JP7632485B2 - Electromagnetic wave absorber - Google Patents

Description

The present invention relates to an electromagnetic wave absorber that absorbs electromagnetic waves, and in particular to an electromagnetic wave absorber used in high-frequency integrated circuit technology.

In high frequency integrated circuit (IC) technology, two adverse effects appear when the operating speed exceeds 1/10 gigahertz. In the first effect, feedback circuits with parasitic inductance and capacitance induce vibrations and cause instability in the ground potential of high frequency ICs, called ground bounce. Ground bounce generates peaks and dips in the signal response and degrades the group delay. The second effect is substrate resonance, which begins at the millimeter-wave bandwidth. When the wavelength of the signal propagating through the IC approaches the size of the chip, additional electromagnetic (EM) wave modes are easily radiated into the substrate, causing oscillations and degradation of the circuit operation due to the energy dispersion of the EM wave signal.

To minimize the effects of ground bounce and substrate resonance and ensure stable operation of high-speed ICs at sub-millimeter wavelengths, the substrate 51 is thinned and a high-density through-substrate via (TSV) system is formed on the backside of the IC chip 5, as shown in FIG. 12.

Ground bounce is suppressed by connecting the ground plane in the IC chip to the module ground 53 using vias 52 and bond wires. Substrate resonance is blocked by thinning the substrate and forming a high density of ground vias through the substrate. The thickness of the thinned substrate, the diameter of the TSVs and the density of the TSVs depend on the operating frequency of the IC circuit.

In commonly used metallized via systems, the resistivity per unit area between the front and back surfaces must be minimized to stabilize the ground potential of the IC chip. Smaller resistivity can be obtained by reducing the via diameter, the spacing between vias, and the thickness of the substrate. Smaller vias are also advantageous for improving layout flexibility and packaging levels of the IC.

In general, absorbing materials can be added to IC chips instead of high-density TSV systems to suppress the effects of ground bounce and substrate resonance and absorb EM wave modes propagating through the substrate of the IC chip. Absorbers can effectively absorb or remove unwanted electromagnetic radiation or scattering, and have potential in radar stealth technology, EM shielding, and energy harvesting.

However, conventional absorbers are thick and heavy, making them practically difficult to use in small IC chips for high-frequency electronic devices.

As described above, in order to suppress board resonance and ground bounce that occur in high-speed ICs, it is necessary to make the board thinner and create high-density TSVs on the back side of the board.

“Broadband metamaterial absorber at mid-infrared using multiplexed cross resonators,” Wei Ma, Yongzheng Wen, and Xiaomei Yu, Opt. Express 21, 30724-30730 (2013). “Broadband metamaterial microwave absorber based on asymmetric sectional resonator structures,” Yongzhi Cheng, Hui Luo and Fu Chen, J. Appl. Phys.127, 214902 (2020). “Experimental demonstration of terahertz metamaterial absorbers with a broad and flat high absorption band,” Li Huang, Dibakar Roy Chowdhury, Suchitra Ramani, Matthew T. Reiten, Sheng-Nian Luo, Antoinette J. Taylor, and Hou-Tong Chen, Opt. Lett. 37, 154-156 (2012). “Perfect Metamaterial Absorber,” N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, Phys. Rev. Lett. 100, 207402. “Wafer-level backside process technology for forming high-density vias and backside metal patterning for 50-μm-thick InP substrate,” Tsutsumi, Takuya & Kosugi, T. & Matsuzaki, H. (2013). 2013 International Conference on Compound Semiconductor Manufacturing Technology, CS MANTECH 2013. 55-58.

However, thinning the substrate and forming the TSV system poses the following challenges with respect to IC layout manufacturing and design:

First, because the fabrication of the TSV system requires etching through the substrate, the design of the IC layout must take into account the implementation of the TSVs on the backside of the IC chip. Since the size of the individual electrical components on the chip is much smaller than the diameter of the individual TSVs, this requires a significant increase in chip size and a simultaneous reduction in the packaging level of the IC chip.

Second, the fabrication of TSVs requires a multi-step process that includes mounting IC chips on a glass substrate using an epoxy adhesive, thinning the substrate using polishing techniques, forming vias using reactive ion etching, and metallizing the backside of the substrate and inside the vias. This complex multi-step process significantly increases the cost and time of fabricating high-speed IC chips.

Third, the fabrication of TSV systems requires precise control of the via hole diameter, which is important for suppressing undesirable EM wave modes. Here, very high etch selectivity is required in the ion reactive etching process to precisely control the TSV diameter.

Fourth, the adhesive commonly used to mount IC chips on glass deteriorates at high temperatures, so control of the substrate temperature during the ion reactive etching process is also essential and must be considered. In this case, the adhesive deteriorates, making it difficult to smoothly remove the thin IC chip from the glass after backside processing, and making it more susceptible to breakage due to mechanical stress.

