Research A rticle
Low Actuating Voltage Spring-Free RF MEMS SPDT Switch
Deepak Bansal,
1,2
Anuroop Bajpai,
1
Prem Kumar,
1
Maninder Kaur,
1
and Kamljit Rangra
1,2
1
Central Electronics Engineering Research Institute (CEERI), Council of Scientic and Industrial Research (CSIR), Pilani,
Rajasthan 333031, I ndia
2
Academy of Scientic and Innovative Research (AcSIR), New Delhi, India
Correspondence should be addressed to Deepak Bansal; dbansal.pu@gmail.com
Received July ; Accepted August
Academic Editor: J it S. Mandeep
Copyright © Deepak Bansal et al. is is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
RF MEMS devices are known to be superior to their solid state counterparts in terms of power consumption and electromagnetic
response. Major limitations of MEMS devices are their low switching speed, high actuation voltage, larger size, and reliability. In the
present paper, a see-saw single pole double throw (SPDT) RF MEMS switch based on anchor-free mechanism is proposed which
eliminates the above-mentioned disadvantages. e proposed switch has a switching time of nsec with actuation voltage of V.
Size of the SPDT switch is reduced by utilizing a single series capacitive switch compared to conventional switches with capacitive
and series combinations. Reliability of t he switch is improved by adding oating metal and reducing stiction between the actuating
bridge and transmission line. Insertion loss and isolation are better than −. dB and − dB, respectively, for GHz to GHz
applications.
1. Introduction
Development of miniature wireless communications devices
like mobile and satellite requires components with minimum
power consumption with better performance. e RF MEMS
devicesareknowntobebettercomparedtosolidstatedevices
due to their low power consumption, high isolation, and
low insertion loss. RF MEMS SPDT switches constitute the
basic building block of the RF MEMS systems like SPT [],
phaseshier[],switchmatrix[],othercommunication
systems [], and so forth. e RF MEMS devices are also
known to have few drawbacks too, for example, low switching
speed (– sec), high pull-in voltage (– V), larger
size, and reliability [, ]. ese issues at present limit the
utilization of the MEMS devices on a larger scale. ese
mechanical parameters are function of spring constant and
pull-in voltage. e conventional anchoring of a rectangular
membrane is through two Euler beams where one end
along the length is xed to the membrane and the other
to anchor post on either side of the membrane. When
a voltage is applied between the movable and the xed
membra ne of the switch as shown in Figure , the bridge
moves down onto the xed ground under the electrostatic
force. At the pull-in voltage, the switch is in the down state
[] which is calculated using D lumped model as shown in
Figure .
From lumped model neglecting higher order eects as
mentioned in [], actuation voltage of the switch is given by
𝑝
=
8
3
27
𝑜
,
()
where is spring constant and and are gap and area of
overlap between the actuation electrode and the bridge.
From (), actuation voltage can be lowered by (i) increas-
ingtheoverlaparea(), (ii) decreasing t he gap (), or (iii)
decreasing spring constant ().
Each method has its own demerits; for example, (i) large
overlap area () enhances stiction forces as stiction is directly
proportional to contact area [, ]. erefore, actuating
electrode area is optimized bas ed on stict ion and restoring
force [].
Smaller gap ()promotesstictionbyreducingbeam
restoring force, which is proportional to the beam displace-
ment. Smaller gap also adversely aects switch isolation
throughenhancedcapacitiveleakage.ebridgegapisalso
limitedbytheavailablefabricationtechnologyandhasbeen
setintherangeof–m.
Hindawi Publishing Corporation
Journal of Electrical and Computer Engineering
Volume 2016, Article ID 7984548, 7 pages
http://dx.doi.org/10.1155/2016/7984548
Journal of Electrical and Computer Engineering
Bridge
k
g
+
−
g
o
F : D lumped model for ac tuation voltage calculation.
A
B
RF IN
RF OUT 2RF OUT 1
Signal
line gap
A
B
20 𝜇m
F : SEM image of a spring-free see-saw RF MEMS capacitive
switch.
Decreasing makes the beam complaint and prone to
stiction. Further, switching time and pull-in voltage are
inversely corelated to spring constant; devices with low pull-
involtageareslowtochangestate.Inthepresentpaper,
a novel RF MEMS series capacitive SPDT switch based on
anchor-free torsional conguration is proposed as shown
in Figure . In the new conguration, as shown in Figures
(a) and (b), anchoring is replaced by spring-free see-saw
mechanism. e conguration has the advantages of low
actuation voltage with high switching speed as discussed in
Section .
2. SPDT Switch Description
e anchor-free see-saw SPDT designed for S-Ku band is
similar to ohmic SPDT except for the contact p oints that
arecapacitiveinthiscase.ecapacitivecontactshavean
advantage of high operating frequency as described in [].
