Diffraction gratings, diffraction grating waveguide system and manufacturing methods for diffraction grating

Abstract

Diffraction gratings (200, 300), a diffraction grating waveguide system (400) and manufacturing methods for a diffraction grating. During diffraction by diffraction gratings (200, 300), magnetic field distributions are concentrated in waist regions (222) of grating units (220, 320), there is a small magnetic field mode spot in each large magnetic field mode spot, and the intensities of green light and blue light in the small magnetic field mode spots may be weaker than those in surrounding regions while the intensity of red light therein may be stronger than that in the surrounding regions. That is to say, during diffraction, the grating units (220, 320) can excite high-order resonance simulation, i.e. large and small optical mode spots, in the magnetic field distributions, which increases the degree of freedom of the grating units (220, 320) regulating the diffraction of incident light. Thus, the diffraction gratings (200, 300) having high color uniformity can be obtained by means of optimization.

Description

Diffractive grating, diffractive grating waveguide system and preparation method of diffractive grating

[0001] The present application claims priority to the Chinese patent application No. 202410862710.9, filed on June 28, 2024, and entitled "A diffractive grating, a diffractive grating waveguide system and a preparation method of the diffractive grating", the content of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] Embodiments of the present application relate to the technical field of diffractive gratings, and in particular to a diffractive grating, a diffractive grating waveguide system and a preparation method of the diffractive grating. BACKGROUND

[0003] Augmented reality (AR) technology is expected to replace mobile phones, notebook computers, tablets and other electronic products, and become a new generation of computing platform. At present, a single-layer diffractive optical waveguide is one of the best solutions to realize lightweight full-color AR glasses. Since the same diffractive grating has dispersion phenomenon for different colors, different incident angles and different polarizations of incident light, the corresponding diffraction efficiency and diffraction angle have large deviations. In the process of design and development, the uniformity of color is one of the most concerned performance indicators of the single-layer diffractive optical waveguide. SUMMARY

[0004] Embodiments of the present application aim to provide a diffractive grating, a diffractive grating waveguide system and a preparation method of the diffractive grating, to solve the problem of large deviation of diffraction efficiency and diffraction angle of the existing diffractive grating for different incident light, so as to improve the color uniformity of the diffractive grating optical waveguide system.

[0005] To solve the above technical problems, the first aspect of the present application provides a diffractive grating, comprising:

[0006] a substrate; a plurality of grating units, each grating unit extending along a first direction and arranged on the substrate along a second direction; each grating unit comprises a bottom, a waist and a top in the direction perpendicular to the substrate in turn, the size of the waist in the second direction is smaller than the size of the bottom and the top in the second direction; the first direction and the second direction are parallel to the substrate, and the first direction is perpendicular to the second direction.

[0007] The second aspect of the present application further provides another diffractive grating, comprising:

[0008] A substrate; a plurality of grating units, the plurality of grating units being arranged on the substrate; each of the grating units comprising, in a direction perpendicular to the substrate, a bottom portion, a waist portion and a top portion in sequence, a size of the waist portion in a first direction being smaller than sizes of the bottom portion and the top portion in the first direction, the first direction being parallel to the substrate.

[0009] A third aspect of the present application provides a diffraction grating waveguide system, comprising:

[0010] An in-coupling grating and an out-coupling grating, the in-coupling grating being the diffraction grating of the first aspect, the out-coupling grating being the diffraction grating of the second aspect, the out-coupling grating being arranged in a direction of arrangement of grating units of the in-coupling grating; wherein the in-coupling grating is configured to receive incident light, and the out-coupling grating is configured to emit out-coupling light.

[0011] A fourth aspect of the present application provides a preparation method of a diffraction grating, for preparing the diffraction grating of any one of the preceding aspects, comprising:

[0012] Selecting a substrate, and coating a to-be-etched layer on the substrate;

[0013] Performing imprint on the to-be-etched layer, so as to form a flat layer and a grating unit layer protruding from the flat layer, the grating unit layer comprising a plurality of to-be-etched grating units;

[0014] Forming a mask layer on a surface of the flat layer and the grating unit layer away from the substrate;

[0015] Etching the to-be-etched grating units in a direction forming an angle with a thickness direction of the substrate, to form grating units comprising, in a direction perpendicular to the substrate, a bottom portion, a waist portion and a top portion in sequence, a size of the waist portion in a first direction being smaller than sizes of the bottom portion and the top portion in the first direction, the first direction being parallel to the substrate;

[0016] Removing the mask layer.

[0017] A fifth aspect of the present application further provides another preparation method of a diffraction grating, for preparing the diffraction grating of any one of the preceding aspects, comprising:

[0018] Selecting a substrate, and depositing a first to-be-etched layer on the substrate;

[0019] Coating a first photoresist layer on the first to-be-etched layer, and performing photoetching on the first photoresist layer, to form a plurality of first photoresist units spaced from each other on the first to-be-etched layer;

[0020] etching the first to-be-etched layer to form a first sub-grating layer, the first sub-grating layer comprising a plurality of first sub-grating units arranged along a first direction;

[0021] coating a second photoresist layer on the first sub-grating layer, and performing photolithography on the second photoresist layer to form a plurality of grooves in the second photoresist layer, the grooves being located one by one directly above the first sub-grating units;

[0022] depositing a second to-be-etched layer on the second photoresist layer and the first sub-grating layer, and performing planarization on the second to-be-etched layer to make the surface of the second to-be-etched layer flush with the surface of the second photoresist layer;

[0023] removing the first photoresist layer and the second photoresist layer to form a plurality of grating units on the substrate, the grating units comprising, in a direction perpendicular to the substrate, a bottom, a waist, and a top in sequence, and the size of the waist in the first direction being smaller than the size of the bottom and the top in the first direction, the first direction being parallel to the substrate.

[0024] Compared with the related art, in the embodiment of the present application, in diffraction, the magnetic field distribution is concentrated in the waist region of the grating unit, there is a small magnetic field spot in a large magnetic field spot, and the intensity of green light and blue light of the small magnetic field spot can be weaker than that of the surrounding area, and the intensity of red light can be stronger than that of the surrounding area. That is, in diffraction, the grating unit can excite high-order resonant modes in the magnetic field distribution, that is, large and small optical spots, which increases the degree of freedom of the regulation of the diffraction of the incident light, and thus a diffraction grating with high color uniformity can be obtained by optimization. BRIEF DESCRIPTION OF DRAWINGS

[0025] One or more embodiments are illustrated by way of example in the figures that are part of this disclosure and which are included to further provide explanatory aspects of the present embodiments. Unless otherwise noted, like elements in the drawings have like reference designations. The drawings provided are not to scale.

