Terahertz coding metasurface antenna unit based on MEMS switch

Abstract

The invention provides a terahertz coding metasurface antenna unit based on an MEMS switch, and belongs to the technical field of radio frequency antennas. The antenna comprises an upper substrate, a radio frequency ground interlayer and a lower substrate. The upper substrate is provided with an MEMS clamped beam switch and a metal resonant structure, and the lower substrate is provided with a direct-current voltage bonding pad; the MEMS clamped beam switch is used for controlling the on-off of a central path in the metal resonant structure; a radio frequency ground via hole connected to the radio frequency ground interlayer is arranged below one metal anchor area in the MEMS clamped beam switch; and a direct-current voltage via hole connected to a direct-current voltage bonding pad is arranged below the pull-down electrode in the MEMS clamped beam switch. The metasurface unit has the broadband characteristic in the terahertz frequency band, and broadband terahertz wave regulation and control can be achieved. The terahertz wave reflector can realize low-loss terahertz wave reflection, and can be applied to terahertz and millimeter wave frequency bands of 350GHz and below.

Description

Technical Field

[0001] The present invention belongs to the technical field of radio frequency antennas, and particularly relates to a terahertz coding metasurface antenna element based on an MEMS switch. Background Art

[0002] Metamaterials or metasurfaces are artificial synthetic materials with periodic microstructures or nanostructures used to interfere with the transmission of electromagnetic waves. They have diverse functions, are easy to implement, and are very thin and light. They can obtain advantages beyond traditional materials by reflecting, absorbing, focusing, or bending electromagnetic waves, and have received extensive attention in recent years.

[0003] The MEMS-based reconfigurable metasurface is an advanced terahertz beam steering technology, which is expected to make up for the lack of phase control devices in the terahertz band and has broad application prospects in the fields of satellite communication systems, 6G intelligent communication systems, broadband network systems, and radar systems. The research on terahertz band beam reconfigurable technology is still in its infancy. Limited by scientific issues such as phase control mechanisms and beamforming methods, and due to system reliability and complexity issues, unit-level control is difficult to achieve in large-scale arrays, seriously affecting its practical applications.

[0004] Currently, there are very few achievements in terahertz band MEMS reconfigurable metasurfaces. The terahertz MEMS metasurface structure is unstable, it is not easy to ensure the processing yield, and the loss generated when acting on terahertz waves is large. For example, in 2011, Hu Tao et al. from Boston University designed a structure-tunable terahertz metamaterial using cantilever beams and used temperature to control the warping angle of the MEMS cantilever beams. This achievement can only achieve the overall control of the metasurface; in 2015, Pitchappa et al. from the National University of Singapore proposed a MEMS metamaterial with row and column control on the basis of the previous research, which can be adjusted independently to a certain extent and has excellent switching performance; in 2017, this research group further improved the performance of the modulator in the underdamped region, covering the entire 360° phase modulation range, but the stability of the cantilever beam needs to be improved, and the uniformity is insufficient in large-scale arrays.

[0005] Due to the current disadvantages of terahertz MEMS metasurfaces such as unstable structure, low adjustable degrees of freedom, and large losses, it is currently difficult to meet the requirements for applications on radio frequency devices such as antennas. The reconfigurable metasurface in the terahertz band has not yet been able to be applied in stable beamforming. Summary of the Invention

[0006] In view of this, the present invention proposes a reconfigurable metasurface unit based on a MEMS structure. The present invention can achieve phase change of a terahertz antenna based on a MEMS structure, can meet the requirements of metasurface unit-level reconfigurability and arbitrary generation of reflected beams in the terahertz frequency band, and the metasurface unit has characteristics such as wide bandwidth, small reflection loss, obvious switching characteristics, stable structure, convenient design and implementation, etc.

