Metasurface reflector

Abstract

The present invention provides a metasurface reflector that exhibits high process resistance and is less susceptible to degradation of its reflective properties. [Solution] The nanostructure comprises a protective layer 8, a first metal layer 10 constituting a nanostructure, a dielectric layer 20, and a second metal layer 30, wherein the protective layer 8 covers the first metal layer 10, the dielectric layer 20 is located between the first metal layer 10 and the second metal layer 30, and the nanostructure 11 contains Ag and Zn, or Ag and Sn.

Description

Technical Field

[0001] The present invention relates to a metasurface reflector.

Background Art

[0002] Currently, eyeglass-type terminals are being considered in AR (Augmented Reality) and VR (Virtual Reality). In particular, in recent years, a retinal scanning display that allows a user to view an image by forming an image of light scanned on the user's retina has attracted attention. In a retinal scanning display, generally, three-color visible light emitted from light sources such as LD (Laser Diode) corresponding to each of R (red), G (green), and B (blue) colors is combined on one optical axis via a PLC (Planer Lightwave Circuit) or the like. The combined three-color visible light is scanned by a MEMS (Micro Electro Mechanical Systems) mirror and reflected by a half mirror in front of the user's eye, and then enters the user's pupil. When this incident light forms an image on the user's retina, the user can view the image.

[0003] As the half mirror, a metasurface reflector is used. A metasurface reflector is a thin film having a nano-level fine structure (nano structure), and functions as a light reflector.

[0004] FIG. 13(a) is a cross-sectional view showing the structure of a conventional metasurface reflector (Prior Art 1). As shown in FIG. 13(a), the outermost surface of the metasurface reflector of Prior Art 1 exposes the metal of the nano structure (see, for example, Patent Documents 1 and 2).

[0005] Also, FIG. 13(b) is a cross-sectional view showing the structure of a conventional metasurface reflector (Prior Art 2). As shown in FIG. 13(b), in the metasurface reflector of Prior Art 2, a process protection layer is formed only on the upper surface of the nano structure.

Prior Art Documents

[0006] [Patent Document 1] U.S. Patent Application Publication No. 2018 / 0113310 [Patent Document 2] Japanese Patent Publication No. 2024-94887 [Overview of the project] [Problems that the invention aims to solve]

[0007] As shown in Figure 13(a), the metasurface reflector of the prior art 1 has an Ag layer 130, an SiO2 layer 120, and an Ag layer 112 formed in that order on a substrate 140. The upper Ag layer 112 is exposed.

[0008] In the conventional metasurface reflector (Technical 1), the outermost nanostructure has exposed metal. Therefore, when forming the nanostructure, regardless of the lift-off or milling process, removing the resist pattern with an organic solvent (e.g., NMP (N-methyl-2-pyrrolidone)) exposes the outermost metal layer (Ag layer 112), leading to corrosion damage as shown in the corroded area 119. (No process resistance)

[0009] As shown in Figure 13(b), the metasurface reflector of the prior art 2 has an Ag layer 130, an SiO2 layer 120, an Ag layer 112, and an Au layer 108 formed in that order on a substrate 140. Only the upper surface of the upper Ag layer 112 is covered with a protective Au layer 108.

[0010] In a conventional structure like the one in Conventional Technology 2, where a protective Au layer 108 is formed only on the upper surface of the Ag layer 112, even if there is no corrosion damage during resist removal, the exposed sides of the Ag layer 112 are left in the air, causing corrosion to progress as shown in the corroded area 119, resulting in a deterioration of the reflective properties. (No resistance to static exposure)

[0011] The present invention has been made in view of the above problems, and aims to provide a metasurface reflector that has high process resistance and is less susceptible to deterioration of reflective properties. [Means for solving the problem]

[0012] To achieve the above objective, the metasurface reflector according to the present invention comprises a protective layer, a first metal layer constituting a nanostructure, a dielectric layer, and a second metal layer, wherein the protective layer covers the first metal layer, the dielectric layer is located between the first metal layer and the second metal layer, and the nanostructure contains Ag and Zn, or Ag and Sn.

[0013] This configuration allows the metasurface reflector of the present invention to suppress sulfidation of the nanostructure itself by containing Ag and Zn, or Ag and Sn, thereby achieving high process resistance. Furthermore, since the protective layer covers the first metal layer, when the resist pattern is removed with an organic solvent during the manufacturing process, the outermost first metal layer is not exposed and subjected to corrosion damage. Therefore, degradation of the reflective properties is less likely to occur.

[0014] In the metasurface reflector according to the present invention, the nanostructure may be a multilayer structure of an Ag layer and a Zn layer, or a multilayer structure of an Ag layer and a Sn layer.

[0015] This configuration allows the metasurface reflector of the present invention to have a multilayer structure in which the first metal layer, which is a nanostructure, consists of an Ag layer and a Zn layer, or an Ag layer and a Sn layer. This suppresses the sulfidation of the first metal layer itself and provides high process resistance. As a result, degradation of the reflective properties is less likely to occur.

[0016] In the metasurface reflector according to the present invention, the nanostructure may be an alloy structure of Ag and Zn, or an alloy structure of Ag and Sn.

[0017] This configuration allows the metasurface reflector of the present invention to suppress sulfidation of the nanostructure itself by using an alloy structure of Ag and Zn, or an alloy structure of Ag and Sn, thereby achieving high process resistance. As a result, degradation of the reflective properties is less likely to occur.