[0026] FIG. 1 is a schematic diagram of a perspective structure of a rectangular diffraction grating;

[0027] FIG. 2 is a schematic diagram of a diffraction light path of a rectangular diffraction grating;

[0028] FIG. 3 is a schematic diagram of a diffraction efficiency curve of a rectangular diffraction grating;

[0029] FIG. 4 is a schematic diagram of a magnetic field distribution of a rectangular diffraction grating in diffraction;

[0030] FIG. 5 is a schematic diagram of an electric field distribution of a rectangular diffraction grating in diffraction;

[0031] Figure 6 is a three-dimensional structural schematic diagram of a one-dimensional diffraction grating according to the first embodiment of this application;

[0032] Figure 7 is a schematic diagram of the diffraction light path of a one-dimensional diffraction grating according to the first embodiment of this application;

[0033] Figure 8 is a cross-sectional schematic diagram of the grating unit of the one-dimensional diffraction grating according to the first embodiment of this application;

[0034] Figure 9 is a schematic diagram of different cross-sections of the grating unit of the one-dimensional diffraction grating according to the first embodiment of this application;

[0035] Figure 10 is a schematic diagram of the diffraction efficiency of a one-dimensional diffraction grating according to the first embodiment of this application;

[0036] Figure 11 is a schematic diagram of the magnetic field distribution of a one-dimensional diffraction grating in the first embodiment of this application during diffraction.

[0037] Figure 12 is a schematic diagram of the electric field distribution of a one-dimensional diffraction grating in the first embodiment of this application during diffraction.

[0038] Figure 13 is a three-dimensional structural schematic diagram of a two-dimensional diffraction grating according to the second embodiment of this application;

[0039] Figure 14 is a three-dimensional structural schematic diagram of the grating unit of the two-dimensional diffraction grating according to the second embodiment of this application;

[0040] Figure 15 is a cross-sectional schematic diagram of a grating unit of a two-dimensional diffraction grating according to the second embodiment of this application;

[0041] Figure 16 is a three-dimensional structural schematic diagram of the diffraction grating waveguide system according to the third embodiment of this application;

[0042] Figure 17 is a three-dimensional structural schematic diagram of another diffraction grating waveguide system according to the third embodiment of this application;

[0043] Figure 18 is a schematic diagram of the fabrication method of the diffraction grating according to the fourth embodiment of this application;

[0044] Figure 19 is a schematic diagram of the fabrication method of the one-dimensional diffraction grating in the fourth embodiment of this application;

[0045] Figure 20 is a schematic diagram of the fabrication method of the two-dimensional diffraction grating in the fourth embodiment of this application;

[0046] Figure 21 is a schematic diagram of the fabrication process of a one-dimensional diffraction grating in the fourth embodiment of this application;

[0047] Figure 22 is a schematic diagram of the fabrication method of the diffraction grating according to the fifth embodiment of this application;

[0048] Figure 23 is a schematic diagram of the fabrication method of the one-dimensional diffraction grating in the fifth embodiment of this application;

[0049] FIG. 24 is a schematic diagram of a method for preparing a two-dimensional diffraction grating in the fifth embodiment of the present application;

[0050] FIG. 25 is a schematic diagram of a method for preparing a one-dimensional diffraction grating in the fifth embodiment of the present application. DETAILED DESCRIPTION

[0051] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the various embodiments of the present application will be described in detail below with reference to the accompanying drawings. However, those of ordinary skill in the art can understand that, in the various embodiments of the present application, many technical details are presented in order to make the present application better understood by the readers. However, the technical solutions claimed by the present application can be implemented even without these technical details and based on various changes and modifications of the following embodiments.

[0052] In the embodiments of the present application, the terms "upper", "lower", "left", "right", "front", "back", "top", "bottom", "inner", "outer", "middle", "vertical", "horizontal", "lateral", "longitudinal", and the like indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings. These terms are mainly used to better describe the present application and its embodiments, and are not intended to limit the indicated devices, elements, or components to necessarily have a specific orientation, or to be constructed and operated in a specific orientation.

[0053] In addition, in addition to being used to indicate the orientation or positional relationship, the above-mentioned partial terms can also be used to indicate other meanings, for example, the term "upper" can also be used to indicate a certain dependent relationship or connection relationship in some cases. Those of ordinary skill in the art can understand the specific meaning of these terms in the present application according to the specific circumstances.

[0054] In addition, the terms "mount", "set", "provided with", "open", "connect", "connected" should be broadly understood. For example, it can be a fixed connection, a detachable connection, or a monolithic structure; it can be a mechanical connection, or an electrical connection; it can be a direct connection, or an indirect connection through an intermediate medium, or an internal communication between two devices, elements, or components. Those of ordinary skill in the art can understand the specific meaning of the above-mentioned terms in the present application according to the specific circumstances.

[0055] In addition, the terms "first", "second", and the like are mainly used to distinguish different devices, elements, or components (the specific types and structures can be the same or different), and are not intended to indicate or imply the relative importance and quantity of the indicated devices, elements, or components. Unless otherwise specified, the meaning of "multiple" is two or more.

[0056] In the present application, a "one-dimensional diffraction grating" refers to a diffraction grating having a plurality of spaced grating units in one direction and only one grating unit in other directions. A "two-dimensional diffraction grating" refers to a diffraction grating having a plurality of spaced grating units in one direction and a plurality of spaced grating units in another direction.

