CN104037027B - A kind of MEMS capacitance switch - Google Patents

Abstract

A MEMS capacitive switch belongs to the field of electronic science and technology. It includes a substrate with an insulating layer, a signal transmission line and ground electrodes on both sides. There is a dielectric layer on the surface of the signal transmission line. Support the driving electrode structure; the driving electrode structure is a "two-wing" step structure, including the first driving electrode in the middle and the second and third driving electrodes on the two wings. , the second and third driving electrodes are respectively connected to the first driving electrode using cantilever beams, the distance between the second and third driving electrodes and the signal transmission line is greater than the distance between the first driving electrode and the signal transmission line; the area of the three driving electrodes increase in turn. The invention can realize three working frequency bands, and has the characteristics of low insertion loss, high isolation and low pull-down voltage, and can be applied to radio frequency or microwave communication systems.

Description

A MEMS capacitive switch

technical field

The invention belongs to the technical field of electronic science and relates to microelectromechanical systems (MEMS), in particular to a MEMS capacitance switch.

technical background

Switches are fundamental components in radio frequency (RF) and microwave communication systems, and radio frequency microelectromechanical (RFMEMS) switches have great potential for application at both the RF and microwave component and system levels. At the component level, radio frequency microelectromechanical (RFMEMS) switches can be used to construct voltage-controlled oscillators, filters (capacitor switches, inductors) and phase shifters, etc., which are indispensable components of modern radar and communication systems. Compared with traditional FET and PIN diode switches, radio frequency microelectromechanical (RFMEMS) switches have the characteristics of low DC power consumption, low insertion loss, high isolation, low intermodulation distortion, wide operating frequency band, low cost, and easy integration.

U.S. Patent No. 7,265,647B2 discloses an adjustable MEMS capacitive switch. As shown in FIGS. Both sides of the transmission line have metal layers 104 and 106 as ground electrodes (the height of the ground electrodes 104 and 106 exceeds the height of the transmission line 108), and the surface of the metal layer 108 as the transmission line is covered with a dielectric layer 109, and one of the ground electrodes 106 surface Covered with a dielectric layer 402, three suspension beam structures 112, 120 and 122 are arranged directly above the transmission line 108. The three suspension beam structures straddle the transmission line 108, and the two ends are respectively placed on the surfaces of the two ground electrodes, and are connected to the electrodes covered with the insulating dielectric layer. form an adjustable capacitor. The structure loads a certain driving voltage to pull down the switch beam above the transmission line to realize the transition of the switch from up to down state. When down, the total capacitance is changed by changing the variable capacitance between the electrode and the cantilever, so that the switch has different resonant frequencies, so that higher isolation can be obtained in different frequency bands. However, all the switch beams of this structure are placed on the same plane. If the distance between the switch beam and the transmission line is too small, the up-state capacitance of the switch will be large, resulting in an increase in the insertion loss of the switch. If the distance between the switch beam and the transmission line is increased Due to the square relationship between the driving voltage and the spacing, the driving voltage of the switch will increase sharply. In addition, the roughness of the dielectric layer has a greater impact on the RF performance of the switch, which will seriously limit the RFMEMS The scope of application of the switch.

Contents of the invention

The invention provides a frequency band adjustable high isolation/low insertion loss MEMS capacitive switch that can work in three frequency bands, has the characteristics of low insertion loss, high isolation, and low pull-down voltage, and can be applied to radio frequency or microwave communication systems middle.