Fifth, as the diameter of the TSV decreases, the etching rate decreases due to the microloading effect, which is prominent at sub-mm wavelengths. Therefore, as the operating frequency range increases, it becomes necessary to adjust the etching parameters, which increases the overall manufacturing time.

Sixth, after the TSVs are formed, metallization is required on the backside of the substrate and inside the vias, typically by electroplating a few micrometers of metal and then reactive ion etching to remove any extraneous seed layer.

As described above, in order to suppress substrate resonance and ground bounce that occur in high-speed ICs, there are problems with IC chip layout design and TSV fabrication in order to reduce substrate thickness and create high-density TSVs on the back surface of the substrate.

In order to solve the above-mentioned problems, the electromagnetic wave absorber of the present invention is an electromagnetic wave absorber arranged on a substrate, and comprises, in order, a metamaterial layer, a spacer layer, and a reflective thin film, and absorbs and suppresses a resonance mode of an electromagnetic wave in the substrate, the metamaterial layer comprises a plurality of metamaterial cells of a subwavelength structure, the metamaterial cells are arranged periodically and designed to resonate at a predetermined frequency, and the structure and period of the metamaterial cells are determined by the wavelength of the incident electromagnetic wave .
In addition, the electromagnetic wave absorber according to the present invention is characterized in that the metamaterial cell is composed of a plurality of sections having different characteristic sizes, and the plurality of sections are arranged asymmetrically.
Moreover, the electromagnetic wave absorber according to the present invention is an electromagnetic wave absorber arranged on a substrate, which comprises, in order, a metamaterial layer, a spacer layer, and a reflective thin film, and which absorbs and suppresses a resonance mode of an electromagnetic wave in the substrate, and is characterized in that the effective dielectric constant and permeability of the metamaterial layer are adjusted so as to match the impedance of the metamaterial layer to the impedance of the substrate and absorb the resonance mode of the electromagnetic wave.
Moreover, the electromagnetic wave absorber according to the present invention is an electromagnetic wave absorber disposed on a substrate, which comprises, in order, a metamaterial layer, a spacer layer, and a reflective thin film, and which absorbs and suppresses a resonance mode of an electromagnetic wave within the substrate, and is characterized in that the difference between the relative dielectric constant of the spacer layer and the relative dielectric constant of the substrate is 2 or more and 11 or less.
Moreover, the electromagnetic wave absorber according to the present invention is an electromagnetic wave absorber arranged on a substrate, which comprises, in order, a metamaterial layer, a spacer layer, and a reflective thin film, and which absorbs and suppresses a resonant mode of an electromagnetic wave within the substrate, the electromagnetic wave having a source within the substrate of a high-speed electronic circuit.

The present invention makes it possible to provide a small electromagnetic wave absorber that does not require a complicated manufacturing process.

FIG. 1 is a schematic cross-sectional view of a semiconductor substrate for a high-speed electronic circuit, which includes an electromagnetic wave absorber according to a first embodiment of the present invention. FIG. 2A is a schematic top view showing the configuration of an electromagnetic wave absorber according to a first embodiment of the present invention. FIG. 2B is a schematic top view showing the configuration of the electromagnetic wave absorber according to the first embodiment of the present invention. FIG. 2C is a schematic top view showing the configuration of the electromagnetic wave absorber according to the first embodiment of the present invention. FIG. 3 is a perspective bird's-eye view showing the structure of a semiconductor substrate for high-speed electronic circuits that includes an electromagnetic wave absorber according to a first embodiment of the present invention. FIG. 4A is a diagram for explaining the characteristics of the electromagnetic wave absorber according to the first embodiment of the present invention. FIG. 4B is a diagram for explaining the characteristics of the electromagnetic wave absorber according to the first embodiment of the present invention. FIG. 5 is a perspective bird's-eye view showing the configuration of a coplanar waveguide including an electromagnetic wave absorber according to a first embodiment of the present invention. FIG. 6 is a diagram for explaining the characteristics of the electromagnetic wave absorber according to the first embodiment of the present invention. FIG. 7 is a diagram for explaining the characteristics of the electromagnetic wave absorber according to the first embodiment of the present invention. FIG. 8 is a diagram for explaining the characteristics of the electromagnetic wave absorber according to the first embodiment of the present invention. FIG. 9 is a diagram for explaining the characteristics of the electromagnetic wave absorber according to the first embodiment of the present invention. FIG. 10 is a diagram for explaining the characteristics of the electromagnetic wave absorber according to the second embodiment of the present invention. FIG. 11 is a perspective bird's-eye view showing the configuration of a coplanar waveguide including an electromagnetic wave absorber according to a third embodiment of the present invention. FIG. 12 is a schematic cross-sectional view of a semiconductor substrate for a conventional high-speed electronic circuit.