[0057] Please refer to FIGS. 1-2, in the AR device, a rectangular grating 102 is usually used as a diffraction element, but the single-layer rectangular grating 102 has a dispersion phenomenon when diffracting incident light of different colors, different incident angles or different polarizations, and the corresponding diffraction efficiency and diffraction angle have a large deviation, resulting in poor color uniformity of the diffracted outgoing light, which affects the display effect of the AR device. For example, as can be seen from FIG. 3, by screening the parameters of the rectangular grating 102 and the substrate 101, a point P1 is found that can make the T+1 diffraction efficiency of R (Red), G (Green) and B (Blue) light similar, wherein the corresponding T+1 diffraction efficiency of RGB light is 4.479%, 5.194% and 9.411% respectively. It can be seen that the diffraction efficiency of RGB light is low and the difference is large, and the dispersion is serious. In order to explore the essential reason for the serious dispersion of the rectangular grating 102, the electric field and magnetic field distribution of the rectangular grating 102 at the point P1 is studied. Please refer to FIG. 4 as well, wherein the magnetic field distribution of B, G and R light of the rectangular grating 102 in the x-z section from left to right in FIG. 4 can be seen. It can be seen that there is a large mode spot in the grating structure, and it moves vertically with the change of the incident wavelength, that is, the resonance intensity difference is large. Please refer to FIG. 5 as well, wherein the electric field distribution of B, G and R light of the rectangular grating 102 in the x-z section from left to right in FIG. 5 can be seen. It can be seen that the distribution of the magnetic field and the electric field in the rectangular grating 102 is similar, there is only one optical mode spot in the rectangular grating 102 at the center position (x=0), and with the increase of the incident wavelength (from B light to R light), the optical mode spot gradually moves upward, that is, the dispersion is obvious.

[0058] In order to solve the problem that the resonance difference of the rectangular grating 102 in the diffraction of light of different colors is large and the dispersion is obvious, an embodiment of the present application provides a diffraction grating, comprising: a substrate; a plurality of grating units, each of the grating units extends along a first direction and is arranged on the substrate along a second direction; in a direction perpendicular to the substrate, each of the grating units comprises a bottom, a waist and a top in sequence, the size of the waist in the second direction is smaller than the size of the bottom and the top in the second direction; the first direction and the second direction are parallel to the substrate, and the first direction is perpendicular to the second direction.

[0059] One embodiment of the present application further provides another diffraction grating, comprising: a substrate; a plurality of grating units arranged on the substrate; each of the grating units comprises, in a direction perpendicular to the substrate, a bottom, a waist and a top in sequence, wherein a size of the waist in a first direction is smaller than sizes of the bottom and the top in the first direction; and the first direction is parallel to the substrate.

[0060] One embodiment of the present application provides a diffraction grating waveguide system, comprising: a coupling-in grating and a coupling-out grating, the coupling-in grating is the first diffraction grating, the coupling-out grating is the second diffraction grating, and the coupling-out grating is arranged in a direction of grating unit arrangement of the coupling-in grating; wherein the coupling-in grating is configured to receive incident light, and the coupling-out grating is configured to emit outgoing light.

[0061] One embodiment of the present application provides a preparation method of a diffraction grating, comprising: selecting a substrate, and coating a to-be-etched layer on the substrate;

[0062] Performing imprint on the to-be-etched layer to form a flat layer and a grating unit layer protruding from the flat layer, wherein the grating unit layer comprises a plurality of to-be-etched grating units;

[0063] Forming a mask layer on a surface of the flat layer and the grating unit layer away from the substrate;

[0064] Etching the to-be-etched grating units in a direction at an angle with a thickness direction of the substrate to form grating units comprising, in a direction perpendicular to the substrate, a bottom, a waist and a top in sequence, wherein a size of the waist in a first direction is smaller than sizes of the bottom and the top in the first direction, and the first direction is parallel to the substrate;

[0065] Removing the mask layer.

[0066] One embodiment of the present application further provides another preparation method of a diffraction grating, comprising:

[0067] Selecting a substrate, and depositing a first to-be-etched layer on the substrate;

[0068] Coating a first photoresist layer on the first to-be-etched layer, and performing photoetching on the first photoresist layer to form a plurality of first photoresist units spaced from each other on the first to-be-etched layer;

[0069] Etching the first to-be-etched layer to form a first sub-grating layer, wherein the first sub-grating layer comprises a plurality of first sub-grating units arranged in a first direction;

[0070] coating a second photoresist layer on the first sub-grating layer, performing photolithography on the second photoresist layer, and forming a plurality of grooves in the second photoresist layer, the grooves being located one by one above the first sub-grating units;

[0071] depositing a second to-be-etched layer on the second photoresist layer and the first sub-grating layer, and performing planarization on the second to-be-etched layer to make the surface of the second to-be-etched layer flush with the surface of the second photoresist unit;

[0072] removing the first photoresist layer and the second photoresist layer to form a plurality of grating units arranged at intervals on the substrate, the grating units sequentially comprising a bottom, a waist, and a top in a direction perpendicular to the substrate, and the size of the waist in a first direction is smaller than the size of the bottom and the top in the first direction, the first direction being parallel to the substrate.

[0073] The diffraction grating provided by the embodiments of the present application has the following advantages. In the diffraction, the magnetic field distribution is concentrated in the waist region of the grating unit, there is a small magnetic field spot in a large magnetic field spot, and the intensity of green light and blue light of the small magnetic field spot can be weaker than that of the surrounding area, and the intensity of red light can be stronger than that of the surrounding area. That is, in the diffraction, the grating unit can excite high-order resonance modes in the magnetic field distribution, that is, large and small optical spots, which increases the degree of freedom of the regulation of the diffraction of the incident light, and thus a diffraction grating with high color uniformity can be obtained through optimization.

[0074] The implementation details of the diffraction grating, the diffraction grating waveguide system, and the preparation method of the diffraction grating of the present embodiment will be described in detail below. The following content is only provided for the implementation details for easy understanding, and is not necessary for implementing the present solution.

[0075] Referring to FIGS. 6 and 7, the first diffraction grating 200 of the first embodiment of the present application comprises a first substrate 210 and a plurality of first grating units 220. Each first grating unit 220 extends along a first direction y and is arranged on the first substrate 210 along a second direction x. In a direction perpendicular to the first substrate 210, each first grating unit 220 sequentially comprises a bottom 221, a waist 222, and a top 223, and the size of the waist 222 in the second direction x is smaller than the size of the bottom 221 and the top 223 in the second direction x. The first direction y and the second direction x are parallel to the first substrate 210, and the first direction y is perpendicular to the second direction x.

[0076] In the present embodiment, in the diffraction, the first grating unit 220 can excite high-order resonance modes in the magnetic field distribution, that is, large and small optical spots, which increases the degree of freedom of the regulation of the diffraction of the incident light, and thus a first diffraction grating 200 with high color uniformity can be obtained through optimization.

