Design, Analysis and Comparison of Various MEMS Switches for Reconfigurable Planar Antenna

Design, Analysis and Comparison of Various MEMS Switches for Reconfigurable Planar Antenna
Acta Polytechnica Hungarica
Vol. 11, No. 10, 2014
Design, Analysis and Comparison of Various
MEMS Switches for Reconfigurable Planar
Antenna
Paras Chawla1*, Rajesh Khanna2
1*
Electronics & Communication Engineering, JMIT, Radaur, Kurukshetra
University, Haryana (India)-135133, [email protected]
2
Department of Electronics & Communication Engineering Thapar University,
Patiala, Punjab-147004, India, [email protected]
Abstract: This paper presents design and analysis of a novel beam for electrostatically
actuated Radio Frequency Micro Electro Mechanical Systems (RF MEMS) shunt switches.
In the proposed beam design, geometrical variations in terms of structure shape, material,
gap, introduction of holes and changing the length and width of anchor have led to good
RF performance. The holes in the beam (maximum up to 60% of total area of upper
electrode) not only make the smooth mechanical movement of the beam but also result in
low spring constant. The designed RF-MEMS switches are actuated electrostatically, and
can be operated at low pull-in voltage, ranging from 0.5 V to 22 V with negligible power
consumption. Proposed RF MEMS switches can easily be integrated in mobile RF front end
section alongwith microstrip antenna in order to provide reconfigurabilty in frequency.
Various shapes of planar antenna are also discussed which are suitable with proposed
shunt switches. The effect of microstrip line and coplanar waveguide as transmission line
are also studied.
Keywords: Electromagnetics; Finite element methods; Microelectromechanical systems;
PolyMUMPS; Optimization
1
Introduction
The fabrication of integrated MEMS device for RF circuits design has a key
impact on small-sized reconfigurable antennas [1, 2, 3]. In the present age,
microelectronic switching take up a important position in make sure
reconfigurability for frequency tuning, beam steering and its shaping. Recently,
MEMS on/off switches have shown very good RF characteristics, including
smaller size, lower insertion loss in on state, almost zero power consumption,
higher isolation in off state, extreme lower intermodulation distortion and weight
[4, 5, 29]. The switch’s capability to achieve well upto 50 GHz is a noteworthy
leap forward over solid-state and mechanical RF switches [6, 30]. The RF MEMS
– 21 –
P. Chawla et al.
Design, Analysis and Comparison of Various MEMS Switches for Reconfigurable Planar Antenna
switches are applied on various shapes and geometry of microstrip antennas like
planar inverted F-shape (PIFA), E-shape, S-shape, spiral, fractal and many more
to achieve different applications. The fractal antenna has feature of self-similar
shapes and can also provide the reason for the design of multi band frequency
antennas [7]. There are numerous fractal geometries like sierpinski carpet,
sierpinski gasket, hilbert curve, koch island, and minkowski etc which have been
used in fractal antennas [7]. Sierpinski gasket is the main predecessor which is
very widely studied. Various beam shapes and materials of capacitive MEMS RF
switches prepared out of chromium, nickel, tantalum [19], tungsten, aluminum,
copper, and gold have so far been presented in previous work for beam design
[11-27]. The essential for low pull-in voltage in MEMS RF switches has often
produced excessive fabrication complication as well as extent of the device.
PolyMUMPs has followed the idea of standard process steps approach as a much
clear path to device functionality and volume production [8]. For larger scale
systems, PolyMUMPs chips act as a standard building unit, where the
microelectromechanical chip is only single piece of the overall building block.
PolyMUMPs technology is also used as a benchmarking tool for statistical studies
and software models, where experimented and measured data from actually
fabricated chips are validated with theories [8, 9]. The key emphasis in this work
is the study and designing of improved beams for RF MEMS shunt switches
which seems compatible with multiband microstrip antennas.
2
Principles and Operation of MEMS RF Switch
MEMS designs are the combination of sensors, mechanical elements, electronics
and actuators part on a semiconductor wafer through surface and bulk microfabrication process. This process generally involves a photo-lithography typed
micro-machining, batch production base fabrication, which usually offers benefits
of lowest cost while producing in huge volume [26, 27]. MEMS RF switches can
be air bridge, or a diaphragm, thin metal conducting cantilever can be considered
from point of mechanics. From radio frequency circuit configuration, it can be
parallel coupled or series coupled with an t-line [10]. The physical connection can
be either resistive i.e. metal-to-metal or capacitive i.e. metal–insulator–metal type.
Each such configuration of switch RF has many advantages in terms of
manufacturability or working performance.
