Pseudo moth-eye structure and method for fabricating the same

Abstract

To provide a pseudo moth-eye structure which can be used as an antenna lens that is thin and is low reflective in a terahertz band, and an antenna lens having the pseudo moth-eye structure in the surface.SOLUTION: A pseudo moth-eye structure 61 includes a base material 7 and a unit pseudo moth-eye structure 1A as a collection of a plurality of pseudo moth-eye elements 2 including a part inclined to the normal of the base material 7. The pseudo moth-eye elements 2 each have a connection part 3A connecting to the base material 7, a body part 4A, and an end part 5A, and are spindle-shaped with a separated part. The unit pseudo moth-eye structure 1A is characterized in that the end parts 3A of the pseudo moth-eye elements 2 converge.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a pseudo-moth-eye structure and a method for producing the same. [Background technology]

[0002] Lenses are used as antennas for wireless communication in the terahertz wave band, and since the higher the refractive index of the lens material, the thinner it can be made, there is a demand for the application of materials with high refractive index, such as silicon. However, since high refractive index materials have a large reflection loss on the surface of the material, structures that control the refractive index, such as moth-eye structures, are attracting attention.

[0003] Patent Document 1 describes the provision of various substrate surfaces, particularly of optical elements, including SiO2 surfaces and quartz glass surfaces, with an anti-reflection arrangement of nearly ideal conical nanostructures, and describes the easy and inexpensive production of moth-eye structures directly on quartz glass by etching.

[0004] Non-Patent Document 1 describes a structure that is fabricated by a femtosecond laser and that controls the refractive index, such as a moth-eye structure, in order to reduce the reflection of silicon. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2013-515970 [Non-patent literature]

[0006] [Non-Patent Document 1] Piao Rong et.al,"Compensation optimization for a static Mars wind imaging Michelson interferometer"[online], Volume 142, July 2021, 106589,Optics and Lasers in Engineering, [Retrieved May 10, 2023], Internet<URL:https: / / doi.org / 10.1016 / j.optlaseng.2021.106584> Summary of the Invention [Problem to be solved by the invention]

[0007] However, it is difficult to form a structure that controls the refractive index, such as a moth-eye structure, on silicon, and expensive processing techniques are required, such as using a femtosecond laser, which is an expensive device. Currently, polypropylene resin, which is easy to handle and has a low refractive index, is used, which has little reflection, but the low refractive index means that the antenna lens is thick. Therefore, an object of the present invention is to provide a pseudo-moth-eye structure that can provide a thin antenna lens with low reflection in the terahertz band, and an antenna lens having the pseudo-moth-eye structure on its surface. [Means for solving the problem]

[0008] The present invention which solves the above problems is as follows. [1] A pseudo-moth-eye structure comprising a substrate and a unit pseudo-moth-eye structure which is an assembly of a plurality of pseudo-moth-eye elements including portions inclined with respect to a normal to the substrate, wherein the plurality of pseudo-moth-eye elements each have a connection portion connecting to the substrate, a main body portion, and a tip portion, and each have a spindle shape with spaced apart portions, and the unit pseudo-moth-eye structure is a pseudo-moth-eye structure characterized in that the tips of each of the pseudo-moth-eye elements converge. [2] A pseudo-moth-eye structure as described in [1], characterized in that the main component of the material is silicon. [3] The pseudo-moth-eye structure according to [1], characterized in that the height from the connection part to the tip of the tip is 50 μm or more. [4] The pseudo-moth-eye structure according to [2], wherein the height from the bottom surface of the connection part to the tip of the tip part is 50 μm or more. [5] An antenna lens having the pseudo-moth-eye structure according to any one of [1] to [4] on at least a part of its surface. [6] A method for producing a pseudo-moth-eye structure, comprising drying a moth-eye structure including a substrate and a plurality of moth-eye elements arranged in a row approximately parallel to a normal line of the substrate, to produce the pseudo-moth-eye structure according to any one of [1] to [4]. [7] A method for producing a pseudo-moth-eye structure according to [6], characterized in that the moth-eye structure is produced by a metal-assisted etching method. Effect of the Invention