[0018] In the metasurface reflector according to the present invention, the nanostructure may be a multilayer structure of an Ag-Zn alloy layer and a Zn layer, or a multilayer structure of an Ag-Sn alloy layer and a Sn layer.

[0019] This configuration allows the metasurface reflector of the present invention to further suppress sulfidation of the nanostructure itself and obtain higher process resistance by making the nanostructure a multilayer structure of Ag-Zn alloy layer and Zn layer, or Ag-Sn alloy layer and Sn layer. As a result, degradation of the reflective properties is less likely to occur.

[0020] In the metasurface reflector according to the present invention, the multilayer structure of the Ag layer and Zn layer of the nanostructure may have a total thickness of 40 nm of the Ag layer and be a multilayer structure of an Ag layer and a 5.4 Å thick Zn layer and an Ag layer, or the multilayer structure of the Ag layer and Sn layer may have a total thickness of 40 nm of the Ag layer and be a multilayer structure of an Ag layer and a 9.4 Å thick Sn layer and an Ag layer.

[0021] With this configuration, the metasurface reflector of the present invention can be made such that the thickness of the Zn layer laminated on the Ag layer in the first metal layer is 5.4 Å × 2 layers, or the thickness of the Sn layer laminated on the Ag layer is 9.4 Å × 2 layers, resulting in an overall Zn content of 3 at% or a Sn content of 3 at% in the first metal layer, thereby obtaining an AgZn laminated structure or AgSn laminated structure with the highest sulfidation resistance.

[0022] In the metasurface reflector according to the present invention, the amount of Zn added to Ag in the Ag-Zn alloy structure of the nanostructure may be 3 at%, and the amount of Sn added to Ag in the Ag-Sn alloy structure may also be 3 at%.

[0023] With this configuration, the metasurface reflector of the present invention can obtain an Ag-Zn alloy or an Ag-Sn alloy with the highest sulfidation resistance by setting the addition amounts of Zn and Sn added to Ag to 3 at% each.

[0024] In the metasurface reflector according to the present invention, the nanostructure may have a configuration including metal units having a trapezoidal shape in plan view.

[0025] With this configuration, the metasurface reflector of the present invention can create a phase delay difference of 0 to 2π because the first metal layer has a trapezoidal pattern.

[0026] In the metasurface reflector according to the present invention, the length of the metal unit in the longitudinal direction is 500 nm or more and 2500 nm or less, the thickness of the metal unit is 10 nm or more and 100 nm or less, and among the short side and the long side parallel to each other in the trapezoidal shape of the metal unit, the length of the short side is 10 nm or more and 200 nm or less, and the length of the long side may be larger than the length of the short side and 100 nm or more and 500 nm or less.

[0027] With this configuration, the metasurface reflector of the present invention can enhance the reflection efficiency for visible light.

[0028] In the metasurface reflector according to the present invention, the dielectric layer may be composed of at least one compound material selected from the group consisting of SiO2, TiO2, MgO, and Al2O3.

[0029] With this configuration, the metasurface reflector of the present invention is such that the dielectric layer having the above configuration performs magnetic resonance through the upper and lower conductive layers, and the incident light incident on the first metal layer can be reflected by the cooperation of the second metal layer, the dielectric layer, and the first metal layer.

[0030] In the metasurface reflector according to the present invention, the protective layer may be configured to cover the upper and side surfaces of the first metal layer.

[0031] With this configuration, the metasurface reflector of the present invention can more reliably prevent corrosion by completely covering the trapezoidally patterned first metal layer with a protective layer.

[0032] In the metasurface reflector according to the present invention, the protective layer may be composed of at least one material selected from the group consisting of Au, Ru, Ir, and TiN.

[0033] With this configuration, the metasurface reflector of the present invention can protect the nanostructure using materials such as Au, Ru, Ir, or TiN, which have very similar optical properties to Au and are inexpensive, as the protective layer material. [Effects of the Invention]

[0034] According to the present invention, it is possible to provide a metasurface reflector that has high process resistance and is less susceptible to deterioration of its reflective properties. [Brief explanation of the drawing]

[0035] [Figure 1] This figure shows a magnified view of a metasurface reflector according to the first embodiment of the present invention, which is attached to the lens of a spectacle-type retinal scanning display. [Figure 2] Figure 1 is a schematic perspective view showing the unit region of the metasurface reflector. [Figure 3] This is a cross-sectional view along line III-III in Figure 2, showing the configuration of the metasurface reflector according to the first embodiment of the present invention. [Figure 4] Figure 2 is a plan view. [Figure 5] This diagram illustrates the reflection principle using a metasurface reflector in a spectacle-type retinal scanning display. [Figure 6]This figure shows the phase change of reflected light at a position along the x-axis of a metasurface reflector. [Figure 7] This figure shows how reflected light converges in relation to incident light due to metal units arranged so that their length in the x-axis direction changes depending on their position in the x-axis direction. [Figure 8] (a) shows the relationship between the angle of reflection and the angle of incidence when the metal unit length is 500 nm, (b) shows the relationship between the angle of reflection and the angle of incidence when the metal unit length is 750 nm, and (c) shows the relationship between the angle of reflection and the angle of incidence when the metal unit length is 1400 nm. [Figure 9] This figure shows the aging deterioration of a trapezoidal patterned metal unit. [Figure 10] This graph shows the change in the reflection angle as a trapezoidal pattern metal unit deteriorates over time, with respect to the length of the trapezoidal pattern metal unit. [Figure 11] This is a cross-sectional view showing the configuration of a metasurface reflector according to a second embodiment of the present invention. [Figure 12] This is a cross-sectional view showing the configuration of a metasurface reflector according to a third embodiment of the present invention. [Figure 13] (a) is a cross-sectional view showing the corrosion state in the case of Conventional Technology 1, which has no protective layer, and (b) is a cross-sectional view showing the corrosion state in the case of Conventional Technology 2, which has an Au protective layer. [Modes for carrying out the invention]