Selection of the switch parameters is made based on available
fabrication technology and electromechanical response of the
switch. Input and output ports are made of ohm Y-shaped
CPW conguration (– – m) for external connections.
e bridge gap, dielectric material, and bridge thickness are
decided by fabrication capabilities. e bridge gap lies in the
range of – m based on nal releasing process, xed at m
in the present design. Silicon dioxide is selected as a dielectric
material due to its well-known behavior. Floating metal area
is selected based on operating frequency and bandwidth as
T : Design parameters used in the proposed switch design.
S.
number
Design parameter Value Unit
CPW (G–S–G) –– m
Anchor dimension 40∗50 m
2
Oxide thickness . m
Oxide dielectric constant .
Floating metal thickness . m
Floatingmetalarea 2∗65∗90 m
2
Bridge thickness m
Bridge length m
Bridge width m
Bridge height above oat metal m
Clamp thickness m
Support thickness m
Support dimensions 10∗100 m
2
Signal line gap m
Actuating electrode area 100∗80 m
2
Chip size 1080∗680 m
2
it decides the capacitive ratio explained in Section []. e
actuation pads and bridge dimensions are xed to keep pull-
involtagelessthanV.ebridgeandclampsaremadeof
thick ( m) gold to avoid curling/buckling of the bridge.
e signal line gap is xed to m, lower gap leads to poor
isolation, and higher gap needs larger width of the bridge to
keep overlap capacitance xed. All the parameters utilized in
designing the SPDT switch are listed in Table . e SPDT
switch utilizes single series capacitive switch which ma kes
its size (. ×. mm
2
)%compactcomparedtothe
literature, for example, . ×mm
2
[], . ×. mm
2
[],
and . ×. mm
2
[].
3. Working of the Switch
Working of the see-saw switch is similar to conventional
switches.Onactuation,thebridgelandsononesideofthe
transmission line as shown in Figures (a) and (b) and
connects the input port to the output port. e other side of
thebridgeisliedupwardsandnosignalispassed.
In OFF state, the switch (bridge) is in an upward tilted
position that results in OFF state capacitance (
o
)of.fF
as shown in Figures (a) and (c):
o
=
𝑜
⋅
Bridge width −Signal line gap/2∗Bridge overlap
Bridge height from oating metal
=
𝑜
(
100−20
)
/2∗5
2
=0.885fF.
()
e capacitance (. fF) between the input and outp ut
lines (reactance /> kfor S-Ku band) provides good
isolation. RF coupling between the signal lines through the
bridge is negligible and can be neglected. Working principle
Journal of Electr ical and Computer Engineering
PECVD oxide
Ground
Ground
Pull-in electrode
Floating metal
Pull-up electrode
Transmission line
Freely suspended Bridge
Gold clamp
Bridge
Ground
Ground
Clamps
(a)
(b)
Conducting rod
OFF state
ON state
F : Cross-sectional view of working the SPDT switch (a)
across AA
and (b) across BB
.
Floating metal Floating metal
Transmission line
Bridge
5𝜇m
65 𝜇m65 𝜇m
20 𝜇m
90 𝜇m
(a) Topviewofthebridgeandcontactarea
Bridge
Oxide Floating metal
Transmission line
65 𝜇m65 𝜇m
(b) Cross-sectional view of the bridge and contact area in ON
state
5𝜇m5𝜇m
(c) Cross-sectional view of the bridge and contact area in
OFF state
F : (a) Top view, (b) cross-sectional view in ON state, and (c)
cross-sectional view in OFF state at the bridge and contact area.
is demonstrated in Figure . In ON state, the bridge tilts and
contacts the metal layer (called oat metal) on the t-line.
is results in an eective overlap capacitance of pF
(area × m
2
) between the bridge and t-line (Figures (a)
and (b)) oering small reactance (< ,forS-Kuband)to
thesignalpassingfrominputtooutputportviathebridge.
4. Fabrication Process Flow
e fabrication process ow for the SPDT switch is illustrated
in Figures (a)–(i). e process begins with growth of
one-micron-thick thermal oxide on high resistive Si wafer.
LPCVD deposited polysilicon which constitutes the actua-
tion electrodes and DC bias lines is doped with phosphorus to
achieve required sheet resistance followed by PECVD oxide
for isolation. A . m thin Ti/TiN layer (oat metal) is sput-
tered followed by Cr/Au seed layer of thickness nm/ nm
and electroplated to get m thickness supports as shown
in Figure (e). A sacricial layer (HIPR) is spin coated to
make a mold for bridge structure followed by again Cr/Au
seed layer of thickness nm/ nm with electroplating to
get m sti bridge structures as shown in Figure (g). e
process of sacricial layer and gold plating is repeated to make
m thick clamps for the bridge as shown in Figure (i). e
nal structure is released with wet etching using critical point
dryer (CPD) machine. e nal fabricated SPDT switch is
showninFigure.