Technical scheme of the present invention is as follows:

A MEMS capacitive switch, as shown in Figures 3 to 5, includes a substrate 7 with an insulating layer 8 on the surface, a signal transmission line 9 made of conductive material in the middle of the surface of the insulating layer 8, and on both sides of the signal transmission line 9 The surface of the insulating layer 8 has a ground electrode 13-1 and 13-2 parallel to the signal transmission line 9, the surface of the signal transmission line 9 is fully covered with a layer of dielectric layer 10, and the surface of the dielectric layer 10 is provided with a first metal covering area 11-2, the second metal covering area 11-1 and the third metal covering area 11-3, wherein the first metal covering area 11-2 is located in the second metal covering area 11-1 and the third metal covering area 11-3 Between; a first fixed anchor point 12-1 is set on the surface of the first ground electrode 13-1, a second fixed anchor point 12-2 is set on the surface of the second ground electrode 13-2, and the first fixed anchor point 12 A driving electrode structure is connected between -1 and the second fixed anchor point 12-2; the driving electrode structure includes three driving electrodes, wherein the first driving electrode 4 is located above the first metal covering region 11-2, and the second The driving electrode 5 is located above the second metal covering region 11-1, the third driving electrode 6 is located above the third metal covering region 11-3, and the area of the first driving electrode 4 is smaller than the area of the second driving electrode 5, and the second driving electrode 6 is located above the third metal covering region 11-3. The area of the electrode 5 is smaller than the area of the third driving electrode 6, and the areas of the three driving electrodes are respectively smaller than the area of the corresponding metal covering layer; among the two sides corresponding to the first driving electrode 4 and the two sides of the signal transmission line 9, one side The first fixed beam 1-1 is connected to the first fixed anchor point 12-1, and the other side is connected to the second fixed anchor point 12-2 by the second fixed beam 1-2; the second driving electrode 5 is connected to the first fixed anchor point 12-1. The driving electrodes 4 are connected by the first cantilever beam 2, and the third driving electrode 6 and the first driving electrode 4 are connected by two second cantilever beams 3; the second driving electrode 5 and the second metal covering area 11-1 The distance between them is equal to the distance between the third driving electrode 6 and the third metal covering region 11-3, and is greater than the distance between the first driving electrode 4 and the first metal covering region 11-2.

The working principle of the present invention is:

The MEMS capacitive switch provided by the present invention has three driving plates, wherein the first driving electrode 4 is a fixed beam electrode, and the second driving electrode 5 and the third driving electrode 6 are cantilevers located on the "two wings" of the fixed beam electrode. The equivalent circuit of the beam electrode is shown in Figure 6: the signal transmission line 9, the dielectric layer 10 and the first metal covering area 11-2 form a fixed capacitance C1, and the signal transmission line 9, the dielectric layer 10 and the second metal covering area 11-1 Constitute a fixed capacitance C2, the signal transmission line 9, the dielectric layer 10 and the third metal covering area 11-3 form a fixed capacitance C3, the first metal covering area 11-2 and the first driving electrode 4 and the air layer between the two constitute a variable The variable capacitance C1', the second metal covering region 11-1 and the second driving electrode 5 and the air layer between the two constitute the variable capacitance C2', the third metal covering region 11-3 and the third driving electrode 6 and the two The air layer between them constitutes the variable capacitor C3'; when the entire MEMS capacitive switch is in the Up state, the existence of the "two-wing" cantilever beam driving electrodes is equivalent to increasing the relative distance between the driving electrodes and the signal line 9, reducing The capacitance in the Up state is reduced, thereby reducing the insertion loss of the entire MEMS capacitive switch. When a bias voltage is applied on the three drive electrodes (the bias pull-down voltage is actually applied between the ground electrode and the signal transmission line, since the ground electrode is electrically identical to the three drive electrodes, the bias voltage can also be said to be applied On the three drive electrodes), the entire MEMS capacitive switch is switched from the Up state to the Down state: when the applied bias voltage is greater than the pull-down voltage of the first drive electrode 4, the first drive electrode 4 is pulled down and covered with the first metal The area 11-2 is in contact, and the second drive electrode 5 and the third drive electrode 6 on the two wings of the first drive electrode 4 are also pulled down a certain distance (because the area of the first drive electrode 4 is smaller than the area of the second drive electrode 5, The area of the second drive electrode 5 is less than the area of the third drive electrode 6, so the pull-down voltage of the first drive electrode 4 is less than the pull-down voltage of the second drive electrode 5, and the pull-down voltage of the second drive electrode 5 is less than that of the third drive electrode 6. Pull-down voltage), but not in contact with the metal coverage area below, at this time the entire MEMS capacitive switch is in the first Down state (the equivalent circuit is shown in Figure 7(a), and the equivalent capacitance is capacitor C1); continue to increase Bias voltage, when the bias voltage is greater than the pull-down voltage of the second drive electrode 5, the second drive electrode is pulled down to be in contact with the second metal covering region 11-1, but the third drive electrode 6 is not completely pulled down , the entire MEMS capacitive switch is in the second Down state (the equivalent circuit is shown in Figure 7(b), and the equivalent capacitance is the parallel connection of capacitor C1 and capacitor C2); then continue to increase the bias voltage, when the bias voltage is greater than During the pull-down voltage of the three drive electrodes 6, the third drive electrode 6 is also pulled down to be in contact with the third metal covering region 11-3, and now the whole MEMS capacitive switch is in the third Down state (equivalent circuit as shown in Fig. 7(c ), the equivalent capacitance is the parallel connection of capacitor C1, capacitor C2 and capacitor C3). Since the entire MEMS capacitive switch has three Down states, corresponding to three different equivalent capacitances, three different resonant frequencies can be obtained, so that the entire MEMS capacitive switch can work in three different frequency bands. In addition, after the electrodes of the fixed beam are pulled down, the effective heights of the drive electrodes on the two wings relative to the signal transmission line 9 decrease accordingly, and then drive is applied to the other two electrodes respectively, that is, the second drive is performed on the basis of the first drive. , can greatly reduce the voltage required for one-time driving; the surface of the signal transmission line 9 is covered with a metal covering layer with an area larger than the area of the corresponding driving electrode on the insulating medium layer corresponding to each driving electrode, when each driving electrode is pulled When it is in contact with the metal covering layer, the effective area of each driving electrode is converted into the area of each metal covering layer, which is equivalent to increasing the relative area of the capacitor, increasing the capacitance value, and improving the isolation; at the same time, due to the metal covering The existence of the layer, as long as the driving electrode is pulled to part of the contact with the metal layer, it is equivalent to the contact of the entire plate with the metal layer, without the need for the entire driving electrode to be pulled down completely, which can greatly reduce the driving voltage while increasing the switching capacitance ratio .

In the Down state, the pull-down voltage V _P of the drive electrode can be expressed as:

Where k is the elastic coefficient of the structure composed of the driving electrode and the support beam, _ε0 is the vacuum dielectric constant, W is the width of the signal transmission line 9, w is the width of the driving electrode, and h is the distance between the driving electrode and the signal transmission line.

When the switch works in the open state (up state), the insertion loss represented by the S ₂₁ parameter is:

${\mathrm{<span\; class="notranslate">\; S</span>}}_{\mathrm{<span\; class="notranslate">\; twenty\; one</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 20</span>}\mathrm{<span\; class="notranslate">\; log</span>}\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; j\&omega;</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; up</span>}}{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}$

When the switch is in the off state (down state), if the operating frequency of the switch is far away from its resonant frequency, since the inductance L of the switch is generally in the order of picohenries and the capacitance C is in the order of picofarads, 1/jωC is much larger than jωL, Therefore, the S ₂₁ parameter of the switch is mainly determined by the off-state (down state) capacitance value C _down of the switch. In the off state, the switch isolation represented by the S ₂₁ parameter is:

${\mathrm{<span\; class="notranslate">\; S</span>}}_{\mathrm{<span\; class="notranslate">\; twenty\; one</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 20</span>}\mathrm{<span\; class="notranslate">\; log</span>}\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; j\&omega;</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; down</span>}}{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}}$

In summary, the MEMS capacitive switch provided by the present invention has the following beneficial effects:

The MEMS capacitive switch provided by the present invention adopts a structure in which a fixed drive electrode and a cantilever drive electrode are combined. The two electrodes are at different levels, forming a "two-wing" step structure, while the traditional switch plates are all in the On the same level, this makes the MEMS capacitive switch provided by the present invention reduce the up-state capacitance, thereby reducing the insertion loss, and at the same time, because of the use of multiple pull-down processes, the pull-down voltage is reduced, and the contradiction between the insertion loss and the driving voltage is alleviated . At the same time, the MEMS capacitive switch provided by the present invention adds a metal covering layer corresponding to the three driving electrodes on the dielectric layer 10 on the surface of the signal transmission line 9, which is equivalent to increasing the relative area between the capacitive plates and increasing the down state capacitance. , improves the isolation, and can effectively reduce the pull-down voltage.

