WO2021261390A1 - Radio wave absorbing sheet and communication device - Google Patents

Abstract

A radio wave absorbing sheet according to the present invention comprises: a first dielectric layer including a first surface and a second surface positioned on the reverse surface thereof to the first surface; a plurality of first conductor layers positioned on the first surface and aligned in a first direction; and a second conductor layer positioned on the second surface. The rate of occupancy of the first conductor layer on the first surface is 0.85 or less.

Description

Radio wave absorption sheet and communication device

The embodiment of the present disclosure relates to a radio wave absorbing sheet and a communication device.

In order to suppress the influence of radio waves on electronic devices, for example, as disclosed in Patent Document 1, a radio wave absorbing sheet that absorbs radio waves is used.

Japanese Patent No. 6063631

In the conventional radio wave absorber, the lower limit of the thickness is easily limited by the wavelength of the radio wave. For example, Patent Document 1 proposes that the thickness of the radio wave absorber for absorbing a radio wave of 6.4 GHz is 1005 μm to 1300 μm.

The embodiment of the present disclosure has been made in consideration of such a point, and an object thereof is to provide a radio wave absorbing sheet that can be easily made thinner.

One embodiment of the present disclosure comprises a first dielectric layer including a first surface and a second surface located on the opposite side of the first surface.
A plurality of first conductor layers located on the first surface and arranged in the first direction,
A second conductor layer located on the second surface is provided.
It is a radio wave absorption sheet in which the occupancy rate of the first conductor layer on the first surface is 0.85 or less.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the dimension of the first conductor layer in the first direction may be less than 5.0 mm.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the thickness of the first dielectric layer may be 300 μm or less.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the relative permittivity of the first dielectric layer may be less than 20.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the dielectric loss tangent of the first dielectric layer may be 0.2 or less.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the thickness of the radio wave absorbing sheet may be 350 μm or less.

The radio wave absorption sheet according to the embodiment of the present disclosure may have a resonance frequency fr located in the range of 20 GHz or more and 110 GHz or less.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the first conductor layer is a circle having a radius r in a plan view.
The resonance frequency fr may be calculated based on the following equations (A1) and (A2).

c is the speed of light, ε _r is the relative permittivity of the first dielectric layer, and h is the thickness of the first dielectric layer.

In the radio wave absorption sheet according to the embodiment of the present disclosure, ^{the ratio of N 2} R to 120 π is 0.8 or more and 1.2 or less.
N ² and R may be calculated based on the following formulas (A3) and (A4).

P is the area of the unit cell, K is a constant, and tan δ is the dielectric loss tangent of the first dielectric layer.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the first conductor layer is a rectangle having a length L and a width W in a plan view, and the width W is a length L or less.
The resonance frequency fr may be calculated based on the following equations (B1), (B2), (B3), and (B4).

c is the speed of light, ε _r is the relative permittivity of the first dielectric layer, and h is the thickness of the first dielectric layer.

In the radio wave absorption sheet according to the embodiment of the present disclosure, ^{the ratio of N 2} R to 120 π is 0.8 or more and 1.2 or less.
N ² and R may be calculated based on the following equations (B5) and (B6).

P is the area of the unit cell, K is a constant, and tan δ is the dielectric loss tangent of the first dielectric layer.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the first conductor layer may include a plurality of openings.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the plurality of first conductor layers are a plurality of first shape layers and a plurality of second shape layers having a shape different from that of the first shape layer in a plan view. And may be included.

In the radio wave absorbing sheet according to the embodiment of the present disclosure, the second dielectric layer including the third surface and the fourth surface located on the opposite side of the third surface,
A third conductor layer located on the fourth surface is provided.
The second conductor layer is located between the second surface of the first dielectric layer and the third surface of the second dielectric layer.
The occupancy rate of the second conductor layer on the second surface may be 0.85 or less.

One embodiment of the present disclosure is
A communication mechanism that transmits or receives radio waves,
A communication device including the above-mentioned radio wave absorbing sheet.

According to the embodiment of the present disclosure, it is possible to provide a radio wave absorbing sheet that can be easily made thinner.

It is a top view which shows one Embodiment of a radio wave absorption sheet. It is sectional drawing which shows the case where the radio wave absorption sheet of FIG. 1 is seen from the direction of II-II. It is a figure which shows the unit cell which constitutes the radio wave absorption sheet of FIG. It is a figure which shows the equivalent circuit of a radio wave absorption sheet. It is sectional drawing which shows an example of the manufacturing method of a radio wave absorption sheet. It is sectional drawing which shows an example of the manufacturing method of a radio wave absorption sheet. It is a figure which shows an example of the cross-sectional shape of the 1st conductor layer of FIG. 6A. It is a figure which shows an example of the cross-sectional shape of the 1st conductor layer of FIG. 6A. It is sectional drawing which shows an example of the communication apparatus provided with the electric wave absorption sheet. It is a top view which shows one Embodiment of a radio wave absorption sheet. It is a figure which shows the unit cell which constitutes the radio wave absorption sheet of FIG. It is a top view which shows one Embodiment of a radio wave absorption sheet. It is a top view which shows one Embodiment of a radio wave absorption sheet. It is a top view which shows one Embodiment of a radio wave absorption sheet. It is sectional drawing which shows the case where the radio wave absorption sheet of FIG. 12 is seen from the XIII-XIII direction. It is a top view which shows one Embodiment of a radio wave absorption sheet. It is sectional drawing which shows the case where the radio wave absorption sheet of FIG. 14 is seen from the XV-XV direction. It is a figure which shows the absorption characteristic of the radio wave absorption sheet of Example 1A. It is a figure which shows the absorption characteristic of the radio wave absorption sheet of Example 1A to 1G. It is a figure which shows the absorption characteristic of the radio wave absorption sheet of Example 2. It is a figure which shows the absorption characteristic of the radio wave absorption sheet of Example 3.

