Optical element

Abstract

This optical element comprises a substrate, a fine structure part provided on a main surface of the substrate and including a plurality of recessed bodies, a top layer provided to the top portion of the fine structure part, and a bottom layer separated from the top layer and provided to the bottom portion of the fine structure part. The shape of each of the plurality of recessed bodies has a columnar shape, a frustum shape, or a combination thereof. The top layer and the bottom layer contain a material different from the plurality of recessed bodies as a main component. The thickness of the top layer and the thickness of the bottom layer are different.

Description

Optical elements

[0001] The present disclosure relates to optical elements.

[0002] In recent years, research and development of optical elements having fine structures such as metalenses has been progressing (see, for example, Patent Document 1 and Non-Patent Document 1).

[0003] US Patent Application Publication No. 2023 / 0194760

[0004] Huang, Luocheng, et al. "Long wavelength infrared imaging under ambient thermal radiation via an all-silicon metalens.", Optical Materials Express, Sep. 2021, Vol. 11, No. 9, pp. 2907-2914

[0005] Conventional optical elements have room for improvement in terms of the aspect ratio of fine structures.

[0006] The present disclosure provides an optical element that can reduce the aspect ratio of a microstructure portion.

[0007] An optical element according to one aspect of the present disclosure comprises a substrate, a microstructure portion provided on a main surface of the substrate, the microstructure portion including a plurality of concave bodies, a first layer provided on the top of the microstructure portion, and a second layer spaced apart from the first layer and provided on the bottom of the microstructure portion, wherein each of the plurality of concave bodies has a shape of a column or a frustum, or a combination thereof, the first layer and the second layer contain a material different from the plurality of concave bodies as a main component, and the thickness of the first layer is different from the thickness of the second layer.

[0008] According to the present disclosure, an optical element capable of reducing the aspect ratio of a fine structure portion can be provided.

[0009] FIG. 1 is a perspective view of an optical element according to an embodiment. FIG. 2 is a perspective view of a unit cell included in the optical element according to the embodiment. FIG. 3 is a side view illustrating the function of the optical element according to the embodiment. FIG. 4 is a cross-sectional view illustrating a first example of a unit cell included in the optical element according to the embodiment. FIG. 5 is a graph illustrating the phase modulation amount of the optical element for incident light having a wavelength of 10 μm when the thicknesses of the top and bottom layers are modulated. FIG. 6 is a graph illustrating the increase in the phase modulation amount of the optical element for incident light having a wavelength of 10 μm when the thicknesses of the top and bottom layers are modulated. FIG. 7 is a graph illustrating the relationship between the phase modulation amount and the depth of the concave body. FIG. 8 is a cross-sectional view illustrating the effect of reducing the aspect ratio of the concave body shown in FIG. 4. FIG. 9 is a cross-sectional view illustrating a second example of a unit cell included in the optical element according to the embodiment. FIG. 10 is a graph illustrating the phase modulation amount of the optical element for incident light having a wavelength of 10 μm when the thicknesses of the top and bottom layers are modulated. Fig. 11 is a diagram showing an increase in the amount of phase modulation of an optical element for incident light with a wavelength of 10 μm when the thicknesses of the top layer and bottom layer are modulated. Fig. 12 is a diagram showing the relationship between the amount of phase modulation and the depth of the concave body. Fig. 13 is a cross-sectional view for explaining the effect of reducing the aspect ratio of the concave body shown in Fig. 9.

[0010] (Summary of the present disclosure) An optical element according to a first aspect of the present disclosure comprises a substrate, a microstructure portion provided on a main surface of the substrate, the microstructure portion including a plurality of concave bodies, a first layer provided on the top of the microstructure portion, and a second layer spaced apart from the first layer and provided on the bottom of the microstructure portion, wherein each of the plurality of concave bodies has a shape of a column or a frustum, or a combination thereof, the first layer and the second layer contain a material different from the plurality of concave bodies as a main component, and the thickness of the first layer is different from the thickness of the second layer.

[0011] Thus, by providing the first and second layers, the transmittance of the optical element can be increased compared to when the first and second layers are not provided. Furthermore, by making the thicknesses of the first and second layers different, the aspect ratio of the concave body can be reduced. Therefore, with the optical element according to this aspect, the aspect ratio of the microstructure portion can be reduced. Furthermore, the transmittance of the optical element can also be increased.

[0012] The aspect ratio is the ratio of the depth of the concave body to the maximum width of the concave body. By reducing the aspect ratio of the microstructure, the optical element can be made thinner. Alternatively, for the same aspect ratio as before, the amount of phase modulation of incident light by the concave body can be made larger than before. Therefore, for example, the optical element can take in more incident light and can be used as a so-called bright lens, that is, a lens with a small maximum F-number.

[0013] An optical element according to a second aspect of the present disclosure is the optical element according to the first aspect, wherein the first layer is at least 36% thicker than the second layer.

[0014] This allows the aspect ratio of the concave body to be reduced. For example, when the concave body is small, the aspect ratio can be effectively reduced.

[0015] An optical element according to a third aspect of the present disclosure is the optical element according to the second aspect, wherein the plurality of concave bodies are arranged two-dimensionally and periodically on the main surface, and the proportion of the area occupied by one concave body with respect to a unit cell corresponding to one period of the arrangement is 7% or more and 65% or less.

[0016] As a result, when the area ratio of the concave body is small, the aspect ratio of the concave body can be effectively reduced by making the first layer provided at the top 36% or more thicker than the second layer provided at the bottom.

[0017] An optical element according to a fourth aspect of the present disclosure is the optical element according to the first aspect, wherein the first layer is at least 33% thinner than the second layer.

[0018] This allows the aspect ratio of the recessed body to be reduced. For example, when the recessed body is large, the aspect ratio can be effectively reduced.

[0019] An optical element according to a fifth aspect of the present disclosure is the optical element according to the fourth aspect, wherein the plurality of concave bodies are arranged two-dimensionally and periodically on the main surface, and the proportion of the area occupied by one concave body with respect to a unit cell corresponding to one period of the arrangement is 40% or more and 65% or less.

[0020] As a result, when the area ratio of the concave body is large, the aspect ratio of the concave body can be effectively reduced by making the second layer provided at the bottom 49% or more thicker than the first layer provided at the top.

