Patent Application
20220003689
The present disclosure relates to an apparatus for generating an X-ray for phase imaging.
An X-ray has a high transmission property in a substance, and imaging using the X-ray is widely used to observe an internal structure of a sample, which corresponds to a scheme of irradiating a sample with an X-ray and projecting the X-ray passing through the sample onto a detector to see through a structure inside the sample, so that a projection image reflecting the X-ray absorption of a sample object is obtained (for example, see Non-Patent Document 1 below).
Further, a scheme of forming a three-dimensional (3D) image of a sample from plural X-ray projection images captured at different directions is known, for example, from Non-Patent Document 2 below. In addition, there has been known a phase imaging technology for generating an X-ray image using a phase change of an X-ray passing through a sample. This technology uses a scheme of detecting the phase of the X-ray using an X-ray grating (for example, Non-Patent Document 3 below). This Non-Patent Document 3 describes a method for manufacturing the X-ray grating. Using X-ray phase imaging allows a clear X-ray image to be obtained using information about a phase change caused by a sample, even for a substance having a low X-ray absorption (a sample made of a substance having a relatively low atomic number, for example, a biological tissue).
A Talbot-Lau interferometer has been known as an example of an X-ray phase imaging apparatus (see Non-Patent Document 4 below). In general, the Talbot-Lau interferometer disposes three gratings in an X-ray path to acquire X-ray phase imaging data. The three gratings are referred to as a grating G0, a grating G1, and a grating G2 in order from a radiation source side. The grating G0 is an absorption grating, and is installed on a downstream side of an X-ray source that emits an X-ray with a certain spread. An X-ray shielding portion and an X-ray transmitting portion are periodically arranged in the grating G0. Due to this property, the grating G0 has a role of geometrically arranging a large number of micro X-ray sources in a pseudo manner at an interval of a grating pitch. In this way, even when a radiation source that generates an X-ray having a low spatial coherence, that is, an X-ray involving multi-wavelength X-rays is used as the radiation source, an X-ray having a high spatial coherence can be irradiated from a micro X-ray source enough to realize the phase imaging, and thus it is possible to realize the phase imaging. That is, the grating G0 may form a part of the radiation source. The grating G1 is a phase grating in which a desired phase change occurs at a wavelength of an X-ray used for imaging by adjusting a material and thickness of the X-ray shielding portion of the grating. The grating G2 is generally an absorption grating, and an installation position or a grating period thereof is adjusted so that a moire image is generated with respect to an image of the grating G1. In a fringe scanning method, while moving one of the grating G1 and the grating G2 relative to the other by a relatively small amount in a period direction of the grating, a changing projection image is acquired by an X-ray image detector, and then data processing of the projection image is performed. In this way, it is possible to acquire a necessary X-ray phase image (so-called phase imaging).
Here, in the grating G0 and the grating G2, a part for completely shielding the X-ray and a part for transmitting without attenuation need to be periodically formed with a predetermined grating pitch in terms of functions. Here, in the case of desiring to increase the spatial resolution of the phase imaging, it is necessary to reduce the grating pitch. However, to effectively shield the X-ray, it is generally necessary to form a heavy metal material sufficiently thick. For this reason, an X-ray shielding portion having a high aspect ratio needs to be formed. Depending on the energy of the X-ray, for example, it is necessary to form a grating pattern having a high aspect ratio of 10 or more, which causes considerable difficulty in manufacturing the grating.
As one solution to the difficulty, a phase imaging measurement scheme referred to as a Lau interferometer has been proposed as described in Non-Patent Document 5 below. According to this scheme, there is an advantage that it is possible to omit the grating G2 that requires creation of a grating having a large area and a high aspect ratio. Furthermore, the grating G0 can be omitted using an X-ray radiation source that generates a grating-patterned X-ray.
Macromolecules 38 (2005), 7197-7200
In the example of Patent Document 1 described above, an anode is formed from a patterned or single-layer thin-film metal material, and the anode is interposed by a support film made of a light element through which an electron beam can easily pass to form a target. By diverting a semiconductor manufacturing technology to formation of the pattern of the thin-film metal material that generates an X-ray, the pattern of the thin-film metal material can be manufactured with submicrometer accuracy. In this technology, an antistatic film for removing an electric charge generated by passage of an electron beam through a target substrate is placed on a surface of the support film. However, conductivity of the thin-film metal material to be the anode of the target is not considered. Further, in this technology, since direct cooling of the target substrate is not considered, it is difficult to apply power of several tens of W or more to the target.
