Band-pass filter sheet

Abstract

Provided is a band-pass filter sheet characterized in that: a first dielectric layer, a metal layer, and a second dielectric layer are laminated in this order starting from the radio wave entry surface side; a mesh structure in which the metal layer is formed in a thin film shape and / or a plurality of closed loop structures which are arranged in a matrix is / are provided; and the thickness D of the first dielectric layer and the second dielectric layer is provided by expression (1). Expression (1): D=λ / 4√ε±20% (where λ = the center wavelength of electromagnetic waves and ε = the permittivity of the dielectric layer)

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 (1 THz = 1000 GHz)) 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 (radome) 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 transparency required for a radome of a radar that uses electromagnetic waves of several hundred GHz or higher, particularly 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 has a first dielectric layer, a metal layer, and a second dielectric layer stacked in this order from the radio wave incident surface side, and has at least one of a mesh structure in which the metal layer is formed as a thin film and a plurality of closed loop structures arranged in a matrix, and is characterized in that a thickness D of the first dielectric layer and the second dielectric layer is given by the following (Equation 1): D=λ / 4√ε±20% (Equation 1) (where λ=center wavelength of the electromagnetic wave, and ε=dielectric constant of the dielectric layer).

[0010] The bandpass filter sheet disclosed in the present application has a structure in which a first dielectric layer, a metal layer, and a second dielectric layer are sequentially stacked, and has at least one of a mesh structure in which the metal layer is formed as a thin film and a plurality of closed loop structures arranged in a matrix, and by specifying the thickness D of the first dielectric layer and the second dielectric layer within a predetermined range based on the frequency of the electromagnetic wave, a high frequency selection effect can be obtained.

[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 metal layer of a first bandpass filter sheet according to the present embodiment; FIG. 3 is a diagram illustrating the frequency characteristics of the first bandpass filter sheet according to the present embodiment; FIG. 4 is a diagram illustrating the configuration of a metal layer of a second bandpass filter sheet according to the present embodiment; FIG. 5 is a diagram illustrating the frequency characteristics 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 according to the present embodiment; FIG. 7 is a diagram illustrating the frequency characteristics of the transmission attenuation of the third bandpass filter sheet according to the present embodiment; FIG. 8 is a diagram illustrating the frequency characteristics of the transmission attenuation of the fourth bandpass filter sheet according to the present embodiment; FIG. 9 is a diagram illustrating the frequency characteristics of the transmission attenuation of the fifth bandpass filter sheet as a comparative example; FIG. 10 is a diagram illustrating the frequency characteristics of the transmission attenuation of the sixth bandpass filter sheet as a comparative example; and FIG. 11 is a diagram illustrating the change in the frequency characteristics of the transmission attenuation when the thickness of the first dielectric layer is changed.

[0012] The bandpass filter sheet disclosed in the present application has a first dielectric layer, a metal layer, and a second dielectric layer stacked in this order from the radio wave incident surface side, and has at least one of a mesh structure in which the metal layer is formed as a thin film and a structure in which a plurality of closed loops are arranged in a matrix, and a thickness D of the first dielectric layer and the second dielectric layer is given by the following (Equation 1): D=λ / 4√ε±20% (Equation 1) (where λ=center wavelength of the electromagnetic wave, and ε=dielectric constant of the dielectric layer).

[0013] With this configuration, the bandpass filter sheet disclosed in the present application can form a bandpass filter with good frequency selection characteristics for high-frequency electromagnetic waves of several hundred GHz or more, by combining the frequency selection effect of the metal layer having at least one of a mesh structure formed in a thin film and a plurality of closed loop structures arranged in a matrix, and the effect of improving the electromagnetic wave absorption characteristics by determining the thicknesses of the first and second dielectric layers based on the frequency λ of the electromagnetic waves to be transmitted.

[0014] In the above bandpass filter sheet, it is preferable that the central wavelength λ of the electromagnetic wave in the formula 1 corresponds to a frequency of the electromagnetic wave of 100 to 400 GHz.

[0015] Furthermore, it is preferable that the thickness of the metal layer formed as a thin film is 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 thickness of the entire bandpass filter sheet, thereby achieving high frequency selectivity.