First Embodiment
An electromagnetic wave absorber according to a first embodiment of the present invention will be described with reference to FIGS.

<Configuration of electromagnetic wave absorber>
The electromagnetic wave absorber according to the present embodiment uses a metamaterial to absorb a wide range of electromagnetic wave modes radiated from a high-speed electronic circuit to a substrate. Hereinafter, the principle and configuration of the present invention will be described in detail with reference to the drawings. This description provides some detailed examples of possible implementations of the present invention, but it should be noted that these details are illustrative and do not limit the scope of the present invention.

Metamaterials are artificial media that derive their properties from embedded subwavelength structures arranged together in configurations similar to atoms in ordinary materials, exhibiting desired values of permittivity and permeability over a measured frequency range, and manipulate electromagnetic (EM) waves. The electromagnetic properties of metamaterials result from the dimensions, geometry, orientation, and arrangement of the circuit's periodic structures, as well as the materials from which those structures are composed.

With proper design, metamaterials can be applied as electromagnetic wave absorbers in a wide range of frequencies. Here, the absorption wavelength and bandwidth can be adjusted by changing the design of the metamaterial cells. A typical structure of an electromagnetic wave absorber consists of a three-layer structure, which has an array of metal subwavelength structures on the surface of a dielectric spacer and a metal ground layer on the back side. Here, the metamaterial periodic structure on the front side is responsible for the electric resonant response of the metamaterial absorber 10, and the coupling of the metal layer and the dielectric spacer layer on the back side determines the magnetic resonant response. Hereinafter, the "electromagnetic wave absorber" is also referred to as the "metamaterial absorber".

Figure 1 shows a cross section of a semiconductor substrate for high-speed electronic circuits that includes an electromagnetic wave absorber 10 according to this embodiment. The electromagnetic wave absorber 10 has a three-layer structure using a metamaterial, and includes a metamaterial layer 11 having subwavelength metamaterial cells 111 periodically arranged on the back surface of a high-speed IC chip substrate 14, an intermediate spacer layer 12, and a lower reflective thin film 13. Here, a source of electromagnetic waves is provided within the substrate of the high-speed electronic circuit.

The subwavelength metamaterial cell 111 is designed to resonate at a predetermined frequency.

Metamaterial structures can be made from metals such as gold, copper and aluminum, as well as other highly conductive materials such as graphene or carbon nanotubes.

The intermediate dielectric spacer layer 12 can be any type of dielectric material, such as silicon-based dielectrics (silicon dioxide, silicon nitride), or polymers such as polyimide or benzocyclobutene (BCB). The thickness of the dielectric spacer 12 is adjusted based on the dielectric constant (dielectric constant) to enhance the absorption of electromagnetic waves by the metamaterial absorber 10.

The lower reflective film 13 is made of metals, metal alloys, and compounds that are thicker than the skin depth of the target radiation (incident electromagnetic wave) to block the transmission of the incident electromagnetic wave. The minimum thickness of the reflective film 13 can be obtained by calculating the skin depth δ.

where ρ is the resistivity of the material used, μ ₀ is the permeability of free space (μ ₀ =4π·10 ^-7 ), μ _r is the relative permeability of the material used (usually equal to 1) and f ₀ is the frequency of the EM wave. For example, a gold film with ρ=2.24 μΩ·cm and μ _r =1 at a frequency of 300 GHz has a skin depth of 0.138 μm. The underlying reflective film 13 must therefore be thicker than this value.

2A-C show examples of metamaterial cells (unit cells) in the electromagnetic wave absorber 10 according to this embodiment. Generally, in this embodiment, the metamaterial cell 111 is composed of one layer of a so-called asymmetric sectional resonator (ASR) structure of various shapes.

For example, the ASR is composed of four sections 112 with different characteristic sizes, denoted A1, A2, A3, and A4, which are combined to form a new metamaterial cell (unit cell) 111, where the characteristic size of each section 112, e.g., the width w, gap g, and perimeter length l _i of each section 112, is related to its respective resonant frequency.

2A and 2C, for example, a material (such as a metal) having a circumferential shape (ring, square, or rectangle) with a width w is divided into four sections 112, and each section 112 is arranged with a distance (gap) g between its end faces. In this case, the sections 112 are arranged so that the distances (perimeter lengths) l _i (l ₁ to l ₄ ) from the center of the metamaterial cell (hereinafter also referred to as "ASR cell") 111 to the outer periphery of each section 112 are different. Here, the width w and gap g of each section 112 are the same. Also, the center of the ASR cell 111 is the intersection of lines connecting the centers of the outer peripheries of the sections 112 facing each other in the cell 111.

