Band-pass filter sheet

Abstract

The present invention achieves a band-pass filter sheet capable of exhibiting excellent frequency-selective characteristics with respect to electromagnetic waves in a high-frequency band ranging from several hundred GHz to the terahertz band. Provided is a band-pass filter sheet in which a first dielectric layer, a first metal layer, a second dielectric layer, a second metal layer, and a third dielectric layer are sequentially laminated from the incident surface side of electromagnetic waves, wherein the first metal layer and the second metal layer are both formed in a thin film shape, one of the first metal layer and the second metal layer has a mesh structure having a mesh shape, the other has a rectangular pattern structure in which a plurality of rectangular patterns are arranged in a matrix form, the arrangement pitch of the mesh structure and the rectangular pattern structure is identical, and the rectangular patterns are arranged in the opening portions of the mesh shape when the band-pass filter sheet is viewed from the thickness direction.

Description

Bandpass Filter Sheet

[0001] The present disclosure relates to a bandpass filter sheet that selectively transmits electromagnetic waves of a predetermined frequency, and in particular to a bandpass filter sheet that selectively transmits electromagnetic waves of high frequencies of 1 GHz or higher.

[0002] In recent years, the use of high-frequency electromagnetic waves, such as centimeter waves with a frequency band of several gigahertz (GHz) and millimeter waves with a frequency band of 30 to 300 gigahertz, has been increasing in mobile communications such as mobile phones, wireless LANs, and electronic toll collection systems (ETC). Furthermore, with the further evolution of the communications environment and the rise of IoT, there is a demand for the development of devices for communications beyond 5G and 6G, and research into technologies that utilize radio waves with frequencies in the terahertz (THz, 1000 GHz and above) band is also progressing.

[0003] Thus, as the frequency of electromagnetic waves used increases, specific materials that are compatible with high-frequency electromagnetic waves are required for transmitting and receiving systems such as antenna circuits, testing equipment devices, and also for noise countermeasures for these devices, prevention of electromagnetic wave leakage, etc. In particular, in devices that handle electromagnetic waves in the terahertz band, location dependency cannot be ignored for all materials used, such as conductors within the circuit, circuit boards, and exterior materials, and strict adjustment and design of length, size, and thickness is required.

[0004] Conventionally, a cover for housing a millimeter-wave radar device that transmits electromagnetic waves of a predetermined frequency in the GHz band and blocks electromagnetic waves of other frequencies has been proposed, which is made by laminating a first component made of a mesh formed from a metal or conductive polymer and a second component made of a dielectric material (Patent Document 1).

[0005] International Publication No. WO2018 / 047937

[0006] For example, in the case of a cover (radome) that houses a radar device such as that described in Patent Document 1, the material must be transparent to electromagnetic waves in the frequency band used for the radar, must be resistant to the external environment, must be rigid enough to maintain an appropriate distance from the antenna circuit, and must have an appropriate thickness to match the wavelength of the electromagnetic waves used for the radar.

[0007] However, because the dielectric constant changes depending on the frequency of the electromagnetic waves that are transmitted, even if the material and configuration are appropriate for millimeter-wave radars that use frequencies of several tens of GHz, it has been difficult to achieve the electromagnetic wave transparency required for a radome for radars that use electromagnetic waves of several hundred GHz or higher, particularly those in the terahertz band, i.e., electromagnetic waves of a specified frequency are less likely to be reflected (low reflectivity) and more electromagnetic waves are able to transmit (high transmittance).

[0008] The present disclosure is intended to solve the above-mentioned problems, and aims to realize a bandpass filter sheet that can exhibit good frequency selection characteristics for electromagnetic waves in the high frequency band from several hundred GHz to the terahertz band.

[0009] In order to solve the above problems, the bandpass filter sheet disclosed in the present application is a bandpass filter sheet in which a first dielectric layer, a first metal layer, a second dielectric layer, a second metal layer, and a third dielectric layer are laminated in this order from the electromagnetic wave incident side, wherein the first metal layer and the second metal layer are both formed as thin films, one of the first metal layer and the second metal layer has a mesh structure in a net shape, and the other has a rectangular pattern structure in which a plurality of rectangular patterns are arranged in a matrix, the mesh structure and the rectangular pattern structure have the same arrangement pitch, and when the bandpass filter sheet is viewed from the thickness direction, the rectangular patterns are arranged within openings in the mesh shape.