Description of drawings

FIG. 1 is a schematic top view structure diagram of a MEMS capacitive switch disclosed in US Patent No. 7,265,647B2.

FIG. 2 is a schematic cross-sectional structural diagram of the A-A connection line in FIG. 1 of the MEMS capacitive switch disclosed in US Patent No. 7,265,647B2.

Fig. 3 is a schematic top view structure diagram of the MEMS capacitive switch provided by the present invention.

FIG. 4 is a schematic cross-sectional structural diagram of the MEMS capacitive switch provided by the present invention along the line A-A in FIG. 3 .

FIG. 5 is a schematic cross-sectional structure diagram of the MEMS capacitive switch provided by the present invention on the line B-B in FIG. 3 .

FIG. 6 is an equivalent circuit diagram of an Up state of the MEMS capacitive switch provided by the present invention.

Fig. 7(a) is an equivalent circuit diagram of the first Down state of the MEMS capacitive switch provided by the present invention.

Fig. 7(b) is an equivalent circuit diagram of the second Down state of the MEMS capacitive switch provided by the present invention.

Fig. 7(c) is an equivalent circuit diagram of the third Down state of the MEMS capacitive switch provided by the present invention.

Fig. 8 is the insertion loss simulation test result of the MEMS capacitive switch Up state provided by the present invention.

FIG. 9 is the simulation test result of the isolation of the MEMS capacitive switch in the Down state provided by the present invention.

detailed description

A MEMS capacitive switch, as shown in Figures 3 to 5, includes a substrate 7 with an insulating layer 8 on the surface, a signal transmission line 9 made of conductive material in the middle of the surface of the insulating layer 8, and on both sides of the signal transmission line 9 The surface of the insulating layer 8 has a ground electrode 13-1 and 13-2 parallel to the signal transmission line 9, the surface of the signal transmission line 9 is fully covered with a layer of dielectric layer 10, and the surface of the dielectric layer 10 is provided with a first metal covering area 11-2, the second metal covering area 11-1 and the third metal covering area 11-3, wherein the first metal covering area 11-2 is located in the second metal covering area 11-1 and the third metal covering area 11-3 Between; a first fixed anchor point 12-1 is set on the surface of the first ground electrode 13-1, a second fixed anchor point 12-2 is set on the surface of the second ground electrode 13-2, and the first fixed anchor point 12 A driving electrode structure is connected between -1 and the second fixed anchor point 12-2; the driving electrode structure includes three driving electrodes, wherein the first driving electrode 4 is located above the first metal covering region 11-2, and the second The driving electrode 5 is located above the second metal covering region 11-1, the third driving electrode 6 is located above the third metal covering region 11-3, and the area of the first driving electrode 4 is smaller than the area of the second driving electrode 5, and the second driving electrode 6 is located above the third metal covering region 11-3. The area of the electrode 5 is smaller than the area of the third driving electrode 6, and the areas of the three driving electrodes are respectively smaller than the area of the corresponding metal covering layer; in the two sides corresponding to the first driving electrode 4 and the two sides of the signal transmission line 9, One side is connected to the first fixed anchor point 12-1 by using the first fixed beam 1-1, and the other side is connected to the second fixed anchor point 12-2 by the second fixed beam 1-2; the second driving electrode 5 is connected to the second fixed anchor point 12-1. One drive electrode 4 is connected with the first cantilever beam 2, and the third drive electrode 6 is connected with the first drive electrode 4 with two second cantilever beams 3; the second drive electrode 5 is connected with the second metal covering area 11- The distance between 1 is equal to the distance between the third driving electrode 6 and the third metal covering region 11-3, and is greater than the distance between the first driving electrode 4 and the first metal covering region 11-2.

