Focusing grating device with large aspect ratio

Abstract

A focusing grating device (100) is described, comprising a substrate (402) and a grating comprising a plurality of grating features (408) on the substrate (402). The grating features (408) are positioned non-perpendicularly to the substrate surface, thereby causing a first focusing direction. The substrate (402) is curved, thereby creating a second focusing direction different from the first focusing direction. An X-ray system (300) comprising such a focusing grating device (100) and a method for producing such a focusing grating device (100) are also described.

Description

Technical Field

[0001] This invention relates to the field of X-ray imaging. More specifically, this invention relates to X-ray gratings with a large aspect ratio for use in differential phase contrast and / or dark-field imaging systems such as those used for X-ray imaging, to corresponding X-ray systems, and to methods for manufacturing such focused X-ray gratings. Background Technology

[0002] Dark-field imaging and differential phase-contrast imaging have attracted significant interest in both medical and non-medical applications, such as security, architecture, and structures, due to their higher sensitivity compared to conventional X-ray imaging, enabling non-destructive imaging of low-density materials. In grating-based differential phase-contrast X-ray imaging or dark-field X-ray imaging, gratings with high aspect ratios (e.g., up to 70) are used. Given this high aspect ratio, these gratings need to be focused toward the X-ray focal spot used in the application. LIGA, a photolithography fabrication technique encompassing photolithography, electroplating, and molding, is currently the manufacturing technology used to produce such high aspect ratio gratings. Typically, gratings are fabricated by vertically growing grating sheets on a flat substrate. Focusing is then accomplished by bending the grating along with the entire substrate. The grating orientation is chosen to be parallel to the bending axis.

[0003] In dark-field computed tomography (CT), the grating is traditionally parallel to the system's rotation axis (A). The grating is arranged in... Figure 1 As shown in the diagram. The problem with this orientation is that the acceptance angle of the source grating G0 is limited. This source grating is located close to the focal spot (approximately 10 cm) and has a high aspect ratio (approximately 70). This high aspect ratio means that the acceptance angle is quite narrow (i.e., α = 14 mrad). This aspect is... Figure 2 As shown in the figure. In dark-field CT applications, the distance D0 from source to source grating G0 is only 10 cm. Basic geometric calculations indicate that the maximum focal spot size S compatible with grating openings of p = 2 μm and an aspect ratio of 50 is S = αD0 = 1.4 mm. If the focal spot size is large, a loss of visibility will occur. In fact, the focal spot is only that small in beams with very small fan angles. For beams with medium fan angles (typically ±30 degrees in medical CT systems), the focal spot is wider because it is a large area on a relatively shallow anode disk (see Figure 1). Figure 1 ).

[0004] US 2007 / 183583 A1 describes a focal detector apparatus and X-ray device for generating projection or tomographic phase-contrast records of an object. The focal detector apparatus includes a radiation source with a focal point arranged on a first side of the object for generating a fan-shaped or conical beam; at least one X-ray grating arranged in the beam path; at least one phase grating arranged on an opposite second side of the object in the beam path, preferably generating an interference pattern of X-rays within a specific energy range; and an analytical detector system that detects the interference pattern generated by the phase grating with at least positional resolution relative to its phase shift.

[0005] One proposed solution to the visibility loss caused by the small acceptance angle of high-contrast gratings is to change the grating orientation to be perpendicular to the system's rotation axis. In this so-called "horizontal grating arrangement," the focal spot size parallel to the grating direction does not affect visibility, and it is suitable for systems, for example, using structured X-ray emission, where a source grating G0 is not required. For systems requiring a source grating, the visibility loss due to the small acceptance angle of the grating still needs to be addressed. Summary of the Invention

[0006] One object of the present invention is to provide a good focusing grating device with a high aspect ratio for use in X-ray imaging, such as differential phase contrast and dark field imaging, as well as a good X-ray imaging system grating device including such focusing and a good method for manufacturing such a focusing grating device.

[0007] The advantage of this invention is that it provides a focusing grating device with a high aspect ratio, wherein focusing is achieved in two non-parallel directions, and the grating allows for a large acceptance angle for radiation used in X-ray imaging systems incorporating such a focusing grating device.

[0008] The advantage of embodiments of the present invention is that it provides a precision focusing grating device with a high aspect ratio, which allows focusing in two different directions according to predetermined focusing requirements without bending the substrate in both directions in a well-defined manner in the final product.