[0010] The bandpass filter sheet disclosed in the present application has a structure in which a first dielectric layer, a first metal layer, a second dielectric layer, a second metal layer, and a third dielectric layer are sequentially laminated, and one of the first metal layer and the second metal layer, each of which is formed as a thin film, has a mesh structure and the other has a rectangular pattern structure, the mesh structure being formed at the same pitch and the rectangular pattern being arranged within the openings of the mesh structure when viewed from the thickness direction of the bandpass filter sheet. As a result, a high frequency selectivity can be achieved, which selectively transmits electromagnetic waves of a predetermined frequency band in a high frequency band of several hundred GHz or more.

[0011] FIG. 1 is a cross-sectional view illustrating the configuration of a bandpass filter sheet according to the present embodiment; FIG. 2 is a diagram illustrating the configuration of a first metal layer and a second metal layer of a first bandpass filter sheet according to the present embodiment; FIG. 3 is a diagram illustrating the frequency characteristics of the transmission attenuation of the first bandpass filter sheet according to the present embodiment; FIG. 4 is a diagram illustrating the configuration of a first metal layer and a second metal layer of a second bandpass filter sheet according to the present embodiment; FIG. 5 is a diagram illustrating the frequency characteristics of the transmission attenuation of the second bandpass filter sheet according to the present embodiment; FIG. 6 is a diagram illustrating the configuration of a metal layer of a third bandpass filter sheet as a comparative example; and FIG. 7 is a diagram illustrating the frequency characteristics of the transmission attenuation of the third bandpass filter sheet according to the present embodiment.

[0012] The bandpass filter sheet disclosed in the present application has a first dielectric layer, a first metal layer, a second dielectric layer, a second metal layer, and a third dielectric layer stacked in this order from the electromagnetic wave incident side, the first metal layer and the second metal layer both being formed as thin films, one of the first metal layer and the second metal layer having a mesh structure, and the other having a rectangular pattern structure in which a plurality of rectangular patterns are arranged in a matrix, the mesh structure and the rectangular pattern structure having the same arrangement pitch, and when the bandpass filter sheet is viewed from the thickness direction, the rectangular patterns are arranged within the openings of the mesh shape.

[0013] With this configuration, the bandpass filter sheet disclosed in the present application can constitute a bandpass filter having frequency selection characteristics that selectively transmit electromagnetic waves of a predetermined frequency in a high frequency band of several hundred GHz or more.

[0014] In the bandpass filter, the metal layer formed as a thin film preferably has a thickness of 5 μm or less, which allows the thickness of the metal layer to be kept within a range that is almost negligible relative to the overall thickness of the bandpass filter sheet, thereby achieving high frequency selectivity.

[0015] Furthermore, it is preferable that the surface resistance of the first metal layer and the second metal layer is 60 Ω / □ or less. By sufficiently reducing the resistance of the metal layer, the attenuation of the electromagnetic wave passing through the metal layer is reduced, thereby achieving high transmission characteristics for electromagnetic waves of a desired frequency.

[0016] Furthermore, it is preferable that the pitch of the mesh structure and the rectangular pattern structure is 0.5 mm or more and 1.5 mm or less, thereby achieving frequency selection characteristics for electromagnetic waves with high frequencies of several hundred GHz or more.

[0017] It is also preferable that the line width of the mesh structure is 15 μm or more and 100 μm or less.

[0018] Furthermore, it is preferable that the first metal layer is formed on one surface of the second dielectric layer, and the second metal layer is formed on the other surface of the second dielectric layer, so that the first metal layer, the second dielectric layer, and the second metal layer can be formed at the same time, and a laminated structure of the bandpass filter sheet including the first dielectric layer and the third dielectric layer can be easily realized.

[0019] It is also preferable that the first dielectric layer, the second dielectric layer, and the third dielectric layer contain at least one of an acrylic resin, a silicone resin, and a urethane resin.

[0020] Furthermore, at least one of the first dielectric layer and the second dielectric layer may be configured to be made up of two or more dielectric layers.

[0021] Hereinafter, the bandpass filter sheet disclosed in the present application will be described with reference to the drawings.

[0022] (Embodiment) FIG. 1 is a cross-sectional view showing the configuration of a bandpass filter sheet according to this embodiment.

[0023] Note that Figure 1 is a diagram provided to facilitate understanding of the configuration of the bandpass filter sheet according to this embodiment, and the sizes of the components shown in the figure, particularly the thickness of each layer, are not necessarily based on reality.

[0024] [Overall structure of bandpass filter sheet] The bandpass filter sheet 100 illustrated in this embodiment is configured by sequentially stacking a first dielectric layer 10, a first metal layer 20, a second dielectric layer 30, a second metal layer, and a third dielectric layer 50 from the incident side of the electromagnetic wave 1.