2B, a material (such as a metal) having a circumferential shape (square or rectangle) with width w is divided into four sections 112, and each section 112 is arranged with a distance (gap) g between its end faces. At this time, the sections 112 are arranged so that the distance (peripheral length) l _i (l ₁ to l ₄ ) from the approximate center line of the ASR cell 111 to the outer periphery of each section 112 is different. Here, the width w and gap g of each section 112 are the same. Also, the approximate center line of the ASR cell 111 is a straight line that is parallel to any one outer periphery and passes through two opposing gaps g.

In this manner, the outer circumferential lengths l _i of the sections 112 are slightly different, forming an asymmetric ASR cell 111 .

Here, the width w and the gap g do not have to be the same. Also, there do not have to be four sections, as long as there are multiple sections.

Although an example has been given in which asymmetric ASR cells 111 are formed by the difference in circumferential length l _i of each section 112, asymmetric ASR cells 111 may also be formed by the difference in width w and gap g of each section 112.

In this embodiment, the metamaterial cell 111 is composed of a plurality of sections 112, each of which is formed by dividing a material having a circular shape. The ASR cell 111 is formed based on the differences in the characteristic sizes (width w, gap g, peripheral length l _i ) of each section 112, and as a result, the resonant frequencies of each section 112 of the ASR cell 111 only need to be slightly different.

Here, the size of the ASR cell 111 may be less than half the size of the target radiation (incident electromagnetic wave). The characteristic size and the difference between them of each section 112 may be set within a range that satisfies the size of this cell 111. For example, when the incident electromagnetic wave is a millimeter wave, it is desirable that the size of the ASR cell 111 is about 0.5 mm or less, and the difference in the characteristic size is 0.001 mm or more and 0.1 mm or less.

The resonant frequencies of each section 112 of the ASR cell 111 are slightly different and when combined produce broader absorption characteristics that are highly advantageous in absorbing the various EM wave modes generated in high speed ICs.

First, adjust the shape, size, period of the cells 111, and arrangement style of the ASR metamaterial cells 111 shown in Figures 2A-C to achieve a desired resonant frequency range. The resonant frequency _fR of the metamaterial can be expressed by Equation (2):

Here, L is the equivalent inductance and C is the equivalent capacitance of the metamaterial cell 111.
The equivalent inductance L and capacitance C can be expressed by the following equations (3) and (4), respectively.

where w is the width of the resonator element, ε ₀ and μ ₀ are the permittivity and permeability of free space, l _i is the external dimension (length) of the section resonator, a and b are numerical factors, ε _r is the relative permittivity (dielectric constant), and h is the thickness of the dielectric spacer layer.

Once the material of the dielectric spacers 12 is selected, the gap between the sections 112 of the ASR structure and the width w of each section 112 is also determined from equations (2)-(4), and the resonant frequency of the ASR metamaterial is proportional to the size of the section resonator associated with the contour length l _i of each section 112, as shown in equation (5).

By varying the geometry of the ASR structure and the thickness of the dielectric spacer 12, the effective permittivity ε and permeability μ of the metamaterial absorber 10 can be independently tuned to impedance match the substrate 14 of the high speed electrical circuit. When EM radiation is incident on the ASR structure, the metamaterial absorber 10 absorbs the EM wave modes through coupled resonance. In the metamaterial absorber 10, the metamaterial layer 11 is a structure that strongly couples to the electric component of the EM wave, while the combination of the spacer layer 12 and the reflective thin film 13 strongly couples to the magnetic component of the EM wave.

3 shows an example of a metamaterial cell (unit cell) obtained by simulation in this embodiment. This metamaterial cell (unit cell) 111 is tuned to 300 GHz and uses a circular ASR metamaterial cell. In addition, a thick indium phosphide (InP) substrate with a dielectric constant _εr1 of 12.4 and a dielectric spacer layer with a dielectric constant _εr2 of 8 and a thickness of 12.5 μm are used.

In FIG. 3, the metamaterial absorber 10 is placed below a thick semiconductor substrate 14, allowing direct transmission of the incident EM wave between the substrate 14 and the metamaterial absorber 10. Simulations of the metamaterial cell 111 used a time-domain solver with normal incidence and appropriate periodic boundary conditions, with the electrical component of the EM wave along the x-axis.

The reflection R(ω) and transmission T(ω) of the metamaterial cell 111 can be calculated from the S parameters that depend on the extraction frequency using equations (6) and (7), respectively.

Here, S ₁₁ (ω) is the reflection coefficient and S ₂₁ (ω) is the transmission coefficient.

The absorption A(ω) of the EM wave passing through the absorber is calculated by equation (8).