[0021] An optical element according to a sixth aspect of the present disclosure is an optical element according to any one of the first to fifth aspects, wherein the refractive index of each of the first layer and the second layer is greater than 1 and less than 5 and is different from the refractive index of the microstructure portion.

[0022] An optical element according to a seventh aspect of the present disclosure is the optical element according to any one of the first to fifth aspects, wherein the first layer and the second layer are each a single-layer film.

[0023] This allows the first and second layers to be both single-layer films, which facilitates film formation, and improves controllability of the thickness of each of the first and second layers, making it easy to realize an optical element having a concave body with a desired transmittance and / or a desired aspect ratio.

[0024] An optical element according to an eighth aspect of the present disclosure is the optical element according to any one of the first to fifth aspects, wherein the first layer and the second layer are each a multilayer film.

[0025] By forming the first and second layers as multilayer films, it is possible to precisely set the transmittance for each wavelength, and for example, it is possible to cut off unnecessary wavelengths.

[0026] An optical element according to a ninth aspect of the present disclosure is the optical element according to any one of the first to fifth aspects, wherein each of the first and second layers contains, as a main component, one or a mixture of two or more selected from the group consisting of silicon, germanium, chalcogenide, chalcohalide, zinc sulfide, zinc selenide, a fluoride compound, thallium halide, sodium chloride, potassium chloride, potassium bromide, cesium iodide, and plastic.

[0027] This allows the optical element to transmit incident light, which is far-infrared light, with high transmittance. Conventionally, glass cannot be used as a material that transmits light in the far-infrared region, and high-refractive-index materials have generally been used. However, when a high-refractive-index material is used, transmittance is, in principle, low. In contrast, the optical element according to the present disclosure can increase the transmittance of incident light by having a first layer and a second layer with different thicknesses, and is therefore useful as an optical element targeting the far-infrared region. For example, in image sensing using the far-infrared region, noise can be reduced and sensing performance can be improved by using an optical element with high transmittance.

[0028] An optical element according to a tenth aspect of the present disclosure is the optical element according to any one of the first to fifth aspects, wherein the substrate and the microstructure contain silicon as a main component, and the plane orientation of the main surface of the substrate is any one of (100), (110), and (111).

[0029] This allows the use of general semiconductor processing techniques that can be applied to silicon, thereby improving the accuracy of the shape and arrangement of the multiple microstructures, and thus enabling the realization of optical elements having concave bodies with desired transmittance and / or desired aspect ratios with high precision.

[0030] An optical element according to an eleventh aspect of the present disclosure is the optical element according to any one of the first to fifth aspects, wherein the thickness of each of the first layer and the second layer is 50 nm or more and 5 μm or less.

[0031] This allows the first and second layers to be easily formed, and the controllability of the thickness of each of the first and second layers is improved, making it easy to realize an optical element having a concave body with a desired transmittance and / or a desired aspect ratio.

[0032] An optical element according to a twelfth aspect of the present disclosure is the optical element according to any one of the first to fifth aspects, wherein the wavelength of light incident on the optical element is λ, the refractive index of a medium surrounding the optical element is n, and the numerical aperture of the optical element is NA=n sin θ ｆ , the maximum half angle of view of the optical element is θ ｉ In this case, the arrangement pitch P of the plurality of concave bodies is

[0033] Meet the following.

[0034] This makes it possible to reduce aberration and suppress reduction in light collection efficiency even for light incident obliquely when the optical element has a lens function.

[0035] Hereinafter, the embodiments will be specifically described with reference to the drawings.

[0036] The embodiments described below are all comprehensive or specific examples. The numerical values, shapes, materials, components, component placement and connection configurations, steps, and step order shown in the following embodiments are merely examples and are not intended to limit the present disclosure. Furthermore, among the components in the following embodiments, components not described in the independent claims are described as optional components.

[0037] Furthermore, each figure is a schematic diagram and is not necessarily an exact illustration. Therefore, for example, the scales of the figures do not necessarily match. Furthermore, in each figure, substantially the same components are given the same reference numerals, and redundant explanations are omitted or simplified.

[0038] Furthermore, in this specification, terms indicating the relationship between elements, such as parallel or perpendicular, terms indicating the shape of elements, such as square or cylindrical, and numerical ranges are not expressions that only express a strict meaning, but are expressions that also include a substantially equivalent range, for example, a difference of about a few percent.

[0039] Furthermore, in this specification, the terms "upward" and "downward" do not refer to the upward direction (vertically upward) and downward direction (vertically downward) in absolute spatial recognition, but are used as terms defined by the concave direction of a concave body. Specifically, the direction from the bottom of a concave body toward the opening is considered "upward," and the opposite direction is considered "downward." In other words, the concave direction of a concave body is considered "downward." Furthermore, the terms "upward" and "downward" are used not only when two components are arranged with a gap between them and another component is present between them, but also when two components are arranged closely together and the two components are in contact with each other.

[0040] In this specification, the term "thickness direction" refers to the thickness direction of the substrate of the optical element, and refers to the direction perpendicular to the main surface of the substrate. Furthermore, unless otherwise specified, the term "plan view" refers to the view from the direction perpendicular to the main surface of the substrate.

[0041] In this specification, the term "major component" refers to the component that is contained in the largest proportion, expressed in mole percent, in a material.

[0042] Furthermore, in this specification, ordinal numbers such as "first" and "second" do not refer to the number or order of components unless otherwise specified, but are used for the purpose of avoiding confusion and distinguishing between components of the same type.

[0043] Furthermore, in this disclosure, the term "light" is not limited to referring to visible light, but is also used to refer to invisible light. Visible light is light with a wavelength of 380 nm or more and 780 nm or less. Invisible light includes ultraviolet light, infrared light, far-infrared light, and radio waves. Ultraviolet light is light with a wavelength of 10 nm or more and 380 nm or less. Infrared light is light with a wavelength of 780 nm or more and 3000 nm (= 3 μm) or less. Far-infrared light is light (electromagnetic waves) with a wavelength of 3 μm or more and 1000 μm (= 1 mm) or less. Radio waves are electromagnetic waves with a wavelength of 1 mm or more.

