Light-transmitting antenna

Abstract

To provide a light-transmitting antenna that can achieve both of high light transmission performance and high radiation efficiency.SOLUTION: A light-transmitting antenna includes a radiation element including a light-transmitting film having a structure in which a first dielectric layer, a metal layer, and a second dielectric layer are laminated in this order. In the light-transmitting film, the first dielectric layer has a thickness of 100 nm to 2000 nm, the thickness of the metal layer is 1 nm to 30 nm, the thickness of the second dielectric layer is 10 nm to 300 nm, and the ratio of the thickness of the first dielectric layer to the thickness of the second dielectric layer is 5:1 to 20:1.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to an optically transparent antenna. [Background technology]

[0002] In recent years, research and development into the Internet of Things (IoT) and fifth-generation mobile communication systems (5G) has been progressing toward the realization of smart cities. To improve communication characteristics, these communication systems use multiple antennas (array antennas) to control the direction of radio wave radiation, such as by concentrating and radiating a beam in one direction. As such antenna arrays are used, electronic devices require more antennas than before, significantly increasing the area and volume required for antenna installation. Furthermore, from the viewpoint of ensuring the radiation characteristics of the antenna, if it becomes desirable to install the antenna on the surface of the device housing rather than inside the housing, problems will arise that will impair the design and aesthetics of the device or system.

[0003] As a means for solving these problems, optically transparent antennas that use transparent conductive films (sometimes referred to as "transparent antennas" or "light-transmitting antennas" in this disclosure) are being considered. If a transparent antenna could be realized, it would be possible to place the antenna on the surface of a product without compromising the design or aesthetics of the product, thereby greatly increasing the degree of freedom in antenna installation.

[0004] To date, various materials, including indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and silver-coated polymer (AgHT), have been investigated as transparent conductive materials, and applications are being promoted in flat panel displays, smart windows, solar cells, gas sensors, electrochromic materials, and more.

[0005] However, transparent conductive films have a trade-off between their electrical conductivity and optical transparency. Antennas using transparent conductive films made of a single material such as ITO, which have a light transmittance of about 80%, have a radiation efficiency of about 30% to 50%, making it difficult to achieve high radiation efficiency. For example, applying a thin metal wire grid to a transparent conductive film can increase the electrical conductivity and improve radiation efficiency, but at the expense of significantly reducing optical transparency.

[0006] On the other hand, transparent conductive films made of multilayer composite materials have been reported. For example, it has been reported that a dielectric-metal-dielectric (DMD) structure, in which a thin metal film with high conductivity such as gold or silver is sandwiched between thin dielectric films such as ITO, can achieve both optical transparency and high conductivity. For example, Non-Patent Documents 1 to 3 report antennas that use materials having a DMD structure (DMD materials). [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Journal of ELECTRONIC MATERIALS, Vol. 42, No. 3, 2013, “Comparison of the Microwave Performance of Transparent Wire Monopole Antennas Based on Silver Films J. HAUTCOEUR et al. [Non-patent document 2] IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 16, 2017, “Transparent Microstrip Patch Antennas With Multilayer and Metal-Mesh Films” Seungman Hong et al. [Non-patent document 3] IEEE ACCESS “Silver Sandwiched ITO Based Transparent Antenna Array for RF Energy Harvesting in 5G Mid-Range of Frequencies” NERMEEN A. ELTRESY et al. Summary of the Invention [Problem to be solved by the invention]

[0008] Although optically transparent antennas using a DMD structure have been proposed as in Non-Patent Documents 1 to 3, an antenna that achieves both higher optical transparency and higher radiation efficiency is desired.

[0009] Therefore, an object of the present disclosure is to provide a light-transmitting antenna that can achieve both high light transmittance and high radiation efficiency. [Means for solving the problem]

