WO2017145175A2

WO2017145175A2 - Method of varying resonant frequency in capacitance type radio frequency (rf) micro-electro-mechanical system (mems) switches

Info

Publication number: WO2017145175A2
Application number: PCT/IN2016/050401
Authority: WO
Inventors: Sarath GOPALAKRISHNAN; Deleep Ramachandran NAIR; Amitava Das GUPTA
Original assignee: INDIAN INSTITUTE OF TECHNOLOGY MADRAS (IIT Madras)
Priority date: 2016-02-24
Filing date: 2016-11-16
Publication date: 2017-08-31
Also published as: WO2017145175A3

Abstract

Embodiments herein provide a method of varying resonant frequency in a capacitance type radio frequency (RF) Micro-Electro-Mechanical System (MEMS) switch. The method includes encapsulating a dielectric layer in the MEMS switch with a floating metal. Further, the method includes varying, the length of the floating metal. Furthermore, the method includes varying the resonant frequency of the MEMS capacitive switch based on the varied length of the floating metal.

Description

"Method of varying resonant frequency in capacitance type radio frequency (RF) Micro-Electro-Mechanical System (MEMS) switches"

FIELD OF INVENTION

[0001] The embodiments herein generally relate to Micro-Electro- Mechanical System (MEMS) switches. More particularly, relates to a method of varying the resonant frequency in a capacitance type radio frequency (RF) MEMS switch. The present application is based on, and claims priority from an Indian Application Number 201641006466 filed on 24^th February 2016, the disclosures of which are hereby incorporated by reference herein

BACKGROUND OF INVENTION

[0002] In general, RF MEMS switches with a stationary dielectric - on-metal (DOM) and a movable beam have widely been used at microwave frequencies to toggle the state of a transmission line. The most commonly used transmission line is a coplanar waveguide (CPW). The CPW has a signal line at the centre and ground planes on either side. The signal line and the ground planes are on the same plane on a substrate. Therefore this configuration is called as a "coplanar" waveguide.

[0003] An RF MEMS switch includes a metallic electrode patterned through a photolithographic process fixed onto the substrate. A dielectric layer is deposited on metallic electrode. This forms the stationary DOM part of the switch. A top electrode is suspended above the stationary DOM part of the switch and is anchored to the ground planes. In the upstate of the switch, there is a finite air gap between the top electrode and the dielectric layer. The top electrode (beam) is connected to zero potential and the bottom electrode is applied with a finite value of direct current (DC) voltage. This develops an electrostatic force of attraction between the two electrodes. The beam suspended above the dielectric-on-metal moves down due to the electrostatic force. When the DC voltage is high enough such that the electrostatic force of attraction is higher than the restoring force of the beam acting in the opposite direction, the beam moves down and makes contact with the dielectric layer. This is termed as down- state of the switch.

[0004] In the conventional RF MEMS switches, the bottom electrode is the signal line of the CPW. The MEMS switch is suspended above the bottom electrode and the movable membrane (beam) is anchored on the ground planes, bridging them across the signal line. The DC voltage to actuate the beam can be applied on the signal line or can be applied onto separate actuation pads dedicated to pull the beam down. In the up-state condition of the beam, RF signal is transmitted from the input port to the output port without being interfered by the beam, because the up-state capacitance due to air is too low to cause any coupling. In the down-state condition of the beam, the air gap is eliminated and the capacitance is increased. At a particular frequency, called the resonant frequency, the impedance due to the down-state capacitance and the inductance of the beam is minimum. At the resonant frequency, the input signal is reflected back due to the low impedance, resulting in high isolation between input and output. However, the interface at the contact of the beam and the bottom electrode is not always perfect which can reduce the down- state capacitance from the expected value. Since the resonant frequency of the switch is dependent on the down-state capacitance, this will increase the value of resonant frequency.

[0005] The above information is presented as background information only to help the reader to understand the present invention. Applicants have made no determination and make no assertion as to whether any of the above might be applicable as Prior Art with regard to the present application.

OBJECT OF INVENTION [0006] The principal object of the embodiments herein is to provide a mechanism for varying a resonant frequency in a capacitance type radio frequency (RF) Micro-Electro-Mechanical System (MEMS) switch.

[0007] Another object of the embodiments herein is to provide a mechanism to encapsulate a dielectric layer in the RF MEMS switch with a floating metal.

[0008] Another object of the embodiments herein is to provide a mechanism for varying the length of the floating metal.

[0009] Another object of the embodiments herein is to provide a mechanism for varying the resonant frequency based on the varied length of the floating metal.

