Patent Publication Number: US-2023139063-A1

Title: Impedance-matching membrane and radio-wave-absorbing body

Description:
TECHNICAL FIELD 
     The present invention relates to an impedance matching film and a radio wave absorber. 
     BACKGROUND ART 
     Conventionally, the technology of matching the impedance of the surface of a radio wave absorber to the characteristic impedance of air by using a predetermined film has been known. Meanwhile, hitherto, there have been attempts to provide transparent radio wave absorbers. 
     For example, Patent Literature 1 describes a radio wave absorber having transparency. In the radio wave absorber, a mesh surface-like object made of conductive fibers is used as a resistive layer. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2000-232320 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     It is thought that impedance matching films having transparency will be required for sensing using high-frequency radio waves such as millimeter waves. In addition, it is thought that impedance matching films having transparency will be required for a wide range of technological fields such as 5th generation mobile communication systems (5G) and the Internet of Things (loT). 
     In order to provide an impedance matching film that can handle high-frequency radio waves and has transparency, it is conceivable to form a plurality of openings in an impedance matching film. In this case, diffraction and interference of light may cause an iridescent pattern in the impedance matching film. It is difficult to say that this is advantageous in terms of the appearance of the impedance matching film. In the technology described in Patent Literature 1, occurrence of such an iridescent pattern is not considered. 
     In view of such circumstances, the present invention provides an impedance matching film that can handle high-frequency radio waves, has transparency, and is advantageous in terms of reducing spatial variations in characteristics related to impedance matching while suppressing occurrence of an iridescent pattern. 
     Solution to Problem 
     The present invention provides an impedance matching film including a plurality of domains, wherein 
     each of the domains has a plurality of openings having different shapes, 
     the pluralities of openings are periodically arranged in a specific direction along a main surface of the impedance matching film in the plurality of domains, and 
     each of sizes of the domains in the specific direction is 50 μm or more. 
     In addition, the present invention provides a radio wave absorber including: 
     the above impedance matching film; 
     a reflector for reflecting radio waves; and 
     a dielectric layer disposed between the impedance matching film and the reflector in a thickness direction of the impedance matching film. 
     Advantageous Effects of Invention 
     The above impedance matching film can handle high-frequency radio waves, has transparency, and is advantageous in terms of suppressing occurrence of an iridescent pattern and spatial variations in characteristics related to impedance matching. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a plan view showing an example of an impedance matching film according to the present invention. 
         FIG.  2    is a plan view showing a domain in the impedance matching film shown in  FIG.  1   . 
         FIG.  3    is a cross-sectional view taken along a line III-III shown in  FIG.  1   . 
         FIG.  4 A  is a plan view showing another example of the domain. 
         FIG.  4 B  is a plan view showing still another example of the domain. 
         FIG.  4 C  is a plan view showing still another example of the domain. 
         FIG.  4 D  is a plan view showing still another example of the domain. 
         FIG.  4 E  is a plan view showing still another example of the domain. 
         FIG.  4 F  is a plan view showing still another example of the domain. 
         FIG.  4 G  is a plan view showing still another example of the domain. 
         FIG.  4 H  is a plan view showing still another example of the domain. 
         FIG.  5 A  is a cross-sectional view showing an example of a radio wave absorber according to the present invention. 
         FIG.  5 B  is a cross-sectional view showing a modification of the radio wave absorber according to the present invention. 
         FIG.  5 C  is a cross-sectional view showing another modification of the radio wave absorber according to the present invention. 
         FIG.  6    is a cross-sectional view showing another example of the radio wave absorber according to the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     If an impedance matching film has a plurality of openings, this is advantageous in terms of imparting transparency to the impedance matching film. For example, if such an impedance matching film can handle high-frequency radio waves, the value of the impedance matching film can be further enhanced. Therefore, the present inventors have studied intensively on an impedance matching film that can handle high-frequency radio waves and has a plurality of openings. In the course of this study, the present inventors have noticed that, when openings having a predetermined size are formed at equal intervals in an impedance matching film, an iridescent pattern is likely to occur due to diffraction and interference of light. The present inventors have further studied and found that occurrence of an iridescent pattern can be suppressed by forming a plurality of openings having different shapes in an impedance matching film. Meanwhile, according to the study by the present inventors, in order to suppress spatial variations in impedance matching, it is also important that the areas of the openings do not considerably vary spatially. Therefore, the present inventors have conducted a great deal of trial and error, have found a condition for suppressing occurrence of an iridescent pattern in an impedance matching film and suppressing spatial variations in characteristics related to impedance matching, and have conceived of an impedance matching film according to the present invention. As used herein, “transparency” means transparency to visible light, unless otherwise described. 
     Embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the following embodiments. 