[0036] Embodiments of the present invention will be described in detail below with reference to the drawings. Note that, for ease of understanding, the scale of the parts in the drawings may differ from the actual scale. In the xyz Cartesian coordinate system set in the drawings, the x-axis and y-axis directions are horizontal, and the z-axis direction is vertical. The positive z-axis direction is also called the upward direction, and the negative z-axis direction is also called the downward direction, but this is unrelated to the direction of gravity. A degree of deviation is permissible in directions such as parallel, perpendicular, orthogonal, horizontal, vertical, up and down, and left and right, as long as it does not impair the effects of the embodiment. Furthermore, the "~" indicating a numerical range means that the values ​​written before and after it are included as the lower and upper limits, respectively.

[0037] [First Embodiment] First, a first embodiment of the present invention will be described. In the following description, the metasurface reflector 1 of this embodiment will be described as being attached as a half-mirror to the lens 51 of a spectacle-type retinal scanning display 100, but its use is not limited to this.

[0038] (composition) Figure 1 is an enlarged view of a metasurface reflector 1 according to a first embodiment of the present invention, attached to the lens 51 of a spectacle-type retinal scanning display 100. As shown in Figure 1, the metasurface reflector 1 is divided into a plurality of unit regions 5. The plurality of unit regions 5 are provided along the inner surface 51a of the lens 51. The plurality of unit regions 5 are arranged in a two-dimensional array in the lateral direction (x-axis direction) and the vertical direction (y-axis direction) of the lens 51. Each unit region 5 is a laminate containing, in the z-axis direction, a first metal layer 10, a dielectric layer 20, and a second metal layer 30 in that order.

[0039] Figure 2 is a schematic perspective view showing a unit region 5. Figure 3 is a cross-sectional view along line III-III in Figure 2, showing the configuration of the metasurface reflector 1 according to the first embodiment of the present invention. As shown in Figures 2 and 3, the metasurface reflector 1 comprises, in order along the z-axis direction, a second metal layer 30, a dielectric layer 20, a first metal layer 10 constituting a nanostructure 11, and a protective layer (process protective layer) 8 on a substrate 40. The protective layer 8 covers the first metal layer 10. The dielectric layer 20 is located between the first metal layer 10 and the second metal layer 30. The first metal layer 10 contains Ag and Zn, or Ag and Sn.

[0040] A chromium (Cr) layer may be provided between the first metal layer 10 and the dielectric layer 20 to improve adhesion. Similarly, Cr layers may be provided between the dielectric layer 20 and the second metal layer 30, and between the second metal layer 30 and the substrate 40. The thickness of the Cr layer is, for example, 3 nm. The first metal layer 10 may be composed of a plurality of nanostructures 11 arranged one-dimensionally or two-dimensionally on the dielectric layer 20. Light incident on the metasurface reflector 1 from the protective layer 8 side is reflected at a predetermined angle depending on the wavelength, angle of incidence, shape of the nanostructures 11, etc. The metasurface reflector 1 may be, for example, a thin plate or film, and may be rectangular, square, polygonal, circular, etc. in plan view.

[0041] The term "light" as used herein refers to visible light, but is not limited to visible light. It may also refer to infrared light, which has a longer wavelength than visible light, or ultraviolet light, which has a shorter wavelength than visible light. The wavelength of visible light is, for example, 380 nm or more and less than 800 nm. The wavelength of infrared light is, for example, 800 nm or more and less than 1 mm. The wavelength of ultraviolet light is, for example, 200 nm or more and less than 380 nm.

[0042] The following describes each component.

[0043] <Circuit board> The substrate 40 is made of, for example, sapphire. The substrate 40 may be, for example, a flexible sheet or a quartz substrate. If the metasurface reflector 1 is used in, for example, a spectacle-type retinal scanning display 100, the substrate 40 may be omitted, and the metasurface reflector 1 may be mounted on the inner surface 51a of the lens 51.

[0044] <Second metal layer> The second metal layer 30 is the base layer. The second metal layer 30 is composed of a metal containing at least one element selected from the group consisting of, for example, gold (Au), copper (Cu), silver (Ag), iridium (Ir), ruthenium (Ru), rhodium (Rh), titanium (Ti), tantalum (Ta), tungsten (W), cobalt (Co), iron (Fe), and nickel (Ni). Preferably, the second metal layer 30 is mainly composed of silver (Ag). The length (thickness) of the second metal layer 30 in the z-axis direction is such that the second metal layer 30 can conduct a resonant current when light is incident on it and can reflect light, and is, for example, 1 nm to 1000 nm, or for example, 200 nm.