[0009] The above-mentioned objectives and advantages are achieved by means of the apparatus and method according to embodiments of the present invention.

[0010] This invention relates to a focusing grating apparatus for X-ray imaging, including dark-field imaging and / or phase-contrast imaging. The focusing grating apparatus includes a substrate and a grating comprising a plurality of grating features positioned on the substrate. The grating features are positioned not perpendicular to the substrate surface, thereby generating a first focusing direction for the X-ray beam, and the substrate is curved, thereby causing a second focusing direction for the X-ray beam, which is different from the first focusing direction. An advantage of embodiments of the invention is that it provides a focusing grating apparatus that allows focusing in two different focusing directions. An advantage of embodiments of the invention is that it provides an accurate and well-defined focusing grating apparatus. Such embodiments can be advantageously used in grating apparatuses for use in differential phase-contrast and / or dark-field X-ray imaging techniques, but the embodiments are not limited thereto. An advantage of embodiments of the invention is that the focusing grating apparatus can be used in systems requiring high energy, such as those needed for medical imaging.

[0011] The substrate can be bent in only a single direction (i.e., in the second direction). One advantage of embodiments of the invention is that focusing is achieved in at least two different focusing directions, while the substrate of the focusing grating device ultimately only needs to be bent in one direction, since it is complex, or even impossible, to bend the substrate precisely in a well-defined manner according to the two-dimensional curved surface shape or in both directions.

[0012] The side edge of at least one grating feature may be such that at least one position is at an angle of at least 1 degree relative to the vertical direction on the substrate.

[0013] By addressing the required focusing of the grating in one dimension during its fabrication process, features for focusing in that dimension are created at the correct dimensions, and then only bending is needed in the other dimension. Since subsequent bending causes deformation, this deformation is limited to that other dimension, rather than deformation in both dimensions. Therefore, the grating according to embodiments of the invention results in smaller overall deformation.

[0014] A focusing grating device can have an aspect ratio of at least 30, such as 50. In some examples, a focusing grating device can have an aspect ratio of 65.

[0015] The bending stress of at least a portion of the grating feature in the second focusing direction can be significantly higher than the bending stress of at least a portion of the grating feature in the first focusing direction. Because the grating feature is at least partially formed into a curved structure, the bending stress of the grating feature in this direction may be less than the bending stress in the curvature direction of the substrate in the final focusing grating device. When referring to significantly higher bending stress in embodiments of the invention, it may refer to bending stress that is, for example, at least 10%, at least 25%, or at least 50% higher than in other directions.

[0016] The grating feature can be a thin sheet of grating.

[0017] The grating features can be made of X-ray absorbing materials. Examples of X-ray absorbing materials include gold, lead, tungsten, or bismuth, but the embodiments are not limited to these. In particular, alloys containing heavy metals can be used. Advantageously, electroplatable high-z materials are used.

[0018] The substrate may include or may be made of a material that is transparent to the X-rays used or a material that absorbs low X-rays, such as any of graphite, glass or Capton.

[0019] The present invention also relates to an X-ray system for grating-based X-ray imaging, comprising at least one focusing grating device as described above.

[0020] X-ray systems can be dark-field imaging systems and / or differential phase-contrast imaging systems.

[0021] An X-ray system may include a focusing grating device located close to the X-ray source, as described above, for example at a distance of less than 20 cm, such as less than 15 cm, such as close to 10 cm.

[0022] The present invention also relates to a method for generating a focusing grating device for focusing an X-ray radiation beam in a first direction and a second direction, wherein the first direction is different from the second direction. The method includes:

[0023] A stack of at least a substrate and a resist layer is obtained;

[0024] Bending of the stack is applied in a first direction;

[0025] The stack is subjected to photolithography when the substrate is bent along the first direction;

[0026] The grating material is electroplated to form grating features;

[0027] Essentially restores the bending of the stack, and

[0028] After the restoration, a further bending of the stack is applied to induce a second focusing direction in the focusing grating device.

[0029] In embodiments of the invention, when referring to substantially restoring the curvature of the stack, it means the step of releasing the curvature of the stack in a manner that restores the stack to its original configuration within permissible tolerances. In some embodiments, this may mean returning the stack to a substantially flat configuration if it was initially flat before the curvature was applied.

