Diffraction grating and mold for forming diffraction grating

Abstract

The present invention provides a diffraction grating capable of improving the uniformity of the grating height without using an etching stopper layer. [Solution] A diffraction grating having a stepped cross-sectional shape on the surface of a light-transmitting substrate, wherein the diffraction grating structure consists of a single member, and the diffraction grating structure has a first diffraction grating section and a second diffraction grating section having different grating periods and number of steps, wherein the grating period of the second diffraction grating section is shorter than the grating period of the first diffraction grating section, the number of steps of the second diffraction grating section is greater than the number of steps of the first diffraction grating section, and the difference between the grating height of the first diffraction grating section and the grating height of the second diffraction grating section is smaller than the minimum height of one step of the first diffraction grating section.

Description

Technical Field

[0001] The present disclosure relates to a diffraction grating and a mold for forming a diffraction grating.

Background Art

[0002] As a diffraction grating, a diffraction grating having a stepped cross-sectional shape is known.

[0003] Conventionally, by repeating lithography and etching N times, a diffraction grating having a stepped cross-sectional shape with 2 to the Nth power steps is formed. Further, when forming a plurality of types of diffraction grating portions having different grating periods, the diffraction grating portion having a short grating period is formed with fewer steps than the diffraction grating portion having a long grating period from the viewpoint of manufacturing limitations. This method is described in, for example, Non-Patent Document 1.

[0004] However, among a plurality of types of diffraction grating portions having different grating periods, the diffraction grating portion having a short grating period has a reduced number of steps, so the diffraction efficiency decreases.

[0005] In addition, due to the characteristics of the etching apparatus, etching conditions, etc., over-etching or under-etching may occur, and the grating height may deviate from the design value. For example, when forming a plurality of types of diffraction grating portions having different grating periods, due to the characteristics of the etching apparatus, etching conditions, etc., in the diffraction grating portion having a short grating period, the grating height may be lower than that of the diffraction grating portion having a long grating period. Similarly in such a case, the grating height becomes non-uniform and the diffraction efficiency decreases.

[0006] In Patent Document 1 and Patent Document 2, as a method for reproducing the design value with high accuracy, a method of arranging an etching stopper layer for each step is described. However, in this method, since it is necessary to form an etching stopper layer for each step in advance, the manufacturing process becomes complicated and requires a long time.

Prior Art Documents

Patent Documents

[0007] [Patent Document 1] Japanese Patent Application Publication No. 8-15510 [Patent Document 2] Japanese Patent Publication No. 2000-181076 [Non-patent literature]

[0008] [Non-Patent Document 1] Zhiyu Zhang et al., “Hybrid-level Fresnel zone plate for diffraction efficiency enhancement” Optics Express Vol.25, Issue26, pp.33676-33687(2017) [Overview of the project] [Problems that the invention aims to solve]

[0009] This disclosure has been made in view of the above circumstances, and its main purpose is to provide a diffraction grating that can improve the uniformity of the grating height without using an etching stopper layer. [Means for solving the problem]

[0010] One embodiment of the present disclosure provides a diffraction grating having a stepped cross-sectional shape on the surface of a light-transmitting substrate, wherein the diffraction grating structure consists of a single member, and the diffraction grating structure has a first diffraction grating portion and a second diffraction grating portion having different grating periods and number of steps, wherein the grating period of the second diffraction grating portion is shorter than the grating period of the first diffraction grating portion, the number of steps of the second diffraction grating portion is greater than the number of steps of the first diffraction grating portion, and the difference between the grating height of the first diffraction grating portion and the grating height of the second diffraction grating portion is smaller than the minimum height of one step of the first diffraction grating portion.

[0011] In this disclosure, it is preferable that the difference between the grating height of the first diffraction grating and the grating height of the second diffraction grating is 15% or less with respect to 100% of the grating height of the first diffraction grating.

[0012] In this disclosure, it is preferable that the number of stages in the first diffraction grating portion is 7 or more.

[0013] One embodiment of the present disclosure provides a mold for forming a diffraction grating, having a pattern structure for replicating the above-mentioned diffraction grating.

[0014] One embodiment of the present disclosure provides a diffraction grating forming mold having a pattern structure with a stepped cross-sectional shape on the surface of a light-transmitting substrate, wherein the pattern structure consists of a single member, and the pattern structure has a first pattern section and a second pattern section with different pattern periods and number of steps, the pattern period of the second pattern section is shorter than the pattern period of the first pattern section, the number of steps of the second pattern section is greater than the number of steps of the first pattern section, and the difference between the pattern height of the first pattern section and the pattern height of the second pattern section is smaller than the minimum height of one step of the first pattern section.

[0015] In this disclosure, it is preferable that the difference between the pattern height of the first pattern portion and the pattern height of the second pattern portion is 15% or less of the pattern height of the first pattern portion (100%).

[0016] In this disclosure, it is preferable that the number of steps in the first pattern section is 7 or more.