[0044] (Embodiment) [Configuration] First, the configuration of an optical element according to an embodiment will be described with reference to Fig. 1, Fig. 2, and Fig. 3. Fig. 1 is a diagram showing an optical element 100 according to the present embodiment. Fig. 2 is a perspective view showing an example of the structure of one unit cell 101 included in the optical element 100 according to the present embodiment. Fig. 3 is a diagram showing the function of the optical element 100 according to the present embodiment.

[0045] As shown in Fig. 1, the optical element 100 includes a substrate 110 and a microstructure including a plurality of concave bodies 120. Note that Fig. 1 shows the concave bodies 120 by cutting out a portion of the optical element 100 to show a cross section.

[0046] 3, the optical element 100 includes a top layer 130, which is an example of a first layer provided on the top of the fine structure, and a bottom layer 140, which is an example of a second layer provided on the bottom of the fine structure and spaced apart from the top layer 130. For convenience of illustration, the top layer 130 and the bottom layer 140 are omitted from FIG.

[0047] The top of the microstructure is a portion that connects the edges of the openings of the multiple recessed bodies 120 and the edges of adjacent recessed bodies 120. The bottom of the microstructure is a portion that corresponds to the so-called valley bottom, and is the bottom surface of each of the multiple recessed bodies 120. The bottom of the microstructure is a portion of the upper surface of the substrate 110 that overlaps with the multiple recessed bodies 120 in a planar view. By providing the top layer 130 and the bottom layer 140, it is possible to increase the transmittance compared to when the top layer 130 and the bottom layer 140 are not provided.

[0048] A microstructure including a plurality of concave bodies 120 is provided on the main surface of the substrate 110. Specifically, the plurality of concave bodies 120 are provided so that the main surface of the substrate 110 forms the bottom surface of the concave body 120. Each of the plurality of concave bodies 120 has a columnar shape. A columnar concave body is also called a column or a "hole." In the optical element 100, a unit element including one concave body 120 is called a "unit cell."

[0049] The optical element 100 is an assembly of unit cells 101 shown in FIG. 2 . The optical element 100 is configured by arranging a plurality of unit cells 101 two-dimensionally. Specifically, the unit cells 101 are unit elements obtained by dividing the main surface of the substrate 110 into a plurality of concave bodies 120, one by one, so that the concave body 120 is located at the center of each unit cell 101, in a planar view of the main surface of the substrate 110. For example, the outer shape of the unit cell 101 corresponds to the area enclosed by the perpendicular bisector of the line segment connecting the centers of two adjacent concave bodies 120. When the concave bodies 120 are arranged at equal intervals in a matrix (i.e., a square lattice), each of the unit cells 101 has an outer shape that is a square area of ​​the same size. The length of one side of the unit cell 101 in a planar view corresponds to the distance between the centers of adjacent concave bodies 120. Depending on the arrangement of the recesses 120, the shape of the unit cell 101 in plan view may not be a square, but may be another polygon such as a regular hexagon or a regular octagon.

[0050] As shown in FIG. 2 , the unit cell 101 includes a portion of the substrate 110, one concave body 120, a portion of the top layer 130, and a bottom layer 140. The substrate 110 and the top layer 130 are provided across the plurality of unit cells 101. The unit cell 101 generates a phase shift in incident light according to the structure of the concave body 120. In this embodiment, the plurality of unit cells 101 have the same structure. The unit cell 101 corresponds to one period of the periodic arrangement of the plurality of concave bodies 120.

[0051] In FIG. 3 , arrows indicate examples of light rays passing through the optical element 100. In this embodiment, the optical element 100 has the function of focusing incident light, similar to a conventional convex lens. Specifically, in the example shown in FIG. 3 , incident light entering the lower surface of the substrate 110 of the optical element 100 is focused by undergoing different phase changes depending on the incident position through the multiple concave bodies 120. To achieve desired focusing characteristics, the shape, size, orientation, etc. of each concave body 120 are appropriately determined. The structure of each concave body 120 can be appropriately determined based on, for example, data indicating the phase profile to be achieved and the results of electromagnetic field simulation. In this way, an optical element having the function of focusing or diverging light using a microstructure can be called a "metalens." A metalens has multiple concave bodies 120 on its surface that are smaller than the wavelength of the incident light, and achieves lens function through the phase shift caused by the concave bodies 120.

[0052] Each of the multiple concave bodies 120 has a size of a subwavelength shorter than the wavelength of light incident on the optical element 100, and can be arranged at subwavelength intervals or periods. The size of the concave body 120 refers to at least one of the maximum width of the concave body 120 in a planar view and the depth of the concave body 120. The interval between the concave bodies 120 is the distance between the centers of two adjacent concave bodies 120 when the main surface of the substrate 110 is viewed in a planar view. The interval between the concave bodies 120 is also called the pitch. The multiple concave bodies 120 may be arranged periodically or non-periodically.

[0053] The maximum width D of the concave body 120 in a planar view (for example, the diameter in the case of a circle) is, for example, 0.1 μm or more and 10 μm or less. The pitch P of the concave body 120 is, for example, 1 μm or more and 10 μm or less. The depth H of the concave body 120 is, for example, 1 μm or more and 50 μm or less. The aspect ratio H / D of the concave body 120 is, for example, 1 or more and 500 or less. Note that these numerical values ​​are merely examples and can be adjusted as appropriate depending on the application or purpose of the optical element 100.

[0054] The optical element 100 can be designed to have desired optical properties for light in a predetermined target wavelength band. The target wavelength band is, for example, a wavelength band defined by specifications. If the lower limit of the target wavelength band is, for example, 1 μm, the size and spacing of the concave bodies 120 can be set to a value shorter than 1 μm. Such microstructures, such as the concave bodies 120 with a nanoscale size smaller than 1 μm, are sometimes called "submicron structures" or "nanostructures." If the target wavelength band is in the infrared region, the size and spacing of the concave bodies 120 may be greater than 1 μm.

