 according to an alternativeembodiment of the present invention. The aperture means 39 in Fig. 4 consists of five circularapertures 38 arranged at a distance x from each other and each having a diameter D. The apertures 38 are arranged along the second direction b. 9Fig. 5 is a view from the sample towards the aperture device 31 according to an alternativeembodiment of the present invention. The aperture means in Fig. 5 consists of one elongated aperture 38, which extends along the elongated along a second direction b.

Fig. 6 is a view from the sample towards the aperture device 31 according to an alternative embodimentof the present invention. The aperture means 39 comprises a plurality of apertures 38 in the end surfacewall 40. The apertures 38 are distributed in the end surface S along the second direction b as well as inthe direction perpendicular to the second direction b. By having the apertures 38 arranged in this way itis possible to collect charged particles from an elongated region, which is slightly wider than the regions for which the aperture device in Fig. 5 is optimal.

Fig. 7 shows in larger detail in cross section a part of the end surface wall 40 with two apertures 38 asshown in Fig. 2. As can be seen in Fig. 7 each aperture has the form of a truncated cone. The diameterof each one of the apertures 38 has an increasing diameter from the end surface S and inwards, i.e.,towards the lens system 13 (Fig. 1). The end surface wall 40 has a thickness T of no more than 200 um,preferably no more than 80 nm, and most preferred no more than 30 um. A thicker end wall makes theapertures 38 longer in the direction perpendicular to the end surface. The end surface wall should be asthin as possible, but a thicker wall is less fragile. A thinner wall reduces electron scattering inside thehole. The flow restriction is primarily through the diameter. The pressure will decrease from the endsurface S and inwards. Thus, even if a high vacuum is sustained inside the lens the pressure will be higherin the aperture. This will provide a longer path in a high pressure environment for the charged particlesto pass. Thus, in order to maximize the number of electrons that pass into the interior of the lens system13 (Fig. 1) it is necessary to minimize the influence of the gas in the apertures 38. This, is achieved by making the end surface wall 40 thin with an increasing diameter D inwards.

Fig. 8 shows in detail from below the aperture means 39 in Fig. 7. As can be seen in Fig. 8 the aperturesare essentially circular with a ragged edge. The ragged edge of the apertures has proven to make it moredifficult for the gas molecules to enter the apertures 38 and thus, contributes to a lower pressure insidethe apertures. The variation A in the radial direction is on the order of 10 % of the diameter D of the aperture 38. Such a ragged edge is formed when the apertures 38 are formed by laser ablation.

The above described embodiments of the invention may be amended in many ways without departing from the scope of the present invention, which is limited only by the appended claims.

Claims

CLAll\/IS

1. An aperture device (31) attachable to a lens system (13) comprising a first end (36), and a second end(37) at a distance from the first end (36), wherein the aperture device comprises an end surface wall (40)with an end surface (S) and an aperture means (39) comprising at least one aperture (38), wherein theaperture device (31) is to be arranged with the end surface (S) facing a sample surface (Ss) of a particleemitting sample (33) which emits particles from a region which is elongated along a first direction (a),wherein the lens system (13) is arranged to form a particle beam of charged particles, emitted from thesample surface (Ss) and entering the lens system (13) through the aperture means (39) at the first end(36) and to transport the charged particles to the second end (37), when the aperture device (31) isattached to the first end of the lens system (13) and the sample surface (Ss) is arranged facing the atleast one aperture (3), characterized in that the aperture means (39) in the end surface (S) is elongated along a second direction(b), wherein the aperture device (31) is to be arranged with the second direction (b) essentially alignedalong the first direction (a) in order to maximize the number of particles that enter the aperture means (39).

2. The aperture device according to claim 1, wherein the aperture means (39) comprises at least twoapertures (38) in the end surface wall (40), wherein the apertures (38) are arranged at different positions along the second direction (b).

3. The aperture device (31) according to claim 1 or 2, wherein the aperture means (39) comprises aplurality of apertures (38) in the end surface wall (40), wherein the apertures (38) are distributed in theend surface (S) along the second direction (b) as well as in the direction perpendicular to the second direction (b).

4. The aperture device (31) according to claim 1, 2 or 3, wherein the apertures (38) are essentially circular.

5. The aperture device (31) according to claim 4, wherein the ratio between the diameter (D) of an aperture (38) and the distance (x) to an adjacent aperture (38) is at least 2, and preferably at least 3.

6. The aperture device (31) according to claim 4 or 5, wherein the diameter of each one of the apertures (38) is less than less than 200 um, preferably less than 100 um, and most preferred, less than 50 um.

7. The aperture device (31) according to any one of the preceding claims, wherein the diameter of each one of the apertures has an increasing diameter from the end surface (S) towards the lens system (13). 11

8. The aperture device (31) according to claim 7, wherein the apertures have been formed by laser ablation.

9. The aperture device (31) according to any one of the preceding claims, wherein the end surface wall(40) has a thickness (T) of no more than 200 um, preferably no more than 80 um, and most preferred no more than 30 um.

10. An analyser arrangement (100) for determining at least one parameter related to charged particlesemitted from a particle emitting sample (31), comprising: - a measurement region (3) comprising an entrance (8) allowing at least a part of said particles to enterthe measurement region (3); - a lens system (13) comprising a first end (36) and a second end (37) arranged at the entrance of themeasurement region (3) at a distance from the first end (36), wherein the lens system (13) is arrangedto form a particle beam from charged particles, emitted from a sample surface (Ss) of a particle emittingsample (S), which enter at the first end (36) and to transport the charged particles to the second end(37), and - an aperture device (31) according to any one of claims 1-9, attached to the first end (36) of the lens system (13).