[0045] <Dielectric layer> The dielectric layer 20 is a layer that functions as a spacer. The dielectric layer 20 is provided on the second metal layer 30. The dielectric layer 20 has a dielectric constant such that it does not hinder the electromagnetic interaction between the first metal layer 10 and the second metal layer 30. The dielectric layer 20 may be composed of a material with a high dielectric constant in order to achieve high reflectivity. The dielectric layer 20 is composed of at least one compound selected from the group consisting of silicon oxide (e.g., SiO2), titanium oxide (e.g., TiO2), magnesium oxide (e.g., MgO), and aluminum oxide (e.g., Al2O3). The length (thickness) of the dielectric layer 20 in the z-axis direction is, for example, 1 nm to 1000 nm, and may be, for example, 40 nm.

[0046] <First metal layer> The first metal layer 10 constitutes a nanostructure 11, which is a nanoscale structure made of metal. The first metal layer 10, together with the second metal layer 30, is a layer that excites electromagnetic resonance. Specifically, the incident electric field of light, an electromagnetic wave incident on the first metal layer 10, resonates through the dielectric layer 20, generating an electric field in the opposite direction in the second metal layer 30. A magnetic field opposite to the incident magnetic field is generated in the dielectric layer 20. As a result, the direction of propagation of light, an electromagnetic wave, is reversed.

[0047] The first metal layer 10 is provided on the main surface of the dielectric layer 20 opposite to the second metal layer 30. Similar to the second metal layer 30, the first metal layer 10 is made of a metal mainly containing at least one element selected from the group consisting of, for example, Au, Cu, Ag, Ir, Ru, Rh, Ti, Ta, W, Co, Fe, and Ni. In this embodiment, the first metal layer 10 is mainly made of Ag.

[0048] Specifically, the nanostructure 11 is, for example, a multilayer structure of an Ag layer and a Zn (zinc) layer, or a multilayer structure of an Ag layer and a Sn (tin) layer.

[0049] In the case of the multilayer structure of the Ag layer 12 and the Zn layer 13, for example, it is a multilayer structure of an Ag layer with a predetermined thickness, a Zn layer 13 with a thickness of 5.4 Å, an Ag layer with a predetermined thickness, a Zn layer 13 with a thickness of 5.4 Å, an Ag layer with a predetermined thickness, and an Ag layer. In the case of the multilayer structure of the Ag layer and the Sn layer, for example, it is a multilayer structure of an Ag layer with a predetermined thickness, a Sn layer with a thickness of 9.4 Å, an Ag layer with a predetermined thickness, a Sn layer with a thickness of 9.4 Å, and an Ag layer with a predetermined thickness. However, in each multilayer structure, the total film thickness of the Ag layer is 40 nm.

[0050] Although alloying Ag with Zn or Sn reduces the initial reflectivity, it can suppress sulfidation more effectively than pure Ag and also suppress the deterioration of reflectivity. As the content of Zn or Sn increases from pure Ag, the deterioration of reflectivity is minimized when the content reaches about 3 at%. Based on this finding, a multilayer structure of an Ag layer and a Zn layer or an Sn layer with performance equivalent to that of an Ag-Zn alloy (Zn: 3 at%) or an Ag-Sn alloy (Sn: 3 at%) was considered, and the film thickness of the Zn layer or Sn layer that can suppress sulfidation and the deterioration of reflectivity was calculated as follows.

[0051] <Thickness of the Zn layer> When the nanostructure 11 is a multilayer structure of an Ag layer and a Zn layer, the film thickness d1 of the Zn layer is obtained by the following formula. d1 = (S1 / (S1 + N)) × D Here, S1 = (mass% of Zn) / (density of Zn) [volume%] N = (mass% of Ag) / (density of Ag) [volume%] D: Thickness of the Ag layer

[0052] When the Zn content is 3 at%, S1 = 0.258 [vol%] and N = 9.357 [vol%]. Thus, for example, when the thickness D of the Ag layer is 40 nm, the thickness d1 of the Zn layer is (0.258 / (0.258 + 9.357)) × 40 [nm] = 1.07 [nm] = 10.7 [Å]. Therefore, when the nanostructure 11 contains two Zn layers, the thickness of one Zn layer is 10.7 / 2 = 5.35 [Å].

[0053] <Thickness of the Sn layer> When the nanostructure 11 has a multilayer structure of an Ag layer and an Sn layer, the thickness d2 of the Sn layer is obtained by the following formula in the same manner as above. d2 = (S2 / (S2 + N)) × D Here, S2 = (mass% of Sn) / (density of Sn) [vol%] N = (mass% of Ag) / (density of Ag) [vol%] D: Thickness of the Ag layer

[0054] When the Sn content is 3 at%, S2 = 0.453 [vol%] and N = 9.219 [vol%]. Thus, for example, when the thickness D of the Ag layer is 40 nm, the thickness d2 of the Sn layer is (0.453 / (0.453 + 9.219)) × 40 [nm] = 1.87 [nm] = 18.7 [Å]. Therefore, when the nanostructure 11 contains two Sn layers, the thickness of one Zn layer is 18.7 / 2 = 9.35 [Å].

[0055] As shown in FIGS. 2 and 4, the first metal layer 10 constituting the nanostructure 11 includes, for example, a metal unit 18 having a trapezoidal shape in a plan view as viewed in the negative z-axis direction.