[0055] Here, the "target wavelength band" refers to the wavelength band of light in which the optical element 100 is intended to be used, and can be determined based on the specifications of the optical element 100 or the specifications of a device in which the optical element 100 is mounted. The target wavelength band may include, for example, at least a portion of the visible light wavelength band (approximately 400 nm to approximately 700 nm). Alternatively, the target wavelength band may include at least a portion of the ultraviolet wavelength band (approximately 10 nm to approximately 400 nm). Alternatively, the target wavelength band may include at least a portion of the infrared wavelength band (approximately 700 nm to approximately 1 mm). Alternatively, the target wavelength band may include at least a portion of the radio wave wavelength band (wavelengths of approximately 1 mm to approximately 1 m). In this disclosure, the term "wavelength" refers to the wavelength in free space unless otherwise specified.

[0056] In one example, the target wavelength band may include at least a portion of the infrared wavelength band from 2.5 μm to 25 μm. The wavelength band from 2.5 μm to 25 μm is used in infrared sensing devices, such as LiDAR (Light Detection and Ranging) sensors or infrared cameras. The target wavelength band may also include the far-infrared wavelength band from 5 μm to 14 μm or the far-infrared wavelength band from 8 μm to 12 μm. The far-infrared wavelength band is used for thermal imaging, etc. The optical element 100 may be used in combination with an image sensor in, for example, an imaging device. The optical element 100 may also be used in telescopes, microscopes, or scanning optical devices. However, the optical element 100 is not limited to these applications.

[0057] The number of concave bodies 120 provided on the main surface of the optical element 100 is determined to an appropriate number depending on the lens characteristics to be achieved. The number of concave bodies 120 is, for example, in the range of 100 to 10,000, and may be less than 100 or more than 10,000 in some cases.

[0058] Here, an example of a design method for an optical element 100 having a lens function will be briefly described. In the optical element 100, a plurality of concave bodies 120 are arranged at intervals shorter than the wavelength of the incident light. A desired phase profile is realized by adjusting the orientation or size of each concave body 120. To increase the focusing efficiency, the pitch P of the concave bodies 120, i.e., the size of the unit cell 101, can be designed to satisfy the Nyquist criterion for sampling (P<λ / 2NA). Here, λ is the design wavelength of the optical element 100, and NA is the numerical aperture of the optical element 100.

[0059] Furthermore, the optical element 100 may be designed to obtain high focusing efficiency even for light that is obliquely incident on the optical element 100. For example, when the refractive index n ｓ The optical element 100 is irradiated with a light beam from a medium (for example, air) having a refractive index n at an incident angle θ ｉ and wave number k ｉ Let k be the wave number corresponding to the shortest wavelength λ in the target wavelength band. ｔ(=2π·n / λ), and the numerical aperture of the optical element 100 is NA=n sin θ ｆ The incident angle θ ｉ is the maximum half angle of view of the optical element 100 (i.e., the maximum angle of incidence of light that can be used in a device including the optical element 100). The maximum angle of incidence of light here may be, for example, the maximum angle of view of a device such as an imaging device, telescope, or microscope that includes the optical element 100, or the maximum scanning angle of a scanning optical device that includes the optical element 100. The maximum half angle of view of the optical element 100 is expressed as θ ｉ The plurality of concave bodies 120 has a maximum of K １ The optical element is formed so as to give the incident light a wave number component (i.e., a spatial frequency component) of .gtoreq.1.

[0060] The maximum spatial frequency component K in the unit cell 101 １ The minimum sampling interval P required to provide the above is determined by the sampling theorem so as to satisfy the following inequality (1):

[0061] Therefore, the interval P between the multiple concave bodies 120 is determined so as to satisfy the following formula (2).

[0062] By arranging the multiple concave bodies 120 so as to satisfy the formula (2), the sampling theorem can be satisfied even for obliquely incident light, making it easier to reproduce an ideal phase. Therefore, it is possible to reduce aberrations and suppress reductions in light collection efficiency. Note that it is not essential for the optical element 100 to have a lens function, so it is not necessary for the optical element 100 to satisfy the formula (2) above.

[0063] In this embodiment, the substrate 110 and the microstructure including the plurality of recesses 120, i.e., the wall surfaces of the plurality of recesses 120, are integrally formed using the same material. The substrate 110 and the wall surfaces of each recess 120 are made of a material containing silicon as the main component and having a (100) crystal plane orientation. The silicon crystal plane orientation may also be (110) or (111). Furthermore, the substrate 110 and the wall surfaces of the recesses 120 may be made of a material other than silicon.

[0064] The thickness of the substrate 110 is, for example, 500 μm. The shape of the substrate 110 is square, as shown in FIG. 1 , and its size is 8 mm × 8 mm. A plurality of recesses 120 are arranged within a circular region with a diameter of 8 mm on the surface of the substrate 110. Note that the thickness and shape of the substrate 110 and the shape and size of the region in which the recesses 120 are arranged are not particularly limited.

[0065] The optical element 100 according to this embodiment includes a top layer 130 and a bottom layer 140 having different thicknesses. The top layer 130 is provided in contact with the top of the microstructure, specifically, the edge of each opening of the plurality of concave bodies 120, and the portion connecting the edges of adjacent concave bodies 120. The bottom layer 140 is provided in contact with the bottom of the microstructure, specifically, the bottom surface (the main surface of the substrate 110) of the plurality of concave bodies 120. By appropriately adjusting the thickness, material, number of layers, etc. of the top layer 130 and the bottom layer 140, the transmittance of incident light can be increased. The transmittance is expressed as the ratio of the intensity of light emitted from the optical element 100 to the intensity of light incident on the optical element 100.

[0066] The thickness of each of the top layer 130 and the bottom layer 140 is 50 nm or more and 5 μm or less. The thickness of at least one of the top layer 130 and the bottom layer 140 may be 100 nm or more, 300 nm or more, or 500 nm or more. The thickness of at least one of the top layer 130 and the bottom layer 140 may be 3 μm or less, 2 μm or less, 1.5 μm or less, or 1 μm or less. The top layer 130 and the bottom layer 140 are each a single-layer film. The refractive index of each of the top layer 130 and the bottom layer 140 differs from the refractive index of the microstructure portion by, for example, a value greater than 1 and less than 5.

[0067] The top layer 130 and the bottom layer 140 contain, as a main component, a material different from that of the microstructure portion, i.e., the wall surface portions of the plurality of concave bodies 120. Each of the top layer 130 and the bottom layer 140 contains, as a main component, one or a mixture of two or more selected from the group consisting of silicon, germanium, chalcogenide, chalcohalide, zinc sulfide, zinc selenide, fluoride compounds, thallium halide, sodium chloride, potassium chloride, potassium bromide, cesium iodide, and plastic. The plastic is, for example, polyethylene.