[0017] One embodiment of the present disclosure provides a diffraction grating having a diffraction grating structure replicated by the above-mentioned diffraction grating forming mold. [Effects of the Invention]

[0018] The diffraction grating of this disclosure has the effect of improving the uniformity of the grating height and thus improving diffraction efficiency. [Brief explanation of the drawing]

[0019] [Figure 1] It is a schematic cross-sectional view illustrating the diffraction grating of the present disclosure. [Figure 2] It is a process diagram illustrating a method for manufacturing a conventional diffraction grating. [Figure 3] It is a process diagram illustrating a method for manufacturing a conventional diffraction grating. [Figure 4] It is a schematic cross-sectional view illustrating a conventional diffraction grating. [Figure 5] It is a schematic cross-sectional view illustrating the diffraction grating of the present disclosure. [Figure 6] It is a schematic cross-sectional view illustrating the diffraction grating of the present disclosure. [Figure 7] It is a schematic cross-sectional view illustrating the diffraction grating of the present disclosure. [Figure 8] It is a schematic cross-sectional view illustrating the diffraction grating of the present disclosure. [Figure 9] It is a process diagram illustrating a method for manufacturing the diffraction grating of the present disclosure. [Figure 10] It is a process diagram illustrating a method for manufacturing the diffraction grating of the present disclosure. [Figure 11] It is a schematic cross-sectional view showing the simulation model of Reference Example 1. [Figure 12] It is a graph showing the simulation results of Reference Example 1. [Figure 13] It is a graph showing the simulation results of Reference Example 2.

Embodiments for Carrying Out the Invention

[0020] Embodiments of this disclosure will be described below with reference to drawings and other figures. However, this disclosure can be implemented in many different ways and should not be interpreted as being limited to the embodiments described below. In addition, in order to make the explanation clearer, the drawings may schematically represent the width, thickness, shape, etc. of each part compared to the actual form, but these are merely examples and should not limit the interpretation of this disclosure. Furthermore, in this specification and each figure, elements similar to those described above with respect to previously shown figures will be denoted by the same reference numerals, and detailed explanations may be omitted as appropriate.

[0021] In this specification, when describing a configuration in which one component is placed on top of another component, the terms "on top" or "below" include, unless otherwise specified, both cases: when one component is placed directly above or below another component so as to be in contact with the other component, and when another component is placed above or below another component via yet another component. Furthermore, in this specification, when describing a configuration in which one component is placed on the surface of another component, the terms "on the surface" include, unless otherwise specified, both cases: when one component is placed directly above or below another component so as to be in contact with the other component, and when another component is placed above or below another component via yet another component.

[0022] The diffraction grating of this disclosure will be described in detail below.

[0023] The diffraction grating of the present disclosure is a diffraction grating having a stepped cross-sectional shape on the surface of a light-transmitting substrate, wherein the diffraction grating structure consists of a single member, and the diffraction grating structure has a first diffraction grating portion and a second diffraction grating portion having different grating periods and number of steps, wherein the grating period of the second diffraction grating portion is shorter than the grating period of the first diffraction grating portion, the number of steps of the second diffraction grating portion is greater than the number of steps of the first diffraction grating portion, and the difference between the grating height of the first diffraction grating portion and the grating height of the second diffraction grating portion is smaller than the minimum height of one step of the first diffraction grating portion.

[0024] Figure 1 is a schematic cross-sectional view illustrating a diffraction grating of the present disclosure. The diffraction grating 1 in Figure 1 has a diffraction grating structure 3 on the surface of a light-transmitting substrate 2, with a stepped cross-sectional shape. A stepped shape refers to a shape having two or more steps. The diffraction grating structure 3 consists of a single member and has a first diffraction grating section 4 and a second diffraction grating section 5 with different grating periods and numbers of steps. The grating period P2 of the second diffraction grating section 5 is shorter than the grating period P1 of the first diffraction grating section 4. Also, the number of steps in the second diffraction grating section 5 is greater than the number of steps in the first diffraction grating section 4. For example, in Figure 1, the number of steps in the first diffraction grating section 4 is 8, and the number of steps in the second diffraction grating section 5 is 9. Furthermore, the difference between the grating height H1 of the first diffraction grating section 4 and the grating height H2 of the second diffraction grating section 5 is smaller than the minimum height D1 of one step in the first diffraction grating section 4.

[0025] The minimum height D1 of a stage in the first diffraction grating section 4 refers to the smallest height among the heights of each stage in the first diffraction grating section 4. Figure 1 shows the case where the height of each stage in the first diffraction grating section 4 is the same for all stages. In this case, the height of all stages in the first diffraction grating section 4 is the minimum height D1. In other words, the minimum height D1 of a stage in the first diffraction grating section 4 may be the height of a stage in the first diffraction grating section 4. In Figure 1, for example, the height of the lowest stage in the first diffraction grating section 4 is taken as the minimum height D1.

