WO2020117123A2

WO2020117123A2 - Aperture device and analyser arrangement

Info

Publication number: WO2020117123A2
Application number: PCT/SE2019/051241
Authority: WO
Inventors: Peter Amann; Anders Nilsson
Original assignee: Scienta Omicron Ab
Priority date: 2018-12-07
Filing date: 2019-12-06
Publication date: 2020-06-11
Also published as: WO2020117123A3

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

APERTURE DEVICE AND ANALYSER ARRANGEMENT

TECHNICAL FIELD

The present invention relates to an aperture device and an analyser arrangement for analysing e.g. the energies, the start directions, the start positions and spin directions of charged particles emitted from a particle emitting sample. In particular, the present invention relates to an aperture device and an analyser arrangement for use in a photoelectron spectrometer of hemispherical deflector type. More specifically, the present invention relates to an aperture device and an analyser arrangement for photoelectron 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 versatile methods for the investigation of surfaces on the atomic scale the electronic and geometrical structure on surfaces and bulk. It provides quantitative information about, e.g., the elemental composition and chemical specificity, e.g., oxidation state, of the surface. At the typical electron energies used in X-ray photoelectron spectroscopy (XPS) (100 eV- 1000 1500 eV) and Hard X-ray PES (HAXPES) (2000 eV - 10 keV) the interaction of the emitted electrons with atoms is strong. This means that the mean free path of the electrons is only on the order of several monolayers, giving XPS exquisite surface sensitivity and several nanometres (nm) for HAXPES. However, photoelectrons are also strongly scattered by gas molecules. 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 the early 70s. The basic approach in most APXPS experiments of today is the use of a differential pumping scheme, where the sample is arranged in a chamber or in an in situ measurement cell. The sample is placed 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 of about one to two aperture diameters from the aperture to ensure that the pressure at the sample surface 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 the sample surface and the aperture should be kept to a minimum. Also, in order to enable a low pressure 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 at 10-30 bar and ammonia production which takes place at 100 bar. However, with such a high pressure the mean free path of photoelectrons is very short. An example of an application in which a high pressure is desirable is the analysis of catalyst surfaces during catalysis. For such an application 1 bar of, e.g., carbon monoxide is a suitable environment. The mean free path for 10 keV electrons in carbon monoxide at 1 bar pressure is about 30 pm. Thus, the distance between the sample surface and the aperture should preferably be on the order of 30 pm to enable a reasonable part of the photoelectrons to pass into the aperture. Due to the extension of the end of the lens system, at which end the aperture is arranged, such a small distance between the aperture and the sample surface will limit the possible angle of incidence of the x-rays on the sample surface. With a small angle of incidence, the area of the sample surface illuminated by x-rays will be elongated and larger. This will result in a lower intensity on the sample surface as the lens system and the detector arrangement will only accept electrons, which are emitted within a limited area perpendicular to the optical axis and within a limited angular range. This will in turn lead to a lower flux of photoelectrons through the aperture, which in turn 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: The Lund 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 with which the aperture may be arranged close to a sample surface while still enabling a high flux of photoelectrons through the aperture device. At 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 is provided. 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 means comprising at least one aperture, wherein the aperture device is to be arranged with the end surface facing a sample surface of a particle emitting sample which emits particles from a region which is elongated 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 first end and to transport the charged particles to the second end, when the aperture device is attached to the first end of the lens system and the sample surface is arranged facing the at least one aperture. The aperture device is characterized in that the aperture means in the end surface is elongated along a second direction, wherein the aperture device is to be arranged with the second direction essentially aligned 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 the sample 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.

In 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 one edge 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 apertures are arranged at different positions along the second direction. If the aperture is constituted by only two apertures the extension of the aperture means in the second direction is the distance from the edge of the first aperture being most distant from the second aperture to the edge of the second aperture being most distant from the first aperture. The extension of the aperture means in the direction perpendicular to the second direction is equal to the extension of one of the apertures in said direction, more precisely of the aperture that has the largest extension in said direction. In case both apertures are circular and have the same diameter, the extension in the second direction is equal to the centre-to-centre distance between 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 apertures are 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 is favourable in that it is easy to fabricate two apertures as each one of the apertures may be fabricated with the methods according to the prior art. In the prior art it is established practice to fabricate single apertures.