[0016] Furthermore, it is preferable that the surface resistance of the metal layer is 60 Ω / □ or less. By sufficiently reducing the resistance of the metal layer, the amount of attenuation of electromagnetic waves passing through the metal layer is reduced, thereby achieving high transmission characteristics for electromagnetic waves of a desired frequency.

[0017] Furthermore, it is preferable that the pitch of the mesh structure or the plurality of closed loop structures arranged in a matrix is ​​0.5 mm or more and 1.5 mm or less, thereby realizing frequency selection characteristics for electromagnetic waves with high frequencies of several hundred GHz or more.

[0018] It is also preferable that the line width of the mesh structure or the closed loop structure is 15 μm or more and 100 μm or less.

[0019] Furthermore, it is preferable that the first dielectric layer and the second 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 metal layer 20, and a second dielectric layer 30 from the incident surface side of the electromagnetic wave 1.

[0025] The thickness D1 of the first dielectric layer 10 and the thickness D2 of the second dielectric layer 30 shown in FIG. 1 both satisfy the requirement of the following (Equation 1): D=λ / 4√ε±20% (Equation 1) (where λ is the central wavelength of the electromagnetic wave, and ε is the dielectric constant of the dielectric layer).

[0026] The metal layer 20 has at least one of a mesh structure formed in a thin film and a structure of a plurality of closed loops arranged in a matrix.

[0027] [Details of Each Member] Next, each member constituting the bandpass filter sheet 100 according to this embodiment will be described.

[0028] <Dielectric Layer> The first dielectric layer 10 and the second dielectric layer 30 of the bandpass filter sheet 100 according to this embodiment can both be formed from various dielectric materials such as acrylic resin, silicone resin, urethane resin, etc. In addition to these, other dielectric materials such as silicone rubber, titanium oxide, polyvinylidene fluoride, polyester resin, glass, and silicone rubber can also be used.

[0029] The first dielectric layer 10 and the second dielectric layer 30 can each be formed as a single layer made of one type of material. Alternatively, they can be formed by stacking two or more layers of the same or different materials. Furthermore, the first dielectric layer 10 and the second dielectric layer 30 can be formed using the same dielectric material, or they can be formed using different dielectric materials, including the number of layers.

[0030] In the bandpass filter sheet 100 of this embodiment shown in FIG. 1, a resin base material for forming a thin metal layer 20 is used as the first dielectric layer 10 .

[0031] The thickness D1 of the first dielectric layer 10 and the thickness D2 of the second dielectric layer 30 are both within the range calculated by D=λ / 4√ε±20% (Equation 1), based on the wavelength λ of the electromagnetic waves to be transmitted by the bandpass filter sheet 100, taking into account the dielectric constant ε of the dielectric material constituting each of the dielectric layers 10 and 30.

[0032] More specifically, when the center frequency of the electromagnetic waves to be transmitted is between 100 GHz and 400 GHz, it is preferable to use a general dielectric material with a relative dielectric constant of about 2 to 3 for the first dielectric layer 10 and the second dielectric layer 30, and to set the thickness to between 120 μm and 230 μm.

[0033] In the bandpass filter sheet 100 of this embodiment shown in FIG. 1 , both the first dielectric layer 10 and the second dielectric layer 30 are made of a translucent, adhesive acrylic-based OCA (Optical Clear Adhesive). Using an adhesive resin material for the dielectric layers 10 and 30 improves workability and reduces the amount of material required when fabricating the bandpass filter sheet 100, which includes the first dielectric layer 10 and the second dielectric layer 30 sandwiched between them by the metal layer 20. This allows the bandpass filter sheet 100 to be produced easily and at low cost. Furthermore, by ensuring that all components constituting the bandpass filter sheet 100, including the first dielectric layer 10 and the second dielectric layer 30, are translucent, the bandpass filter sheet 100 as a whole can achieve a predetermined translucency (e.g., a total light transmittance of 50% or more). This translucency as a whole allows for a bandpass filter sheet that can be used in applications requiring design or applications where visibility into the interior of an antenna tester is important.