[0010] The means for achieving the above object include the following aspects. <1> a radiating element including a light-transmitting film having a structure in which a first dielectric layer, a metal layer, and a second dielectric layer are laminated in this order; The light-transmitting film is a light-transmitting antenna, wherein the first dielectric layer has a thickness of 100 nm to 2000 nm, the metal layer has a thickness of 1 nm to 30 nm, the second dielectric layer has a thickness of 10 nm to 300 nm, and the ratio of the thickness of the first dielectric layer to the thickness of the second dielectric layer is 5:1 to 20:1. <2> The metal layer has a porous structure with an average pore size of 10 nm to 60 nm. <1> The optically transparent antenna according to claim 1. <3> The metal layer contains one or more metals selected from the group consisting of gold, silver, and copper. <1> or <2> The optically transparent antenna according to claim 1. <4> The first dielectric layer and the second dielectric layer each independently comprise one or more materials selected from the group consisting of indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and silver-coated polymer (AgHT). <1> ~ <3> 10. The optically transparent antenna according to claim 9, wherein: [Effects of the Invention]

[0011] According to the present disclosure, a light-transmitting antenna that can achieve both high light transmittance and high radiation efficiency is provided. [Brief explanation of the drawings]

[0012] [Figure 1] 10 is a photograph comparing the light transmittance of each conductive film. [Figure 2] FIG. 1 is a diagram showing the transmittance of each conductive film in the wavelength range of 400 nm to 700 nm, which includes the visible light region. [Figure 3] 1 is a FE-SEM image showing a single layer ITO film. [Figure 4] 1 is a FE-SEM image showing a two-layer ITO / Ag film. [Figure 5] FIG. 1 is a schematic diagram showing the configuration of a monopole antenna prototyped as an example. [Figure 6] FIG. 6 is a diagram showing a reflection characteristic S11 of the antenna shown in FIG. [Figure 7] FIG. 10 is a diagram showing the radiation pattern at 3.2 GHz of an antenna prototyped as an example. DETAILED DESCRIPTION OF THE INVENTION

[0013] An embodiment that is an example of the present disclosure will be described. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range may be replaced by the upper or lower limit of another numerical range described in stages, or may be replaced by a value shown in an example.

[0014] The light-transmitting antenna according to the present disclosure has a structure (DMD structure) in which a first dielectric layer, a metal layer, and a second dielectric layer are stacked in this order, and is equipped with a radiating element including a light-transmitting film (DMD film) that satisfies the following relationships (1) to (4): (1) The thickness of the first dielectric layer is 100 nm to 2000 nm. (2) The thickness of the metal layer is 1 nm to 30 nm. (3) The thickness of the second dielectric layer is 10 nm to 300 nm. (4) The ratio of the thickness of the first dielectric layer to the thickness of the second dielectric layer is 5:1 to 20:1. In the present disclosure, "light transmissive," "light-transmitting type," or "transparent" means that the transmittance is 50% or more, preferably 60% or more, and more preferably 70% or more, in the wavelength range of 400 nm to 700 nm, including the visible light region. In addition, in this disclosure, a "dielectric layer" is a layer made of a material classified as a semiconductor or an insulator. Generally, they are classified as conductors (metals), semiconductors, and insulators, and when classified by conductivity, they are as follows: conductors: 10 6 S / m or more, semiconductor: 10 6 From 10 -6 S / m, insulator: 10 S / m or less. The "dielectric layer" in this disclosure is a layer made of a semiconductor or insulator other than a conductor (metal), and is preferably transparent or light-transmitting because it constitutes part of a light-transmitting antenna (transparent antenna).

[0015] The inventors of the present disclosure have previously investigated the radiation efficiency of an antenna made of a single-layer ITO with respect to film thickness and conductivity (sheet resistance) through both electromagnetic field analysis and experiments. The inventors of the present disclosure have clarified the electrical mechanism by which radiation efficiency does not depend only on the conductivity (sheet resistance) of the material itself, but also on the fact that when the film thickness is thin, loss due to the skin effect becomes significant, resulting in a significant decrease in radiation efficiency.

[0016] The inventors of the present disclosure further conducted research into achieving both optical transparency and radiation efficiency when applying an optically transparent film with a DMD structure (sometimes referred to as a "DMD film" in the present disclosure) to an antenna, and discovered that by providing a radiating element with a DMD film that satisfies the specific relationships (1) to (4) above, it is possible to obtain an optically transparent antenna that exhibits significantly high values ​​for both optical transparency and radiation efficiency. First, each layer of the light-transmitting film (DMD film) included in the radiating element of the light-transmitting antenna according to the present disclosure will be described.