[0007] The object of the present invention is achieved as follows:

[0008] A terahertz coding metasurface antenna unit based on a MEMS switch includes an upper substrate and a lower substrate. The lower surface of the upper substrate and the upper surface of the lower substrate both have metal layers, and the two metal layers are welded together to form a radio frequency ground sandwich layer (3);

[0009] The upper surface of the upper substrate has a metal resonant structure (1). The metal resonant structure (1) includes a large loop (8) located in the middle, two end branches (9) located on both sides, and a central passage (7) passing through the large loop and communicating with the two end branches. The central passage (7) is disconnected at the middle position;

[0010] The lower surface of the lower substrate has a DC voltage pad (6);

[0011] A MEMS fixed beam switch (2) for controlling the on / off of the central passage is provided at the center of the large loop; the MEMS fixed beam switch (2) includes a switch membrane bridge (11), two metal anchor areas (10) located at both ends of the switch membrane bridge (11), and a pull-down electrode (12) provided below the switch membrane bridge (11); a radio frequency ground via hole (4) connecting the metal anchor area (10) and the radio frequency ground sandwich layer (3) is provided below one metal anchor area (10); a DC voltage via hole (5) connecting the pull-down electrode (12) and the DC voltage pad (6) is provided below the pull-down electrode (12), and there is a gap between the radio frequency ground sandwich layer (3) and the DC voltage via hole (5).

[0012] Further, the material of the upper substrate is fused quartz, and the material of the lower substrate is glass or silicon.

[0013] Further, the switch membrane bridge (11) is made of a metal material or a metal-dielectric composite material; a hollow hole is opened on the switch membrane bridge (11), so that the maximum horizontal line width of the switch membrane bridge (11) does not exceed 10 microns.

[0014] Further, the large loop (8) is rectangular, the central passage (7) is located on the central axis of the long side of the large loop (8), the end branches (9) are parallel to the long side of the large loop (8), and the entire metal resonant structure (1) is symmetric about the central axis of the long side and the central axis of the short side of the large loop (8).

[0015] Further, the MEMS clamped beam switch (2) is located within the central enclosed area of the large loop (8). The long side direction of the switch membrane bridge (11) is perpendicular to the central passage (7), and an electrode isolation layer is covered on the pull-down electrode (12). When the switch membrane bridge (11) is pulled down by the pull-down electrode (12), the metal part on the switch membrane bridge (11) contacts the middle break point of the central passage (7), thereby connecting the central passage.

[0016] Further, the thickness of the upper substrate does not exceed one hundred micrometers.

[0017] The beneficial effects of the present invention are as follows:

[0018] 1. The metasurface unit of the present invention has broadband characteristics in the terahertz frequency band. The phase difference curve adjusted by the switch on-off shows a relatively slow change in the frequency domain, and broadband terahertz wave regulation can be achieved.

[0019] 2. The present invention can achieve low-loss terahertz wave reflection. The reflection loss of the metasurface unit is less than 1 dB in both phase switching states, and it can be applied to the terahertz and millimeter wave frequency bands below 350 GHz.

[0020] 3. The structure of the present invention is novel and unique, and can be obtained by conventional processing. Description of the Drawings

[0021] Figure 1 is the overall structure schematic diagram of the embodiment of the present invention;

[0022] Figure 2 is Figure 1 the top layer graphic schematic diagram of

[0023] Figure 3 is the electromagnetic response equivalent circuit diagram of the embodiment of the present invention;

[0024] Figure 4 is the side view of the MEMS switch of the embodiment of the present invention;

[0025] Figure 5 is the structure change simulation data diagram of the MEMS switch under different potential differences in the embodiment of the present invention;

[0026] Figure 6 is the S-parameter curve diagram of the unit of the embodiment of the present invention;

[0027] Figure 7 is the phase curve diagram of the unit of the embodiment of the present invention. Detailed Embodiments

[0028] The present invention will be further described below with reference to the accompanying drawings.

[0029] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0030] A terahertz-coded metasurface antenna unit based on an MEMS switch, as Figure 1 and 2 shown, includes a metal resonant structure 1 disposed on the upper surface of the substrate, an MEMS fixed beam switch 2, a radio frequency ground interlayer 3 located between two layers of the substrate, and a DC voltage pad 6 located on the lower surface of the substrate.