When the external pull-in is applied on the actuation pad, an electrostatic power is
generated on the switch beam [28, 32, 34]. The reaction force (F) balance the
generated electrostatic force produced in beam structure. At two-third air gap, the
switch beam becomes unstable and results in breakdown at the lower transmission
line in down-state position [29-34]. The pull-in voltage be influenced by the value
spring constant of beam shape, gap and electrode area [1, 28]. In this research we
have proposed various beam shape designs to lesser the k value.
– 22 –
Acta Polytechnica Hungarica
Vol. 11, No. 10, 2014
Calculation of k for meander formed Serpentine flexure beam [5] is specified as
underk 
48GJ
 GJ
l 
l l
 EI
2
a
a
x
b

n

n 
for
3
3l
GJ
l l
EI
(1)
b
a
b
x
where n is the amount of meanders used in the serpentine beam flexure,
3
𝐺 = 𝐸⁄2(1 + 𝑣) represent torsion modulus, 𝐼𝑥 = 𝑤𝑡 ⁄12 denotes moment of
inertia. Further la, lb and w are primary length, secondary length and width of the
meander, E is the elasticity, v belongs to Poisson’s ratio and the torsion constant is
specified by,
1
192 𝑡
3
𝛱5 𝑤
𝐽 = 𝑡 3 𝑤 (1 −

∑𝑖=1,𝑜𝑑𝑑
1
𝑖5
𝑡𝑎𝑛ℎ (
𝑖𝛱𝑤
2𝑡
))
(2)
For the situation where 𝑙𝑎 ≫ 𝑙𝑏 , the k of the serpentine beam flexure turn into
𝑘 ≈ 4𝐸𝑤 (𝑡⁄(𝑛𝑙 )3 )
𝑎
2.1
(3)
Design of Transmission Lines
Coplanar waveguide (CPW) is a three-conductor on one-sided t-line. CPW has
center conductor alongwith two grounds lying in same plane, reducing the effects
of coupling and allowing for easy addition of shunt and series elements. Since
microwave ICs are basically coplanar in construction and therefore CPW lines are
used commonly as circuit parts and be integrated lines. At mm-wave frequencies,
CPW deals the prospective of lesser conductor and radiation losses as related to
microstrip lines. CPW has important advantage of varying magnitudes of the t-line
without effecting the value of characteristic impedance [6, 10, 28].
An estimated formula [10], for the typical impedance of the CPW, assuming t is
very small, 0 < k < 1, and h >> w, is
Zo 
p
1
30  2
[ln( 2
( r  1) / 2
1
p
p
)]1 ohms
w
w  2s
(4)
(5)
where w is center strip width, s is slot width, and εre is relative dielectric constant
of the material.
An empirical calculation for effective εre [10] is mention as-
– 23 –
P. Chawla et al.
re 
Design, Analysis and Comparison of Various MEMS Switches for Reconfigurable Planar Antenna

0.25  p  
h
 pw 
 0.04  0.7 p  1 0.1 r 
 
tanh1.785 log 1.75 
100  
2 
w
 h 

 r 1 
(6)
Further, microstrip transmission may be considered as a two wire line and is
further viewed the most extensively used as a planar microstrip transmission line
[29-31]. One side of the design is free to access for mounting on the hymite or
other packaged devices and further geometrical construction lends itself well to
printed circuit board patterning methods to define the complete circuit. Planar
transmission line has been used extensively in microwave and millimeter circuits
and systems.
The structure is complex and due to which the mathematical expression of per unit
length dimensions is difficult to calculate. The approximated analytical expression
of effective relative permittivity is given as
1 0 WwV 2
 p( g 0  g )
2
g2
(7)
where, g – separation between upper and bottom electrode and go – relaxed gap.
Microwave post-parameters which ought to be optimized for MEMS switch are the
return loss, isolation, insertion loss (IL) and switching operating frequency. The IL
is due to misalliance of the impedance of t-line and RF switch. The contact beam
resistance and metallization loss will also added to the IL. RF MEMS influence on
above mentioned microwave post-parameters results will be included in
preceeding sections.
3
Switch Design and Simulation Results
We have proposed four different beam structures designs, whose micro fabrication
steps are explained by using the modeling action involved in the designing of
MEMS switches. A thorough modeling and analysis of all four designed switches
has been offered based on multi-physics coventorware software, take out their key
performance characteristics and the results are intuitionistic. We have also used
this software to recognize the relationship between shape, material and actuated
voltage of RF switch. Further, variation of displacement in switch i.e. from
maximum z-direction to minimum z-direction w.r.t. voltage is shown, when the
switch is electrostatically actuated.