[0034] When the first dielectric layer 10 and the second dielectric layer 30 are made of a resin material such as OCA that has adhesive properties, it is preferable that the adhesive strength to the metal layer is 3 N / 10 mm or more.

[0035] Of course, the laminated structure of the first dielectric layer 10, the metal layer 20, and the second dielectric layer 30 can also be constructed using an adhesive material such as a double-sided adhesive sheet or adhesive.

[0036] <Metal Layer> The metal layer 20 of the bandpass filter sheet 100 shown in this embodiment is a thin film layer disposed between the first dielectric layer 10 and the second dielectric layer 30. The metal layer 20 has either a mesh structure (grid-like), a closed-loop structure in which a plurality of rectangular closed loops are arranged in a matrix of vertical and horizontal rows, or a composite structure including a mesh structure and a closed-loop structure.

[0037] The surface resistance of the metal layer 20 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, a value of about 0.01 Ω / □ is preferable.

[0038] 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 value of the metal layer 20 is 0.1 Ω / □ or more and 80 Ω / □ or less.

[0039] Furthermore, it is possible to reduce transmission loss by making the thickness of the metal layer 20 thin enough to be negligible as a "layer" compared with the thicknesses D1 and D2 of the first dielectric layer 10 and second dielectric layer 30. For this reason, the thickness of the metal layer 20 is preferably 5 μm or less, and more preferably 3 μm or less.

[0040] In order to selectively transmit electromagnetic waves having frequencies of several hundred GHz or more through the bandpass filter sheet 100, the pitch, i.e., the repetition interval, of the mesh structure and closed loop structure of the metal layer 20 is preferably 0.5 mm or more and 1.5 mm or less. In addition, the width (line width) for forming the mesh structure and closed loop structure is preferably 15 μm or more and 100 μm or less.

[0041] The metal layer 20 can be formed using inorganic conductive materials such as ITO (indium tin oxide), ATO (antimony-doped tin oxide), and carbon black; organic conductive materials such as PEDOT-PSS, polyacetylene, and polythiophene; and metal plating films such as electroless plating films and electrolytic plating films.

[0042] More specifically, methods for forming the metal layer include various printing methods, such as inkjet printing and screen printing, on a resin substrate such as polyethylene terephthalate (PET), a method in which a conductive material is applied to the entire surface of a substrate sheet, and then a mask is applied to the area where the metal layer 20 is to be formed, and the conductive material is removed from the area not covered by the mask using acid or the like, and a method in which laser light is irradiated to remove the conductive material from areas other than the metal layer 20. Furthermore, a wide variety of conventional pattern formation methods suited to the characteristics of the conductive material to be used can be employed, such as a photolithography method using a photocurable resin and a mask pattern, and 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 20 is to be formed using light or heat, and then the electroless plating catalyst solution and the electroless plating solution are sequentially brought into contact with each other to form an electroless plating film on the metal layer 20 area and then an electrolytic plating film thereon.

[0043] As the base material of the metal layer 20, in addition to the PET exemplified above, various resins, rubber materials, and even various dielectrics (insulators) such as paper and wood can be used.

[0044] In addition, in order to achieve the desired shape of the metal layer 20, i.e., the desired pitch and line width, and further to adjust the thickness of the metal layer 20 to achieve the desired surface resistance value, it is preferable to form the metal layer 20 using a printing method such as inkjet printing or screen printing.

[0045] When a closed loop structure is adopted as the metal layer 20, it is preferable that the error in the size of each closed loop, i.e., the length of one side of the rectangle and the spacing between the closed loops, is within ±5% of the placement pitch of the closed loop structure, and that the error in the mesh line width in the mesh structure and the line width when a closed loop structure is used is also within ±5% of the mesh pitch.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] The metal layer of the first bandpass filter sheet shown in Figure 2 has a closed-loop structure in which multiple rectangular closed loops are arranged in a matrix in the vertical and horizontal directions, with the arrangement pitch a of the rectangular closed loops being 0.70 mm, the outer side length b of one rectangular closed loop being 0.66 mm, the inner side length c being 0.59 mm, the line width d being 35 μm, and the thickness being 1 μm. In this case, the surface resistance of the metal layer 20 is 60 Ω / □.