[0031] Among them, the metal resonant structure 1 includes a central passage 7, a large loop 8 and a terminal branch 9. The central passage 7 is controlled to be turned on and off by the MEMS fixed beam switch 2; the MEMS fixed beam switch 2 is located at the center of the large loop 8, and the long side direction of the beam is perpendicular to the central passage, and the on and off of the central passage is realized in a metal contact manner. The switch itself has broadband and low-loss characteristics in the terahertz frequency band.

[0032] The MEMS fixed beam switch 2 is located at the central position of the large loop 8 of the metal structure 1. The long side direction of the switch is perpendicular to the central passage 7. It includes a metal anchor region 10, a switch membrane bridge 11, an electrostatic pull-down electrode 12 and an electrode isolation layer 13. The metal layer of the switch membrane bridge 11 is connected to the metal anchor region 10 and spans over the pull-down electrode 12 to form a suspended structure, realizing an electrostatic force drive structure; the MEMS fixed beam switch 2 is axisymmetric. The switch membrane bridge 11 can be designed as a pure metal material or a metal-dielectric composite material as required. When designed as a composite material, the metal layer therein must be connected to the metal anchor region, cover the electrostatic pull-down electrode, and form a metal contact across the central passage to conduct the signal path when pulled down; one of the two metal anchor regions is connected to the radio frequency ground via hole 4, and the electrostatic pull-down electrode 12 is connected to the DC voltage pad 6 through the DC voltage via hole 5; in order to facilitate the release of the sacrificial layer during the processing of the switch, holes are opened on the switch membrane bridge 11 to penetrate the entire membrane bridge structure to achieve hollowing, so as to ensure that the maximum horizontal line width of the switch membrane bridge is between 5 and 10 microns.

[0033] The metasurface antenna element is a radio frequency structure composed of three layers of surface metal, an MEMS switch structure, two layers of low-loss substrates, and interlayer metallized vias. The conduction of the element is remotely controlled by a specific voltage. Among them, one of the two types of interlayer vias connects the electrostatic pull-down electrode of the MEMS switch to the DC voltage pad, which is called the DC voltage via, and the other connects the metal anchor area of the MEMS switch and the radio frequency ground of the intermediate metal layer, which is called the radio frequency ground via. The vias can be realized by through-hole processes such as TGV and TSV and metal filling processes. The diameter of the through-holes needs to be ensured to be below 50 microns to achieve a high-density drilled substrate. The taper of the through-holes needs to reach at least 85°, and the metallization does not require filling. The two layers of low-loss substrates are tightly bonded together by methods such as Au-Au, Au-Sn, Cu-Cu, and Cu-Sn to ensure that the DC voltage via 5 can smoothly apply the potential difference from the bottom layer to the top layer, and the top and bottom layers of metal need to achieve electrical conduction.

[0034] The thickness of the upper substrate does not exceed one hundred microns and can be achieved by means such as mechanical and chemical mechanical thinning. The planarization manufacturing of the switch membrane bridge can be realized by means of the sacrificial layer process.

[0035] During use, the state of the MEMS fixed beam switch 2 can be controlled by applying a voltage to the DC voltage pad 6; the state of the MEMS fixed beam switch 2 can change the shape of the metal structure 1, and the radio frequency characteristics of the metal resonant structure 1 are determined by the large loop 8 with variable size and the end stub 9.

[0036] In this example, the size of the terahertz-coded metasurface antenna element is approximately half the wavelength corresponding to the center frequency. Among them, both the metal resonant structure 1 and the MEMS fixed beam switch 2 are axisymmetric. The metal resonant structure 1 only generates an on-off phase difference for the incident electromagnetic wave parallel to the central path 7; the metal resonant structure 1 has a switchable central path 7, and its length 14 affects the operating frequency of the entire antenna element. When the length 14 increases, the operating frequency decreases; the metal resonant structure 1 also has a large loop 8 with variable size and an end stub 9, and their sizes 18, 19, and 20 jointly determine the reflection phase difference in the on and off states. Optimizing the three sizes together makes the phase difference reach the optimal value; the final reflection phase of the antenna element is also related to the incident angle of the electromagnetic wave, and the optimization is only carried out for normal incidence.