[0077] In the present application, the refractive index of the first substrate 210 and the first grating unit 220 is not specifically limited, and the size relationship of the refractive index of the two is also not specifically limited. The refractive index of the first substrate 210 can be less than, equal to, or greater than the refractive index of the first grating unit 220, preferably less than the refractive index of the first grating unit 220. Among them, the refractive index of the first grating unit 220 can be between 1.6-2.0.

[0078] Optionally, the first substrate 210 can be composed of an optical waveguide, optical glass or a silicon wafer, etc., as long as it can meet the actual use requirements, and the present application does not make specific limitations.

[0079] In the present embodiment, the first grating unit 220 is a long strip structure extending along the first direction y, and a plurality of first grating units 220 are arranged at equal intervals along the second direction x to form a one-dimensional diffraction grating.

[0080] Please refer to FIG. 8, in the present embodiment, each first grating unit 220 includes a first part 224 and a second part 225 which are sequentially stacked on the first substrate 210, the position where the first part 224 and the second part 225 meet is the waist 222 of the first grating unit 220, the cross section of the first part 224 along the second direction x is a first trapezoid, the cross section of the second part 225 along the second direction x is a second trapezoid, and the top edge of the first trapezoid coincides with the bottom edge of the second trapezoid to form a polygonal cross section. Specifically, the first trapezoid and the second trapezoid can be isosceles trapezoid, non-isosceles trapezoid or right trapezoid, and the shapes of the first trapezoid and the second trapezoid can be the same or different. In this way, when manufacturing the first diffraction grating 200, the specific shape of the first grating unit 220 can be selected according to actual needs, so that the first diffraction grating 200 can meet various application requirements.

[0081] Again refer to FIG. 8, and refer to FIG. 9, for the polygonal cross section, it has 7 basic geometric parameters: a1, a2, b1, b2, c, d1 and d2. It can be understood that when any 6 geometric parameters are selected, the 7th geometric parameter is calculated, therefore, in the specific design, any 6 geometric parameters can be taken as adjustable parameters, so as to design different shapes of cross section.

[0082] In some embodiments, the polygonal cross-section is a pentagonal cross-section. For example, the first trapezoid and the second trapezoid can both be right-angled trapezoids, in which the length of the bottom side of the first trapezoid is greater than the length of the top side, the length of the bottom side of the second trapezoid is less than the length of the top side, the length of the top side of the first trapezoid is equal to the length of the bottom side of the second trapezoid, and the right-angled legs of the first trapezoid and the second trapezoid are connected. In this way, the leg portion 222 of the cross-section of the first grating unit 220 in the arrangement direction (the second direction x) is smaller than the bottom portion 221 and the top portion 223, and the profile of the cross-section is pentagonal. In this case, the first grating unit 220 as a whole is a pentagonal prism extending in the first direction y.

[0083] In some embodiments, the pentagonal cross-section is provided with a chamfer at the connection between at least two adjacent sides. In other words, any one of the edges of the first grating unit 220 in the form of a pentagonal prism can be provided with a chamfer, and preferably a round chamfer. It can be understood that the edges of the two end faces of the first grating unit 220 in the first direction y can also be provided with a chamfer.

[0084] In some other embodiments, the polygonal cross-section is a hexagonal cross-section. For example, when the first trapezoid and the second trapezoid are both isosceles trapezoids, the length of the bottom side of the first trapezoid is greater than the length of the top side, the length of the bottom side of the second trapezoid is less than the length of the top side, and the length of the top side of the first trapezoid is equal to the length of the bottom side of the second trapezoid. In this way, the leg portion 222 of the cross-section of the first grating unit 220 in the arrangement direction (the second direction x) is smaller than the bottom portion 221 and the top portion 223, and the profile of the cross-section is hexagonal. It can be understood that when the first trapezoid and the second trapezoid are different trapezoids, a hexagonal cross-section can also be formed. When the first trapezoid and the second trapezoid are the same isosceles trapezoids, the cross-section of the first grating unit 220 in the arrangement direction is funnel-shaped. In this case, the first grating unit 220 as a whole is a hexagonal prism extending in the first direction y.

[0085] In some embodiments, the hexagonal cross-section is provided with a chamfer at the connection between any two adjacent sides. In other words, any one of the edges of the first grating unit 220 in the form of a hexagonal prism can be provided with a chamfer, and preferably a round chamfer. It can be understood that the edges of the two end faces of the first grating unit 220 in the first direction y can also be provided with a chamfer.

[0086] In the following, the first grating unit 220 will be described with a funnel-shaped cross-section, but this is not limiting.

[0087] Referring to FIG. 10, the parameters of the funnel-shaped first grating unit 220 and the first substrate 210 are screened, and when the incident angle of the incident light is 0, a point P2 is found, which can make the T+1 diffraction efficiencies of the R (Red), G (Green) and B (Blue) lights close to each other. The T+1 diffraction efficiencies of the corresponding RGB lights are 26.327%, 25.842% and 26.208% respectively. It can be seen that the diffraction efficiencies of the RGB lights of the first grating unit 220 are all higher than those of the rectangular grating 102, and the differences are small, and the dispersion is not obvious.

[0088] Referring to FIG. 11, the magnetic field distributions of the B, G and R lights of the first grating unit 220 in the x-z section are sequentially shown from left to right in FIG. 11. It can be seen that the resonance intensities of the funnel-shaped first grating unit 220 are close to each other, which is the main reason why the T+1 diffraction efficiencies of the three wavelengths are close to each other. In addition, it can be seen that the magnetic field distribution of the first grating unit 220 is more concentrated in the area where the waist 222 is located, and there is a small magnetic field spot in a large magnetic field spot, and for the G and B lights, the intensity of the small magnetic field spot can be weaker than that of the surrounding area, and for the R light, the intensity of the small magnetic field spot can be stronger than that of the surrounding area.

[0089] Referring to FIG. 12, the electric field distributions of the B, G and R lights of the first grating unit 220 in the x-z section are sequentially shown from left to right in FIG. 12. It can be seen that the funnel-shaped first grating unit 220 forms two optical spots at the central position (x=0), and the two optical spots gradually separate as the incident wavelength increases.

[0090] The above results show that for different incident lights (RGB), the funnel-shaped first grating unit 220 can excite a high-order resonance mode at the central position (x=0), i.e., has two optical spots, thereby increasing the regulation degree of freedom of the first grating unit 220 for the diffraction of the incident light, and further optimizing the first diffraction grating 200 with high color uniformity, i.e., the T+1 diffraction efficiencies of the R, G and B lights are close to each other.