In the technology of Patent Document 2, the thin-film metal material serving as the anode is formed in a striped pattern to ensure electric conduction, and heat is applied to a water-cooled metal block via a diamond layer with high thermal conductivity disposed under the thin-film metal material, thereby intending to improve electron beam applied power and X-ray focal position accuracy. However, this technology does not particularly consider extraction of an X-ray in a stripe pattern shape, and proposes to extract an X-ray in parallel with a stripe at a low angle of about 3 to 12 degrees with respect to the target surface. The stripe pattern in the technology of Patent Document 2 is formed by a semiconductor manufacturing technology as in Patent Document 1. A lower limit of a stripe width is set to about 5 μm.
The technology of Patent Document 3 described above is related to the Talbot-Lau interferometer. This document describes a technology for generating an X-ray in a stripe pattern shape using a rotating target. In addition, Patent Document 4 describes an apparatus for generating an X-ray in a stripe pattern shape using a rotating target or a fixed target. In a rotating target type X-ray generating apparatus, by cooling a target portion irradiated with an electron beam while rotating the target portion at a high speed of 6,000 to 12,000 rpm in an actual example, high-power electron beam irradiation is allowed, and the X-ray generation intensity is increased. However, in this technology, rotation-induced vibration blur occurs in a rotation target rotation axis in a direction perpendicular or parallel to the rotation axis. Since there is a limit to the machining accuracy, even when the machining accuracy is improved, it is considered that a shake amplitude of 2 to 3 μm may occur. Therefore, the rotating target type X-ray generating apparatus is not suitable for high spatial resolution imaging. Patent Documents 3 and 4 propose a scheme of reducing an apparent axis blurring amount (rotational axis direction blurring mount) to 1/10 by setting an X-ray extraction angle to 6 degrees in the rotating target type X-ray generating apparatus. In addition, Patent Document 4 proposes a scheme of reducing an apparent plane size of an X-ray focal point while increasing the electron beam irradiation power by setting the X-ray extraction angle to about 6 degrees in the fixed target type. However, in such a technology, there is a problem that a visual field size of the phase imaging by the Talbot-Lau interferometer is limited.
Incidentally, conventionally, in the case of irradiating a target having a grating structure with an electron beam, the electron beam is irradiated from a direction perpendicular to a target surface. In this case, a portion (substrate portion), in which there is no metal pattern for X-ray generation, hardly does not contributes to X-ray generation.
As a result of various studies on improvement of X-ray generation efficiency, the inventors have found that X-ray generation efficiency can be greatly improved by setting a depth (M) of a metal pattern for X-ray generation (X-ray generating portion) in a predetermined range after tilting irradiation of an electron beam. The disclosure has been made based on this finding.
(Item 1) An X-ray generating apparatus for X-ray phase imaging using an X-ray excited by an electron beam irradiated from an electron source onto a target,
D≤M≤D+r,
(Item 2) The X-ray generating apparatus according to item 1, in which a ratio (a:b) of a grating width (a) of the X-ray generating portions to a grating width (b) of the target substrate is set to 1:2, and
a grating pitch (a+b) is set to be equal to or less than a penetration distance of the X-ray excitation electrons passing through both the target substrate and the X-ray generating portions.
(Item 3) The X-ray generating apparatus according to item 1 or 2, in which the penetration depth (D) is calculated by the following equation:
where n is the number of X-ray generating portions or target substrates between the plural grooves through which the X-ray excitation electrons pass (n≥1), and ψ is a tilt angle of the electron beam in a plane parallel to a surface of the target substrate.
(Item 4) The X-ray generating apparatus according to any one of items 1 to 3, in which the irradiation angle (θ) is set to a value between 10° and 75°.
(Item 5) The X-ray generating apparatus according to item 1, including an X-ray tube for phase imaging, a direction (ψ) of irradiating the electron beam for X-ray excitation inclined at a predetermined angle with respect to the perpendicular to the target substrate being set to such an angle (ψ) that 60% or more of the electron beam reaches an X-ray generating metal portion while the electron beam for X-ray excitation irradiates the target substrate and passes through an inside.
According to the disclosure, an electron beam applied to a substrate portion is also applied to an X-ray generating portion after penetrating a substrate, and thus an X-ray can be efficiently generated.