[0030] Performing a photolithography process may include applying a mask. An advantage of embodiments of the present invention is that the mask is applied when the substrate is bent along a first direction to obtain grating features and slits of constant width between these features.

[0031] In the photolithography process, the steps of developing the resist layer and performing electroplating can be completed before the bending of the stack is substantially restored.

[0032] Providing a stack including a resist may include providing a stack of resist layers having a thickness varying along a first direction. Furthermore, the process may include using a curved mask. The required spacing can be designed to account for the use and irradiation of the mask during bending. Since bending the mask affects the spacing, this can be taken into account when designing the mask to correct for changes in spacing caused by the bending of the developing mask.

[0033] The present invention also relates to the use of the focusing grating device as described above for X-ray imaging. The use of the focusing grating device can be used for differential phase-contrast imaging and / or dark-field imaging.

[0034] Specific and preferred aspects of the invention are set forth in the appended independent and dependent claims. Features from the dependent claims may be suitably combined with features from the independent claim and other dependent claims, and not merely features expressly stated in the claims.

[0035] These and other aspects of the invention will become apparent and will be illustrated with reference to the embodiments described below. The reference numerals in the accompanying drawings are used in conjunction with the figures. Attached Figure Description

[0036] Figure 1 The illustration shows a prior art grating arrangement for a dark-field computed tomography (CT) X-ray imaging system, illustrating the orientation of the grating.

[0037] Figure 2 The diagram illustrates the use of Figure 1 The angle of reception obtained by the grating in the arrangement shown, and the corresponding maximum focal spot size that can be used with such a grating.

[0038] Figure 3 illustrates a focusing grating device according to an embodiment of the present invention.

[0039] Figure 4 The illustration shows a grating support that can be used in an embodiment of the invention to generate the required curvature during the manufacturing process of a focusing grating device.

[0040] Figure 5 The illustration shows an X-ray system including a focusing grating device according to an embodiment of the present invention.

[0041] Figure 6The illustrations depict different steps in the manufacturing process of a focusing grating device according to an embodiment of the present invention.

[0042] In different accompanying drawings, the same reference numerals refer to the same or similar elements. Detailed Implementation

[0043] The invention will now be described with reference to specific embodiments and particular drawings, but the invention is not limited thereto, but is defined only by the claims. The described drawings are merely illustrative and not restrictive. In the drawings, the dimensions of some elements may be exaggerated and are not drawn to scale for illustrative purposes. Dimensions and relative dimensions do not correspond to actual reductions in practice for carrying out the invention.

[0044] Furthermore, the terms first, second, etc. (e.g., first direction and second direction) used in the specification and claims are used to distinguish between similar elements and are not necessarily used to describe an order in time, space, sequence, or any other way. It should be understood that the terms thus used are interchangeable where appropriate, and embodiments of the invention described herein can operate in orders other than those described or illustrated herein.

[0045] It should be noted that the term "comprising" as used in the claims should not be construed as limiting to the means listed herein; it does not exclude other elements or steps. The words "a" or "an" do not exclude a plurality.

[0046] Throughout this description, references to "an embodiment" or "an embodiment" mean that a particular feature, structure, or characteristic described in connection with that embodiment is included in at least one embodiment of the invention. Furthermore, in one or more embodiments, particular features, structures, or characteristics may be combined in any suitable manner, as will be apparent to those skilled in the art based on this disclosure.

[0047] Numerous specific details are set forth in the description provided herein. However, it should be understood that embodiments of the invention can be practiced without these specific details. In other instances, well-known methods, structures, and techniques have not been shown in detail so as not to obscure the understanding of this specification.

[0048] In a first aspect, the present invention relates to a focusing grating device. A focusing grating device can be advantageously used in X-ray imaging applications to focus X-ray radiation beams, such as differential phase-contrast imaging or dark-field imaging, but embodiments are not limited thereto. In the context of the invention, a focusing grating device refers to a grating device that focuses X-ray radiation to a point by providing structural features of a grating that focuses in two directions. FIG3 shows a schematic diagram of a focusing grating device 100 according to an embodiment of the present invention. The focusing grating device 100 includes a substrate 402 and a grating. The grating has a plurality of grating features 408 located on the substrate 402. Figure 3A As shown, the grating feature 408 is positioned not perpendicular to the surface of the substrate 402, thereby introducing a first focusing direction for the incoming X-ray radiation beam. Figure 3A This illustrates that, in the described scenario, grating feature 408 focuses radiation onto a line. In the final focusing grating device 100, the substrate is curved, and by bending ( Figure 3B The curvature direction (indicated schematically by a double arrow) causes a second focusing direction that differs from the first focusing direction. Figure 3B The illustration shows how grating feature 408 focuses radiation onto a single point.