[0025] In the bandpass filter sheet 100 shown in this embodiment, the first metal layer 20 and the second metal layer 40 are formed as thin films, and one of the first metal layer 20 and the second metal layer 40 has a mesh structure in a mesh shape (lattice shape), while the other has a rectangular pattern structure in which a plurality of rectangular patterns are arranged vertically and horizontally in a matrix shape. The arrangement pitch of the matrix structure and the arrangement pitch of the rectangular pattern structure are the same, and further, when the bandpass filter sheet is viewed from the thickness direction, the rectangular patterns are arranged within the openings of the mesh shape.

[0026] [Details of Each Member] Next, each member constituting the bandpass filter sheet 100 according to this embodiment will be described.

[0027] <Dielectric Layer> The first dielectric layer 10, the second dielectric layer 30, and the third dielectric layer 50 of the bandpass filter sheet 100 according to this embodiment can all be formed from various dielectric materials such as acrylic resin, silicone resin, urethane resin, etc. In addition to these, other dielectric materials such as polyester resins such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polybutylene terephthalate (PBT), silicone rubber, titanium oxide, polyvinylidene fluoride, polyester resin, glass, and silicone rubber can also be used.

[0028] The first dielectric layer 10, the second dielectric layer 30, and the third dielectric layer 50 can each be formed as a single layer made of a single material. Alternatively, they can be formed by stacking two or more layers of the same or different materials. The first dielectric layer 10, the second dielectric layer 30, and the third dielectric layer 50 can all be made of the same dielectric material, or they can be made of different dielectric materials, including the number of layers.

[0029] The thicknesses of the dielectric layers 10, 30, and 50 of the bandpass filter sheet 100 according to this embodiment are not particularly limited and can be appropriately selected within a range that achieves the required strength for the bandpass filter sheet 100 as a whole and the required flexibility and translucency for the bandpass filter sheet 100 as a whole. Furthermore, the thicknesses of the first dielectric layer 10, the second dielectric layer 30, and the third dielectric layer 50 may be the same in part or all, or may be different from one another. The thickness of the second dielectric layer 30 is preferably 10 μm or more and 100 μm or less in order to ensure a sufficient spacing between the first metal layer 20 and the second metal layer 40. If the thickness of the second dielectric layer 30 is less than 10 μm, it becomes difficult to obtain a clear difference between the non-transmission region and the transmission region. If the thickness of the second dielectric layer 30 is greater than 100 μm, the transmission loss that occurs during transmission through the second dielectric layer 30 becomes large, which is undesirable. Similarly, the first dielectric layer 10 and the third dielectric layer 50 may be thin, but are preferably 5 μm or more in order to protect the metal layers (20, 40). If they are too thick, the loss during transmission will be large, so they are preferably 100 μm or less, more preferably 50 μm or less.

[0030] In the bandpass filter sheet 100 of this embodiment shown in Fig. 1, a first metal layer 20 is formed by printing on one surface of the second dielectric layer 30 (the upper surface in Fig. 1 on the side where the electromagnetic wave 1 is incident), and a second metal layer 40 is formed by printing on the other surface of the second dielectric layer 30 (the lower surface in Fig. 1 on the side opposite to the side where the electromagnetic wave 1 is incident), so that the second dielectric layer 30 serves as a resin substrate for forming the two metal layers 20, 40. When the second dielectric layer 30 is configured as a resin substrate for forming the two metal layers 20, 40 in this way, it is preferable to use PET (polyethylene terephthalate) because of its stability as a resin material and its ease of availability.

[0031] 1 , the first dielectric layer 10 and the third dielectric layer 50 are both configured as a laminate of PET and a translucent, adhesive acrylic-based OCA (Optical Clear Adhesive). This allows the adhesive properties of the OCA to bond the PET layers (first dielectric layer 10 and third dielectric layer 50) to the laminate of the second dielectric layer 30 and the first metal layer 20 and second metal layer 40 formed on both sides of the second dielectric layer 30. This improves workability and reduces the amount of material required when fabricating the bandpass filter sheet 100, which is a laminate of the first dielectric layer 10, the first metal layer 20, the second dielectric layer 30, the second metal layer 40, and the third dielectric layer 50. This allows the bandpass filter sheet 100 to be easily manufactured at low cost.

[0032] Furthermore, by making all of the members constituting the bandpass filter sheet 100 translucent, it is possible to achieve a predetermined translucency (for example, a total light transmittance of 50% or more) for the entire bandpass filter sheet 100. By making the entire bandpass filter sheet translucent, it is possible to obtain a bandpass filter sheet that can be used in applications where design is required or where it is effective to be able to see into the inside of an antenna tester, for example.

