1. Divya Verma, Ajay Kaushik / International Journal of Engineering Research and Applications
(IJERA) ISSN: 2248-9622 www.ijera.com
Vol. 2, Issue 5, September- October 2012, pp.391-394
Analysis of RF MEMS Capacitive Switch based on a Fixed-Fixed
Beam Structure
Divya Verma*, Ajay Kaushik**
*,**MMEC, Maharishi Markandeshwar University, Mullana, Haryana(India),
ABSTRACT
RF MEMS has evolved over the past adjustment of a separate RF device or component,
decade and it has emerged as a potential such as variable capacitors, switches, and
technology for wireless, mobile and satellite filters.There has been great research effort on Radio
communication and defence applications. RF Frequency Micro-Electro- Mechanical Systems (RF
MEMS provides an opportunity to revolutionize MEMS) switches because they have many
the wireless communication. This paper advantages over p-i-n diode or field effect transistor
describes the Performance of low loss Fixed- (FET) switches [5]. RF MEMS switches show
Fixed RF MEMS capacitive switch . The RF attractive electrical performance characteristics that
MEMS capacitive Fixed-Fixed switch exhibit are critically needed in the next generation RF
lower losses, better reliability, and good switches with high isolation, very low insertion loss,
performance at higher frequencies. RF MEMS wide bandwidth operation and excellent linearity [6,
switches can be classified based on their 7 and 8]. This makes it ideal to enable a plethora of
actuation mechanisms into categories such as wireless appliances operating in the home/ground,
electrostatic, electromagnetic and thermal. Most mobile, and space spheres such as handsets, base
of the RF-MEMS switches reported to date have stations, and satellites.
used electrostatic actuation , which normally
requires high actuation voltages. In this paper a The main existing challenge in use of RF MEMS
fixed-fixed RF MEMS capacitive switch is switches is high value of actuation voltage. As the
designed to achieve low actuation voltage and to high actuation voltage requires high voltage drive
analyse their performance parameters. circuits which degrades life time and induces
malfunction by charge trapping problem. So, in this
Keywords: Capacitive, electrostatic actuation, pull- paper we have focused in the reduction of actuation
in voltage, RF MEMS switch. voltage by studying the various parameters which
effect the actuation voltage. In this paper proposed
I. INTRODUCTION RF MEMS capacitive switch based on fixed-fixed
Wireless communication has made an beam structure which shows an improvement in
explosive growth of emerging consumer markets, as characteristics at higher frequencies. Here, we
well as in military applications of RF, microwave, propose a switch which uses fixed-fixed shape beam
and millimetre-wave circuits and systems. These and its parameters are analyzed. It has wide
include wireless personal communication systems, potential with multiband support for different
wireless local area networks, satellite applications like K and Ka band which is to be sight
communications, automotive electronics, etc. In for different satellite communication. It is also
these systems, the RF switch is one of the essential supposed to support next generation mobile terminal
components to handle RF signals [1,2]. RF MEMS applications.
is an emerging technology that promises the
potential of revolutionizing RF and microwave II. RF MEMS SWITCH
system implementation for the next generation of Switch is the basic element that connect
telecommunication applications [3]. Its low power, or disconnect the electrical connection. There are
better RF performance, large tuning range, and two basic switches used in RF to millimeter-wave
integration capability are the key characteristics circuit design: the shunt switch and the series
enabling system implementation with potential switch. The series MEMS switch is excellent for
improvements in size, cost, and increased RF-40 GHz applications with a typical isolation of
functionality. 50 dB at 1 GHz, and 30 dB at 10 GHz [9]. The shunt
design is excellent at 10-100 GHz applications, with
The term RF MEMS refers to the design and a typical isolation of 17 dB at 10 GHz and 35-40 dB
fabrication of MEMS for RF integratedcircuits. It at 30-40 GHz for a capacitance of 4 pF . From a
should not be interpreted as the traditional MEMS mechanical point of view, MEMS switches can be a
devices operating at RF frequencies [4].MEMS thin metal cantilever, air bridge, or diaphragm, from
devices in RF MEMS are used for actuation or RF circuit configuration point of view, it can be
series connected or parallel connected with an RF
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2. Divya Verma, Ajay Kaushik / International Journal of Engineering Research and Applications
(IJERA) ISSN: 2248-9622 www.ijera.com
Vol. 2, Issue 5, September- October 2012, pp.391-394
transmission line [4,10]. The contact condition can membrane. At (2/3g0), the increase in the
be capacitive (metal–insulator–metal) or resistive electrostatic force is greater than the increase in the
(metal-to-metal), polar ceramics such as restoring force, resulting in the beam position
(Ba,Sr)TiO3 - BST and designed to open the line or becoming unstable and collapse of the beam to the
shunt it to ground upon actuation of the MEMS down-state position. The pull-down (also called
switch. Each type of switch has certain advantages pull-in) voltage is found to be
in performance or manufacturability. Main
8𝑘
mechanical operations of RF MEMS switches 2𝑔 𝑜 𝑔𝑜 3
depends mainly on spring constant of material used 𝑉𝑝 𝑉 = 𝑉 = 27𝜖0 𝑊. 𝑤
3
i.e. k. We always require to have less k i.e. less stiff
material because the deflection of beam depends on
8𝑘
spring constant k and we need more deflection with 𝑔𝑜 3
given force for an given RF MEMS Switch. In this = 27𝜖0 𝐴 (2)
paper we have used Fixed-Fixed type beam shape
with holes to lower the k value [10].