Since transmission is eliminated by the reflective thin film 13, which is thicker than the skin depth δ shown in equation (1), and the transmission value is zero in the entire measured frequency range, equation (8), which represents absorption, is simplified to equation (9).

4A shows an example of a simulated absorption spectrum of the metamaterial absorber 10 at a frequency of 300 GHz, where the absorption was calculated according to equation (9) from the extracted reflection coefficient S ₁₁ (ω) obtained from the simulation shown in FIG.

The optimized absorber has a broadband absorption of 30 GHz, observed between 285 GHz and 315 GHz, where the broadband absorption width (30 GHz) is measured as the bandwidth for 90% or greater absorption, similar to that of a conventional broadband absorber. Furthermore, due to the nearly symmetrical design of the ASR cell 111, the simulated absorption showed similar absorption for both the transverse electric (TE) 102 and transverse magnetic (TM) 101 polarization modes.

As shown in Fig. 4B, very high absorption was achieved by impedance matching between the metamaterial absorber structure and the semiconductor substrate 14 of the high-speed circuit. The effective impedance was calculated according to Equation (10) with _S21 (ω)=0 based on the extracted reflection coefficient _S11 (ω) obtained from the simulation in Fig. 4.

In Fig. 4B, on average, the real part Re(z) 103 of the relative impedance is about 1 and the imaginary part Im(z) 104 is about 0 in the wideband frequency range observed in Fig. 4A. When good impedance matching is obtained, _Zin (ω) = _ZS = 1, so the input impedance ( _Zin ) of the metamaterial absorber 10 according to this embodiment is almost equal to the impedance ( _ZS ) of the substrate 14 in the wideband frequency range. Therefore, the absorption rate of the metamaterial absorber 10 according to this embodiment is almost 100%.

As shown in Figure 5, a coplanar waveguide 20 with a metamaterial absorber 10 on its back surface is used to evaluate the optimized metamaterial cell shown in Figure 3 in a more practical condition.

Typically, the structure of the coplanar waveguide 2 has a waveguide port 25, 26 at one end and the other end of the waveguide 20, respectively, and is composed of a surface metal layer and a bottom surface ground layer. In the coplanar waveguide 2, the propagation direction of the electromagnetic radiation is different compared to the simulation of the metamaterial cell 111, since it is from the first waveguide port 25 to the second waveguide port 26. To test the metamaterial absorber 10, the bottom surface ground layer of the coplanar waveguide 20 is replaced with the metamaterial absorber 20 designed as described above. The metamaterial layer 21 is added to the back surface of the substrate 24 together with the dielectric spacer layer 22. Furthermore, a reflective thin film 23 is added to the back surface (the bottom layer of the coplanar waveguide 20). The reflective thin film 23 also serves as the ground layer of the coplanar waveguide 20.

<Characteristics of electromagnetic wave absorbers>
Figure 6 shows the frequency dependent transmission coefficient _S21 (ω) for three different coplanar waveguide structures. In the first case 201, the coplanar waveguide 2 is equipped with an optimized structure of the metamaterial absorber 20. In the second case 202, the metamaterial layer 21 is removed from the absorber 20, leaving only the spacer dielectric layer 22 and the reflective thin film 23. In the third case 203, the material of the spacer dielectric layer 22 is replaced with the same material as the substrate 24 to simulate the inclusion of the metamaterial layer 21 in the semiconductor substrate 24.

In the first case 201, a high-intensity resonant peak was observed, and it was confirmed that the electromagnetic waves transmitted through the waveguide were absorbed in a certain frequency range.

In the second case 202, the absorber without the metamaterial layer does not affect the transmission of the EM wave modes. Therefore, typical coplanar waveguide transmission of the EM wave occurs with minimal loss.

In the third case 203, flat transmission characteristics were observed in the measured frequency range, so that the metamaterial absorber in which the dielectric spacer layer 22 is replaced with the same material as the substrate 24 (InP in this embodiment) does not absorb EM wave modes.

The effects on the operation of the optimized metamaterial absorber structure are discussed below.

In the coplanar waveguide 2, the orientation of the electric field toward the metamaterial cell 211 is different from that of the simulated single metamaterial cell 111 shown in Figure 3. In the single metamaterial cell 111 shown in Figure 3, the electric field vector E is parallel to the surface of the metamaterial absorber 10, so that LC resonance is easily induced in the gap of the ASR structure.

On the other hand, in a coplanar waveguide, due to the structure of the waveguide and its operating principle, the electric field vector E is oriented perpendicular to the surface.

Normally, when the electric field vector E is oriented perpendicular to the surface of the metamaterial, no resonance occurs because the perpendicular electric field E (electric field with electric field vector E) cannot couple into the metamaterial gap. However, a small additional frequency shift can be observed to cause resonance to occur, as shown in Figure 6.