[0026] As a conventional method for manufacturing diffraction gratings, a method for forming a diffraction grating structure having a 2 to the power of N step shape is described by repeating lithography and etching N times. Figures 2(a) to (j) and 3(a) to (b) are process diagrams illustrating a conventional method for manufacturing diffraction gratings. As shown in Figure 2(a), a resist layer 103a is formed on a substrate 102, and a pattern is exposed by irradiating it with an energy ray 105 such as ultraviolet light through a mask 104. As shown in Figure 2(b), a resist pattern 103b is formed. As shown in Figure 2(c), the substrate 102 is etched. As shown in Figure 2(d), the resist pattern 103b is removed. This forms a 2-step step shape. As shown in Figures 2(e) to (h), the formation of the resist layer 103a, pattern exposure, etching of the substrate 102, and removal of the resist pattern 103b are further performed to form a 4-step step shape. As shown in Figures 2(i) to (j) and Figures 3(a) to (b), the resist layer 103a is formed, the pattern is exposed, the substrate 102 is etched, and the resist pattern 103b is removed to form an 8-step staircase shape. In this way, by repeating lithography and etching three times, a diffraction grating structure with 2 to the power of 3, or 8, steps can be formed.

[0027] In conventional methods for manufacturing diffraction gratings, when forming multiple types of diffraction grating sections with different grating periods, the diffraction grating sections with shorter grating periods are formed with fewer stages than the diffraction grating sections with longer grating periods, from the viewpoint of manufacturing limitations.

[0028] In conventional methods for manufacturing diffraction gratings, if the number of stages in the diffraction grating section with a short grating period is reduced, when the number of stages in the diffraction grating section with a long grating period is 2 to the power of N, the number of stages in the diffraction grating section with a short grating period becomes, for example, 2 to the power of (N-1). For example, as shown in Figure 4(a), when the number of stages in the diffraction grating section 111 with a long grating period is 2 to the power of 3, or 8 stages, the number of stages in the diffraction grating section 112 with a short grating period becomes 2 to the power of 2, or 4 stages. Also, for example, as shown in Figure 4(b), when the number of stages in the diffraction grating section 111 with a long grating period is 16 stages (2 to the power of 4), the number of stages in the diffraction grating section 112 with a short grating period becomes 2 to the power of 3, or 8 stages. In this case, the grating height h2 of the diffraction grating section 112 with a short grating period becomes lower than the grating height h1 of the diffraction grating section 111 with a long grating period by the height d1 of one stage in the diffraction grating section 111 with a long grating period. Therefore, in the diffraction grating section 112 with a short grating period, the number of stages decreases, resulting in a reduction in diffraction efficiency.

[0029] Furthermore, in the manufacturing method of diffraction gratings, over-etching or under-etching may occur depending on the characteristics of the etching equipment and etching conditions, causing the grating height to deviate from the design value. For example, when forming multiple types of diffraction grating sections with different grating periods in the manufacturing method of diffraction gratings, the grating height of the diffraction grating section with a shorter grating period may be lower than that of the diffraction grating section with a longer grating period, depending on the characteristics of the etching equipment and etching conditions. In such cases, the grating height becomes non-uniform, and the diffraction efficiency decreases.

[0030] In this disclosure, the grating period of the second diffraction grating is shorter than that of the first diffraction grating, but the number of stages in the second diffraction grating is greater than that of the first diffraction grating. Furthermore, by increasing the number of stages in the second diffraction grating section, which has a shorter grating period, the difference between the grating height of the first diffraction grating section and the grating height of the second diffraction grating section can be made smaller than the minimum height of one stage in the first diffraction grating section, thereby making the grating height uniform. Therefore, it is possible to improve the diffraction efficiency.

[0031] The following describes the configurations of the diffraction gratings of this disclosure.

[0032] 1. Diffraction grating structure The diffraction grating of this disclosure has a diffraction grating structure with a stepped cross-sectional shape on the surface of a light-transmitting substrate.

[0033] The diffraction grating structure has a first diffraction grating section and a second diffraction grating section with different grating periods and number of stages. The grating period of the second diffraction grating section is shorter than that of the first diffraction grating section, the number of stages in the second diffraction grating section is greater than that of the first diffraction grating section, and the difference between the grating height of the first diffraction grating section and the grating height of the second diffraction grating section is smaller than the minimum height of one stage in the first diffraction grating section.

[0034] The difference between the grating height of the first diffraction grating and the grating height of the second diffraction grating is smaller than the minimum height of one stage in the first diffraction grating. This improves the uniformity of the grating height of the diffraction grating structure and increases the diffraction efficiency. The height of one stage in the first diffraction grating will be described later.

[0035] The difference between the lattice height of the first diffraction grating and the lattice height of the second diffraction grating is preferably 15% or less, more preferably 9% or less, and even more preferably 6% or less, relative to the lattice height of the first diffraction grating (100%). A difference of 15% or less enhances the uniformity of the lattice height of the diffraction grating structure and increases diffraction efficiency. A difference of 9% or less, or 6% or less, further enhances diffraction efficiency.

[0036] The grating height of the first diffraction grating section is set appropriately according to the application of the diffraction grating. Generally, if the height of each stage is the same for all stages, it can be set to a grating height H1, which can be obtained from the following equations (1) and (2). H1 = D1 × (L-1) (1) D1 = (2 × λ) / (L × 2 × Δn) (2) D1 is the height of one stage. L is the number of stages. λ is the wavelength of the incident light. Δn is the difference between the refractive index n1 of the first diffraction grating and the refractive index n2 of the medium in contact with the first diffraction grating, and is the value obtained by subtracting the refractive index n2 of the medium in contact with the first diffraction grating from the refractive index n1 of the first diffraction grating. For example, when the wavelength λ of the incident light is 940 nm, the number of stages L is 8, and the refractive index difference Δn is 0.5, the grating height H1 is 1645 nm.

