International Journal of Engineering Trends and Technology (IJETT) – Volume 25 Number 4- July 2015
Design and EM Modeling of RF MEMS Switches
Anil Kumar Chaurasia1, Dr. Rajesh Mehra 2
M.E Scholar & Associate Professor
Department of Electronics and communication Engineering
National Institute of Technical Teacher Training and Research, Chandigarh, INDIA-160019
Abstract: Microelectromechanicalsystem is a new
invention for system integration of sensors, actuators and
signal processing. A research on this is presented on design
optimization, Reliability, dielectric Charging problem,
stiction and anti-stiction in RF MEMS. The reliability in
MEMS (micro electro-mechanical system) has steadily
developed in the recent year. The reliability of switch is
reduced due to the charging effects. Charging effects
occurred due to geometry and material of the device.
Geometry of the switch can improve switch reliability by
reducing the effect of stress gradient and intrinsic biaxial
stress on the capacitive switch membrane. Topology
optimization is used to minimizing the stress curling and
stress stiffening. Dielectric charging and its effect are
extensively addressed. Use of advanced dielectric material,
an improvement in mechanical design and application of
dielectric material is discussed. The failure modes in MEMS
are discussed contain wear, stiction, environmentally
induced failure, crystallographic defect, fracture,
degradation of dielectrics, creep, parasitic capacitance,
packaging, Delimitation and electric related failure.
Keywords: RF MEMS switch, Reliability, Stiction,
Dielectric
charging,
Self-assembled
monolayers,
delimitation
I. Introduction
In modern era, the specific drift towards the miniaturized
energy-efficient devices and the rapidly growing of telecommunication industries have led to expeditious RF MEMS
devices [1]. Micro electro-mechanical systems are the devices
that integrates electrical and mechanical element. MEMS
switches can be actuated by thermal actuation,
electromagnetic actuation, piezoelectric actuation and
electrostatic actuation, but electrostatic actuation is
predominantly preferred due to almost zero power
consumption, more auspicious reliable de-vices and less
convoluted manufacturing process [2]. However, RF MEMS
switches faces several disadvantages including hot switching,
high actuation voltage in high power applications. MEMS also
face reliability problem that are linked with RF signal power
level. Now a days lifetime of MEMS switch is under intense
research [3].MEMS switches can be designed using various
configurations based on actuation mechanism, movement,
circuit configuration and contact type. The new wireless
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standards are requiring devices with low power consumption,
large bandwidth, high linearity, excellent isolation, very low
insertion loss, small footprint slow cost, high quality factor,
low intermodulation distortion and light weight. MEMS
devices have another advantages are compatible with
integrated circuits [4], [5] for RF mixed signal environments,
analog and digital [6]–[8]. The combination of RF MEMS
series and shunt switches are used to achieve high isolation
and multi-band devices [9]. Broadband nature and less
insertion loss and high isolation has the most demanding
parameters in wireless industries. To improve these
parameters float metal concept, fixed central capacitor and
asymmetric structure design has been utilized. OFF and ON
state can be achieved by varying the capacitance between the
movable membrane and the signal line. The ratio of
capacitance in the down state to up state defines the figure of
merit. By increasing the value of capacitance in the down state
and decrease the capacitance in the up state resulted good RF
parameters [10]. In RF MEMS switch, electromagnetic
modelling is used to determine the electrical parameter like
resistance, capacitance, and inductance from the measurement
of S-parameter [11]. In capacitive shunt switch, the electrical
parameter can be defined using CLR model. The reliability
issues are directly related to both electrical and mechanical
phenomenon. The reliability of a MEMS switch faces many
limitations such as: packaging optimization [12], single
switches mechanical response and various charging
mechanism [13]. Mechanical characteristics of MEMS
devices varied due to device dimension. By decreasing the
dimension of MEMS devices, the ratio of total surface area to
volume increases [14]. Biaxial stress and stress gradient can
be improved by the geometry of the switch. topology
optimization distributes the material in a reputed manner and
is used to generate the location of cut outs, size, shapes and
overall structure. So as to reduce the mechanical compliances
[15]. Parametric optimizations signify the trade-off between
vertical deflections and stress stiffening for the symmetric
switch and justified the displacement of anchor from finite
element analysis [16]. The Cantilever type beam in switch
design provides high temperature stability and dielectric less
design results in excellent performance and high reliability
operation [17]. One more reliability issues in MEMS is
mechanical shock. During fabrication operation process
MEMS capacitive switches exposed to shock can cause to
induce shock loads. The shock loads can induces very high
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dynamic load on the membrane causing the chipping, fracture
problem and cracking. Also effects the electrical and
mechanical failure of the devices
III. FABRICATION OF MEMS SWITCHES
Micromachining technique is used in fabricating the MEMS
three dimensional geometry. The fabrication technology
passes through lithography, deposition process and etching.