[0091] The second embodiment of the present application is substantially the same as the first embodiment, and the main difference is that, in the present embodiment, referring to FIG. 13, the array of the second grating units 320 of the second diffraction grating 300 is arranged on the second substrate 310 to form a two-dimensional diffraction grating. Optionally, the second grating units 320 in the present embodiment can be arranged in a square array, a hexagonal array or other array arrangement, which is not limited in the present embodiment.

[0092] In the embodiment, each second grating unit 320 is arranged perpendicularly to the second substrate 310 and is generally columnar. For example, the second grating unit 320 can be prism-shaped with a thin waist or cylinder-shaped with a thin waist. The cylinder-shaped second grating unit 320 is taken as an example for description.

[0093] Referring to FIG. 14, in the embodiment, the first part 321 of the second grating unit 320 has a first trapezoidal cross section in the direction perpendicular to the second substrate 310, and the second part 322 has a second trapezoidal cross section in the direction perpendicular to the second substrate 310. The shapes of the first trapezoidal cross section and the second trapezoidal cross section can be the same or different. When the first trapezoidal cross section and the second trapezoidal cross section are the same isosceles trapezoidal cross section, the first part 321 and the second part 322 of the second grating unit 320 are both circular truncated cones, and the second part 322 is arranged above the first part 321. In this case, the cross section of the second grating unit 320 in the direction perpendicular to the second substrate 310 is funnel-shaped hexagonal cross section. When the first trapezoidal cross section and the second trapezoidal cross section are both right-angled trapezoidal cross sections and the right-angled sides of the two trapezoidal cross sections are connected, the cross section is pentagonal.

[0094] Referring to FIG. 15, similarly, whether the cross section is pentagonal or hexagonal, the junctions of any two adjacent sides are chamfered, preferably rounded. That is, any side of the second grating unit 320 can be chamfered or rounded.

[0095] Since the second grating unit 320 in the embodiment also has a design in which the waist size is smaller than the sizes of the bottom and the top, the second grating unit 320 in the embodiment has the same technical effects as the first grating unit 310 in the first embodiment.

[0096] The third embodiment of the present application provides a diffraction grating waveguide system 400, as shown in FIG. 16, which includes a coupling-in grating 410 and a coupling-out grating 420. The coupling-in grating 410 is the first diffraction grating 200 described in the first embodiment, and the coupling-out grating 420 is the second diffraction grating 300 described in the second embodiment. The coupling-out grating 420 is arranged in the direction (second direction x) of the arrangement of the first grating unit 220 of the coupling-in grating 410 (first diffraction grating 200) and is spaced apart from the coupling-in grating 410. The coupling-in grating 410 is used to receive incident light, and the coupling-out grating 420 is used to emit outgoing light.

[0097] Referring again to FIG. 16, in some embodiments, the first grating unit 220 and the second grating unit 320 described above can be arranged on the same third substrate 430. In this way, the volume of the diffraction grating waveguide system 400 can be reduced, which is conducive to the miniaturization design of electronic devices carrying the diffraction grating waveguide system 400.

[0098] Referring to FIG. 17, in some other embodiments, the first diffraction grating 200 and the second diffraction grating 300 can be separately arranged on the third substrate 430 if necessary, i.e., the third substrate 430 is provided with the first substrate 210 separately carrying the first grating unit 220 and the second substrate 310 separately carrying the second grating unit 320.

[0099] The diffraction grating waveguide system 400 of the present embodiment adopts the first diffraction grating 200 and the second diffraction grating 300 as described above, and thus, in the diffraction process, the diffraction grating waveguide system 400 can excite high-order resonance modes, thereby increasing the degree of freedom of the diffraction grating waveguide system 400 in regulating the diffraction of the incident light and improving the color uniformity of the diffraction grating waveguide system 400.

[0100] Referring to FIGS. 18-21, the fourth embodiment of the present application provides a preparation method for preparing the first diffraction grating 200 or the second diffraction grating 300, which comprises:

[0101] S100. Selecting a fourth substrate 510 and coating a first etching layer 520 on the fourth substrate 510;

[0102] S200. Imprinting the first etching layer 520 to form a first flat layer 521 and a first grating unit layer 522 protruding from the first flat layer 521, the first grating unit layer 522 comprising a plurality of first etching grating units 5221;

[0103] S300. Forming a mask layer 530 on the surface of the first flat layer 521 and the first grating unit layer 522 away from the fourth substrate 510;

[0104] S400. Etching the first etching grating unit 5221 in a direction at an angle to the thickness direction of the fourth substrate 510 to form a grating unit comprising a bottom, a waist and a top in sequence in a direction perpendicular to the fourth substrate 510, and the size of the waist in the second direction x is smaller than the size of the bottom and the top in the second direction x, and the second direction x is parallel to the fourth substrate 510;

[0105] S500. Removing the mask layer 530.

[0106] For step S100, the optical waveguide plate, optical glass or silicon wafer, etc. is usually selected as the fourth substrate 510 according to the actual application requirements, and then a first etching layer 520 is coated on the fourth substrate 510. In order to improve the uniformity of the coating, the first etching layer 520 can be prepared by using the spin coating method, and when the grating unit of a preset height is prepared, the thickness of the first etching layer 520 can be set to be equal to or slightly greater than the numerical value of the preset height, and preferably the thickness of the first etching layer 520 is set to be greater than the numerical value of the preset height.

[0107] For step S200, different imprinting molds are selected according to the diffraction grating to be prepared, which is one-dimensional or two-dimensional. When the diffraction grating to be prepared is a one-dimensional diffraction grating, there is S201. The first to-be-etched layer 520 is imprinted using a mold having a plurality of square grooves arranged in parallel, so that the first to-be-etched layer 520 forms a first flat layer 521 and a first grating unit layer 522 protruding from the first flat layer 521, the first grating unit layer 522 having a plurality of first to-be-etched grating units 5221 extending along a first direction y and arranged at intervals along a second direction x. More specifically, each first grating unit 5221 is a quadrangular prism extending along the first direction y, and the size of the quadrangular prism in the thickness direction of the fourth substrate 510 can be slightly greater than the preset height value of the grating unit.

[0108] Then, step S300 is performed, specifically, a layer of metal film 540 can be deposited on the first flat layer 521 and the first grating unit layer 522 as a mask, and the metal film layer 540 can be formed of molybdenum or chromium.

