50120140507011

INTERNATIONAL JOURNAL OF COMPUTER ENGINEERING &
International Journal of Computer Engineering and Technology (IJCET), ISSN 0976-6367(Print),
ISSN 0976 - 6375(Online), Volume 5, Issue 7, July (2014), pp. 73-84 © IAEME
TECHNOLOGY (IJCET)
ISSN 0976 – 6367(Print)
ISSN 0976 – 6375(Online)
Volume 5, Issue 7, July (2014), pp. 73-84
© IAEME: www.iaeme.com/IJCET.asp
Journal Impact Factor (2014): 8.5328 (Calculated by GISI)
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73

IJCET
© I A E M E
SIMULATED RF MEMS DOUBLE-POLE DOUBLE-THROW SWITCH
USING A NOVEL SEESAW STRUCTURE
Mohammed Al-Amin, Sufian Yousef, Barry Morris, Hassan Shirvani
Anglia Ruskin University, Bishop Hall Lane,
Chelmsford, Essex, CM1 1SQ, United Kingdom.

ABSTRACT
This paper explores the modelling and simulation of a Radio Frequency Micro Electro-
Mechanical Systems (RF MEMS) switch using Finite Element modelling and analysis (FEM and
FEA) tools on a novel seesaw design, providing Double-Pole Double-Throw (DPDT) functionality.
This optimises the capabilities of the seesaw design structure for use in mobile communication
systems and devices. After researching other available seesaw designs, it was realised that an
improvement could be achieved by applying additional contacts within a 3D plane.
During the development of the DPDT seesaw switch, a low electrostatic actuation voltage of
14 V was achieved. This provided the switch with improved compatibility with voltages closer to
those used in integrated circuits for mobile systems.
The switch is a progression of existing Single-Pole Single-Throw (SPST) seesaw switches,
with an additional set of upper and lower contacts at each end of the seesaw, offering DPDT
switching capability within the space envelope. The length, height and width of the switch is 41 μm,
7.6 μm and of 9 μm respectively, which is a suitable size for fabrication and conforms to the
Microscale, from 1 μm to 100 μm.
Copper Bulk General (Cu) was chosen for the pivot material on its merits of possessing good
electrical conductivity and optimised flexibility and stiffness for elastic recovery.
The simulation, using Intellisuite, attained a working switch design, with an ‘Air-Gap’ of
1μm between the contacts, thus providing isolation when the switch is open-circuited.
Keywords: DPDT, Electrostatic, Pivot, RF MEMS, Radio Frequency Micro Electro-Mechanical
Systems, Seesaw, Simulated, Switch,

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1. INTRODUCTION

As technology progresses, there is an ever increasing need for large-scale integration of RF
MEMS switching devices within mobile communication systems. The aim of the research was to
create a switch for mobile devices to allow switching between multiple different protocols or
frequencies. The design also needed to have low power consumption for increased battery life and
direct interfacing capabilities with mobile components, without additional circuitry. Achieving this
leads to reduced cost and high reliability. With the use of RF MEMS switches, it is possible to meet
all the criteria in a micro space envelope.
Micro Electro-Mechanical Systems, also known as MEMS, is an emerging technology, which
is finding its way into a number of applications, such as gyroscopes, sensors, digital imaging and
mobile communications. The properties of materials change considerably from the macro-scale to
the micro-scale. For example electrostatic forces become more significant, while the mass to surface
area ratio becomes less significant. MEMS technology takes advantage of these small scale
properties by being able to use simple electrostatic plates to develop actuation forces, and relatively
increased surface area (with respect to mass) for heat dissipation, which in turn improves reliability.
MEMS can be broken down into sub-fields, which include:
MOEMS (Micro-Opto Electro-Mechanical Systems): used for optical imaging such as, digital light
projection.
Bio MEMS: an example of this is ‘Lab-on-a-Chip’ (LOC) where numerous biological tests can be
carried out more efficiently than traditional testing techniques.
MEMS Audio: used for microphonic sensors in commercial, studio microphones and mobile devices.
MEMS Sensors: these include the detection of movement (in the x, y and z axes), heat, velocity and
acceleration.
RF MEMS: used for mobile phones, mobile base stations, satellites and other communication
devices.
Currently, MEMS devices are at a disadvantage when it comes to size constraints, as most RF
MEMS switches are too large to be implemented into an integrated circuit and are packaged
separately. This causes difficulty when creating smaller mobile devices. RF MEMS are commonly
known for reliability issues [7] due to the moving components. There is a high chance of material
fatigue and breakage [6], MEMS devices are also known to have a high voltage actuation (up to
100V) [2, 8], which is due to electrostatic actuation and design size. This requires the device to
provide a separate voltage source to be stepped up from a low voltage to a high voltage and causes
the mobile device to consume more power than needed due the control circuitry and power
conversion inefficiency. This paper takes these challenges into account, for the design of the RF
MEMS switch, in order to overcome them.
The paper concentrates on RF MEMS, more specifically, it addresses the use of RF MEMS
switching on mobile phone devices with the goal to operate at lower voltages than those used in
existing portable devices. The device was required to fit within the MEMS scale of 1 μm – 100 μm,
with ease of fabrication for future integration into microchips.

