An X-type upper electrode type MEMS single-pole six-throw switch

Abstract

The application belongs to the technical field of radio frequency MEMS, and particularly relates to an X-shaped upper electrode type MEMS single-knife six-pole switch, which comprises a substrate, a microwave transmission line, a switch assembly and an air bridge. The microwave transmission line is arranged on the substrate. The microwave transmission line comprises an input signal line, a power divider, an output signal line and a ground line. The input signal line is arranged in one piece. The output signal line is arranged in six pieces. The input signal line is connected with the power divider. The power divider is connected with the output signal line through the switch assembly. The switch upper electrode of the application adopts an X-shaped double-end fixed beam structure. The anchor point is connected with the straight plate structure through a right-angle curved beam. The length of the upper electrode is indirectly increased. The elastic coefficient of the beam is reduced. The driving voltage is further reduced. The lower electrode top end adopts a semicircular structure. A funnel-shaped gradient structure is adopted at the connection position of the signal line. The two structures can reduce the coupling capacitance between the upper electrode and the lower electrode and improve the isolation of the switch. In addition, the three circular contacts are connected in parallel. The contact resistance is small. The current density is more uniform. The purpose of improving the insertion loss and improving the switch power capacity is achieved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of radio frequency MEMS, and particularly relates to an X-shaped upper electrode type MEMS single-pole six-throw switch. BACKGROUND

[0002] The demand for miniaturization, multi-channel and wide frequency band in the field of modern wireless communication promotes the progress of radio frequency MEMS technology. The devices (such as switches, filters, delay lines and attenuators) based on radio frequency MEMS technology have outstanding advantages in insertion loss, linearity, reliability, volume and the like, and are widely used in radar scanning, electronic countermeasures, satellite communication, microwave testing and the like.

[0003] The single-pole multi-throw switch can be integrated with multiple devices to realize multi-channel, adjustable and multifunctional microwave integrated circuits, and gradually becomes an ideal choice in the military and civilian fields. Under the impetus of this development trend, the single-pole multi-throw switch also faces new opportunities and challenges. On the one hand, the traditional radio frequency switch is mostly developed by using a PIN tube, and a single-pole multi-throw switch with high isolation, large power capacity and small opening voltage can be obtained. However, the PIN switch has the disadvantages of large size, high power consumption, large insertion loss and poor linearity. For example, in 2016, a compact structure PIN diode single-pole nine-throw switch was designed by Sichuan University. The switch has excellent isolation and switching time performance, but has a large insertion loss, a size of more than 28 cm2, and a working frequency band of only 800 MHz. In comparison, the MEMS switch can well improve the above problems and improve the utilization rate of radio frequency system signals.

[0004] On the other hand, most of the single-pole multi-throw switches in the MEMS field realize high frequency, low insertion loss and high isolation function, and the performance of the multi-channel (single-pole six-throw and above) MEMS switch is relatively small. For example, in 2015, a radio frequency MEMS single-pole seven-throw switch was proposed by Limoges University, and the working frequency band is only between DC~10GHz, and the isolation is small. In summary, the radio frequency MEMS single-pole multi-throw switch still has urgent application requirements and broad research space. SUMMARY

[0005] In view of the technical problems of the traditional single-pole multi-throw switch, such as large size, high insertion loss, low isolation, low working frequency and poor linearity, the application provides an X-shaped upper electrode type MEMS single-pole six-throw switch. The working frequency of the switch can cover the L~Ka band, has good radio frequency performance, meets the miniaturization and multi-channel requirements, and has high reliability.

[0006] In order to solve the above technical problems, the technical scheme adopted by the application is as follows:

[0007] The application discloses an X-shaped upper electrode type MEMS single-blade six-pole switch, which comprises a substrate, a microwave transmission line, a switch assembly, and an air bridge, wherein the microwave transmission line is arranged on the substrate, the microwave transmission line comprises an input signal line, a power divider, six output signal lines and a ground line, the input signal line is arranged in one, the output signal lines are arranged in six, the input signal line is connected with the power divider, and the power divider is connected with the output signal lines through the switch assembly.