[0068] Below, a case where the top layer 130 is thicker than the bottom layer 140 and a case where the top layer 130 is thinner than the bottom layer 140 will be described based on specific simulation results.

[0069] [First Example (Top Layer>Bottom Layer)] FIG. 4 is a cross-sectional view showing a first example of one unit cell 101 included in the optical element 100 according to this embodiment.

[0070] In the example shown in FIG. 4, the top layer 130 is thicker than the bottom layer 140. That is, the thickness of the top layer 130 is T Ｔ and the thickness of the bottom layer 140 is T Ｂ In this case, T Ｔ >T Ｂ is.

[0071] 5 is a graph showing the phase modulation amount of the optical element 100 for incident light with a wavelength of 10 μm when the thicknesses of the top layer 130 and the bottom layer 140 are modulated. In FIG. 5, the horizontal axis represents the thickness T of the top layer 130. Ｔ The vertical axis represents the thickness T of the bottom layer 140. Ｂ The phase modulation amount is a value obtained by subtracting the phase of light incident on the optical element 100 from the phase of light emitted from the optical element 100 based on the incident light.

[0072] In the simulation of the phase modulation amount, the shape, height H, and pitch P of the concave body 120 are fixed. Also, the composition and refractive index of each of the wall surface portion of the concave body 120, the top layer 130, and the bottom layer 140 are fixed. The thickness T of the top layer 130 Ｔ , the thickness T of the bottom layer 140 Ｂand the width D of the concave body 120 are modulated.

[0073] Specific simulation conditions are as follows: the target wavelength is 10 μm; the wall surface of the concave body 120 is made of silicon; the top layer 130 and the bottom layer 140 are single-layer films of zinc sulfide (ZnS); and the shape of the concave body 120 is cylindrical. The depth H of each concave body 120 is 30 μm. The pitch P of the concave bodies 120, i.e., the size of the unit cell 101, is 2.8 μm.

[0074] The thickness T of the top layer 130 Ｔ and the thickness T of the bottom layer 140 Ｂ are modulated in 0.1 μm increments within a range of 0 μm or more and 1.5 μm or less. The width (diameter) D of each concave body 120 is modulated in 0.07 μm increments within a range of 0.80 μm or more and 2.56 μm or less. The width D of the concave body 120 can be converted into the area occupied by one concave body 120 relative to the unit cell 101 (i.e., the area ratio). When the width D is 0.80 μm, the area ratio is approximately 7%, and when the width D is 2.56 μm, the area ratio is approximately 65%.

[0075] The phase modulation amount shown in FIG. 5 is T Ｔ and T Ｂ Under the condition above, the width D of each concave body 120 is modulated in increments of 0.07 μm within the range of 0.80 μm to 2.56 μm, and this is the maximum value among the phase modulation amounts obtained according to the value of the width D.

[0076] In FIG. Ｔ =T Ｂ The range to the right of the line (positive direction of the horizontal axis) is T Ｔ >T Ｂ As can be seen from FIG. 5, the phase modulation amount is Ｔ >T Ｂ In addition, in the range to the right of the oblique line, the thickness T Ｂ When the thickness T of the top layer 130 is fixed, ＴIt can be seen that the larger the value, the larger the amount of phase modulation.

[0077] 6 is a diagram showing an increase in the amount of phase modulation of the optical element 100 for incident light with a wavelength of 10 μm when the thicknesses of the top layer 130 and the bottom layer 140 are modulated. Ｔ =T Ｂ Compared to the case where Ｔ >T Ｂ Specifically, the increase in the amount of phase modulation shown in FIG. 6 is a value obtained by comparing the amount of phase modulation and T Ｔ =T Ｂ 6, the increase in the amount of phase modulation was approximately 0.5 rad (≈π / 6) at most.

[0078] Although only the case where the design wavelength is 10 μm is illustrated here, similar simulations were performed for incident light having a certain intensity or more throughout the entire design wavelength range of 5 μm to 14 μm and for incident light having a certain intensity or more throughout the entire design wavelength range of 8 μm to 12 μm, and the results confirmed an increase in the amount of phase modulation similar to that for 10 μm. Specifically, the increase in the amount of phase modulation in the range of 5 μm to 14 μm was approximately 0.36 rad, and the increase in the amount of phase modulation in the range of 8 μm to 12 μm was approximately 0.48 rad.

[0079] Fig. 7 is a diagram showing the relationship between the amount of phase modulation and the depth H of the concave body 120. In Fig. 7, the horizontal axis represents the amount of phase modulation, and the vertical axis represents the depth H of the concave body 120. As shown in Fig. 7, there is a linear relationship between the depth H of the concave body 120 and the amount of phase modulation. Specifically, the amount of phase modulation increases as the concave body 120 becomes deeper.

[0080] As shown in Figures 5 and 6, when the top layer 130 is thicker than the bottom layer 140, the maximum value of the phase modulation amount is approximately 0.5 rad. From the relationship shown in Figure 7, the depth H corresponding to 0.5 rad is approximately 0.8 µm. In other words, it can be seen that making the top layer 130 thicker than the bottom layer 140 has the same effect of increasing the phase modulation amount as making the depth H of the concave body 120 deeper by approximately 0.8 µm. In other words, when a comparative example is used in which the top layer 130 and the bottom layer 140 have the same thickness, the concave body 120 of the first example, in which the depth H is 0.8 µm shorter than that of the concave body 120 of the comparative example, can ensure the same phase modulation amount as the concave body 120 of the comparative example.

[0081] Fig. 8 is a cross-sectional view illustrating the effect of reducing the aspect ratio of the concave body 120 shown in Fig. 4. The aspect ratio is expressed as H / D. As shown in Fig. 8, the aspect ratio of the concave body 120 according to the first example can be reduced by 1.0 compared to the comparative example.

[0082] The thickness T of the top layer 130 Ｔ and the thickness T of the bottom layer 140 Ｂ Difference T Ｔ -T Ｂ is, for example, the thickness T Ｔ This allows the increase in the amount of phase modulation to be increased. Ｔ -T Ｂ is, for example, the thickness T Ｔ It may be 10% or more, 20% or more, or 30% or more of the difference T Ｔ -T Ｂ is, for example, the thickness T Ｔ It may be 80% or less, 70% or less, or 60% or less.