[0109] Then, for step S400, there is S401. The first to-be-etched grating unit 5221 is etched on at least one side of the extension direction (the first direction y) of the first to-be-etched grating unit 5221. For example, the sidewall of the first to-be-etched grating unit 5221 is etched at an angle that is an acute angle with the thickness direction of the fourth substrate 510 on the left side and / or the right side of the first to-be-etched grating unit 5221, so that the size of the waist of the first to-be-etched grating unit 5221 is smaller than that of the bottom and the top. Optionally, the etching method can be dry etching, wet etching or other etching methods, which are not limited in the present application.

[0110] For example, using dry etching, the left and right sides of the first to-be-etched grating unit 5221 are etched at an angle of 45° with the thickness direction of the fourth substrate 510, the top surface and the side surface of the first to-be-etched grating unit 5221 can be etched into a shape with an included angle of 45°, and the sidewall of the first to-be-etched grating unit 5221 can be etched into two inclined walls with an included angle of 90°, at this time, a1 and a2 of the grating unit are the same size. During etching, different shapes of grating units can be formed by adjusting the composition and proportion of etching gas, as well as the temperature, power, angle and other parameters of etching. In some embodiments, the composition of the etching gas can include chlorine (Cl2), trifluoromethane (CHF3), sulfur fluoride (SF6), hydrogen peroxide (H2Br), carbon tetrafluoride (CF4), argon (Ar) and oxygen (O2), etc. When the etching angles of the sidewalls of the first to-be-etched grating unit 5221 on both sides are different, the values of a1 and a2 of the formed grating unit are also different.

[0111] After the etching is completed, step S500 is performed to complete the fabrication of the one-dimensional diffraction grating.

[0112] When the diffraction grating to be prepared is a two-dimensional diffraction grating, there is S202. The first etching layer 520 is imprinted using a mold having a plurality of arrayed columnar grooves, so that the first etching layer 520 forms a first flat layer 521 and a first grating unit layer 522 protruding from the first flat layer 521, and the first grating unit layer 522 has a plurality of arrayed columnar first etching grating units 5222. Taking the cylindrical first etching grating unit 5222 as an example, at this time, the height of the cylindrical first etching grating unit 5222 can be equal to or slightly greater than the preset height value.

[0113] Then, step S300 is performed, and specifically, a layer of metal film 540 can be deposited on the first flat layer 521 and the first grating unit layer 522 as a mask.

[0114] Then, for step S400, there is S402. During etching of the first etching grating unit 5222, the fourth substrate 510 is rotated so that the sidewall of the first etching grating unit 5222 perpendicular to the first flat layer 521 is uniformly etched. By rotating the fourth substrate 510, the sidewall of the first etching grating unit 5222 can be etched, so that the grating unit forms a funnel-shaped grating with two circular table stacks.

[0115] After the etching is completed, step S500 is performed to complete the fabrication of the two-dimensional diffraction grating.

[0116] Referring to FIGS. 22-25, a fifth embodiment of the present application provides another method for preparing the first diffraction grating 200 or the second diffraction grating 300, which includes:

[0117] F100. A fifth substrate 610 is selected, and a second etching layer 620 is deposited on the fifth substrate 610;

[0118] F200. A first photoresist layer 630 is applied on the second etching layer 620, and the first photoresist layer 630 is subjected to photolithography to form a plurality of spaced first photoresist units 631 on the second etching layer 620;

[0119] F300. The second etching layer 620 is etched to form a second sub-grating layer 621, and the second sub-grating layer 621 includes a plurality of spaced second sub-grating units 6211;

[0120] F400. A second photoresist layer 640 is coated on the second sub-grating layer 621, and the second photoresist layer 640 is subjected to photolithography to form a plurality of first grooves 641, each of which is located directly above a second sub-grating unit 6211.

[0121] F500. A third to-be-etched layer 650 is deposited on the second photoresist layer 640 and the second sub-grating layer 621, and the third to-be-etched layer 650 is subjected to planarization to make the surface of the third to-be-etched layer 650 flush with the surface of the second photoresist layer 640.

[0122] F600. The first photoresist layer 630 and the second photoresist layer 640 are removed to form a plurality of grating units arranged at intervals on the fifth substrate 610, each of which includes a bottom, a waist and a top in sequence in a direction perpendicular to the fifth substrate 610, and the size of the waist in the second direction x is smaller than that of the bottom and the top in the second direction x, and the second direction x is parallel to the fifth substrate 610.

[0123] For step F100, it is substantially the same as S100 described above. In step F100, the second to-be-etched layer 620 deposited on the fifth substrate 610 is a metal oxide, such as zirconium oxide, titanium oxide or niobium oxide, etc.

[0124] For step F200, an electron beam glue or other photoresist can be used and arranged on the second to-be-etched layer 620 by spin coating. When the first photoresist layer 630 is subjected to photolithography, different mask plates can be selected according to the type of the diffraction grating to be prepared. For example, if a one-dimensional diffraction grating is to be prepared, F201. a mask plate with a strip-shaped mask pattern is selected to cover the first photoresist layer 630, and the first photoresist layer 630 is subjected to exposure to form a plurality of strip-shaped first photoresist units 631 arranged at intervals along the second direction x, and each of the first photoresist units 631 has a second groove 632 between any two adjacent first photoresist units 631, and the second groove 632 exposes part of the second to-be-etched layer 620.

[0125] At this time, for step F300, there is F301. The power distribution of the etching light source or the exposure time of different regions of the second layer to be etched 620 is regulated to form a second sub-grating layer 621, and the second sub-grating layer 621 includes a plurality of second sub-grating units 6211 which extend in the same direction as the first photoresist unit 631 and have a first trapezoidal cross section along the second direction x. That is, the second sub-grating unit 6211 extends along the first direction y and is arranged and distributed in intervals along the second direction x. During etching, the power of the etching light source can be symmetrically distributed, with more power in the middle and gradually decreasing power on both sides; or, during etching, some regions of the second layer to be etched 620 can be concentratedly etched, and when the regions on both sides of the etched region are etched, the etching time can be gradually reduced along the front line away from the region. In this way, a second groove 632 with a trapezoidal cross section along the second direction x can be etched on the second layer to be etched 620, and a plurality of second sub-grating units 6211 with a first trapezoidal cross section can be formed. After the etching of the second layer to be etched 620 is completed, the first photoresist layer 630 is removed.