[0056] The trapezoidal metal unit 18 has a length L in the x-axis direction (hereinafter referred to as the "longitudinal direction") perpendicular to both the parallel short and long sides of the trapezoidal shape, for example, between 500 nm and 2500 nm. The thickness d of the metal unit 18 in the z-axis direction is for example between 10 nm and 100 nm. Of the parallel short and long sides in the trapezoidal shape of the metal unit 18, the length W1 of the short side is for example between 10 nm and 200 nm, and the length W2 of the long side is greater than the length W1 of the short side, for example between 100 nm and 500 nm. In this case, the reflectivity for visible light can be increased.

[0057] The first metal layer 10 may include a plurality of metal units 18 arranged two-dimensionally in the x-axis and y-axis directions (see Figures 1 and 7). The spacing between two adjacent metal units 18 in the x-axis direction is set so that the wavefront of the reflected light is continuous. This spacing should be such that the two metal units 18 do not touch, for example, it should be set to less than half the wavelength of the incident light (laser light Ls). The spacing is, for example, about 20 nm. The plurality of metal units 18 are formed, for example, by photolithography.

[0058] <Protective layer> The process protection layer 8 covers the top and sides of the first metal layer 10. The process protection layer 8 is composed of at least one material selected from the group consisting of, for example, Au, Ru, Ir, and TiN. The thickness of the process protection layer 8 is, for example, 10 nm. The thickness ratio of the process protection layer 8 to the first metal layer 10 is, for example, 20% or less.

[0059] (Reflection by metasurface reflectors) Next, the principle of light reflection by the metasurface reflector 1 according to this embodiment will be explained.

[0060] Figure 3 shows how laser light Ls is incident on the metasurface reflector 1 at an incident angle θi, and how reflected light Lr is reflected at a reflection angle θr. As shown in Figure 3, the incident angle θi is the angle between the normal to the surface irradiated by the laser light Ls and the incident direction of the laser light Ls. The reflection angle θr is the angle between the normal to the surface irradiated by the laser light Ls and the emission direction of the reflected light Lr. In a plane containing the laser light Ls and the reflected light Lr, if the reflected light Lr is emitted on the opposite side of the normal from the incident light (laser light Ls), the reflection angle θr is expressed as a positive value. If the reflected light Lr is emitted on the same side of the normal as the incident light (laser light Ls), the reflection angle θr is expressed as a negative value.

[0061] As shown in Figure 1, the metasurface reflector 1 has a plurality of unit regions 5. Each unit region 5 has a metal unit 18 configured to reflect laser light Ls at a reflection angle θr corresponding to the position where the unit region 5 is located, when laser light Ls is incident at an incident angle θi corresponding to the position where the unit region 5 is located (Figure 2). For example, as will be explained later, the reflection angle θr of each unit region 5 is set so that the laser light Ls reflected by each unit region 5 (i.e., reflected light Lr) passes through the center of the pupil PP in the user's eyeball E. Therefore, the incident angle θi and the reflection angle θr are determined by the position where the unit region 5 is located. The unit region 5 is configured so that an incident angle θi and a reflection angle θr corresponding to the position where the unit region 5 is located are obtained.

[0062] Figure 5 is a diagram illustrating the reflection principle by the metasurface reflector 1 in a spectacle-type retinal scanning display 100. As shown in Figure 5, for example, when the user's pupil PP is facing forward, the unit region 5 located from position Pa to position Pc in the x-axis direction is used. The laser light Ls reflected by the unit region 5 located at position Pa corresponds to the pixel at the right edge of the image projected onto the retina RE. Position Pb is located midway between position Pa and position Pc, and the laser light Ls reflected by the unit region 5 located at position Pb corresponds to the pixel in the center of the image. The laser light Ls reflected by the unit region 5 located at position Pc corresponds to the pixel at the left edge of the image.

[0063] Specifically, in the unit region 5 located at position Pa, laser light Ls is incident at an incident angle θi of 30°, and is reflected at a reflection angle θr of 5°, emitting as reflected light Lr. In the unit region 5 located at position Pb, laser light Ls is incident at an incident angle θi of 40°, and is reflected at a reflection angle θr of -5°, emitting as reflected light Lr. In the unit region 5 located at position Pc, laser light Ls is incident at an incident angle θi of 50°, and is reflected at a reflection angle θr of -10°, emitting as reflected light Lr.

[0064] Figure 6 shows the phase change of reflected light Lr at different positions along the x-axis of the metasurface reflector 1. As shown in Figure 6, the width of the metal unit 18 increases along the x-axis from width W1 to width W2. The phase change at each position along the x-axis of the metal unit 18 is substantially the same as the phase change caused by a square metal body (shown by a dashed line) with sides of the same length as the width at that position in a plan view. The larger the area of ​​the square metal body in a plan view, the larger the phase change (phase delay) at that position. Thus, since the laser light Ls is reflected with different phase changes depending on the position along the x-axis, a wavefront is formed by interference between the reflected light. That is, a plane wave is generated that propagates in a direction determined by the relationship between the position along the x-axis and the phase change. The reflection angle changes depending on the length (pattern length) of the metal unit 18 in the x-axis direction.

[0065] The length Lx of each unit region 5 shown in Figure 4 is determined by the wavelength λ of the object being reflected and the incident angle θi and reflection angle θr corresponding to the position where the unit region 5 is located. The length L of the metal unit 18 in the x-axis direction is the same as or slightly shorter than the length Lx of the unit region 5 in the x-axis direction. Therefore, the length L of the metal unit 18 in the x-axis direction is determined by the wavelength λ of the object being reflected and the incident angle θi and reflection angle θr corresponding to the position where the metal unit 18 is located in the unit region 5.

