NEAR-FIELD ELECTROMAGNETIC WAVE ABSORBING FILM

Abstract

A near-field electromagnetic wave absorbing film comprising a thin metal film formed on a surface of a plastic film; the thin metal film being provided with plural lines of laser-etched openings extending and crossing in two directions; pluralities of laser-etched openings being arranged in each line such that at least part of adjacent laser-etched openings are separated; the laser-etched opening lines extending in two directions crossing at an angle of 45-90°; thin metal film portions remaining after forming the laser-etched openings being composed of wide remaining portions partitioned by the laser-etched opening lines, and bridge-like remaining portions connecting adjacent wide remaining portion; and the bridge-like remaining portions having widths of 20 μm or less; thereby having electric resistance of 50-300 Ω/100 cm.sup.2 and light transmittance (measured with laser rays having a wavelength of 660 nm) of 30-80%.

Claims

1. A near-field electromagnetic wave absorbing film comprising a plastic film, and a single- or multi-layered thin metal film formed on a surface of the plastic film; said thin metal film being provided with plural lines of laser-etched openings extending and crossing in two directions; pluralities of said laser-etched openings being arranged in each line such that at least part of adjacent laser-etched openings are separated; said laser-etched opening lines extending in two directions crossing at an angle of 45-90°; each of said laser-etched opening lines being provided with pluralities of laser-etched openings merged into integral openings; thin metal film portions remaining after forming said laser-etched openings being composed of wide remaining portions partitioned by said integral openings, and bridge-like remaining portions connecting adjacent wide remaining portions; and said bridge-like remaining portions having widths of 20 μm or less; whereby said near-field electromagnetic wave absorbing film has electric resistance of 50-300 Ω/100 cm.sup.2 and light transmittance (measured with laser rays having a wavelength of 660 nm) of 30-80%.

2. The near-field electromagnetic wave absorbing film according to claim 1, wherein said bridge-like remaining portions have an average width of 2-15 μm.

3. (canceled)

4. The near-field electromagnetic wave absorbing film according to claim 1, wherein said laser-etched openings have diameters of 100 μm or less.

5. The near-field electromagnetic wave absorbing film according to claim 1, wherein the centerline distance between adjacent laser-etched opening lines is 1.5-5 times the diameters of said laser-etched openings.

6. The near-field electromagnetic wave absorbing film according to claim 1, wherein the thickness of said thin metal film is 10-300 nm.

7. The near-field electromagnetic wave absorbing film according to claim 1, wherein said thin metal film is made of at least one metal selected from the group consisting of aluminum, copper, silver, tin, nickel, cobalt, chromium, and alloys thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 is a partial enlarged plan view showing a near-field electromagnetic wave absorbing film according to one embodiment of the present invention.

[0021] FIG. 2(a) is a cross-sectional view taken along the line A-A in FIG. 1.

[0022] FIG. 2(b) is a cross-sectional view taken along the line B-B in FIG. 1.

[0023] FIG. 3 is a partial enlarged view of FIG. 1.

[0024] FIG. 4 is a partial enlarged plan view showing a near-field electromagnetic wave absorbing film according to another embodiment of the present invention.

[0025] FIG. 5 is a partial enlarged view of FIG. 4.

[0026] FIG. 6(a) is a perspective view showing an apparatus for measuring the electric resistance of a near-field electromagnetic wave absorbing film.

[0027] FIG. 6(b) is a plan view showing the measurement of the electric resistance of a near-field electromagnetic wave absorbing film using the apparatus of FIG. 6(a).

[0028] FIG. 6(c) is a cross-sectional view taken along the line C-C in FIG. 6(b).

[0029] FIG. 7(a) is a plan view showing a system for evaluating the electromagnetic wave absorbability of a near-field electromagnetic wave absorbing film.

[0030] FIG. 7(b) is a partially cross-sectional front view showing a system for evaluating the electromagnetic wave absorbability of a near-field electromagnetic wave absorbing film.

[0031] FIG. 8 is a photomicrograph showing the near-field electromagnetic wave absorbing film of Example 1.

[0032] FIG. 9 is a photomicrograph showing the near-field electromagnetic wave absorbing film of Comparative Example 1.

[0033] FIG. 10 is a photomicrograph showing the near-field electromagnetic wave absorbing film of Comparative Example 2.