Micromachining is the set of fabrication and design tool that
form structure and precisely machines [19]. In
micromachining there are three techniques used. These are:
surface micromachining, bulk micromachining and LIGA
A. Surface micromachining
Fig. 1. Schematic and cross-section of
capacitive shunt switch.
II. RF MEMS DEVICE STRUCTURE
The RF MEMS geometry is a very complex process,
because it is very necessary to optimizing and designing the
coupling between micro-electronics and micro-machined
circuits. The lifetime and reliability of RF MEMS switches are
strongly dependent on operational environment, design and
fabrication. Fabrication process has a significant impact on
performance characteristics and geometry design.MEMS
switch can be of two types, based on membrane design: fixedfixed beam and cantilever beam type switch. The geometry
consist of cantilever beams on either side of transmission line.
When both the cantilever beam are in the up position, the
switch is in ON state. Insertion loss is achieved in ON state.
By pulling down the right, left, and both cantilever beam in
down state, the OFF state can be achieved through
electrostatic actuation. Isolation characteristics are defined in
OFF state.
The RF MEMS capacitive shunt switch geometry is illustrated
in fig.1. The fixed-fixed beam shows insertion loss in the ON
state and isolation in the OFF state. The electrical and
mechanical characteristics of the RF MEMS capacitive shunt
switch is expressively depend on its geometry. In RF MEMS
shunt configuration, output and input RF ports are connected
to each other. The geometry of the switch is based on 50. The
cantilever design results in high reliability and very high
temperature stability in comparison of fixed-fixed beam.
Optimized RF MEMS switch having cantilever beam has
lower actuation voltage, high stiffness and excellent
performance [18].
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Surface micromachining is used to make the thin micromechanical structure on the the top of the surface of the
wafer. Surface micromachining technique is different from
bulk micromachining due to the structure built on the wafer
instead of within the substrate. Thin layers of sacrificial
material are patterned and deposited on the surface of the
wafer. Each layer is patterned by lithography process and
etched (either dry etching or wet etching) before next layer is
deposited. This technique can involves many layers with a
different mask to produce the different pattern on every layer.
Surface micromachining technique requires a set of sacrificial
material, structural material and chemical etchants. Sacrificial
materials have good mechanical properties to avoid the device
failure during the fabrication process including low residual
stress, good adhesive and etchants. The common sacrificial
materials are silicon nitride, silicon dioxide, resists,
polysilicon, polyimide and so forth. Plasma etching is used to
remove the sacrificial layer instead of wet etching using
chemical gases such as CF4, Sf6, CHF3 with the other natural
gasses such as Ar or H2, O2. However plasma etching lefts a
large amount of undercut of mask due to isotropic fluorine
atom etching of the silicon atom which is known to be very
high compared with vertical etch induced by the ion
bombardment [20].
B. bulk micromachining
Bulk micromachining is the technology that removes the
bulk substrate. This process removes the holes in the substrate
in the backside of the wafer and large pits. Bulk micromachining was developed in 1960 and the principle of bulk
micromachining is to remove select amount of silicon from
the substrate to form the membrane on the one side of a wafer,
holes, trenches or other structures [21].
C. LIGA
The acronym of LIGA is lithographic galvanoformung
abformung. LIGA is a process in which thick photoresist are
exposed to X-rays that produce moulds and used for 3 D
structures/
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IV. ELECTROMAGNETIC MODELLING
The schematic of RF MEMS shunt switch is shown in fig. 2.
