Terahertz MEMS composite membrane switch based on glass substrate

Abstract

The invention discloses a terahertz MEMS composite membrane switch based on a glass substrate, and belongs to the field of terahertz wireless communication equipment. The MEMS switch is of a three-dimensional structure and sequentially comprises a metal ground, a glass substrate, a coplanar waveguide signal transmission line, a silicon nitride dielectric layer and an MEMS composite film from bottom to top. The MEMS composite film is equivalent to a clamped beam structure, and an anchor area of the MEMS composite film is connected with a signal ground of the CPW; the composite film is composed of a gold film and a silicon dioxide film, the composite film, a silicon nitride dielectric layer below and a CPW signal line form a parallel plate capacitor, bias voltage is loaded through a direct-current voltage signal transmission line connected with an anchor area, the height of the MEMS composite film is changed, then the plate capacitance value is changed, and switching between the on state and the off state of a transmission signal is achieved. The MEMS switch of the composite film has the advantages of low insertion loss and high isolation in terahertz frequency band application, and can be applied to radio frequency devices such as phase shifters, attenuators, reconfigurable filters and the like.

Description

Technical Field

[0001] The present invention is a terahertz MEMS composite thin film switch based on a glass substrate, which can be applied to radio frequency devices such as phase shifters, attenuators, and reconfigurable filters. It is an important component of terahertz phased array antennas and terahertz metasurface systems, and belongs to the field of terahertz wireless communication equipment. Background Art

[0002] A MEMS (Micro-electro-mechanical system) switch is a three-dimensional mechanical electronic structure fabricated using semiconductor technology and measuring in the micron / nanometer range. Its function is to control the on / off switching of signals in communication links by changing the switch state. Depending on the actuation method, MEMS switches can be categorized as electrostatic, thermal, electromagnetic, and inertial. Depending on the number of circuits they can control, they can be divided into single-pole, single-throw (SPST) and single-pole, multiple-throw (SPMT) switches. Traditionally, MEMS switches are primarily used in millimeter-wave communication systems, serving as key components in phase shifters, attenuators, and reconfigurable filters. They hold significant research value in fields such as communications, radio navigation and positioning, and radio remote sensing. In recent years, with the commercialization of 5G, research on various 6G technologies has been steadily underway, with terahertz communication being a key technology in this research. Terahertz MEMS switches primarily address the application needs of terahertz phased array antennas and terahertz intelligent metasurface systems, and related research has garnered widespread attention since 2012.

[0003] At present, the structural forms of MEMS switches are mainly divided into two categories: (1) contact type. For example, patent CN111584310B proposes a contact type RF MEMS switch with reconfigurable driving voltage and its preparation method. This type of MEMS switch has a simple principle and is easy to process, but has a large insertion loss and is not suitable for terahertz communication systems; (2) capacitive type. For example, in 2016, Nair et al. designed a capacitive MEMS switch that can be used in the frequency range of 10-40GHz, with a driving voltage of 18.75V and an insertion loss of <0.3dB. This type of switch is mostly a fixed-beam structure of a single material, with large insertion loss and high driving voltage.

[0004] Based on the above problems, this patent proposes a terahertz MEMS composite thin film switch on a glass substrate, which aims to reduce insertion loss and driving voltage value. It has a simple structure and is easy to manufacture. It can be applied to devices such as terahertz phase shifters, attenuators, and reconfigurable filters. Summary of the Invention

[0005] To address the issues of high insertion loss and high driving voltage in existing single-metal thin-film terahertz MEMS switches, the present invention designs a terahertz MEMS composite thin-film switch based on a glass substrate. The implementation principle is as follows: A coplanar waveguide (CPW) transmission line is used as the basic structure, and a MEMS composite thin-film clamped beam is designed on this basis. A DC bias circuit is used to control the beam to different height states. Theoretical model analysis shows that this relatively changes the capacitance value of the entire structure, thereby changing the transmission characteristics of the entire structure and realizing signal on-off switching.

