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SPHERICAL ABERRATION ADJUSTMENT CATHODE LENS, SPHERICAL ABERRATION CORRECTION ELECTROSTATIC LENS, ELECTRON SPECTROSCOPE, AND PHOTOEMISSION ELECTRON MICROSCOPE

Foreign code F210010491
File No. (S2020-0105-N0)
Posted date 2021年7月29日
Country 世界知的所有権機関(WIPO)
International application number 2020JP047288
International publication number WO 2021125297
Date of international filing 令和2年12月17日(2020.12.17)
Date of international publication 令和3年6月24日(2021.6.24)
Priority data
  • 特願2019-227788 (2019.12.17) JP
Title SPHERICAL ABERRATION ADJUSTMENT CATHODE LENS, SPHERICAL ABERRATION CORRECTION ELECTROSTATIC LENS, ELECTRON SPECTROSCOPE, AND PHOTOEMISSION ELECTRON MICROSCOPE
Abstract Provided is an electrostatic lens that allows an uptake angle of approximately ±90° or a very large uptake angle over a wide energy range from 0 to 2 keV. In a cathode lens 19 configured of a point source electrode 12 in which a point source 11 is arranged on a lens axis and a draw-in electrode 13 for drawing in charged particles generated from the point source 11, the point source electrode 12 is a planar electrode perpendicular to the lens axis (or such a planar electrode additionally having a convex portion axially symmetrical with respect to the lens axis direction in which the point source is arranged in the upper center of the convex portion), the draw-in electrode 13 has a protruding wall portion 13b that protrudes axially symmetrically in the axial direction from a base surface 13a, which is perpendicular to the lens axis, and that has an opening surface formed at the end portion thereof, and a planar grid portion 16 is formed at the opening surface, or an opening surface is formed at a tapered protruding wall portion extending from the end portion of the protruding wall portion and expanding outward and at the expanded end portion.
Outline of related art and contending technology BACKGROUND ART
In electron spectrometers, sensitivity along with energy resolution is one of the most important performance. In the measurement of photoelectrons or Auger electrons, in a case where the signal is weak and largely buried in noise, the retention time must be greatly increased in order to obtain a sufficient SN ratio (Signal to Noise ratio). However, not only does this prevent efficient measurement, but there is less a need to limit the continuous measurement time due to time constraints such as the use time of the radiation facility and the duration of the excitation light source. In addition, for samples prone to radiation damage or samples prone to aging, such as organic materials, measurements for long periods of time are prevented, and in many cases, weak signals are not sufficiently captured. In the research of new semiconductor materials and superconducting materials, it is known that advanced doping techniques are used, and even very small amounts of dopants can lead to significant changes in the material. Capturing weak signals from such dopants is critical in the development of new materials.
In addition, in research on polycrystalline materials, multimagnetic domain materials, and the like in which a large number of micro, nano devices, and micro domains are formed, micro region analysis using a micro probe or a limited visual field aperture becomes important. However, when an attempt is made to measure a smaller region, a decrease in signal intensity cannot be avoided. As a result, in the development of new electron spectroscopy devices, it is an important point to improve sensitivity.
In an electron spectrometer, in addition to measuring the energy distribution of electrons emitted from a sample, the emission angle distribution can be measured. When measuring the energy distribution, information on the composition of the elements is obtained, and when measuring the emission angle distribution, composition information in the depth direction and electronic state information are obtained. In addition, because the momentum in the in-plane direction of the sample is preserved in the photoelectron emission process, information on the momentum of electrons in the substance can be obtained by measuring the kinetic energy and emission angle of the photoelectron. By irradiating a sample with ultraviolet rays or X rays, adjusting the energy to the valence band, and measuring the kinetic energy and the emission angle distribution of the photoelectrons, the energy band structure of the substance can be evaluated, and the properties of the substance can be substantially determined. Furthermore, in the emission of photoelectrons from the inner shell, when the kinetic energy becomes hundreds of eV or more, a strong peak called a forward convergence peak appears in the direction connecting the photoelectron emitting atom and the scattering atom surrounding the photoelectron emitting atom. By measuring this peak over a wide range of angles, it is possible to directly capture the appearance of the atomic arrangement around a specific atom. It is also possible to determine the interatomic distance from the diameter of the diffraction ring formed around the forward convergent peak. As described above, the measurement of the emission angle distribution using an electron spectrometer makes it possible to obtain detailed information on the atomic level that is difficult with other analytical techniques, making it very powerful in developing new materials and studying unknown physical property mechanisms.