[0008] The switch assembly comprises an upper electrode, a first lower electrode, a second lower electrode, a driving electrode, a lead wire, a first anchor point, a first contact group and a second contact group, the upper electrode is fixed on the substrate through the first anchor point, the tail end of the first lower electrode is connected with the power divider, the top end of the first lower electrode is fixed with the first contact group, the tail end of the second lower electrode is connected with the output signal lines, and the top end of the second lower electrode is fixed with the second contact group; the upper electrode is located above the driving electrode, the first contact group and the second contact group.

[0009] The upper electrode is in an X-shaped structure, the upper electrode comprises a straight plate structure and a right-angle curved beam structure, and the end corner of the straight plate structure is fixedly connected with the initial end of the right-angle curved beam structure; a release hole is arranged on the straight plate structure, the release hole comprises strip-shaped release holes and square release holes, the square release holes are arranged in an array form, there are 7 rows in the transverse direction and 2 columns in the longitudinal direction; two strip-shaped release holes are symmetrically arranged on the two sides of the square release hole array in the transverse direction; the right-angle curved beam structure is connected by three rectangular strips, the rectangular strips are fixedly connected in a 90-degree backfolding mode, and the tail end of the right-angle curved beam structure is connected with the first anchor point.

[0010] The first lower electrode and the second lower electrode are arranged on the surface of the substrate, the tail end of the first lower electrode and the second lower electrode is connected with the output signal lines and the power divider in a funnel-shaped gradual change structure, the top end of the first lower electrode and the second lower electrode is in a semicircular structure, and each port of the power divider is evenly arranged around a disc.

[0011] The driving electrode and the lead wire are fixed on the surface of the substrate, the driving electrode is arranged between the first lower electrode and the second lower electrode, the driving electrode is opposite to the middle part of the straight plate structure, the driving electrode is electrically connected with the lead wire, and the driving electrode applies a driving voltage through the lead wire.

[0012] The first contact group and the second contact group are respectively fixed on the top end of the first lower electrode and the second lower electrode, the first contact group and the second contact group are located directly below the longitudinal two ends of the straight plate structure, the first contact group and the second contact group each comprise three circular contacts, and the three circular contacts are evenly arranged around the semicircular side edge of the top end of the first lower electrode and the second lower electrode.

[0013] The end of the input signal line is provided with an input port, and the end of the output signal line is provided with an output port, the output ports are arranged in sequence clockwise, and the output ports are one port, two ports, three ports, four ports, five ports and six ports respectively.

[0014] The surface of the substrate is provided with a ground wire, and the ground wire is scattered around the profile of the input signal line, the output signal line and the lead wire.

[0015] The air bridge includes a first air bridge, a second air bridge and a third air bridge, the first air bridge, the second air bridge and the third air bridge are fixed on the ground wire through a second anchor point, a third anchor point and a fourth anchor point respectively, the first air bridge is arranged above the input signal line, the second air bridge is arranged above the adjacent two lead wires, and the third air bridge is arranged adjacent to the upper electrode of the switch assembly.

[0016] The second anchor point and the fourth anchor point are two groups of fixed air bridges, and the third anchor point is three groups of fixed air bridges.

[0017] Compared with the prior art, the present application has the beneficial effects that:

[0018] The switch upper electrode of the present application adopts an X-shaped double-end fixed beam structure, the anchor point and the straight plate structure are connected through a right-angle curved beam, the length of the upper electrode is indirectly increased, the elastic coefficient of the beam is reduced, and the driving voltage is reduced. The release hole of the switch upper electrode reduces the elastic stiffness of the straight plate structure and the air damping in the switch deformation process on the one hand, improves the switching speed of the switch, and on the other hand, the existence of the release hole can make the release of the sacrificial layer more sufficient in the process of manufacturing, and improve the reliability of the switch. The top end of the lower electrode of the switch of the present application adopts a semicircular structure, and the end is connected with a funnel-shaped transition zone, the isolation degree of the switch is improved by reducing the coupling capacitance. In addition, the switch is provided with three circular contacts, so that the current distribution is more uniform, and the switch failure problem is effectively improved. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only exemplary, and those skilled in the art can also obtain other implementation drawings according to the provided drawings without creating any creative labor.