[0017] (First dielectric layer) The first dielectric layer has a thickness in the range of 100 nm to 2000 nm. From the viewpoints of light transmittance and radiation efficiency, the thickness of the first dielectric layer is preferably 150 nm to 1500 nm, more preferably 200 nm to 1000 nm, and even more preferably 250 nm to 500 nm. The first dielectric layer can be made of materials known for use in DMD films, such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), silver-coated polymer (AgHT), etc. The first dielectric layer may be made of two or more materials.

[0018] (metal layer) The metal layer is located between the first and second dielectric layers and has a thickness in the range of 1 nm to 30 nm. From the viewpoints of light transmittance and radiation efficiency, the thickness of the metal layer is preferably 2 nm to 20 nm, and more preferably 3 nm to 10 nm. As a material for forming the metal layer, a metal having high conductivity, such as gold, silver, copper, or other materials known as metal layers for DMD films, can be used. The metal layer may be porous and have holes, or may have very thin portions. If the metal layer has a porous structure, the average diameter of the holes is preferably 10 nm to 60 nm. It is presumed that the random holes in the metal layer allow part of the constituent material of the second dielectric layer formed thereon to penetrate into the holes in the metal layer, contributing to improvements in light transmittance, conductivity, and radiation efficiency.

[0019] (Second dielectric layer) The second dielectric layer has a thickness in the range of 10 nm to 300 nm. From the viewpoints of light transmittance and radiation efficiency, the thickness of the second dielectric layer is preferably 15 nm to 200 nm, and more preferably 20 nm to 50 nm. The second dielectric layer can be made of the same material as the first dielectric layer, including indium tin oxide (ITO), fluorine-doped tin oxide (FTO), silver-coated polymer (AgHT), and other materials known for use in DMD films. The second dielectric layer may also be made of two or more materials.

[0020] (Relationship between the first and second dielectric layers) The first and second dielectric layers each have a thickness within the ranges described above, but the ratio (t1:t2) of the thickness of the first dielectric layer (t1) to the thickness of the second dielectric layer (t2) must be 5:1 to 20:1. From the viewpoints of light transmittance and radiation efficiency, the ratio (t1:t2) of the thicknesses of the dielectric layers is preferably 7:1 to 15:1, and more preferably 8:1 to 12:1.

[0021] The materials constituting the first and second dielectric layers can be selected independently, i.e., the first and second dielectric layers can be composed of the same material or different materials.

[0022] The light-transmitting antenna according to the present disclosure is configured such that the radiating element includes a light-transmitting film having a structure in which a first dielectric layer, a metal layer, and a second dielectric layer that satisfy the thickness relationship described above are stacked in this order in the thickness direction. Since the light-transmitting film in the present disclosure has a thickness of less than 3 μm, it is difficult to use it independently as a radiation element. Therefore, it is preferable to form the light-transmitting film in the present disclosure on a light-transmitting support, such as a glass substrate or a plastic substrate, to form a radiation element.

[0023] The shape of the radiating element in the present disclosure is not particularly limited, and the shape of a radiating element in a known antenna can be adopted. The shape of the light-transmitting film arranged on the substrate in a plan view may be, for example, rectangular, square, circular, semicircular, triangular, trapezoidal, or a combination thereof. Alternatively, the light-transmitting film may be a mesh-like shape.

[0024] The number of radiating elements in the antenna according to the present disclosure is not limited, but for example, in the case of a light-transmitting antenna having two radiating elements, the light-transmitting films combined as each radiating element may be of the same or different shapes, and may be of the same size or different sizes (similar shapes).

[0025] In the case of a light-transmitting antenna having two radiating elements, one of the two radiating elements is a ground conductor, which is a good conductor such as copper (including gold-plated copper).

[0026] The method for forming the light-transmitting film in the radiating element of the light-transmitting antenna according to the present disclosure is not particularly limited, and examples thereof include sputtering, film deposition, vacuum deposition, CVD (chemical vapor deposition), coating, spin coating, and spraying. In particular, sputtering is preferred from the viewpoint of accurately forming each layer to a predetermined thickness. Using a sputtering device, the first dielectric layer, metal layer, and second dielectric layer may be sequentially formed on a transparent support such as a glass substrate so as to satisfy the above-mentioned conditions (1) to (4).