[0037] In this embodiment, the MEMS fixed beam switch 2 is specially optimized. Combining process feasibility, multi-physics field simulation calculations are used to realize the surface process structure in the central gap of the large loop 8, and further realize the geometric shape reconfiguration of the central path 7.

[0038] The MEMS clamped beam switch 2 supports the entire switch structure through metal anchor regions 10 made of metals such as gold and aluminum. The switch membrane bridge 11 is flatly mounted on the two metal anchor regions, and the distance between its lower surface and the upper surface of the metal pattern is about 1 micron. The potential difference between the pull-down electrode 12 and the switch membrane bridge 11 causes an electrostatic adsorption force, which makes the center of the switch membrane bridge 11 bend downward until it contacts the electrode isolation layer 13 covering the pull-down electrode 12 and the central path 7. The typical MEMS clamped beam switch 2 has a downward-convex metal contact to ensure sufficient metal contact. The typical MEMS clamped beam switch laminated structure is as shown in Figure 4 This switch uses fused quartz as the substrate material, gold as the material for the pull-down electrode 12 and the metal layer of the switch membrane bridge 11, and silicon nitride as the material for the electrode isolation layer 13. The switch membrane bridge 11 also has a silicon oxide layer as the top layer material of the composite beam.

[0039] The typical radio frequency principle of the MEMS switch is as shown in Figure 3 This switch is driven by a DC voltage to realize the selection of two current paths. Due to the introduction of the switch beam, the equivalent circuit includes bilateral parasitic capacitances C B and parasitic inductances L T . When the switch is closed, the central path 7 presents a resistance R C . When the switch is open, it presents a high isolation structure with the double capacitances C U in series with the beam inductance L B . The MEMS switch in this embodiment has the following characteristics: the switch beam and the controlled path are perpendicular to each other, the break point is divided into two and distributed on both sides of the pull-down electrode, and the double break points reduce the open-circuit capacitance, thus realizing the characteristics of high frequency and high isolation.

[0040] The working mode of the metasurface antenna unit is as follows:

[0041] When the MEMS clamped beam switch 2 is open, the central path 7 fails. At this time, the response current of the antenna unit flows along the large loop 8 from one end stub 9 to the other, and the current path is relatively long, generating unique reflected electromagnetic characteristics in the frequency domain and producing a section of changing phase curve. On the contrary, if there is a potential difference sufficient to initiate the pull-down between the pull-down electrode 12 of the MEMS clamped beam and the metal layer of the switch membrane bridge 11, the MEMS clamped beam switch conducts. At this time, the central path 7 is complete, and the response current of the antenna unit flows along the central path 7, which is completely different from the state when the switch is open, generating another section of changing phase curve in the frequency domain.

[0042] In the overall structure of the antenna unit shown in Figure 1 , the radio frequency ground sandwich layer 3 with the same size as the entire antenna unit is used to reflect the incident beam and direct it to the space in the upper half plane. The resonant characteristics of the antenna unit are related to the opening size of the radio frequency ground sandwich layer 3. The smaller the opening ring, the greater the unit reflectivity.

[0043] The on and off states of the MEMS clamped beam switch represent two deflection states of the metasurface antenna element.

[0044] To reduce losses, metals with low resistivity, such as gold, are used for the metal material, and materials with low losses, such as fused silica glass, are used for the upper substrate.

[0045] The graphic structure of the top-layer metasurface metal has an important impact on the performance of the metasurface element, specifically manifested as follows:

[0046] a) The on-off position of the metasurface graphic affects the reconfigurable phase difference and directly affects the programmable effect. Therefore, the metal structure and the central disconnection position are unique designs;

[0047] b) The length of the graphic parallel to the incident electric field affects the resonant frequency of the element, thereby affecting the reflection amplitude and phase of the element;

[0048] c) The sizes of the large loop and the end stub affect the resonant frequency of the element and the phase difference between the two operating states. Only through optimization can the phase difference reach the optimal value of 180°.