– 24 –
Acta Polytechnica Hungarica
3.1
Vol. 11, No. 10, 2014
Modeling Action of Various RF MEMS Switch
Table 1 shows the processed file of design-1 RF switch. The switch is designed on
silicon substrate of dimension 280×150 µm2, relative dielectric constant 11.9, and
thickness 10 µm which is covered with silicon nitrate of thickness 0.2 µm. The
microstrip transmission line are made up of titanium (Ti) of thickness 0.5 µm and
complete geometry with dimension and equivalent circuit (in down state) is shown
in Figure 1(a-c). Holes are used, in three out of four designed switches, in upper
beam electrode to increase the switching mechanism, reduce the squeeze film
damping and lowering the mass of beam. The electrostatic actuation can be
decreased using: reduction in the spring constant of beam, minimize the air gap,
and increase the actuation area.
Table 1
Process file of design-1 capacitive switch
Sr.
No.
1.
Substrate
Layer
Name
Substrate
2.
Stack Material
Nitride
3.
Planar Fill
Waveguid
e
4.
5.
6.
7.
Straight Cut
Planar Fill
Straight Cut
Conformal
Shell
Straight Cut
Straight Cut
Sacrifice
8.
9.
10.
Step Name
Material
Name
Silicon
(100)
Silicon
Nitride
(SIN)
Titanium
Thicknes
s (µm)
10
1.0
Beam
Gold
0.5
– 25 –
Ground
0.5
BPSG
(a)
Photoresist
0.2
Sacrifical
BPSG
Mask Name
Waveguide
+
Anchor
-
Beam
Holes
+
-
P. Chawla et al.
Design, Analysis and Comparison of Various MEMS Switches for Reconfigurable Planar Antenna
(b)
(c)
(d)
Figure 1
(a) 2-D layout with dimension of beam (b) 3-D layout of ‘Design-1’ capacitive switch. Equivalent
circuit of Design-1 in (c) up position and (d) down position.
The equivalent model of shunt MEMS switch is used to find the L, R and C
parameters, where L is the inductance, R is the beam resistance and C is the
capacitance of bridge in up and down state. In this work, the actual circuit of
proposed “Design-1” structure of Fig. 1(b) is generated by means of HSPICE
– 26 –
Acta Polytechnica Hungarica
Vol. 11, No. 10, 2014
model file. The HSPICE file from HFSS solver is exported in Advanced Design
System (ADS) software to validate the results through circuit and layout approach.
The equivalent model shown in Fig. 1 (c) and 1 (d), the bridge is represented by
the two short segments of transmission line and a lumped LCR model of the
bridge having a different up- and down-position capacitance value [1]. The bridge
capacitance in down state is due to the bridge-dielectric and transmission line
active overlapping area. Further, up-state is due to the active overlapping area
between the beam and the conductor. The up-state capacitance (Cup) is always less
than the down-state capacitance (Cdown). This effect is due to the collapse of
MEMS upper electrode/bridge on the dielectric surface, when an electrostatic
actuation is applied between the transmission line and the switch bridge. The
equivalent generated electrostatic force causes the largely increase in the switch
beam capacitance. The LCR model of shunt capacitive MEMS switch obeys the
rules as an inductor above the series resonant frequency, as a capacitor below this
frequency and decreases as a series resistance at resonance. Further, in up-position
of switch, the inductance plays no role and only shunt capacitance has a major role
to define/model the MEMS bridge. Although, it is observed in down-position of
the MEMS bridge, the inductance plays a significant role [1].
For design-1 MEMS switch analysis, the resonant frequency occurs at 22 GHz for
Cup = 0.0556 pF and Lup = 0.942 nH, when it is in on-position. For Cdown = 3806.63
pF and Ldown = 0.0073 nH, the switch resonate at 1 GHz frequency when the
MEMS bridge is in down or off position. According to these circuit values, the
insertion loss measured 0.34 dB at 4 GHz and 0.98 dB at 8 GHz in on-condition.
The values of isolation are 67.6 dB at 4 GHz and 73.6 dB at 8 GHz in offcondition. The electromagnetics parameters are optimized by varying the shunt
inductance and capacitance values. Except the characteristics impedance (Z0 = 50
ohm), other circuit values are permitted to vary the isolation and insertion loss
results calculations. It has been observed that by decreasing the Cup from 0.0556
pF to 55.6 fF and by increasing Lup from 0.942 nH to 29.42 nH, the insertion value
improves and value 0.22 dB at 4 GHz and 0.61 dB at 8 GHz, respectively.