[0027] The optically transparent antenna according to the present disclosure exhibits high optical transparency and high radiation efficiency, and specifically can achieve both an optical transmittance of 75% or more and a radiation efficiency of 80% or more. Although the applications of the light-transmitting antenna according to the present disclosure are not limited, the radiation efficiency of the light-transmitting antenna according to the present disclosure is at the same level as small antennas built into portable devices and wearable devices, and the antenna may be installed on the surface of a product without impairing the design or aesthetics of the product. Furthermore, the light-transmitting antenna according to the present disclosure is extremely practical because it does not obstruct visibility even when installed on the windshield of an automobile, for example. [Example]

[0028] The light-transmitting antenna according to the present disclosure will be described in more detail below with reference to examples, but the light-transmitting antenna according to the present disclosure is not limited to the following examples.

[0029] Example 1 (ITO / Ag / ITO transparent conductive film deposition) A 300 nm ITO film (thickness, the same applies below) was formed on a 0.7 mm thick non-alkali glass substrate, and a 5 nm Ag film and a 30 nm ITO film were further formed on the ITO film. The ITO and Ag films were deposited using a facing target sputtering system. The film formation conditions were: substrate temperature: room temperature, sputtering gas pressure: 4 mTorr, Ar gas flow rate: 20 sccm, and Ar + 10% O2 gas flow rate: 6 sccm, controlled by a mass flow controller. The sputtering current was constant at 500 mA, and the deposition rates of ITO and Ag were 80 nm / min and 60 nm / min, respectively, to produce a transparent conductive film with a DMD (ITO 300 nm / Ag 5 nm / ITO 30 nm) structure. The light transmittance and conductivity of the conductive film at each film formation stage were measured.

[0030] (light transmittance) Figure 1 shows photographs comparing the optical transmittance of conductive films at each stage of film formation: (a) ITO 300 nm, (b) ITO 300 nm / Ag 5 nm, and (c) DMD film (ITO 300 nm / Ag 5 nm / ITO 30 nm). Figure 2 shows the transmittance of the conductive film shown in Figure 1 in the wavelength range of 400 nm to 700 nm, which includes the visible light region. As can be seen from Figures 1 and 2, a single-layer 300 nm ITO film achieves an average transmittance of 82.3%. When a two-layer ITO / Ag film is formed by depositing 5 nm of Ag on top of this single-layer ITO film, not only is there shielding from the Ag particles, but reflection due to the metallic luster becomes significant, and the average transmittance drops to 59.7%.

[0031] On the other hand, in a three-layer DMD (ITO / Ag / ITO) film, in which an ITO film is deposited on top of an ITO / Ag film, the ITO film functions as an anti-reflection layer, significantly improving the average transmittance, with maximum and average transmittances of 76.7% and 74.0%, respectively. It is noteworthy that the transmittance of the DMD film from 435 nm to 450 nm is equivalent to that of a single-layer ITO film, and in particular, the transmittance of the DMD film from 443 nm to 445 nm is 74.7%, exceeding that of a single-layer ITO film. This suggests that the transmittance of the DMD film is determined not by just one or two layers of the DMD film, but by the interaction of the three layers.

[0032] (conductivity) Table 1 summarizes the sheet resistance and conductivity of each film shown in Figure 1. The sheet resistance was measured by the four-terminal method.

[0033] [Table 1]

[0034] As can be seen from Table 1, the resistance of a single layer of ITO film is 5.2 Ω / sq (6.4 × 10 5 The sheet resistance of the ITO / Ag film was 4.7 Ω (7.0 × 10 5S / m), and the DMD film reduces it to 3.4 Ω / sq (8.8 × 10 5 S / m). When silver nanoparticles sandwiched between ITO films are randomly dispersed and arranged like islands, interactions occur between adjacent particles, resulting in plasmon resonance, which is typified by phase compensation and optical resonance. The high transmittance and high conductivity of the ITO / Ag / ITO film are thought to be due to this plasmon resonance.

[0035] (SEM observation) Figure 3 shows a field emission scanning electron microscope (FE-SEM) image of a single-layer ITO film, and Figure 4 shows a two-layer ITO / Ag film. In Figure 3, the film is characterized by separate domains, which is characteristic of ITO. In contrast, in Figure 4, Ag particles are deposited thinly on the ITO film, leaving numerous randomly spaced holes. This distribution, which allows Ag and ITO to easily interact with each other, is thought to induce plasmon resonance, resulting in good optical and electrical properties.