The RF MEMS shunt switch has a lumped CLR model and
two short sections signal line. The geometry of the switch is
suspended at a gap height g above the dielectric layer on the
signal line [22]. The width of the dielectric thickness is t d
having dielectric constant r. The length, thickness and width of
the structure are L m, w m and t m respectively. The width of
the transmission line is W m.The ratio of up-state capacitance
to the down state capacitance defines the RF response of the
MEMS capacitive switch. The transmission line are of length
( W2 + l).Where l is the distance form edge of the MEMS
bridge to the reference plane [23].
V. RELIABILITY RESEARCH
eliminates the ablation failure mode but also reduce
the stiction problem [21], [22].
Fig. 2. Cross-section of RF MEMS capacitive shunt switch.
Reliability of a switch is affected by mainly four
parameters: contact degradation, mechanical failure,
environment and dielectric stiction. The environmental effect
can create the un-wanted films on the switch surface, this will
cause the stiction problem and actuation problem. These
effects can controlled by clean room procedures careful
packaging. Packaging plays very important role in
performance of RF MEMS switches. Packaging provides
protection against physical environment such as reactive
element, particles and moistures. In RF MEMS capacitive
shunt switches, the dielectric stiction is the main failure mode.
High electrostatic fields causes the charge to tunnel into
dielectric layer, where the charge remains for a long amount
of time. This process happens due to very long recombination
times. Mechanical failure of switches is still a serious cause of
geometry failures
1) Reliability issues In RF MEMS reliability issues in
manufacturing and design are the major challenges
for further developing the RF devices [17]. Issues in
RF MEMS design: In micro engine, the pin joint
plays a crucial role for this device [18]. To evaluate
the reliability of the design, developed the
accelerated stress experiments for these pin joints
and analysed the surface coating effects. The
experimentally shows that the gap between the
surfaces is the critical parameter for reliability of the
micro engine [19]. Mechanical property of the switch
geometry plays a important role in reliability of
MEMS switch. The mechanical properties of alumina
atomic-layer-deposited (ALD) for silicon (si)
substrate of RF MEMS devices is studied by [20].
Besides irreversible stiction, another mode of failure
called dominant failure mode occurred due to the
joule heating induced ablation. By using the diamond
like carbon material for electrodes, not only
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Fig. 3. Top view of capacitive shunt switch.
experimented the roughening effect on reliability
improvement of the adhesive bond for RF MEMS
manufacturing application. The investigated result shows that
roughening can improve adhesive bonding strength as well as
Fig. 4. Equivalent circuit model of RF MEMS capacitive
shunt switch.
Provides the sticky characteristics. [23] Simulated and
analysed basically two concept soft coating and nonlinear
spring to enhanced the shock protection for RF MEMS.
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Nonlinear spring can able to reduce the impulses by more than
90% while soft coating process can reduce impulses by 40%
[24]. [25] Developed a nano scale based testing on an AFM
technique to investigate the effect of temperature and
specimen size. Temperature and specimen size is important
for design database of RF MEMS material. An experimental
investigation on the effect of packaged shell and packaged die
adhesive on reliability and the performance of RF MEMS
[25]. The experimental results shows that if the thicker die
adhesive, then residual stress increment is also smaller and
hence piezoresistance variation is also small. Adhesion and
friction of SC-Si with two polymers and oxide layer called
poly (dimethylsiloxane) and poly (methylmethacrylate) is
stud-ied by [26]. The dependency of the each polymer on
relative humidity, sliding velocity and the rest time was
analysed [17]. The experimental results show that both the
polymers are highly hydrophobic. The adhesive force is not
dependent to rest time and relative humidity and the friction
coefficient of both of them are lower than the silicon. The
authors investigated that both the polymers can be used
successfully in RFMEMS.
2) Issues in manufacturing process: In RF MEMS yield
improvement has been challenging issues. It is defined as
the fraction of the manufactured parts that are usable and
they are not failed prior to the customer shipment. The
relationship between can be used to enhance the
reliability [27]. By reduc-ing the contact area adhesion,
strictions and friction performance can be improved.
