This application claims the right of foreign priority to Japanese Patent Application No. 2015-096104, filed May 8, 2015 by at least one common inventor, and to Japanese Patent Application No. 2015-130414, filed Jun. 29, 2015 by at least one common inventor, both of which are incorporated herein by reference in their respective entireties.
The present invention relates to a hard X-ray photoelectron spectroscopy apparatus. More particularly, the invention relates to configuration of an analyzer, a sample, and an X-ray source in a laboratory hard X-ray photoelectron spectroscopy apparatus.
An Electron is emitted by irradiation of high-energy light to a substance.
Orbits of groups of electrons orbiting with the shallowest energies have large radii and thus overlap electron orbits of the nearest neighbor atoms. This overlap provides the bonding strength, allowing the atoms to bond to become a solid. When a vacuum level (the minimum energy when a free electron is placed in a vacuum) is set to a point of origin, the electron with higher energy will be set free from restraint of atomic nucleus. Sufficiently high energy is given to the bound electron from outside by high energy photon irradiation, thereby enabling an electron wave to become free and be released from the solid surface. This is called photoelectron emission.
As shown in
The emitted electrons are analyzed by an energy analyzer (analyzer) (1) to obtain a spectrum reflecting the densities of bound states (left) of the electron inside the solid, which shows the electronic structure inside the sample solid and its chemical bonding property, as schematically indicated on the right side of the figure. A method of analyzing the electronic states and the chemical states using this is called photoelectron spectroscopy. High-resolution measurement using synchrotron radiation X-ray has been recently performed all over the world. A laboratory apparatus using AlK α rays (1.49 keV) as an excitation source also has been commercially available and very widely used for not only research and development but also as chemical analyses.
Thus, photoelectron spectroscopy is a very useful analytical method and has been widely used, but has a big problem. When a solid is irradiated with an X-ray, the X-ray penetrates inside of the solid to generate a photoelectron. If the photoelectron is generated at a position shallow from the solid surface, it can be released from the surface as it is without being disturbed, as shown in
If energy of the photons used for excitation of photoelectron is higher, the kinetic energy of the photoelectron will become larger, and accordingly the mean free path of the photoelectron will also become larger, as seen in
Hard X-ray photoelectron spectroscopy using synchrotron radiation is now popular to many users as a very powerful means for research and analysis of substances, but the competitive rates for obtaining a beam time are very high and application chances of the experimental proposals are opened only twice a year. Additionally, the experiment can be performed only in synchrotron radiation facilities. Therefore, a hard-X-ray photoelectron spectroscopy available in laboratories has been strongly needed.
Simulation result shows that when the irradiation spot to the stationary water-cooled Cr target is set to 100 microns and electron beam irradiation output exceeds 50W, sublimation of Cr will be nonnegligible. In order to increase any further electron beam output, it is necessary to use a water-cooled target rotating at high speed. In a configuration where an X-ray source is contained in an analysis chamber, incorporating the rotary target would impose great restrictions on space and mechanism, and is thus impossible. Additionally, incorporating X-rays into a monochromater crystal assembly at as large solid angle of X-ray acceptance as possible would be necessary when the X-ray is dispersed and focused on the sample. This also cannot be significantly improved due to spatial restrictions in a configuration where the X-ray source (40) mounted to a flange (50) is contained in the analysis chamber (14). Recently, experimental methods to raise the pressure in an analytical chamber to almost atmospheric pressure and observe the photoelectron spectra under controlled atmosphere, so called NAP (Near Ambient Pressure photoelectron spectroscopy) or HiPP (High Pressure Photoelectron spectroscopy) have been actively used. In this measuring method, gas is introduced into the analytical chamber, and thus X-ray source in
As a result of keen examination, the inventors have successfully solved conventional problems by solving the above problems (size of the X-ray source (i.e., size of X-ray monochromater crystals) is restricted due to size of the analysis chamber, and the vacuum of X-ray source and the vacuum of the analysis chamber cannot be separated).