Hereinafter, embodiments for carrying out the disclosure will be described in detail with reference to the accompanying drawings. As a premise of the description of the present embodiment, first, a principle of X-ray generation by electron beam excitation will be described in detail with reference to
(Principle of X-ray generation by electron beam excitation) When an electron beam is irradiated on a solid, an X-ray (characteristic X-ray) generated due to electron transition of an atom included in the solid and an X-ray generated by bremsstrahlung emitted from an electron damped by an electric field of the atom (bremsstrahlung X-ray) are generated. While an X-ray of a specific wavelength is generated as the characteristic X-ray, an X-ray of a continuous wavelength having smaller energy (longer wavelength) than maximum energy corresponding to an acceleration voltage of an electron beam is generated as the bremsstrahlung X-ray. Hereinafter, a description will be mainly focused on the bremsstrahlung X-ray.
With regard to generation of an X-ray in a solid by an electron beam applied to a surface of the solid, studies for describing a region of X-ray generation in the solid using a mathematical expression have been conducted for a long time. In particular, an equation shown in Non-Patent Document 7 is known to correspond to energy of an irradiation electron beam in a range from 10 keV to 1,000 keV. The equation presented in this thesis shows good agreement with measured values of a lot of theses reported on actual X-ray generation.
When each numerical value is put in Equation (2), a relativistic correction term of energy is added, and λs=0.182 is set as a numerical value obtained from measurement, the following Equation (3), which is evaluated to closely match the measured value at 10 to 1,000 keV, is obtained.
XD is expressed by the following Equation (4) using the maximum penetration depth R of the electrons. Here, γ is a value representing loss of an electron diffusion distance due to the atomic number of the target material.
According to Equations (3) and (4), by determining the electron beam acceleration voltage (E0), the atomic number (Z) of the element included in the X-ray target for generating the X-ray, the atomic weight (A) thereof, and the density (ρ) of the X-ray target, a region where the X-ray is generated from the solid irradiated with the electron beam can be obtained as a numerical value. Here, when a numerical value useful in designing an X-ray target shape is set to r=R−XD, it can be described that an X-ray is generated in a spherical region having a radius r from the penetration distance XD of electrons from the target surface.
To form a grating-shaped X-ray source, it is necessary to have an X-ray target structure in which portions having low X-ray generation efficiency and portions having high X-ray generation efficiency are alternately arranged. According to Non-Patent Document 8, the intensity Q of bremsstrahlung is represented by Expression (5). Here, P is a polynomial representing motion of an electron decelerated to the energy E.
Q(hv/E)=P(hv/E)Z2/E (5)
An important point is that the bremsstrahlung intensity is proportional to the square of the atomic number (Z) of the element serving as the X-ray target. Therefore, to form a grating-shaped radiation source, it is necessary to alternately arrange a substance including an element having a low atomic number and a substance including an element having a large atomic number.
The low atomic number elements that can be used as a material of the substrate for the X-ray target due to characteristics thereof actually correspond to Be, B, C (DI, diamond), C (DLC, diamond-like carbon), BN, SiC, AlN, etc. In obtaining a numerical value using Expressions (3) and (4), for a compound, an average atomic number and an average atomic weight are used for an atomic number and an atomic weight. Table 1 illustrates R, XD, and r with respect to the electron beam energy (keV) of the above materials. Here, the unit of the numerical value is μm.
In the present embodiment, a 0.8 μm grating-shaped groove can be manufactured, and an N-type SiC substrate having a low resistance (up to 20 mΩ cm2) was used as a target wafer. A SiC wafer is used to form a power control semiconductor element, has high heat resistance and has been developed for a process to allow fine processing similarly to a Si wafer. However, even in the case of Be, B, C (DI, diamond), C (DS, diamond-like carbon), BN, and AlN listed in Table 1, when a conductive film for allowing a current to flow on a surface is formed using a processing process thereof, usage as a substrate for the X-ray target is similarly allowed.
(X-ray generating apparatus of present embodiment) Next, the X-ray generating apparatus of the present embodiment (hereinafter, may be simply referred to as “generating apparatus” or “apparatus”) will be described with further reference to
(Target) As illustrated in
On a surface of the target substrate 36, a plurality of grooves 361 (see
The X-ray generating portions 32 are arranged in a grating shape by being embedded in the plurality of grooves 361 formed in the target substrate 36. Further, the X-ray generating portions 32 is made of a metal such as tungsten (W), tantalum (Ta), platinum (Pt) or gold (Au), or an alloy thereof. In the present embodiment, description will be made below on the assumption that tungsten (hereinafter sometimes referred to as W or metal W) is used.