Similarly, in down position the optimized isolation has been achieved 87.68 at 4
GHz and 93.59 dB at 8 GHz, by increasing Ldown from 0.0073 nH to 73 pH and at
the same time by increasing Cdown from 3806.63 pF to 38.06 nF. For these
optimized circuit elements, the switch resonates at 3.94 GHz and 0.95 GHz in onand off-condition, respectively.
The DC-contact shunt design-2 and design-3 switches designing steps are same in
up-position as that of design-1 except that the silicon nitride film is removed
below the metal bridge. As a result, there is a contact of metal to metal layer
among the transmission line and the ground surface. In the down-position of these
two DC contact shunt MEMS switches results in an RShLdown equivalent circuitmodel in parallel with the transmission line. Shunt resistance (RSh) is the addition
of the metal bridge resistance and the contact resistance. The LRC parameters
extraction of both switches follows the same method as the design-1 capacitive
– 27 –
P. Chawla et al.
Design, Analysis and Comparison of Various MEMS Switches for Reconfigurable Planar Antenna
shunt MEMS switch. The design-2 up-state results show that by increasing
inductance from 0.352nH to 3.5 nH, the insertion loss 0.378 dB at 4 GHz and 0.10
dB at 8 GHz can be achieved. The isolation value in the down-state of design-2
switch can be optimized up to 45.9 dB at 4 GHz and 45.8 dB at 8 GHz by
decreasing the shunt inductance from 0.0123 nH to 12.3 pH. Similar statement has
been observed for design-3 switch. The optimized results of insertion loss (0.36
dB at 4 GHz and 0.096 at 8 GHz) and isolation (39.6 dB at 4 GHz and 39.6 dB at
8 GHz) can be achieved by selecting Lup equal to 3.57 nH and Ldown equal to 0.122
pH, respectively. Further, the simulated up state capacitance and isolation results
are verified from following equations [1],
|𝑆11 |2 ≅
2 𝑍2
𝑤2 𝐶𝑢𝑝
𝑜
(8)
4
and
(
|𝑆21 |2 ≅
2𝑅𝑠 2
𝑍𝑜
(
)
2√2𝑅𝑠
𝑍𝑜
2𝑤𝐿 2
{ ( 𝑍𝑜 )
𝑓𝑜𝑟 𝑤𝐿 ≪ 𝑅𝑠
)
2
𝑓𝑜𝑟 𝑤𝐿 = 𝑅𝑠
(9)
𝑓𝑜𝑟 𝑤𝐿 ≫ 𝑅𝑠
Finally, the design-4 capacitive shunt MEMS switch results show optimized
insertion loss i.e. 0.051 dB at 4 GHz and 0.198 dB at 8 GHz, when Lup is equal to
0.0542 nH and Cup is equal to 76.8 fF. In the down position, the optimized
isolation i.e. 22.41 dB at 4 GHz and 22.66 dB at 8 GHz can be achieved by
selecting circuit component values as Ldown equal to 0.023 nH and Cdown equal to
8.12 pF, respectively.
Figure 2 show the 2D layout with dimensions of other three designed MEMS
shunt RF switches. The equivalent circuit of design-2 DC shunt MEMS switch is
shown in Figure 3.
(a)
– 28 –
Acta Polytechnica Hungarica
Vol. 11, No. 10, 2014
(b)
(c)
Figure 2
(a) 2-D layout with dimension of ‘Design-2’ (b) 2-D layout with dimension of ‘Design-3’ (c) 2-D
layout with dimension of ‘Design-4’ switch
(a)
– 29 –
P. Chawla et al.
Design, Analysis and Comparison of Various MEMS Switches for Reconfigurable Planar Antenna
(b)
Figure 3
Equivalent circuit of Design-2 MEMS shunt switch in (a) up position and (b) down position
3.2
Analysis of Various RF MEMS Switches – Multiphyics
Properties
After designing various RF MEMS switches are analysed. The voltage versus
charge has been shown for various beam structures in Table 2. The voltage on
beam is applied 1V w.r.t. ground in each case. The charge values on various
beams are 0.185 pC, 0.355 pC, 0.354 pC & 0.311 pC, respectively. Further the
Tables 3 to 6 show the capacitance matrix of various beam structures of switches.