Hereinafter, the configuration of the radio wave absorbing sheet 10 according to the embodiment of the present disclosure will be described in detail with reference to the drawings. It should be noted that the embodiments shown below are examples of the embodiments of the present disclosure, and the present disclosure is not construed as being limited to these embodiments. Further, in the present specification, terms such as "board", "base material", "sheet" and "film" are not distinguished from each other based only on the difference in names. For example, "base material" and "base material" are concepts including members that can be called sheets or films. Furthermore, the terms used herein, such as "parallel" and "orthogonal", and the values of length and angle, which specify the shape and geometric conditions and their degrees, are bound by a strict meaning. Instead, the interpretation shall be made to include the range in which similar functions can be expected. Further, in the drawings referred to in the present embodiment, the same parts or parts having similar functions may be designated by the same reference numerals or similar reference numerals, and the repeated description thereof may be omitted. Further, the dimensional ratio of the drawing may differ from the actual ratio for convenience of explanation, or a part of the configuration may be omitted from the drawing.

Hereinafter, embodiments of the present disclosure will be described. First, the background will be described.

In recent years, radio waves in the frequency band of about 30 GHz or more and 300 GHz or less, called millimeter waves, have begun to be used in various fields. Examples in the field are 5th generation mobile communication systems, mobiles, radars for collision prevention systems for automobiles, medical biosensing, and the like. On the other hand, as the use of radio waves increases, there is concern that problems such as malfunction of electronic devices and deterioration of the communication environment will occur.

The radio wave absorption sheet is known as one of the products that solves these problems. The radio wave absorption sheet can absorb noise and unnecessary radio waves emitted from electronic devices. By using the radio wave absorbing sheet, effects such as prevention of malfunction of electronic devices and improvement of communication environment can be expected.

On the other hand, the thickness of the conventional radio wave absorption sheet is large. For example, the radio wave absorbing sheet proposed in Patent Document 1 described above has a thickness of 1 mm or more. For this reason, it has not been easy to install a radio wave absorbing sheet in an electronic device that is required to be miniaturized and thinned. Further, in Patent Document 1, a frequency band of 6.4 GHz or less is assumed, and the frequency band of the millimeter wave band is not supported.

In consideration of such a background, in this embodiment, we propose a radio wave absorption sheet that is easy to realize thinning and can correspond to the frequency band of the millimeter wave band.

FIG. 1 is a plan view showing an embodiment of the radio wave absorbing sheet 10. FIG. 2 is a cross-sectional view showing a case where the radio wave absorbing sheet 10 of FIG. 1 is viewed from the II-II direction. The radio wave absorbing sheet 10 includes a first dielectric layer 20 including a second surface 22 located on the opposite side of the first surface 21 and the first surface 21, and a plurality of first conductor layers 30 located on the first surface 21. And a second conductor layer 40 located on the second surface 22.

The radio wave absorbing sheet 10 absorbs radio waves incident on the radio wave absorbing sheet 10 from the first surface 21 side. For example, the radio wave absorbing sheet 10 absorbs radio waves by utilizing resonance. By utilizing resonance, the thickness of the radio wave absorbing sheet 10 can be made smaller than that of a conventional radio wave absorber.

The frequency of the radio wave targeted by the radio wave absorption sheet 10 is, for example, 20 GHz or more. The frequency of the radio wave targeted by the radio wave absorbing sheet 10 is, for example, 110 GHz or less, and may be 80 GHz or less. Such radio waves are used in 5th generation mobile communication systems, radar devices for automobiles, and the like.

Each component of the radio wave absorbing sheet 10 will be described.

The first dielectric layer 20 contains a material having an insulating property. The insulating material may be an organic material or an inorganic material. The insulating material may be a combination of an organic material and an inorganic material.

As the inorganic material, silicon oxide _(SiO 2), silicon nitride _{(Si 3 N} 4), silicon oxynitride (SiON), aluminum oxide _{(Al 2 O} 3), aluminum nitride (AlN), silicon carbide (SiC), Silicon nitride carbide (SiCN), carbon-added silicon oxide (SiCO), borosilicate glass, quartz glass and the like can be used.

Organic materials include polyimide, epoxy resin, benzocyclobutene resin, polyamide, phenol resin, fluororesin, liquid crystal polymer, polyamideimide, polybenzoxazole, cyanate resin, aramid resin, polyolefin, polyester, BT resin, polyacetal, and polybutylene. Telephthalate, syndiotactic polystyrene, polyphenylene sulfide, polyether ether ketone, polyether nitrile, polycarbonate, polyphenylene ether polysulfone, polyether sulfone, polyarylate, polyetherimide and the like can be used. Further, fibers, fillers and the like may be included in the layers of these organic materials. The material of the fiber and the filler may be an insulating material such as glass, talc, mica, silicon oxide, aluminum oxide, titanium oxide, or a conductive material such as carbon or metal.

As the material of the first dielectric layer 20, a compound or a mixture containing both an organic material and an inorganic material may be used. Examples are silicone resins, FR-4, FR-5 and the like.

The first dielectric layer 20 may have a heat resistance of about 150 ° C. For example, the glass transition point or the deflection temperature under load of the material of the first dielectric layer 20 may be 150 ° C. or higher. As a result, when the radio wave absorbing sheet 10 is installed in an electronic device including a component whose temperature rises to about 150 ° C., the radio wave absorbing sheet 10 can function appropriately.

_{The relative permittivity ε r} of the first dielectric layer 20 may be less than 20.0, may be 15.0 or less, may be 10.0 or less, and may be 5.0 or less. You may. By utilizing resonance, the first dielectric layer 20 can absorb radio waves even when the relative permittivity ε _r of the first dielectric layer 20 is lower than that of the conventional radio wave absorber. _{The relative permittivity ε r} of the first dielectric layer 20 may be 2.0 or more, 3.0 or more, or 5.0 or more.

The dielectric loss tangent tan δ of the first dielectric layer 20 may be 0.20 or less, 0.15 or less, 0.10 or less, or 0.05 or less. good.

Generally, when the dielectric layer contains a filler having conductivity at a high density, the relative permittivity and the dielectric loss tangent of the dielectric layer increase. On the other hand, when the density of the filler is high, there may be a problem that the dielectric layer becomes brittle. Further, as the density of the filler increases, the distribution of the filler tends to be uneven. If the distribution of the filler becomes non-uniform, voids may be generated in the dielectric layer or the characteristics of the dielectric layer may vary.