[0050] It was assumed that the first dielectric layer and the second dielectric layer were both made of acrylic OCA with a dielectric constant ε = 2.5 (center λ = 1 mm), and the center frequency of the electromagnetic wave to be transmitted was 300 GHz (wavelength λ ≈ 1 mm). However, taking into account actual use, the thicknesses D1 and D2 of the first and second dielectric layers were calculated as 150 μm, rather than the value of D calculated strictly from (Equation 1), which was 158 μm.

[0051] Fig. 3 shows the frequency characteristics of the first bandpass filter sheet having the metal layer shown in Fig. 2. Fig. 3(a) shows the frequency characteristics of the transmission attenuation, which indicates, in dB, the amount of attenuation of electromagnetic wave energy that is transmitted through the back surface of the bandpass filter sheet among the energy of electromagnetic waves incident on the bandpass filter sheet, and Fig. 3(b) shows the frequency characteristics of the return attenuation, which indicates, in dB, the energy of electromagnetic waves that is reflected to the front surface side of the bandpass filter sheet relative to the energy of electromagnetic waves incident on the bandpass filter sheet.

[0052] As shown in Figure 3(a), the frequency characteristics of the transmission attenuation of the first bandpass filter sheet show that frequencies around 300 GHz, which are set as the transmission frequency, exhibit a low transmission attenuation of -1.00 dB for electromagnetic waves in the 250 GHz to 300 GHz band. On the other hand, for electromagnetic waves in the 30 GHz to 190 GHz and 340 GHz frequency bands above and below the 300 GHz frequency band, a high transmission attenuation of -2.00 dB or more is obtained. This confirms that a transmission range (a band with low transmission attenuation) that transmits electromagnetic waves of the desired frequency and a blocking range (a band with high transmission attenuation) that blocks the transmission of electromagnetic waves in the frequency bands above and below this range are realized. Note that "low transmission attenuation" here means that the rate at which electromagnetic waves are attenuated by transmission is small, and the rate at which electromagnetic waves are transmitted is large. On the other hand, "high transmission attenuation" means that the rate at which electromagnetic waves are attenuated by transmission is large, and the rate at which electromagnetic waves are transmitted is small.

[0053] In the frequency characteristics of the return loss of the first bandpass filter sheet shown in Figure 3(b), the return loss peaks at -29 dB at 284 GHz, which is the center frequency set for transmission through the sheet. Furthermore, a high return loss of greater than -10.00 dB is achieved in the frequency band from 230 GHz to 330 GHz, around 300 GHz. This confirms that the first bandpass filter sheet suppresses the reflectance of electromagnetic waves in the frequency band that is desired to be transmitted, allowing a greater amount of electromagnetic waves to be transmitted without being reflected.

[0054] 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.

[0055] The metal layer of the second bandpass filter sheet shown in Fig. 4 has a mesh structure in which a grid pattern is formed from a conductive material, with the repeat pitch of the mesh structure, i.e., the distance e between the centers of the lines constituting the mesh, being 0.70 mm in both the vertical and horizontal directions, the width of the mesh openings (the length inside the mesh structure) f being 0.63 mm, the line width g of the lines constituting the mesh being 35 µm, and the thickness being 1 µm. In this case, the surface resistance of the metal layer 20 is 60 Ω / □.

[0056] The first dielectric layer and the second dielectric layer were calculated with a dielectric constant ε of 2.5 (center λ=1 mm) and thicknesses D1 and D2 of 150 μm, similar to the first bandpass filter sheet described above.

[0057] Fig. 5 shows the frequency characteristics of the second bandpass filter sheet having the metal layer shown in Fig. 4. Fig. 5(a) shows the frequency characteristics of the transmission attenuation, which indicates the amount of attenuation of the energy of the electromagnetic wave transmitted through the back surface of the bandpass filter sheet, and Fig. 5(b) shows the frequency characteristics of the return attenuation, which indicates the amount of attenuation of the energy of the electromagnetic wave reflected from the front surface of the bandpass filter sheet.