Surface elasticity has a great influence on the interaction
forms and the effective module of the nonporous
materials [28]. [29] Conducted a experiment on a 2terminal carbon nano tube based NEMS switch with the
closed loop feedback control mechanism. The experiment
consist of pull-in/ pull-out test by the multi-walled carbon
nano tube that is welded to the probe attached to the nano
scaled manipulator actuated by electrostatically. The
current-voltage curve predicted hysteretic loop between
pull-out and pull-in processes. Both the experiments and
theoretical modelling confirms the bi-stability of the
structure. Based on the failure mechanism, the authors
investigated that the fundamental understanding of the
failure modes as the function of configuration parameter
is of extreme importance to manufacture a reliable
MEMS device [30]. By using a CMOS process tested
carefully in transformer oil [31]. Developed the 3terminal NEMS switching device. The authors used a
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liquid medium to improve the number of reliable
switching and to reduce the operation voltage. Due to a
reduction of arcing suppression, elimination of exposure
to oxygen and moisture and surface adhesion force
reliability was improved by this method. Micro structured
coating helps the micro structures to show the higher
resistance to fatigue, oxidation, lower friction, formation
of lubricious tribofilms and larger fracture toughness
[17].[18] Concluded that how to achieve hardness, low
strength, significant residual stress and high surface
roughness of polycrystalline mono-metallic films used in
MEMS devices. High resolution transmission electron
microscopy, X ray diffraction, Nano indentation, AFM
and transmission electron microscopy techniques are used
to investigated the existence of an amorphous nano
crystalline micro structures that shows metallic
conductivity, sub nano meter root mean square roughness
and better hardness [19]. The result shows that there is
high correlation between elastic modulus, hardness with
wear resistance and friction coefficients.
3) Reliability testing: Now a days the users require very high
reliability. The MEMS industry group (MIG) including
the dozens of companies in MEMS industry published
their annual report on the entitled “Focus on reliability”.
The theme of the report is “demonstration of reliability is
required by customers” [20]. For understanding the
simulation model and theoretical concept experimental
investigation is required.
There is a need for measurement methods that are able to
evaluating strain fields [12]. The first reliability test on
micromachined surface of the micro engines developed at the
Sandia National Laboratories is presented by [21]. A total of
41 micro-engine were stressed at 36 thousands rpm. The
functionality of micro-engine was inspected at 60 rpm. Low
failure rate, no wear out regions and infant mortality were
observed. [15] Was observed the wear out of the contact
surfaces and improve the one of the failure mode of microengines. Control and measurement of residual stresses are the
main issue of reliability analysis of MEMS devices. The
residual stress can result in fracture, micro-structural change,
delimitation and excessive deformation of micro-structure
during failure of device during the operation [14]. A new
method for measurement of residual stresses in MEMS
devices with a very high local spatial resolution is proposed
by Sabat et al. 2007. This proposed method utilized the
combined features of imaging-milling of the (FIM) focused
ion beam equipment to scaled down the hole drilling method
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to micro scale level. In fabrication, the mechanical property of
the materials can affects their reliability and performance [13].
It is very difficult to measure and evaluate the mechanical
properties of thin films for MEMS devices due to forces,
displacement and small size involved. A micro-tensile testing
System is developed in this area by Han et al. 2006 to
measure the mechanical properties of the thin films of Au used
in MEMS devices. The issues in reliability like accurate strain
measurement and specimen handing were successfully
improved and the elastic modulus, tensile strength and stressstrain curve of Au films are successfully derived [12].
To evaluate the reliability of RF MEMS devices techniques
like accelerated life twisting should be developed. The formulated failure criteria and mechanical responses for a large
class of the shock loaded MEMS devices is analysed by [11].
The MEMS were modeled as the micro structures attached to
the elastic substrate and shock were modelled as the pulses of
acceleration applied to substrate over a finite time of range.
The study of many MEMS structures and shock loads shows
that with the duration in 50 to 5000 ms, the substrate response
are like the rigid bodies and the substrate is expected to be
resist against the stress wave induced damages. Humidity and
the impacts of temperature on aging process of the vapour
deposited SAM- coated and electrostatic actuated RF MEMS
devices is studied by [10]. Degradation of the surface coating
was observed when the device was stressed at 300 C and
humidity of 500. The study shows that both humidity and
temperature are responsible for failures and they can
accelerate degradation process.