The present invention aims to solve the above problems (size of the X-ray source (i.e., size of X-ray monochromater crystals) is restricted due to size of the analysis chamber, and the vacuum of X-ray source and the vacuum of the analysis chamber cannot be separated).
The hard X-ray photoelectron spectroscopy apparatus according to an aspect of the present invention comprises, in the light of the attached claims, an X-ray source (3), an analyzer (6), a sample manipulator (2), an analysis chamber (14), and vacuum evacuation systems, wherein, in a three dimensional space defined by a XYZ rectangular coordinate axis system where Z axis is defined as a direction parallel to the plane of a plate-like sample (5) and X axis and Y axis are defined as directions perpendicular to that direction, the sample (5) is arranged to be rotatable around the Z axis by the above-described sample manipulator (2) or be rotated at predetermined angle by a jig,
the above-described X-ray source (3) comprises;
an electron gun (3b) which accelerates and further focuses electrons,
a target (7) which is irradiated with the electrons accelerated and focused by the above-described focusing electron gun (3b) on to generate an X-ray,
a monochromater crystal assembly (9), wherein the monochromater crystal assembly meets the Bragg condition of X-ray diffraction in X-Y plane to diffract/reflect and monochromatize the X-ray generated in the above-described target (7) and extract characteristic X-rays only, and on the other hand, the electron-beam-irradiation position on the target (7), the center of the monochromater crystal assembly (9), and center of the sample (5) are arranged on a Rowland circle (see
a vacuum chamber (14) for installing these components,
wherein the monochromater crystal assembly (9) used for monochromatization with diffraction and reflection of the above-described X-ray source (3) is located on the Rowland circle (see
wherein the above-described Rowland circle (see
Referring to
In the present invention, X-ray source conceptually comprises the target (7), the electron gun (3b), the monochromater crystal assembly (9), and the vacuum chamber (14).
The above-described X-ray source (3) comprises;
the electron gun (3b) contained in a vacuum vessel (3a) which accelerates and further focuses electrons,
the target (7) contained in a vacuum vessel (7a) which is irradiated with the electrons accelerated and focused by the above-described focusing electron gun (3b) to generate an X-ray, the monochromater crystal assembly (9) having a toroidal surface configured to be an ellipse in X-Y plane having the center of the target (7) and the center of the sample (5) as two of the focuses and a spherical surface in Z axial direction which is obtained by rotation of the above mentioned ellipse around a line connecting the center of the target (7) and the center of the sample (5), in order to diffract/reflect and monochromatize the X-ray beam (103) generated in the above-described target (7), and extract characteristic X-rays beam (105) only, and, a vacuum vessel (10) for installing these components, wherein the above-described analyzer (6) is such that its optical axis (11) is arranged perpendicular to the incident direction of X-ray (namely, X axial direction in
As mentioned later for the above-described configuration, it is possible to maximize the yield of photoelectron emission and the photoelectron collection efficiency in the analyzer (6). Actually, as described later, considering anisotropy of the photoelectron emission by unpolarized light X-ray irradiation generated by electron beam irradiation and attenuation due to inelastic scattering of the photoelectron within a sample, if the analyzer (6) is placed within a range of ±36 degree angle around the x axis direction in a X-Y plane and within a range of ±49 degree angle in a X-Z plane, it can secure about 65% of the photoelectron signal intensity of that the optimum configuration can secure, and thus can withstand practical use.
It is also possible to analyze depth direction based on a dependency of the photoelectron signal intensity on take off angle from the sample surface. When this approach is employed, it is possible to measure the take off angle dependence of the photoelectron intensity by placing the analyzer (6) in the optimal configuration, i.e., settling an incident direction of the X-ray to Y-axis, the optical axis of the analyzer (6) to the X direction, and the aperture of the entrance slit (6S) of the analyzer (6) parallel to Y axial direction, arranging the sample (5) at a few angle from the Y-axis so that the X-ray is obliquely incident on the surface of the sample (5), further adding a Y′ axis perpendicular to Z axis and parallel to the surface of the sample (5), rotating the sample (5) around the Y′ axis with respect to the optical axis of the analyzer (6) without substantially changing the shape of the irradiation area of the X-ray extending in parallel to the Y axis on the sample (5), thereby protecting decrease in the photoelectron collection efficiency of the analyzer (6) due to the sample rotation.