Presently, although the emission angle distribution is measured by various methods, electrostatic hemispherical analyzers that capture and analyze electrons in a specific direction have been widely used in the evidence from the 1960 s to the 1990 s, when "high-resolution photoelectron spectroscopy" has been established by kya-SEGERN-et al. While this electrostatic hemispherical analyzer can achieve high energy resolution, measurement of the emission angle distribution is performed while rotating the sample or the analyzer little by little. therefore, it takes a great deal of time to acquire data over a wide solid angle, and it has been difficult to use a wide region of inverse lattice space as a measurement target.
In addition, "angle-resolved photoelectron spectroscopy" has been developed using two different approaches from around 1990. One way is to introduce a new technique in the development flow of "high-resolution photoelectron spectroscopy" described above, and another way is to develop a new method focusing on the angular distribution measurement.
Fig. 16 is a schematic diagram of an angularly resolved electron spectrometer developed under the former concept. The basic part of the angularly resolved electron spectrometer 100 includes an input lens 102 that takes in and converges electrons emitted from the sample 101, an electrostatic hemispherical portion (CHA) 103 including an inner sphere and an outer sphere, a slit 104 provided at an inlet of the electrostatic hemispherical portion 103, and a detector 105 provided at an outlet of the electrostatic hemispherical portion 103. The energy resolution is adjusted by changing both or either of the deceleration ratio and the slit width of the input lens. The collection can include a "microspectral mode" (Transmission mode in which two-dimensional location-dependent information of the energy spectrum is obtained at a time; Also referred to as Spatial mode), and an "angularly resolved spectral mode" (Diffraction mode, also referred to as Angular mode) in which two-dimensional information of the angular distribution of the energy spectrum is obtained at once, are used.
The switching between the microspectral mode and the angularly resolved spectral mode is done by the lens voltage. The lens voltage is adjusted so that the image plane is at the slit position in the microspectral mode, while the diffraction plane (angle distribution) is at the slit position in the angularly resolved spectral mode. In the angularly resolved spectral mode, at the exit of the electrostatic hemisphere 103, an energy dispersion is formed in a direction connecting the entrance and exit, and a one-dimensional angular distribution is formed in a direction perpendicular thereto. Here, the angular range that can be measured at a time depends on the entrance angle of the input lens. Electron spectroscopy apparatuses having the basic configuration described above have been developed, sold, and widely used by several manufacturers. however, an apparatus using a normal input lens has a large spherical aberration, which makes it impossible to measure a wide range of emission angle distributions at once.