A quantitative accelerated life testing program is developed
by [9] and achieved significant result in 2 months. A
combined stress test of tilting and a classical test of vibration
at the high temperature and the electrical signal was
performed. This study reveals that under worst condition, the
failure rate of the devices are below than 10 7 h 1. The
electrostatic discharge accelerated test is proposed by [8] for
RF MEMS devices. The MEMS devices were investigated at
the temperature of 100 and 150 C, during the range of 100h up
to 1 year to simulate a declining condition of up to 20 years.
All the sampled parts passed wire bond pull test and die shear
test requirement and every package passed the experiment.
Only the two packages showed abnormal measurement in
residual gas analysis, but the region still remained unknown.
For batch fabrication of MEMS devices, testability is another
very important issue. An easy to implement failure detection
method is proposed by [7], applicable for any type of the
electrostatic stiction limited micro actuators. This method is
based on detection of the pull-in current peak value of an
operating MEMS device at the lower frequency. During mass
production, this method is used for the automated online
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reliability testing. An experimental technique and a new test
device is presented by [6] to study fatigue life of the nanoscale
silicon nitride thin films used in nano mechanical systems.
The fatigue test of RF MEMS devices in liquid environment is
presented by [5].They fabricated and designed the MEMS
tensile specimen while being in the saline solution. This
proposed can be used for the evaluation of Bio MEMS in the
biological liquids. To analyse the performance and reliability
of MEMS de-vices, testing technique is used on the
mechanical properties of thin films. To improve the reliability
of the mechanical structures by bulging test as the nondestructive testing technique is experimental tested by [4].
This proposed experimental method can be used for the
characterization of thickness and geometry of any types of
membrane. [3] Inspected the maximum rating for the shock
test on a commercial off shelf MEMS accelerometer and it is
compared with the published maximum rating for acceleration
4) Evaluation: one dimension of the geometry is reduced to
nano range and the two other are remains large then the
structure is called as a quantum well. By reducing the two
dimension of the geometry to nano range and the other remain
same, then the nano structure is called as a quantum wire. By
reducing all the three dimension of the geometry to a nano
range, then the structure is called quantum dot. [2]
Investigated and developed the techniques for assessing the
reliability of 1-D nano-components. After then, he
experimented to developed the techniques to assess a 2-D
nano-components, e.e., nano-discs and nano-films [1]
As the feature size are reduced up to 10 nm, scaling goes up
with serious restriction [17]. The low switching speed restrict
their use in the applications such as RF (radio frequency)
where the high speed is not required. The voltage up-converter
components are required due to their large actuation voltage
requirement. By downscaling MEMS to NEMS, these
restrictions (actuation voltage and switching speed) are
eliminated [18]. An evaluation and systematic analysis of
(CNT) carbon nanotube based NEMS devices are presented
and discussed their advantages by [19].
Due to the scarcity of data, Bayesian approach is of more
importance in reliability of the nanoscale structures [20].
While some research has been investigated, still there is a
room to apply this tool and philosophy in the reliability
assessment of MEMS devices. [20] Developed and
experimented a full Bayesian analysis on change point, cost
optimal burn in time and hazard rate for a nano-scale high
dielectric constant gate dielectric film. Yuan et al.2010 used
Weibull exponential distribution to inference and plot the Lshaped hazards rate function.
They observed this function for nano-electronic devices. The
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posterior and prior total expected costs can be minimized by
optimizing and evaluating the burn-in time using the proposed
model
A flexible nonparametric Bayesian approach is presented
by [22] for modelling the L-shaped hazard rate functions. Its
change point for the novel nano-electronic device called metal
oxide semiconductor (MOS) capacitor with the fixed oxide
high dielectric constant gate dielectric. This proposed method
can be used to examine the reliability of novel nano-electronic
devices. When the failure mechanism are not known, then the
current parametric reliability design are not applicable, and
the limited data are available.