[0009] By having the pseudo-moth-eye structure according to the present invention on the surface, the reflection from the high refractive index crystalline silicon surface can be reduced, thereby making it possible to miniaturize the antenna lens for the terahertz band. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1A is a schematic diagram showing a moth-eye structure, and FIG. 1B is a schematic diagram showing a unit pseudo-moth-eye structure according to one embodiment of the present invention. [Diagram 2] (a) Schematic diagram of a low refractive index lens with evaluation of its reflection and thickness (◯, ×), (b) schematic diagram of a high refractive index lens with evaluation of its reflection and thickness (◯, ×), (c) schematic diagram of a high refractive index lens with a moth-eye structure with evaluation of its reflection, thickness and processability (◯, ×), and (c´) schematic diagram of the moth-eye structure formed in (c). [Diagram 3]This is an overview of the method for fabricating a moth-eye structure (metal assisted chemical etching method), and shows a schematic diagram of (a-1) the preparation of silver (electroless plating method), (a-2) an SEM image of (a-1), (b-1) a partially enlarged view of (a-1), and (b-2) the progressed state of the reaction in (b-1). [Figure 4] FIG. 1 shows (a-1) an SEM image of a sample with a pseudo-moth-eye structure (n-type, Example 1) viewed from the tip, (a-2) an SEM image of a cross section of (a-1), (b) an SEM image of a cross section of another sample with a pseudo-moth-eye structure (p-type, Example 2), and (c) an SEM image of a cross section of yet another sample with a pseudo-moth-eye structure (p-type, Example 3). [Diagram 5] FIG. 1 shows (a) a cross-sectional SEM image of a sample with a pseudo-moth-eye structure, (b-1) a cross-sectional SEM image of the wire apex, (b-2) a cross-sectional SEM image of the wire bottom, and (c) an SEM image of a sample with a pseudo-moth-eye structure (resistivity 20 kΩ cm, Example 4) viewed from the tip. [Figure 6] FIG. 1 is a diagram showing a measurement device for radio waves reflected and transmitted through a sample in response to a transmitted radio wave. [Figure 7] 1A and 1B are diagrams showing the change in reflectance and the change in transmittance depending on whether or not a pseudo-moth-eye structure is present. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiments, and changes, modifications, and improvements can be made without departing from the scope of the invention.

[0012] 1(a), the motheye structure 1 includes a base material (substrate) 7 and a plurality of motheye elements including a motheye element 2 (2') that are substantially horizontal with respect to a normal line (vertical line) of the base material (substrate) 7, and each of the plurality of motheye elements includes spaced apart portions. The motheye element 2 (2') has a curved spindle shape and includes a connection portion 3 (3'), a main body portion 4 (4'), and a tip portion 5 (5'). That is, the motheye structure 1 includes the base material (substrate) 7 and a plurality of motheye elements that stand in a forest substantially perpendicular to the base material (substrate) 7.

[0013] The unit pseudo-moth-eye structure 1A is a collection of multiple pseudo-moth-eye elements 2A (2'A) including portions inclined with respect to the normal to the base material (substrate) 7A. Each of the multiple pseudo-moth-eye elements 2A (2'A) has a connection portion 3A (3'A) connecting to the base material 7, a main body portion 4A (4'A), and a tip portion 5A (5'A), and is spindle-shaped with spaced apart portions, and the unit pseudo-moth-eye structure 1A is formed by the tip portions 5'A, etc. of the multiple pseudo-moth-eye elements converging. Therefore, the overall shape of the unit pseudo-motheye structure 1A is a substantially cone shape with the aggregate of the connection parts 3A of the multiple pseudo-motheye elements as the bottom surface. The unit pseudo-motheye structures 1A, etc. are arranged adjacent to each other to form a pseudo-motheye structure 61 (see FIG. 4).

[0014] As shown in FIG. 1(b), if the height of the pseudo moth-eye unit structure 1A (from the bottom surface of the connection portion 3A (3'A) to the tip of the tip portion 5A (5'A)) having a substantially conical shape is h mm, the height h is preferably 0.05 mm (50 μm) or more, more preferably 0.1 mm (100 μm) or more, from the viewpoint of smoothly matching the impedance in silicon and the impedance in space and reducing reflection. If the diameter of the bottom surface of the pseudo moth-eye unit structure 1A is d, the area of ​​the bottom surface of the pseudo moth-eye unit structure 1A can be calculated from the diameter d. Since the entire area occupied by the bottom surfaces of the pseudo moth-eye unit structures 1A arranged adjacent to each other is the area of ​​the pseudo moth-eye structure, it is possible to fabricate a pseudo moth-eye structure having a large area of, for example, 8 inches and a high pseudo moth-eye structure. The height of the pseudo moth-eye structure can be treated as being equal to the height of the pseudo moth-eye unit structure or the length of the pseudo moth-eye element.