The above-described target (7) is preferably a Cr target, and Ag and Ti can also be chosen as a target when AgL α rays (2.98 keV) or TiK α rays (4.51 keV) are used.
The above-described monochromater crystal assembly (9) preferably consists of a crystal selected from a group consisting of ionic crystals such as LiF and NaCl, quartz, and semiconductors of Ge, Si, or GaAs.
When the CrK α rays are used, a reflection plane of the above-described monochromater crystal assembly (9) may be preferably a Ge422 reflection plane or a LiF222 reflection plane.
It is preferable to accelerate an electron to 20-30 keV and focus the electron beam to about spot of 100 micrometers or less with the above-described electron gun.
The hard X-ray photoelectron spectroscopy apparatus according to second aspect of the present invention is configured such that, the above-described analysis chamber (14) and the above-described X-ray source (3) are integrated, an analysis chamber part (14a) and the X-ray source (3a) are arranged in the same structure (20), a vacuum area of the analysis chamber part and the X-ray source is divided by a partition (12), the X-ray is guided through the X-ray window (13) provided at the partition (12) to the analysis chamber (14).
Additionally, the hard X-ray photoelectron spectroscopy apparatus according to third aspect of the present invention is configured such that, the X-ray source (3) is separated from the analysis chamber (14) in a vacuum, the target (7) is used as a rotary anticathode, making the best of the advantage that the target (7) can be placed outside the analysis chamber (14), and is then excited with a high-output focusing electron gun (3a), so that the intensity and the density of the X-ray flux will be higher than those obtained by using a stationary target (7) by a digit.
There has been two big problems in the above-described prior art, which is: size of the X-ray source (i.e., size of X-ray monochromater crystals) and size of the mechanism of a target part) is restricted due to size of the analysis chamber (6) and higher output is limited, accordingly; the vacuum of the X-ray source and the vacuum of the analysis chamber cannot be separated. In an experimental method, called NAP (Near Ambient Pressure photoelectron spectroscopy) or HiPP (High Pressure Photoelectron spectroscopy), gas is introduced into the analytical chamber and thus X-ray source (3) in
However, the hard X-ray photoelectron spectroscopy apparatus according to the first aspect of the present invention comprises a configuration having the X-ray source and the analysis chamber separated, and thus can solve all of these problems. In the hard X-ray photoelectron spectroscopy, photoionization cross sections are smaller than those obtained in the conventional photoelectron spectroscopy, and thus it is also indispensable to practically increase the photoelectron collection efficiency as much as possible. Using the hard X-ray photoelectron spectroscopy apparatus, according to the first aspect of the present invention, it is possible not only to separate the vacuum of the X-ray source and that of the analysis chamber, but also to maximize the photoelectron collection efficiency.
The hard X-ray photoelectron spectroscopy apparatus according to the second aspect of the present invention is configured such that the above-described analysis chamber (14) and the above-described X-ray source (3) are integrated, the analysis chamber part (14a) and the X-ray source (3) are arranged in the same structure, and a vacuum area of the analysis chamber part (14a) and the X-ray source (3) is divided by the partition (12), so that it has remarkable effects that enable downsizing of the whole apparatus, separation of the vacuum areas of X-ray source (3) and the analysis chamber part (14a) (photoelectron analysis part), and maximization of the photoelectron intensity. Furthermore, it is possible to realize a configuration which can be used by changing two X-rays having different energy by applying another invention “X-ray generator and analyzer” (U.S. Pat. No. 5,550,082, inventors: KOBAYASHI, Keisuke; YAMAZUI, Hiromichi, IWAI, Hideo, OBATA, Masaaki).