An experimental and analytical study on reliability of the
micro-mirror with the interdigitated cantilevers are able of
symmetrical bidirectional rotation are presented by [23]. They
investigated that, the reliability of these devices can be improved by using the bending interdigitated cantilever instead
of the conventional twisting hinge. The experiment shows that
the von mises stress for cyclic rotation in the micro-mirror
with twisting hinge structure is of two times greater than the
stress in the micro-mirror with the interdigitated cantilever
beam. [24] Evaluate a series of two component and single
component ionic liquids ultra-thin films used in MEMS
devices and study their surface properties and formation by
using
ellipsometric
thickness
measurement,
X-ray
photoelectron spectra and AFM. The nano-tribological
behaviours and adhesive of the films were examined by a
colloidal probe. Their study can help to design the ionic liquid
films.
The reliability of 3C-SiC cantilever beam using the
dynamic Raman spectroscopy that enables the direct data
collection of the Weibull fracture test on MEMS devices. The
obtained measurement resolution and the primary results
examines that Raman spectroscopy is a suitable approach to
measure dynamic strains induced in 3C-SiC MEMS geometry
[26]. [25] Study the effects of plasma-enhanced CVD
(chemical vapour deposition) on the dielectric charging of
silicon nitride films used in MEMS devices. A high
correlation in the electrical properties of the silicon nitride
films obtained from both the techniques was observed. This
proposed method can be used to determine the dielectric layer
which is more reliable for electrostatic actuated MEMS
devices. A technique based on mix-mode transient circuit
simulation to examine the robustness of ESD protection in
NEMS devices.
Due to the very broad range of loading rates and types, it is
very important to develop the techniques for analysing the
dynamic failure of Au RF MEMS geometry.[27] analysed the
dynamic failure of MEMS devices over a broad range of
loading types and rates. Three investigated method were
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developed for analysing dynamic response of The MEMS
devices. They used to determine the maximum threshold value
of the dynamic loading rates where no loss can be observed.
[26] Analysed the effect of process variations on the device.
Investigated the techniques to model and analyse the
reliability of the MEMS devices. They proposed the system
level reliability based on surface methodology. They
presented experiment and simulation based lifetime estimation
method for component, material and system levels. The study
in-vestigated that thermo-mechanically reliable design for the
micro-system can be achieved by combined computational
and experimental approach.
B. MEMS failure modes and mechanism
The failure phenomenon occurred when the switch
geometry failed to actuate. The major failure modes in RF
MEMS switches are: creep, electromigration,pitting of contact
surfaces,
delamination,
stiction,fracture,
electrostatic
discharge and wear [9]. [Huang et al.] Updated review on
failure mechanism such as wear, creep, fatigue. Failure in RF
MEMS can cause due to mechanical, electrical, biological,
chemical and thermal.
1) Stiction in MEMS: Stiction in RF MEMS has been a
catastrophic failure mode in switches. Surface roughness and
environmental conditions can cause the stiction problem. The
large surface to volume ratio of RF MEMS switches makes
the interfacial friction, wear and stiction. The self-assembled
monolayers (SAMs) and hydrophobic films are able to release
stiction problem [8]. A wear resistant anti-stiction coating is
highly desirable for preventing the RF MEMS switches from
stiction, wear and friction.
VI. DESIGN OPTIMIZATION
The modification can only affect the membrane of the
switch. For a fixed value of central conductor and all other
parameters are varying, we can observed the performance of
RF MEMS switch. In the ON state, the insertion loss is varied
with capacitance. By decreasing the capacitance in the ON
state, we can increase the insertion loss. By varying only the
bridge width and all other parameters are kept fixed, the upstate capacitance is effectively varied and hence changes the
insertion loss of the switch. The magnitude of S11 increases
with the increase in bridge width.
In the OFF state both the capacitance and inductance
determine the switch response. If we vary the bridge width
and all other parameter kept fixed then the variation of
resonant frequency can be observed. Both the capacitance and
inductance are varied, if the variation of bridge occurred along
the length. The bridge inductance is determined by that
portion of the bridge which is over the CPW slot and is
independent of that portion which is over the centre
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conductor. The centre conductor bridge area determines the
capacitance. Hence the resonant frequency varied with the
bridge width. The study observed that narrow bridge width
and wider CPW slot results in large inductance value. The
spring constant varies linearly with bridge width. So, pull in
voltage is actually independent of bridge width. Changing the
electrode area by varying the bridge width can tune the switch
into different frequencies.