On the other hand, as a method of focusing on angularly resolved measurements, electron spectroscopy apparatuses that measure two-dimensional emission angle distributions at once have been developed from around 1990. Fig. 17 is a schematic diagram of a two-dimensional spherical mirror analyzer (DIANA: Display-type Spherical Mirror Analyzer) developed by Ohigai et al. (NpL 1). In DIANA110, electrons emitted from the sample 112 as a result of irradiation with radiated light (SR) light source, light from an UV lamp, or an electron beam from an electron gun 111 are transmitted through the hemispherical grid 113 and the ring-shaped electrode (114, 115), draw an elliptical orbit therein, pass through the hemispherical grid 113 again when the orientation is changed by 180 °, and exit out of the electric field to converge to the position of the aperture 116. The ring-shaped electrode (114,115) is provided with the function of forming a spherically symmetric electric field and simultaneously with the function of blocking electrons having a higher energy than the pass energy. Electrons having a lower energy than the pass energy are partially passed through the aperture 116 as a result of the spherically symmetric electric field being reversed in orientation. Therefore, a high pass filter for blocking electrons having energy lower than the pass energy is provided in front of the screen 117. As illustrated in Fig. 17, the high pass filter includes a plurality of grids including a blocking potential grid 118. Only electrons having a pass energy and a nearby energy can pass through the high pass filter, and a two-dimensional emission angle distribution in which the energy is selected is displayed on the screen 117. The DIANA has an uptake angle of ± 50 ° or ± 60 ° and is characterized by being able to measure the two-dimensional release angle distribution in one degree without distortion over its large solid angle. The practical strength was demonstrated in Non-Patent Documents 2 and 3) and in Non-Patent Documents 4 and 5). Here, the configured pass energy ranges from a few 10 eV to 1 keV. There were no analyzers capable of measuring a ± 50 ° emission angle distribution at once in this broad energy range, long in addition to the DIANA.
However, DIANA has a problem in that the energy resolution is not very good at about 0.5% of the pass energy. Atomic structure analysis such as photoelectron holographic or stereoscopic atomic photograph is performed by setting the kinetic energy of the photoelectron to approximately 500~ 1000 eV, but in this case, the energy resolution is approximately a few eV~ 5 eV. At an energy resolution of approximately several eV~ 5 eV, it is difficult to separate and distinguish electronic states with close energy peaks such as chemical shifts and spin orbital splits.
Therefore, a new two-dimensional electronic analyzer (DELMA: Display-type ellipsoidal mesh analyzer) has been developed (NpL 6). Fig. 18 shows a schematic diagram of a two-dimensional electronic analyzer (DELMA). In the DELMA120, a WAAEL (Wide-acceptance-angle electrostatic lens), which is a wide angle electrostatic type lens 121, is newly developed and used as an objective lens. At the inlet of the WAAEL is provided with an ellipsoidal spheroidal mesh electrode 122, which achieves an uptake angle of ± 45 °. Unlike DIANA110 in Fig. 17, in DELMA120, a basic portion includes a lens, and a plurality of Einzel lenses 123 are disposed after the wide angle electrostatic type lens 121 (WAAEL). The energy analysis is performed by inserting the aperture 124 at the exit location of the wide angle electrostatic lens 121 (WAAEL). The possible energy resolution in this case is approximately 0.3%.
In DELMA120, measurements are performed in a "microimaging mode" that can obtain a two-dimensional real space image of a sample and in an "angle distribution mode" that can obtain two-dimensional emission angle distribution information; In the microimaging mode, a magnified image of the sample is projected onto the screen of detector 129, and in the angular distribution mode, a two-dimensional emission angular distribution spanning ± 45 ° is projected onto the screen of detector 129. By observing the magnified image of the sample in the microimaging mode, aligning the region to be examined with the optical axis, inserting the aperture (124,127) for the purpose of limiting the field of view, and switching to the angular distribution mode, it is possible to measure the two-dimensional emission angular distribution from a specific microregion. As shown in Fig. 18 (2), the DELMA is combined with the electrostatic hemisphere (CHA) 125, similar to the angularly resolved electron spectrometer shown in Fig. 16, allowing for high energy resolution measurements. The electrostatic hemisphere (CHA) 125 is combined so that its sample plane coincides with the detector entrance plane 128 of the DELMA120. When measuring with high energy resolution, the detector of the DELMA is retracted and the two-dimensional emission angle distribution is incident on the input lens of the CHA. At this time, by forming slits (not illustrated) in the inlet of the CHA, one-dimensional angular distribution in which energy is resolved can be obtained in the detector (not illustrated) provided at the outlet of the CHA. The DELMA120 lens system is provided with an electrostatic deflector 126, and it is possible to obtain a two-dimensional emission angle distribution with high energy resolution by placing the two-dimensional emission angle distribution into the CHA while shaking it, obtaining a large number of one-dimensional angle distributions, and synthesizing them. A method for obtaining a two-dimensional emission angle distribution by scanning using the electrostatic deflector 126 as described above has also been introduced into the angularly resolved electron spectrometer illustrated in Fig. 16, and is now widely used.