A depth M (see
Here,
r is a difference (r=R−XD) between a maximum penetration depth (R) of X-ray excitation electrons irradiated as an electron beam in the X-ray generating portion and a penetration distance (XD) of the X-ray excitation electrons in the X-ray generating portion; and
D is a penetration depth of X-ray excitation electrons passing through the X-ray generating portion and the target substrate in a vertical direction of the target substrate.
In the present embodiment, a ratio a:b of a grating width a of the X-ray generating portions 32 (see
The penetration depth D in the present embodiment can be calculated by the following equation (the same as Equation (8) described later).
Here,
n is the number of X-ray generating portions or target substrates between a plurality of grooves through which the X-ray excitation electrons pass (n≥1);
θ is an irradiation angle of the electron beam with respect to a perpendicular to the target substrate surface (so-called tilt angle in the period direction); and
ψ is a tilt angle of the electron beam in a plane parallel to the surface of the target substrate (so-called grating direction tilt angle).
A scheme of obtaining these angles θ and ψ is illustrated in
The irradiation angle θ in the present embodiment is set to a value in a range of 10° to 75°, more preferably in a range of 15° to 65°.
The metal plate 37 has an X-ray emission hole 38 and a cooling water channel 39. The cooling water channel 39 has an inlet 391 and an outlet 392.
(Electron source) The electron source 2 is configured to be able to extract a grating-shaped X ray in a direction perpendicular to the target substrate 36 by irradiating the target 3 with the electron beam 23 inclined at a predetermined irradiation angle θ (see
(X-ray tube body) The X-ray tube body 1 includes an X-ray extraction window 34 for extracting the X-ray beam 35 from the X-ray tube body 1.
(High-voltage power supply) The high-voltage power supply 4 includes a filament power supply 41 and a bias power supply 42.
Details of each element described above in the present embodiment will be described later as an operation of the present embodiment.
(Example of target creation method) Next, an example of a processing process for producing the target substrate 36 will be described with further reference to
By developing the photosensitive resist material film 200 irradiated with the UV light, the pattern of the UV mask 110 is transferred to this film 200, and a part of the photosensitive resist material film 200 remains on the SiO2 film 300. By dry-etching this film, only a part of the SiO2 film 301 covered with the resist film is left, and a state illustrated in
Subsequently, the X-ray generating portions 32 made of metal are embedded in the wafer 400 in which the grooves 361 are formed. As described above, W (Z=74) is used as the X-ray generating portions 32 of the present embodiment. SiC is a 1:1 stoichiometric compound of Si (Z=14) and C (Z=6). According to the above Equation (5), the X-ray generation intensity from a portion W is about 47 times smaller than that of SiC. As described above, Ta (Z=73), Pt (Z=78), and Au (Z=79) can be used as the metal.
A specific embedding method of the X-ray generating portions 32 will be described with further reference to
Subsequently, a TiN film 403 and a Ti film 404 are formed on the surface formed by polishing as illustrated in
(Electron beam irradiation method) Next, a description will be given of an electron beam irradiation method for efficiently extracting striped X-rays from the target 3 having the grating-shaped X-ray generating portions 32. First, a principle will be discussed below.
In the X-ray generation model illustrated in
The X-ray generation target according to the present embodiment is included in the X-ray source for phase imaging. In this case, it is known that a ratio of a grating width of the target substrate to a metal grating width is appropriately 2:1 (See Non-Patent Document 9 described above).
The material of the target substrate 36 is set to C (DI, diamond) or SiC, the material of the X-ray generating portions 32 is set to W, and the X-ray generation region when the target substrate 36 is perpendicularly irradiated with electrons at an applied voltage of 140 kV is illustrated in
According to Table 1, when C (DI) is used as the substrate, the electron beam vertically irradiated on the surface of the target substrate 36 (non-metal portion between the X-ray generating portions 32) generates a bremsstrahlung X-ray in a spherical region having a radius r1 of about 25 μm around a position OD at a depth of about 40 μm. In this instance, the electron beam 23 applied to the target substrate 36 does not diffuse to the X-ray generating portions 32 made of the W metal, and thus does not generate X-rays from the X-ray generating portions 32. When SiC is used as the target substrate 36, bremsstrahlung X-rays are generated from a spherical region having a radius r2 of about 35 μm around the position OD at the depth of about 40 μm similarly to C (DI). Here, a part of the electron beam 23 vertically irradiating the target substrate 36 is diffused to the X-ray generating portions 32 corresponding to the W metal. Electrons reaching the X-ray generating portions 32 by diffusion generate more bremsstrahlung X-rays from the W metal, and lose energy at a distance r3 of about 30 μm from OD.