[0058] As shown in Figure 5(a), the frequency characteristics of the transmission attenuation of the second bandpass filter sheet show a low transmission attenuation of -1.00 dB for electromagnetic waves in the frequency band of 250 GHz to 300 GHz, which is the frequency band around 300 GHz set as the transmission frequency, indicating good transmission of the desired electromagnetic waves. On the other hand, a transmission attenuation of greater than -2.00 dB is achieved for electromagnetic waves in the frequency bands below 180 GHz and above 335 GHz, which are the frequency bands above and below the transmission frequency band. Thus, it can be seen that the second bandpass filter sheet shown in Figure 4 also achieves a transmission range (a band with low transmission attenuation) that transmits electromagnetic waves of the desired frequency well, and a stop range (a band with high transmission attenuation) that blocks the transmission of electromagnetic waves in the frequency bands above and below that.

[0059] Furthermore, in the frequency characteristics of the return loss of the second bandpass filter sheet shown in Fig. 5(b), similar to the frequency characteristics of the return loss of the first bandpass filter sheet shown in Fig. 3(b), the return loss reaches a peak value of -29 dB at 284 GHz, which is set as the center frequency for transmission through the sheet, and the return loss is greater than -10.00 dB in the frequency band from 225 GHz to 330 GHz. This confirms that the reflectance of electromagnetic waves in the frequency band that is desired to be transmitted is also suppressed in the second bandpass filter sheet, and more electromagnetic waves are transmitted without being reflected.

[0060] FIG. 6 is a schematic diagram showing the configuration of the metal layer of the third bandpass filter sheet used in examining the frequency characteristics.

[0061] The metal layer 20 of the third bandpass filter sheet shown in Figure 6 has a structure that combines a mesh structure and a closed loop structure, in which a rectangular closed loop is formed within a mesh structure in which a grid-like pattern is formed using a conductive material.

[0062] 6, the repetition pitch h of the mesh structure is 0.70 mm in both the vertical and horizontal directions, the width of the mesh openings (the inner length of the mesh structure) i is 0.63 mm, the line width j of the lines constituting the mesh is 70 μm, the outer length k of one side of one rectangular closed loop is 0.55 mm, the inner length l is 0.48 mm, the line width m is 35 μm, and the thickness of the metal layer is 1 μm. The surface resistance of the metal layer 20 in this case is 60 Ω / □.

[0063] The first dielectric layer and the second dielectric layer were calculated with a dielectric constant ε of 2.5 (center λ=1 mm) and thicknesses D1 and D2 of 150 μm, similar to the first bandpass filter sheet and the second bandpass filter sheet described above.

[0064] FIG. 7 shows the frequency characteristics of the transmission attenuation of the third bandpass filter sheet having the metal layer shown in FIG.

[0065] As shown in FIG. 7 , the frequency characteristics of the transmission attenuation of the third band-pass filter sheet, like those of the first and second band-pass filter sheets described above, achieve a low transmission attenuation of −1.00 dB for frequencies between 250 GHz and 300 GHz, which are electromagnetic waves in the vicinity of 300 GHz that are set as the frequencies to be transmitted, and achieve a transmission attenuation of greater than −2.00 dB for electromagnetic waves in the frequency bands above and below this frequency band, that is, 180 GHz or less and 335 GHz or more.

[0066] In this way, it can be confirmed that the third bandpass filter sheet, whose metal layer configuration is shown in Figure 6, also has a transmission range (a band with low transmission attenuation) that transmits electromagnetic waves of a desired frequency, and a stop range (a band with high transmission attenuation) that blocks the transmission of electromagnetic waves in the frequency bands above and below that.

[0067] FIG. 8 shows the frequency characteristics of the transmission attenuation of the fourth bandpass filter sheet.