[0049] When constructing the MEMS clamped beam switch, the structural characteristics of the clamped beam have an important impact on the final switch performance and the final realization of the element reflection, specifically manifested as follows:

[0050] a) The vertical distance between the pull-down electrode and the switch membrane bridge and the size of the pull-down electrode directly affect the switch isolation and the pull-down potential difference. Adjusting to the appropriate size enables the overall miniaturization and high isolation of the switch, thereby increasing the element reflectivity and minimizing the loss of the metasurface antenna;

[0051] b) The overlapping length of the switch membrane bridge and the central passage affects the switch isolation when off, and the width affects the resonant characteristics when on. Both states need to be considered simultaneously to optimize the switch and element performance, thereby achieving the best antenna reflection beam;

[0052] c) The length of the via connected to the pull-down electrode and the opening size of the RF ground sandwich also determine the resonant state of the element. When the switch is pulled down, there is also RF current coupling in the pull-down electrode, resulting in resonance between the current and the metallized via. Reducing the opening ring of the RF ground sandwich can achieve the effect of isolating resonance.

[0053] Since the metasurface element is coupled with the switch, voltage bias structure, etc. during specific implementation, resulting in differences in resonant characteristics in the two states, designing a reasonable MEMS switch form, feeding structure, and metasurface element is of great significance for improving the performance of terahertz programmable metasurfaces. The final structural parameters of the metasurface element are the result of comprehensive optimization.

[0054] Next, select a size combination for the example description (the following data units are micrometers):

[0055] Size 14 = 280, Size 15 = 85, Size 16 = 450, Size 17 = 70, Size 18 = 200, Size 19 = 430, Size 20 = 300;

[0056] The total thickness of the dielectric substrate is 400, and the metal layer thickness of the microstrip line and the metal ground is 1.

[0057] At this time, the electrostatic force simulation diagram of the MEMS switch in the metasurface unit is as follows:

[0058] Figure 5 What is shown in is the structural change of the MEMS switch in this embodiment under different potential differences. It shows that when the switch membrane bridge is subjected to the electrostatic force generated by a potential difference below 9V, the contact can be pulled down by 1 micrometer, making the central path structure where it is located conductive.

[0059] The S-parameters of the unit of the MEMS switch in the on and off states are as follows:

[0060] Figure 6 What is shown in is the S-parameters of the metasurface unit based on the MEMS switch in different working states. The four curves are respectively the reflection coefficient and the transmission coefficient of the switch in the on and off states. The results show that when the switch is in the on state, the transmission coefficient of the metasurface unit in the 307 - 327 GHz frequency band is less than -18 dB, and the reflection loss is less than 1 dB; while when the switch is off, the transmission coefficient of the metasurface unit is less than -22 dB in the 300 - 350 GHz frequency band, and the reflection loss is less than 1 dB.

[0061] The reflection phases of the unit of the MEMS switch in the on and off states are as follows:

[0062] Figure 7 What is shown in is the reflection phase characteristic of the above metasurface unit. The two curves are respectively the phases of the switch in the on and off states. The results show that when the switch is off, the reflection phase curve of the unit shows a convex upward characteristic, while after the switch is closed, the phase curve becomes concave downward. The phase difference between these two states is 180° ± 10° in the 307 - 327 GHz frequency band.

[0063] It can be seen that the metasurface unit controlled by the MEMS fixed - beam switch can achieve the 1 - bit programmability of the reflection phase in the terahertz frequency band.

[0064] The present invention realizes a reconfigurable metasurface unit with different phase responses through a low-loss switch. The potential of the bottom electrode of the switch is raised through a DC bias structure, and the electrostatic force generated by the potential difference between the bottom electrode of the switch and the metal part of the switch beam is used for adsorption, and the metal resonant structure is turned on in cooperation with the metal contact on the beam; when the central path of the metal resonant structure is turned on, the reflection response and the non-conducted state present a 180° phase difference in a large bandwidth.