[0066] In this embodiment, the lengths Lx of the unit regions 5 included in the same arrangement in the x-axis direction are different from each other, and the lengths L in the x-axis direction of the metal units 18 included in the same arrangement in the x-axis direction are also different from each other.

[0067] The length Ly of each unit region 5 is a predetermined fixed value. The length Ly is slightly larger than the width W2. The length Ly may also be the length obtained by adding the resolution of the exposure apparatus used to form the metal unit 18 (e.g., 100 nm) to the width W2, for example, set to 600 nm. The widths W1 and W2 of each metal unit 18 are predetermined fixed values. As described above, the width W1 is set to near the resolution of the exposure apparatus used to form the metal unit 18 (e.g., 100 nm). The width W2 is set to a length (e.g., 350 nm) that yields a phase difference of substantially 360° (2π radians) from the phase of the reflected light Lr in width W1.

[0068] Figure 7 illustrates a mirror made of metal units 18, which are nanostructures. Multiple metal units 18 arranged in the x-axis direction have varying lengths in the x-axis direction depending on their position in the x-axis direction. For example, the movable mirror 62 in Figure 5 ensures that even if the position in the x-axis direction from which the laser beam Ls is incident changes, the reflected light Lr remains focused. In this way, by accumulating patterns in which the longitudinal length (metal unit length) of the metal units 18 is varied, multiple reflection angles spanning both positive and negative directions can be created.

[0069] Figure 8(a) shows the relationship between the angle of incidence and the angle of reflection when the metal unit length is 500 nm, Figure 8(b) shows the relationship between the angle of incidence and the angle of reflection when the metal unit length is 750 nm, and Figure 8(c) shows the relationship between the angle of incidence and the angle of reflection when the metal unit length is 1400 nm. As can be seen from Figure 8(a), when the laser beam Ls is incident on the right end of the mirror in Figure 7, if the metal unit length is 500 nm, the angle of incidence is 47° and the angle of reflection is -20°. As can be seen from Figure 8(b), when the laser beam Ls is incident on the center of the mirror in Figure 7, if the metal unit length is 750 nm, the angle of incidence is 45° and the angle of reflection is 0°. As can be seen from Figure 8(c), when the laser beam Ls is incident on the left end of the mirror in Figure 7, if the metal unit length is 1400 nm, the angle of incidence is 44° and the angle of reflection is +20°. In this way, in the mirror shown in Figure 7, where multiple metal units are arranged so that the length of the metal units changes in the x-axis direction, the reflected light generated at positive and negative angles is focused. In the retinal scanning display 100 shown in Figure 5, laser light Ls can be incident on the metasurface reflector 1 from an oblique angle, and the reflected light Lr can be focused to the position of the pupil PP in the front.

[0070] (Effects of corrosion on nanostructures) Figure 9 shows the aging deterioration of a trapezoidal metal unit 18. It shows the state in which the trapezoidal metal unit 18 has deteriorated over time, corroding and shrinking in area from its original trapezoidal shape to become metal unit 18'. When the shape of the metal unit 18 changes from its original trapezoidal shape due to aging, the reflection angle becomes inconsistent and the intensity of reflected light decreases.

[0071] Figure 10 is a graph showing the change in the reflection angle with respect to the length of the trapezoidal pattern metal unit 18 when the trapezoidal pattern metal unit 18 deteriorates over time. The solid line shows the theoretical curve of metal unit length-reflection angle. When the shape of the trapezoidal pattern metal unit 18 changes due to deterioration over time, the relationship between metal unit length and reflection angle shifts, for example, from the solid line to the upper or lower dashed line, or fluctuates between the upper and lower dashed lines.

[0072] In this embodiment, the metasurface reflector 1 has a nanostructure 11 that contains Zn or Sn in Ag, thereby suppressing the sulfidation of the first metal layer 10, which is the nanostructure 11, and achieving high process resistance. Furthermore, since the process protection layer 8 covers the first metal layer 10, including its sides, when the resist pattern is removed with an organic solvent, the outermost surface of the first metal layer 10 is not exposed and subjected to corrosion damage. Therefore, degradation of the reflective properties is less likely to occur.

[0073] (Manufacturing process) The metasurface reflector 1 is obtained by sequentially fabricating a second metal layer 30, a dielectric layer 20, a first metal layer 10, and a process protection layer 8 on a substrate 40 using techniques such as sputtering and photolithography. The substrate 40 may be a base material such as a sapphire substrate, a flexible sheet, or a quartz substrate.

[0074] Specifically, a second metal layer 30 is formed on the substrate 40 by vacuum deposition using a method such as DC (Direct Current) sputtering. For the formation of the second metal layer 30, a metal material is used that is composed of any metal selected from the set consisting of Au, Cu, Ag, Ir, Ru, Rh, Ti, Ta, W, Co, Fe, and Ni, or a metal alloy containing at least one element selected from the above set. The second metal layer 30 is formed with a film thickness of, for example, 1 nm to 1000 nm. When silver is used as the metal material, the film thickness of the second metal layer 30 is, for example, 200 nm.

[0075] Next, a dielectric layer 20 is formed on the second metal layer 30. Specifically, the dielectric layer 20 is formed by vacuum deposition using a method such as RF (Radio Frequency) sputtering. For the formation of the dielectric layer 20, dielectric materials such as silicon dioxide (SiO2), titanium dioxide (TiO2), magnesium oxide (MgO), or aluminum oxide (Al2O3), which can be formed in semiconductor processes, are used. The dielectric layer 20 is formed with a thickness of, for example, 1 nm to 1000 nm. When silicon dioxide is used as the dielectric material, the thickness of the dielectric layer 20 is, for example, 40 nm.