In the present embodiment, since the use of resonance, even when the relative dielectric constant of the first dielectric layer 20 as described above epsilon _r and the dielectric loss tangent tanδ is limited, the excellent radio wave absorption performance Can be expressed. Therefore, it is possible to avoid the problem caused by the high density of the filler.

The thickness h may be, for example, 300 μm or less, 250 μm or less, 200 μm or less, 150 μm or less, or 100 μm or less. By utilizing resonance, radio waves can be absorbed even when the thickness h of the first dielectric layer 20 is smaller than that of the conventional radio wave absorber.

The first dielectric layer 20 may be composed of a single layer or may be composed of a plurality of layers.

_{As a method for measuring the relative permittivity ε r} and the dielectric loss tangent tan δ of the first dielectric layer 20, an open resonator method can be used. Measurement systems that perform the open resonator method include a network analyzer, a millimeter wave doubler, a millimeter wave detector, and a Fabry-Perot resonator. As the Fabry-Perot resonator, FPR-40, FPR-50, FPR-60, FPR-75, PFR-90, FPR-110 and the like of Keycom Co., Ltd. can be used depending on the frequency. As a measuring instrument for measuring the thickness of the first dielectric layer 20, a length measuring machine can be used, and for example, a Digimicro manufactured by Nikon Corporation can be used. If the thickness cannot be measured by the length measuring machine, the thickness of the first dielectric layer 20 may be calculated based on the image of the cross section of the sample of the radio wave absorbing sheet 10. A scanning electron microscope can be used as a measuring instrument for measuring an image.

Although not shown, an insulating layer such as an adhesive layer may exist between the first surface 21 of the first dielectric layer 20 and the first conductor layer 30. Similarly, an insulating layer such as an adhesive layer may exist between the second surface 22 of the first dielectric layer 20 and the second conductor layer 40.

Although not shown, the interface between the first surface 21 of the first dielectric layer 20 and the first conductor layer 30 may be roughened. For example, the first surface 21 may include irregularities. This makes it possible to improve the adhesion between the first surface 21 and the first conductor layer 30. Rough surface treatment includes wet etching, blasting, polishing, plating and the like. Similarly, the interface between the second surface 22 of the first dielectric layer 20 and the second conductor layer 40 may be roughened. For example, the second surface 22 may include irregularities.

The first conductor layer 30 will be described. The first conductor layer 30 contains a material having conductivity. For example, the first conductor layer 30 includes copper (Cu), gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), tin (Sn), aluminum (Al), and the like. It includes metals such as nickel (Ni), chromium (Cr), titanium (Ti), molybdenum (Mo), tungsten (W), and tantalum (Ta), or alloys using these. The first conductor layer 30 may contain an organic conductor such as a conductive polymer, a conductive fiber such as a carbon nanotube (CNT), or the like.

The thickness T1 of the first conductor layer 30 may be 1 μm or more, or 5 μm or more. The thickness of the first conductor layer 30 may be 50 μm or less, or 20 μm or less.

Next, the shape and arrangement of the first conductor layer 30 will be described. In the example shown in FIG. 1, the first conductor layer 30 is a circle having a radius r in a plan view. "Plane view" means to see the radio wave absorbing sheet 10 along the direction orthogonal to the in-plane direction of the first dielectric layer 20.

As shown in FIG. 1, the plurality of first conductor layers 30 may be arranged in the in-plane direction of the first dielectric layer 20. For example, the plurality of first conductor layers 30 may be arranged in the first direction D1 in the first period P1. The plurality of first conductor layers 30 may be arranged in the second direction D2 in the second period P2. The second direction D2 is a direction different from the first direction D1, for example, a direction orthogonal to the first direction D1. The first period P1 is the distance between the center points C0 of two adjacent first conductor layers 30 in the first direction D1. The second period P2 is the distance between the center points C0 of the two adjacent first conductor layers 30 in the second direction D2. The first period P1 and the second period P2 may be the same or different. The cycles P1 and P2 are, for example, larger than 1.5 mm and may be 2.0 mm or more, or may be 2.5 mm or more.

The configuration of the plurality of first conductor layers 30 is determined so that resonance occurs at the frequency of the target radio wave. For example, the radius r, the periods P1 and P2 of the first conductor layer 30 are determined so that the occupancy rate of the first conductor layer 30 on the first surface 21 is 0.85 or less. The occupancy rate may be 0.75 or less, 0.60 or less, 0.50 or less, or 0.30 or less. The occupancy rate may be 0.05 or more, or 0.10 or more.

The occupancy rate may be calculated by dividing the total area of the plurality of first conductor layers 30 by the area of the region where the first conductor layers 30 are distributed. The area of the region where the first conductor layer 30 is distributed may be the area of a quadrangle 35 surrounding the plurality of first conductor layers 30, as shown by reference numeral 35 in FIG. 1, for example.

When a plurality of first conductor layers 30 are arranged periodically, the occupancy rate may be calculated by dividing the area of one first conductor layer 30 by the area of the unit cell 12. FIG. 3 is a perspective view showing the unit cell 12. The unit cell 12 includes one first conductor layer 30, a first dielectric layer 20, and a second conductor layer 40, and has an area corresponding to one first conductor layer 30. For example, the unit cell 12 is a quadrangle including a pair of first sides extending in the first direction D1 and a pair of second sides extending in the second direction D2 in a plan view. The length of the first side is equal to the first period P1 and the length of the second side is equal to the second period P2. In the example shown in FIG. 3, the area of the unit cell 12 is P1 × P2. The area of the first dielectric layer 20 is πr ² . Therefore, the occupancy rate is πr ² / (P1 × P2).