     As shown in  FIG.  1   , an impedance matching film  10  includes a plurality of domains  11 . As shown in  FIG.  2   , each domain  11  has a plurality of openings  12  having different shapes. The pluralities of openings  12  are periodically arranged in a specific direction along main surfaces  10   f  of the impedance matching film  10 . The size of each domain  11  in the specific direction is 50 μm or more. The domain  11  is a region that has a plurality of openings  12  having different shapes and is delimited, for example, on the basis of the periodicity of the periodic arrangement in a specific direction of the pluralities of openings  12  having different shapes. As used herein, “having different shapes” means that, even when one of two openings  12  to be compared is shifted parallel, the one of the two openings  12  does not completely overlap the other of the two openings  12 . For example, when one of two openings  12  to be compared is shifted parallel so as to overlap the other of the two openings  12 , if the area of the overlapping portion of the two openings  12  is 90% or less of the area of the one of the two openings  12 , the one of the two openings  12  can be regarded as not completely overlapping the other of the two openings  12 . The size of each domain  11  in the specific direction is the distance between the ends of the pluralities of openings  12  in the specific direction of the two domains  11  adjacent to each other in the specific direction. 
     In an impedance matching film, when openings having a predetermined size and the same shape are formed at equal intervals, an iridescent pattern is likely to occur due to diffraction and interference of light. On the other hand, in the impedance matching film  10 , since the pluralities of openings  12  having different shapes are periodically arranged in the specific direction, the sizes of the openings  12  in the specific direction vary periodically. In addition, since the impedance matching film  10  has the plurality of domains  11  such that the pluralities of openings  12  are periodically arranged in the specific direction along the main surfaces  10   f  of the impedance matching film  10 , the areas of the openings  12  do not considerably vary spatially. Therefore, in the impedance matching film  10 , spatial variations in characteristics related to impedance matching is easily suppressed. 
     In the impedance matching film  10 , the specific direction may include a plurality of alignment directions intersecting each other. The pluralities of openings  12  are periodically arranged, for example, in three alignment directions Da, Db, and Dc intersecting each other. Accordingly, in a wide range of the impedance matching film  10 , occurrence of an iridescent pattern can be suppressed, and spatial variations in characteristics related to impedance matching can be reduced. The plurality of alignment directions intersecting each other may be orthogonal to each other. For example, any one of a size La of each domain  11  in the alignment direction Da, a size Lb of each domain  11  in the alignment direction Db, and a size Lc of each domain  11  in the alignment direction Dc is 50 μm or more. Desirably, the sizes La, Lb, and Lc are 50 μm or more. 
     A frame defining the pluralities of openings  12  may, for example, form a plurality of linear patterns aligned in the specific direction such that the linear patterns appear in the alignment direction Da and the alignment direction Db, or may form a plurality of dotted patterns aligned in the specific direction such that the dotted patterns appear in the alignment direction Dc. In either case, since the size of each domain  11  in the specific direction is 50 μm or more, occurrence of an iridescent pattern can be suppressed. Meanwhile, in the case where the frame defining the pluralities of openings  12  forms linear patterns in the specific direction, the size of each domain  11  in the specific direction is more desirably 50 μm or more in terms of suppressing occurrence of an iridescent pattern. 
     As shown in  FIG.  2   , for example, the openings  12  are adjacent to each other in each domain  11 . In other words, the shapes of the openings  12  adjacent to each other are different from each other. Accordingly, occurrence of an iridescent pattern is more easily suppressed in the impedance matching film  10 . 
     The upper limit of the size of each domain  11  in the specific direction is not limited to a specific value. 
     An average value PA of the perimeters of the plurality of openings  12  included in each domain  11  is not limited to a specific value. The average value P A  is, for example, 5000 μm or less. Accordingly, the impedance matching film  10  easily handles high-frequency radio waves. The average value P A  can be determined by dividing the total of all the perimeters of the plurality of openings  12  included in each domain  11  by the number of the plurality of openings  12 . 
     The average value P A  is desirably 4000 μm or less, more desirably 3000 μm or less, further desirably 2000 μm or less, and particularly desirably 1000 μm or less. The average value PA is, for example, 10 μm or more, and may be 20 μm or more, may be 50 μm or more, or may be 100 μm or more. 
     As shown in  FIG.  2   , all the shapes of the plurality of openings  12  included in the domain  11  may be different from each other. Accordingly, occurrence of an iridescent pattern is more easily suppressed in the impedance matching film  10 . 
     The number of openings  12  included in each domain  11  is not limited to a specific value. The number of openings  12  included in each domain  11  is, for example, 3 or more, and may be 4 or more, or may be 5 or more. The number of openings  12  may be a square number or an integer other than a square number. 
     The shape of each domain  11  is not limited to a specific shape. The shape of each domain  11  is, for example, a polygonal shape formed such that the domains  11  having the same shape exist on a plane without gaps therebetween. The shape of each domain  11  is, for example, a square shape or a rectangular shape. 
     The sheet resistance of the impedance matching film  10  is not limited to a specific value. The impedance matching film  10  has, for example, a sheet resistance of 200 to 1000 Ω/□. Accordingly, good impedance matching for high-frequency radio waves is easily performed by the impedance matching film  10 . The sheet resistance of the impedance matching film  10  can be measured, for example, according to the eddy current method. 
     The sheet resistance of the impedance matching film  10  may be 250 Ω/□ or more, may be 300 Ω/□ or more, or may be 350 Ω/□ or more. The sheet resistance of the impedance matching film  10  may be 950 Ω/□ or less, may be 900 Ω/□ or less, or may be 850 Ω/□ 0  or less. 
     A thickness t of the impedance matching film  10  is not limited to a specific value. The thickness t is, for example, 5 nm or more. In this case, the sheet resistance of the impedance matching film  10  is less likely to vary over a long period of time, and the impedance matching film  10  easily exhibits high durability. 