[0006] The object of the present invention is achieved like this:

[0007] A terahertz MEMS composite thin film switch based on a glass substrate, comprising a metal ground, a glass substrate and a coplanar waveguide transmission line; the coplanar waveguide and the metal ground are respectively closely attached to the upper surface and the lower surface of the glass substrate; and a composite thin film switch;

[0008] The coplanar waveguide is divided into three parts, namely a coplanar waveguide ground, a coplanar waveguide underground and a coplanar waveguide transmission line; the coplanar waveguide ground and the coplanar waveguide underground are symmetrical to each other, the coplanar waveguide transmission line is located on the coplanar waveguide ground and the coplanar waveguide underground, and there is no contact between the three;

[0009] The composite film switch spans the coplanar waveguide transmission line and includes a silicon dioxide film, a metal film, and a metal bridge pier. Two metal bridge piers are provided, with metal bridge pier a located in a defect avoidance area on the coplanar waveguide ground, and metal bridge pier b located on the upper surface of the coplanar waveguide ground. One end of the silicon dioxide film overlaps metal bridge pier b, and the other end is connected to metal bridge pier a through the metal film. The metal film is tightly attached to the lower surface of the silicon dioxide film, extending from metal bridge pier a to metal bridge pier b without contacting metal bridge pier b.

[0010] The metal film is suspended directly above the coplanar waveguide transmission line, and a silicon nitride dielectric layer is provided on the upper surface of the coplanar waveguide transmission line; the coplanar waveguide signal transmission line and the metal bridge pier a are both connected to the corresponding driving electrodes through corresponding DC bias lines.

[0011] Furthermore, the silicon dioxide film has a rectangular structure, wherein the middle position is a pull-down portion, and rectangular hollows are provided on both sides; a plurality of circular through holes are provided between the rectangular hollows and the pull-down portions for releasing the sacrificial layer.

[0012] Compared with the background technology, the present invention has the following advantages:

[0013] To address the high insertion loss and high drive voltage issues of existing terahertz MEMS switches, the present invention designs a switch beam structure made of a metal and silicon oxide composite film, which can significantly reduce the switch's insertion loss. It also designs a crane-leg-shaped beam structure with four fixed ends, which can reduce the switch's elastic coefficient and, in turn, the switch's drive voltage. Based on the characteristics of the three-dimensional MEMS composite thin film fixed beam, when a drive voltage is applied through a DC bias circuit, the beam is positioned at different heights, changing the equivalent circuit parameters of the entire structure and, in turn, the plate capacitance, enabling switching between the on and off states of the transmitted signal. This patent also proposes a processing technique for the MEMS composite thin film switch to ensure the feasibility of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solution of this patent, the following is a brief description of the drawings required for use in the embodiments.

[0015] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0016] Figure 2 A top view of the MEMS switch of the present invention;

[0017] Figure 3 for Figure 2 A-A' cross-section diagram;

[0018] Figure 4 is an equivalent circuit diagram of the MEMS switch of the present invention;

[0019] Figure 5 Schematic diagram of the positions of the silicon dioxide film and the metal film in the MEMS switch of the present invention;

[0020] Figure 6 yes Figure 2 Dimensioned drawings of parts;

[0021] Figure 7 A dimension diagram of the silicon dioxide film in the present invention;

[0022] Figure 8 This is a force analysis diagram of the silicon dioxide film in the present invention;

[0023] Figure 9 Insertion loss curve of the switch;

[0024] Figure 10 This is the isolation curve of the switch.

[0025] In the figure: 1. MEMS switch, 2. Metal ground, 3. Glass substrate, 4. Coplanar waveguide, 5. Silicon nitride dielectric layer, 6. Crane leg structure, 7. Metal pier, 8. Metal film, 9. Silicon dioxide film, 10. Driving electrode. DETAILED DESCRIPTION

[0026] The technical solution adopted in the present invention is:

[0027] The MEMS switch is a three-dimensional structure consisting, from bottom to top, of a metal ground, a glass substrate, a coplanar waveguide (CPW), a silicon nitride dielectric layer, and a MEMS composite film. The MEMS composite film is equivalent to a fixed-beam structure, with its anchor regions (i.e., the bridge piers at each end of the beam) connected to the CPW signal ground. The composite film (i.e., the beam structure) is composed of a metal film and a silicon dioxide film, forming a parallel plate capacitor with the underlying silicon nitride dielectric layer and the CPW signal transmission line. Applying a bias voltage through a DC voltage signal transmission line connected to the anchor region changes the height of the MEMS composite film, and thus the plate capacitance, enabling switching between the on and off states of the transmitted signal.