[0034] FIG. 11 is a graph showing the relations between S.sub.11 and the frequency of incident electromagnetic waves in the near-field electromagnetic wave absorbing films of Example 1 and Comparative Examples 1 and 2.

[0035] FIG. 12 is a graph showing the relations between a transmission attenuation power ratio Rtp and the frequency of incident electromagnetic waves in the near-field electromagnetic wave absorbing films of Example 1 and Comparative Examples 1 and 2.

[0036] FIG. 13 is a photomicrograph showing the near-field electromagnetic wave absorbing film of Example 2.

[0037] FIG. 14 is a photomicrograph showing the near-field electromagnetic wave absorbing film of Comparative Example 3.

[0038] FIG. 15 is a photomicrograph showing the near-field electromagnetic wave absorbing film of Comparative Example 4.

[0039] FIG. 16 is a graph showing the relations between S.sub.11 and the frequency of incident electromagnetic waves in the near-field electromagnetic wave absorbing films of Example 2 and Comparative Examples 3 and 4.

[0040] FIG. 17 is a graph showing the relations between a transmission attenuation power ratio Rtp and the frequency of incident electromagnetic waves in the near-field electromagnetic wave absorbing films of Example 2 and Comparative Examples 3 and 4.

[0041] FIG. 18 is a graph showing the relation between a noise absorption ratio P.sub.loss/P.sub.in and the frequency of incident electromagnetic waves in the near-field electromagnetic wave absorbing film of Example 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] The embodiments of the present invention will be explained in detail referring to the attached drawings, and it should be noted that explanation concerning one embodiment is applicable to other embodiments unless otherwise mentioned. Also, the following explanation is not restrictive, and various modifications may be made within the scope of the present invention.

[0043] [1] Near-Field Electromagnetic Wave Absorbing Film

[0044] FIGS. 1 and 2 show a near-field electromagnetic wave absorbing film 1 according to a preferred embodiment of the present invention. This near-field electromagnetic wave absorbing film 1 comprises a single- or multi-layered thin metal film 11 formed on a surface of a plastic film 10, the thin metal film 11 being provided with plural lines of laser-etched openings 12 extending and crossing in two directions.

[0045] (1) Plastic Film

[0046] Resins forming the plastic film 10 are not particularly restrictive as long as they have sufficient strength, flexibility and workability in addition to insulation, and they may be, for instance, polyesters (polyethylene terephthalate, etc.), polyarylene sulfide (polyphenylene sulfide, etc.), polyamides, polyimides, polyamideimides, polyether sulfone, polyetheretherketone, polycarbonates, acrylic resins, polystyrenes, polyolefins (polyethylene, polypropylene, etc.), etc. From the aspect of strength and cost, polyethylene terephthalate (PET) is preferable. The thickness of the plastic film 10 may be about 10-100 μm.

[0047] (2) Thin Metal Film

[0048] Though not limited as long as they are conductive, metals forming the thin metal film 11 are preferably aluminum, copper, silver, tin, nickel, cobalt, chromium, and alloys thereof, particularly aluminum, copper, nickel and these alloys, from the aspect of corrosion resistance and cost. The thickness of the thin metal film is preferably 10-300 nm, more preferably 20-200 nm, most preferably 30-150 nm. The thin metal film 11 can be formed by vapor deposition (physical vapor deposition such as vacuum vapor deposition, sputtering, ion plating, etc.; or chemical vapor deposition such as plasma-CVD, thermal-CVD, photo-CVD, etc.), plating, or foil-bonding.

[0049] When the thin metal film 11 has a single layer, the thin metal film 11 is preferably made of aluminum or nickel from the aspect of conductivity, corrosion resistance and cost. When the thin metal film 11 has pluralities of layers, one may be made of a non-magnetic metal, and the other may be made of a magnetic metal. The non-magnetic metal may be aluminum, copper, silver, tin or an alloy thereof, and the magnetic metal may be nickel, cobalt, chromium or an alloy thereof. As long as their total thickness is within the above range, the non-magnetic metal layer and the magnetic metal layer are not restricted in thickness.