[0037] The grating height of the second diffraction grating can be the same as the grating height of the first diffraction grating described above.

[0038] The grating height of the second diffraction grating may be higher or lower than the grating height of the first diffraction grating, or it may be the same as the grating height of the first diffraction grating, as long as the difference between it and the grating height of the first diffraction grating is within a predetermined range.

[0039] The lattice height of each diffraction grating is the distance from the top to the bottom of each diffraction grating in the thickness direction of the diffraction grating. For example, as shown in Figure 1, the lattice height H1 of the first diffraction grating 4 is the distance from the top to the bottom of the first diffraction grating 4, and the lattice height H2 of the second diffraction grating 5 is the distance from the top to the bottom of the second diffraction grating 5.

[0040] The height of each stage in the first diffraction grating is set appropriately according to the grating height, number of stages, and approximate shape of the first diffraction grating. For example, if the height of each stage is set to be the same for all stages, it can be set to the height D1 of each stage obtained from equation (2) above. If the height of each stage is set to be the same for all stages, for example, when the wavelength λ of the incident light is 940 nm, the number of stages L is 8, and the refractive index difference Δn is 0.5, the height D1 of each stage will be 235 nm.

[0041] The height of one stage in the second diffraction grating can be the same as the height of one stage in the first diffraction grating described above.

[0042] In this disclosure, the difference between the grating height of the first diffraction grating and the grating height of the second diffraction grating is small, and the number of stages in the second diffraction grating is greater than the number of stages in the first diffraction grating. Therefore, if the height of each stage is set to be the same for all stages, the height of one stage in the second diffraction grating is smaller than the height of one stage in the first diffraction grating.

[0043] In each diffraction grating section, the height of each stage may be the same for all stages, or it may be different for each stage.

[0044] In the second diffraction grating section, for example, if the number of stages in the second diffraction grating section is N, and the sections from top to bottom are labeled as 1st, 2nd, ..., Nth, then if the height of each stage is the same for all stages except the Nth stage, the height of the Nth stage may be the same as or different from the heights of the other stages. If the height of the Nth stage is different from the heights of the other stages, the height of the Nth stage may be higher or lower than the heights of the other stages. For example, in Figures 5(a) and (b), the number of stages in the second diffraction grating section is 9, and the heights D2a of stages 1 to 8 are the same. In Figure 5(a), the height D2b of the 9th stage is lower than the heights D2a of stages 1 to 8. In Figure 5(b), the height D2b of the 9th stage is higher than the heights D2a of stages 1 to 8.

[0045] The grating period of the second diffraction grating is shorter than that of the first diffraction grating. The difference between the grating periods of the first and second diffraction gratings is not particularly limited as long as the grating period of the second diffraction grating is shorter than that of the first diffraction grating, and can be set appropriately depending on the application of the diffraction grating.

[0046] The lattice period of the first diffraction grating is set appropriately according to the application of the diffraction grating. For example, when the wavelength λ of the incident light is 940 nm, the number of stages L is 8, and the refractive index difference Δn is 0.5, the lattice period of the first diffraction grating can be in the range of 1.0 μm to 215.4 μm when the diffraction angle is between 0.25° and 70°, when the diffraction angle is between 0.25° and 50°, when the lattice period of the first diffraction grating can be in the range of 1.2 μm to 215.4 μm, and when the diffraction angle is between 0.25° and 30°, when the lattice period of the first diffraction grating can be in the range of 1.9 μm to 215.4 μm. If the lattice period of the first diffraction grating is too short, the lattice period of the second diffraction grating will be even shorter, which may make it difficult to manufacture the diffraction grating from the standpoint of manufacturing limits.

[0047] The lattice period of the second diffraction grating should be shorter than the lattice period of the first diffraction grating. If the lattice period of the second diffraction grating is too short, it may become difficult to manufacture the diffraction grating from the standpoint of manufacturing limitations.

[0048] The lattice period of each diffraction grating section is the distance from the outer endpoint of the top surface to the outer endpoint of the bottom surface in the plane direction of the diffraction grating. For example, as shown in Figure 1, the lattice period P1 of the first diffraction grating section 4 is the distance from the outer endpoint of the top surface to the outer endpoint of the bottom surface of the first diffraction grating section 4, and the lattice period P2 of the second diffraction grating section 5 is the distance from the outer endpoint of the top surface to the outer endpoint of the bottom surface of the second diffraction grating section 5. For example, as shown in Figures 1 and 6(a), for grooves in the diffraction grating structure 3 where the groove depth increases with each step, the lattice periods are P1, P2, and P. As shown in Figure 6(b), for grooves in the diffraction grating structure 3 where the groove depth increases or decreases with each step, the lattice period is not determined.

[0049] The width of each stage in the first and second diffraction grating sections is set appropriately according to the grating period and number of stages in the first and second diffraction grating sections, respectively. Note that if the width of each stage is too small, it may become difficult to manufacture the diffraction grating from a manufacturing limit perspective.