A. Effect of holes on the beam
Holes in the beams are used to reduce the Air film damping
and to increase the switching speed of the MEMS switch.
ligament efficiency, is used to characterized the perforation
pattern. The holes in the membrane reduce the young’s
modulus and residual stress.
B. T-line characteristics
figure. The transmission line consist of thin metallic strip
deposited on surface of the dielectric film with the two
conducting ground plane which are parallel to the strip [5].
For example, with the signal space,S of 60 um and and the
signal line width,W of 60 um, and dielectric thickness is 11.9
um, determines the impedance, Z0, which must be equal to
50 . The height of silicon substrate is found to be 96 um at 50 .
C. Effect of bridge thickness
The switch resistance comprises of the two components, Rs
and Rs1. Rs1 is due to the signal line loss and can be calculated
as
Rs1 = 2 Z0I
Where is the line loss. Rs is due to the MEMS bridge. If the
thickness of the bridge is smaller than the two skin depth, the
resistant of the switch is constant with the frequency and
the thickness of the bridge is greater than two skin depth. The
The return loss in the both states, isloation in the OFF state
and the insertion loss in ON state are the parameters which are
to be measured for RF performances [7]. The mismatch
between the switch and characteristics impedance of the line
causes the insertion loss [6]. The insertion loss of the switch is
also affected by the beam metallization and contact resistance .
switch resistant varied with frequency as the function f due to
the skin depth effect. is the free space permeability. is the
metal conductivity. The spring constant K is the function of t3,
thus the pull in voltage increases exponentially with thickness
of bridge t.
VII. DESIGN OF RF MEMS SWITCH FOR
RELIABLE OPERATION
In the present work, the switch is designed for an actuation
voltage of 4 Volt. The CPW dimensions for the proposed
design are chosen as 50/100/50 um. The width of the beam is
chosen 100 um so that the area of the actuating
VIII. RESULTS AND DISCUSSIONS
A process flow is designed for fabrication of the proposed
switch on a silicon substrate. The proposed fabrication process
is designed using three masks shown in Fig.3. The masks for
the proposed design are generated in Ansoft HFSS. Mask-1 is
used for two processes, to etch the CPW in Aluminium and to
etch the dielectric layer in Silicon Nitride. Mask-2 is used to
etch the posts for the membrane (Aluminium) and Mask-3 is
used to create serpentine switch membrane in Aluminium.
Fig. 5. Holes in the membrane design
Fig. 6. The CPW signal line configuration.
The coplanar wave guide (CPW) facilitate the insertion loss
of both shunt and series passive and active devices. The
transmission line characteristics are very much dependent on
the conductor spacing, S, width, W, height of substrate, H and
substrate permittivity, r to obtain the characteristics
impedance, Z0. The configuration of signal line is shown in
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The fabrication process starts with the evaporation of thin
metal films of 1 um Aluminium onto the silicon substrate. The
metal film is patterned using Mask-1 with a photo resist,
followed by reactive ion etching to realise the structure. In all
the processes sacrificial etching of the photo resist is
performed. A silicon nitride layer is deposited on top of the
Aluminium to act as an isolating structure between the switch
membrane and the central conductor of the CPW line. The
dielectric layer is deposited using chemical vapour deposition
(CVD) and patterned using Mask-1.A layer of Aluminium is
evaporated on to the dielectric layer to form the post and
patterned using Mask-2 followed by etching of aluminium. To
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International Journal of Engineering Trends and Technology (IJETT) – Volume 25 Number 4- July 2015
create the gap between the CPW and the switch membrane, a
sacrificial layer of PSG is deposited with planarization as the
mode of deposition. Aluminum is deposited on the planarized
sacrificial layer of PSG and patterned using Mask-3. The
aluminium layer is partially etched off to realize the
serpentine switch membrane. Finally, the switch membrane
structural element is released by the performing the etching
of the PSG sacrificial layer. The top view of the proposed
serpentine structure after sacrificial etching.