[0033] When a resin material having adhesive properties such as OCA is used for the first dielectric layer 10 and the third dielectric layer 50, the adhesive strength to the metal layers (20, 40) is preferably 3 N / 10 mm or more. Of course, the laminated structure of the first dielectric layer 10, the metal layer 20, the second dielectric layer 30, the second metal layer 40, and the third dielectric layer 50 can also be formed using an adhesive material such as a double-sided adhesive sheet or adhesive.

[0034] <Metal Layer> The first metal layer 20 and the second metal layer 40 of the bandpass filter sheet 100 shown in this embodiment are thin film layers respectively disposed between the first dielectric layer 10 and the second dielectric layer 30, and between the second dielectric layer 30 and the third dielectric layer 50. One of the two metal layers 20, 40 has a mesh structure having a mesh shape (lattice-like), while the other has a rectangular pattern structure in which a plurality of rectangular patterns are arranged in a matrix in the vertical and horizontal directions. Either the mesh structure or the rectangular pattern structure may be the first metal layer 20, i.e., the layer located on the side of the incident surface of the electromagnetic wave 1.

[0035] The pitch of the mesh structure, which is one metal layer, i.e., the distance between the center lines of the lines forming the grid, and the pitch of the rectangular pattern structure, which is the other metal layer, i.e., the distance between the centers of adjacent rectangular patterns, are formed to be the same. Note that the amount of pitch deviation due to manufacturing errors, etc., is within the allowable range of about ±5%.

[0036] Furthermore, the pitch error of the mesh structure in the first metal layer 20 and the second metal layer 40 and the pitch error of the rectangular pattern structure are also similarly within the allowable range of about ±5%.

[0037] The pitch of the mesh structure and the rectangular pattern structure, i.e., the repetition interval, is determined according to the frequency of the electromagnetic wave 1 to be absorbed by the bandpass filter sheet 100. The inventors' studies have confirmed that it is preferable to form the arrangement interval between the mesh shapes of the mesh structure and the rectangular patterns of the rectangular pattern structure at a pitch approximately the same as the wavelength of the electromagnetic wave 1 to be absorbed by the bandpass filter sheet. In the bandpass filter sheet 100 according to this embodiment shown in Figure 1, the frequency of the electromagnetic wave to be absorbed is 300 GHz (wavelength λ ≈ 1 mm), so the pitch of the mesh structure and the rectangular pattern structure is approximately 1 mm, and is preferably 0.5 mm or more and 1.5 mm or less.

[0038] Furthermore, it is desirable that the line width of the mesh shape in the mesh structure be as narrow as possible in order to keep the amount of transmission attenuation of electromagnetic waves generated in the mesh part as low as possible, and it is preferable that the width be 15 μm or more and 100 μm or less so that the mesh shape can be formed correctly.

[0039] Furthermore, when the bandpass filter sheet 100 is viewed in its thickness direction, i.e., from the direction of the arrow in FIG. 1 along which the electromagnetic wave 1 is incident, each rectangular pattern constituting the rectangular pattern structure is located within a mesh-shaped opening in the mesh structure. In this case, a deviation of the center positions of the mesh structure and the rectangular pattern structure is permissible up to about 50% of the pitch. However, as described above, it is necessary to avoid the rectangular pattern of the rectangular pattern structure being located within the mesh-shaped opening of the mesh structure, i.e., to avoid overlapping between the mesh structure and the rectangular pattern structure. Furthermore, the rectangular pattern in the rectangular pattern structure may be a rectangular closed-loop pattern with a central opening, or a solid pattern with no central opening and the entire rectangle made of a metal layer.

[0040] The surface resistance of both the first metal layer 20 and the second metal layer 40 is preferably 60 Ω / □ or less. If the surface resistance is higher than 60 Ω / □, it becomes difficult to obtain high selective transmission characteristics for electromagnetic waves of the desired frequency. The lower limit of the surface resistance is preferably low from the viewpoint of electromagnetic wave transmission characteristics, and is ideally 0 Ω / □. However, taking into consideration the manufacturing process for realizing the pattern shape of the thin film, it is thought to be about 0.01 Ω / □.

[0041] In the bandpass filter sheet 100 shown as this embodiment, the center frequency of the electromagnetic waves to be selectively transmitted is set to 300 GHz, and in this case the surface resistance values ​​of the first metal layer 20 and the second metal layer 40 are 0.1 Ω / □ or more and 60 Ω / □ or less.