The development of multilayer design and accumulation of a metal layer into the
process permitted a unlimited increase of the MEMS switch on-position
capacitances. By introduction of the metal layer, prominent to a Con/Coff ratio
higher which is at maximum 50 times more than the ones attained without this
feature [4]. Maximum and minimum displacement of various switches in X, Y, Z
directions are shown in Table 7. The reaction forces in three directions of various
designed switches have been shown in Table 8. The reaction forces that developed
at the fixed ends is a result of pressure load is exerted in downward direction. The
occurrence of natural vibration at some specific frequencies results the existence
of modal shapes. Such vibrating response assumes the properties of physical
system. At equilibrium, the resonant frequencies of MEMS upper electrode is
calculated. The different mode shapes and their concern frequencies are very
much resembles the characteristics of under-damped response.
Table 2
Charge values of all four designed switches at beam voltage = 1V
Charge value (pC)
Design-1
1.85 × 10-1
Design-2
3.55 × 10-1
Design-3
3.54 × 10-1
Design-4
3.11 × 10-1
– 30 –
Acta Polytechnica Hungarica
Vol. 11, No. 10, 2014
Table 3
Capacitance matrix (pF) of ‘Design-1’ switch
Ground
CPW
Beam
Ground
2.933 × 10-1
-1.226 × 10-1
-1.707 × 10-1
CPW
-1.226 × 10-1
1.373 × 10-1
-1.466 × 10-2
Beam
-1.707 × 10
-1.466 × 10
1.853 × 10-1
-1
-2
The MEMS beam switch response should be prefered below the mode 1 resonant
frequency in order to avoid peak in displacement. So, the prime objective is
always increase the performance of switch upper beam structure first mode
resonance frequency. Figure-3 show the harmonic analysis and the modes of
design-1 switch.