According to the hard X-ray photoelectron spectroscopy apparatus according to the third aspect of the present invention, in a structure where the monochromater crystal assembly (9), the electron gun (3b) and the target (7) are separated from the vacuum area of the analysis chamber (14) and placed outside, it is possible to excite a high-speed rotating water-cooled target (7) with a high-output focusing electron gun (3b) and to realize a monochromatized CrK α-ray source with the flux intensity and flux density which are 10 times or more of an output obtained by using the stationary target (7).
The CrKα-ray has a wide bandwidth of about 2-3 eV, and a plurality of different specific emission lines such as Kβ-ray are included in closer energies. Also, the Bremsstrahlung X-ray extends to a high energy area and thus it cannot be used for an excitation source of photoelectron spectroscopy as it is. Thus, it is necessary to be monochromatized by X-ray diffraction with a single crystal. While the diffraction can be achieved with various kinds of crystals, an angle of diffraction (20) closer to 180 degrees is advantageous since the energy width of the monochromatized X-ray becomes wider as incident and reflection directions are separated away from the normal direction of the crystal surface. Also, it is necessary to increase the size of the monochromater crystals in order to obtain as many monochromatized X-ray fluxes as possible. Moreover, considering the performance of the spectroscopy, a good crystal with little defect and distortion is required to reduce the spectroscopic resolution.
The selection of the monochromater crystals which meet the above conditions is limited. As a practical problem, crystals which are commercially available, have good crystallinities, enable large area wafer polishing, and are stable, are only ionic crystals such as LiF and NaCl, semiconductors such as Ge, Si, GaAs, and InSb and quartz, and oxides such as ZnO. Furthermore, there is a relationship among band width LE, a diffraction angle θ of the monochromatized X-ray, radius R in Rowland circle (C) (see FIG. 9 described later), and size x of a diffraction direction of a crystal, as shown in formula below.
x=√{square root over (ΔE/E)}×√{square root over (2)}×R tan θ [Formula 1]
Wherein, E is energy of a photon of the X-ray, and ΔE is a band width of the monochromatized X-rays. It is necessary to increase the size of the crystal in order to obtain as large X-ray flux dispersed as possible. For this purpose, it is advantageous to utilize diffraction reflection having as a large diffraction angle as possible. Under this condition, in the case of the CrKα-ray, Ge422 reflection (2θ=165.35 degrees) or LiF222 reflection (2θ=162.05 degrees) is appropriate. LiF is difficult to be handled since it has deliquescency. Thus, the inventors decided to use Ge.
A monochromater crystal assembly (9) is manufactured such that, a glass substrate has a toroidal surface polished so that an ellipse focusing on a position of the target (7) and a position of the sample (5) comes into contact with Rowland circle (C) satisfying Rowland conditions (see
In the case of normal incidence with a CrKα X-ray, photoelectrons are excited in an area, 10 μm deep from the surface of the sample.