[0050] (3) Laser-Etched Opening Lines

[0051] As shown in FIGS. 1-3, each laser-etched opening 12 is a substantially circular opening formed by evaporating a metal by irradiating the thin metal film 11 with laser beam spots, without boring the plastic film 10. The diameter D of each laser-etched opening 12 is preferably 100 μm or less, more preferably 40-80 μm.

[0052] Plural lines of laser-etched openings 12 are arranged in the thin metal film 11, such that they extend and cross in two directions. The thin metal film 11 is partitioned by crossing laser-etched opening lines 12a, 12b to individual wide remaining portions 13. In each laser-etched opening line 12a, 12b, pluralities of laser-etched openings 12 are arranged such that at least part of adjacent laser-etched openings are separated, so that there remains a narrow bridge-like remaining portion 14 between separated adjacent laser-etched openings 12 of the thin metal film 11. Accordingly, portions of the thin metal film remaining after forming the laser-etched openings 12, which are called remaining thin metal film portions, are composed of wide remaining portions 13 and narrow bridge-like remaining portions 14.

[0053] In both laser-etched opening lines 12a, 12b, at least part of adjacent laser-etched openings 12 are preferably separated. The widths W of the bridge-like remaining portions 14 are 20 μm or less, preferably 1-15 μm. Accordingly, the maximum width Wmax of the bridge-like remaining portions 14 is 20 μm, preferably 15 μm. The average width Way of the bridge-like remaining portions 14 is 2-15 μm, preferably 3-10 μm.

[0054] In order that the bridge-like remaining portions 14 have different widths W, the thin metal film 11 may be irradiated with laser beam spots at different intervals.

[0055] Though a laser beam spot per se is circular, a region of the thin metal film evaporated by a laser beam spot is not completely circular, but tends to have slightly irregular contour due to interference between adjacent laser-etched openings 12. For example, when adjacent laser-etched openings 12 are not separated, both laser-etched openings 12, 12 are likely merged, or bridge-like remaining portions 14 likely have too small widths. This appears to be due to the fact that the evaporated metal is solidified between adjacent laser beam spots, resulting in too narrow bridge-like remaining portions 14. Because laser-etched openings 12 actually formed by evaporating the thin metal film 11 do not have the same interval even though laser beam spots are arranged with the same interval, bridge-like remaining portions 14 tend to have different widths W within a range of 20 μm or less. Accordingly, the near-field electromagnetic wave absorbing film of the present invention 1 has excellent electromagnetic wave absorbability in a wide frequency range.

[0056] The laser-etched opening lines 12a, 12b extending in two directions cross with an angle θ of 45-90°. This provides high electromagnetic wave absorbability with reduced anisotropy. When the crossing angle θ is less than 45°, sufficient electromagnetic wave absorbability cannot be exhibited. When the crossing angle θ is 90°, the maximum electromagnetic wave absorbability is obtained. The preferred crossing angle θ is 60-90°.

[0057] The centerline distance T between adjacent laser-etched opening lines 12a, 12b largely affects the sizes of wide remaining portions 13, which in turn affect the electric resistance (thus, electromagnetic wave absorbability) of the near-field electromagnetic wave absorbing film 1. Accordingly, the centerline distance T between the laser-etched opening lines 12a, 12b should be set to obtain desired electric resistance. Specifically, the centerline distance T between the laser-etched opening lines 12a, 12b is preferably 100-400 μm, more preferably 150-300 μm.

[0058] FIGS. 4 and 5 show a near-field electromagnetic wave absorbing film 100 according to another preferred embodiment of the present invention. This near-field electromagnetic wave absorbing film 100 has pluralities of laser-etched openings 12, which are integrally merged into elongated openings 112 [portions from which the thin metal film disappears (evaporated)]. As a result, thin metal film portions remaining after forming the laser-etched openings 12 are composed of wide remaining portions 13 partitioned by the integral openings 112, and bridge-like remaining portions 14 connecting adjacent wide remaining portions 13. In this embodiment, because of relatively small wide remaining portions 13, remaining thin metal film portions 11a composed of the wide remaining portions 13 and the bridge-like remaining portions 14 look like a network as a whole.

[0059] In this embodiment, the thin metal film 11 may be irradiated with laser beam spots at different intervals, such that part of adjacent laser-etched openings 12 are integrally connected.