     The thickness t of the impedance matching film  10  may be 10 nm or more or may be 15 nm or more. The thickness t is, for example, 500 nm or less. Accordingly, warpage of the impedance matching film  10  is easily suppressed, so that cracks are less likely to occur in the impedance matching film  10 . The thickness t may be 450 nm or less or may be 400 nm or less. 
     An opening ratio in the impedance matching film  10  is not limited to a specific value. The opening ratio in the impedance matching film  10  may be, for example, 40% or more. On the other hand, the opening ratio in the impedance matching film  10  is desirably 65% or more. Accordingly, the impedance matching film  10  easily has high transparency. The opening ratio in the impedance matching film  10  is a ratio Sa/(Sa+Sb) of an opening area Sa of the pluralities of opening  12  to a sum Sa+Sb of the opening area Sa of the pluralities of opening  12  and an area Sb of the non-opening portion of the impedance matching film  10  when the impedance matching film  10  is viewed in a plan view. 
     The opening ratio in the impedance matching film  10  is more desirably 70% or more and further desirably 75% or more. The opening ratio in the impedance matching film  10  is, for example, 99% or less, and may be 98% or less, or may be 97% or less. 
     The shapes of the pluralities of openings  12  are not limited to specific shapes. The pluralities of openings  12  each have, for example, a square shape or a rectangular shape in a plan view. 
     The material forming the impedance matching film  10  is not limited to a specific material. The material forming the impedance matching film  10  may be an inorganic material such as metals, alloys, and metal oxides, or may be an organic material such as electroconductive polymers and carbon nanotubes. 
     The impedance matching film  10  may be a film having a plurality of through holes formed therein and having a uniform thickness, or may be a woven fabric. The fiber forming the woven fabric may be an organic material such as electroconductive polymers and carbon nanotubes, or may be an inorganic material such as metals and alloys. 
     As shown in  FIG.  3   , the impedance matching film  10  may be formed, for example, on one main surface of a substrate  22 . In this case, the impedance matching film  10  can be provided by an impedance matching film-attached film  15 . The impedance matching film  10  may be provided alone without using the substrate  22 . 
     The substrate  22  serves, for example, as a support for supporting the impedance matching film  10 . The impedance matching film  10  in the impedance matching film-attached film  15  can be produced, for example, by forming the plurality of openings  11  by laser processing, etching, or the like in a non-porous film formed on one main surface of the substrate  22  by a film forming method such as sputtering. In some cases, a non-porous film for the impedance matching film  10  may be formed by a film forming method such as ion plating or coating (for example, bar coating). 
     The substrate  22  has, for example, a thickness of 10 to 150 μm, and desirably has a thickness of 15 to 100 μm. Accordingly, the flexural rigidity of the substrate  22  is low, and it is possible to suppress wrinkling or deformation of the substrate  22  when forming the impedance matching film  10 . 
     Each domain  11  may be configured, for example, as shown in  FIG.  4 A ,  FIG.  4 B ,  FIG.  4 C ,  FIG.  4 D ,  FIG.  4 E ,  FIG.  4 F ,  FIG.  4 G , and  FIG.  4 H . 
     As shown in  FIG.  4 A  to  FIG.  4 E , the domain  11  may have openings  12  having the same shape as long as the domain  11  has a plurality of openings  12  having different shapes. In addition, as shown in  FIG.  4 C  to  FIG.  4 E , openings  12  having the same shape may be adjacent to each other. Even in such a case, occurrence of an iridescent pattern is easily suppressed as compared to an impedance matching film having only openings having the same shape and formed at equal intervals as the openings. 
     As shown in  FIG.  4 F , in the domain  11 , the shape of each opening  12  may be a circular shape. In this case, the shape of the domain  11  is, for example, a parallelogram shape. On the other hand, the shape of the domain  11  may be a square shape or a rectangular shape. 
     As shown in  FIG.  4 G , in the domain  11 , the shape of each opening  12  may be a regular hexagonal shape. In this case, the shape of the domain  11  is, for example, a parallelogram shape. On the other hand, the shape of the domain  11  may be a square shape or a rectangular shape. 
     As shown in  FIG.  4 H , in the domain  11 , the shape of each opening  12  may be a triangular shape. In this case, the shape of the domain  11  is, for example, a hexagonal shape formed by linearly symmetrically arranging a pair of parallelograms such that the parallelograms share one side. 
     In the domain  11 , the shape of each opening  11  may be another polygonal shape or an elliptical shape. The shape of the domain  11  may be another polygonal shape. 
     As shown in  FIG.  1   , the impedance matching film  10  is formed, for example, by only one type of the domains  11 . The impedance matching film  10  may be formed by two or more types of domains  11 . In this case, in the specific direction, the same type of domains  11  may be adjacent to each other, or a plurality of types of domains  11  may alternately exist. 
     As shown in  FIG.  5 A , a radio wave absorber  1   a  can be provided, for example, using the impedance matching film  10 . The radio wave absorber  1   a  includes the impedance matching film  10 , a reflector  30  for reflecting radio waves, and a dielectric layer  20 . The dielectric layer  20  is disposed between the impedance matching film 10  and the reflector  30  in the thickness direction of the impedance matching film  10 . 
     The radio wave absorber  1   a  is, for example, a λ/4 radio wave absorber. The radio wave absorber  1   a  is designed such that, when radio waves of a wavelength λ 0  to be absorbed by the radio wave absorber  1   a  are incident on the radio wave absorber  1   a , radio waves resulting from reflection on the front surface of the impedance matching film  10  (front surface reflection) and radio waves resulting from reflection on the reflector  30  (back surface reflection) interfere with each other. In the λ/4 radio wave absorber, as shown in the following equation (1), the wavelength λ 0  of the radio waves to be absorbed is determined according to a thickness t of the dielectric layer and a relative permittivity ε r  of the dielectric layer. That is, the radio waves of the wavelength to be absorbed can be set by adjusting the relative permittivity and the thickness of the dielectric layer as appropriate. In the equation (1), sqrt(ε r ) means the square root of the relative permittivity ε r . 
       λ 0 =4 t ×sqrt(ε r )   Equation (1)
 