[0015] As will be described later, since the pseudo-moth-eye structure 61 is formed by drying the moth-eye structure 1, the dimensions of the moth-eye elements and the substrate tend to shrink due to the loss of solvent and other substances contained in the moth-eye structure during drying.

[0016] As shown in Figure 2, lenses used as antennas for wireless communication in the terahertz wave band can be classified into (a) low refractive index lenses, (b) high refractive index lenses, and (c) high refractive index lenses with a moth-eye structure. A low refractive index lens 20 has a high ratio of transmitted radio waves 20WT and a low ratio of reflected radio waves 20WR relative to radio waves W from the radio wave source S, i.e., it has low reflection, and therefore has good transparency (◯). On the other hand, because of its low refractive index, the lens is thick and large (×).

[0017] The high refractive index lens 21 has a low ratio of transmitted radio waves 21WT and a high ratio of reflected radio waves 21WR with respect to the radio waves W from the radio wave source S, i.e., it is highly reflective (high reflection loss), and therefore has poor transparency (x). On the other hand, due to its high refractive index, the lens is thin and slim (o). The high refractive index lens 22 with a moth-eye structure has a high ratio of transmitted radio waves 22WT and a low ratio of reflected radio waves 22WR with respect to the radio waves W from the radio wave source S, i.e., it is low reflective, and therefore has good transparency (o), and due to the moth-eye structure, the lens is thin and slim (o). However, due to limitations on the material that can be used (silicon), it is poor in processability (x). Here, the moth-eye structure 1C formed on the entire surface of the high refractive index lens 22 with a moth-eye structure is a collection of curved spindle shapes regularly arranged in a lattice shape with their apexes facing up, as shown in Figure (c') when viewed obliquely.

[0018] It is difficult to form a moth-eye structure on silicon, and expensive processing techniques such as femtosecond lasers are required. For this reason, polypropylene resin, which is easy to handle and has low reflection but a low refractive index, is currently used. The refractive index of silicon is about 3.4, and that of polypropylene resin is about 1.5.

[0019] As shown in Fig. 3, the moth-eye structure 1 can be fabricated. That is, silver is fabricated by electroless plating, and then the fabricated silicon nanowires are made into the moth-eye structure 1 by metal-assisted chemical etching. In the process of fabricating silver, when a silicon wafer 30 is immersed in a solution of hydrogen fluoride (HF) and silver nitrate (AgNo3), Ag is adsorbed onto the surface of the silicon wafer 20 (Fig. 3 (a-1) and (b-1)). In the SEM image of the surface viewed from above, it can be observed as shown in Fig. 3 (a-2).

[0020] Ag adsorbed on the surface of the silicon wafer 30 is oxidized by hydrogen peroxide (H2O2) to become silver ions. The silver ions and fluoride ions from hydrogen fluoride (HF) oxidize silicon (Si) of the silicon wafer to silicon hexafluoride (SiF6 2-) is dissolved. As a result, the portion of the silicon wafer that is not dissolved becomes silicon nanowires that are connected in the normal direction of the base material (substrate) 30, i.e., in a direction approximately perpendicular to the base material (substrate) 30 (FIGS. (b-1) and (b-2)). The silicon nanowires become moth-eye elements, and a plurality of silicon nanowires (moth-eye elements) stand in a forest to form a moth-eye structure 1. As a result, the diameter of the moth-eye (diameter of the connection parts 3, 3') and the like can be controlled by the spacing between the silicon nanowires and the like.

[0021] The unit pseudo-moth-eye structure 1A can be produced by completing the solution etching for producing the silicon nanowires and drying the moth-eye structure 1, whereby the liquid such as the solvent contained in the moth-eye structure 1 evaporates and the silicon nanowires aggregate due to surface tension.