Meanwhile, the electrons applied to the surface of W corresponding to the X-ray generating portions 32 generate bremsstrahlung X-rays from a spherical region having a radius r4 of about 15 μm around the position OD at a depth of about 4.5 μm. Approximately 30 to 40% of the electrons irradiated to W corresponding to the X-ray generating portions 32 are recoil electrons and do not contribute to generation of X-rays. In addition, referring to the electrons applied to the surface of W corresponding to the X-ray generating portions 32, when the distance between the X-ray generating portions 32 is sufficiently smaller than a radius 15 μm of a braking X-ray generation region of the W metal alone, and a ratio of a width of a target substrate portion to a width of the X-ray generating portions 32 (W metal portion) is 2:1, the generation region of bremsstrahlung X-rays is widened as indicated by r of 2DI/W or 2SiC/W shown in Table 1. A radius r5 of this generation region is larger than r4 as illustrated in
As described above, according to the principle study on the X-ray generation region illustrated in
On the other hand, in the present embodiment, by irradiating the target 3 with the electron beam 23 inclined at a predetermined angle θ, the electron beam applied to the target substrate 36 portion generates bremsstrahlung X-rays in the X-ray generating portions (W metal portion) 32, which uses a characteristic in which the material of the target substrate 36 has a longer electron penetration distance XD of the incident electron beam than that of the metal portion (for example, W), and the electron beam irradiated on the target substrate 36 can be transmitted longer through the target substrate material. Hereinafter, this point will be described in more detail.
An explanatory diagram of a depth D from a surface into which the electron beam 23 passing through the X-ray generating portions 32 (W metal grating) having the width a and the target substrate 36 portion having the width b (hereinafter may be indicated by a symbol “K”) can penetrate is illustrated as
Similarly, the electron beam 23 incident on the target substrate 36 portion illustrated in
By irradiating the X-ray generating portions 32 with the electron beam at an angle equal to or greater than θmin, bremsstrahlung X-rays can be generated to a deeper portion when compared to the case of irradiating the W metal alone (case of θ=0°). Further, in the present embodiment, the X-ray generating portions 32 can generate bremsstrahlung X-rays using the electron beam incident on the target substrate 36 portion. Here, a condition when transmitting the X-ray generating portions 32 and substrate portions (portions adjacent to the X-ray generating portions 32) by a length of n sheets is expressed by the following Equation (7). In addition, the electron beam penetration depth D is represented by the following Equation (8). Here, n≥1.
In the above Equations (7) and (8), under the condition that n is an integer, regardless of the position of the electron beam applied to the X-ray generating portions 32 (W metal portion) of the X-ray target and the target substrate 36 portion, the penetration depth is the same. When n is a non-integer value of 1 or more, the penetration depth of the electron beam irradiated on the X-ray generating portions 32 (W metal portion) becomes smaller than the penetration depth of the electron beam irradiated on the target substrate 36 portion. However, the average of the penetration depth in the entire electron beam irradiation portion including the substrate 36 portion and the X-ray generating portion 32 (W metal portion) is substantially the same as a value obtained by Equations (7) and (8).
In addition, to achieve irradiation of an electron beam in which n becomes 1 or more, it is necessary that the electron beam penetrates at least up to a length corresponding to one period of the grating pitch. Under this condition, a maximum length of the grating pitch (a+b) of the X-ray target 3 can be determined. A maximum value Pmax of the grating pitch (a+b) when the ratio of the width of the X-ray generating portions 32 (W metal portion) to the width of the SiC target substrate 36 portion is 1:2 and n=1 is shown for each electron beam energy in Table 2 below.
Here, in the present embodiment, the grating pitch (a+b) is set to be equal to or less than the penetration distance XD of the X-ray excitation electrons passing through both the target substrate 36 and the X-ray generating portions 32 as described above. That is, the grating pitch (a+b) is set to a narrow distance enough to allow the X-ray excitation electrons to pass through both the target substrate 36 and the X-ray generating portions 32.
The actually employable grating pitch (a+b) is determined by the electron beam irradiation angle θ and a number n based on that the electron beam penetrates up to a length corresponding to n times the grating pitch, and is determined by the following Equation (9). Here, it is presumed that ψ=0°. In addition, Pmax is a maximum value of the grating pitch.
According to Equation (9), 0≤sin θ≤1 from a range of θ that can be actually taken, and therefore, the maximum value of the grating pitch of the X-ray target is present. The Pmax when the applied voltage is 240 kV is about 26 μm. However, considering the general applied voltage (140 kV) and the irradiation angle θ of the electron beam (usually less than 90°), it is desirable that a+b≤10 μm.