[0060] The sizes of wide remaining portions 13 may vary depending on the widths W and number of bridge-like remaining portions 14, the type and thickness of the thin metal film 11 (electric resistance), etc. In general, larger wide remaining portions 13 are obtained when the thin metal film 11 is formed by a metal having larger electric resistance, and when the thin metal film 11 is thinner. Intensive research has revealed that the electromagnetic wave absorbability of the near-field electromagnetic wave absorbing film 1 depends on the size and electric resistance of thin metal film portions remaining after forming laser-etched openings 12 (remaining thin metal film portions 11a=wide remaining portions 13+bridge-like remaining portions 14). Specifically, with electric resistance of 50-300 Ω/100 cm.sup.2 and light transmittance (measured with laser rays having a wavelength of 660 nm) of 30-80%, the near-field electromagnetic wave absorbing film 1 has excellent absorbability to wide-frequency electromagnetic waves, with low anisotropy.

[0061] (4) Electric Resistance

[0062] The electric resistance of the near-field electromagnetic wave absorbing film 1 having remaining thin metal film portions 11a is measured by a DC two-terminal method under pressure (simply called “under-pressure two-terminal method”), using an apparatus shown in FIGS. 6(a)-6(c). Specifically, a square test piece TP1 (10 cm×10 cm) of the near-field electromagnetic wave absorbing film 1 is placed with its remaining thin metal film portions 11a above on a flat, hard insulation surface, a pair of electrodes 120, 120 each comprising an electrode body portion 121 of 10 cm in length, 1 cm in width and 0.5 mm in thickness, and an electrode extension 122 of 1 cm in width and 0.5 mm in thickness extending from a center side of the electrode body portion 121 are attached to opposing side portions of the square test piece TP1. A transparent acrylic plate 130 of 10 cm×10 cm×5 mm is placed on the test piece TP1 and both electrodes 120, 120, such that it completely covers them, and a cylindrical weight 140 (3.85 kg) of 10 cm in diameter is placed on the transparent acrylic plate 130, to measure current flowing between both electrode extensions 222, 222 to determine the surface resistance of the near-field electromagnetic wave absorbing film 1.

[0063] The near-field electromagnetic wave absorbing film 1 should have electric resistance in a range of 50-300 Ω/100 cm.sup.2. When the electric resistance is less than 50 Ω/100 cm.sup.2 or more than 300 Ω/100 cm.sup.2, the near-field electromagnetic wave absorbing film 1 does not have sufficient electromagnetic wave absorbability. The electric resistance of the near-field electromagnetic wave absorbing film 1 is preferably 60-250 Ω/100 cm.sup.2, more preferably 80-200 Ω/100 cm.sup.2.

[0064] (5) Light Transmittance

[0065] The electromagnetic wave absorbability of the near-field electromagnetic wave absorbing film 1 depends on the area ratio of the remaining thin metal film portions 11a (wide remaining portions 13+bridge-like remaining portions 14). Because the remaining thin metal film portions 11a have substantially no light transmittance, the area ratio of the remaining thin metal film portions 11a can be expressed by the light transmittance of the near-field electromagnetic wave absorbing film 1. The light transmittance of the near-field electromagnetic wave absorbing film 1 is measured with laser rays having a wavelength of 660 nm.

[0066] When the light transmittance of the near-field electromagnetic wave absorbing film 1 should be in a range of 30-80%. When the light transmittance is less than 30%, the area ratio of the remaining thin metal film portions 11a is too large, resulting in high reflectance of electromagnetic waves, and thus low electromagnetic noise absorbability. On the other hand, when the light transmittance is more than 80%, the area ratio of the remaining thin metal film portions 11a is too small, resulting in insufficient electromagnetic wave absorbability. The light transmittance of the near-field electromagnetic wave absorbing film 1 is preferably 35-70%.

[0067] (6) Protective Layer

[0068] A protective plastic layer (not shown) is preferably formed on a thin metal film having plural lines of laser-etched openings 12 extending and crossing in two directions. A plastic film for the protective plastic layer may be the same as the plastic film 10. The thickness of the protective plastic layer is preferably about 10-100 μm. To prevent detachment, a plastic film is preferably heat-laminated to the near-field electromagnetic wave absorbing film 1 as a protective layer. When the protective plastic layer is formed by a PET film, the heat lamination temperature may be 110-150° C.