[0049] By introducing a focusing direction through the shape and position of the grating features, the substrate in the final focusing grating device 100 only needs to be bent in another direction, providing two focusing directions for the incident radiation beam, such as... Figure 3B As shown in the diagram. The substrate 402 used in the focusing grating device 100 can be made of a material that is transparent to the X-rays used or a material that absorbs low X-rays, such as graphite, glass, or... Any one of them.

[0050] The focusing grating device 100 according to embodiments of the present invention can have a large aspect ratio. For example, in some embodiments, the focusing grating device 100 can have an aspect ratio of at least 30, such as at least 50, such as up to at least 65, or even up to at least 70. They can be used, for example, typically for focusing toward an X-ray focal spot.

[0051] According to an embodiment of the present invention, during the manufacturing process of the grating device, the substrate 402 is bent along a bending direction before the grating feature 408 is formed, and straightened again after the grating feature 408 is formed, such that the grating feature 408 is positioned not perpendicular to the substrate surface 402. This introduces a first focusing direction caused by the grating feature 408. Figure 3BAs shown, by bending the substrate in another direction (schematically represented by double arrows) after all features have been generated, and maintaining this bending in the final focusing grating device 100, the focusing grating device 100 provides two focusing directions. In some embodiments, the bending radius of the substrate can be in the range of 5 to 15 cm. Reference will be made later. Figure 6 The manufacturing process of focusing grating equipment is described in detail.

[0052] In at least some embodiments, the grating features 408 are shaped such that the side edges of at least one of the grating features 408 form an angle of at least 0.1 degrees, for example at least 0.5 degrees, or for example at least 1 degree, relative to a vertical direction on the substrate 402 at at least one location. Due to their manufacturing method, in the final focusing grating device 100, the bending stress in at least a portion 408 of the grating features in the second focusing direction can be significantly higher than the bending stress in at least a portion 408 of the grating features in the first focusing direction.

[0053] To temporarily induce curvature in substrate 402, specifically during the fabrication of the grating apparatus, and more specifically during the photolithography process, a grating support can be used to hold the substrate to the grating support during specific processing steps. The curvature of the substrate can be relieved by removing it from the grating support. In an exemplary embodiment, the grating support can be as follows: Figure 4 The diagram illustrates a grating support 200 comprising at least two support members 202, 204 to which a focusing grating device 100 can be mounted. The focusing grating device 100 can be coupled to the grating support 200 in any suitable manner, such as by fasteners like adhesives (e.g., glue), screws, rivets, clamps, etc. In various embodiments, the grating support 200 may include one or more of a support surface, support member, support bracket, etc.

[0054] In an embodiment of the invention, the substrate 402 can be made large during manufacturing, thus facilitating its clamping in the retainer. After manufacturing, the substrate 402 can be cut to a suitable size.

[0055] To induce curvature in the substrate (and thus the grating device), applied after the photolithography process and retained in the final product, a grating support can be used to which the grating device can be fixed. Such a grating support can be similar to the grating support described for inducing bending during grating fabrication, i.e., during the photolithography process. It should be understood that the grating support can be selected such that an appropriate degree of bending occurs. This additional bending of the substrate, and therefore the grating device, is typically induced during mounting on the grating support and is retained during use by keeping the grating device mounted on the grating support when used for imaging. Therefore, the grating support can also be considered part of the grating device.

[0056] As those skilled in the art will understand, the substrate will be mounted on the grating support in different orientations to cause focusing in two different directions.

[0057] The grating feature 408 can be a grating sheet, although other shapes can also be used, depending on the type of grating envisioned. The grating feature 408 is typically made of an X-ray absorbing material, such as gold, lead, tungsten, or bismuth. In certain embodiments, alloys containing heavy metals may be used. Advantageously, high-Z materials that can be electroplated may be used. The properties of the grating feature 408, such as its spacing and height, can be selected according to the optical characteristics required by the system.