[0050] In this disclosure, since the grating period of the second diffraction grating is shorter than that of the first diffraction grating, and the number of stages in the second diffraction grating is greater than that of the first diffraction grating, the width of one stage in the second diffraction grating is usually smaller than the width of one stage in the first diffraction grating.

[0051] In each diffraction grating section, the width of each stage may be the same for all stages, or it may be different for each stage.

[0052] The number of stages in the second diffraction grating is greater than the number of stages in the first diffraction grating. The difference between the number of stages in the first diffraction grating and the number of stages in the second diffraction grating is not particularly limited as long as the number of stages in the second diffraction grating is greater than the number of stages in the first diffraction grating; for example, it may be 1 or more, or 1 to 4. The difference between the number of stages in the first diffraction grating and the number of stages in the second diffraction grating is usually 1.

[0053] The number of stages in the first and second diffraction gratings is preferably 7 or more, more preferably 12 or more, and even more preferably 16 or more. A number of stages of 7 or more can increase diffraction efficiency. A number of stages of 12 or more, or 16 or more, can further increase diffraction efficiency. However, there are limits to the effect of increasing the number of stages on improving diffraction efficiency. From the viewpoint of the number of processes, the number of stages in the first and second diffraction gratings is preferably 16 or less.

[0054] The number of stages in each diffraction grating section is the number of surfaces parallel to the surface of the light-transmitting substrate in each diffraction grating section. For example, in Figure 1, the first diffraction grating section 4 has 8 stages, and the second diffraction grating section 5 has 9 stages.

[0055] The diffraction grating structure only needs to have at least a first diffraction grating section and a second diffraction grating section with different grating periods and number of stages.

[0056] A diffraction grating structure may have multiple types of first diffraction grating sections, each with the same number of stages but different grating periods. In this case, the grating period of the second diffraction grating section is shorter than the grating period of all the first diffraction grating sections, and the number of stages in the second diffraction grating section is greater than the number of stages in all the first diffraction grating sections. For example, in Figure 7(a), the diffraction grating structure 3 has multiple types of first diffraction grating sections 4a and 4b, each with the same number of stages (8 stages) but different grating periods P1a and P1b. For example, in Figure 8(b), the diffraction grating structure 3 has multiple types of first diffraction grating sections 4a, 4b, and 4c, each with the same number of stages (8 stages) but different grating periods P1a, P1b, and P1c. In such cases, the design values ​​of the grating heights H1a, H1b, or H1a, H1b, H1c for each of the first diffraction grating sections 4a, 4b or 4a, 4b, 4c are usually the same.

[0057] Figure 8(b) is a cross-sectional view of Figure 8(a), showing an example where the diffraction grating is a diffractive Fresnel lens.

[0058] When a diffraction grating structure has multiple types of first diffraction grating sections, each having the same number of stages but different grating periods, the number of types of first diffraction grating sections is not particularly limited as long as it is two or more, and can be appropriately selected according to the application of the diffraction grating.

[0059] The diffraction grating structure may have multiple types of second diffraction grating sections, each with the same or different number of stages and different grating periods. In this case, the grating periods of all second diffraction grating sections are shorter than those of the first diffraction grating section, and the number of stages of all second diffraction grating sections is greater than that of the first diffraction grating section. For example, in Figure 7(b), the diffraction grating structure 3 has multiple types of second diffraction grating sections 5a and 5b, each with the same number of stages (9 stages) and different grating periods P2a and P2b. In this case, the grating heights H2a and H2b of each second diffraction grating section 5a and 5b may be the same or different, as long as the difference between the grating height H1 of the first diffraction grating section 4 and the grating heights H2a and H2b of each second diffraction grating section 5a and 5b is within a predetermined range.

[0060] When a diffraction grating structure has multiple types of second diffraction grating sections, each with the same or different number of stages and different grating periods, the number of types of second diffraction grating sections is not particularly limited as long as it is two or more, but is usually between two and four.

[0061] A diffraction grating structure has a stepped cross-sectional shape. A diffraction grating structure may also approximate a sloped surface with a stepped shape. For example, a diffraction grating structure may approximate a smooth surface with a stepped shape, or a curved surface with a stepped shape.

[0062] A diffraction grating structure is composed of a single component, that is, one component. Therefore, a diffraction grating structure is not composed of multiple components, unlike conventional structures that have an etching stopper layer.

[0063] The diffraction grating of this disclosure is not particularly limited as long as the diffraction grating structure consists of a single component, and the light-transmitting substrate and the diffraction grating structure may be integrated or separate. When the light-transmitting substrate and the diffraction grating structure are integrated, it means that the light-transmitting substrate and the diffraction grating structure are composed of a single layer. When the light-transmitting substrate and the diffraction grating structure are separate, the diffraction grating may have a light-transmitting substrate and a light-transmitting layer disposed on one surface of the light-transmitting substrate and constituting the diffraction grating structure.

[0064] The light-transmitting substrate is not particularly limited as long as it is transparent to light of the target wavelength. The material of a light-transmitting substrate that can transmit visible light may be, for example, glass, quartz, resin, etc. The material of a light-transmitting substrate that can transmit infrared light may be, for example, silicon, etc.