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Figure 1: Cross sectional representation of the RF MEMS seesaw switch in the ‘off-state’
The research undertaken is a novel concept, using similar ideas of seesaw theory from K.
Jongseok (2007) et al [4] and J. M. Cabral A. S. Holmes (2006) [1]. These authors created an
approach to design, with the seesaw concept, but as a Single- Pole Single-Throw (SPST) switch with
similar voltages. The research in hand takes the SPST concept and adds addition contacts on a 3D
plane. This provides the seesaw switch with Double-Pole Double-Throw (DPDT) connectivity and in
turn gives the device the flexibility of selecting four protocols or frequencies. Multiple seesaw
switches can be configured and utilised to provide an extensive switching selection of more than four
configurations. This innovation has not been attempted, to date, and quadruples the connectivity
within a reduced space envelope.
The decision of using the seesaw concept over others, such as the wobble motor principle
switch, designed by S. Pranonsatit (2006) et al [8], is that the design allows for multiple connectivity
within a reduced area. Also, with the ability to reduce the size for lower actuation voltages. The
wobble motor does provide Single-Pole 8-Throw (SP8T) switching, but sacrifices size and ease of
fabrication, due to manually attaching the cartwheel.
The procedure for development of the RF MEMS switch required the use of Finite Element
Modelling (FEM) for designing the switch and Finite Element Analysis (FEA) for simulation.
FEM modelling focuses on a two dimensional (2D) layout of the structure with the intent of
creating a three dimensional (3D) design model. FEA simulation uses the FEM to provide stress,
displacement and electromagnetic outcomes of the structure, for a given voltage input.
The use of FEA software tools provides an expedient approach to design and simulation,
while reducing cost to a minimum. These software tools can be used on a high specification
computer to provide computational data of the structure of a MEMS device, without requiring
external equipment or development time and cost. The software tools provide high precision analysis
with regular updates.
Intellisuite, created by Intellisense, was selected because of its software capabilities. It is able
to provide a comprehensive materials database, efficient Solid and Process modelling tools and an
effective Thermo-Electromechanical analysis tool [3] [5], with a graphical user interface (GUI) tool
for users to develop and analyse MEMS devices. The software is designed for MEMS development
only, but provides a simple CAD FEM tool with a validation tool to detect any errors in the mesh.

Intellisuite uses an intuitive method of designing, allowing the user to draw the shape on a grid with
a mouse.
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The properties of the materials used in MEMS are crucial to the functionality of the design,
since the use of inappropriate materials can cause a malfunction or damage to the device and its
peripheral components. Common materials used in RF MEMS simulations are Silicon Bulk General
(Si), Copper Bulk General (Cu) and Aluminium Bulk General (Al) [2] [4].
The operating principles of this RF MEMS switch relies on an electrostatic force to close the
contacts within an RF circuit (Figure 3). This force (F) depends on the following equation:

(1)
Where,
V = Supply voltage
= Dielectric constant of the ‘Air Gap’
A = Area of the electrostatic plates
d = Distance between the electrostatic plates
F = Force between the electrostatic plates
Figure 2: Graphical representation of the force parameters
The simulation takes into account fringe capacitance, which affects the forces on the beam.
Fringe capacitance should be added to the equation as a constant (C) to provide improved accuracy
to the result. The simulation required a voltage of 14 V to achieve a sufficient pulling force, using a
copper pivot (Figure 6), with a pivot thickness of 0.0476 μm. The only parameters, which can be
changed in equation 1, to increase this force, are the area (A) and the distance (d) between the
parallel plates. As the seesaw is a symmetrical design, a degree of flexibility is required to ensure
that the contacts on each side of the seesaw are closed simultaneously to provide maximum contact
surface area, for a low resistance [4].