Fig. 19 (1) (2) illustrates a spherical aberration correction electrostatic type lens described in PTL 1 and NpL 7, and Fig. 20 (1) (2) illustrates a spherical aberration correction deceleration type lens described in PTL 2 and NpL 8. The spherical aberration correction electrostatic type lens illustrated in Fig. 19 (1) (2) is an Eintzel type lens using an ellipsoidal mesh electrode, and electrons having the same kinetic energy as when exiting the sample are obtained at the exit. The spherical aberration correction slowing down lens shown in Fig. 20 (1) (2) is a slowing down lens using an ellipsoidal mesh electrode, and electrons of about 1/5 kinetic energy when exiting the sample are obtained at the exit. As shown in FIGS. 19 (2) and 20 (2), calculations show that both lenses are able to correct spherical aberration over capture angles of up to ± 60 °. Each of the ellipsoidal mesh electrodes has a partial shape of an ellipsoid of revolution with the major axis of the ellipse as the axis of revolution, and occupies approximately half of the ellipsoid of revolution when the capturing angle is ± 50 ° as illustrated in FIGS. 19 (1) and 20 (1), and more than half of the ellipsoid of revolution when the capturing angle is about ± 55 °~ ± 60 ° as illustrated in FIGS. 19 (2) and 20 (2). It is difficult to manufacture such elliptical mesh electrodes with narrow inlets with high definition, high transmittance, and high precision. Moreover, as illustrated in FIGS. 19 (1) and 20 (1), even with an ellipsoid mesh electrode in which approximately half of the spheroid has a mesh shape, it is not easy to manufacture a mesh having a large ratio (d/R) between the mesh depth d and the opening radius R of the mesh electrode illustrated in Fig. 19 (3) with high definition, high transmittance, and high precision. In the lenses shown in FIGS. 19 (1) (2) and 20 (1) (2), the ratio (d/R) of the mesh depth d to the opening radius R of the mesh electrode is 1.5~ 2.4.
The spherical aberration correction electrostatic type lens illustrated in Fig. 19 is used in the two-dimensional electronic analyzer DELMA120 illustrated in Fig. 18. However, the uptake angle is set to ± 45 °. A design with an uptake angle of ± 50 ° or ± 60 ° reduces the space around the sample and significantly limits the angle of incidence of the irradiation beam irradiating the sample. When the angle of the irradiation beam from the sample surface is small, the irradiation spot has a shape elongated in the incident direction. In order to obtain high energy resolution and high sensitivity, it is desirable that the irradiation spot be as small as possible. At DELMA, when the sample is directly opposite the lens, the angle of the irradiation beam from the sample plane is 15 °.
We have already developed a parallel beam two-dimensional electronic analyzer with a wide capture angle (see Patent Document 3). Fig. 21 is a schematic diagram of a collimated beam two-dimensional electronic analyzer. The basic part of the collimated beam two-dimensional electronic analyzer 130 consists of an ellipsoidal mesh electrode 132, a plurality of axisymmetric electrodes 133 a~ 133 e, a planar collimator electrode 134, and a detector 135. Electrons emitted from the sample 131 are taken up into the analyzer, collimated over an uptake angle of ± 60 °, and incident on the planar collimator electrode 134. The planar collimator electrode 134 is a plate or a thin film having myriad long holes. when the walls and surfaces of the holes are coated with an electron absorbing material such as graphite, only the pass energy and electrons in the vicinity of the pass energy pass through the planar collimator electrode 134, and the other electrons collide with the planar collimator electrode 134 and are absorbed by the electron absorbing material. Thus, energy analysis is possible, but the energy resolution and the sensitivity are in a countered relationship, and there is a problem in that if an attempt is made to increase the aspect ratio of the holes in the planar collimator electrode 134 to obtain high energy resolution, a significant decrease in detection sensitivity cannot be avoided.