[0065] When a diffraction grating has a light-transmitting substrate and a light-transmitting layer, the light-transmitting layer is not particularly limited as long as it is transparent to light of the target wavelength. The material of the light-transmitting layer that can transmit visible light may be, for example, silicon oxide, silicon nitride, silicon oxynitride, resin, etc.

[0066] The resin may be, for example, an ultraviolet-curable resin, a thermosetting resin, an electron beam-curable resin, or an ultraviolet-curable or thermosetting spin-on glass. The ultraviolet-curable resin may be, for example, a resin mainly composed of urethane acrylate, polyester acrylate, epoxy acrylate, polyether acrylate, polythiol, butadiene acrylate, etc. By forming the optically transparent layer with an ultraviolet-curable resin, a diffraction grating structure can be easily formed by imprinting or the like.

[0067] An adhesion layer may be formed between the light-transmitting substrate and the light-transmitting layer. The adhesion layer may be, for example, a silane coupling agent. By forming an adhesion layer, the adhesion between the light-transmitting substrate and the light-transmitting layer can be improved. Improved adhesion can suppress delamination between the light-transmitting substrate and the light-transmitting layer in the imprint method. Furthermore, improved adhesion is often advantageous when used as a diffraction grating for a long period of time.

[0068] 2. Diffraction grating The diffraction grating of this disclosure may be, for example, a transmission-type diffraction grating.

[0069] The diffraction grating of this disclosure is preferably, for example, a diffractive Fresnel lens. In a diffractive Fresnel lens, as shown in Figures 8(a) and (b), for example, the grating period of the diffraction grating portion decreases from the center to the outer edge. Therefore, conventionally, the diffraction grating portion at the outer edge of a diffractive Fresnel lens is formed with a reduced number of stages from the viewpoint of manufacturing limitations. When the number of stages is reduced using a conventional method for manufacturing diffraction gratings, the grating height becomes low, as described above. For example, depending on the characteristics of the etching apparatus and etching conditions, the grating height may be low in the diffraction grating portion with a short grating period, that is, in the diffraction grating portion with a small width per stage. Thus, the grating height tends to be non-uniform in diffractive Fresnel lenses. In contrast, as described above, the diffraction grating of this disclosure can have a uniform grating height, and it is possible to improve the diffraction efficiency.

[0070] A diffractive Fresnel lens is also known, for example, a harmonic Fresnel lens.

[0071] 3. Mold for forming diffraction gratings The diffraction grating structure described above can also be used as a pattern structure for a diffraction grating forming mold in the imprint method. When used as a pattern structure, the following terms shall be interpreted: Diffraction grating structure shall be interpreted as pattern structure. Diffraction grating shall be interpreted as a diffraction grating forming mold. Grating period shall be interpreted as pattern period. First diffraction grating section shall be interpreted as first pattern section. Second diffraction grating section shall be interpreted as second pattern section. Grating height shall be interpreted as pattern height.

[0072] The diffraction grating forming mold of the present disclosure is a diffraction grating forming mold having a pattern structure with a stepped cross-sectional shape on the surface of a light-transmitting substrate, wherein the pattern structure consists of a single member, and the pattern structure has a first pattern section and a second pattern section having different pattern periods and number of steps, wherein the pattern period of the second pattern section is shorter than the pattern period of the first pattern section, the number of steps of the second pattern section is greater than the number of steps of the first pattern section, and the difference between the pattern height of the first pattern section and the pattern height of the second pattern section is smaller than the minimum height of one step of the first pattern section.

[0073] The pattern structure has a first pattern section and a second pattern section with different pattern periods and number of stages. The pattern period of the second pattern section is shorter than that of the first pattern section, the number of stages in the second pattern section is greater than that of the first pattern section, and the difference between the pattern height of the first pattern section and the pattern height of the second pattern section is smaller than the minimum height of one stage in the first pattern section.

[0074] The difference between the pattern height of the first pattern section and the pattern height of the second pattern section is smaller than the minimum height of one layer in the first pattern section. This makes it possible to improve the uniformity of the grating height of the diffraction grating structure in a diffraction grating formed by the imprint method, thereby improving the diffraction efficiency.

[0075] Furthermore, the difference between the pattern height of the first pattern section and the pattern height of the second pattern section is preferably 15% or less, more preferably 9% or less, and even more preferably 6% or less, relative to 100% of the pattern height of the first pattern section. When the above difference is 15% or less, the uniformity of the grating height of the diffraction grating structure can be increased in the diffraction grating formed by the imprint method, and the diffraction efficiency can be increased. When the above difference is 9% or less, or 6%, the diffraction efficiency can be further increased in the diffraction grating formed by the imprint method.

[0076] The number of stages in the first pattern section and the second pattern section is preferably 7 or more, more preferably 12 or more, and even more preferably 16 or more. A number of stages of 7 or more allows for higher diffraction efficiency in the diffraction grating formed by the imprint method. A number of stages of 12 or more, or 16 or more, allows for even higher diffraction efficiency in the diffraction grating formed by the imprint method. However, there are limits to the effect of increasing the number of stages on improving diffraction efficiency. From the viewpoint of the number of processes, the number of stages in the first pattern section and the second pattern section is preferably 16 or less, for example.