[0076] Next, a metal layer (hereinafter referred to as the "outermost metal layer") which will become the first metal layer 10 is formed on the dielectric layer 20. The outermost metal layer may be a multilayer structure of, for example, an Ag layer and a Zn layer, and is formed by a method such as sputtering, similar to the second metal layer 30. The outermost metal layer is formed with a thickness of, for example, 1 nm to 1000 nm. The thickness of the outermost metal layer is, for example, 40 nm.

[0077] Next, a first metal layer 10 (multiple nanostructures 11) is formed by a photolithography process and an etching process. Specifically, a liquid resist is applied to the outermost metal layer using a spin coater or the like, and the applied liquid resist is dried to form a resist film (photoresist). Then, a pattern corresponding to the nanostructures 11 is transferred to the resist film using an exposure device such as a KrF exposure machine or an electron beam lithography device. Then, the pattern transferred to the resist film is developed using a developer. Finally, the parts of the outermost metal layer that are not covered by the pattern are removed by ion milling, and then the resist film is removed. This forms the first metal layer 10. The widths W1, W2 and length L of each metal unit 18 are, for example, 10 nm to 1000 nm.

[0078] Next, a process protection layer 8 is formed on the first metal layer 10, including the sides, using a method such as sputtering. At this time, areas where the process protection layer 8 is not to be formed are masked. Through these steps, the metasurface reflector 1 is formed.

[0079] Depending on the application, the metasurface reflector 1 may be formed directly on the lens of eyeglasses or a half-mirror instead of on the substrate 40. The formation method is the same as the method for forming the metasurface reflector 1 on the substrate 40.

[0080] [Second Embodiment] Next, a second embodiment of the present invention will be described. Figure 11 is a cross-sectional view showing the configuration of the metasurface reflector 2 according to the second embodiment of the present invention. In the second embodiment, the configuration of the first metal layer 10 differs from that of the first embodiment. The other configurations are the same as in the first embodiment, and the same reference numerals are used for the same components, and their descriptions are omitted as appropriate.

[0081] The first metal layer 10 constituting the nanostructure 11 is an alloy structure of Ag and Zn, or an alloy structure of Ag and Sn.

[0082] In the case of an alloy structure of Ag and Zn, preferably, the amount of Zn added to Ag is 3 at%. In the case of an alloy structure of Ag and Sn, preferably, the amount of Sn added to Ag is 3 at%.

[0083] As mentioned above, alloying Ag with Zn or Sn reduces its initial reflectivity, but it suppresses sulfidation more effectively than pure Ag, thus reducing the degradation of reflectivity due to corrosion. As the Zn or Sn content is increased from pure Ag, the degradation of reflectivity is minimized at a content of around 3 at%.

[0084] Furthermore, the principle behind the reflection of light by the metasurface reflector 2 is that the nanostructure 11 and the second metal layer 30 resonate through the dielectric layer 20, thereby enhancing the reflection. Although there is a decrease in initial reflectivity, the only layer whose structure is modified for corrosion protection is the first metal layer 10, and considering the resonance principle of the metasurface reflector 2, the impact of alloying on the degradation of reflection intensity is considered to be small.

[0085] By making the first metal layer 10 constituting the nanostructure 11 an alloy structure of Ag and Zn, or an alloy structure of Ag and Sn, the sulfidation of the nanostructure 11 itself can be suppressed, and high process resistance can be obtained. As a result, degradation of the reflective properties is less likely to occur.

[0086] [Third Embodiment] Next, a third embodiment of the present invention will be described. Figure 12 is a cross-sectional view showing the configuration of the metasurface reflector 3 according to the third embodiment of the present invention. In the third embodiment, the configuration of the first metal layer 10 differs from that of the first embodiment. The other configurations are the same as in the first embodiment, and the same reference numerals are used for the same components, and their descriptions are omitted as appropriate.

[0087] The first metal layer 10 constituting the nanostructure 11 is a multilayer structure of an Ag-Zn alloy layer 15 and a Zn layer 13, or a multilayer structure of an Ag-Sn alloy layer and a Sn layer.

[0088] By making the first metal layer 10 constituting the nanostructure 11 a multilayer structure of Ag-Zn alloy layer and Zn layer, or an Ag-Sn alloy layer and Sn layer, the sulfidation of the nanostructure 11 itself can be further suppressed, and higher process resistance can be obtained. As a result, degradation of the reflective properties is less likely to occur.

[0089] [Examples of application] The metasurface reflectors 1, 2, and 3 according to embodiments of the present invention can be applied, for example, to a retinal projection device 60 or a retinal scanning display (near-eyewearable device) 100 equipped with a retinal projection device 60.

[0090] The retinal scanning display 100 shown in Figure 5 is a device that superimposes images onto the field of view of the real world. The retinal scanning display 100 is, for example, a head-mounted device. In this example, it is a glasses-type device, but it can take the form of goggles, a hat, or a helmet. Examples of retinal scanning displays 100 include smart glasses such as AR glasses and MR (Mixed Reality) glasses. The retinal scanning display 100 in Figure 5 includes a glasses frame 50, lenses 51 attached to the rim 52, and a retinal projection device 60 attached to the temple 53.