In FIG. 1, reference numeral S1 represents the dimension of the first conductor layer 30 in the first direction D1. The dimension S1 is, for example, less than 5.0 mm, may be 4.0 mm or less, may be 3.0 mm or less, may be 2.0 mm or less, or may be 1.5 mm or less. It may be 1.0 mm or less. Reference numeral S2 represents the dimension of the first conductor layer 30 in the second direction D2. The dimension S2 is, for example, less than 5.0 mm, may be 4.0 mm or less, may be 3.0 mm or less, may be 2.0 mm or less, or may be 1.5 mm or less. It may be 1.0 mm or less. In the example shown in FIG. 1, the dimensions S1 and S2 are the diameters of the first conductor layer 30.
The conditions regarding the range of these dimensions may be satisfied by both dimensions S1 and S2, or may be satisfied by only one of them. For example, the dimension S1 may be less than 5.0 mm, but the dimension S2 may be 5.0 mm or more. Similarly, the above-mentioned conditions regarding the range of the period P1 and the period P2 may be satisfied by both the period P1 and the period P2, or may be satisfied by only one of them.

Next, the second conductor layer 40 will be described. The second conductor layer 40 contains a material having conductivity. As the material of the second conductor layer 40, the material exemplified by the first conductor layer 30 can be used. The material of the second conductor layer 40 may be the same as or different from the material of the first conductor layer 30.

The thickness T2 of the second conductor layer 40 may be 1 μm or more, or 5 μm or more. The thickness of the second conductor layer 40 may be 50 μm or less, or 20 μm or less.

The second conductor layer 40 faces the first conductor layer 30 in the thickness direction of the first dielectric layer 20. As a result, an electric field can be generated between the first conductor layer 30 and the second conductor layer 40.

The second conductor layer 40 may be spread so as to overlap the plurality of first conductor layers 30 in a plan view. For example, the second conductor layer 40 may be located over the entire second surface 22 of the first dielectric layer 20. Although not shown, the second conductor layer 40 may have openings, slits, or the like.

The thickness of the entire radio wave absorbing sheet 10 is, for example, 350 μm or less, may be 300 μm or less, may be 250 μm or less, or may be 200 μm or less. Since the thickness h of the first dielectric layer 20 is small, the overall thickness of the radio wave absorbing sheet 10 is also small. The thickness of the entire radio wave absorbing sheet 10 may be 5 μm or more, 10 μm or more, 20 μm or more, or 50 μm or more.

By reducing the thickness of the entire radio wave absorbing sheet 10, restrictions on the installation location of the radio wave absorbing sheet 10 can be reduced. That is, the degree of freedom in the layout of the radio wave absorbing sheet 10 is increased. Further, the thermal resistance of the radio wave absorbing sheet 10 in the thickness direction can be reduced. Therefore, when the radio wave absorbing sheet 10 is installed in the electronic device, it is possible to suppress the temperature rise of the electronic component caused by the radio wave absorbing sheet 10. Thereby, for example, the radio wave absorbing sheet 10 can be installed in an electronic device including a component whose temperature rises to about 150 ° C.

Next, the operation of the radio wave absorbing sheet 10 will be described. FIG. 4 is a diagram showing an equivalent circuit of the radio wave absorption sheet 10. The equivalent circuit includes a primary circuit 14, a secondary circuit 15, and a transformer 16 that connects the primary circuit 14 and the secondary circuit 15.

The primary circuit 14 represents a radio wave propagating in the atmosphere. The primary circuit 14 has a characteristic impedance based on the permittivity of the atmosphere. The secondary circuit 15 represents a resonance circuit composed of a first dielectric layer 20, a first conductor layer 30, and a second conductor layer 40. Resonant circuits include, for example, resistors, capacitors and coils connected in parallel. The transformer 16 is composed of a plurality of first conductor layers 30.

The resonance circuit will be described. As described above, the first conductor layer 30 is a circle having a radius r in a plan view. In this case, the resonance frequency fr of the resonance circuit is calculated based on the following equations (A1) and (A2).

c is the speed of light. r _eff is the effective radius of the first conductor layer 30 when the fringe effect at the end of the first conductor layer 30 is taken into consideration.

As described above, the frequency of the radio wave targeted by the radio wave absorbing sheet 10 is 20 GHz or more and 110 GHz or less. In this case, the radio wave absorbing sheet 10 is configured so that the resonance frequency fr is 20 GHz or more and 110 GHz or less. For example, when the frequency of the radio wave targeted by the radio wave absorbing sheet 10 is 79 GHz, the thickness h and the relative permittivity ε _{r of the first dielectric layer 20 so that the resonance frequency fr becomes the same as or close to 79 GHz.} In addition, the radius r of the first conductor layer 30 is determined. When the thickness h and the relative permittivity ε _{r of} the first dielectric layer 20 are predetermined, the radius r of the first conductor layer 30 is determined so that the resonance frequency fr becomes a desired value. As a result, the target radio wave can be absorbed by using resonance.

Next, the transformer 16 will be described. In order for the radio wave absorbing sheet 10 to effectively absorb radio waves, it is preferable that the characteristic impedance of the secondary circuit 15 is the same as or close to the characteristic impedance of the primary circuit 14. When the resonance circuit of the secondary circuit 15 satisfies the resonance condition, the characteristic impedance of the secondary circuit 15 is N ² R. N is the turns ratio of the transformer 16. R is the resonance resistance of the secondary circuit 15.

N ² and R are calculated based on the following formulas (A3) and (A4).

P is the area of the unit cell 12. K is a constant. ^{Πr 2} / P in the formula (A3) represents the occupancy rate of the first conductor layer 30 on the first surface 21. The formula (A3) means that the occupancy rate of the first conductor layer 30 and N ² are in a proportional relationship.

Assuming that the characteristic impedance in the atmosphere is the same as the characteristic impedance in vacuum, the impedance of the primary circuit 14 is 120π. Wave absorbing sheet 10 is preferably arranged ^{to N 2} R is the same as 120Pai, or approximate. For example, the dielectric loss tang tan δ of the first dielectric layer 20, the radius r of the first conductor layer 30, and the area P of the unit cell 12 are set so that the ratio ^{of N 2 R to 120 π is 0.8 or more and 1.2 or less.} It is determined. When the dielectric loss tangent tan δ of the first dielectric layer 20 and the radius r of the first conductor layer 30 have already been determined, the area P of the unit cell 12 is set so that the ratio ^{of N 2 R to 120 π becomes a desired value.} It will be adjusted. For example, the first period P1 and the second period P2 of the first conductor layer 30 are adjusted. As a result, the radio wave absorbing sheet 10 can effectively absorb radio waves. The ratio of N ² R to 120 π may be 0.9 or more and 1.1 or less, or 0.95 or more and 1.05 or less.