[0069] [2] Electromagnetic Wave Absorbability of Near-Field Electromagnetic Wave Absorbing Film

[0070] (1) Transmission Attenuation Power Ratio

[0071] Using a system comprising a 50-Ω microstripline MSL (64.4 mm×4.4 mm), an insulating substrate 220 supporting the microstripline MSL, a grounded electrode 221 attached to a lower surface of the insulating substrate 220, conductive pins 222, 222 connected to both ends of the microstripline MSL, a network analyzer NA, and coaxial cables 223, 223 connecting the network analyzer NA to the conductive pins 222, 222 as shown in FIGS. 7(a) and 7(b), a test piece TP2 of each near-field electromagnetic wave absorbing film 1 is adhered to the microstripline MSL to measure its reflected wave power S.sub.11 and transmitted wave power S.sub.21 to an input electromagnetic wave of 0.1-6 GHz, thereby determining its transmission attenuation power ratio Rtp by the following formula (1):

Rtp=−10×log [10.sup.S21/10/(1−10.sup.S11/10)]  (1).

[0072] (2) Noise Absorption Ratio

[0073] In the system shown in FIGS. 7(a) and 7(b), input power P.sub.in=reflected wave power S.sub.11+transmitted wave power S.sub.21+absorbed power (power loss) P.sub.loss. Accordingly, the noise absorption ratio P.sub.loss/P.sub.in is determined by subtracting the reflected wave power S.sub.11 and the transmitted wave power S.sub.21 from the input power P.sub.in, and dividing the resultant power loss P.sub.loss by the input power P.sub.in.

[0074] The present invention will be explained in more detail referring to Examples below without intention of restriction.

Example 1, and Comparative Examples 1 and 2

[0075] Each thin Al film 11 as thick as 80 nm was formed on a PET film 10 as thick as 16 μm by a vacuum vapor deposition method, and provided with plural lines of laser-etched openings 12 of 60 μm in diameter extending and crossing in two directions by a 3-Axis hybrid laser marker (MD-X1000 available from Keyence Corporation), to produce a near-field electromagnetic wave absorbing film 1 shown in FIGS. 8-10.

[0076] The electric resistance of a square test piece TP1 (10 cm×10 cm) cut out of each near-field electromagnetic wave absorbing film 1 was measured by the method described in Section [1] (4). The light transmittance of each near-field electromagnetic wave absorbing film 1 was measured with laser rays having a wavelength of 660 nm. The results are shown in Table 1.

TABLE-US-00001 TABLE 1 No. Example 1 Com. Ex. 1 Com. Ex. 2 Corresponding Figure FIG. 8 FIG. 9 FIG. 10 Crossing Angle θ .sup.(1) (°) 90 90 90 Maximum Width Wmax.sup.(2) (μm) 15 60 0 Average Width Wav.sup.(2) (μm) 8 50 0 Electric Resistance (Ω/100 cm.sup.2) 100 2 ∞ Light Transmittance (%) 70 25 50 Note: .sup.(1) The crossing angle θ of laser-etched opening lines. .sup.(2)Measured in bridge-like remaining portions of the thin metal film.

[0077] A test piece TP2 (55.2 mm×4.7 mm) cut out of each near-field electromagnetic wave absorbing film 1 was adhered to a microstripline MSL in the system shown in FIGS. 7(a) and 7(b), to measure reflected wave power S.sub.11 and transmitted wave power S.sub.21 relative to input power P.sub.in in a frequency range of 0.1-6 GHz. The reflected wave power S.sub.11 and the transmitted wave power S.sub.21 were measured by the method described in Section [2] (1) and (2), to determine S.sub.1, and a transmission attenuation power ratio Rtp in a frequency range of 0.1-6 GHz. The S.sub.1, and the transmission attenuation power ratio Rtp in a frequency range of 0.1-6 GHz are shown in FIGS. 11 and 12, respectively. As is clear from FIG. 11, too much wave power was reflected in Comparative Example 1 having too few laser-etched openings 12 (too low electric resistance and light transmittance). As is clear from FIG. 12, the transmission attenuation power ratio Rtp was low in Comparative Example 2 having excessive laser-etched openings 12 (too high electric resistance).