     The radio wave absorber  1   a  is configured to be able to absorb radio waves in a predetermined frequency range of 10 GHz or more, for example. Examples of the frequency ranges of radio waves that can be absorbed by the radio wave absorber  1   a  are as follows. The following radio waves are under consideration for use as radio waves for 5G in various countries.
     27.5 to 29.5 GHz   27.5 to 28.35 GHz   24.25 to 24.45 GHz   24.75 to 25.25 GHz   37 to 38.6 GHz   38.6 to 40 GHz   47.2 to 48.2 GHz   64 to 71 GHz   24.25 to 27.5 GHz   40.5 to 43.5 GHz   66 to 71 GHz   24.75 to 27.5 GHz   37 to 42.5 GHz   27.5 to 29.5 GHz   31.8 to 33.4 GHz   37 to 40.5 GHz   

     Other examples of the frequency ranges of the radio waves that can be absorbed by the radio wave absorber  1   a  are as follows. The following radio waves can be used as radio waves for a millimeter wave radar.
     21.65 to 26.65 GHz   60 to 61 GHz   76 to 77 GHz   77 to 81 GHz   94.7 to 95 GHz   139 to 140 GHz   

     The radio wave absorber  1   a  has, for example, an absorption peak frequency of 10 GHz or more. This allows absorption of desired high-frequency radio waves. 
     An absorption peak frequency f P  is the frequency of a radio wave whose return loss |S| is the maximum for the radio wave absorber  1   a . The return loss |S| is the absolute value of S calculated by the following equation (2). In the equation (2), P 0  is the power of transmitted radio waves when radio waves are incident on a measurement target at a predetermined incident angle, and P i  is the power of received radio waves in this case. The value of the return loss |S| for the radio wave absorber  1   a  is determined, for example, with the value of the return loss |S| when radio waves are incident on a plate of a reference metal such as aluminum at a predetermined incident angle being regarded as 0 dB. In the radio wave absorber  1   a , front surface reflection of radio waves having the absorption peak frequency f p  occurs properly, and the radio wave absorber  1   a  can satisfactorily absorb the radio waves having the absorption peak frequency f P . 
         S [dB]=10×log| P   i   /P   0 |  Equation (2)
 