In the present embodiment, a tilt direction of the electron beam irradiation on the X-ray target (that is, on the W metal pattern) is perpendicular to the surface of the X-ray generating portions 32 (that is, ψ=0°) in projection onto the surface of the X-ray generating portions 32 (see
In the present embodiment, since the irradiation electron beam 23 is inclined, the electron beam 23 incident on the X-ray generating portions 32 (W metal portion) passes through the X-ray generating portions 32 (W metal portion), and penetrates the target substrate 36 made of a light element. For this reason, the electron beam penetration depth D from the substrate surface becomes deeper than that in the case of the metal W alone. In addition, the electron beam 23 incident on the target substrate 36 made of a light element passes through the target substrate 36 portion, and then enters the X-ray generating portions 32 (W metal portion). Thus, also in this case, the electron beam penetration depth D is deeper than in the case of the metal W alone. Then, in the target 3 including the target substrate and the metal portion, a portion generating the braking X-ray can be located deep from the surface, and the volume of a braking X-ray generating spherical region can be increased. Then, when compared to a case where the electron beam 23 is irradiated perpendicularly to the target substrate 36 (θ=0°), the intensity of X-ray generation is increased.
When the penetration depth (D) of a 140 kV electron beam 23 can be changed from 4.5 μm to 10 μm, the volume for generating the braking X-ray increases by about 20% when the X-ray generating portions 32 correspond to the W metal. In the case of vertical irradiation (θ=0°), a maximum length R (=XD r) in the depth direction as a distance for generating bremsstrahlung X-rays in the W metal portion is about 20 (XD(W) of
Next, a relationship between the depth of the X-ray generating portions 32 (W metal portion) that mainly generate X-rays and the intensity of the emitted X-rays will be considered with reference to
In the vertical irradiation, the electron beam that does not irradiate the X-ray generating portions 32 reaches a depth corresponding to the penetration distance XD(K) in the target substrate 36, and the electrons generating X-rays diffuse from this location. A case where the applied voltage of the electron beam is set to 140 kV, and M is smaller than the depth (4.527 μm) of the penetration distance XD(W) into W, that is, M<XD(W) is taken as an example. Here, when the target substrate 36 is SiC, as can be seen from Table 1, the penetration distance XD(K) at the target substrate 36 is 40.255 μm, and the diffusion limit r(K) (=R−XD) is 34.940 μm. Therefore, when M is 5.315 μm or less, which is a difference between the penetration distance XD(K) and the diffusion limit r(K), the electrons of the electron beam vertically irradiated on the target substrate do not reach the X-ray generating portions 32, and thus X-rays are not generated due to the electrons.
Meanwhile, the electron beam irradiating the X-ray generating portions 32 penetrates the W metal as the X-ray generating portions 32, so that a penetration distance in the depth direction of the target substrate 36 decreases. That is, in this case, even when M<XD(W), X-rays are generated from the X-ray generating portions 32. In addition, in the case of XD(W)≤M≤XD(W)+r(W), which corresponds to a range (A) of M of
Next, a case where θmin≤θ, M is sufficiently large, and an electron beam can pass through both the X-ray generating portions 32 and the target substrate 36 will be considered. When the target is irradiated with an electron beam at θ=15.7°, the penetration depth D of the electron beam is 10.7 μm from Equation (8). When θ is the same, the penetration depth D is the same in the case of the electron beam irradiating the target substrate 36 and in the case of the electron beam irradiating the X-ray generating portions 32. Here, when M is equal to or greater than the penetration depth D, all the electrons diffusing into the X-ray generating portions 32 involve in generation of the braking X-ray.
The value of D differs depending on the irradiation angle of the electron beam. When the applied voltage is 140 kV, D=9.3 μm at θ=32.8° and D=6.5 μm at θ=54.3°. Therefore, in the oblique electron beam irradiation, a state such as direct irradiation of the target substrate in the case of vertical irradiation (a state in which a lot of electrons do not contribute to X-ray generation and the efficiency is low) does not occur. That is, all the irradiated electron beams contribute to the generation of X-rays from the W metal. However, when M>D+r(W), there is no electron diffusion that contributes to X-ray generation even when M is increased, and thus the X-ray intensity is saturated at M=D+r(W).