Example 2, and Comparative Examples 3 and 4

[0078] Each thin Ni film 11 as thick as 50 nm was formed on a PET film 10 as thick as 16 μm by a vacuum vapor deposition method, and provided with plural lines of laser-etched openings 12 of 60 μm in diameter extending and crossing in two directions by a 3-Axis hybrid laser marker (MD-X1000 available from Keyence Corporation), to produce a near-field electromagnetic wave absorbing film 1 shown in FIGS. 13-15. The electric resistance and light transmittance of each near-field electromagnetic wave absorbing film 1 were measured by the same methods as in Example 1. The results are shown in Table 2.

TABLE-US-00002 TABLE 2 No. Example 2 Com. Ex. 3 Com. Ex. 4 Corresponding Figure FIG. 13 FIG. 14 FIG. 15 Crossing Angle θ .sup.(1) (°) 90 90 90 Maximum Width Wmax.sup.(2) (μm) 15 60 0 Average Width Wav.sup.(2) (μm) 7 50 0 Electric Resistance (Ω/100 cm.sup.2) 60 6 ∞ Light Transmittance (%) 40 30 50 Note: .sup.(1) The crossing angle θ of laser-etched opening lines. .sup.(2)Measured in bridge-like remaining portions of the thin metal film.

[0079] The S.sub.11, transmission attenuation power ratio Rtp and noise absorption ratio P.sub.loss/P.sub.in of each near-field electromagnetic wave absorbing film 1 in a frequency range of 0.1-6 GHz were measured by the same methods as in Example 1. The S.sub.11 and the transmission attenuation power ratio Rtp in a frequency range of 0.1-6 GHz are shown in FIGS. 16 and 17, respectively. The noise absorption ratio P.sub.loss/P.sub.in of Example 2 is shown in FIG. 18. As is clear from FIG. 16, too much wave power was reflected in Comparative Example 3 having too few laser-etched openings 12 (too low electric resistance and light transmittance). As is clear from FIG. 17, the transmission attenuation power ratio Rtp was low in Comparative Example 4 having excessive laser-etched openings 12 (too high electric resistance). Further, as is clear from FIG. 18, Example 2 meeting the requirements of the present invention with respect to laser-etched openings 12, electric resistance and light transmittance exhibited a high noise absorption ratio P.sub.loss/P.sub.in.

Effects of the Invention

[0080] The near-field electromagnetic wave absorbing film of the present invention has excellent electromagnetic wave absorbability with small anisotropy, because laser-etched opening lines are formed in two directions in a thin metal film; pluralities of laser-etched openings being arranged in each line such that at least part of adjacent laser-etched openings are separated; thin metal film portions remaining after forming the laser-etched openings being composed of wide remaining portions partitioned by the laser-etched opening lines, and narrow bridge-like remaining portions connecting the wide remaining portions, so that it has electric resistance of 50-300 Ω/100 cm.sup.2 and light transmittance (measured with laser rays having a wavelength of 660 nm) of 30-80%. The near-field electromagnetic wave absorbing film of the present invention having such features is suitably usable as a noise suppression sheet for communications apparatuses such as cell phones, smartphones, wireless LAN, etc., and electronic appliances such as computers, etc.

DESCRIPTION OF REFERENCE NUMERALS

[0081] 1: Near-field electromagnetic wave absorbing film [0082] 10: Plastic film [0083] 11: Thin metal film [0084] 11a: Remaining thin metal film portion [0085] 12: Laser-etched opening [0086] 12a, 12b: Laser-etched opening line [0087] 13: Wide remaining portion [0088] 14: Bridge-like remaining portion [0089] 112: Integral opening [0090] 120: Electrode [0091] 121: Electrode body portion [0092] 122: Electrode extension [0093] 130: Transparent acrylic plate [0094] 140: Cylindrical weight [0095] 220: Insulating substrate [0096] 221: Grounded electrode [0097] 222: Conductive pin [0098] 223: Coaxial cable [0099] D: Diameter of laser-etched opening [0100] W: Width of bridge-like remaining portion [0101] T: Centerline distance between adjacent laser-etched opening lines [0102] TP1, TP2: Test piece of near-field electromagnetic wave absorbing film [0103] MSL: Microstripline [0104] NA: Network analyzer