[0022] The drying conditions are preferably as follows: drying with an air gun using nitrogen or dry air, or drying in air at 100 degrees or higher for 5 minutes or more. EXAMPLES

[0023] Using n-type (phosphorus-doped) and p-type (boron-doped) silicon wafers (1 kohm·cm to 50 kohm·cm), a moth-eye structure including a plurality of silicon nanowires, i.e., an n-type moth-eye structure, a p-type moth-eye structure, and a p-type moth-eye structure, were formed as follows, and then dried to produce the corresponding pseudo-moth-eye structure 61 (Example 1), pseudo-moth-eye structure 71 (Example 71), and pseudo-moth-eye structure 81 (Example 3). Since the higher the resistance of the silicon wafer used, the smaller the absorption loss (14 Dec 2015 | Vol. 23, No. 25 | DOI:10.1364 / OE.23.031977 | OPTICS EXPRESS 31979), a 20 kohm·cm silicon wafer was used to form an i-type (non-doped) moth-eye structure, and then dried to produce the pseudo-moth-eye structure 91 (Example 4). The drying conditions were as follows: drying with nitrogen or dry air using an air gun, or drying in air at 100 degrees or higher for 5 minutes or more. In the process of making silver particles, the experimental conditions were H2O: 40ml, HF: 10ml, AgNO3: 120mg, and the etching time was ≧10s (60s). In the process of making silicon nanowire arrays, the experimental conditions were H2O: 400ml, HF: 100ml, H2O2: 9ml to 80ml, and the etching time was ≧10min (10min). The experimental conditions for removing silver particles after nanowire formation were HNO3: 10ml, H2O: 50ml, and the etching time was 20min. Furthermore, in the process of removing oxide film, the experimental conditions were HF: 2ml, H2O: 80ml, and the etching time was ≧10min. In order to completely remove silver, the silver particle removal and oxide film removal processes were repeated three times. Note that the etching time in parentheses indicates the time in the example.

[0024] The pseudo-moth-eye structure 61 (Example 1) has an aggregation width at the tip of 10 μm to 30 μm and a length of 110 μm. The pseudo-moth-eye structure 71 (Example 2) has an aggregation width of 10 μm to 50 μm and a length of 100 μm. The pseudo-moth-eye structure 81 (Example 3) has an aggregation width of 1 μm to 10 μm and a length of 70 μm. The pseudo-moth-eye structure 91 (Example 4) has an aggregation width of 2 μm to 10 μm and a length of 70 μm.

[0025] 4(a-1) and (a-2), the pseudo-moth-eye structure 61 (Example 1) includes a substrate 67 and a plurality of unit pseudo-moth-eye structures including a unit pseudo-moth-eye structure 62 (62'). The pseudo-moth-eye element 63 has a connection portion 64, a main body portion 65, and a tip portion 66.

[0026] Comparing with the scale (10 μm) of FIG. 4(a-1), the diameter of the unit pseudo-moth-eye structure 62 (62') etc. was about 15 to 35 μm. Also, comparing with the scale (50 μm) of FIG. 4(a-2), it was found that the height of the unit pseudo-moth-eye structure 62 (62') etc., i.e., the pseudo-moth-eye structure 61, was about 100 μm. According to FIG. 4(a-2), the inclination of the pseudo-moth-eye element 63 with respect to the normal to the substrate 67 was mainly at its tip.

[0027] 4(b) shows that the height of unit pseudo-moth-eye structure 72 (72'), i.e., pseudo-moth-eye structure 71, is about 300 μm. According to FIG. 4(b), the inclination of the pseudo-moth-eye element with respect to the normal to substrate 77 was mainly at its tip, similar to Example 1.

[0028] 4(c) , it was found that the height of unit pseudo-moth-eye structure 82 (82'), i.e., pseudo-moth-eye structure 81, was about 100 μm. According to FIG. 4(c), the inclination of the pseudo-moth-eye element with respect to substrate 87 was from the connection part to the tip part, unlike in Examples 1 and 2.

[0029] Comparing with the scale (20 μm) of FIG. 5(a), it was found that the height of the unit pseudo-moth-eye structure 92 (92'), i.e., the pseudo-moth-eye structure 91, was about 70 μm. This was also found from FIG. 5(b-1) and (b-2). Also, FIG. 5(c) showed that the aggregation width was 1 μm to 10 μm.

[0030] 6 was used to measure radio waves reflected and transmitted by a measurement sample (specimen) 110 in response to a transmitted radio wave W. The measurement device 100 has a radio wave transceiver 101 and a transmitting antenna 102. The measurement sample (specimen) 110 was placed in contact with the transmitting antenna 102, and the transmitted radio wave W transmitted from the radio wave transceiver 101 was applied to the measurement sample (specimen) 110.