[0028] MEMS switches are realized through micro-nano metal surface processing technology. The specific processing steps are as follows:

[0029] Step 1: Surface cleaning of quartz substrate with a thickness of 400um;

[0030] Step 2: Magnetron sputtering a DC bias line on a quartz substrate. The DC bias line material is Cr and has a thickness of 50 μm.

[0031] Step 3: Process a SiN protective layer on the high-resistance line using a PECVD (plasma enhanced chemical vapor deposition) system with a thickness of 0.1 μm.

[0032] Step 4: Sputter the seed layer of the CPW transmission line and the driving electrode, and then thicken it by electroplating to a thickness of 1 μm;

[0033] Step 5: Grow a SiN dielectric layer at the capacitor location using a PECVD system with a thickness of 0.1 μm.

[0034] Step 6: Spin-coat a polyimide sacrificial layer with a thickness of 2 μm;

[0035] Step 7: Etch the anchor area structure on the sacrificial layer, then sputter the seed layer and thicken it by electroplating. The anchor area height is 1 micron.

[0036] Step 8: Prepare a gold cantilever beam with a thickness of 0.5 μm by sputtering a seed layer and thickening it with electroplating;

[0037] Step 9: PECVD system grows SiO2 composite beam with a thickness of 1 μm;

[0038] Step 10: Release the sacrificial layer to obtain a suspended MEMS clamped beam.

[0039] The terahertz MEMS composite thin film switch obtained based on the above preparation method includes a metal ground, a quartz glass substrate, a coplanar waveguide transmission line, two driving electrodes (including a DC bias voltage transmission line), a silicon nitride dielectric layer, and a MEMS composite thin film clamped beam; the coplanar waveguide is composed of three parts, namely, the coplanar waveguide upper signal ground (coplanar waveguide upper surface), the coplanar waveguide signal transmission line, and the coplanar waveguide lower signal ground (coplanar waveguide lower surface).

[0040] The metal ground is located on the lower surface of the quartz glass substrate;

[0041] The coplanar waveguide transmission line and the driving electrode are located on the upper surface of the quartz glass substrate;

[0042] The silicon nitride dielectric layer is located on the surface of the coplanar waveguide signal line;

[0043] The MEMS composite film fixed-support beam includes two symmetrically positioned metal anchor areas (metal bridge piers), a metal film and a silicon dioxide film; the gold film is connected to the driving electrode through one anchor area; the silicon dioxide film is located on the upper surface of the metal film and is connected to the signal ground of the coplanar waveguide through another anchor area; the MEMS composite film is suspended above the dielectric layer.

[0044] The MEMS composite film beam is configured as a crane-leg structure with four ends fixed to reduce the elastic coefficient of the beam; a through hole is provided in the main body of the beam to facilitate the release of the sacrificial layer during processing and further reduce the elastic coefficient of the beam.

[0045] The following is a more specific embodiment:

[0046] like Figures 1 to 3 The figure shows a top view of a terahertz MEMS composite thin film switch, which consists of a coplanar waveguide transmission line and a MEMS composite thin film beam. Figure 6 According to the central operating frequency, the basic dimensions of the coplanar waveguide transmission line are designed to be G / S / G = 10 / 75 / 10μm. Due to the loading of MEMS switches, in order to achieve impedance matching, a defective ground structure is designed with the dimensions of w1 = 30μm, w2 = 50μm, w3 = 12.5μm, l1 = 70μm, l2 = 120μm. There is a layer of thickness t on the transmission line. d =0.1μm silicon nitride dielectric layer, the switch is suspended at a height of g=1μm above the dielectric layer; the dimensions of the MEMS composite film beam are L=60μm, W=300μm, and the thickness is t=1μm; the substrate material is quartz glass with a thickness of h=400μm;