[0020] The structures, proportions, sizes, etc. shown in the specification are only used to cooperate with the disclosed content, to be understood and read by those skilled in the art, and are not used to limit the implementation conditions of the application, so they do not have technical significance. Any modification of the structure, change of the proportion relationship or adjustment of the size, without affecting the effect and purpose that can be achieved by the application, should still fall within the scope covered by the disclosed technology.

[0021] Figure 1 The overall structure diagram of the single-blade six-pole switch of the application;

[0022] Figure 2 The structure top view of the switch assembly of the application;

[0023] Figure 3 The structure side view of the switch assembly of the application;

[0024] Figure 4 The structure top view of the lower electrode, contact and driving electrode of the application;

[0025] Figure 5 The structure diagram of the circular contact group of the application;

[0026] Figure 6 The structure diagram of the X-shaped upper electrode of the application;

[0027] Figure 7 The panoramic top view of the single-blade six-pole switch of the application, which peels off the upper electrode, the first anchor point and the air bridge;

[0028] Figure 8 The insertion loss diagram of the single-blade six-pole switch of the application when one port is on;

[0029] Figure 9 The isolation diagram of the other ports of the single-blade six-pole switch of the application when one port is on;

[0030] Figure 10 The insertion loss diagram of the single-blade six-pole switch of the application when two ports are on;

[0031] Figure 11 The isolation diagram of the other ports of the single-blade six-pole switch of the application when two ports are on;

[0032] Figure 12 The insertion loss diagram of the single-blade six-pole switch of the application when three ports are on;

[0033] Figure 13 The isolation diagram of the other ports of the single-blade six-pole switch of the application when three ports are on.

[0034] Wherein: 1 is a substrate, 2 is an input signal line, 201 is an input port, 3 is a power divider, 4 is an output signal line, 401 is a one-port, 402 is a two-port, 403 is a three-port, 404 is a four-port, 405 is a five-port, 406 is a six-port, 5 is an upper electrode, 6 is a first lower electrode, 7 is a second lower electrode, 8 is a driving electrode, 9 is a lead wire, 10 is a first anchor point, 11 is a first contact group, 12 is a second contact group, 13 is a straight plate structure, 131 is a strip-shaped release hole, 132 is a square release hole, 14 is a right-angle curved beam structure, 15 is a ground wire, 16 is a first air bridge, 17 is a second air bridge, 18 is a third air bridge, 19 is a second anchor point, 20 is a third anchor point, and 21 is a fourth anchor point. DETAILED DESCRIPTION

[0035] In order to make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments of the present application. These descriptions are only used to further explain the features and advantages of the present application, and are not used to limit the claims of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0036] The specific embodiments of the present application will be further described in detail below with reference to the accompanying drawings and embodiments. The following embodiments are used to illustrate the present application, but are not used to limit the scope of the present application.

[0037] The terms "first", "second", "third", "fourth", "fifth", "sixth", "seventh" and "eighth" are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second", "third", "fourth", "fifth", "sixth", "seventh" and "eighth" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "a plurality of" is two or more, unless otherwise specified.

[0038] In the description of the present application, it should be noted that, unless otherwise explicitly specified and limited, the terms "mounting", "connection", "connecting" should be understood in a broad sense, for example, it can be fixed connection, or detachable connection, or integral connection; it can be mechanical connection, or electrical connection; it can be direct connection, or indirect connection through an intermediate medium, or internal communication of two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0039] In the present embodiment, as shown in Figure 1 , Figure 7As shown, it comprises a substrate 1, a microwave transmission line, a switch assembly, and an air bridge. The microwave transmission line is arranged on the substrate 1, wherein the microwave transmission line comprises an input signal line 2, a power divider 3, six output signal lines 4, and a ground line 15. The input signal line 2 has one line, the output signal line 4 has six lines, and the input signal line 1 is connected to the power divider 3, and the power divider 3 is communicated with the output signal line 4 through the switch assembly. Overall, the single-pole six-throw switch has one input end and six output ends, and is distributed in a tree shape, scattered in a ring shape, and has uniform angles between the ports. The ports are connected by the circular power divider located in the center and controlled by the switch assembly of each branch to control the on-off of the branch signal.