[0042] Furthermore, transmission loss can be reduced by making the thickness of the first metal layer 20 and the second metal layer 40 thinner than the thickness of the laminated first dielectric layer 10, second dielectric layer 30, and third dielectric layer 50. For this reason, the thickness of the first metal layer 20 and the second metal layer 40 is preferably 5 μm or less, and more preferably 3 μm or less.

[0043] The first metal layer 20 and the second metal layer 40 can both be formed using a metal plating film such as an electroless plating film or an electrolytic plating film, or a metal paste.

[0044] More specifically, methods for forming the metal layers (20, 40) include a method of printing a metal paste onto a substrate made of a resin such as polyethylene terephthalate (PET) using various printing methods such as an inkjet method or a screen printing method, a method of applying the above-mentioned metal material to the entire surface of a substrate sheet, and then masking the area where the metal layer is to be formed and removing the conductive material from the area not covered by the mask using an acid or the like, and a method of irradiating laser light onto areas other than the metal layer to remove the conductive material.

[0045] In addition, a wide variety of conventional pattern formation methods can be adopted that are suited to the characteristics of the metal material being used, such as a photolithography method using a photocurable resin and a mask pattern, or a method in which a deactivator that inhibits the function of an electroless plating catalyst is applied to the entire surface of the substrate, and then the deactivator is removed from the area where the metal layer is to be formed using light or heat, and then the electroless plating catalyst solution and the electroless plating solution are brought into contact sequentially to form an electroless plating film on the metal layer area and then an electrolytic plating film on top of that.

[0046] In addition, as the substrate used when forming the metal layer, in addition to the PET exemplified above, various resins, rubber materials, and even various dielectrics (insulators) such as paper and wood can be used.

[0047] In addition, it is preferable to form the first metal layer 20 and the second metal layer 40 using a printing method such as inkjet printing or screen printing in order to achieve the desired shape, i.e., the desired pitch and line width, and further to adjust the thickness of the metal layers 20 and 40 to achieve the desired surface resistance value.

[0048] A bandpass filter sheet with high frequency selectivity can be obtained by keeping the error in each of the sizes of the rectangular patterns arranged in a matrix, that is, the length of one side of the rectangle and the arrangement interval (pitch), within about ±5% of the arrangement pitch of the rectangular patterns, formed as a rectangular pattern structure in the first metal layer 20 or the second metal layer 40. Similarly, it is preferable that the error in the line width of the mesh portion of the mesh structure formed as the first metal layer 20 or the second metal layer 40, or the line width when a closed loop structure with an opening in the center is used as the rectangular pattern, is also within ±5% of the pitch in the mesh structure or rectangular pattern structure.

[0049] Furthermore, if there is a difference in size between the vertical and horizontal directions of the matrix structure or rectangular pattern shape, anisotropy may occur with respect to the electromagnetic wave 1 incident on the bandpass filter sheet 100. For this reason, it is preferable that the first metal layer 20 and the second metal layer 40 have the same size in the vertical and horizontal directions, and it is also preferable that the difference in size between the vertical and horizontal directions is no more than about ±5% of the pitch of the mesh structure or rectangular pattern structure.

[0050] EXAMPLES Hereinafter, the results of a study conducted on the frequency characteristics of electromagnetic waves of the bandpass filter sheet according to this embodiment will be described.

[0051] In the following studies, simulations were performed using full-wave three-dimensional electromagnetic field software "Ansys HFSS (product name: manufactured by ANSYS, Inc.)" using the finite element method.

[0052] FIG. 2 is a schematic diagram showing the configuration of the metal layer of the first bandpass filter sheet used in examining the frequency characteristics.

[0053] The metal layer of the first bandpass filter sheet shown in Figure 2 has a rectangular pattern structure 21 with a closed loop structure in which the first metal layer 20 on the incident surface side of the electromagnetic wave 1 has a rectangular pattern with multiple central openings, and closed loop shapes are arranged in a matrix in the vertical and horizontal directions, and the second metal layer 40 located on the back side relative to the incident surface of the electromagnetic wave 1 is formed as a mesh structure 41.

[0054] In addition, since Figure 2 shows the bandpass filter sheet 100 as viewed from the first dielectric layer 10 side, which is the incident side of the electromagnetic wave 1, the mesh structure 41 located on the back side of the second dielectric layer 30 is shown by dashed lines.

[0055] The sizes of the metal patterns 21, 41 of the first metal layer 20 and the second metal layer 40 were set as follows, assuming that the frequency of the electromagnetic waves to be transmitted by the bandpass filter sheet is 280 GHz, in order to obtain conditions that allow for good transmission of electromagnetic waves with a wider bandwidth.