[0031] In the measuring device 100, a measurement sample (specimen) 110 was placed in contact with the receiving antenna 104 on the opposite side of the transmitting antenna 102, and the transmitted radio wave WT was measured by the radio wave receiver 105.

[0032] The corresponding frequency of the transmission radio wave W was 220 GHz to 280 GHz (WR-3 standard waveguide). The measurement sample (specimen) 110 used was as follows, with a thickness of 550 μm and a size of 2 cm square. Figure 7 shows the changes in reflectance and transmittance. Figure 7(a) shows that at frequencies of 220 GHz to 280 GHz, the reflectance was about 0% to about 28% with the pseudo-moth-eye structure (Example 4), whereas it was about 70% to about 55% without the pseudo-moth-eye structure (i-type silicon wafer). Figure 7(b) shows that the transmittance was about 95% to about 52% with the pseudo-moth-eye structure (Example 4), whereas it was about 28% to about 37% without the pseudo-moth-eye structure (i-type silicon wafer).

[0033] In other words, when a transmission wave is applied to the pseudo-moth-eye structure, it is possible to reduce the proportion of reflected waves and increase the proportion of transmitted waves. Furthermore, as the frequency of the transmission wave increases, there is a tendency for the proportion of reflected waves to increase and the proportion of transmitted waves to decrease. As described above, the pseudo-moth-eye structure can be fabricated by forming silicon nanowires using metal assisted chemical etching on a silicon wafer and drying them, without requiring expensive silicon processing techniques. In other words, it has become pseudo-easy to form a moth-eye structure on silicon, which was previously difficult, and the effect of reducing reflection in the terahertz band has been confirmed. [Industrial Applicability]

[0034] The possibility of using silicon lenses allows the lens thickness to be reduced, which in turn allows the packaging volume to be reduced. This technology reduces the reflection loss that is an issue in this case, and improves the lens antenna gain, thereby enabling the realization of high performance communication systems. [Explanation of symbols]

[0035] 1: Moth-eye structure 2, 2´: Motheye element 3, 3´: Connection part 4, 4´: Main body 5, 5´: Tip 7, 7A, 67, 77, 87: Base material (substrate) 1A, 62, 62´, 72, 72´, 82, 82´: Unit pseudo-moth-eye structure 2A, 2´A, 63: Pseudo-moth-eye element 3A, 3´A, 64: Connection of pseudo-moth-eye elements 4A, 4´A, 65: Main body of pseudo-moth-eye element 5A, 5´A, 66: Tip of pseudo-moth-eye element 6A: Substrate (substrate) with unit pseudo-moth-eye structure 20: Low refractive index lens 21: High refractive index lens 22: High refractive index lens with moth-eye structure 30: Silicon wafer (base material (substrate)) 61, 71, 81, 91: Pseudo moth-eye structure 100: Measuring device 101: Radio transmitter / receiver 102: Transmitting antenna 104: Receiving antenna 105: Radio receiver 110: Measurement sample (specimen) S:Radio wave source W: Transmitted radio wave WR: Reflected radio waves WT: Transmission radio wave

Claims

1. A pseudo-moth-eye structure comprising a substrate and a unit pseudo-moth-eye structure which is an assembly of a plurality of pseudo-moth-eye elements including a portion inclined with respect to a normal to the substrate, wherein the plurality of pseudo-moth-eye elements each have a connection portion connected to the substrate, a main body portion, and a tip portion, and each have a spindle shape having spaced apart portions, and the unit pseudo-moth-eye structure is characterized in that the tips of each of the pseudo-moth-eye elements converge.

2. 2. The pseudo-moth-eye structure according to claim 1, characterized in that the main component of the material is silicon.

3. 2. The pseudo-moth-eye structure according to claim 1, wherein the height from the connection portion to the tip of the tip portion is 50 μm or more.

4. 3. The pseudo-moth-eye structure according to claim 2, wherein the height from the bottom surface of the connection portion to the tip of the tip portion is 50 μm or more.

5. 5. An antenna lens having the pseudo-moth-eye structure according to claim 1 on at least a portion of its surface.

6. A method for producing a pseudo-moth-eye structure, comprising drying a moth-eye structure including a substrate and a plurality of moth-eye elements arranged substantially parallel to a normal line of the substrate, thereby producing the pseudo-moth-eye structure according to any one of claims 1 to 4.

7. 7. The method for producing a pseudo-moth-eye structure according to claim 6, wherein the pseudo-moth-eye structure is produced by a metal-assisted etching method.

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