Table 4
Capacitance matrix (pF) of ‘Design-2’ switch
Ground
Wgnd1
Wcon1
Wgnd2
Wcon2
Beam
Ground
2.265
-1.015
-9.926 × 10-2
-1.014 × 10-1
-9.925 × 10-1
-3.705 × 10-2
Wgnd1
-1.015
Wcon1
-6.908 × 10-5
-2.707 × 10-5
-9.193 × 10-5
-1.457 × 10-1
-6.908 × 10
-5
1.126 × 10
-1.076 × 10
-4.404 × 10
-7
-1.319 × 10-2
-2.707 × 10
-5
-1.076 × 10
-4
-9.458 × 10
-5
-1.456 × 10-1
-9.193 × 10
-5
-4.404 × 10
-7
-1
-1.319 × 10-2
-1.457 × 10
-1
-1.319 × 10
-2
1.161
-9.926 × 10
-2
Wgnd2
-1.014 × 10
-1
Wcon2
-9.925 × 10
-1
Beam
3.705 × 10
-2
-1
-4
1.160
-8.458 × 10
-5
1.126 × 10
-1.456 × 10
-1
-1.319 × 10
-2
3.547 × 10-1
Table 5
Capacitance matrix (pF) of ‘Design-3’ switch
Ground
Wgnd1
Wcon1
Wgnd2
Wcon2
Beam
Ground
2.264
-1.015
-9.874 × 10-2
-1.014
-9.873 × 10-2
-3.770 × 10-2
Wgnd1
-1.015
-6.908 × 10-5
-2.707 × 10-5
-9.193 × 10-5
-1.457 × 10-1
Wcon1
-9.874 × 10
1.121 × 10
-1.219 × 10
-7.423 × 10
-7
-1.324 × 10-2
-9.832 × 10
-5
-1.449 × 10-1
1.161
-2
7.514 × 10
-5
-2.806 × 10
-5
-1
-1.219 × 10
-4
-4
Wgnd2
-1.014
Wcon2
-9.873 × 10-2
-9.832 × 10-5
-7.423 × 10-7
-9.842 × 10-5
1.121 × 10-1
-1.325 × 10-2
Beam
-3.770 × 10
-1.450 × 10
-1.324 × 10
-1.449 × 10
-1.325 × 10
3.542 × 10-1
-2
-1
-2
1.159
-1
-2
Table 6
Capacitance matrix (pF) of ‘Design-4’ switch
Ground
Beam
Wgnd1
Wgnd2
Conductor
Ground
1.979
-7.739 × 10-2
-8.110 × 10-1
-8.109 × 10-1
-2.801 × 10-1
Beam
-7.739 × 10-2
3.110 × 10-1
-9.949 × 10-2
-1.017 × 10-1
-3.233 × 10-2
Wgnd1
-8.110 × 10
-1
9.949 × 10
9.106 × 10
-8.968 × 10
-5
-2.933 × 10-5
Wgnd2
-8.109 × 10
-1
-1
-2.931 × 10-5
Conductor
-2.801 × 10
-1
-2
-1
-1.017 × 10
-1
-3.233 × 10
-2
-8.968 × 10
-5
9.128 × 10
-2.933 × 10
-5
-2.931 × 10
– 31 –
-5
3.125× 10-1
P. Chawla et al.
Design, Analysis and Comparison of Various MEMS Switches for Reconfigurable Planar Antenna
Table 7
Maximum & minimum displacement (in µm) of vector components of all designed switches
Design-1
Design-2
Design-3
Design-4
Max
Min
Max
Min
Max
Min
Max
Min
Node
Displacement
1.881
0
1.011
0
9.876 ×
10-1
0
1.666 ×
10-1
0
Node X
Displacement
9.445 ×
10-3
-9.530
× 10-3
4.221
-4.221
3.974
-3.974
4.40
-4.40 ×
10-1
Node Y
Displacement
2.335 ×
10-3
2.241 ×
10-3
3.682
-3.682
3.509
-3.509
6.859 ×
10-1
-6.859
× 10-1
Node Z
Displacement
1.017 ×
10-3
-1.881
1.399 ×
10-1
-1.011
× 10-2
1.356 ×
10-1
-9.876
× 10-1
2.806 ×
10-3
-1.666
× 10-1
Table 8
Vector components of force (in N/m2) on various parts of all designed switches
Design-1
Design-2
Design-3
Design-4
Fx
Fy
Fz
Fx
Fy
Fz
Fx
Fy
Fz
Fx
Fy
Fz
2.219
× 10-1
2.585
× 10-1
9.358
5.567
2.225
7.668
-5.593
2.236
7.861
-2.556
×101
5.611
4.337
Anchor
2
-2.219
× 102
-2.590
× 10-1
9.363
5.567
-2.225
7.668
-5.593
-2.236
7.861
-2.556
×101
-5.611
4.337
Anchor
3
-
-
-
-5.567
2.225
7.668
5.593
-2.236
7.861
2.556
×101
5.611
4.337
Anchor
4
-
-
-
-5.567
-2.225
7.668
5.593
2.236
7.861
2.556
×101
-5.611
4.337
Anchor
1
(a)
(b)
– 32 –
Acta Polytechnica Hungarica
Vol. 11, No. 10, 2014
(c)
(d)
(e)
(f)
Figure 3
Harmonic analysis of design-1 switch (a) - (f) shows the six modes from 1 to 6
3.3
Analysis of MEMS Switches-Electromagnetics Properties
The electromagnetic properties of the switches are calculated by using HFSS
software. The switches are simulated in both down and up states, fig. 5 and 6
shows the S-parameters results i.e. insertion, return loss and isolation of all
designed switches.
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P. Chawla et al.
Design, Analysis and Comparison of Various MEMS Switches for Reconfigurable Planar Antenna
(a)
(b)
Figure 5
Simulated S-parameter of Designed switches (a) insertion loss and (b) Return loss in on-state.
(a)
(b)
Figure 6
Simulated S-parameter of Designed switches (a) Isolation loss and (b) Return loss in off-state.
4
Comparison among RF MEMS Switches
The above version of four proposed switches are chosen for comparison since all
the switches has shunt configuration, almost similar dimensions and they are
simulated for the study of their electromagnetic as well electromechanical
characteristics, while the difference among them in beams structures are
considered. Typical dimensions of all designed switch beam lengths are ranging
from 250 to 400 µm and thickness ranging from 0.5 to 1.5 µm. The “Design-3”
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Acta Polytechnica Hungarica
Vol. 11, No. 10, 2014
switch show a significant improvement over the rest of three designed switches in
defined operational band i.e. 1 to 8 GHz. So, its performance comparison is
attempted at beginning with others in terms of electromechanical and
electromagnetic properties as shown in Table 9.
Due to ease of fabrication, the “design-1” is best suited. As the shape of the beam
in particular structure is “fixed-fixed” type it has relatively higher spring constant
as compared with other structures. Small diameter holes of 5µm are introduced on
beam which lower the mass, release some amount of resuidal stress and reduce
Young’s modulus [1]. The pull-in voltage calculated in this manner is lies between
3.15 to 3.5 V. The lower mass turn yields a upper resonant frequency which is
shown in generated harmonic modes (as in Fig. 3).
The serpentine spring shaped beam is considered in rest of the designs. Such
variations in beams can be used to down effective value of spring constant in RF
switch. The constructional difference between “Design-2” and “Design-3” are that
in latter structure holes of 5 µm diameter are introduced. As a result a significantly
lower actuation voltage 0.5 V instead of 22.9 V in “Design-3” was measured,
combined with negligible up-state capacitance. Using 3D electrostatic simulations
results, it is confirmed that the area of the holes are filled by fringing fields.