Next, a method for manufacturing the radio wave absorbing sheet 10 will be described. 5 to 6B are views showing an example of a manufacturing process of the radio wave absorbing sheet 10.

First, the first dielectric layer 20 is prepared. The first dielectric layer 20 may be a flexible member or a rigid member. Subsequently, the first conductor layer 30 is formed on the first surface 21 of the first dielectric layer 20. Further, the second conductor layer 40 is formed on the second surface 22 of the first dielectric layer 20. The method for forming the first conductor layer 30 and the second conductor layer 40 is not particularly limited. For example, the first conductor layer 30 and the second conductor layer 40 may be formed by a plating method, a vapor deposition method, a sputtering method, or the like. The first conductor layer 30 and the second conductor layer 40 may be formed by attaching the metal foil to the first dielectric layer 20. The first conductor layer 30 and the second conductor layer 40 may be formed by printing the conductive paste on the first dielectric layer 20. The method for forming the first conductor layer 30 and the method for forming the second conductor layer 40 may be the same or different.

Subsequently, a patterning step of processing the first conductor layer 30 and dividing the first conductor layer 30 into the plurality of first conductor layers 30 described above is carried out. For example, as shown in FIG. 5, a plurality of resist layers 37 are formed on the first conductor layer 30. The positions of the plurality of resist layers 37 correspond to the positions of the plurality of first conductor layers 30 shown in FIGS. 1 and 2, respectively. Subsequently, the first conductor layer 30 is processed by wet etching using the resist layer 37 as a mask. As a result, as shown in FIG. 6A, a plurality of first conductor layers 30 arranged in the in-plane direction of the first surface 21 can be obtained.

6B and 6C are examples of cross-sectional shapes of the first conductor layer 30 and the resist layer 37 of FIG. 6A, respectively. As shown in FIG. 6B, the first conductor layer 30 may include a portion whose dimensions decrease toward the first surface 21. As shown in FIG. 6C, the first conductor layer 30 may include a portion whose dimensions increase toward the first surface 21. Although not shown, the dimensions of the first conductor layer 30 may be constant regardless of the distance from the first surface 21. In any case, the dimension of the first conductor layer 30 at the position closest to the first surface 21 affects the absorption characteristics of the radio wave absorbing sheet 10. Therefore, the above-mentioned dimension S1 is measured at the position closest to the first surface 21.

After that, the resist layer 37 is removed. In this way, the radio wave absorbing sheet 10 including the first dielectric layer 20, the plurality of first conductor layers 30, and the second conductor layer 40 can be manufactured.

According to the present embodiment, since the radio wave is absorbed by using resonance, the thickness of the first dielectric layer 20 can be made smaller than that of the conventional radio wave absorber. Therefore, the thickness of the entire radio wave absorbing sheet 10 can be reduced. As a result, the radio wave absorbing sheet 10 becomes lighter.

FIG. 7 is a diagram showing an example of a communication device 100 provided with a radio wave absorbing sheet 10. The communication device 100 includes a housing 110, a communication mechanism 120 located inside the housing 110 to transmit or receive radio waves, and a control mechanism 130 located inside the housing 110 to control the communication mechanism 120. To prepare for. The communication mechanism 120 transmits or receives the radio wave E having the above-mentioned frequency targeted by the radio wave absorption sheet 10.

In FIG. 7, dotted lines with reference numerals 10A to 10F show an example of the arrangement of the radio wave absorbing sheet 10, respectively. For example, as indicated by reference numeral 10A, the radio wave absorbing sheet 10 may be arranged between the communication mechanism 120 and the control mechanism 130. As shown by reference numeral 10B, the radio wave absorbing sheet 10 may be arranged between the front surface 111 of the housing 110 and the communication mechanism 120. As shown by reference numeral 10C, the radio wave absorbing sheet 10 may be attached to the front surface 111 of the housing 110. As shown by reference numeral 10D, the radio wave absorbing sheet 10 may be arranged between the rear surface 112 of the housing 110 and the communication mechanism 120. As indicated by reference numeral 10E, the radio wave absorbing sheet 10 may be attached to the rear surface 112 of the housing 110. As shown by reference numeral 10F, the radio wave absorbing sheet 10 may be attached to the side surface 113 of the housing 110.

According to the present embodiment, since the thickness of the radio wave absorbing sheet 10 is small, the degree of freedom in arranging the radio wave absorbing sheet 10 inside the housing 110 is high.

It is possible to make various changes to the above-described embodiment. Hereinafter, modification examples will be described with reference to the drawings as necessary. In the following description and the drawings used in the following description, the same reference numerals as those used for the corresponding portions in the first embodiment will be used for the portions that can be configured in the same manner as in the above-described embodiment. , Omit duplicate explanations. Further, when it is clear that the action and effect obtained in the above-described embodiment can be obtained in the modified example, the description thereof may be omitted.

(First modification)
FIG. 8 is a plan view showing the radio wave absorbing sheet 10 according to the first modification. As shown in FIG. 8, the first conductor layer 30 may be a rectangle having a length L and a width W in a plan view. The width W may be the same as the length L or may be smaller than the length L.

In the example shown in FIG. 8, the side of the first conductor layer 30 having the length L extends in the first direction D1, and the side of the first conductor layer 30 having the width W extends in the second direction D2. It is extended. Although not shown, the side of the first conductor layer 30 having a length L may extend in a direction different from that of the first direction D1.

FIG. 9 is a perspective view showing the unit cell 12 in this modified example. The unit cell 12 includes one rectangular first conductor layer 30. As in the case of the above-described embodiment, the characteristics of the resonance circuit realized by the radio wave absorbing sheet 10 are determined based on the configuration of the unit cell 12.

In this modification, the resonance frequency fr is calculated based on the following equations (B1), (B2), (B3), and (B4).

_{The ε re} (L) of the equation (B2) is calculated by substituting L for x of the equation (B3). _{The ε re} (W) of the equation (B2) is calculated by substituting W for x in the equation (B3). _Le is the effective length of the first conductor layer 30 when the fringe effect at the end of the first conductor layer 30 is taken into consideration.