[0040] Further, as shown in the figure, Figures 1-4 the switch assembly comprises an upper electrode 5, a first lower electrode 6, a second lower electrode 7, a driving electrode 8, a lead wire 9, a first anchor point 10, a first contact group 11, and a second contact group 12. The upper electrode 5 is fixed on the substrate 1 through the first anchor point 10. The end of the first lower electrode 6 is connected to the power divider 3, and the top end is fixed with the first contact group 11. The end of the second lower electrode 7 is connected to the output signal line 4, and the top end is fixed with the second contact group 12. The upper electrode 5 is located at a certain distance above the driving electrode 8, the first contact group 11, and the second contact group 12.

[0041] Further, preferably, as shown in the figure, Figure 6 the overall structure of the upper electrode 5 is X-shaped, comprising a straight plate structure 13 and a right-angled curved beam structure 14. The end angle of the straight plate structure 13 is fixedly connected to the beginning end of the right-angled curved beam structure 14. The upper electrode 5 belongs to a double-end fixed beam structure. When a voltage is applied to the driving electrode 8, the straight plate structure 13 will bend and deform. When the voltage reaches a threshold value, the two ends of the straight plate structure 13 will be in close contact with the contacts, and the switch will be turned on. The right-angled curved beam structure 14 reduces the elastic coefficient of the beam by increasing the length of the upper electrode and the inward tension, thereby reducing the static electric force required to drive the switch.

[0042] Further, preferably, as shown in the figures, Figure 2 , Figure 6 the size of the straight plate structure 13 is 124 μm × 65 μm. The straight plate structure 13 is provided with release holes, including strip-shaped release holes 131 and square release holes 132. The square release holes 132 are arranged in an array form, with 7 rows in the horizontal direction and 2 columns in the vertical direction. The length of each release hole is 6 μm, and the spacing is 8 μm. Two strip-shaped release holes 131 are symmetrically arranged on both sides of the array of square release holes 132, with a size of 105 μm × 6 μm, and the distance between the strip-shaped release holes 131 is 31 μm. The increase of the release holes reduces the elastic stiffness of the straight plate structure 13 and the air damping in the switching process of the switch, and improves the switching speed of the switch. On the other hand, the existence of the release holes can make the sacrificial layer release more fully during the process of manufacturing, thereby improving the reliability of the switch.

[0043] Further, the right-angle bend beam structure 14 is connected by three rectangular strips with a width of 13 μm and a length of 28 μm, 28 μm and 32 μm respectively, and the rectangular strips are connected in a 90-degree backfolding mode. The right-angle bend beam structure 14 is connected with the first anchor point 10.

[0044] Further, preferably, as shown in Figure 4 , the first lower electrode 6 and the second lower electrode 7 are arranged on the surface of the substrate 1, and the opposite area of the first lower electrode 6, the second lower electrode 7 and the upper electrode 5 is 289 μm 2 . The end of the first lower electrode 6 and the second lower electrode 7 is connected with the output signal line 4 and the power divider 3 by using a funnel-shaped transition structure, and the top of the first lower electrode 6 and the second lower electrode 7 is in a semicircular structure with a radius of 10 μm. The semicircular structure reduces the opposite area of the upper and lower electrodes, and the funnel-shaped transition structure effectively reduces the coupling capacitance between the upper and lower electrodes, so that the isolation of the switch is improved.

[0045] Further, as shown in Figure 7 , the driving electrode 8 and the lead wire 9 are fixed on the surface of the substrate 1, the driving electrode 8 is arranged between the first lower electrode 6 and the second lower electrode 7, the driving electrode 8 is opposite to the middle part of the straight plate structure 13, the driving electrode 8 is electrically connected with the lead wire 9, and the driving electrode 8 applies a driving voltage through the lead wire 9.