SUMMARY

[0010] Accordingly the embodiments herein provide a method of varying the resonant frequency in a capacitance type radio frequency (RF) Micro-Electro-Mechanical System (MEMS) switch. The method includes encapsulating the dielectric layer in the MEMS switch with a floating metal. Further, the method includes varying the length of the floating metal. Furthermore, the method includes varying the resonant frequency based on the varied length of the floating metal.

[0011] Accordingly the embodiments herein provide a capacitance type radio frequency (RF) Micro-Electro-Mechanical System (MEMS) switch. The switch includes a substrate, a lower electrode formed on the substrate, a dielectric layer formed on the lower electrode, a floating metal encapsulated on the dielectric layer and a beam connected to a ground. The length of the floating metal is varied to vary the resonant frequency.

[0012] These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

BRIEF DESCRIPTION OF FIGURES

[0013] This invention is illustrated in the accompanying drawings, throughout which like reference letters indicate corresponding parts in the various figures. The embodiments herein will be better understood from the following description with reference to the drawings, in which:

[0014] FIG. la illustrates a schematic of a capacitance type radio frequency (RF) Micro-Electro-Mechanical System (MEMS) switch in an up-state of a beam, according to an embodiment as disclosed herein;

[0015] FIG. lb illustrates a schematic of the capacitance type RF MEMS switch in an actuated down- state of beam, according to an embodiment as disclosed herein;

[0016] FIG. 2 is a flow chart illustrating method of varying a resonant frequency in a capacitance type RF MEMS switch in the down- state of beam, according to an embodiment as disclosed herein;

[0017] FIG. 3 is a graph showing the variation of a down-state capacitance (C¾ with a length of the floating metal (LFM), according to an embodiment as disclosed herein;

[0018] FIG. 4 illustrates an RF MEMS switch with a varying length of the floating metal (L_FM), according to an embodiment as disclosed herein;

[0019] FIG. 5 is a graph showing the variation of isolation with the length of the floating metal (LFM), according to an embodiment as disclosed herein; [0020] FIG. 6 is a graph comparing the measured isolation with the simulated values for various values of the length of the floating metal, according to an embodiment as disclosed herein;

[0021] FIG. 7 illustrates a schematic of an RF MEMS multi frequency (MF) switch with segregated floating metals of different lengths, according to an embodiment disclosed herein;

[0022] FIG. 8 illustrates a top-view of an RF MEMS switch with segregated floating metals of different lengths, according to an embodiment disclosed herein;

[0023] FIG. 9 is a graph showing the variation of isolation with frequency of RF MEMS switch for the down- state condition of beam 1, beam 2 and beam 3 respectively, according to an embodiment as disclosed herein; and

[0024] FIG. 10 is a graph showing the variation of isolation with frequency of RF MEMS switch when all the beams are in the up- state and when all the beams are in the down- state, according to an embodiment as disclosed herein.

DETAILED DESCRIPTION OF INVENTION

[0025] The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well- known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. Also, the various embodiments described herein are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. The term "or" as used herein, refers to a nonexclusive or, unless otherwise indicated. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein can be practiced and to further enable those skilled in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

[0026] The embodiments herein provide a method of varying a resonant frequency in a capacitance type radio frequency (RF) Micro- Electro-Mechanical System (MEMS) switch. The method includes encapsulating a dielectric layer in the MEMS switch with a floating metal. Further, the method includes varying, the length of the floating metal. Furthermore, the method includes varying the resonant frequency based on the varied length of the floating metal.

[0027] In an embodiment, the dielectric layer in the RF MEMS capacitive switch is encapsulated. This includes depositing a metal on top of the dielectric layer to form a stationary MIM capacitor.

[0028] Unlike the conventional systems and methods, the proposed method provides a mechanism for varying the resonant frequency of the RF MEMS capacitive switch by only varying the length of the floating metal encapsulating the dielectric layer. The proposed method provides more degrees of freedom as the resonant frequency of the switch is solely dependent on length of the floating metal. This capability to vary the frequency of operation in the design of the switch eases the design of the switches.

[0029] Further, the proposed method can be used in coplanar waveguide (CPW) signal transmission. The proposed method can be used in CPW to toggle the state of a transmission line. In an embodiment, the state of the switch (upstate or downstate) can be determined and the signal whether transmitted or blocked can be estimated based on the state of the switch.

[0030] Referring now to the drawings and more particularly to FIGS. 1 through 10 where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments.

[0031] FIG. la illustrates a schematic of a capacitance type radio frequency (RF) Micro-Electro-Mechanical System (MEMS) switch in an up-state of a beam, according to an embodiment as disclosed herein. In an embodiment, as depicted in the FIG. la, the RF MEMS switch 100 includes a substrate 102, actuation pads 116 positioned on the substrate 102, posts/insulator 110 insulating the actuation pads 116 from the ground plane 112, a signal line (an electrode layer) 104 positioned on the substrate 102, a dielectric layer 106 positioned on the signal line 104, a floating metal 114 positioned on the dielectric layer 106, a beam (top electrode) 108 positioned on ground lines 112.