[0083] Second Example (Top Layer<Bottom Layer) FIG. 9 is a cross-sectional view showing a second example of one unit cell 101 included in the optical element 100 according to the present embodiment.

[0084] In the example shown in Figure 9, the top layer 130 is thinner than the bottom layer 140. That is, the thickness of the top layer 130 is T Ｔand the thickness of the bottom layer 140 is T Ｂ In this case, T Ｔ <T Ｂ is.

[0085] 10 is a diagram showing the phase modulation amount of the optical element 100 for incident light with a wavelength of 10 μm when the thicknesses of the top layer 130 and the bottom layer 140 are modulated. In FIG. 10, the horizontal axis represents the thickness T of the top layer 130. Ｔ The vertical axis represents the thickness T of the bottom layer 140. Ｂ represents.

[0086] In the simulation of the phase modulation amount, the shape, height H, and pitch P of the concave body 120 are fixed. Also, the composition and refractive index of each of the wall surface portion of the concave body 120, the top layer 130, and the bottom layer 140 are fixed. These fixed values ​​are the same as in the first example. The thickness T of the top layer 130 Ｔ , the thickness T of the bottom layer 140 Ｂ and the width D of the concave body 120 are modulated.

[0087] The thickness T of the top layer 130 Ｔ and the thickness T of the bottom layer 140 Ｂ are modulated in 0.1 μm increments within a range of 0 μm or more and 1.5 μm or less. The width (diameter) D of each concave body 120 is modulated in 0.07 μm increments within a range of 1.81 μm or more and 2.56 μm or less. The width D of the concave body 120 can be converted into the area occupied by one concave body 120 relative to the unit cell 101 (i.e., the area ratio). When the width D is 1.81 μm, the area ratio is approximately 40%, and when the width D is 2.56 μm, the area ratio is approximately 65%.

[0088] The phase modulation amount shown in FIG. 10 is T Ｔ and T Ｂ Under the condition above, the width D of each concave body 120 is modulated in increments of 0.07 μm within the range of 1.81 μm to 2.56 μm, and this is the maximum value among the phase modulation amounts obtained according to the value of the width D.

[0089] In FIG. Ｔ =T ＢThe range to the left of the line (negative direction of the horizontal axis) is T Ｔ <T Ｂ As can be seen from FIG. 10, the phase modulation amount when T Ｔ <T Ｂ In addition, in the range to the left of the oblique line, the thickness T Ｂ When the thickness T of the top layer 130 is fixed, Ｔ It can be seen that the smaller the value, the higher the amount of phase modulation.

[0090] 11 is a diagram showing an increase in the amount of phase modulation of the optical element 100 for incident light with a wavelength of 10 μm when the thicknesses of the top layer 130 and the bottom layer 140 are modulated. Ｔ =T Ｂ Compared to the case where Ｔ <T Ｂ Specifically, the increase in the amount of phase modulation shown in FIG. 11 is a value obtained by comparing the amount of phase modulation and T Ｔ =T Ｂ 11, the increase in the amount of phase modulation was approximately 0.26 rad (≈π / 12) at most.

[0091] Although only the case where the design wavelength is 10 μm is illustrated here, similar simulations were performed for incident light having a certain intensity or more throughout the entire design wavelength range of 5 μm to 14 μm and for incident light having a certain intensity or more throughout the entire design wavelength range of 8 μm to 12 μm, and the results confirmed an increase in the amount of phase modulation similar to that for 10 μm. Specifically, the increase in the amount of phase modulation in the range of 5 μm to 14 μm was 0.23 rad, and the increase in the amount of phase modulation in the range of 8 μm to 12 μm was 0.34 rad.

[0092] Fig. 12 is a diagram showing the relationship between the amount of phase modulation and the depth H of the concave body 120. In Fig. 12, the horizontal axis represents the amount of phase modulation, and the vertical axis represents the depth H of the concave body 120. As shown in Fig. 12, there is a linear relationship between the depth H of the concave body 120 and the amount of phase modulation. Specifically, the amount of phase modulation increases as the concave body 120 becomes deeper.

[0093] As shown in Figures 10 and 11, when the top layer 130 is thinner than the bottom layer 140, the maximum value of the phase modulation amount is approximately 0.26 rad. From the relationship shown in Figure 12, the depth H corresponding to 0.26 rad is approximately 0.4 µm. In other words, it can be seen that making the top layer 130 thinner than the bottom layer 140 has the same effect of increasing the phase modulation amount as making the depth H of the concave body 120 deeper by approximately 0.4 µm. In other words, when a comparative example is used in which the top layer 130 and the bottom layer 140 have the same thickness, the concave body 120 of the second example, in which the depth H is 0.4 µm shorter than that of the concave body 120 of the comparative example, can ensure the same phase modulation amount as the concave body 120 of the comparative example.

[0094] Fig. 13 is a cross-sectional view illustrating the effect of reducing the aspect ratio of the concave body 120 shown in Fig. 9. The aspect ratio is expressed as H / D. As shown in Fig. 9, the aspect ratio of the concave body 120 according to the second example can be reduced by 0.5 compared to the comparative example.

[0095] The thickness T of the bottom layer 140 Ｂ and the thickness T of the top layer 130 Ｔ Difference T Ｂ -T Ｔ is, for example, the thickness T Ｂ This allows the increase in the amount of phase modulation to be increased. Ｂ -T Ｔ is, for example, the thickness T Ｂ It may be 10% or more, 20% or more, or 30% or more of the difference T Ｂ -T Ｔ is, for example, the thickness T Ｂ It may be 80% or less, 70% or less, or 60% or less.

[0096] [Other Examples] Table 1 shows the thickness T of the top layer 130. Ｔ、 The thickness T of the bottom layer 140 Ｂ , and the height H of the concave body 120 is set to a specific value. Ｔ >T Ｂ Examples 6 and 7 show the case of T Ｔ <T Ｂ The left arrow in the table indicates that the content is the same as the column on the left.