     The radio wave absorber  1   a  exhibits, for example, a return loss of 10 dB or more, and desirably exhibits a return loss of 20 dB or more, in a predetermined frequency range of 10 GHz or more. 
     The reflector  30  is not limited to a specific form as long as the radio waves to be absorbed can be reflected. The reflector  30  is, for example, a transparent conductive film. In this case, the reflector  30  has transparency, and the entire radio wave absorber  1   a  is easily made transparent. The material forming the transparent conductive film may be an inorganic material such as metals including aluminum, etc., alloys, and metal oxides, or may be an organic material such as electroconductive polymers and carbon nanotubes. The reflector  30  may be an opaque conductive film. The material forming such a conductive film may be an inorganic material such as metals including aluminum, etc., alloys, and metal oxides, or may be an organic material such as electroconductive polymers and carbon nanotubes. 
     The transparent conductive film has, for example, a plurality of openings  31  formed along main surfaces of the transparent conductive film. This configuration allows the reflector  30  to properly reflect radio waves to be absorbed and makes it easier for the reflector  30  to have desired transparency. The transparent conductive film may be a non-porous film. 
     In the case where the reflector  30  has the plurality of openings  31 , the impedance matching film  10  may be a film having a plurality of through holes formed therein and having a uniform thickness, or may be a woven fabric. The fiber forming the woven fabric may be an organic material such as electroconductive polymers and carbon nanotubes, or may be an inorganic material such as metals and alloys. 
     The shapes of the plurality of openings  31  in the reflector  30  are not limited to specific shapes. Each of the shapes of the plurality of openings  31  may be, for example, a triangular shape, a quadrilateral shape such as a square shape and a rectangular shape, a hexagonal shape, another polygonal shape, a circular shape, or an elliptical shape in a plan view. 
     The arrangement of the plurality of openings  31  in the reflector  30  is not limited to a specific arrangement. The plurality of openings  31  may be arranged such that, for example, the centers of the plurality of openings  31  form a planar lattice such as a square lattice and a parallelogram lattice. 
     The dielectric layer  20  has, for example, a relative permittivity of 2.0 to 20.0. In this case, it is easy to adjust the thickness of the dielectric layer  20 , and it is easy to adjust the radio wave absorption performance of the radio wave absorber  1   a . The relative permittivity of the dielectric layer  20  is, for example, a relative permittivity at 10 GHz measured according to the cavity resonance method. 
     The dielectric layer  20  is formed, for example, from a predetermined polymer. The dielectric layer  20  contains, for example, at least one polymer selected from the group consisting of ethylene-vinyl acetate copolymer, vinyl chloride resin, urethane resin, acrylic resin, acrylic urethane resin, acrylic-based elastomer, polyethylene, polypropylene, silicone, polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyimide, and cycloolefin polymer. In this case, it is easy to adjust the thickness of the dielectric layer  20 , and the production cost of the radio wave absorber  1   a  can be kept low. The dielectric layer  20  can be produced, for example, by hot-pressing a predetermined resin composition. 
     The dielectric layer  20  may be formed as a single layer, or may be formed of a plurality of layers made of the same material or different materials. In the case where the dielectric layer  20  has n layers (n is an integer equal to or greater than 2), the relative permittivity of the dielectric layer  20  is determined as follows, for example. A relative permittivity ε i  of each layer is measured (i is an integer from 1 to n). Next, ε i ×(t i /T) is obtained by multiplying the measured relative permittivity ε i  of each layer by the ratio of a thickness t i  of the layer to a total thickness T of the dielectric layer  20 . The relative permittivity of the dielectric layer  20  can be determined by adding up ε i ×(t i /T) of all the layers. 
     As shown in  FIG.  5 A , the dielectric layer  20  includes, for example, a first layer  21 , a second layer  22 , and a third layer  23 . The first layer  21  is disposed between the second layer  22  and the third layer  23 . The first layer  21  contains, for example, at least one polymer selected from the group consisting of ethylene-vinyl acetate copolymer, vinyl chloride resin, urethane resin, acrylic resin, acrylic urethane resin, polyethylene, polypropylene, silicone, polyethylene terephthalate, polyethylene naphthalate, polycarbonate, polyimide, and cycloolefin polymer. 