As an alternative to having two or more apertures, the aperture may be elongated. An elongated aperture is elongated along the second direction. The shape of the elongated aperture can vary. It 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 apertures are distributed in the end surface along the second direction as well as in the direction perpendicular to the second direction. Such an arrangement may be favourable for example in the case where the apertures are small in relation to the region on the sample which emits particles. The diameter of the aperture determines the minimum achievable distance between the aperture and the sample surface if a high pressure is to be maintained at the sample surface. According to established theories a distance between the sample surface and the aperture being twice the diameter of the aperture enables a pressure at the sample surface of 99 % of the pressure at a very large distance from the aperture, when a vacuum is present on the opposite side of the aperture. At a distance being equal to the diameter of the aperture the pressure is 95 % of the pressure at a large distance from the aperture. Thus, the possible distance between the aperture and the sample surface is approximately equal to the diameter of the aperture to maintain a reasonable pressure at the sample surface. For a very small distance between the sample surface and the aperture, a very small diameter of the aperture is necessary. Thus, if the region on 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, most theories 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 1.5, preferably at least 2, and most preferred at least 3. This ratio is derived from theoretical calculations. A smaller distance than 1.5 times the diameter of an aperture introduces, according to theoretical calculations, so-called cross talk between the apertures. In case established theories are not necessary to use, it is, of course possible to have a shorter distance between the apertures. The diameter of an aperture is not limited by the invention. However, if a small distance between the aperture and the sample surface is desirable the diameter of an aperture may be is less than 200 pm, preferably less than 100 pm, and most preferred, less than 50 pm. The diameter of the aperture determines the minimum achievable distance between the 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 surface towards the lens system. This is favourable in that the aperture in this case is defined by the opening in the surface facing the sample surface, i.e., an increasing size of the aperture in the direction away from the sample surface does not affect the effective aperture size. An increasing diameter contributes to the pressure 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 from the side of the end surface wall facing away from the sample surface as this produces the desired conical shape of the aperture. Laser ablation is also advantageous in that the edges of the aperture becomes ragged. According to experiments, ragged edges seem to be favourable in that they make it more difficult for 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 pm, preferably no more than 800 pm, and most preferred no more than 30 pm. A thin end surface wall is favourable in that the problems with a high pressure in the apertures compared to the interior of the lens system are decreased by making the distance with high pressure shorter. In other words the pressure on the inside of the aperture decreases very rapidly with an increasing distance from the aperture. However, the pressure in the aperture 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 for determining 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 a part of said particles to enter the measurement region, a lens system comprising a first end and a second end 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 particle emitting sample, which enter at the first end and to transport the charged particles to the second end. The analyser arrangement also comprises an aperture device according to the first aspect of the invention and any of the features described with reference to the first aspect, attached to the first end of the lens system. The aperture device may or may not be an active part of the lens system. By active is meant 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.