[0036] (antenna performance) We prototyped a monopole antenna, which is the basic structure of an antenna, and evaluated its performance as an antenna. Figure 5 shows the configuration of the prototype monopole antenna. Monopole antennas and dipole antennas are well-known basic antennas and are suitable for evaluating antenna performance. In particular, with a monopole antenna, the Wheeler method can be used, making it easy to measure the radiation efficiency. Here, the radiating element length l = 20 mm, width w = 5 mm, ground conductor width d = 80 mm, and feed point spacing g = 0.5 mm. The thickness of the transparent conductive film is the same as that shown in Figure 1, 300 nm for the single-layer ITO and 335 nm for the three-layer DMD (ITO / Ag / ITO).

[0037] This antenna resonates at approximately 3 GHz, which is roughly halfway between frequency bands such as wireless LAN (2.45 GHz, 5 GHz), 4G (2 GHz, 3.4 GHz), and 5G (3.6 GHz to 4.2 GHz). We adopted an antenna that resonates near 3 GHz because of its applicability to these wireless technologies and the ease of antenna prototyping.

[0038] Figure 6 shows the reflection characteristics S of the prototype antenna shown in Figure 5. 11 The reflection characteristic of the antenna is shown as S 11 is one of the indicators for evaluating antenna performance, and is generally 11 <-10 dB is considered to be the standard for good antenna performance. For comparison, the results of electromagnetic field analysis (simulated) based on the measured sheet resistance are also shown. As can be seen from FIG. 6, the antenna with the DMD film according to the present disclosure resonates at 3.2 GHz, and the S 11 =-30.9dB is obtained.

[0039] Table 2 shows the results of measuring the radiation efficiency of the prototype antenna using the Wheeler method.

[0040] [Table 2]

[0041] As can be seen from Table 2, the radiation efficiencies of the single-layer ITO, double-layer ITO / Ag film, and triple-layer DMD film were 68.0%, 75.5%, and 81.4%, respectively. Using an antenna with a DMD film within the scope of this disclosure instead of a single-layer ITO film significantly improved radiation efficiency. This 80% radiation efficiency is equal to or better than the radiation efficiency of small antennas built into portable and wearable devices, and can be said to have achieved good radiation efficiency.

[0042] Furthermore, Figure 7 shows the radiation pattern of the prototype antenna at 3.2 GHz. For comparison, Figure 7 also shows the radiation pattern obtained from electromagnetic field analysis. In Figure 7, the square marks represent experimental (measured) data, and the solid lines represent the results of electromagnetic field analysis. As can be seen from Figure 7, (A) the xy plane is omnidirectional in the horizontal plane, and (B) the zx plane and (C) the zy plane are shaped like an eight, a radiation pattern typical of a monopole antenna, and it can be said that good radiation characteristics have been obtained.

[0043] There are some discrepancies between the analytical values ​​(Simulated) and the measured values ​​(Measured) in Figures 6 and 7 and Table 2. This is thought to be because connectors and cables were connected for measurement purposes in the experiment, and these connectors and cables were not modeled in the electromagnetic field analysis.

[0044] An antenna using a DMD film of ITO 300 nm / Ag 5 nm / ITO 30 nm, which is an example of a DMD film in this disclosure, has an optical transmittance of 76.7% and a radiation efficiency of 81.4%, achieving both high transparency and radiation efficiency.

Claims

1. a radiating element including a light-transmitting film having a structure in which a first dielectric layer, a metal layer, and a second dielectric layer are laminated in this order; a light-transmitting antenna, wherein the light-transmitting film has a first dielectric layer having a thickness of 300 nm, a metal layer having a thickness of 5 nm, and a second dielectric layer having a thickness of 30 nm, and wherein the light-transmitting film has a transmittance of 70% or more in the range from 443 nm to 445 nm, and a radiation efficiency of 80% or more as measured by the Wheeler method.

2. 2. The light-transmitting antenna according to claim 1, wherein the metal layer has a porous structure with an average pore size of 10 nm to 60 nm.

3. 3. The optically transparent antenna according to claim 1, wherein the metal layer contains one or more metals selected from the group consisting of gold, silver, and copper.

4. The optically transparent antenna according to any one of claims 1 to 3, wherein the first dielectric layer and the second dielectric layer each independently contain one or more materials selected from the group consisting of indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and silver-coated polymer (AgHT).

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