[0097] In Example 3, the thickness T of the top layer 130 Ｔ is 1.40 μm, and the thickness of the bottom layer 140 T Ｂ In this case, T Ｔ / T Ｂ = 1.36, and T Ｔ is T Ｂ The concave portions 120 are 36% thicker than the unit cell 101. The pitch P of the concave portions 120 is 2.80 μm, and the depth H of the concave portions 120 is 30 μm. The width (diameter) D of the concave portions 120 is modulated in the range of 0.85 μm to 2.54 μm. When the width D of the concave portions 120 is 0.85 μm, the area ratio of one concave portion 120 to the unit cell 101 is approximately 7%, and when the width D is 2.54 μm, the area ratio is approximately 65%.

[0098] The substrate 110 and the wall surface of the recess 120 are made of silicon, and the top layer 130 and the bottom layer 140 are made of zinc sulfide (ZnS).

[0099] Under the above conditions, when the width D of each concave body 120 is modulated in the range of 0.85 μm to 2.54 μm, the maximum value of the transmittance of the optical element 100 for incident light with a wavelength of 10 μm is 97.8%. Ｔ =T Ｂ With the other conditions the same, the maximum value of the transmittance is 95.1%. With no film formation, i.e., without the top layer 130 and the bottom layer 140, and with the other conditions the same, the maximum value of the transmittance is 76.6%. Ｔ =T Ｂ Compared to the case where T Ｔ＝ 1.40 μm, T Ｂ= 1.03 μm, the increase in the amount of phase modulation is 0.10 rad. Furthermore, the aspect ratio can be reduced by 0.18 compared to the comparative example.

[0100] The above has been described for Example 3, but the same applies to Examples 4 to 7.

[0101]

[0102] [Effects, etc.] As described above, in the optical element 100 according to the present embodiment, the transmittance of the optical element 100 can be increased by providing the top layer 130 and the bottom layer 140. Furthermore, by making the thicknesses of the top layer 130 and the bottom layer 140 different, the aspect ratio of the concave body 120 can be made smaller than when the thicknesses of the top layer 130 and the bottom layer 140 are equal. When the ratio of the area of ​​the concave body 120 to the unit cell 101 in plan view is smaller than a predetermined value, a configuration in which the top layer 130 is thicker than the bottom layer 140 is advantageous in terms of improving transmittance. Furthermore, when the ratio of the area of ​​the concave body 120 to the unit cell 101 in plan view is larger than a predetermined value, a configuration in which the top layer 130 is thinner than the bottom layer 140 is advantageous in terms of improving transmittance. The predetermined value here is, for example, 20% or more and 40% or less.

[0103] The smaller aspect ratio makes it possible to reduce the thickness of the optical element 100. Alternatively, when the aspect ratio is the same as before, the amount of phase modulation of incident light by the concave body 120 can be made larger than before. Therefore, for example, the optical element 100 can take in more incident light, and can be used as a so-called bright lens, that is, a lens with a small maximum aperture.

[0104] Furthermore, because the top layer 130 and the bottom layer 140 have different thicknesses, the conditions for controlling the thickness of each layer in the film formation process are relaxed. That is, the thicknesses of the top layer 130 and the bottom layer 140 can be made different more easily than when the top layer 130 and the bottom layer 140 have the same thickness. In particular, when the aspect ratio of the concave body 120 is large, it is difficult to make the top layer 130 and the bottom layer 140 have the same thickness. For example, when the aspect ratio of the concave body 120 is 1 or greater, an optical element 100 including a top layer 130 and a bottom layer 140 with different thicknesses is useful from the perspective of ease of manufacture.

[0105] Although the first and second examples use a combination of silicon and zinc sulfide, the present invention is not limited to this. By performing a similar simulation when using other combinations of materials, the thicknesses of the top layer 130 and the bottom layer 140 and the area ratio of the concave body 120 can be set within appropriate ranges.

[0106] Furthermore, the shape of each of the multiple concave bodies 120 is not limited to a cylindrical shape. Each of the multiple concave bodies 120 may have a shape other than a cylindrical shape, such as a columnar shape or a frustum shape, or a combination thereof. The columnar shape is a cylindrical or elliptical cylinder, or a rectangular column shape such as a square column, a hexagonal column, or an octagonal column. The frustum shape is a circular or elliptical frustum shape, or a truncated pyramid shape such as a square pyramid, a hexagonal pyramid, or an octagonal pyramid. In the case of a frustum shape, the area of ​​the bottom surface is larger than the area of ​​the top surface, but the reverse may also be true. Furthermore, a combination of a columnar shape and a frustum shape is, for example, a configuration in which a columnar body and a frustum-shaped body are stacked in the height direction. In other words, the concave body 120 may have a shape in which the top surface of one of a columnar body and a frustum-shaped body is in contact with the bottom surface of the other. The number of combined columnar bodies and frustum-shaped bodies may be three or more.

[0107] [Manufacturing Method] The optical element 100 can be manufactured using a general semiconductor manufacturing technique such as lithography, etc. For example, the optical element 100 can be manufactured by the following method.

[0108] First, a silicon substrate having a (100) crystal plane orientation on its surface is prepared as the substrate 110. Note that the "surface" refers to one of the main surfaces of the silicon substrate. When a microstructure including multiple recesses 120 is integrally formed with the substrate 110, the "surface" of the silicon substrate becomes the tip surface of the top of the microstructure.

[0109] Next, a positive resist is applied to the surface of the silicon substrate by a method such as spin coating. Subsequently, light or an electron beam is irradiated to desired locations, and then a development process is performed. As a result, the resist is removed from the locations irradiated with light or an electron beam. This silicon substrate is then subjected to SF ６ The etching is performed using a reactive ion etching technique or the like using an etching gas such as a gas. This etches the surface of the silicon substrate where the resist has been removed. Then, a wet process using a resist remover or the like is performed. ２ Residual resist on the surface of the silicon substrate is removed by a dry process such as ashing, thereby forming a microstructure including a plurality of recesses 120 on the main surface of the substrate 110. The microstructure corresponds to the upper layer portion of the prepared silicon substrate.