[0058] In a second aspect, the present invention relates to an X-ray system comprising at least the focusing grating device 100 as described in the first aspect. The X-ray system may be, for example, a dark-field imaging system or a differential phase-contrast imaging system, but embodiments are not limited thereto. The X-ray system can be any system that benefits from using the focusing grating device 100 with a large aspect ratio.

[0059] Exemplary X-ray systems that can benefit from the present invention include Figure 5As shown in the diagram. This X-ray system 300 may include a generally fixed gantry 302 housing a rotating gantry 304 rotatably supported by the fixed gantry and rotating about a z-axis around an inspection area 306. It also includes an X-ray radiation source 308 that generates a focal spot 310, thereby emitting radiation through the inspection area 306. A radiation-sensitive detector array 312 may be positioned opposite the radiation source 308 opposite the inspection area 306. The radiation detector array 312 detects radiation passing through a field of view 314 and an object 315 therein and generates a signal indicating it. The system typically also includes an X-ray imaging interferometer comprising one or more X-ray gratings, such as three gratings: a source grating G0 near the X-ray source, which may typically be a focusing grating device 100, as described in the first aspect of the invention; an absorber grating 320 near the detector G2; and a phase grating or absorption grating configured depending on whether it is configured with a conventional geometry, an inverted geometry, or a symmetrical geometry. The X-ray system 300 may also include a grating support 318 for creating a specific distance between gratings (G0, G1), maintaining the grating shape—as described above in the first aspect—and for maintaining the grating position. Further features and advantages of the source grating, and optionally other gratings, can be described as in the embodiments of the first aspect. The X-ray system 300 may also include, for example, a low-energy filter, a bowtie attenuator, and a beam collimator. Figure 5 (Not shown in the image). The X-ray system 300 typically also includes an object support 324, such as a couch, for supporting the object 315 in the field of view 314. Furthermore, a general-purpose computing system or computer may be used as an operator console 326 and may include a reconstruction processor 322. Further features may be known to those skilled in the art.

[0060] In a third aspect, the present invention relates to a method of manufacturing a focusing grating device 100. This method is particularly suitable for manufacturing the focusing grating device 100 as described in the first aspect. According to the invention, the method relates to manufacturing a focusing grating device 100 for focusing in a first focusing direction and a second focusing direction, wherein the first focusing direction is different from the second focusing direction. According to an embodiment, the method includes obtaining a stack of at least a substrate 402 and a resist layer 404. The method further includes applying a bend to the stack in the first direction, performing a photolithography process on the stack while the substrate 402 is bent, electroplating a grating material to form a grating feature 408, and substantially restoring the bend in the first direction. The latter results in the focusing grating device providing focus in one direction, caused by the created grating feature. The manufacturing method further includes, after the restoring the bend, applying a further bend to the stack in a separate bending direction (the second direction) to induce a second focusing direction in the focusing grating device. This additional focusing direction is caused by the additional bend of the stack, which is retained in the final focusing grating device.

[0061] By way of illustration, the embodiments of the present invention are not limited thereto; see reference. Figure 6 The present invention illustrates standard and optional features of an embodiment of the method for manufacturing a focusing grating device 100.

[0062] According to an exemplary embodiment, the method includes obtaining a stack of at least a substrate 402 and a resist layer 404. Therefore, the substrate 402 is typically a substrate that is substantially transparent to X-rays or has a low X-ray absorption coefficient. The resist layer 404 can be any suitable resist that allows photolithography processes to be performed.

[0063] In the next step, the method includes applying a mask 406. The mask can be any mask suitable for performing photolithography processes known for use in manufacturing grating devices for X-rays.

[0064] According to embodiments of the invention, the invention may include applying bending of the stack in a first direction. The latter will allow focusing to occur in one direction via the grating feature. Figure 6 Part A shows the partially fabricated mask 406 obtained in this step, illustrating a curved substrate 402 (curvature schematically represented by double arrows), a curved resist layer 404, and a curved mask 406.

[0065] The method further includes developing and etching the resist layer 404 while the substrate 402 is bent to create a negative pattern in the resist layer 404, thereby subsequently producing grating features 408. The latter, as... Figure 6 As shown in part B, mask feature removal was also applied.

[0066] After forming the negative pattern of the anti-corrosion feature, the method further includes optionally electroplating a grating material while bending is still applied to form the grating feature 408.