[0091] The retinal projection device 60 shown in Figure 5 is a device that directly projects (draws) an image onto the retina RE of a user wearing a retinal scanning display 100. The retinal projection device 60 is mounted on the retinal scanning display 100. The retinal projection device 60 includes a light source unit 61, a movable mirror 62, and a metasurface reflector 1, 2, or 3.

[0092] The light source unit 61 emits laser light of a color and intensity corresponding to the pixels of the image projected onto the retina RE. For example, a full-color laser module is used as the light source unit 61. The light source unit 61 includes, for example, a red laser diode, a green laser diode, a blue laser diode, a near-infrared laser diode, and a multiplier that combines the laser light emitted from each laser diode into a single laser beam. The light source unit 61 emits the combined laser beam.

[0093] The movable mirror 62 is a component for scanning with laser light Ls.

[0094] Metasurface reflectors 1, 2, and 3 are components that project an image onto the retina RE of the user wearing the retinal scanning display 100 by reflecting laser light Ls that has passed through the movable mirror 62 and irradiating the reflected light Lr onto the retina RE. No image is displayed on the metasurface reflectors 1, 2, and 3.

[0095] The metasurface reflector relating to this disclosure is not limited to the embodiments described above.

[0096] In the above embodiment, when the nanostructure 11 has a multilayer structure of an Ag layer and a Zn or Sn layer, two Zn or Sn layers were used. However, if the desired film thickness can be achieved, there may be other layers, such as one or three layers. The film thickness of the Zn or Sn layer is determined, for example, so that the total content of the Zn or Sn layer in the nanostructure 11 is an optimal value for sulfur resistance, for example, about 3 at%.

[0097] Furthermore, if the nanostructure 11 is a multilayer structure of Ag-Zn alloy layers and Zn layers, or an Ag-Sn alloy layer and Sn layers, the thickness of each Ag-Zn alloy layer and Zn layer is determined, for example, such that the total content of the Zn or Sn layer in the nanostructure 11 is optimal for sulfidation resistance, for example, about 3 at%.

[0098] As described above, the present invention has the effect of high process resistance and less deterioration of reflective properties, and is useful for metasurface reflectors in general. [Explanation of symbols]

[0099] 1, 2, 3 Metasurface reflectors 5 Unit Area 8. Process protection layer (protection layer) 10 1st metal layer 11 Nanostructures 12 Ag layer 13 Zn layer or Sn layer 15. Ag-Zn alloy or Ag-Sn alloy 18 Metal Units 19 Corroded parts 20 Dielectric layer 30 Second metal layer 40 circuit boards 50 eyeglass frames 51 lenses 51a Inner surface 52 rim 53 Temple 60 Retinal projection device 61 Light source unit 62 Movable Mirror E Eyeball PP Pupil RE (Retina) 100 Retinal Scanning Display

Claims

1. A metasurface reflector comprising a protective layer, a first metal layer constituting a nanostructure, a dielectric layer, and a second metal layer, wherein the protective layer covers the first metal layer, the dielectric layer is located between the first metal layer and the second metal layer, and the nanostructure contains Ag and Zn, or Ag and Sn.

2. The metasurface reflector according to claim 1, wherein the nanostructure is a multilayer structure of an Ag layer and a Zn layer, or a multilayer structure of an Ag layer and a Sn layer.

3. The metasurface reflector according to claim 1, wherein the nanostructure is an alloy structure of Ag and Zn, or an alloy structure of Ag and Sn.

4. The metasurface reflector according to claim 1, wherein the nanostructure is a multilayer structure of an Ag-Zn alloy layer and a Zn layer, or a multilayer structure of an Ag-Sn alloy layer and a Sn layer.

5. The metasurface reflector according to claim 2, wherein the multilayer structure of the Ag layer and Zn layer of the nanostructure has a total thickness of 40 nm of the Ag layer and is a multilayer structure of an Ag layer and a Zn layer of 5.4 Å thickness and an Ag layer, and the multilayer structure of the Ag layer and Sn layer has a total thickness of 40 nm of the Ag layer and is a multilayer structure of an Ag layer and a Sn layer of 9.4 Å thickness and an Ag layer.

6. The metasurface reflector according to claim 3, wherein in the alloy structure of Ag and Zn of the nanostructure, the amount of Zn added to Ag is 3 at%, and in the alloy structure of Ag and Sn, the amount of Sn added to Ag is 3 at%.

7. The metasurface reflector according to any one of claims 1 to 6, wherein the nanostructure includes a trapezoidal metal unit in plan view.

8. The longitudinal length of the aforementioned metal unit is between 500 nm and 2500 nm. The thickness of the metal unit is 10 nm or more and 100 nm or less. The metasurface reflector according to claim 7, wherein, in the trapezoidal shape of the metal unit, of the parallel short and long sides, the length of the short side is 10 nm or more and 200 nm or less, and the length of the long side is greater than the length of the short side and is 100 nm or more and 500 nm or less.

9. The dielectric layer is SiO ２ , TiO ２ MgO, Al ２ O ３ The metasurface reflector according to claim 1, comprising at least one compound material selected from the group consisting of the following.

10. The metasurface reflector according to claim 1, wherein the protective layer covers the upper and side surfaces of the first metal layer.

11. The metasurface reflector according to claim 1, wherein the protective layer is composed of at least one material selected from the group consisting of Au, Ru, Ir, and TiN.

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