On the other hand, since approximately 2000, analyzers that combine an energy filter with a photoelectron microscope (PEEM: Photoemission Electron microscopy) have been developed. Fig. 22 is a schematic configuration diagram of an analyzer combining a PEEM and a high pass filter (NpL 9). A scaled-up image or a two-dimensional emission angle distribution of the sample 142 is projected onto the screen 141 of the analyzer 140, depending on the lens voltage. Mapping of the Fermi surface can be performed by providing a high pass filter. Furthermore, analyzers combining an electrostatic hemispherical analyzer with a lens system of PEEM have been developed, making it possible to efficiently perform ARPES (Angle Resolved Photoelectron Spectroscopy) measurements with high energy resolution. In both these analyzers and similar analyzers using the technique described in PEEM, a high voltage of several 10 kV is applied between the sample 142 directly facing each other and the inlet of the objective lens 143, and electrons emitted from the sample are accelerated by a strong acceleration electric field at the same time as the electron is emitted and then incorporated into the objective lens 143. A large uptake angle has thus been achieved. Specifically, for low energy electrons of a few 10 eV or less, a capture angle of nearly ± 90 ° is achieved. However, due to problems with the power supply and withstand voltage, it is difficult to further increase the acceleration voltage from several 10 kV. As a result, there is a problem in that for electrons of several 10 eV or more, the uptake angle decreases significantly as the kinetic energy increases.
Fig. 23 is a simple calculation result for explaining this. Shows the electron trajectory when a uniform accelerating electric field is applied between the sample surface and the objective lens inlet. The dotted line is a virtual image line taken from the objective lens inlet. Fig. 23 (1) is a result when the ratio (eUa/E0) of the accelerating voltage Ua multiplied by the charge elemental amount e to the kinetic energy E0 of electrons at the sample surface is 1000, for example, when E0 is 20 eV with respect to the accelerating voltage 20 kV. In this case, it can be seen that the virtual line converges to almost one point. In addition, it can be seen that the exit angle on the sample surface is greatly reduced on the virtual image surface. As a result, the spherical aberration due to the objective lens is significantly reduced, and a very large capture angle is achieved. Fig. 23 (2) is a result when the ratio (eUa/E0) is 10, for example, when E0 with respect to the accelerating voltage 20 kV is 2 keV. In this case, a large spherical aberration occurs in the virtual image surface. It can also be seen that the incident angle on the objective lens significantly increases compared to the case in Fig. 23 (1). As a result, large spherical aberration occurs even in the objective lens in combination with the cathode lens, and the uptake angle is significantly reduced.
  • Applicant
  • ※All designated countries except for US in the data before July 2012
  • INTER-UNIVERSITY RESEARCH INSTITUTE CORPORATION NATIONAL INSTITUTES OF NATURAL SCIENCES
  • Inventor
  • MATSUDA, Hiroyuki
  • MATSUI, Fumihiko
IPC(International Patent Classification)
Specified countries National States: AE AG AL AM AO AT AU AZ BA BB BG BH BN BR BW BY BZ CA CH CL CN CO CR CU CZ DE DJ DK DM DO DZ EC EE EG ES FI GB GD GE GH GM GT HN HR HU ID IL IN IR IS IT JO JP KE KG KH KN KP KR KW KZ LA LC LK LR LS LU LY MA MD ME MG MK MN MW MX MY MZ NA NG NI NO NZ OM PA PE PG PH PL PT QA RO RS RU RW SA SC SD SE SG SK SL ST SV SY TH TJ TM TN TR TT TZ UA UG US UZ VC VN WS ZA ZM ZW
ARIPO: BW GH GM KE LR LS MW MZ NA RW SD SL SZ TZ UG ZM ZW
EAPO: AM AZ BY KG KZ RU TJ TM
EPO: AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR
OAPI: BF BJ CF CG CI CM GA GN GQ GW KM ML MR NE SN ST TD TG

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