5. Mechanical Characteristics
e RF MEMS devices have a better electromagnetic response
compared to the solid state devices but mechanical param-
eters like biasing voltage, speed, and reliability are inferior
which limit their applications. e proposed spring-free
system has addressed the above drawbacks as follows.
5.1. Actuation Voltage. e proposed switch design is free
from the spring act ion, and acting restoring force is neg-
ligible. e only gravitational force has control over the
actuation. Electrostatic force required to operate the switch
must overcome the gravitational force as given below:
𝑒
=
1
2
𝑜
2
act
2
𝑜
≥,
()
where is a gravitational constant.
From (), required actuating voltage is
act(min)
=
2
2
𝑜
𝑜
.
()
For the proposed switch parameters as listed in Table ,
actuation voltage turns out to be . V. However, on measure-
ments in open environment, actuation voltage turns out to be
V due to bridge damping and friction b etween hinges and
clamps which is much less than actuation voltage reported
intheliterature[–].Comparisonsforsize,pull-involtage,
switching time, and number of switches utilized in the present
SPDT switch are described in Table .
5.2. Switching Speed. Resonancefrequencyandswitching
speed of the SPDT switch are extracted from D nonlinear
model given by
2
2
+
+=
𝑒
,
()
Journal of Electrical and Computer Engineering
ON state
OFF state
+
−
(a)
Spring free see-saw mechanism
mg
mg
(b)
OFF state
ON state
+
−
(c)
F : Schematic of dierent states of the bridge during the see-saw mechanism: (a) port “ON” and port “OFF,” (b) both “OFF,” and
(c) port “OFF” and port “ON.”
ermal oxide
(a)
Polysilicon electrode
(b)
Oxide deposition
(c)
Floating metal
(d)
Electroplated support
(e)
Sacricial layer-1
(f)
Electroplated bridge
(g)
Sacricial layer-2
(h)
Electroplated clamps
(i)
F : Fabrication process ow for the SPDT switch.
Journal of Electr ical and Computer Engineering
T : Literature survey of size, pull-in voltage, switching t ime, and number of switches used in the SPDT switch.
SN Device parameter e proposed see-saw design
Literature
Value Year Ref
Size (mm
2
)
1.08∗0.68
5.2∗3.0 []
3.8∗1.1 []
3.5∗1.5 []
0.68∗0.88 []
Pull-in voltage (V)
[]
[]
. []
[]
[]
Switching speed (s) .
– []
. []
. []
[]
Numberofswitches
[]
[]
[]
[]
[]
where , ,andare mass of bridge, damping coecient, and
the spring constant of the anchor, respectively. For the spring-
free system (=0), switching time is calculated by [, ]
𝑝𝑖
=
2
3
𝑜
3
𝑜
2
act
,
()
where
𝑜
is initial gap, is area, and
act
is applied actuation
voltage between electrode and bridge. Damping constant for
the rectangular plate is given by []
=
3
2
2
3
𝑜
,
()
where is the coecient of viscosity.
From () and (), calculated switching time is nsec
at an actuation voltage of V under vacuum (. torr)
conditions.
5.3. Power Handling. e proposed design has a low actu-
ation voltage of . V which leads to self-actuation of the
bridge as explained in [, ]. Self-actuation by RF power
andactuationvoltagearecloselycorrelatedtoeachother.e
electrostaticforceduetoRFpowerisgivenby()and()[].
Consider
𝑒
=
1
2
𝑜
2
2
dc-eq
,
()
where
dc-eq
=
𝑝𝑘
2
=
0
.
()
e proposed SPDT switch with design parameters men-
tioned in Table wil l self-actuate at RF power of mWatt.
See-saw mechanism with the second electrode as shown in
Figure is used to avoid self-actuation. Two electrodes are
placed on opposite sides to control self-actuation. By applying
sucient voltage (>RFvoltage),thatis,VformWattat
opposite electrode, self-actuation is controlled.
5.4. Reliability. e switch reliability is a function of the
large number of parameters like stiction, power handling,
temperature, stress, the number of switches, contact forces,
creep, and so forth which make it dicult to quantify. In the
present design, the switch reliability is improved by control-
ling three parameters out of all reliability parameters: stiction
force, power handling, and the number of switches. e
proposed switch design has reduced OFF state contact area
by times compared to conventional switches as explained
in [–]. Stiction forces that are directly proportional to
cont act area [, ] are reduced by a factor of . S econdly,
power handling is further dependent upon many parameters
like temperature, cold/hot switching, stress, self-actuation,
and so forth. e see-saw conguration with the second
electrode has increased power handling from self-actuation
perspective only. irdly, numbers of SPST switches utilized
in the SPDT conguration are reduced to one which has
improved reliability by reducing the switch dependency.
6. Electromagnetic Analysis
e SPDT switch is designed in High Frequency Structural
Simulator (HFSS) for C to Ku band applications. e elec-
tromagnetic responses of the RF MEMS see-saw switch are
similar to a conventional anchored switch []. OFF state