However, among them, photoelectrons only in an area, about 10 nm deep from the surface of the sample (5), can escape from the surface to produce photoelectron spectrum without scattering. Therefore, most of the X-rays become useless. In order to avoid this situation, it is necessary to adopt a configuration where X-rays are preferably obliquely incident to the surface of the sample (5) and absorbed in a region as close as possible to this surface (see
Furthermore, it is necessary to take anisotropy of emission intensity of the photoelectron into consideration. Since, in the hard X-ray photoelectron spectroscopy, consequently energies of the X-ray photoelectrons are high, the anisotropy patterns in angular intensity distribution are different from those in the conventional spectroscopy (see
In the hard X-ray photoelectron spectroscopy, the s orbital states contribute to a spectrum the most. When an angle between an incident direction of the X-rays and an emission direction of the photoelectron is set as θ, a photoionization cross section to determine photoelectron intensity reaches to the maximum in a direction vertical to the incident direction of the X-ray (θ=90 degrees), as illustrated in
In the case of the X-ray of unpolarized light, with this configuration, in an X-Z plane determined by the X-axis and the axis (Z-axis) included in the surface of the sample (5) vertically to an incident direction of the X-ray (Y-axis), a photoionization cross section has no angular dependence. However, if the analyzer (6) is inclined from the surface of the sample (6) by angle φ, attenuation due to inelastic scattering becomes large, so that intensity of the photoelectron collected by the analyzer (6) has angular dependence indicated in
As described in detail above, the larger a take off angle of the photoelectrons from the sample (5) (measured from a direction perpendicular to the sample surface) becomes, the more photoelectron intensity is attenuated, and there is a technique utilizing this effect to analyze a depth profiles of compositions and chemical bonding states of the sample from the take off angle dependence of photoelectron intensity. In the case where the sample is rotated around the Z-axis by an application of this technique to the laboratory hard X-ray photoelectron spectroscopy, the conditions for oblique incidence of the X-ray into the sample (5) are broken, leading to extreme attenuation of signal intensities along with the escape angle. This problem can be relieved by changing the take off angle with an axis (Y′-axis) which rotates the sample being provided in a longitudinal direction of an elongated footprint of X-rays on the sample.
With a laboratory X-ray light source, the X-rays emitted from the target become spread according to a cosine rule. In order to take in this largely spreading X-rays as much as possible, the size of the monochromater crystal assembly in a direction vertical to an energy dispersion direction is increased as much as possible. In practical, at 730 mm in diameter of the Rowland circle (C) (see
Also, the laboratory X-ray source is unpolarized light, and thus, when we attempt to satisfy these conditions in a laboratory, an optimal relative spatial configuration of an X-ray source (3)-analyzer (6)-sample (5) is determined uniquely (see
Since a cylinder type of the analysis chamber as shown in
With this shape, the purpose of taking in as many X-ray fluxes as possible is achieved (it is because that, if the crystal is enlarged to the energy dispersion direction, monochromaticity of the X-ray becomes deteriorated, and thus the crystal only in the direction vertical to the energy dispersion direction is enlarged). If sizes of the both crystals are decreased at the sacrifice of X-ray flux, this configuration is no longer limited, but the practicality will be decreased. Even if LiF222 is used instead of Ge422, a relative configuration will be uniquely determined in a similar manner. There are other characteristic X-rays which can be used for hard X-ray photoelectron spectroscopy, such as Ag—Lα rays (2.98 keV) and Ti-Kα rays (4.51 keV) as well as a CrKα-ray. However, relatively considering the obtained band width of the X-ray and intensity of the X-ray beam, etc., CrKα-ray has the highest practicality when combined with the monochromater crystals.
Another embodiment will be explained below which satisfies the conditions for the above-mentioned spatial configurations.
The inventors adopts in this embodiment the knowledge relating to another invention “X-Ray Generator and Analyzer” (U.S. Pat. No. 5,550,082; Inventors: KOBAYASHI, Keisuke, YAMAZUI, Hiromichi, IWAI, Hideo, and KOBATA, Masaaki), in which a configuration of the double-ray source switching and utilizing an AlKα-ray and CrKα-ray is suggested.
The target (7) is irradiated with an electron beam by the electron gun (3b) to generate an X-ray. There is an area coated with Al and Cr on the substrate of the target (7). The target (7) is linearly movable in a direction, allowing for a to-be-irradiated area with an electron beam to be selected for Al or Cr coated part. This allows for the AlKα-ray or CrKα-ray to be selected and generated. Each X-ray is monochromatized by monochromater crystals for AlKα-ray (9a) or monochromater crystals for CrKα-ray (9b) and arranged to be obliquely incident on the surface of the sample (5). In this case, a Rowland circle of a spectroscope for AlKα-ray and CrKα-ray is designed to intersect at the same two points, i.e., a position of the target (7) and a position of the sample (5). Thus, whichever X-rays are selected, the focusing position of the X-rays does not change and it is not necessary to readjust the positions of the sample (5) and the analyzer (6) (see
In order to efficiently receive photoelectrons emitted from this elongated area with the analyzer, the aperture (107) of the entrance slit (6S) of the analyzer (6) enables the analyzer (6) (see
According to the embodiment 2, superior effects can be achieved which satisfy all of the followings; to make the whole apparatus compact; to separate the vacuum of the X-ray source (3a) and that of the analysis chamber part (photoelectron analysis part) (14a); and to maximize photoelectron intensity. This allows for an application to a HiPP/NAPP measuring device. Furthermore, since the size of the Rowland circle (C) of the X-ray spectrometer could be decreased to the half of the embodiment 1, decreasing the required area of the monochromater crystal (9) assembly to one fourth, there is also an advantage that the cost can be significantly lowered. Such advantage can be achieved by a design where the analysis chamber (14) and the X-ray source (3) are integrated while the vacuums are separated.