The electrons penetrating the target substrate 36 due to the vertical irradiation of the electron beam reach the depth XD(K) along a penetration direction of the electrons, and then diffuse inside the target substrate 36 within the range of r(K). Thus, in the case of M>XD(K)−r(K), the electrons reach the X-ray generating portions 32 and generate bremsstrahlung X-rays. For this reason, when the grating pitch (a+b) of the target 3 is a value sufficiently smaller than a+b obtained by Equation (9), the X-ray intensity generated under the condition that the electron beam is vertically irradiated becomes the X-ray intensity at θ=0° including the substrate irradiation in
On the other hand, in the present embodiment, when D determined by Equation (8) depending on the irradiation angle (that is, the depth in the direction perpendicular to the substrate surface) is used, the embedding depth M is excellent in X-ray generation efficiency under the same applied voltage and current conditions in the range of D≤M≤D+r(W) (range (A) of
A relationship between the electron beam irradiation angle θ and the X-ray intensity shown in
(Operation of target) Next, an operation of the target 3 irradiated with the electron beam 23 will be described with reference to
In the present embodiment, the target substrate 36 is electrically and thermally bonded to the water-cooled metal plate 37 by metal bonding. Assuming that the thermal conductivity of the N-type SiC included in the target substrate 36 is 150 W/mK, even in a case where 1 kW of heat is generated on a front surface portion, when a temperature difference is obtained based on Fourier's law by approximating with 1D heat conduction in the case of removing heat from a back side, a temperature difference between a front and a back of the target substrate 36 becomes about 80 to 90° C. Aluminum (Al) or copper (Cu), which is a common metal material used for the water-cooled metal plate 37, has a thermal expansion coefficient of 16 to 23 ppm, which is larger than 3.7 ppm for a SiC wafer and 4.5 ppm for the W metal. Thus, metal bonding may not be sufficiently performed. Furthermore, there is a possibility that a joint between the substrate 36 and the metal plate 37 is broken or the SiC wafer is broken due to the thermal stress caused by turning ON/OFF the electron beam power. Thus, in the present embodiment, as the material of the substrate 36, it is possible to use a material in which aluminum and Si fine particles are thermally compressed to suppress the thermal expansion coefficient. Since the thermal conductivity of this material is 150 W/mK and the thermal expansion coefficient is about 7 ppm, a structure that reduces thermal stress can be realized.
In the present embodiment, the maximum surface temperature of the substrate 36 is maintained at about 200° C. or less by using the SiC target substrate 36, setting the cooling water temperature to 20 to 30° C., and setting maximum power of the applied electron beam to about 2 kW. In this way, it is possible to suppress damage to an embedding structure of the fine X-ray generating portions 32. For cooling an X-ray tube loaded with an electron beam load of 2 kW, a water-cooled cooling system having a proven track record in cooling X-ray diffraction tubes is commercially available. By circulating the cooling water using this cooling system, the target 3 can be stably cooled. When this cooling method is used, an X-ray tube using an electron beam to which power of 2 kW or more is applied can be realized depending on the irradiation diameter of the electron beam 23 and the design of the cooling system. Note that in the present embodiment, the cooling method using water cooling is described. However, as a cooling method, it is possible to use not only water but also liquid cooling using oil or liquid metal as a refrigerant, or an air cooling system using gas.
In the water cooling structure of
(X-ray tube) Here, the X-ray tube for phase imaging in which the above-described target 3 is installed will be described in more detail with reference to
This X-ray tube is of a type that extracts X-rays from the back surface of the target substrate 36. The electron source 2 in which the filament 21 for generating thermoelectrons and the electron lens 22 are disposed inside the X-ray tube body 1 whose inside is evacuated to a high vacuum. The X-ray target 3 having a pattern of the X-ray generating portions 32 on a surface is installed to face the electron source 2. The tilt angle of the electron beam 23 irradiating the surface of the target 3 is set to a predetermined angle θ. A cross-sectional shape of the thermoelectrons generated by the filament 21 heated by the filament power supply 41 is adjusted by the bias power supply 42 and the electron lens 22, and the thermoelectrons are accelerated by a high voltage of the high-voltage power supply 4 to irradiate the X-ray target 3. The electron beam irradiates the target substrate 3 at a predetermined tilt angle θ from an oblique direction, and the electron beam penetrates to a predetermined depth from the target surface by the acceleration voltage of the electrons.