[0056] As shown by the respective symbols in Figure 2, the pitch a of the mesh structure 41 is 0.60 mm, the inner spacing b of the mesh shape of the mesh structure 41 is 0.56 mm, the line width c of the mesh structure 41 is 0.04 mm, the outer length d of the rectangular closed loop that constitutes the rectangular pattern structure 21 is 0.42 mm, the inner width (opening size) e of the closed loop is 0.34 mm, and the line width f of the closed loop is 0.04 mm.

[0057] As shown in Figure 2, the centers of the rectangular closed loops constituting the rectangular pattern structure 21 are formed so as to be located at the centers of the mesh-shaped openings of the mesh structure 41, and the pitch of the rectangular pattern structure 21, i.e., the arrangement pitch of the rectangular closed loops, is 0.60 mm, the same as the pitch a of the mesh structure 41.

[0058] The first dielectric layer 10 and the third dielectric layer 50 are both 85 μm thick laminate films made of a 75 μm thick PET sheet and a 10 μm thick acrylic OCA sheet placed on the metal layer 20, 40 side. The second dielectric layer 30 is made of a 100 μm thick PET sheet. Calculations were performed assuming that the dielectric constant ε of the acrylic OCA is approximately 2.5 (center λ = 1 mm) and the dielectric constant ε of the PET is 3.0 (center λ = 1 mm).

[0059] FIG. 3 shows the frequency characteristics of the first bandpass filter sheet including the first metal layer and the second metal layer shown in FIG.

[0060] FIG. 3 shows frequency characteristics of transmission attenuation, which indicates the attenuation, in dB, of the energy of electromagnetic waves incident on the bandpass filter sheet that is transmitted through the rear surface of the bandpass filter sheet.

[0061] As shown in Figure 3, the frequency characteristics of the transmission attenuation of the first bandpass filter sheet show that the transmission attenuation over an extremely wide frequency band from 185 GHz to 420 GHz, centered around 280 GHz, which was set as the frequency to be transmitted, is suppressed to -5 dB or less, forming a good transmission range, and in the frequency bands on both sides of this, blocking ranges are formed in which electromagnetic waves with a transmission attenuation of -8 dB or more are absorbed and transmission is suppressed, and it was confirmed that a bandpass filter sheet that selectively transmits electromagnetic waves of a predetermined frequency can be realized.

[0062] FIG. 4 is a schematic diagram showing the configuration of the metal layer of the second bandpass filter sheet used in examining the frequency characteristics.

[0063] The metal layer of the second bandpass filter sheet shown in Figure 4 has a rectangular pattern structure 21 in which the first metal layer 20 on the incident surface side of the electromagnetic wave 1 has solid rectangular patterns with no opening in the center arranged in a matrix in the vertical and horizontal directions, and the second metal layer 40 located on the back side relative to the incident surface of the electromagnetic wave 1 is formed as a mesh structure 41, similar to the first bandpass filter sheet.

[0064] Like Figure 2, Figure 4 also shows the bandpass filter sheet 100 as viewed from the first dielectric layer 10 side, which is the incident side of the electromagnetic wave 1, and therefore the mesh structure 41 located on the back side of the second dielectric layer 30 is shown by dashed lines.

[0065] The sizes of the metal patterns of the first metal layer 20 and the second metal layer 40 were set as follows, assuming that the frequency of the electromagnetic waves to be transmitted by the bandpass filter sheet is 280 GHz, the same as the first bandpass filter sheet, in order to obtain conditions that allow for good transmission of electromagnetic waves over a wider bandwidth.

[0066] As shown by the symbols in Figure 4, the pitch a of the mesh structure 41 was 0.50 mm, the inner spacing b of the mesh shape of the mesh structure 41 was 0.46 mm, the line width c of the mesh structure 41 was 0.04 mm, and the length d of one side of the rectangular fill pattern that constitutes the rectangular pattern structure 21 was 0.12 mm.

[0067] As shown in Figure 4, the center of the rectangular fill pattern that constitutes the rectangular pattern structure 21 is formed so as to be located at the center of the mesh-shaped opening portion of the mesh structure 41, and the arrangement pitch of the filled rectangular patterns of the rectangular pattern structure 21 is 0.50 mm, the same as the pitch a of the mesh structure 41.

[0068] The first dielectric layer 10 and the third dielectric layer 50 were both made of PET sheets with a thickness of 23 μm, and the second dielectric layer 30 was made of a PET sheet with a thickness of 50 μm. The calculation was performed assuming that the dielectric constant ε of PET was 3.0 (center λ=1 mm).