Yet another embodiment will be explained below which satisfies the conditions for the above-mentioned spatial configuration. In the embodiment 1, in order to achieve large X-ray flux, a structure is adopted which achieves a large acceptance angle of the monochromater crystal assembly, but there is a spatial restriction for this structure. It is considered that the output of the electron gun (3b) exciting the target for further increase of the X-ray flux is increased. However, if the output of the electron beam is increased exceeding the cooling capacity of the target (7), the target layer (7) will be damaged, since the most of energy of the electron beam turn into heat within the target layer (7). If the size of the spot (footprint (FP)) on the target (7) of the electron beam is increased, the density of the generated heat decreases, thereby preventing the damage to the target (7). However, the spot size on the target (7) of the electron beam corresponds to the size of the X-ray spot (footprint (FP)) on the sample (5) as it is, which degrades energy resolution and spatial resolution of the photoelectron spectrum. Also if the spot (footprint (FP)) size on the sample increases, a photoelectron image which is enlarged by the electron lens (8) of the analyzer (6) and projected on the entrance slit (6S) of the analyzer (6) becomes larger than the aperture of the slit (6S), resulting in a decrease in photoelectron signal intensity detected by the analyzer (6).
In order to fulfill the requests to sustain the spot (footprint (FP)) size on the target (7) of the electron beam to 100 microns or less and increase X-ray intensity, paying attention to the area enclosed by the sign a in
The rotating anticathode type of the X-ray source has been commonly used for an X-ray diffractometer, etc. and already been the known art. It is configured to accelerate electron rays generated from a linear filament and irradiate an elongated area on a target by focusing the electron beam in a one-dimensional direction by a simply-structured electrode, and extract them out from a direction along a long-extending X-ray generation area at a low angle (typically 6 degrees). This known system has an advantage that an apparent spot size of an X-ray can be decreased by the simply-structured electron gun. But on the other hand, the rate of utilization of the X-ray is low and the obtained X-ray flux is decreased for increased output of the electron gun, which does not meet the purpose of this embodiment. Here, in the rotating anticathode type of the X-ray source in this embodiment 3, unlike these known systems, a hard X-ray photoelectron spectroscopy with high throughput was achieved by irradiating the target with an electron gun (3b) equipped with convergent lens which can two-dimensionally focused electron beam to about 100 microns or less and generating 100-micron size of focusing X-ray spot with high intensity.
In the above-mentioned conventional art, there were two big problems that the size of the X-ray source (i.e., the size of the monochromater crystals of the X-ray, and the size and structure of the target mechanism) is limited due to the size of the analysis chamber and that the vacuum of the X-ray source and that of the analysis chamber cannot be separated. Also, in an experimental technique referred to as NAP (Near Ambient Pressure Photoelectron spectroscopy) or HiPP (High Pressure Photoelectron spectroscopy), gas is introduced into an analysis chamber. In this situation, the X-ray source in
Number | Date | Country | Kind |
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2015-096104 | May 2015 | JP | national |
2015-130414 | Jun 2015 | JP | national |
Number | Date | Country | |
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Parent | 14970182 | Dec 2015 | US |
Child | 17732301 | US |