In “Design-4” the actuation area is reduced as compared with second and third
design. The higher conductance and the smaller contact area ensure better
electromagnetic characteristics such as insertion loss and isolation in the 1 to 8
GHz operating frequency range. Further, in this article, the evaluation of the all
new propsed switches can be attained via a straight comparison with the switches
as presented by authors of [11-27].
A low-loss microwave capacitive shunt MEMS switch using 5 meanders
serpentine spring folded suspensions is reported earlier [11] to achieve low
actuation voltage of 9 V together with the large area of capacitive actuators, so
that to maintain sufficient isolation measured in off-state and on-to-off ratio
capacitance is 48. A low actuation voltage of 6V MEMS switch by using low
spring constant folded suspension beam mechanical structure for high frequency
applications has also been reported [12]. A capacitive shunt switch which consider
a see-saw type movable structure to implement a shunt (capacitive) across the
CPW with a gap of 10 µm among the electrodes and the movable part and further
an actuation electrostatic of 80 V has been reported earlier [13]. Mingxin Song et
al. [14] have showed that both the torsion deflection and ordinary bending are
influenced by the relation of arm thickness and width. Lower actuation voltage of
1.5 V is achieved by using extended torsion arm size and extended driven arm
size. A capacitive MEMS switches with different meander spring beams which
provide low-spring constant as well as low pull-in voltage has also been reported
[15]. Depending on the serpentine design, the actuation voltages were achieved
from 1.9-7.0 V. A low-loss MEMS RF switch using low actuation voltage and
better mechanical stability are also known [16]. The switch MEMS has an
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P. Chawla et al.
Design, Analysis and Comparison of Various MEMS Switches for Reconfigurable Planar Antenna
isolation value of - 40 dB at 10 GHz and the pull-in voltage achieved is merely 3
V. Three different membrane designs using different spring constant with singlemeander, two-meander and six-strip springs to reduce the effect of residual stress
and pull-in voltage have been measured and compared [17]. Shunt switch is
actuated by electrostatic and electromagnetic forces combination for low voltage
and power application has also been reported [18]. The coupling capacitance
Coff/Con ratio is approximately 62.5, the isolation of 20.7 dB and IL of 0.85 dB is
achieved at 19.5 GHz. A shunt capacitive MEMS RF switches in III–V
technology have been fabricated and designed [19] using tantalum pentoxide
(Ta2O5) and tantalum nitride (TaN) for the dielectric layers and the actuation lines,
respectively. The thin metallic membrane is used for a capacitive membrane
microwave switch [20]. Electromagnetic model for capacitive shunt switch for
millimeter-micrometer wave applications has also been reported [21]. The IL in
the up-position is corresponding to the CPW t-line loss. It is also revealed that
sudden rise in the value of down position isolation is 20+ dB could be attained
with the selection of the exact LC series resonant frequency of the RF switch. The
equivalent LCR circuit of the shunt RF switch used in the structure of altered 2and 4-bridge “cross” switches from 10 to 40 GHz has also been given [22]. The
300µm long gold bridge is used and the gap height reduced from 3.5 to 1.5 µm to
lower the pull-in voltage from 50 to 15 V. The IL in the up-position is 0.2–0.4 dB
and down- position capacitance of 2.2 pF resulted in a high value of isolation. A
RF MEMS TiO2 capacitive switch with high capacitive ratio & low-actuation
voltage on high resistivity silicon (HRS) material weighted down on a semisuspended CPW showed an attenuation of 0.1dB/mm in the t-line [23].
The development of multilayer design methods and their properties by studying
the behavior of the capacitive MEMS switches at three unlike stages is pioneered
by F. Giacomozzi et al. [24]. The shunt switch capacitance was 4.6 pF and by
introduction of the floating metal layer the bridge realized a capacitance of near
7.7 pF, important to a Con/Coff ratio higher than 200 which is maximum around
50 times more than the ones reached for the identical designs without this feature.
Capacitive switches on microwave laminate having MEMS device monolithically
integrated with other elements offering adaptability and re-configurability features
have also been reported [25]. The dual warped beam type switches demonstrate in
[26], who showed an off-to-on capacitive ratio of up to 170, exhibiting very good
RF performance. The effect of varying the different geometric physical
dimensions on the S-parameters has been demonstrated by Bahmanyar et al. [27].