[0068] The fourth bandpass filter sheet, whose transmission loss is shown in Fig. 8, has a pattern configuration of the metal layer 20 that combines a mesh structure and a closed loop structure, with rectangular closed loops formed within a mesh structure formed in a grid pattern using a conductive material, similar to the configuration of the third bandpass filter sheet shown in Fig. 6. However, in the fourth bandpass filter sheet, after the metal layer is formed, heat treatment is performed before the dielectric layer is overlaid, thereby reducing the surface resistance of the metal layer to 1.0 Ω / □.

[0069] In the fourth bandpass filter sheet, the first dielectric layer was a four-layer laminated sheet with a thickness of 143 μm, consisting of a 10 μm thick acrylic OCA sheet, a 100 μm thick PET sheet, a 10 μm thick acrylic OCA sheet, and a 23 μm thick PET sheet. The second dielectric layer was a three-layer laminated sheet with a thickness of 133 μm, consisting of a 100 μm thick PET sheet, a 10 μm thick acrylic OCA sheet, and a 23 μm thick PET sheet. The acrylic OCA sheet had a dielectric constant ε of 2.5, and the PET sheet had a dielectric constant ε of 3.00 (center λ = 1 mm).

[0070] 8, the frequency characteristics of the transmission attenuation of the fourth bandpass filter sheet are smaller than −2.50 dB over the extremely wide frequency band from 140 GHz to 370 GHz, which is different from the transmission attenuation of the frequency bands above and below this range. In this way, by adjusting the dielectric constants and thicknesses of the first and second dielectric layers, it is possible to greatly widen the frequency bandwidth of the transmission range through which electromagnetic waves are transmitted.

[0071] 9A and 9B show frequency characteristics of the fifth bandpass filter sheet, where Fig. 9A shows the frequency characteristics of the transmission attenuation calculated from the energy of the electromagnetic wave transmitted through the back surface of the bandpass filter sheet, and Fig. 9B shows the frequency characteristics of the return attenuation calculated from the energy of the electromagnetic wave reflected from the front surface of the bandpass filter sheet.

[0072] The fifth bandpass filter sheet is a comparative bandpass filter sheet that does not include a second dielectric layer.

[0073] The metal layer of the fifth bandpass filter sheet, whose frequency characteristics are shown in Fig. 9, has a mesh structure with a grid pattern, with a repetition pitch e (see Fig. 4) of 0.90 mm in both the vertical and horizontal directions, a width f (see Fig. 4) of the mesh openings (the inner length of the mesh structure) of 0.86 mm, a line width g (see Fig. 4) of the lines constituting the mesh of 40 µm, and a thickness of 1 µm. The surface resistance of the metal layer in this case is 20 Ω / □.

[0074] The first dielectric layer was an acrylic OCA sheet having a dielectric constant ε=2.5 (center λ=1 mm) and a thickness D1 of 250 μm.

[0075] 9(a), the fifth bandpass filter sheet without the second dielectric layer exhibits a transmission attenuation of less than −2.96 dB in the frequency band of 315 GHz or less, while exhibiting a transmission attenuation of more than −3.08 dB in the frequency band of 340 GHz or more.

[0076] As described above, it can be seen that the fifth bandpass filter sheet, which does not have a second dielectric layer, does not have sufficient frequency characteristics as a bandpass filter, in that the transmission attenuation does not become large in frequency bands lower than the specified frequency band, and blocking bands (bands with high transmission attenuation) through which electromagnetic waves are difficult to transmit are formed on both sides of the transmission band of the specified frequency band.

[0077] Furthermore, the return loss shown in FIG. 9(b) shows a low return loss of −10 dB in the frequency band from 50 GHz to 300 GHz, which is lower than the set frequency of 284 GHz, and a high return loss of −11 dB or more in the frequency band of 330 GHz or higher.

[0078] From these results, it is considered important that the bandpass filter sheet disclosed herein has both the first and second dielectric layers in order to achieve good element ranges in the frequency bands above and below the frequency band (transmission band) to be transmitted. Furthermore, from the calculation results of the frequency characteristics of the fifth bandpass filter sheet shown in Figure 9, it can be inferred that the frequency bands higher than the frequency band set by the first dielectric layer located on the electromagnetic wave incident side of the metal layer and lower than the frequency band set by the second dielectric layer located on the opposite side from the incident side have a significant effect on both the transmission characteristics and the reflection characteristics.