[0126] After that, for step F400, there is F401. The power distribution of the etching light source or the exposure time of different regions of the second photoresist layer 640 is regulated to make the second photoresist layer 640 form a plurality of first grooves 641 which extend in the same direction as the second sub-grating unit 640, each first groove 641 has a second trapezoidal cross section along the second direction x, and the top edge of the first trapezoid coincides with the bottom edge of the second trapezoid. A second photoresist layer 640 is coated on the second sub-grating layer 621, and the total height of the second photoresist layer 640 can be the same as or slightly greater than the height of the grating unit to be made. Then, the second photoresist layer 640 is etched using the same mask plate as in step F201, specifically, the part of the second photoresist layer 640 above the second sub-grating unit 6211 is etched away to form a first groove 641, and the second sub-grating unit 6211 is exposed, and the first groove 641 has a second trapezoidal cross section along the second direction x.

[0127] After that, for step F500, a third layer to be etched 650 is deposited on the second photoresist layer 640 and the second sub-grating layer 621, so that the third layer to be etched 650 completely fills the first groove 641, and the material of the third layer to be etched 650 is the same as that of the second layer to be etched 620. After the third layer to be etched 650 is solidified, the third layer to be etched 650 is planarized so that the surface of the third layer to be etched 650 is flush with the second photoresist layer 640. The third layer to be etched 650 includes a plurality of third sub-grating units 651 arranged in intervals along the second direction x, the second sub-grating unit 651 is located directly above the second sub-grating unit 6211, and is connected to the second sub-grating unit 6211.

[0128] Finally, for step F600, the first photoresist layer 630 and the second photoresist layer 640 can be completely removed by lithography or the like, at this time, the second sub-grating unit 6221 and the third sub-grating unit 651 together constitute a grating unit, the waist of the grating unit has a size smaller than the size of the bottom and the top.

[0129] When the type of the grating to be prepared is a two-dimensional diffraction grating, for step F200, there is F202. A mask plate with a plurality of array-distributed circular mask patterns is selected to cover the first photoresist layer 630, and the first photoresist layer 630 is exposed to form a plurality of array-distributed cylindrical first photoresist units 631 on the first etching layer 620. The specific array distribution form of the first photoresist units 631 is not limited, which can be a right-angle array distribution or other types of distribution.

[0130] After that, for step F300, there is F302. The power distribution of the etching light source or the exposure time of different regions of the second etching layer 620 is regulated to form a second sub-grating layer 621, which includes a plurality of array-distributed second sub-grating units 6211 with a first trapezoidal cross section along the second direction x. Specifically, after etching the first photoresist layer 630, the first photoresist layer 631 has a grid-shaped second groove 632, which exposes a part of the second etching layer 620. By regulating the power distribution of the etching light source to etch this part of the region, or by regulating the exposure time of this part of the region, the second etching layer 620 located directly below the cylindrical second photoresist unit 631 forms a circular truncated cone shape or a structure similar to a circular truncated cone shape. The power regulation of the etching light source or the exposure time (etching time) regulation of the etched region of the second etching layer 620 can refer to the above content, which will not be repeated here. After the second etching layer 620 is etched, a plurality of array-distributed circular truncated cone shapes or structures similar to circular truncated cone shapes are formed.

[0131] After that, for step F400, there is F402. The power distribution of the etching light source or the exposure time of different regions of the second photoresist layer is regulated to form a plurality of array-distributed grooves, each groove has a second trapezoidal cross section along the second direction, and the top edge of the first trapezoid coincides with the bottom edge of the second trapezoid. More specifically, after etching the second etching layer 620, a second photoresist layer 640 is applied, and the total thickness of the second photoresist layer 640 can be equal to or slightly greater than the height of the grating unit to be prepared. After the second photoresist layer 640 is solidified, a mask plate with a plurality of array-distributed circular transparent patterns is used to perform lithography on the second photoresist layer 640 to form a plurality of circular truncated cone shapes or structures similar to circular truncated cone shapes, which are located directly above the second sub-grating unit 6211 and expose the second sub-grating unit 6211.

[0132] After that, for step F500, a third to-be-etched layer 650 is deposited on the second photoresist layer 640 and the second sub-grating layer 621, so that the third to-be-etched layer 650 completely fills the first groove 641, and the material of the third to-be-etched layer 650 is the same as that of the second to-be-etched layer 620. After the third to-be-etched layer 650 is solidified, the third to-be-etched layer 650 is planarized, so that the surface of the third to-be-etched layer 650 is flush with the second photoresist layer 640. The third to-be-etched layer 650 includes a plurality of arrayed third sub-grating units 651, the third sub-grating units 651 are located directly above the second sub-grating units 6211, and are connected with the second sub-grating units 6211.

[0133] Finally, for step F600, the first photoresist layer 630 and the second photoresist layer 640 can be completely removed by photolithography or the like. At this time, the second sub-grating unit 6221 and the third sub-grating unit 651 jointly constitute a grating unit, and the size of the waist of the grating unit is smaller than the size of the bottom and the top.

[0134] The above describes the diffraction grating, the diffraction grating waveguide system and the preparation method of the diffraction grating provided by the embodiments of the present application in detail. The principles and implementation manners of the present application are described by applying specific examples. The above implementation manners are only used to help understand the idea of the present application, and there will be changes in the specific implementation manners and application ranges. In summary, the content of the present specification should not be understood as a limitation of the present application.

Claims

1. A diffraction grating, characterized in that, include: substrate; A plurality of grating units, each of the grating units extending along a first direction and arranged along a second direction on the substrate; In a direction perpendicular to the substrate, each grating unit sequentially includes a bottom, a waist, and a top. The dimension of the waist in the second direction is smaller than the dimensions of the bottom and the top in the second direction. The first direction and the second direction are parallel to the substrate, and the first direction is perpendicular to the second direction.

2. The diffraction grating according to claim 1, characterized in that, Each grating unit includes a first part and a second part stacked sequentially on the substrate. The position where the first part and the second part meet is the waist of the grating unit. The cross-section of the first part along the second direction is a first trapezoid, and the cross-section of the second part along the second direction is a second trapezoid. The top edge of the first trapezoid and the bottom edge of the second trapezoid coincide to form a polygonal cross-section.

3. The diffraction grating according to claim 2, characterized in that, The polygonal cross section is a hexagonal cross section, and the junction of at least two adjacent sides of the hexagonal cross section is chamfered; Alternatively, the polygonal cross-section may be a pentagonal cross-section, wherein at least two adjacent sides of the pentagonal cross-section are chamfered.