     In the radio wave absorber  1   a , the second layer  22  serves as a substrate for the impedance matching film  10 . The second layer  22  is disposed, for example, at a position closer to the reflector  30  than the impedance matching film  10  is. As shown in  FIG.  5 B , the second layer  22  may be disposed at a position farther from the reflector  30  than the impedance matching film  10  is. In this case, the dielectric layer  20  is composed of the first layer  21  and the third layer  23 . In this case, the impedance matching film  10  and the dielectric layer  20  are protected by the second layer  22 , and the radio wave absorber  1   a  has high durability. In this case, for example, the impedance matching film  10  may be in contact with the first layer  21 . The material of the second layer  22  is, for example, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), acrylic resin (PMMA), polycarbonate (PC), polyimide (PI), or cycloolefin polymer (COP). Among them, the material of the second layer  22  is desirably PET in terms of the balance among good heat resistance, dimensional stability, and manufacturing cost. 
     In the radio wave absorber  1   a , the third layer  23  supports the reflector  30 , for example. In this case, the reflector  30  may be produced, for example, by forming a film on the third layer  23  using a method such as sputtering, ion plating, or coating (for example, bar coating). Furthermore, the plurality of openings  31  may be formed by laser processing, etching, or the like. As shown in  FIG.  5 A , the third layer  23  is disposed, for example, at a position closer to the impedance matching film  10  in the radio wave absorber  1   a  than the reflector  30  is, and forms a part of the dielectric layer  20 . As shown in  FIG.  5 C , the third layer  23  may be disposed at a position farther from the impedance matching film  10  than the reflector  30  is. In this case, for example, the reflector  30  is in contact with the first layer  21 . 
     As the material of the third layer  23 , for example, the materials exemplified as the material of the second layer  22  can be used. The material of the third layer  23  may be the same as or different from the material of the second layer  22 . The material of the third layer  23  is desirably PET in terms of the balance among good heat resistance, dimensional stability, and manufacturing cost. 
     The third layer  23  has, for example, a thickness of 10 to 150 μm, and desirably has a thickness of 15 to 100 μm. Accordingly, the flexural rigidity of the third layer  23  is low, and it is possible to suppress wrinkling or deformation of the third layer  23  when forming the reflector  30 . The third layer  23  may be omitted in some cases. 
     The first layer  21  may be composed of a plurality of layers. In particular, in the case where the first layer  21  is in contact with at least one of the impedance matching film  10  and the reflector  30  as shown in  FIG.  5 B  or  FIG.  5 C , the first layer  21  can be composed of a plurality of layers. 
     The first layer  21  may have adhesiveness, or may not necessarily have adhesiveness. In the case where the first layer  21  has adhesiveness, an adhesive layer may be disposed in contact with at least one of both main surfaces of the first layer  21 , or adhesive layers may not necessarily be disposed in contact with both main surfaces of the first layer  21 , respectively. In the case where the first layer  21  does not have adhesiveness, adhesive layers are desirably disposed in contact with both main surfaces of the first layer  21 , respectively. In the case where the dielectric layer  20  includes the second layer  22 , even if the second layer  22  does not have adhesiveness, adhesive layers may not necessarily be disposed in contact with both main surfaces of the second layer  22 , respectively. In this case, an adhesive layer can be disposed in contact with one main surface of the second layer  22 . In the case where the dielectric layer  20  includes the third layer  23 , even if the third layer  23  does not have adhesiveness, adhesive layers may not necessarily be disposed in contact with both main surfaces of the third layer  23 , respectively. In this case, an adhesive layer can be disposed in contact with at least one main surface of the third layer  23 . Each adhesive layer contains, for example, a rubber-based adhesive agent, an acrylic-based adhesive agent, a silicone-based adhesive agent, or a urethane-based adhesive agent. The thickness of each adhesive layer containing the adhesive agent is not limited to a specific value, and is, for example, 3 to 50 μm, and desirably 5 to 30 μm. 
     The radio wave absorber la may contain at least one of a dielectric loss material and a magnetic loss material. In other words, the radio wave absorber  1   a  may be a dielectric loss radio wave absorber or a magnetic loss radio wave absorber. The dielectric layer  20  may contain at least one of a dielectric loss material and a magnetic loss material. The material forming the impedance matching film  10  may be magnetic. 
     The radio wave absorber  1   a  can be modified in various respects. For example, the radio wave absorber  1   a  may be modified into a radio wave absorber  1   b  shown in  FIG.  6   . The radio wave absorber  1   b  is configured in the same manner as the radio wave absorber  1   a  except for the portions that are particularly described. The components, of the radio wave absorber  1   b , identical to or corresponding to the components of the radio wave absorber  1   a  are indicated by the same reference characters, and the detailed descriptions thereof are omitted. The descriptions given for the radio wave absorber  1   a  are also applicable to the radio wave absorber  1   b  unless there is a technical inconsistency. 
     As shown in  FIG.  6   , the radio wave absorber  1   b  further includes an adhesive layer  40   a . In the radio wave absorber  1   b , the reflector  30  is disposed between the dielectric layer  20  and the adhesive layer  40   a.    