[0110] Furthermore, a film is formed using a material different from that of the concave bodies 120 so as to cover the microstructure including the plurality of concave bodies 120. For example, zinc sulfide (ZnS) is formed on each of the top and bottom of the microstructure. The film formation is performed by EB (Electron Beam) evaporation, sputtering, CVD (Chemical Vapor Deposition), or the like. The thicknesses of the top layer 130 and the bottom layer 140 can be adjusted by adjusting the film formation conditions. For example, the bottom layer 140 can be made thicker than the top layer 130 by applying a bias voltage perpendicular to the substrate 110 during film formation.

[0111] Through these steps, an optical element 100 can be fabricated, which includes a substrate 110, a plurality of concave bodies 120, and a top layer 130 and a bottom layer 140 having different thicknesses.

[0112] When the substrate 110 and the concave body 120 are formed using different materials, a film containing a material different from that of the substrate 110 may be formed on the main surface of the substrate 110 and patterned to form the concave body 120. The film may be formed by epitaxial growth, vapor deposition, sputtering, CVD, coating, or the like.

[0113] Other Embodiments While optical elements according to one or more aspects have been described above based on embodiments, the present disclosure is not limited to these embodiments. As long as they do not deviate from the gist of the present disclosure, various modifications conceivable by those skilled in the art to the present embodiments and embodiments constructed by combining components of different embodiments are also included within the scope of the present disclosure.

[0114] For example, in the above embodiment, an example has been described in which the multiple unit cells 101 included in the optical element 100 have the same structure, but the multiple unit cells 101 may include unit cells having a structure different from the other unit cells 101. For example, the multiple unit cells 101 may include unit cells that differ from the other unit cells 101 in at least one of the size, shape, and material of the concave body 120. Alternatively, the multiple unit cells 101 may include unit cells that differ from the other unit cells 101 in at least one of the thickness, material, and number of layers of at least one of the top layer 130 and the bottom layer 140.

[0115] Furthermore, in the above embodiment, an example was shown in which the bottom of the microstructure is part of the main surface of the substrate 110, but this is not limited thereto. For example, the microstructure may include a plurality of concave bodies 120 and a base portion that connects and supports the wall portions of the plurality of concave bodies 120. The base portion is laminated on the main surface of the substrate 110. The base portion is formed using the same material as the wall portions of the plurality of concave bodies 120. In other words, the refractive index of the base portion is the same as the refractive index of each of the wall portions of the plurality of concave bodies 120. In this case, the upper surface of the base portion (the main surface opposite the substrate 110) becomes the bottom of the concave body 120.

[0116] Furthermore, for example, the top layer 130 and the bottom layer 140 may each be a multilayer film. In this case, the top layer 130 and the bottom layer 140 have, for example, the same film configuration. The thickness of each of the multiple films included in the top layer 130 may be different from the thickness of the corresponding film included in the bottom layer 140. Alternatively, the thickness of at least one of the multiple films included in the top layer 130 may be different from the thickness of the corresponding film included in the bottom layer 140, and the thickness of at least one other of the multiple films included in the top layer 130 may be the same as the thickness of the corresponding film included in the bottom layer 140.

[0117] Furthermore, the top layer 130 and the bottom layer 140 may contain different materials as their main components.

[0118] Furthermore, a layer containing the same material as the top layer 130 or the bottom layer 140 as a main component may be provided on the side surface of the concave body 120 .

[0119] Furthermore, various modifications, substitutions, additions, omissions, etc. can be made to the above-described embodiments within the scope of the claims or their equivalents.

[0120] The present disclosure can be used in devices having optical elements such as lenses, such as cameras, LiDAR sensors, projectors, AR (Augmented Reality) displays, telescopes, microscopes, and scanning optical devices.

[0121] 100 Optical element 101 Unit cell 110 Substrate 120 Concave body 130 Top layer 140 Bottom layer

Claims

1. An optical element, comprising: a substrate; a fine structure portion provided on a main surface of the substrate and including a plurality of concave bodies; a first layer provided on a top portion of the fine structure portion; and a second layer provided on a bottom portion of the fine structure portion and spaced apart from the first layer, wherein each shape of the plurality of concave bodies has a columnar shape, a frustum shape, or a combination thereof, the first layer and the second layer mainly contain materials different from those of the plurality of concave bodies, and a thickness of the first layer is different from a thickness of the second layer.

2. The optical element according to claim 1, wherein the first layer is 36% or more thicker than the second layer.

3. The optical element according to claim 2, wherein the plurality of concave bodies are two-dimensionally and periodically arranged on the main surface, and a ratio of an area occupied by one of the concave bodies to a unit cell corresponding to one period of the arrangement is 7% or more and 65% or less.

4. The optical element according to claim 1, wherein the first layer is 33% or more thinner than the second layer.

5. The optical element according to claim 4, wherein the plurality of concave bodies are two-dimensionally and periodically arranged on the main surface, and a ratio of an area occupied by one of the concave bodies to a unit cell corresponding to one period of the arrangement is 40% or more and 65% or less.

6. The optical element according to any one of claims 1 to 5, wherein a refractive index of each of the first layer and the second layer is greater than 1 and different from a refractive index of the fine structure portion within a range of less than 5.

7. The optical element according to any one of claims 1 to 5, wherein each of the first layer and the second layer is a single-layer film.

8. The optical element according to any one of claims 1 to 5, wherein each of the first layer and the second layer is a multilayer film.

9. The optical element according to any one of claims 1 to 5, wherein each of the first layer and the second layer mainly contains one selected from the group consisting of silicon, germanium, chalcogenide, chalcohalide, zinc sulfide, zinc selenide, fluoride compound, thallium halide, sodium chloride, potassium chloride, potassium bromide, cesium iodide, and plastic, or a mixture of two or more thereof.

10. The optical element according to any one of claims 1 to 5, wherein the substrate and the fine structure portion mainly contain silicon, and a plane orientation of the main surface of the substrate is any one of (100), (110), and (111).

11. The thickness of each of the first layer and the second layer is 50 nm or more and 5 μm or less. The optical element according to any one of claims 1 to 5.

12. Let the wavelength of the light incident on the optical element be λ, the refractive index of the medium surrounding the optical element be n, and the numerical aperture NA of the optical element be NA = nsinθ ｆ , where θ is the maximum half-angle of the optical element ｉ . When this is the case, the array period P of the plurality of concave bodies satisfies The optical element according to any one of claims 1 to 5

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