where V is the voltage applied between the beam
Calculation for spring constant for Fixed-Fixed and electrode, A= Ww is the electrode area, g0 is the
shaped beam is given below. zero-bias bridge height, ∈0 is the permittivity of air.
Fixed-Fixed flexure As shown in Eq. (2), the pull down voltage depends
𝑡 3 on the spring constant of beam structure, and, beam
𝑘 = 4𝐸𝑤 (1)
𝑙 gap g0 and electrode area A [12]. There are two
Where k is a spring constant, E is a Young’s approaches to reduce the actuation voltage: A first
modulus, l is the length of the beam, t is the approach in lowering the actuation voltage is to
thickness of the beam. In many MEMS switches, increase the actuation area. Increasing the area is not
small diameter holes (3–8 mm) are defined in the a practical solution because the compactness is the
beam to reduce the squeeze film damping and prevailing issue and adoption of MEMS technology
increase the switching speed of the MEMS switch. is to achieve the miniaturization. The second
The hole area can be up to 60% of the total surface alternative, which offers the maximum design
area of the MEMS structure. The holes also result in flexibility for a low-to-moderate actuation voltage,
a lower mass of the beam, which in turn yields a is to lower the switch spring constant, hence,
higher mechanical resonant frequency[10]. designing a compliant switch. To reduce the
actuation voltage, the key is beam structure of low
spring constant k.
III. RESULTS
A. RF MEMS Design and Analysis
Figure 2 shows the voltage and charge
values on conductor calculated and measured for
fixed-fixed based RF MEMS switch. Since
Coventorware software could synthesize the
multiply factors, such as electrostatic-forces, pull-
down voltages, Young’s modulus, and other
Figure1: Fixed-Fixed beam based RF-MEMS switch vector values could are obtained. Figure 3 shows
capacitance matrix which shows self-capacitance
1) Electrostatic Actuation: terms (located on the diagonal of the capacitive
When the voltage is applied between a matrix) should be positive and mutual-capacitance
fixed-fixed beam and the pull down electrode, an terms (off-diagonal elements) should be negative
electrostatic force is induced on the beam. The according to the ConventorWare’s convention. A
electrostatic force applied to the beam is found by Capacitance Matrix dialog that deviates from this
considering the power delivered to a time-dependent rule is an indication that the mesh needs to be
capacitance. This electrostatic force is approximated refined. Figure 4 shows pull-in voltage ranges for
as being distributed evenly across the beam section fixed-fixed beam based RF MEMS switch. The
above the electrode. As this electrostatic force is graph in figure 5 shows charge produced on a
applied to the beam, the beam membrane starts to beam with different values of voltages of a
deflect downward, decreasing the gap g and capacitive MEMS switch.
increasing the electrostatic pressure on the
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3. Divya Verma, Ajay Kaushik / International Journal of Engineering Research and Applications
(IJERA) ISSN: 2248-9622 www.ijera.com
Vol. 2, Issue 5, September- October 2012, pp.391-394
applied to many other devices, including tunable
filters, other antenna geometries, signal splitters or
military applications.
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4. Divya Verma, Ajay Kaushik / International Journal of Engineering Research and Applications
(IJERA) ISSN: 2248-9622 www.ijera.com
Vol. 2, Issue 5, September- October 2012, pp.391-394
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