[0065] Using this phase difference, a 1-bit large-scale metasurface array can be realized. The reconfigurable metasurface unit has a high reflectivity and a low transmittance. In the frequency band of 300 - 350 GHz, the reflection coefficient is greater than -1 dB, and the transmission coefficient is less than -15 dB; in the frequency band of 307 - 327 GHz, the phase difference between different switching states is 180° ± 10°. Compared with other terahertz metasurfaces, the present invention can achieve unit-level reconfiguration and large-angle phase regulation, and has the characteristics of high integration and good consistency. Its planar microstrip structure can realize rapid manufacturing.

[0066] The above are only several specific examples. If metasurface units with different center frequencies are to be obtained, different parameters can be adjusted according to the specific implementation manners. For example, the working center frequency can be adjusted by adjusting the length dimension of the unit pattern, the key dimensions of the switch, and the metallized via feeding structure, and the reflection coefficient and the phase difference can be adjusted; the reconfigurable metasurface unit can also be realized through other on-off switches.

Claims

1. A terahertz coding metasurface antenna unit based on an MEMS switch, characterized in that It includes an upper substrate and a lower substrate. The lower surface of the upper substrate and the upper surface of the lower substrate both have metal layers, and the two metal layers are welded together to form a radio frequency ground sandwich layer (3); The upper surface of the upper substrate has a metal resonant structure (1). The metal resonant structure (1) includes a large loop (8) located in the middle, two end stubs (9) located on both sides, and a central passage (7) passing through the large loop and communicating with the two end stubs. The central passage (7) is disconnected at the middle position; The lower surface of the lower substrate has a DC voltage pad (6); A MEMS fixed beam switch (2) for controlling the on / off of the central passage is provided at the center of the large loop. The MEMS fixed beam switch (2) includes a switch membrane bridge (11), two metal anchor regions (10) located at both ends of the switch membrane bridge (11), and a pull-down electrode (12) provided below the switch membrane bridge (11); a radio frequency ground via (4) connecting the metal anchor region (10) and the radio frequency ground sandwich layer (3) is provided below one metal anchor region (10); a DC voltage via (5) connecting the pull-down electrode (12) and the DC voltage pad (6) is provided below the pull-down electrode (12), and there is a gap between the radio frequency ground sandwich layer (3) and the DC voltage via (5).

2. The terahertz-coded metasurface antenna unit based on an MEMS switch according to claim 1, wherein The material of the upper substrate is fused quartz, and the material of the lower substrate is glass or silicon.

3. The terahertz coding metasurface antenna unit based on the MEMS switch according to claim 1, wherein The switch membrane bridge (11) is made of metal or metal-dielectric composite material; the switch membrane bridge (11) is provided with a hollowed-out hole, so that the maximum horizontal line width of the switch membrane bridge (11) does not exceed 10 microns.

4. The terahertz coding metasurface antenna unit based on an MEMS switch according to claim 1, wherein The large loop (8) is rectangular, the central passage (7) is located on the central axis of the long side of the large loop (8), the end stub (9) is parallel to the long side of the large loop (8), and the entire metal resonant structure (1) is symmetric about the central axes of the long side and the short side of the large loop (8).

5. The terahertz coding metasurface antenna unit based on an MEMS switch according to claim 4, wherein The MEMS fixed beam switch (2) is located in the central closed area of the large loop (8). The long side direction of the switch membrane bridge (11) is perpendicular to the central passage (7). An electrode isolation layer is covered on the pull-down electrode (12); when the switch membrane bridge (11) is pulled down by the pull-down electrode (12), the metal part on the switch membrane bridge (11) contacts the middle break point of the central passage (7), so that the central passage is connected.

6. The terahertz coding metasurface antenna unit based on an MEMS switch according to claim 1, wherein The thickness of the upper substrate does not exceed one hundred microns.

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