Also in this modification, the frequency of the radio wave targeted by the radio wave absorbing sheet 10 is 20 GHz or more and 110 GHz or less. In this case, the radio wave absorbing sheet 10 is configured so that the resonance frequency fr is 20 GHz or more and 110 GHz or less. For example, when the frequency of the radio wave targeted by the radio wave absorbing sheet 10 is 79 GHz, the thickness h and the relative permittivity ε _{r of the first dielectric layer 20 so that the resonance frequency fr becomes the same as or close to 79 GHz.} Further, the length L and the width W of the first conductor layer 30 are determined. When the thickness h and the relative permittivity ε _{r of} the first dielectric layer 20 are predetermined, the length L and the width W of the first conductor layer 30 are determined so that the resonance frequency fr becomes a desired value. To. As a result, the target radio wave can be absorbed by utilizing resonance.

In this modification, N ² and R are calculated based on the following equations (B5) and (B6).

K is a constant. The LW / P of the formula (B5) represents the occupancy rate of the first conductor layer 30 on the first surface 21. The formula (B5) means that the occupancy rate of the first conductor layer 30 and N ² are in a proportional relationship.

Also wave absorption sheet 10 of this modification, preferably, configured so that N 2 ^R is the same as 120Pai, or approximate. For example, the dielectric loss tangent tan δ of the first dielectric layer 20, the length L of the first conductor layer 30, the width W, and the unit cell 12 so that the ratio ^{of N 2 R to 120 π is 0.8 or more and 1.2 or less.} Area P is determined. When the length L and width W of the dielectric loss tangent tan δ of the first dielectric layer 20 and the first conductor layer 30 have already been determined, the unit cell 12 so that the ratio ^{of N 2 R to 120 π becomes a desired value.} Area P is adjusted. For example, the first period P1 and the second period P2 of the first conductor layer 30 are adjusted. As a result, the radio wave absorbing sheet 10 can effectively absorb radio waves.

(Second modification)
FIG. 10 is a plan view showing the radio wave absorbing sheet 10 according to the second modification. As shown in FIG. 10, the first conductor layer 30 may include a plurality of openings 33. As a result, the radio wave absorbing sheet 10 becomes lighter. Further, a part of visible light can pass through the first conductor layer 30.

The specific configuration of the first conductor layer 30 including the opening 33 is arbitrary. For example, the first conductor layer 30 may be in the form of a so-called mesh, which is formed by combining a plurality of linear layers.

Although not shown, the second conductor layer 40 may also include a plurality of openings. As a result, a part of visible light can pass through the first conductor layer 30. When the first dielectric layer 20 is transparent, a part of visible light can pass through the radio wave absorbing sheet 10. As a result, it is possible to prevent the radio wave absorbing sheet 10 from being conspicuous.

(Third modification example)
FIG. 11 is a plan view showing the radio wave absorbing sheet 10 according to the third modification. As shown in FIG. 11, the shapes of the plurality of first conductor layers 30 may be different. For example, the plurality of first conductor layers 30 may include a plurality of first shape layers 31 and a plurality of second shape layers 32 having a shape different from that of the first shape layer 31 in a plan view. .. The first shape layer 31 has a dimension S11 in the first direction D1 and a dimension S12 in the second direction D2. The second shape layer 32 has a dimension S21 in the first direction D1 and a dimension S22 in the second direction D2. In the example shown in FIG. 10, the dimension S21 is larger than the dimension S11, and the dimension S22 is larger than the dimension S12.

The unit cell 12 including the first shape layer 31 constitutes a resonance circuit having a first resonance frequency fr1. The unit cell 12 including the second shape layer 32 constitutes a resonance circuit having a second resonance frequency fr2. The second resonance frequency fr2 is different from the first resonance frequency fr1. According to this modification, since the radio wave absorbing sheet 10 has two resonance frequencies, the radio wave absorbing sheet 10 can have two absorption peaks.

(Fourth modification)
FIG. 12 is a plan view showing the radio wave absorbing sheet 10 according to the fourth modification. FIG. 13 is a cross-sectional view showing a case where the radio wave absorbing sheet of FIG. 12 is viewed from the XIII-XIII direction. The radio wave absorbing sheet 10 includes a second dielectric layer 50 including a fourth surface 52 located on the opposite side of the third surface 51 and the third surface 51, and a third conductor layer 60 located on the fourth surface 52. May be provided. The second dielectric layer 50 is a first dielectric so that the second conductor layer 40 is located between the second surface 22 of the first dielectric layer 20 and the third surface 51 of the second dielectric layer 50. It is laminated on the layer 20. The third surface 51 of the second dielectric layer 50 may be in contact with the second conductor layer 40. The third conductor layer 60 faces the first conductor layer 30 and the second conductor layer 40 in the thickness direction of the first dielectric layer 20.

As shown in FIG. 12, the second conductor layer 40 may be divided into a plurality of pieces on the second surface 22 of the first dielectric layer 20. In other words, the radio wave absorbing sheet 10 may include a plurality of second conductor layers 40 located on the second surface 22 of the first dielectric layer 20. In this case, as shown in FIG. 12, the second conductor layer 40 does not have to face the first conductor layer 30 in the thickness direction of the first dielectric layer 20. The occupancy rate of the second conductor layer 40 on the second surface 22 is, for example, 0.85 or less, 0.75 or less, and 0.60 or less, as in the case of the first conductor layer 30. It may be 0.50 or less, or 0.30 or less. The occupancy rate may be 0.05 or more, or 0.10 or more. The occupancy rate of the second conductor layer 40 on the second surface 22 may be larger than the occupancy rate of the first conductor layer 30 on the first surface 21.

The second dielectric layer 50 contains a material having an insulating property. As the material of the second dielectric layer 50, the material exemplified by the first dielectric layer 20 can be used. The material of the second dielectric layer 50 may be the same as or different from the material of the first dielectric layer 20.