[0046] Further, preferably, as shown in Figure 4 , 5 , the first contact group 11 and the second contact group 12 are fixed on the top of the first lower electrode 6 and the second lower electrode 7 respectively, the first contact group 11 and the second contact group 12 are located directly below the longitudinal two ends of the straight plate structure 13, the first contact group 11 and the second contact group 12 each include three circular contacts with a radius of 2.5 μm, and the three circular contacts are uniformly arranged around the semicircular side of the top of the first lower electrode 6 and the second lower electrode 7. The sharp corners of the traditional square contacts can generate high current density, which can reduce the power handling capacity of the switch. In contrast, the current distribution of the circular contacts is more uniform. In addition, the three contacts are connected in parallel, which can reduce the insertion loss by reducing the contact resistance, and can also avoid the phenomenon of micro-dissolution caused by high temperature of the contacts.

[0047] Further, as shown in Figure 7 , the input signal line 2 corresponds to the input port 201, the output signal line 4 corresponds to the output port, and the output port is sequentially arranged clockwise and is recorded as the first port 401, the second port 402, the third port 403, the fourth port 404, the fifth port 405 and the sixth port 406. The ports located at the symmetrical positions have very similar radio frequency performance. The first port 401 and the sixth port 406 form an acute angle with the input port 201, which causes large loss, so the radio frequency performance is slightly worse than that of the other ports.

[0048] Further, the surface of the substrate 1 is provided with a ground wire 15, which is scattered around the input signal line 2, the output signal line 4 and the lead wire 9.

[0049] Further, the air bridge includes a first air bridge 16, a second air bridge 17 and a third air bridge 18, which are fixed on the ground wire 15 through a second anchor point 19, a third anchor point 20 and a fourth anchor point 21 respectively, the first air bridge 16 is arranged above the input signal line, the second air bridge 17 is arranged above two adjacent lead wires 9, and the third air bridge 18 is arranged close to the upper electrode 5 of the switch assembly. The air bridge is arranged to improve the influence caused by the irregularity and discontinuity of the ground wire.

[0050] Further, the ports of the power divider 3 are evenly arranged around the disc.

[0051] Further, the first anchor point 10 is used for fixing the upper electrode, the second anchor point 19, the third anchor point 20 and the fourth anchor point 21 are used for fixing the air bridge, the second anchor point 19 and the fourth anchor point 21 are used for fixing the air bridge in a group of two, and the third anchor point 20 is used for fixing the air bridge in a group of three.

[0052] The working principle of the embodiment is as follows: the driving voltage is applied to the driving electrode 8 through the lead wire 9, the upper electrode 5 is gradually bent and deformed under the action of the electrostatic force, when the voltage reaches the threshold value, the straight plate structure 13 at both ends is in contact with the contact point, and the switch is in the closed state, so that the signal gating can be realized; when the driving voltage is removed, the upper electrode 5 will return to the initial position due to the elastic force, the upper electrode 5 is separated from the contact point, and the switch is in the open state, so that the signal gating cannot be realized. Each output branch is provided with a switch assembly, and the driving voltage is controlled during work to meet the gating of any branch.

[0053] Figure 8 , Figure 9 , Figure 10 The insertion loss of the one-port 401, the two-port 402 and the three-port 403 of the single-pole six-throw switch of the embodiment is shown in the table.

[0054] When the one-port 401 of the single-pole six-throw switch of the embodiment is closed, the isolation degrees of the other ports are as shown in Table 1. Figure 11 When the two-port 402 of the single-pole six-throw switch of the embodiment is closed, the isolation degrees of the other ports are as shown in Table 2. Figure 12 When the three-port 403 of the single-pole six-throw switch of the embodiment is closed, the isolation degrees of the other ports are as shown in Table 3. Figure 13As shown; similarly, by the symmetry relationship analysis method, the isolation of the X-type upper electrode MEMS single-pole six-throw switch satisfies ≥37 dB@26.5 GHz, ≥29 dB@40 GHz.

[0055] The above only describes the preferred embodiments of the present application in detail, but the present application is not limited to the above-mentioned embodiments, and various changes can be made within the knowledge of those skilled in the art without departing from the purpose of the present application, and all the changes shall be included in the protection scope of the present application.