[0079] In designing the fifth bandpass filter sheet, the size of the mesh structure and the thickness of the first dielectric layer were changed from the design values ​​of the second bandpass filter sheet so that a more preferable transmission attenuation amount could be achieved without the second dielectric layer. However, from the results shown in Fig. 9, it was confirmed that it is difficult to realize a bandpass filter sheet that can transmit the desired electromagnetic waves in a configuration without the second dielectric layer.

[0080] Next, the influence of the thicknesses D1 and D2 of the first and second dielectric layers deviating from the condition D=λ / 4√ε±20% in (Equation 1) was examined.

[0081] FIG. 10 is a diagram showing frequency characteristics of transmission attenuation of the sixth bandpass filter sheet.

[0082] In the sixth bandpass filter sheet, compared to the second bandpass filter sheet shown in Figure 4, the configuration of the metal layer is not changed, but the thickness D1 of the first dielectric layer and the thickness D2 of the second dielectric layer are increased by 23% to 185 µm, exceeding the 20% condition of (Equation 1) for the thickness (D1, D2 =) 150 µm of the second bandpass filter sheet.

[0083] The frequency characteristics of the transmission attenuation of the sixth bandpass filter sheet shown by the solid line in Fig. 10 are compared with the frequency characteristics of the transmission attenuation of the second bandpass filter sheet shown by the dashed line in Fig. 10, and the frequency at which the transmission attenuation is smallest is 220 GHz, which is a change from the frequency of 275 GHz for the second bandpass filter sheet. This change is just over 20%, which is almost the same as the change in the thicknesses D1 and D2 of the first and second dielectric layers. Therefore, it is considered that the more the thickness D of the dielectric layer deviates from the condition of (Equation 1), the greater the deviation of the frequency at which the transmission attenuation is smallest from the frequency of the electromagnetic waves that are desired to be transmitted.

[0084] In the case of the sixth bandpass filter sheet shown in FIG. 10 , the value of the transmission attenuation at 274 GHz, which is the frequency at which the transmission attenuation is smallest in the second bandpass filter sheet, is −1.5 dB, which is approximately −0.8 dB larger, and considering that less electromagnetic wave is transmitted, the range of ±20% specified in (Equation 1) is considered to be the permissible range of the dielectric layer thicknesses (D1, D2) for realizing a bandpass filter sheet that allows good transmission of electromagnetic waves of a desired frequency.

[0085] Furthermore, the sixth bandpass filter sheet has the smallest transmission attenuation value of approximately −1.0 dB, which is larger than −0.8 dB for the second bandpass filter sheet. This is thought to be because the mesh structure of the metal layer remains the same as the design optimized for transmitting electromagnetic waves of a frequency of 284 GHz, which corresponds to a realistic thickness of 150 μm for the usable acrylic OCA used as the dielectric layer.

[0086] FIG. 11 is a diagram showing the influence of the transmission attenuation on the frequency characteristics when the thickness of the first dielectric layer is changed.

[0087] FIG. 11 shows the frequency characteristics of the transmission attenuation when the thickness D2 of the second dielectric layer in the second bandpass filter sheet described above is constant at 150 μm, and the thickness D1 of the first dielectric layer is set to 50 μm (reference numeral 101 in FIG. 11 ), 100 μm (reference numeral 102 in FIG. 11 ), 150 μm (reference numeral 103 in FIG. 11 ), 200 μm (reference numeral 104 in FIG. 11 ), 250 μm (reference numeral 105 in FIG. 11 ), and 300 μm (reference numeral 106 in FIG. 11 ).

[0088] As shown in Figure 11, in the frequency characteristics shown by 102 and 104, where the thickness D1 of the first dielectric layer is 100 μm (reference numeral 103), which is a deviation of approximately 30% from the thickness of 150 μm, and where the thickness D1 is 200 μm, respectively, the frequency at which the transmission attenuation is smallest is shifted by approximately 35 GHz, which corresponds to approximately 15%. Thus, it is believed that the frequency at which the transmission attenuation is smallest changes by approximately half the deviation (proportion) between the thickness D1 of the first dielectric layer and the thickness D2 of the second dielectric layer. Therefore, to realize a bandpass filter sheet that effectively transmits electromagnetic waves in a desired frequency band, it is preferable that the deviation in thickness between the first and second dielectric layers be 30% or less, and more preferably 20% or less.