The thickness h2 of the second dielectric layer 50 may be, for example, 300 μm or less, 250 μm or less, 200 μm or less, 150 μm or less, or 100 μm or less. The thickness h2 may be 5 μm or more, 10 μm or more, 20 μm or more, or 50 μm or more.

The third conductor layer 60 contains a material having conductivity. As the material of the third conductor layer 60, the material exemplified by the first conductor layer 30 can be used. The material of the third conductor layer 60 may be the same as or different from the material of the first conductor layer 30.

The thickness T3 of the third conductor layer 60 may be 1 μm or more, or 5 μm or more. The thickness of the third conductor layer 60 may be 50 μm or less, or 20 μm or less.

The first conductor layer 30 and the third conductor layer 60, which face each other with the first dielectric layer 20 and the second dielectric layer 50 interposed therebetween, constitute a first resonance circuit having a first resonance frequency. do. Further, the second conductor layer 40 and the third conductor layer 60, which face each other with the second dielectric layer 50 interposed therebetween, form a second resonance circuit having a second resonance frequency. Also in this modification, since the radio wave absorbing sheet 10 has two resonance frequencies, the radio wave absorbing sheet 10 can have two absorption peaks.

(Fifth modification)
FIG. 14 is a plan view showing the radio wave absorbing sheet 10 according to the fifth modification. FIG. 15 is a cross-sectional view showing a case where the radio wave absorbing sheet of FIG. 14 is viewed from the XV-XV direction.

12 and 13 show an example in which the first conductor layer 30 and the second conductor layer 40 do not overlap in the thickness direction of the first dielectric layer 20. However, the present invention is not limited to this, and as shown in FIGS. 14 and 15, the first conductor layer 30 and the second conductor layer 40 may overlap in the thickness direction of the first dielectric layer 20. ..

The first conductor layer 30 has a dimension S1 in the first direction D1 and a dimension S2 in the second direction D2. The second conductor layer 40 has a dimension S3 in the first direction D1 and a dimension S4 in the second direction D2. The dimension S1 may be larger than the dimension S3. The dimension S2 may be larger than the dimension S4. As shown in FIG. 14, the second conductor layer 40 may have a contour surrounding the first conductor layer 30 in a plan view.

The first conductor layer 30 and the second conductor layer 40 facing each other with the first dielectric layer 20 interposed therebetween constitute a first resonance circuit having a first resonance frequency. Further, the second conductor layer 40 and the third conductor layer 60, which face each other with the second dielectric layer 50 interposed therebetween, form a second resonance circuit having a second resonance frequency. Also in this modification, since the radio wave absorbing sheet 10 has two resonance frequencies, the radio wave absorbing sheet 10 can have two absorption peaks.

Next, the embodiment of the present disclosure will be described more specifically by way of examples, but the embodiment of the present disclosure is not limited to the description of the following examples as long as the gist of the present disclosure is not exceeded.

(Example 1)
First, a laminated board in which copper foils were provided on both sides of a glass epoxy base material was prepared. The thickness of the glass epoxy base material was 100 μm, and the thickness of the copper foil was 12 μm. The glass epoxy substrate functions as the first dielectric layer 20 described above. The copper foil functions as the first conductor layer 30 and the second conductor layer 40 described above. The relative permittivity of the glass epoxy substrate was 4.2, and the dielectric loss tangent was 0.02.

Subsequently, the above-mentioned plurality of resist layers 37 were formed on one of the copper foils. Subsequently, the copper foil was processed by wet etching using the resist layer 37 as a mask. In this way, the radio wave absorbing sheet including the first dielectric layer 20, the plurality of first conductor layers 30 located on the first surface 21, and the second conductor layer 40 located on the second surface 22. 10 was made. The shape and arrangement of the first conductor layer 30 in a plan view are as follows.
-Shape of the first conductor layer 30: Circular with a radius of 0.506 mm-First period P1: 2.712 mm
・ Second cycle P2: 2.712 mm
The occupancy rate of the first conductor layer 30 is 0.109. The characteristic impedance N ² R of the secondary circuit 15 is calculated to be 120 π.

Subsequently, the absorption characteristics of the radio wave absorption sheet 10 were evaluated. Specifically, the amount of reflection attenuation of the radio wave in the radio wave absorbing sheet 10 was measured by the free space method. As a measuring instrument, a vector network analyzer PNA N5222B manufactured by Keysight and a frequency multiplier WR12 manufactured by Virginia Diodes were used. The WR12 is a device for expanding the frequency range of PNA N5222B.

In addition, the amount of reflection attenuation of the radio wave absorption sheet 10 was calculated based on the simulation. As the simulation software, HFSS manufactured by ANSYS was used. The measurement result by the free space method and the calculation result by the simulation were equivalent. The result of the simulation is shown in FIG. In the graph of FIG. 16, the horizontal axis indicates the frequency f, and the vertical axis indicates the reflection attenuation amount d. BW1 is the width of the absorption peak at the position where the reflection attenuation amount is −15 dB. The width BW1 was 0.56 GHz. The depth H of the absorption peak was 57.6 dB.

(Example 1B to Example 1G)
Under the condition that the cycles P1 and P2 of the first conductor layer 30 of Example 1A were changed, the absorption characteristics of the radio wave absorbing sheet 10 were evaluated based on the simulation. The cycles P1, P2 in Examples 1B, 1C, 1D, 1E, 1F and 1G are 2.1 mm, 2.3 mm, 2.5 mm, 2.7 mm, 2.9 mm and 3.1 mm. The shape of the first conductor layer 30 is the same as that of Example 1A. Therefore, the resonance frequency fr of the resonance circuit of the secondary circuit 15 is 79.4 GHz as in the case of Example 1A. On the other hand, the characteristic impedance N ² R of the secondary circuit 15 is different from that of Example 1A. Therefore, in the example of Examples 1B ~ Example 1G, the characteristic impedance N ^{2 R} of the secondary circuit 15 does not coincide with the the characteristic impedance of the primary circuit 14 120π.

The simulation results of Example 1A to Example 1G are summarized in FIG. Even when the characteristic impedance N ² R of the secondary circuit 15 does not match the characteristic impedance 120π of the primary circuit 14, an absorption peak occurs. ^{The larger the deviation between the characteristic impedance N 2} R of the secondary circuit 15 and 120 π, the smaller the depth of the absorption peak.