[0032] FIG. lb illustrates a schematic of the capacitance type RF MEMS switch in an actuated down- state of beam, according to an embodiment as disclosed herein.

[0033] While not shown, a control circuit is typically coupled to the beam 108, and the signal line 104 or the actuation pads 116. In operation, the control circuit can apply a direct current (DC) voltage between the beam 108, and the signal line 104 or the actuation pads 116, thereby creating an electric field that actuates the beam 108 from the default upstate position (as shown in the FIG. la) to the actuated down-state position (as shown in the FIG. lb). In the down-state position, the RF MEMS switch 100 can provide high capacitive coupling (e.g., closed position for RF MEMS switch 100 where RF signal is shunt to ground 112). When the DC bias voltage is removed, the beam 108 returns to the default or the up-state position that provides low capacitive coupling (e.g., open position for RF MEMS switch 100 where RF signal is transmitted).

[0034] In an embodiment, as depicted in FIG. lb, the down-state capacitance (C¾ of a stationary metal-insulator-metal (MIM) capacitor switch is dependent on three parameters of the MIM capacitor which include relative permittivity of the dielectric layer (e_r ), the area of the MIM capacitor (Ad) and the thickness of the dielectric layer (t), or the like. The area of the MIM capacitor is the product of the width of the dielectric layer 106 which is the same as the width of the floating metal (WFM) H4 and the length of the dielectric layer 106 which is the same as the length of the floating metal (LFM) 114. The resonant frequency of the switch 100 (f₀) depends on the down-state capacitance (Cd) and the inductance of the beam (L_S) 108.

c_a = (2)

A_d = W_FML_FM (3)

[0035] The relative permittivity and thickness of the dielectric layer 106 are the fixed parameters. The width of the floating metal 114 cannot be changed because it is same as the width of the signal line of the CPW 104.

[0036] In an embodiment, the resonant frequency f₀ as given by Eq. (1) is dependent only on the down-state capacitance (Cd) as given by Eq. (2) which in turn depends only on the length of the floating metal (LFM) 114 as given by Eq. (3). Therefore, the resonant frequency f₀ of the RF MEMS switch 100 can be varied by varying only the length of the floating metal (L_FM) 114.

[0037] FIG. 2 is a flow chart illustrating a method 200 for varying resonant frequency of RF MEMS switch 100, according to an embodiment as disclosed herein. In an embodiment, at step 202, the method 200 includes encapsulating the dielectric layer 106 with a floating metal layer 114 as depicted in FIG 1 (b). At step 204, the method 200 includes varying the length of the floating metal 114. At step 206, the method 200 includes obtaining a desired resonant frequency based on the varied length of the floating metal 114 at step 204.

[0038] The various actions, acts, blocks, steps, or the like in the method 200 may be performed in the order presented, in a different order or simultaneously. Further, in some embodiments, some of the actions, acts, blocks, steps, or the like may be omitted, added, modified, skipped, or the like without departing from the scope of the invention.

[0039] FIG. 3 is a graph showing the variation of down-state capacitance (C¾ with the length of the floating metal (LFM) 114, according to an embodiment as disclosed herein. In an embodiment, the resonant frequency changes proportionately as the length of the floating metal 114 changes. This indicates that a wide range of values for resonant frequency can be achieved by elongating the floating metal 114.

[0040] FIG. 4 illustrates a RF MEMS switch 100 with a varying length of the floating metal (LFM) 114, according to an embodiment as disclosed herein. In an embodiment, the switch 100 structure is simulated in High Frequency Structural Simulator (HFSS) to demonstrate the effect of frequency tunability. The lengths of the floating metal 114 chosen for simulation are 140 μιη, 180 μιη and 240 μιη.

[0041] FIG. 5 is a graph showing the variation of isolation with the length of the floating metal (L_FM) 114, according to an embodiment as disclosed herein. In an embodiment, with the variation in the length of the floating metal 114, the resonant frequency is shifting. For LFM ⁼ 140 μιη, the resonant frequency is 8.8 GHz. With an increase in the length from 140 μιη to 180 μιη and 240 μιη, the resonant frequency changes to 7.7 GHz and 6.6 GHz respectively. [0042] FIG. 6 is a graph comparing the measured isolation with the simulated values for various values of the length of the floating metal 114, according to an embodiment as disclosed herein. In an embodiment, the device is realized and characterized using manual probe station. The devices for which the results are obtained are shown in FIG. 6. As the length of the floating metal 114 is changed from 140 μιη to 180 μιη, the resonant frequency has also changed from 8.223 GHz to 7.226 GHz. The results show a fine match with the simulated values. This validates the fact that RF MEMS capacitive switches 100 can be used to make tunability easier to achieve any frequency of operation.