[0067] Subsequent methods include, for example Figure 6 As shown in section C, the curvature of the stack is essentially restored (schematically represented by double arrows). As a result, and as can be seen in the figure, the grating feature 408 is positioned not perpendicular to the surface of the substrate 402, thereby inducing a first focusing direction for the radiation beam. Figure 6 Part D shows Figure 6 Different views of the steps shown in part C.

[0068] The method may also include removing the remaining features from the resist layer to maintain the focus grating.

[0069] In a further method step, the grating may be pre-formed or mounted on a grating support to generate a second focusing direction, which is typically different from the focusing direction caused by the curved grating feature. Figure 6 Part E illustrates the further bending of the stack that causes the second focusing direction (the bending direction is from...). Figure 6 The double arrows in the diagram schematically indicate part E), thereby generating the focusing grating device 100.

[0070] In some embodiments, providing a stack may include providing a stack of resist layers having a thickness that varies along a first bending direction, to account for the fact that the thickness of the resist layer may vary on the substrate by bending.

[0071] The aforementioned method may include different steps of the LIGA process, which is a photolithography manufacturing technology that includes photolithography, electroplating, and molding, and is currently commonly used to manufacture such gratings with a large aspect ratio.

[0072] In an embodiment of the invention, the substrate can be enlarged during manufacturing to make it easier to clamp in the retainer. After manufacturing, the substrate can be cut to a suitable size.

Claims

1. A focusing grating device (100) for X-ray imaging, the X-ray imaging including dark-field imaging and / or phase-contrast imaging, the focusing grating device comprising: Substrate (402), and A grating, comprising a plurality of grating features (408) positioned on the substrate (402). The grating feature (408) is positioned not perpendicular to the surface of the substrate, thereby causing a first focusing direction for the X-ray radiation beam. Furthermore, the substrate (402) is curved, thereby causing a second focusing direction for the X-ray radiation beam, which is different from the first focusing direction.

2. The focusing grating device (100) according to claim 1, wherein The substrate (402) is bent only in a single direction.

3. The focusing grating device (100) according to claim 1 or 2, wherein The side edges of at least one of the grating features (408) are such that they form an angle of at least 1 degree relative to the vertical direction on the substrate for at least one location.

4. The focusing grating device (100) according to claim 1 or 2, wherein The focusing grating device (100) has an aspect ratio of at least 30.

5. The focusing grating device (100) according to claim 4, wherein The focusing grating device (100) has an aspect ratio of at least 50.

6. The focusing grating device (100) according to claim 1 or 2, wherein The bending stress in the second focusing direction of at least a portion of the grating feature (408) is significantly higher than the bending stress in the first focusing direction of at least a portion of the grating feature.

7. The focusing grating device (100) according to claim 1 or 2, wherein, The grating feature (408) is a grating sheet.

8. The focusing grating device (100) according to claim 1 or 2, wherein, The grating feature (408) is made of X-ray absorbing material.

9. The focusing grating device (100) according to claim 1 or 2, wherein, The substrate (402) comprises any one or a combination of graphite, glass, or Capton®.

10. An X-ray system (300) for grating-based X-ray imaging, comprising at least one focusing grating device (100) according to any one of claims 1-9, wherein, The X-ray system includes a dark-field imaging system and / or a phase-contrast imaging system.

11. A method for manufacturing a focusing grating device (100) for focusing an X-ray radiation beam in a first focusing direction and a second focusing direction, wherein the first focusing direction is different from the second focusing direction, The method includes: A stack of at least a substrate (402) and a resist layer (404) is obtained; Apply bending to the stack in the first focusing direction; A photolithography process is performed on the stack when the substrate (402) is bent in the first focusing direction; Electroplating the grating material to form grating features (408) substantially restores the curvature of the stack, and After restoring the bending, an additional bending is applied to the stack to induce a second focusing direction in the focusing grating device (100).

12. The method for manufacturing a focusing grating device (100) according to claim 11, wherein, The photolithography process includes applying a mask (406).

13. The method for manufacturing a focusing grating device (100) according to claim 11 or 12, wherein, The photolithography process includes a step of developing the resist layer (404), and the electroplating is performed before substantially restoring the curvature of the stack.

14. The method for manufacturing a focusing grating device (100) according to claim 11 or 12, wherein, Providing a stack including a resist layer (404) includes providing a stack of a resist layer (404) having a thickness that varies along the first focusing direction.

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