In addition, in the third case described above, although the same metamaterial layer 21 is included, the occurrence of resonance is suppressed by making the material of the dielectric spacer 22 the same as that of the substrate 24.

Thus, in the coplanar waveguide 2 of this embodiment, the occurrence of resonance in the metamaterial absorber 20 is influenced by additional factors.

From equations (3) and (4), since the resonant frequency is proportional to the relative dielectric constant _εr of the substrate 24, the shift of the resonant peak for various substrates 24 with different relative dielectric constant values can be clearly obtained.

7 shows the calculation results of the relationship between the position of the resonance peak of the metamaterial layer 21 and the change in the relative permittivity _εr2 of the dielectric spacer layer 12 in two different high-speed IC semiconductor substrates 24. Calculations were performed for the case where the dielectric constant _εr1 of the semiconductor substrate 24 is 12.4 204 and for the case where the dielectric constant _εr1 is 6 205. In both cases of the two substrates, a shift of the resonance peak position to a lower frequency is observed by increasing the value of the relative permittivity _εr2 of the dielectric spacer layer 22. The shift change is larger for substrates with smaller values of the relative permittivity _εr1 .

6, no resonant peaks are observed upon removal of metamaterial layer 21 or replacement of the material of spacer layer 22 with the material of substrate 24. Thus, metamaterial layer 21 and spacer layer 22 are necessary for the generation of resonant peaks and absorption of transmitted EM waves.

The shift in the resonant peak shown in Figure 7 is due to the transmission of an electric field E through a waveguide having a metamaterial layer 21 disposed at the interface of two dielectric materials having different dielectric constants.

To explain the shift in the resonant peak, first consider an interface between two dissimilar dielectrics under an electric field. For an EM wave propagating in a waveguide, the electric field E is oriented perpendicular to the upper and lower metal layers, generating an electric flux density D. The electric flux density vector D is defined by equation (11):

Here, ε is the relative dielectric constant (dielectric constant) of the dielectric film and E is the electric field vector.

When the interface between the substrate 24 and the spacer layer 22 is composed of two materials with different dielectric constants, a change in the electric field E occurs upon transmission across the dielectric boundary.

The electric flux vector D at normal incidence is continuous across a dielectric boundary, resulting in a discontinuity in the resultant electric field E related to the dielectric constant, which can be separated into a continuous tangential electric (E _T ) field mode parallel to the horizontal plane of the waveguide, i.e., the interface, and a perpendicular electric field E (E _N ).

The electric field E _T is oriented in the plane of the metamaterial cell 211, so that resonance is induced in the metamaterial gap.

Furthermore, since the emergence of modes of the tangential electric field E _T strongly depends on the change in the dielectric constant and the propagation of the electric field E in these dielectrics, a resonance shift due to the difference between ε _r1 and ε _r2 is observed.

It is also observed that the larger the difference in the dielectric constants, the larger the difference in the electric field propagation and the larger the change in the direction of the electric field vector E, resulting in a stronger tangential electric field component. This results in an increase in the intensity of the resonant peak and stronger absorption.

8 shows the dependence of the resonant peak intensity on the relative dielectric constant ( _εr2 ) of the spacer layer 22 in the substrate ( _εr1 = 12.4) 24. When the relative dielectric constant _εr2 of the spacer layer 22 is about 12.4, i.e., approximately the same as the relative dielectric constant _εr1 of the substrate 24, the resonant peak intensity is at a minimum. In other words, the greater the difference between the relative dielectric constant _εr1 of the substrate 24 and the relative dielectric constant _εr2 of the spacer layer 22, the higher the peak intensity observed.

For example, the peak intensity is high when the dielectric constant _εr1 of the substrate 24 is 12.4 and the dielectric constant _εr2 of the spacer layer 22 is about 1 to 10 or about 15 to 20. In other words, the resonance peak intensity is high when the difference between the dielectric constant _εr1 of the substrate 24 and the dielectric constant _εr2 of the spacer layer 22 is about 2 or more and 11 or less.

Thus, when the materials of the substrate 24 and the spacer layer 22 are the same or similar, the occurrence of tangential electric modes is limited and resonances in the metamaterial layer 21 are not induced due to the lack of a well-established dielectric interface.

As described above, by designing the dielectric interface, the shift of the resonant peak in the coplanar waveguide 2 can be predicted and additional tuning of the metamaterial absorber 20 can be easily performed to exhibit broadband absorption in the required frequency range.

First Example
As a first embodiment of the present invention, an example in which the metamaterial absorber 10 according to the first embodiment is applied to an InP substrate coplanar waveguide intended for a high-speed IC will be described.