The X-ray target 3 of the present embodiment is water-cooled, and thus desirably set to a ground potential together with the X-ray tube body 1. By setting the target potential to the ground potential, it is possible to reduce a distance between the target substrate 36 and the X-ray extraction window 34 attached to the X-ray tube body 1. Further, in the Lau interferometer proposed in Non-Patent Document 6, it is necessary to install the target substrate corresponding to G0 and the phase grating G1 at a short distance. However, in the X-ray tube of the present embodiment, it is possible to establish a Lau interferometer by disposing a phase grating 5 disposed outside the X-ray extraction window 34 near the target substrate 36.
In the present embodiment, the target substrate 36 and the phase grating 5 can be disposed close to each other, so that the X-ray tube ball body 1 and the phase grating 5 can be integrally coupled to each other.
A power supply apparatus 9 includes the high-voltage power supply 4, the filament power supply 41, and the bias power supply 42, and can supply a necessary voltage to the electron source 2 via a cable 91.
A control apparatus 11 can dynamically control operation timing and operation content of the water cooling apparatus 10 and the power supply apparatus 9 in accordance with preset content or in response to a detection result from an appropriate sensor (not illustrated).
As described above, in the present embodiment, a embedding depth of an X-ray target having a structure in which a metal is embedded in a target substrate made of a light element serving as a support substrate has been studied. A range of arrival (R), a penetration depth (XD), etc. of electrons irradiated on the substrate and the X-ray generating portions 32 as the electron beam are studied in detail, and it is possible to obtain optimum ranges of a depth of the metal pattern irradiated with the electron beam and an irradiation angle on the target wafer on the side where the pattern is formed to increase the X-ray dose generated from the metal pattern.
From a result of study of the range of arrival (R) and the penetration depth (XD) of the irradiated electron beam in the target, in a case where W is used as metal embedded in the substrate, it is desirable that the irradiation angle (θ) on the target wafer satisfies a condition of sin θ>a/XD with respect to the width (a) of W. XD changes with the irradiated electron beam energy (E, acceleration voltage). For example, when E=140 kV, the minimum θmin=10.2 degrees when a=0.8 μm. The most desirable angle is θ=12.5 degrees. In this instance, the optimum embedding depth (M) of the X-ray generating portions 32 generating bremsstrahlung X-rays is 26.7 μm. After the numerical values are determined in this way, grooves having a width of 0.8 μm can be formed at a pitch of 2.4 μm on the surface by a semiconductor/MEMS process, for example, by a plasma etching process using XeF2 gas using a low-resistance (up to 20 mΩ cm2)N-type SiC substrate as a target wafer. After depositing W using the CVD method on a wafer having grooves formed in a grating shape and filling the grooves with W metal, the W metal at a position other than in the grooves on the target wafer is removed by polishing, so that a target wafer in which a grating-shaped W metal is embedded can be manufactured.
Next, an X-ray generating apparatus according to a second embodiment of the disclosure will be described with reference to
In the 2D pattern, regular repetition in a 2D direction is required. As an example of realizing such a 2D pattern, the present embodiment considers two types corresponding to a case where the number of closest grating points from a certain grating point is four (
When the electron beam is inclined in a first approaching direction 321 (
Here, since the metal grating is closest to the first approaching direction, this direction is considered in the same way as the direction of the metal grating of the 1D grating, and the angle ψ is taken as
Similarly, in the case of the hexagonal grating (Example 2), when the electron beam is inclined in the first approaching direction 321, electrons having a width of about 1.6a (about 53% of the whole) among the electrons irradiating the target substrate portion of the target substrate portion do not X-ray excite the X-ray generating portions 32. Even in this case, the volume ratio of the X-ray generating portions 32 irradiated with X-rays is about 3.8 times that of the perpendicular electron beam irradiation. Therefore, it is possible to increase X-ray excitation of the X-ray generating portions 32 by about 3.8 times. In the hexagonal grating (Example 2), the target substrate is directly X-ray excited in a portion of a width 1×a (about 33% of the whole) in the direction 322 of the grating point corresponding to second approaching. On the other hand, in the third approaching direction 323, gratings having the diameter a overlap on the X-ray projection plane, and the X-ray generating portions 32 are excited by using all the electron beams. In this way, the X-ray intensity about 10 times as high as that of vertical irradiation can be obtained.
Similarly to the case of the square grating, the metal grating is closest to the first approaching direction. Thus, this direction is considered in the same way as the direction of the metal grating of the 1D grating, and the angle ψ is taken as illustrated in
In the second embodiment, since the configuration and advantages other than those described above are the similar to those in the first embodiment, further detailed description of the second embodiment will be omitted. It should be appreciated that the various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2020/006082 | 2/17/2020 | WO | 00 |