     For example, the radio wave absorber  1   b  can be adhered to a predetermined article by pressing the radio wave absorber  1   b  against the article with the adhesive layer  40   a  brought into contact with the article. Accordingly, a radio wave absorber-attached article can be obtained. 
     The adhesive layer  40   a  contains, for example, a rubber-based adhesive agent, an acrylic-based adhesive agent, a silicone-based adhesive agent, or a urethane-based adhesive agent. The radio wave absorber  1   b  may further include a release liner (not shown). In this case, the release liner covers the adhesive layer  40   a . The release liner is typically a film that can maintain the adhesive strength of the adhesive layer  40   a  when covering the adhesive layer  40   a  and that can easily be peeled from the adhesive layer  40   a . The release liner is, for example, a film made of polyester resin such as PET. By peeling the release liner, the adhesive layer  40   a  becomes exposed, allowing the radio wave absorber  1   b  to be adhered to an article. 
     EXAMPLES 
     Hereinafter, the present invention will be described in more detail by means of Example. The present invention is not limited to the following Example. First, evaluation methods for the Example and Comparative Example will be described. 
     [TEM Observation] 
     Cross-sectional observation samples of a non-porous film according to each of the Example and the Comparative Example and an alloy film in an alloy film-attached film according to each of the Example and the Comparative Example were prepared using a focused ion beam processing observation apparatus (product name: FB-2000A, manufactured by Hitachi High-Tech Corporation). Then, the cross-sectional observation samples were observed using a field emission transmission electron microscope (product name: HF-2000, manufactured by Hitachi High-Tech Corporation), and the thickness of the non-porous film according to each of the Example and the Comparative Example was measured. The thickness of the non-porous film was regarded as the thickness of the alloy film in the alloy film-attached film according to each of the Example and the Comparative Example. 
     [Sheet Resistance] 
     The sheet resistance of the alloy film in the alloy film-attached film according to each of the Example and the Comparative Example was measured by the eddy current method according to JIS Z 2316 using a non-contact type resistance measurement device NC-80LINE manufactured by NAPSON CORPORATION. The results are shown in Table 2. 
     [Appearance Check] 
     In a state where the alloy film-attached film of the sample according to each of the Example and the Comparative Example was irradiated with light from a white light source, whether or not an iridescent pattern was observed was checked. When an iridescent pattern was not observed, the film was evaluated as “A”, and when an iridescent pattern was observed, the film was evaluated as “X”. The results are shown in Table 2. 
     [Radio Wave Absorption Performance] 
     With reference to JIS R 1679: 2007, radio waves having frequencies of 60 to 90 GHz were made incident at an incident angle of 0° on the sample according to each of the Example and the Comparative Example fixed to a sample holder, using a vector network analyzer manufactured by ANRITSU CORPORATION, and a return loss |S| at each frequency was determined according to the above equation (2). Instead of the sample according to each of the Example and the Comparative Example, an aluminum plate was fixed to the sample holder, a return loss |S| when radio waves were incident perpendicularly on the plate was regarded as 0 dB, and the return loss |S| of each sample was determined. The plate had a face dimension of 30 cm square, and the thickness of the plate was 5 mm. The maximum value of the return loss |S| in each sample and the frequency (absorption peak frequency f p ) at which the maximum value was exhibited were determined. When the maximum value of the return loss |S| is 10 dB or more, the sample can be evaluated to have good radio wave absorption performance. The results are shown in Table 2. 
     Example 1 
     DC magnetron sputtering was performed using an Al (aluminum) target material and an Si (silicon) target material and using argon gas as a process gas, to form an Al—Si alloy film on a PET film. In the DC magnetron sputtering, discharge involving the Al (aluminum) target material and discharge involving the Si (silicon) target material were performed simultaneously. Thus, a non-porous film according to Example 1 was formed on the PET film. The non-porous film had a thickness of 30 nm. Next, using a metal laser patterning machine, openings were formed in the non-porous film according to Example 1 such that a domain having 16 openings and having a square shape with 280 pm square was repeatedly formed along a main surface of the non-porous film. In each domain, 16 openings each having a square or rectangular shape with a dimension shown in Table 1 were formed in a matrix. In each cell of Table 1, the value on the left side indicates the dimension of the opening in the direction extending along the rows of the matrix, and the value on the right side indicates the dimension of the opening in the direction along the columns of the matrix. The distance between the nearest openings was 10 μm. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Column 
               