Conclusions
In this article, four different MEMS RF switches are designed, analyzed and
compared. We have studied the relations of actuated voltage, Young’s modulus,
material composition, Con/Coff and electrostatic-forces of shunt MEMS switches
with the help of coventor software. Minimum pull-in voltage calculated for
serpentine beam switch with hole (“Design-3”) is found to be around 0.5 V. The
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Acta Polytechnica Hungarica
Vol. 11, No. 10, 2014
lower pull-in voltage is desirable in those circuits which require less power
consumption like RF MEMS based mobile front-end section. The effect of various
Table 9
Comparison of various RF MEMS switch design (operating band 1 to 10 GHz)
Ref.
Pull-in
voltage
Return loss in upstate (S11)
Insertion loss (S21)
Return loss in
down-state (S11)
Isolation (S21)
[11]
9V
21 dB at 10GHz
0.11dB at 10GHz
0.98dB at 10 GHz
15 dB at 10 GHz
[12]
6V
-
-
-
-
[13]
5V
-
0.5 dB at 10 GHz
<0.5 dB at 10 GHz
28 dB at 2.3 GHz
[14]
1.5 V
-
-
-
-
[15]
1.9- 7 V
-
-
-
-
[16]
3V
10 dB at 10 GHz
0.37 dB at 10 GHz
-
40dB at 10 GHz
[18]
3.7 V
15 dB at 10 GHz
0.3 dB at10 GHz
0.99 dB at 10 GHz
12.5dB at 10 GHz
[19]
15–20 V
10 dB at 10 GHz
0.6 dB at 10 GHz
0.4 dB at 10 GHz
12.5dB at 10 GHz
[20]
50 V
2.5 dB at 10 GHz
0.16 dB at 10 GHz
0.2 dB at 10 GHz
15 dB at 10 GHz
[21]
20-50 V
20 dB at 10 GHz
0.07–0.1 dB from
1 to 10 GHz
0.005–0.015 dB
from 1 to 10 GHz
12-14 dB at 10
GHz
[22]
15 to 25
V
22 dB at 10 GHz
0.1–0.3 dB from 1
to 10 GHz
1-2 dB at 10 GHz
8 dB at 10 GHz
[23]
8V
-
0.4 dB at 10 GHz
-
24 dB at 10 GHz
[24]
< 10 V
35 dB at 10 GHz
0.1 dB at 10 GHz
-
20 dB at 10 GHz
[25]
30-40 V
12 dB at 10 GHz
<0.4dB at 10 GHz
0.2 dB at 10 GHz
18dB at 10 GHz
[26]
27 V
-
0.2 dB at 10.5
GHz
-
41 dB at 10.5 GHz
[27]
-
-
0.65dB at 1-10
GHz
-
12dB to 38dB at
1-10 GHz
Design1
3.193.5V
13.80 dB @ 4 GHz
9.09 dB @ 8 GHz
0.22 dB @ 4 GHz
0.61 dB @ 8 GHz
0.003 dB @ 4 GHz
0.003 dB @ 8 GHz
87.7 dB @ 4 GHz
73.6 dB @ 8 GHz
Design2
22.823.1V
11.40 dB @ 4 GHz
17.10 dB @ 8 GHz
0.38 dB @ 4 GHz
0.10 dB @ 8 GHz
0.004 dB @ 4 GHz
0.004 dB @ 8 GHz
45.9 dB @ 4 GHz
45.8 dB @ 8 GHz
Design3
0.510.52V
11.50 dB @ 4 GHz
17.20 dB @ 8 GHz
0.36 dB @ 4 GHz
0.09 dB @ 8 GHz
0.009 dB @ 4 GHz
0.009 dB @ 8 GHz
40.1 dB @ 4 GHz
40.2 dB @ 8 GHz
Design4
0.630.75V
20.30 dB @ 4 GHz
14.40 dB @ 8 GHz
0.05 dB @ 4 GHz
0.19 dB @ 8 GHz
0.675 dB @ 4 GHz
0.659 dB @ 8 GHz
23.1 dB @ 4 GHz
23.4 dB @ 8 GHz
values of electrostatic force on displacement is also calculated. As electrostatic
force is increased, displacement of beam decreased. Further, the effect of beam
– 37 –
P. Chawla et al.
Design, Analysis and Comparison of Various MEMS Switches for Reconfigurable Planar Antenna
thickness on pull-in voltage has also studied. The thickness of beam had direct
relationship with pull-in voltage, i.e. as the thickness of beam increased, the pullin voltage also increased. This article laid the foundation for proposing a new
improved RF MEMS on/off switches for reconfigurable planar antenna, and that is
the subject of our future work.
Acknowledgements
This work was supported by National Program on Micro and Smart Systems
(NPMASS) and also MANCEF, New Mexico, USA for providing coventoreware,
comsol and other useful softwares.
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