[0089] Furthermore, the inventors' investigations confirmed that the mesh structure formed on the metal layer and the closed loop structure arranged in a matrix are within the acceptable range for a bandpass filter sheet as long as the error in the line width formed and the error in the pitch of the arrangement pattern are both within approximately 10%.

[0090] As described above, in the bandpass filter sheet according to this embodiment, the thicknesses D1 and D2 of the first and second dielectric layers are defined as D=λ / 4√ε±20% (Equation 1) based on the frequency λ to be transmitted and the dielectric constants of the first and second dielectric layers, thereby enabling the bandpass filter sheet to have good frequency selective transmission characteristics for high-frequency electromagnetic waves of several hundred GHz or more.

[0091] In the above study, the characteristics in the 300 GHz band were examined, but for electromagnetic waves of higher frequencies in the terahertz band, a bandpass filter sheet with similarly good frequency selective transmission characteristics can be realized by setting the thicknesses D1 and D2 of the first and second dielectric layers within the range of (Equation 1), and more preferably by designing them to match the frequency of the electromagnetic waves that pass through the mesh structure or closed loop structure formed in the metal layer.

[0092] Furthermore, because it has good selective transmission properties for electromagnetic waves in high frequency bands of several hundred GHz or more, it can 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.

[0093] In particular, since it is possible to realize a wideband filter function that transmits electromagnetic waves, it can also be used effectively as an antenna radome (exterior body of a radio wave transmitter / receiver) that forms the electromagnetic wave emission and incidence surface of a device that houses an antenna.

[0094] Furthermore, by forming the pattern on the metal layer in a mesh structure or a closed loop structure arranged in a matrix, it is possible to ensure high light transmittance in the visible light range, making it suitable for use in applications where design is required or where it is necessary to visualize the inside, such as in testing equipment for the antenna itself.

[0095] The bandpass filter sheet disclosed in the present application comprises a first dielectric layer, a thin-film metal layer, and a second dielectric layer, and by setting the thicknesses of the first and second dielectric layers to D = λ / 4√ε ± 20% (Equation 1), it is possible to realize a bandpass filter sheet with high frequency selective transmission characteristics for electromagnetic waves in the high-frequency band of several hundred GHz or more.

[0096] REFERENCE SIGNS LIST 1 (incident) electromagnetic wave 10 first dielectric layer 20 metal layer 30 second dielectric layer 100 bandpass filter sheet

Claims

1. A bandpass filter sheet, comprising a first dielectric layer, a metal layer, and a second dielectric layer, laminated in this order from the radio wave incident surface side, the metal layer having at least one of a mesh structure formed in a thin film and a plurality of closed loop structures arranged in a matrix, and the thickness D of the first dielectric layer and the second dielectric layer is given by the following (Formula 1): D = λ / 4√ε ± 20% (Formula 1) (where λ = central wavelength of the electromagnetic wave, ε = dielectric constant of the dielectric layer).

2. The bandpass filter sheet according to claim 1, wherein the central wavelength λ of the electromagnetic wave in (Equation 1) is a value corresponding to an electromagnetic wave frequency of 100 to 400 GHz.

3. The bandpass filter sheet according to claim 1 or 2, wherein the metal layer formed in a thin film has a thickness of 5 μm or less.

4. The bandpass filter sheet according to any one of claims 1 to 3, wherein the surface resistance of the metal layer is 60 Ω / □ or less.

5. A bandpass filter sheet according to any one of claims 1 to 4, wherein the mesh structure or the plurality of closed loop structures arranged in a matrix has a pitch of 0.5 mm or more and 1.5 mm or less.

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

7. A bandpass filter sheet according to any one of claims 1 to 6, wherein the first dielectric layer and the second 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 and the second dielectric layer is made of two or more dielectric layers.

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