[0077] The light-transmitting substrate and the pattern structure may be integrated or separate. The integrated configuration includes a master mold. The separate configuration includes a replica mold.

[0078] The other components of the diffraction grating forming mold of this disclosure are the same as those described above as a diffraction grating structure.

[0079] 4. Method for manufacturing diffraction gratings and molds for forming diffraction gratings The method for manufacturing a diffraction grating according to this disclosure is not particularly limited as long as it can manufacture a diffraction grating having the above-described diffraction grating structure on the surface of a light-transmitting substrate, but it is preferable that the method involves repeatedly performing lithography and etching to form the structure layer by layer.

[0080] Figures 9(a) to (j) and 10(a) to (f) are process diagrams showing an example of a method for manufacturing a diffraction grating according to the present disclosure. As shown in Figure 9(a), a resist layer 11a is formed on a light-transmitting substrate 2, and a pattern is exposed by irradiating it with an energy ray 13 such as ultraviolet light through a mask 12. As shown in Figure 9(b), a resist pattern 11b is formed. As shown in Figure 9(c), the light-transmitting substrate 2 is etched. As shown in Figure 2(d), the resist pattern 11b is removed. This forms the first stage. As shown in Figures 9(e) to (h), the formation of the resist layer 11a, pattern exposure, etching of the light-transmitting substrate 2, and removal of the resist pattern 11b are further performed. This forms the second stage. Although not shown, the formation of the resist layer 11a, pattern exposure, etching of the light-transmitting substrate 2, and removal of the resist pattern 11b are repeated. This forms the third to seventh stages. As shown in Figures 9(i) to (j) and Figures 10(a) to (b), the resist layer 11a is formed, the pattern is exposed, the light-transmitting substrate 2 is etched, and the resist pattern 11b is removed. This forms the 8th stage and creates the first diffraction grating portion 4. As shown in Figures 10(c) to (f), the resist layer 11a is formed, the pattern is exposed, the light-transmitting substrate 2 is etched, and the resist pattern 11b is removed. This forms the 9th stage and creates the second diffraction grating portion 5. In this way, for example, by repeating lithography and etching (N+1) times, a diffraction grating structure having N stages of first diffraction grating portions and (N+1) stages of second diffraction grating portions can be formed.

[0081] When forming the final stage of the second diffraction grating, the light-transmitting substrate can be etched until the difference between the grating height of the second diffraction grating and the grating height of the first diffraction grating is smaller than the minimum height of one stage of the first diffraction grating.

[0082] When designing a diffraction grating, for example, a diffraction grating structure may be formed on the surface of a light-transmitting substrate in advance, and then a diffraction grating section (hereinafter sometimes referred to as a specific diffraction grating section) may be identified in which the difference in grating height from the first diffraction grating section is greater than or equal to the minimum height of one stage in the first diffraction grating section, and the design may be carried out for that specific diffraction grating section. For example, in the specific diffraction grating section, the difference in grating height from the first diffraction grating section can be made smaller than the minimum height of one stage in the first diffraction grating section by narrowing the width of one stage and increasing the number of stages.

[0083] 5. Method for replicating diffraction gratings The diffraction gratings of this disclosure may be replicated by an imprint method using a diffraction grating forming mold. A method for replicating a diffraction grating by an imprint method will be described. For example, an ultraviolet-curable resin is applied to a light-transmitting substrate. A diffraction grating forming mold is pressed onto the uncured ultraviolet-curable resin, and the ultraviolet-curable resin is filled into the pattern structure. Ultraviolet light is irradiated through the diffraction grating forming mold to cure the ultraviolet-curable resin. The diffraction grating forming mold is released from the ultraviolet-curable resin, and the diffraction grating is completed. The completed diffraction grating has a diffraction grating structure replicated by the diffraction grating forming mold.

[0084] The diffraction grating of this disclosure may have a diffraction grating structure replicated by a diffraction grating forming mold. The diffraction grating structure replicated by the diffraction grating forming mold is a structure inverted from the pattern structure. Inversion means that the concave parts of the diffraction grating structure correspond to the convex parts of the pattern structure, and the convex parts of the diffraction grating structure correspond to the concave parts of the pattern structure.

[0085] As mentioned above, in the case of a single replication, the diffraction grating structure will be an inverted version of the pattern structure. If it is desired that the diffraction grating structure and the pattern structure be identical, a total of two replications may be performed, including the manufacture of a replica mold. In other words, the diffraction grating may be replicated using the replica mold created by the imprint method, and then further replicated using the imprint method.

[0086] The diffraction grating forming mold of this disclosure may have a pattern structure for replicating the diffraction grating. The pattern structure for replicating the diffraction grating is a structure that is an inversion of the diffraction grating structure. In other words, when manufacturing the diffraction grating forming mold, a structure that is an inversion of the diffraction grating structure may be designed as the pattern structure. Because the pattern structure is a structure that is an inversion of the diffraction grating structure, the desired diffraction grating can be produced in a single replication.