[0069] FIG. 5 shows the frequency characteristics of a second bandpass filter sheet including the first metal layer and the second metal layer shown in FIG.

[0070] Similar to FIG. 3, FIG. 5 shows the frequency characteristics of the transmission attenuation, which indicates the attenuation, in dB, of the energy of electromagnetic waves incident on the bandpass filter sheet that is transmitted through the back surface of the bandpass filter sheet.

[0071] As shown in Figure 5, the frequency characteristics of the transmission attenuation of the second bandpass filter sheet show that the transmission attenuation is suppressed to -5 dB or less over an extremely wide frequency band from 100 GHz to 485 GHz, centered around 280 GHz, which is the set frequency to be transmitted, forming a good transmission range. Furthermore, in the frequency bands on either side of this, below 100 GHz and above 480 GHz, there are blocking regions where electromagnetic waves with a transmission attenuation exceeding -5 dB are absorbed and transmission is suppressed. Thus, it was confirmed that the second bandpass filter sheet also selectively transmits electromagnetic waves of a predetermined frequency.

[0072] FIG. 6 is a schematic diagram showing the configuration of the metal layer of a third bandpass filter sheet as a comparative example shown in the above-described embodiment.

[0073] The metal layer of the third bandpass filter sheet shown in Figure 6 does not have a first metal layer 20 on the incident surface side of the electromagnetic wave 1, and has a mesh structure 41 formed as a second metal layer 40 located on the back side relative to the incident surface of the electromagnetic wave 1.

[0074] 6, the mesh structure 41 located on the back surface side of the second dielectric layer 30 is indicated by a broken line, similar to FIGS.

[0075] The size of the mesh structure 41 of the second metal layer 40 was set as follows, in order to obtain conditions for allowing good transmission of electromagnetic waves over a wider bandwidth, assuming a frequency of 280 GHz as the frequency of the electromagnetic waves to be transmitted, similar to the first bandpass filter sheet and the second bandpass filter sheet.

[0076] As indicated by the symbols in FIG. 6, the mesh pitch a of the mesh structure 41 was 0.42 mm, the inner spacing b of the mesh structure 41 was 0.38 mm, and the line width c of the mesh structure 41 was 0.04 mm.

[0077] The first dielectric layer 10 was a 33 μm-thick laminate consisting of a 23 μm-thick PET sheet and a 10 μm-thick acrylic OCA sheet, with a 110 μm-thick laminate consisting of a 100 μm-thick PET sheet and a 10 μm-thick acrylic OCA sheet stacked directly on top of this as the second dielectric layer 30. A mesh-structured second metal layer 40 was sandwiched between the layers, and the third dielectric layer 50 was a 133 μm-thick laminate consisting of a 100 μm-thick PET sheet, a 10 μm-thick acrylic OCA sheet, and a 23 μm-thick PET sheet stacked together. The dielectric constant ε of the acrylic OCA was approximately 2.5 (center λ = 1 mm), and the dielectric constant ε of the PET was 3.0 (center λ = 1 mm).

[0078] FIG. 7 shows the frequency characteristics of the third bandpass filter sheet shown in FIG. 6 as a comparative example.

[0079] Similar to Figures 3 and 5, Figure 7 shows the frequency characteristics of the transmission attenuation, which indicates the attenuation, in dB, of the energy of electromagnetic waves incident on the bandpass filter sheet that is transmitted through the back surface of the bandpass filter sheet.

[0080] As shown in Fig. 7, the frequency characteristics of the transmission attenuation of the third bandpass filter sheet show that the transmission attenuation is -5 dB or less in the frequency range above 100 GHz. On the other hand, on the high frequency side, in the range of 480 GHz or more shown in Fig. 6, no stop band where the transmission attenuation of electromagnetic waves exceeds -5 dB is formed, and it was not possible to form a bandpass filter sheet that selectively transmits predetermined frequencies.

[0081] As such, it can be seen that the third bandpass filter sheet, which does not have the first metal layer, does not have sufficient frequency characteristics as a bandpass filter, in that the transmission attenuation does not increase in frequency bands higher than the specified frequency band, and blocking zones through which electromagnetic waves are difficult to transmit are formed on both sides of the transmission zone of the specified frequency band.

[0082] From these results, it is considered important that the bandpass filter sheet disclosed in the present application has two metal layers, a first metal layer and a second metal layer, in order to ensure that the frequency bands above and below the frequency band (transmission band) to be transmitted are good blocking bands.