In the following, preferred embodiments of the invention will be described with reference to the appended 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 an embodiment 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. In the photoelectron spectrometer 1 of the hemispherical type, a central component is the measurement region 3 in which the energies of the electrons are analysed. The measurement region 3 is formed by two 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 the region between the hemispheres 5 with a direction close to perpendicular to the base plate 7 are deflected by the electrostatic field, and those electrons having a kinetic energy within a certain range defined by the deflecting field will reach a detector arrangement 9 after having travelled through a half circle. In a typical instrument, the electrons are transported from their source (typically a sample 33 with a sample surface Ss (Fig. 2) that emits electrons after excitation with photons, electrons or other particles) to the entrance 8 of the hemispheres by an electrostatic lens system 13. The lens system 13 comprises an optical axis 15, a first end 36, and a second end 37 at a distance along the optical axis 15 from 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 13 at the first end 36 and to transport the charged particles to the second end 37. The lens system 13 also comprises a plurality of lenses L1-L3 having a common and substantially straight optical axis 15. The photoelectrons 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 of the 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 emitted within 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, the sample 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. The aperture device 31 comprises an end surface wall 40 with an end surface S and an aperture means 39 comprising two apertures 38. The aperture device 31 is attached to the first end 36 of the lens system 13. The aperture device 31 is arranged with the end surface S facing a sample surface Ss of a particle emitting sample 33, which emits particles from a region (Fig. 2), and which is elongated along a first direction a. Fig 3 is a view towards the end surface of the aperture device 31 in Fig. 2. The aperture means 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 exposed to x-rays is. The region 11 which is illuminated with x-rays corresponds to the region on the sample surface Ss from which electrons are emitted. It is to be noted that said region 11 which is exposed to x-rays is on the sample surface Ss and not on the end surface S of the aperture device 39, as may be erroneously construed from Fig. 3. Thus, the aperture device 31 is arranged with the second direction b essentially aligned along the first direction a in order to maximize the number of charged particles that enter the aperture means 39, for this given aperture means 39. It is possible to further increase the number of charged particles that enter the aperture means 39 by altering the shape of the aperture means 39. In the embodiment shown in Fig. 3 the apertures 38 are circular and have a diameter D. The apertures 38 are arranged at a distance x from each other. The ratio between the distance x between the apertures 38 and the diameter D of the apertures should be at least 2 and preferably at least 3. This minimum ratio has been determined to make the so called cross-talk between the apertures so small that it might be ignored according to established theories. The absence of cross-talk here means that the pressure distribution at the sample surface below each aperture is the same as it would be with only one circular aperture in the aperture device 31. The diameter D of each one of the apertures 38 is less than 200 pm, preferably less than 100 pm, and most preferred, less than 50 pm. The diameter D of the aperture should be small to allow the aperture to be placed close to the sample while maintaining a sufficiently high pressure at the sample. According to presently established theories the distance x between the aperture 38 and the sample surface Ss should be kept at twice the diameter D of the aperture to achieve a sufficiently high pressure at the sample surface Ss. The pressure drops at the sample surface Ss, when the sample surface Ss is arranged closer to the aperture 38 than twice the diameter D of the aperture 38. The pressure drop is predictable. Thus, a predictable pressure is achievable for distances d between the sample surface and the aperture 38 being as small as equal to the diameter D of the aperture 38. To achieve a predetermined pressure at the sample surface Ss when the aperture 38 is arranged at a distance d equal to the diameter D of the aperture 38 from the sample surface Ss, the pressure, at a distance d of twice the diameter D, has to be higher than the desired pressure.

The apertures 38 in the embodiment of Fig. 3 are circular. It 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 alternative

embodiment of the present invention. The aperture means 39 in Fig. 4 consists of five circular apertures 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. Fig. 5 is a view from the sample towards the aperture device 31 according to an alternative

embodiment 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 embodiment of the present invention. The aperture means 39 comprises a plurality of apertures 38 in the end surface wall 40. The apertures 38 are distributed in the end surface S along the second direction b as well as in the direction perpendicular to the second direction b. By having the apertures 38 arranged in this way it is 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 as shown in Fig. 2. As can be seen in Fig. 7 each aperture has the form of a truncated cone. The diameter of 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 pm, preferably no more than 80 nm, and most preferred no more than 30 pm. A thicker end wall makes the apertures 38 longer in the direction perpendicular to the end surface. The end surface wall should be as thin as possible, but a thicker wall is less fragile. A thinner wall reduces electron scattering inside the hole. The flow restriction is primarily through the diameter. The pressure will decrease from the end surface S and inwards. Thus, even if a high vacuum is sustained inside the lens the pressure will be higher in the aperture. This will provide a longer path in a high pressure environment for the charged particles to pass. Thus, in order to maximize the number of electrons that pass into the interior of the lens system 13 (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 apertures are essentially circular with a ragged edge. The ragged edge of the apertures has proven to make it more difficult for the gas molecules to enter the apertures 38 and thus, contributes to a lower pressure inside the apertures. The variation D 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

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 the aperture device (31) is to be arranged with the end surface (S) facing a sample surface (Ss) of a particle emitting 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 the sample 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) is attached to the first end of the lens system (13) and the sample surface (Ss) is arranged facing the at least 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 aligned along 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 two apertures (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 a plurality of apertures (38) in the end surface wall (40), wherein the apertures (38) are distributed in the end 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 1.5, preferably at least 2, and most preferred 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 pm, preferably less than 100 pm, and most preferred, less than 50 pm.

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).

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 pm, preferably no more than 80 pm, and most preferred no more than 30 pm.

10. An analyser arrangement (100) for determining at least one parameter related to charged particles emitted from a particle emitting sample (31), comprising:

- a measurement region (3) comprising an entrance (8) allowing at least a part of said particles to enter the measurement region (3);

- a lens system (13) comprising a first end (36) and a second end (37) arranged at the entrance of the measurement region (3) at a distance from the first end (36), wherein the lens system (13) is arranged to form a particle beam from charged particles, emitted from a sample surface (Ss) of a particle emitting sample (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).