[0087] This disclosure is not limited to the embodiments described above. The embodiments described above are illustrative. Any configuration that is substantially identical to the technical idea described in the claims of this disclosure and produces similar effects is included within the technical scope of this disclosure. [Examples]

[0088] Let's explain Example 1. The exact coupled wave theory described in "Numerical Analysis of Diffractive Optical Elements and Their Applications" (supervised by Kashiko Kodate, Maruzen Publishing, 2011) was used for the analysis simulation of the diffraction efficiency of the diffraction grating. The exact coupled wave theory is also known as RCWA or Rigorous Coupled Wave Analysis. The grating period of the diffraction grating was set to 50 μm, the wavelength of the incident light to 940 nm, and the refractive index difference to 0.5. Assuming that the height of each stage was the same for all stages, the grating height was set to 16 stages. Specifically, from equations (1-1) and (2-1), the grating height H 16L I set it. H 16L =D × (L-1) (1-1) D = (2 × λ) / (L × 2 × Δn) (2-1) D is the height of one stage. L is the number of stages. λ is the wavelength of the incident light. Δn is the difference between the refractive index n1 of the diffraction grating and the refractive index n2 of the medium in contact with the diffraction grating, and is the value obtained by subtracting the refractive index n2 of the medium in contact with the diffraction grating from the refractive index n1 of the diffraction grating. Since the wavelength λ of the incident light is 940 nm, the number of stages L is 16, and the refractive index difference Δn is 0.5, the grating height H 16L This becomes 1762.5 nm. For example, as shown in Figures 11(a) to (c), the lattice height H 16L The above values ​​were used for the N-stage diffraction grating model. The number of stages N was set to 2 to 16. For example, Figures 11(a) to (c) show models with 16 stages, 8 stages, and 4 stages, respectively, and the grating height H of all models is the same. 16L The values ​​above are used. This simulation analyzes how the diffraction efficiency changes when the number of stages N is changed, when the grating height is set to 16 stages. The simulation results are shown in Figure 12. As shown in Figure 12, the diffraction efficiency was 85% or higher when the number of stages N in the diffraction grating was 7 or more. It was confirmed that increasing the number of stages improves the uniformity of the grating height and thus improves the diffraction efficiency.

[0089] Let's explain example 2. For the analysis simulation of the diffraction efficiency of the diffraction grating, we used RCWA as described in "Numerical Analysis of Diffractive Optical Elements and Their Applications" (supervised by Kashiko Kodate, Maruzen Publishing, 2011). The grating period of the diffraction grating was set to 50 μm, the wavelength of the incident light to 940 nm, and the refractive index difference to 0.5. Assuming that the height of each stage was the same for all stages, the grating heights were set for 2, 4, 8, 12, and 16 stages, respectively. Specifically, the grating height H was set using equations (1-2) and (2-2). H = D × (L-1) (1-2) D = (2 × λ) / (L × 2 × Δn) (2-2) The symbols in equations (1-2) and (2-2) above are the same as the symbols in equations (1-1) and (2-1) above. For example, when the number of stages L is 16, the wavelength λ of the incident light is 940 nm and the refractive index difference Δn is 0.5, so the lattice height H is 1762.5 nm. For example, when the number of stages L is 8, the wavelength λ of the incident light is 940 nm and the refractive index difference Δn is 0.5, so the lattice height H is 1645 nm. For each case where the lattice height H is set to M stages (M=2, 4, 8, 12, 16), a model of an N-stage diffraction grating was used. The number of stages N was set to 2 to 16. For example, Figures 11(a) to (c) show the models for 16 stages, 8 stages, and 4 stages (number of stages N=16, 8, 4) when the lattice height is set to 16 stages (number of stages M=16), and the lattice height H of all models is 16L The values ​​are set to the same predetermined values. This simulation analyzes how the diffraction efficiency changes when the number of stages M is changed when the lattice height is set to M stages (M=2, 4, 8, 12, 16). The simulation results are shown in Figure 13. As shown in Figure 13, it was shown that the diffraction efficiency increases when the number of stages M is 8 or more, that is, when the lattice height is designed to be 8 stages or more. [Explanation of Symbols]

[0090] 1 ... Diffraction grating 2 … Light-transparent substrate 3. Diffraction grating structure 4 … First diffraction grating section 5 ... Second diffraction grating section D1… Minimum height of one stage in the first diffraction grating section H1 ... Grating height of the first diffraction grating H2 ... Grating height of the second diffraction grating section P1 ... Grating period of the first diffraction grating P2 ... Grating period of the second diffraction grating

Claims

[Claim 1] A diffraction grating having a stepped cross-sectional shape on the surface of a light-transmitting substrate, The diffraction grating structure consists of a single member, The diffraction grating structure has a first diffraction grating section and a second diffraction grating section having different grating periods and number of stages. The lattice period of the second diffraction grating is shorter than the lattice period of the first diffraction grating. The number of stages in the second diffraction grating is greater than the number of stages in the first diffraction grating. A diffraction grating in which the difference between the grating height of the first diffraction grating and the grating height of the second diffraction grating is smaller than the minimum height of one stage of the first diffraction grating.

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