Fig. 7. OFF state S Parameters showing Isolation and
Return Loss
seen for the centre part of the switch membrane, as evident
from the colour scheme.
Fig.8 shows the variation of the capacitance formed by the
upper switch membrane and the lower electrode (centre
conductor of the CPW). As the actuation voltage increases the
switch membrane is pulled towards the bottom electrode,
thereby resulting an increase in the capacitance. The capacitance increases many times after the pull in as the switch
membrane gets snapped to the lower electrode. Fig.8 shows
that after pull-in, the capacitance remains at 130.6 fF and this
is the down state capacitance of the switch.
Fig. 9. Magnitude of S11 and S21 in OFF state of RF
Fig.
10. Magnitude
MEMS
switchof S11 and S21 in OFF state of RF MEMS switch
In the upstate position of the switch, that is when no
actuation voltage is applied, the capacitance is seen to be
103 fF. Therefore, the capacitance ratio for the proposed
design is 12.67. The results of the electromechanical analysis
of the proposed switch are described in Table 3.
IX. CONCLUSION
Fig. 8. ON State S Parameters showing Insertion Loss
and Return Loss
Fig.6 to Fig.7 show the results of the EM analysis of the
proposed switch using HFSS. Fig. 5 shows the result of the
pull-in analysis and the maximum possible displacement of 1
um is obtained for 4.0 V. It may be noted that a switch with
same dimensions and using a fixed -fixed flexure for the
switch would need actuation voltage as high as 5V as evident
from Fig.6. Fig.7 shows the deformation experienced when
4.0 V is applied to the central conductor of the CPW (which
acts as the lower electrode), and the maximum displacement is
ISSN: 2231-5381
We intend to accommodate a research on MEMS reliability
by covering extant literature on design optimization, offers a
starting point for researchers and pinpoint the ideas for future
research. A systematic and comprehensive survey on the
Fig. 10. Magnitude of S11 and S21 in ON state of RF
MEMS switch
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International Journal of Engineering Trends and Technology (IJETT) – Volume 25 Number 4- July 2015
reliability research has been presented. Over the last decade
MEMS have been vastly expanded, as reviewed in this survey.
Although a research on reliability is still incomplete. A lot of
research is required to understand the reliability issues in
MEMS. There is still a lack of research on system level
reliability. As reviewed in this survey, the methods for burn-in
analysis and accelerated life testing is of great importance to
Parameter
Value
CPW Lines
50/100/50
Length of Membrane
300 um
Width of Membrane
100 um
Gap
1.5 um
Beam length (Horizontal)
13 um
Beam length (Vertical)
40 um
Thickness of Beam
2 um
TABLE II
MECHANICAL COMPONENTS OF THE PROPOSED RF MEMS SWITCH
Parameter
Value
Young’s Modulus
70 MPa
Poisson’s Ratio (v)
0.35
Sheer Modulus (G)
26e6
X-Axis Moment of Inertia (Ix) 0.2e-12
Y-Axis Moment of Inertia (Iy)
1.3e-24
Polar Moment of Inertia (Ip)
1.5e-24
Torsional Constant (J)
0.6e-24
TABLE III
SIMULATION RESULTS
Parameter
Fig. 11. Group Delay S11 in ON and OFF state of RF
MEMS switch
Value
Pull in Voltage (Vp)
4V
Up State Capacitane
103 fF
Down State Capacitance 1.3 pF
Capacitance Ratio
13
--------------------------------------REFERENCES
Fig. 12. Group Delay S21 in ON and OFF state of RF
MEMS switch
facilitate the further commercialization of MEMS devices. An
electromagnetic modeling in RF MEMS is used to
evaluate the RF performance in the down-state and up-state.
As reviewed, the isolation bandwidth can be obtained by
varying the inductive section with large dimension. Sam is an
most effective measure to prevent sriction in MEMS devices
and reduces the surface energy. There is an unlimited demand
of reliable anti-stiction MEMS device
TABLE I
STRUCTURE PARAMETER OF PROPOSED RF MEMS SWITCH
ISSN: 2231-5381
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