In the simulated device, the InP substrate 14 has a relative dielectric constant ε _r1 =12.4 and a thickness t _s =55 μm. The InP substrate parameters and its thickness are based on values used in real InP high-speed ICs. The top layer of the coplanar waveguide is a 2 μm thick gold film. The dimensions of the waveguide are determined by the total size of the metamaterial absorber 10 (3×7 cells) placed on the backside of the substrate 14, and are equal to 1.89 mm long and 0.81 mm wide.

The metamaterial absorber 10 is placed on the back side of the InP substrate 14. The metamaterial layer 11 is made of a gold thin film with a thickness of 200 nm. The parameters of the ASR cell 111 are the radius of each section 112 as follows: l ₁ =0.126 mm, l ₂ =0.116 mm, l ₃ =0.129 mm, and l ₄ =0.119 mm. The width w=0.045 mm, the gap size g=0.018 mm, and the cell period size a=0.27 mm. The parameters are adjusted to obtain broadband absorption at 300 GHz. The spacer layer 12 is a silicon nitride film with ε _r2 =8 and a thickness of 12.5 μm. Finally, a gold thin film with a thickness of 200 nm is formed on the bottom side of the spacer layer 12. The material has a thickness sufficient to block all transmission for electromagnetic waves with frequencies above 150 GHz.

The effects of this embodiment will be described below. Figure 9 shows the results of calculations of the electromagnetic wave modes propagating through the coplanar waveguide in this embodiment for an InP/SiN waveguide 301 without a metamaterial layer, an InP/SiN waveguide 302 with a metamaterial layer 11, and an InP/InP waveguide 303 with a metamaterial layer 11. The electric field mode is shown on the left in the figure, and the magnetic field mode is shown on the right.

As shown in Figure 9, the electromagnetic wave mode propagating through the coplanar waveguide 302 in this embodiment is largely absorbed. The electromagnetic wave passes through the coplanar waveguide having the metamaterial absorber 10 on the back surface and is gradually absorbed. When there is no metamaterial layer 301 or when the same dielectric material is used for the spacer layer 12 303, no absorption of the electromagnetic wave mode is observed.

This embodiment mitigates the trade-off between better suppression of substrate resonance and ground bounce on the one hand and complex multi-step processing and smaller packaging of IC circuit elements on the other.

In this way, according to this embodiment, good suppression of electromagnetic wave modes can be achieved by a simple process without forming TSVs, so the mechanical strength of the substrate is increased and higher cost-effectiveness is achieved. In addition, since TSVs are not required, layout design is simplified and the mounting level of high-speed ICs can be significantly improved.

Furthermore, different parts of the metamaterial absorber 10 can be freely tuned by varying the materials in each layer, the design of the metamaterial cells, and the thickness of each layer, allowing it to be tailored for various frequency ranges and different material-based high-speed IC circuits.

Second Example
In the electromagnetic wave absorber according to the second embodiment of the present invention, an active metamaterial absorber is used as the metamaterial absorber, instead of the passive absorber used in the first embodiment.

Resonance occurs when the electric field E of the incident electromagnetic wave is localized parallel to the gap in the metamaterial cell, and the gap acts as an equivalent capacitance according to equation (2).

Therefore, by adding a variable capacitor diode (varactor) or transistor as a controllable variable capacitance in the gap region between the sections of the metamaterial cell, the resonant frequency of the metamaterial can be changed and actively tuned.

As shown in FIG. 10, in the metamaterial absorber of this embodiment, the high absorption of the electromagnetic wave mode is shifted towards different frequency ranges. Changing the lumped capacitance value C in the metamaterial absorber from 0 fF to 5 fF shifts the broadband absorption peak towards lower frequencies. The center frequency of the metamaterial cell shifts from 300 GHz when C is 0 fF to 265 GHz when C is 5 fF.

Furthermore, the absorption band for an absorption rate of 90% or more decreases from 30 GHz when C is 0 fF to about 20 GHz when C is 5 fF. In this embodiment, since the absorption peak can be shifted only to low frequencies, once the maximum operating frequency is determined for the passive metamaterial absorber structure, the resonant frequency can be actively adjusted in the frequency range below the maximum operating frequency.

Active control of the absorption characteristics is achieved by controlling the bias voltage applied to the active components, so that in cases where the high-speed IC is capable of operating at a wide range of frequencies over a wide bandwidth, the active metamaterial absorber can be tuned to suppress the resonant substrate modes that arise.

This embodiment achieves similar effects to the first embodiment while increasing the operational freedom of application of a single design and improving the cost-effectiveness of manufacturing a single IC and metamaterial absorber that operates over a wider frequency range.