[0043] FIG. 7 illustrates a schematic of an RF MEMS MF switch 100 with segregated floating metals of different lengths, according to an embodiment disclosed herein. In an embodiment, MF switch 100 is developed by using an array of beams 108 in the same device. The geometry of the beam 108 is the same. The length of the floating metal 114 and the dielectric layer 106 is different underneath each beam 108. In other words, each MIM capacitor has a different down-state capacitance and therefore a different resonant frequency. Figure 7 shows the schematic of a MF switch that can operate at three different frequencies.

[0044] FIG.8 illustrates a top-view of an RF MEMS switch 100 with segregated floating metals of different lengths, according to an embodiment disclosed herein. In an embodiment, the length of the floating metal 114 underneath beam 1, beam 2 and beam 3 are 10 μιη, 20 μιη and 30 μιη respectively. Therefore, each beam is selective to a different frequency fl, f 2 and f 3 respectively.

[0045] At a time, only one switch is considered to be in down-state position and the isolation is found for various values of frequency. This simulation was done in HFSS and the isolation vs. frequency for the three cases: only beam 1 is in the down-state, only beam 2 is in the down-state and only beam 3 is in the down-state. The results are shown in FIG. 9.

[0046] FIG. 9 is a graph showing the variation of isolation with frequency of RF MEMS switch 100 for the down- state condition of beam 1, beam 2 and beam 3 respectively, according to an embodiment as disclosed herein. For beam 1 where L_FM = 10 μιη, the resonant frequency is fl = 23 GHz, for beam 2 where L_FM = 20 μιη, the resonant frequency is fl = 16.6 GHz and for beam 3 where L_FM = 30 μιη, the resonant frequency is fl = 14.6 GHz. This validates the working of an RF MEMS MF switch 100.

[0047] FIG. 10 is a graph showing the variation of isolation with frequency of RF MEMS switch 100 when all the beams are in the up-state and when all the beams are in the down- state, according to an embodiment as disclosed herein. The isolation at resonance is > 30 dB in magnitude which is appreciable for an RF MEMS capacitive switch 100. The insertion loss is <1 dB in magnitude until 25 GHz for which this MF switch 100 is designed. These switches can be used for multi-frequency applications with a single set of beams.

[0048] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

Claims

STATEMENT OF CLAIMS We claim:

1. A method of varying a resonant frequency in a capacitance type radio frequency (RF) Micro-Electro-Mechanical System (MEMS) switch, the method comprising:

encapsulating a dielectric layer in said MEMS switch with a floating metal;

varying a length of said floating metal;

varying said resonant frequency based on said varied length.

2. The method of claim 1, varying said resonant frequency based on said varied length includes computing a down-state capacitance between a beam in said RF MEMS switch and said floating metal.

3. The method of claim 1, wherein said method further comprising:

varying length of each said floating metal among a plurality of said floating metals, encapsulated on each said dielectric layer among a plurality of said dielectric layers in said RF MEMS switch;

operating each of said beam among a plurality of said beams; and varying resonant frequency associated with each said beam based on said operation.

4. The method of claim 3, wherein each said beam is in a down-state.

5. The method of claim 3, wherein each said beam is in an up-state.

6. A capacitance type radio frequency (RF) Micro-Electro-Mechanical System (MEMS) comprising:

a substrate;

a lower electrode formed on said substrate;

a dielectric layer formed on said lower electrode;

a floating metal encapsulated on said dielectric layer;

a beam connected to a ground; and wherein length of said floating metal is varied to vary said resonant frequency.

7. The capacitance type RF MEMS switch of claim 6, wherein said resonant frequency is varied by computing a down-state capacitance between a beam in said RF MEMS switch and said floating metal after varying said length.

8. The capacitance type RF MEMS switch of claim 6, wherein said resonant frequency is varied by:

varying length of each said floating metal among a plurality of said floating metals encapsulated on each said dielectric layer among a plurality of said dielectric layers in said RF MEMS switch;

operating each of said beam among a plurality of said beams in said RF MEMS switch; and

varying resonant frequency associated with each said beam based on said operation.

9. The capacitance type RF MEMS switch of claim 8, wherein each said beam among said plurality of beams is in a down-state.

10. The capacitance type RF MEMS switch of claim 8, wherein each said beam among said plurality of beams is in an up-state.