[0083] As described above, the bandpass filter sheet according to this embodiment comprises two metal layers, a first metal layer and a second metal layer, one of which has a mesh structure and the other has a rectangular pattern structure. The mesh structure and the rectangular pattern structure are arranged at the same pitch, and the mesh structure and the rectangular pattern structure do not overlap when viewed from the thickness direction. This makes it possible to realize a bandpass filter sheet with good frequency selective transmission characteristics for high-frequency electromagnetic waves of several hundred GHz or more.

[0084] In the above study, the characteristics in the 300 GHz band were examined, but a bandpass filter sheet with similarly good frequency selective transmission characteristics can also be realized for electromagnetic waves of higher frequencies in the terahertz band by designing the shape of the mesh structure in one of the first metal layer and the second metal layer and the rectangular pattern structure in the other, i.e., the arrangement pitch and the size of the rectangular pattern, to match the frequency of the electromagnetic waves to be transmitted.

[0085] Furthermore, the bandpass filter sheet disclosed in the present application has good selective transmission characteristics for electromagnetic waves in high frequency bands of several hundred GHz or more, and can therefore be used effectively in various devices that use electromagnetic waves in such high frequency bands, such as antennas, circuit boards, electronic communication equipment, and inter-server communication devices.

[0086] Furthermore, since it is possible to realize a wideband filter function that transmits electromagnetic waves, it can also be used effectively as an antenna radome (an outer casing for a radio wave transmitter / receiver) that forms the electromagnetic wave emission and incidence surface of a device that houses an antenna.

[0087] Furthermore, since the pattern formed as the metal layer has a mesh structure or a rectangular pattern structure, it is possible to ensure high light transmittance in the visible light range by using a translucent material as the dielectric layer, and the material can be suitably used in applications where design is required or where it is necessary to visualize the inside, such as a testing machine for the antenna itself.

[0088] The bandpass filter sheet disclosed in the present application has a first dielectric layer, a first metal layer, a second dielectric layer, a second metal layer, and a third dielectric layer stacked in this order from the electromagnetic wave incident side, with one of the first metal layer and the second metal layer formed as a thin film having a mesh structure and the other having a rectangular pattern structure, with the arrangement pitch of the mesh structure and the rectangular pattern structure being the same and with the respective patterns not overlapping, thereby making it possible to realize a bandpass filter sheet with wide and high frequency selective transmission characteristics for electromagnetic waves in a high frequency band of several hundred GHz or more.

[0089] REFERENCE SIGNS LIST 1 (incident) electromagnetic wave 10 first dielectric layer 20 first metal layer 30 second dielectric layer 40 second metal layer 50 third dielectric layer 100 bandpass filter sheet

Claims

1. A bandpass filter sheet in which a first dielectric layer, a first metal layer, a second dielectric layer, a second metal layer, and a third dielectric layer are laminated in this order from the electromagnetic wave incident side, wherein the first metal layer and the second metal layer are both formed as thin films, one of the first metal layer and the second metal layer has a mesh structure in a net shape, and the other has a rectangular pattern structure in which a plurality of rectangular patterns are arranged in a matrix shape, the mesh structure and the rectangular pattern structure have the same arrangement pitch, and when the bandpass filter sheet is viewed in the thickness direction, the rectangular patterns are arranged within the openings of the mesh shape.

2. The bandpass filter sheet according to claim 1, wherein the first metal layer and the second metal layer formed as thin films have a thickness of 5 μm or less.

3. A bandpass filter sheet according to claim 1 or 2, wherein the surface resistance of the first metal layer and the second metal layer is 60 Ω / □ or less.

4. A bandpass filter sheet according to any one of claims 1 to 3, wherein the mesh structure and the rectangular pattern structure have an arrangement pitch of 0.5 mm or more and 1.5 mm or less.

5. A bandpass filter sheet according to any one of claims 1 to 4, wherein the line width of the mesh structure is 15 μm or more and 100 μm or less.

6. A bandpass filter sheet according to any one of claims 1 to 5, wherein the first metal layer is formed on one side of the second dielectric layer, and the second metal layer is formed on the other side of the second dielectric layer.

7. A bandpass filter sheet according to any one of claims 1 to 6, wherein the first dielectric layer, the second dielectric layer, and the third dielectric layer contain at least one of an acrylic resin, a silicone resin, and a urethane resin.

8. A bandpass filter sheet according to any one of claims 1 to 7, wherein at least one of the first dielectric layer, the second dielectric layer, and the third dielectric layer is made up of two or more dielectric layers.

Images