(Example 2)
The absorption characteristics of the radio wave absorption sheet 10 configured under the following conditions were evaluated based on a simulation. As the material of the first dielectric layer 20, the first conductor layer 30, and the second conductor layer 40, glass epoxy, copper, and copper are assumed as in the case of Example 1. The occupancy rate of the first conductor layer 30 is 0.403. The characteristic impedance N ² R of the secondary circuit 15 is calculated to be 120 π.
-Thickness of the first conductor layer 30: 12 μm
-Shape of the first conductor layer 30: Circular with a radius of 1.49 mm-First period P1: 4.16 mm
・ Second cycle P2: 4.16 mm
-Thickness of the second conductor layer 40: 12 μm
-Thickness of the first dielectric layer 20: 100 μm
Relative permittivity of the first dielectric layer 20: 4.2
Dielectric loss tangent of first dielectric layer 20: 0.02

The simulation results are shown in FIG. The resonance frequency fr was 28.2 GHz. The depth H of the absorption peak was 40.4 dB, and the width BW1 was 0.22 GHz.

(Example 3)
The absorption characteristics of the radio wave absorption sheet 10 configured under the following conditions were evaluated based on a simulation. As the material of the first dielectric layer 20, the first conductor layer 30, and the second conductor layer 40, glass epoxy, copper, and copper are assumed as in the case of Example 1. The occupancy rate of the first conductor layer 30 is 0.102. The characteristic impedance N ² R of the secondary circuit 15 is calculated to be 120 π.
-Thickness of the first conductor layer 30: 12 μm
-Shape of the first conductor layer 30: Square with a side length of 0.85 mm-First period P1: 2.66 mm
・ Second cycle P2: 2.66 mm
-Thickness of the second conductor layer 40: 12 μm
-Thickness of the first dielectric layer 20: 100 μm
Relative permittivity of the first dielectric layer 20: 4.2
Dielectric loss tangent of first dielectric layer 20: 0.02

The simulation results are shown in FIG. The resonance frequency fr was 79.35 GHz. The depth H of the absorption peak was 44.09 dB, and the width BW1 was 0.6 GHz.

10 Radio wave absorbing sheet 12 Unit cell 14 Primary circuit 15 Secondary circuit 16 Transformer 20 First dielectric layer 21 First surface 22 Second surface 30 First conductor layer 31 First shape layer 32 Second shape layer 33 Opening 35 Distribution area 37 Resist layer 40 Second conductor layer 50 Second dielectric layer 51 Third surface 52 Fourth surface 60 Third conductor layer 100 Communication device 110 Housing 120 Communication mechanism 130 Control mechanism D1 First direction D2 Second Direction P1 1st cycle P2 2nd cycle

Claims (15)

1. A first dielectric layer including a first surface and a second surface located on the opposite side of the first surface,
   A plurality of first conductor layers located on the first surface and arranged in the first direction,
   A second conductor layer located on the second surface is provided.
   A radio wave absorbing sheet in which the occupancy rate of the first conductor layer on the first surface is 0.85 or less.
2. The radio wave absorbing sheet according to claim 1, wherein the dimension of the first conductor layer in the first direction is less than 5.0 mm.
3. The radio wave absorbing sheet according to claim 1, wherein the thickness of the first dielectric layer is 300 μm or less.
4. The radio wave absorbing sheet according to claim 1, wherein the relative permittivity of the first dielectric layer is less than 20.
5. The radio wave absorbing sheet according to claim 1, wherein the dielectric loss tangent of the first dielectric layer is 0.2 or less.
6. The radio wave absorbing sheet according to claim 1, wherein the thickness of the radio wave absorbing sheet is 350 μm or less.
7. The radio wave absorption sheet according to any one of claims 1 to 6, which has a resonance frequency fr located in the range of 20 GHz or more and 110 GHz or less.
8. The first conductor layer is a circle having a radius r in a plan view.
   The resonance frequency fr is calculated based on the following equations (A1) and (A2).
   

   

   The radio wave absorption sheet according to claim 7, wherein c is the speed of light, ε _r is the relative permittivity of the first dielectric layer, and h is the thickness of the first dielectric layer.
9. The ratio of N ² R to 120 π is 0.8 or more and 1.2 or less.
   N ² and R are calculated based on the following formulas (A3) and (A4).
   

   

   The radio wave absorption sheet according to claim 8, wherein P is the area of the unit cell, K is a constant, and tan δ is the dielectric loss tangent of the first dielectric layer.
10. The first conductor layer is a rectangle having a length L and a width W in a plan view, and the width W is a length L or less.
    The resonance frequency fr is calculated based on the following equations (B1), (B2), (B3), and (B4).
    

    

    The radio wave absorption sheet according to claim 7, wherein c is the speed of light, ε _r is the relative permittivity of the first dielectric layer, and h is the thickness of the first dielectric layer.
11. The ratio of N ² R to 120 π is 0.8 or more and 1.2 or less.
    N ² and R are calculated based on the following equations (B5) and (B6).
    

    

    The radio wave absorption sheet according to claim 10, wherein P is the area of the unit cell, K is a constant, and tan δ is the dielectric loss tangent of the first dielectric layer.
12. The radio wave absorption sheet according to any one of claims 1 to 6, wherein the first conductor layer includes a plurality of openings.
13. Any of claims 1 to 6, wherein the plurality of first conductor layers include a plurality of first shape layers and a plurality of second shape layers having a shape different from that of the first shape layer in a plan view. The radio wave absorption sheet described in item 1.
14. A second dielectric layer including a third surface and a fourth surface located on the opposite side of the third surface, and
    A third conductor layer located on the fourth surface is provided.
    The second conductor layer is located between the second surface of the first dielectric layer and the third surface of the second dielectric layer.
    The radio wave absorption sheet according to any one of claims 1 to 6, wherein the occupancy rate of the second conductor layer on the second surface is 0.85 or less.
15. A communication mechanism that transmits or receives radio waves,
    A communication device comprising the radio wave absorbing sheet according to any one of claims 1 to 6.

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