Claims

1. An X-type top electrode MEMS single-pole six-throw switch, characterized in that: The system includes a substrate (1), microwave transmission lines, a switching assembly, and an air bridge. The microwave transmission lines are disposed on the substrate (1) and include an input signal line (2), a power divider (3), an output signal line (4), and a ground line (15). There is one input signal line (2) and six output signal lines (4). The input signal line (2) is connected to the power divider (3), and the power divider (3) is connected to the output signal line (4) through the switching assembly. The switching assembly includes an upper electrode (5), a first lower electrode (6), a second lower electrode (7), and a drive. The upper electrode (5) is fixed to the substrate (1) via the first anchor point (10), the end of the first lower electrode (6) is connected to the power divider (3), and the top of the first lower electrode (6) is fixed with the first contact group (11). The end of the second lower electrode (7) is connected to the output signal line (4), and the top of the second lower electrode (7) is fixed with the second contact group (12). The upper electrode (5) is located between the driving electrode (8) and the first contact group. (11) Above the second contact group (12); the first contact group (11) and the second contact group (12) are located directly below the longitudinal ends of the straight plate structure (13). The first contact group (11) and the second contact group (12) each include three circular contacts, which are evenly arranged around the semi-circular side of the top of the first lower electrode (6) and the second lower electrode (7); the upper electrode (5) is an X-shaped structure, which includes a straight plate structure (13) and a right-angle curved beam structure (14). The end corners of the straight plate structure (13) and the right-angle curved beam are connected. The beginning of the structure (14) is fixedly connected; the straight plate structure (13) is provided with a release hole, the release hole includes a strip release hole (131) and a square release hole (132), the square release hole (132) is arranged in an array, with 7 rows in the horizontal direction and 2 columns in the vertical direction; the two strip release holes (131) are arranged symmetrically on both sides of the array of square release holes (132); the right angle curved beam structure (14) is composed of three rectangular strips connected together, the rectangular strips are folded back at 90 degrees and fixedly connected, and the end of the right angle curved beam structure (14) is connected to the first anchor point (10).

2. The X-type upper electrode MEMS single-pole six-throw switch according to claim 1, characterized in that: The first lower electrode (6) and the second lower electrode (7) are both disposed on the surface of the substrate (1). The ends of the first lower electrode (6) and the second lower electrode (7) are connected to the output signal line (4) and the power divider (3) with a funnel-shaped gradient structure. The top of the first lower electrode (6) and the second lower electrode (7) are semi-circular structures. Each port of the power divider (3) is evenly spread out with the disk as the center.

3. The X-type top electrode MEMS single-pole six-throw switch according to claim 1, characterized in that: The driving electrode (8) and the lead wire (9) are both fixed on the surface of the substrate (1). The driving electrode (8) is disposed between the first lower electrode (6) and the second lower electrode (7). The driving electrode (8) is directly opposite the middle of the straight plate structure (13). The driving electrode (8) is electrically connected to the lead wire (9). The driving electrode (8) applies a driving voltage through the lead wire.

4. The X-type upper electrode MEMS single-pole six-throw switch according to claim 1, characterized in that: The input signal line (2) has an input port (201) at its end, and the output signal line (4) has an output port at its end. The output ports are arranged in a clockwise order, and the output ports are respectively port 1 (401), port 2 (402), port 3 (403), port 4 (404), port 5 (405), and port 6 (406).

5. The X-type upper electrode MEMS single-pole six-throw switch according to claim 1, characterized in that: The surface of the substrate (1) is provided with ground lines (15), which are scattered around the substrate with the outlines of input signal lines (2), output signal lines (4) and leads (9).

6. The X-type upper electrode MEMS single-pole six-throw switch according to claim 1, characterized in that: The air bridge includes a first air bridge (16), a second air bridge (17), and a third air bridge (18). The first air bridge (16), the second air bridge (17), and the third air bridge (18) are fixed on the ground wire (15) through the second anchor point (19), the third anchor point (20), and the fourth anchor point (21), respectively. The first air bridge (16) is mounted above the input signal line (2), the second air bridge (17) is mounted above two adjacent leads (9), and the third air bridge (18) is adjacent to the upper electrode (5) of the switch assembly.

7. The X-type top electrode MEMS single-pole six-throw switch according to claim 6, characterized in that: The second anchor point (19) and the fourth anchor point (21) are fixed air bridges in pairs, and the third anchor point (20) is a fixed air bridge in three pairs.

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