[0047] like Figure 4The figure shows the equivalent circuit diagram of the MEMS switch. When a DC voltage is applied between the MEMS beam and the transmission line, the electrostatic force generated causes the MEMS beam to sink into the dielectric layer, and the switch capacitance increases by hundreds of times (i.e., C d ), the capacitance effect causes the transmitted signal to be reflected back, which is equivalent to a reflection switch. When no bias voltage is applied, the MEMS beam rebounds to its original position. At this time, the capacitance (i.e. C u ) decreases, and the signal can pass through the switch.

[0048] The transmission characteristics (i.e., S parameters) of a MEMS switch are correlated with its electrical parameters and can be calculated using the following formula:

[0049] The transfer matrix ABCD of the switch can be expressed as follows:

[0050]

[0051] Where:

[0052] M1 - circuit parameters at the input end of the transmission line;

[0053] M2 - circuit parameters at the output end of the transmission line;

[0054] M3——Circuit parameters of switch membrane;

[0055] The calculation formulas for the above circuit parameters are as follows:

[0056]

[0057] Y3(down-state)=[(jωC d ) -1 +jωL+R s ] -1

[0058] Y3(up-state)=[(jωC u ) -1 +jωL+R s ] -1

[0059] Where:

[0060] (Z1, θ1) – characteristic impedance and electrical length of the transmission line input;

[0061] (Z2, θ2) – characteristic impedance and electrical length at the output end of the transmission line;

[0062] C d ——The capacitance value of the switch membrane in the pull-down state;

[0063] C u ——The capacitance value of the switch membrane in the suspended state;

[0064] L——inductance value of switch membrane;

[0065] R s ——Resistance value of the switch membrane;

[0066] According to the transmission line theorem, the electrical parameter expression of the switch can be obtained:

[0067]

[0068] like Figure 2 The figure shows a schematic diagram of a MEMS composite thin film beam. This invention changes the traditional single metal beam MEMS switch design approach, employing a composite thin film of metal and silicon dioxide beams, and employing a glass substrate with low dielectric loss for signal transmission. This significantly reduces the switch's transmission losses. The metal film is fabricated on the anchor region and has a thickness of 0.5 μm. The silicon dioxide film is fabricated on the metal film and has a thickness of 1 μm. All structural metal materials are low-resistivity metals, such as gold and aluminum.

[0069] like Figure 7 The figure shows a top view of the MEMS composite film beam. The crane leg type fixed beam structure is processed above the coplanar waveband transmission line (1). One end of the crane leg type fixed beam structure is connected to the DC bias line through the anchor area, and the other end is connected to the coplanar waveguide signal ground through the anchor area. In order to reduce the driving voltage, the crane leg type structure is designed to connect the main structure of the beam to the anchor area. At the same time, a through hole is opened on the main structure of the beam, on the one hand to facilitate the release of the sacrificial layer during the processing, and on the other hand to further reduce the elasticity of the switch and reduce the driving voltage value. The various dimensions of the MEMS composite film beam are, w = 15μm, l s =50μm,l c =160μm, R=6μm;

[0070] like Figure 8 The figure shows a force analysis diagram of a MEMS clamped beam. The driving voltage of the MEMS switch can be calculated from the beam's elastic coefficient. The beam's elastic coefficient is divided into two parts: one is the beam's stiffness elasticity k1, and the other is the process residual stress elastic coefficient k2. The elastic coefficient k of the crane-leg clamped beam described in this patent is calculated using the following formula:

[0071]

[0072] k=k1+k2

[0073] Where:

[0074] E——Young's modulus;

[0075] w——width of the switch beam;

[0076] t——thickness of switch beam;

[0077] l s , l——the length of the switch beam crane leg branch and the entire beam;

[0078] σ——biaxial residual stress

[0079] ν——Poisson's ratio

[0080] The working process of the MEMS switch described in the present invention is as follows:

[0081] The drive electrodes are connected to a PCB test board via gold wire bonding, allowing for a DC power supply network to be designed on the PCB. The drive electrodes on one side are connected to an anchor area of the switch, while the drive electrodes on the other side are connected to the CPW signal lines. A dielectric layer is fabricated on the CPW signal lines to prevent breakdown caused by the connection between the MEMS switch beam and the signal lines. When no bias voltage is applied, the transmission signal passes through the CPW normally. As the bias voltage gradually increases, the electrostatic force bends the MEMS beam, increasing the switch capacitance by hundreds of times. This capacitive effect causes the transmission signal to be reflected back. As the bias voltage gradually decreases, the MEMS beam rebounds to its original position, allowing the transmission signal to pass normally.

[0082] The transmission characteristic curve of the MEMS switch is obtained using 3D simulation software:

[0083] like Figure 9 The figure shows the insertion loss curve of the switch. When the switch is not driven by voltage, the signal is transmitted normally through the CPW. Transmission loss primarily comes from the coplanar waveguide impedance mismatch. As shown in the figure, the insertion loss of the switch is less than -1dB in the frequency range of 0.1THz to 0.12THz, and less than -3dB in the frequency range of 0.1THz to 0.14THz. At 0.1THz, the insertion loss of the switch is less than -0.5dB, which is very low.

[0084] like Figure 10 The figure shows the isolation curve of the switch. When the switch is driven by a voltage, the electrostatic force exerted on the MEMS beam contacts the dielectric layer, causing a sharp increase in capacitance and causing the transmitted signal to be reflected. The isolation performance at this point reflects the switch's off-state performance. As shown in the figure, within the frequency range of 0.1THz to 0.14THz, the switch's isolation is less than -20dB, indicating good isolation performance in the off-state.

[0085] The above-mentioned specific implementation can be locally adjusted in different ways by those skilled in the art without departing from the principles and purpose of the present invention. For example, the size of the MEMS switch beam of the coplanar waveguide can be adjusted to adjust the operating frequency band. The scope of protection of the present invention is based on the claims and is not limited to the above-mentioned specific implementation. All implementation schemes within its scope are subject to the constraints of the present invention.

Claims

1. A terahertz MEMS composite thin film switch based on a glass substrate, comprising a metal ground, a glass substrate, and a coplanar waveguide transmission line; the coplanar waveguide and the metal ground are respectively closely attached to the upper and lower surfaces of the glass substrate; characterized in that: Also includes composite membrane switches; The coplanar waveguide is divided into three parts, namely a coplanar waveguide ground, a coplanar waveguide underground and a coplanar waveguide transmission line; the coplanar waveguide ground and the coplanar waveguide underground are symmetrical to each other, the coplanar waveguide transmission line is located on the coplanar waveguide ground and the coplanar waveguide underground, and there is no contact between the three; The composite film switch spans the coplanar waveguide transmission line and includes a silicon dioxide film, a metal film, and a metal bridge pier. Two metal bridge piers are provided, with metal bridge pier a located in a defect avoidance area on the coplanar waveguide ground, and metal bridge pier b located on the upper surface of the coplanar waveguide ground. One end of the silicon dioxide film overlaps metal bridge pier b, and the other end is connected to metal bridge pier a through the metal film. The metal film is tightly attached to the lower surface of the silicon dioxide film, extending from metal bridge pier a to metal bridge pier b without contacting metal bridge pier b. The metal film is suspended directly above the coplanar waveguide transmission line, and a silicon nitride dielectric layer is provided on the upper surface of the coplanar waveguide transmission line; the coplanar waveguide signal transmission line and the metal bridge pier a are both connected to the corresponding driving electrodes through corresponding DC bias lines.

2. The terahertz MEMS composite membrane switch based on a glass substrate according to claim 1, characterized in that: The silicon dioxide film has a rectangular structure, with a pull-down portion in the middle and rectangular hollows on both sides. Multiple circular through holes are provided between the rectangular hollows and the pull-down portions for releasing the sacrificial layer.

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