In the above scheme, the dimensions of each part are as follows:

The length×width×thickness of the signal transmission line 9 is 1mm×100μm×2μm, the insulating dielectric layer 10 is 0.35μm in thickness, and the material is silicon nitride (Si ₃ N ₄ ), two ground electrodes 12-1, 12-2 The length×width×thickness of the first driving electrode 4 is 1㎜×60μm×2μm, the length×width×thickness of the first driving electrode 4 is 40μm×60μm×2μm, and the length×width×thickness of the two fixed beams 1-1, 1-2 is 60 μm×10 μm×2 μm, the length×width×thickness of the second driving electrode 5 is 60 μm×60 μm×2 μm, the length×width×thickness of the cantilever beam 2 or 3 is 60 μm×10 μm×2 μm, the length of the third driving electrode 6 is × width × thickness is 100 μm × 60 μm × 2 μm, the distance between the first driving electrode 4 and the signal transmission line 9 is 2 μm, the distance between the second and third driving electrodes and the signal transmission line 9 is 6 μm, and the thickness of the three metal covering layers is 0.05 μm, the length×width of the first metal covering layer is 100 μm×100 μm, and the length×width of the second and third metal covering layers are 400 μm×100 μm.

The simulation results of the performance parameters of the switch are shown in Figure 8 and Figure 9. The MEMS capacitive switch can work in three frequency bands within 30 GHz, and the insertion loss of each frequency band is less than 0.25dB, and the isolation is higher than 37dB.

Claims (1)

1. A micro-mechanical system capacitive switch, comprising a substrate (7) with an insulating layer (8) on the surface, a signal transmission line (9) made of conductive material at the middle position of the insulating layer (8) surface, in the signal transmission line (9) The surface of the insulating layer (8) on both sides has a first ground electrode (13-1) and a second ground electrode (13-2) parallel to the signal transmission line (9), respectively, on the surface of the signal transmission line (9) The whole layer is covered with a dielectric layer (10); It is characterized by: The surface of the dielectric layer (10) is provided with a first metal covering area (11-2), a second metal covering area (11-1) and a third metal covering area (11-3), wherein the first metal covering area ( 11-2) Located between the second metal covering area (11-1) and the third metal covering area (11-3); a first fixed anchor point (12-1) is set on the surface of the first ground electrode (13-1) 1), a second fixed anchor point (12-2) is set on the surface of the second ground electrode (13-2), between the first fixed anchor point (12-1) and the second fixed anchor point (12-2) A driving electrode structure is connected between them; the driving electrode structure includes three driving electrodes, wherein the first driving electrode (4) is located above the first metal covering area (11-2), and the second driving electrode (5) is located on the second Above the metal covering area (11-1), the third driving electrode (6) is located above the third metal covering area (11-3), and the area of the first driving electrode (4) is smaller than the area of the second driving electrode (5) , the area of the second driving electrode (5) is smaller than the area of the third driving electrode (6), and the areas of the three driving electrodes are respectively smaller than the area of the corresponding metal covering area; the first driving electrode (4) and the signal transmission line ( 9) Of the two sides corresponding to both sides, one side uses the first fixed beam (1-1) to connect with the first fixed anchor point (12-1), and the other side uses the second fixed beam (1-2) to connect with The second fixed anchor point (12‐2) is connected; the second driving electrode (5) is connected to the first driving electrode (4) by the first cantilever beam (2), and the third driving electrode (6) is connected to the first driving electrode (4). The electrodes (4) are connected by two second cantilever beams (3); the distance between the second driving electrode (5) and the second metal covering area (11‐1) and the distance between the third driving electrode (6) and The distances between the three metal covering regions (11-3) are equal and greater than the distance between the first driving electrode (4) and the first metal covering region (11-2).