4. A diffraction grating, characterized in that, substrate; A plurality of grating units are arranged in an array on the substrate; in a direction perpendicular to the substrate, each grating unit sequentially includes a bottom, a waist, and a top, wherein the dimension of the waist in a first direction is smaller than the dimensions of the bottom and the top in the first direction; the first direction is parallel to the substrate.

5. The diffraction grating according to claim 4, characterized in that, include: Each grating unit includes a first part and a second part stacked sequentially on the substrate. The position where the first part and the second part meet is the waist of the grating unit. The cross-section of the first part along the first direction is a first trapezoid, and the cross-section of the second part along the first direction is a second trapezoid. The top edge of the first trapezoid and the bottom edge of the second trapezoid coincide to form a polygonal cross-section.

6. The diffraction grating according to claim 5, characterized in that, The polygonal cross section is a hexagonal cross section, and the junction of at least two adjacent sides of the hexagonal cross section is chamfered; Alternatively, the polygonal cross-section may be a pentagonal cross-section, wherein at least two adjacent sides of the pentagonal cross-section are chamfered.

7. A diffraction grating waveguide system, characterized in that, include: The coupling-in grating and the coupling-out grating, wherein the coupling-in grating is a diffraction grating as described in any one of claims 1-3, and the coupling-out grating is a diffraction grating as described in any one of claims 4-6, wherein the coupling-out grating is spaced apart from the coupling-in grating in the arrangement direction of the grating units of the coupling-in grating; wherein the coupling-in grating is used to receive incident light, and the coupling-out grating is used to emit outgoing light.

8. A method for fabricating a diffraction grating, characterized in that, Select a substrate and coat a layer to be etched onto the substrate; Imprinting is performed on the layer to be etched to form a planar layer and a grating unit layer protruding from the planar layer, wherein the grating unit layer includes a plurality of grating units to be etched. A mask layer is formed on the surface of the planarization layer and the grating unit layer facing away from the substrate; The grating unit to be etched is etched in a direction that forms an angle with the thickness direction of the substrate, forming a grating unit that includes a bottom, a waist and a top in a direction perpendicular to the substrate, wherein the size of the waist in a first direction is smaller than the size of the bottom and the top in the first direction, and the first direction is parallel to the substrate. Remove the mask layer.

9. The preparation method according to claim 8, characterized in that, The grating unit to be etched is etched in a direction at an angle to the thickness direction of the substrate, forming a grating unit that sequentially includes a bottom, a waist, and a top in a direction perpendicular to the substrate, wherein the dimension of the waist in the first direction is smaller than the dimensions of the bottom and the top in the first direction. Specifically: The grating unit to be etched is etched on at least one side of the two sides of the extension direction of the grating unit to be etched.

10. The preparation method according to claim 8, characterized in that, The grating unit to be etched is etched in a direction at an angle to the thickness direction of the substrate, forming a grating unit that sequentially includes a bottom, a waist, and a top in a direction perpendicular to the substrate, wherein the dimension of the waist in the first direction is smaller than the dimensions of the bottom and the top in the first direction. Specifically: During the etching of the grating unit to be etched, the substrate is rotated so that the sidewalls of the grating unit to be etched that are perpendicular to the planar layer are uniformly etched.

11. A method for fabricating a diffraction grating, characterized in that, Select a substrate and deposit a first layer to be etched on the substrate; A first photoresist layer is coated on the first layer to be etched, and photolithography is performed on the first photoresist layer to form a plurality of mutually spaced first photoresist units on the first layer to be etched. The first layer to be etched is etched to form a first sub-grating layer, the first sub-grating layer including a plurality of first sub-grating units arranged along a first direction; A second photoresist layer is coated on the first sub-grating layer, and photolithography is performed on the second photoresist layer to form multiple grooves, each groove being located directly above the first sub-grating unit. A second etchable layer is deposited on the second photoresist layer and the first sub-grating layer, and the second etchable layer is planarized so that the surface of the second etchable layer is flush with the surface of the second photoresist layer. The first photoresist layer and the second photoresist layer are removed to form a plurality of grating units that are spaced apart from each other on the substrate. The grating units include a bottom, a waist and a top in a direction perpendicular to the substrate, and the size of the waist in a first direction is smaller than the size of the bottom and the top in the first direction. The first direction is parallel to the substrate.

12. The preparation method according to claim 11, characterized in that, The etching of the first layer to be etched forms a first sub-grating layer, the first sub-grating layer comprising a plurality of mutually spaced first sub-grating units, specifically: The power distribution of the etching light source or the exposure time of different regions of the first layer to be etched is adjusted to form a first sub-grating layer. The first sub-grating layer includes a plurality of first sub-grating units that extend in the same direction as the first photoresist unit and have a first trapezoidal cross section along the first direction.

13. The preparation method according to claim 12, characterized in that, A second photoresist layer is coated on the first sub-grating layer, and photolithography is performed on the second photoresist layer to form multiple grooves. Each groove is located directly above the first sub-grating unit. Specifically: By adjusting the power distribution of the etching light source or the exposure time of different regions of the second photoresist layer, the second photoresist layer forms a plurality of grooves extending in the same direction as the first sub-grating unit. The cross-section of each groove along the first direction is a second trapezoid, and the top edge of the first trapezoid coincides with the bottom edge of the second trapezoid.

14. The preparation method according to claim 11, characterized in that, The etching of the first layer to be etched forms a first sub-grating layer, the first sub-grating layer comprising a plurality of mutually spaced first sub-grating units, specifically: The power distribution of the etching light source or the exposure time of different regions of the first layer to be etched is adjusted to form a first sub-grating layer. The first sub-grating layer includes multiple arrayed first sub-grating units with a cross-section of a first trapezoid along the first direction.

15. The preparation method according to claim 14, characterized in that, A second photoresist layer is coated on the first sub-grating layer, and photolithography is performed on the second photoresist layer to form multiple grooves. Each groove is located directly above the first sub-grating unit. Specifically: By adjusting the power distribution of the etching light source or the exposure time of different regions of the second photoresist layer, the second photoresist layer forms a plurality of arrayed grooves, each groove having a cross-section of a second trapezoid along the first direction, and the top edge of the first trapezoid coinciding with the bottom edge of the second trapezoid.

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