            
           
           
               
               
               
               
               
            
               
                 Row 
                 First column 
                 Second column 
                 Third column 
                 Fourth column 
               
               
                   
               
               
                 First row 
                 30 μm, 30 μm 
                 30 μm, 50 μm 
                 30 μm, 70 μm 
                 30 μm, 90 μm 
               
               
                 Second row 
                 50 μm, 30 μm 
                 50 μm, 50 μm 
                 50 μm, 70 μm 
                 50 μm, 90 μm 
               
               
                 Third row 
                 70 μm, 30 μm 
                 70 μm, 50 μm 
                 70 μm, 70 μm 
                 70 μm, 90 μm 
               
               
                 Fourth row 
                 90 μm, 30 μm 
                 90 μm, 50 μm 
                 90 μm, 70 μm 
                 90 μm, 90 μm 
               
               
                   
               
            
           
         
       
     
     DC magnetron sputtering was performed using an ITO target material containing 10 weight % of SnO 2  and using argon and oxygen as process gases, to form an ITO film on a PET film. Then, the ITO film was annealed under the condition of a temperature of 150° C. for 1 hour to polycrystallize the ITO, to obtain a reflector-attached film. The sheet resistance of the reflector of the reflector-attached film was 20 Ω/□. Next, an acrylic resin having a relative permittivity of 2.6 was molded so as to have a thickness of 550 pm, to obtain an acrylic resin layer A. The alloy film-attached film according to Example 1 was put on the acrylic resin layer A such that the alloy film of the alloy film-attached film according to Example 1 was in contact with the acrylic resin layer A. Next, the reflector-attached film was put on the acrylic resin layer A such that the ITO in the reflector-attached film was in contact with the acrylic resin layer A. Thus, a sample according to Example 1 was obtained. 
     Comparative Example 1 
     A non-porous film according to Comparative Example 1 was formed on a PET film in the same manner as Example 1. The thickness of the non-porous film was 30 nm. Next, using a metal laser patterning machine, a plurality of square-shaped openings were formed at equal intervals in the non-porous film according to Comparative Example 1 so as to form a square lattice, to obtain an alloy film-attached film according to Comparative Example 1. In a plan view of the alloy film-attached film according to Comparative Example 1, the size of each opening in the direction in which the plurality of openings were arranged at equal intervals was 35 μm, and the distance between the nearest openings was 10 μm. 
     A sample according to Comparative Example 1 was obtained in the same manner as Example 1, except that the alloy film-attached film according to Comparative Example 1 was used instead of the alloy film-attached film according to Example 1. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Size of 
                 Average 
                 Distance 
                   
                   
                 Absorption 
                 Maximum 
               
               
                   
                 domain 
                 value of 
                 between 
                   
                 Presence/ 
                 peak 
                 value of 
               
               
                   
                 in specific 
                 perimeters 
                 nearest 
                 Sheet 
                 absence of 
                 frequency 
                 return 
               
               
                   
                 direction 
                 of openings 
                 openings 
                 resistance 
                 iridescent 
                 fp 
                 loss 
               
               
                   
                 [μm] 
                 [μm] 
                 [μm] 
                 [Ω/□] 
                 pattern 
                 [GHz] 
                 [dB] 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Ex. 1 
                 280 
                 240 
                 10 
                 400 
                 A 
                 77 
                 40 
               
               
                 Comp. 
                 — 
                 140 
                 10 
                 400 
                 X 
                 77 
                 40 
               
               
                 Ex. 1