<Third Example>
In the electromagnetic wave absorber according to the third embodiment of the present invention, the metamaterial absorber is modified to increase the absorption of the substrate resonant modes in the IC chip. A single cell of the metamaterial absorber can absorb up to 100% of the EM incident radiation of a certain wavelength depending on the geometry of the resonant metamaterial cell. Thus, the broadband characteristics of the metamaterial absorber are increased by combining ASR cells (unit cells) of different sizes.

FIG. 11 shows a coplanar waveguide 40 with an electromagnetic wave absorber according to this embodiment. In the electromagnetic wave absorber according to this embodiment, a collection (group) 41 of metamaterial cells tuned to slightly different resonant frequencies are connected. Since the metamaterial absorber 10 is formed on the entire back surface of the chip, the propagating EM waves are almost completely absorbed in a certain optimized frequency range. By gradually changing the size (characteristic size, e.g., g, w, l ₁ to l ₄ in FIG. 2A) of the metamaterial geometric shape of each group (first group to nth group) 41_1 to 41_n, a small shift of the resonant frequency f _Ri (i=1 to n) is obtained. As a result, a broadband absorption characteristic is obtained.

Thus, in this embodiment, an assembly of metamaterial cells with different characteristic sizes is arranged to form adjacent lines of metamaterial assemblies with shifted resonant frequencies. This configuration increases the absorption bandwidth since the propagating EM wave modes are absorbed over a broad frequency range (one or more frequencies).

The metamaterial absorber of this embodiment has the same effect as the first embodiment and can operate over a wider frequency range.

<Fourth Example>
An electromagnetic wave absorber according to a fourth embodiment of the present invention includes an ASR structure having different resonant frequencies, and has a multi-layer laminate structure including symmetric metamaterial cells.

This configuration combines the absorption characteristics of each metamaterial cell or collection of metamaterial cells, increasing the bandwidth of the absorption characteristics, i.e. improving the broadband absorption of the substrate resonant modes in the IC.

The metamaterial absorber achieves even broader bandwidth absorption through a synergistic effect between the characteristics of the ASR structure and the multilayer structure of the metamaterial cell.

The metamaterial absorber of this embodiment has the same effect as the first embodiment and can operate over a wider frequency range.

In the embodiment of the present invention, examples of the structure, dimensions, materials, etc. of each component in the configuration of the absorbent body and the manufacturing method are shown, but the present invention is not limited to these. Anything that can exert the function and effect of the absorbent body will do.

The present invention can be applied to high-frequency integrated circuits.

10: Electromagnetic wave absorber 11: Metamaterial layer 12: Spacer layer 13: Reflective thin film 14: Substrate

Claims (8)

An electromagnetic wave absorber disposed on a substrate,
a metamaterial layer, a spacer layer, and a reflective thin film,
Absorbing and suppressing resonant modes of electromagnetic waves within the substrate ;
The metamaterial layer comprises a plurality of subwavelength structure metamaterial cells;
the metamaterial cells are arranged periodically and designed to resonate at a predetermined frequency;
An electromagnetic wave absorber , characterized in that the structure and period of the metamaterial cells are determined by the wavelength of the incident electromagnetic wave . the metamaterial cell is composed of a plurality of sections having different characteristic sizes;
2. The electromagnetic wave absorber of claim 1 , wherein the plurality of sections are asymmetrically arranged. An electromagnetic wave absorber disposed on a substrate,
a metamaterial layer, a spacer layer, and a reflective thin film,
Absorbing and suppressing resonant modes of electromagnetic waves within the substrate;
13. An electromagnetic wave absorber, comprising : a metamaterial layer having an effective dielectric constant and a magnetic permeability adjusted to match an impedance of the metamaterial layer to an impedance of the substrate and to absorb a resonant mode of the electromagnetic wave. An electromagnetic wave absorber disposed on a substrate,
a metamaterial layer, a spacer layer, and a reflective thin film,
Absorbing and suppressing resonant modes of electromagnetic waves within the substrate;
13. An electromagnetic wave absorber, wherein a difference between the relative dielectric constant of said spacer layer and the relative dielectric constant of said substrate is 2 or more and 11 or less. An electromagnetic wave absorber disposed on a substrate,
a metamaterial layer, a spacer layer, and a reflective thin film,
Absorbing and suppressing resonant modes of electromagnetic waves within the substrate;
The electromagnetic wave absorber is characterized in that the electromagnetic waves are generated from a source within a substrate of a high-speed electronic circuit. the metamaterial layer couples to an electric component of the electromagnetic wave;
6. The electromagnetic wave absorber according to claim 1 , wherein the spacer layer and the reflective thin film are coupled to a magnetic component of the electromagnetic wave. The electromagnetic wave absorber according to any one of claims 1 to 6, wherein the reflective thin film is thicker than a skin depth of the electromagnetic wave. A high-speed electronic circuit comprising the electromagnetic wave absorber according to any one of claims 1 to 7 .

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