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Figure 3: Cross sectional representation of the activated RF MEMS seesaw switch in the ‘on-state’
The design, which has been created, incorporates most of the research developments within
one structure, these are: low voltage actuation, reduced von-mises stress and increased switching
functionally within a space envelope of the micrometre range. To understand the characteristics of
the design, the research adopted an empirical, simulation based approach, in order to provide
effective results, using Intellisuite software to model and simulate the RF MEMS seesaw switch.
2. SUITABILITY OF THE DESIGN FOR MOBILE COMMUNICATION
The research conducted has concentrated on optimising the best features of existing designs
and incorporating them into a seesaw switch structure. The dimensions of the space envelope allows
the design to be incorporated into mobile communication devices. RF MEMS provides innate
advantages over conventional solid state switching materials; for example:
• Low insertion loss, due to direct contact of low impedance materials
• Low power consumption, due to voltage activation rather than current activation
• Immunity to current leakage, due to no current path
• High isolation, due to the ‘Air-Gap’ between contacts
Compared to semiconductors, the ‘on-state’ resistance of RF MEMS is innately linear,
because of its ohmic contacts. The seesaw mechanism relies on the elastic recovery forces of the
pivot and is controlled by using two independent pull-down electrodes.
3. DESIGN AND DEVELOPMENT
To accommodate the Microscale between 1 μm to 100 μm, a length of 41 μm was chosen to
be an appropriate value, enabling the design to be large enough to facilitate fabrication and small
enough for increased switching speed. The distance between the beam contacts and the fixed
contacts is 1 μm when in the ‘off-state’ (Figure 1).
The seesaw pivot was designed using single polarity supply voltages across each pair of
electrostatic plates, which were driven alternately with pulsed voltage waveforms. This allows
reduced external control circuitry and circuit complexity. With the use of the DPDT switch design
[1] [4], the RF MEMS seesaw device accommodates two distinct radio frequencies, which
communicate simultaneously. By taking into account the area of the device (41 μm x 9 μm or 369
μm2, which is shown in Table II), this increases functionality within the space envelope. With the
use of elastic recovery, the device is set to the ‘off-state’ (Figure 1) without any voltages being
applied.

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The pivot is the thinnest component of the structure, at a thickness of 0.0476 μm, which
causes fabrication constraints. This restricts the design for use only with advanced fabrication
techniques, such as 32 nm fabrication. The seesaw provides an advantage during the etching process
because of its simple design, as it allows the etching solution to run though the structure without
being held in the gaps.
Table I: Seesaw Dimensions
Seesaw Dimensions
Elements Dimensions (μm)
Air Gap 1
Beam Length 41
Beam Height 4
Beam Width 5
The seesaw RF MEMS switch enables switching between a dual input and output
configuration, depending on the application. For primary use, it is configured for switching between
two RX (receive) and TX (transmit) frequency bands. Other configurations may be used by
employing RF mixers at the input to the seesaw RF MEMS switch, and provides simultaneous RX
and TX for two distinct RF frequency bands. This in turn, provides a dual RX/TX switch with a total
of four frequencies. The seesaw RF MEMS switch is designed to be used for mobile communication
devices for common protocols such as GSM, Wi-Fi, 3G, 4G, WiMAX, Bluetooth, GPS and many
other protocols. Depending on the capacity of the antennas, all protocols can be implemented, as
most of them use frequencies that are less than 5.8 GHz.
Figure 4: Activated RF MEMS seesaw switch. An earlier prototype simulation, of an oblique view
in the ‘on-state’ with colour coded displacement
The seesaw switch has the versatility of being connected into three configurations i.e. SPST
DPDT or Single-Pole Double-Throw (SPDT) switching without any changes to the seesaw structure.
This can be achieved by selecting the appropriate contact terminals.
Using multiple RF MEMS seesaw switches with its three configurable modes, it is possible to
achieve switching between multiple different protocols to create a switching matrix for GSM, 2G,
3G, 4G, Bluetooth, Wi-Fi and GPS.

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4. TECHNIQUES OF THE SWITCH

The RF MEMS device employs two SPDT switches, which are mounted at each end of a
beam. The beam is balanced on a fixed central pivot to provide a seesaw mechanism. The pivot gives
the seesaw elastic recovery of the beam, to a central position, when the switch is in the off-state. The
pulling force, developed by the electrostatic plates, need to exceed the force of elastic recovery in
order for the switch to make contact. During operation, each electrostatic plate is activated
alternately to control the seesaw motion. Since the switches are mechanically linked via the beam,
they are inversely synchronised with each other and may be considered as one Double-Pole Double-
Throw (DPDT) switch.
The seesaw motion of the beam is controlled by two complementary, electrostatic control
signals, with a duty-cycle of 50 %, in the form of digital voltage pulses. The electrostatic forces,
generated by these pulses, are used to alternately pull down each side of the seesaw to activate the
switches. Four pairs of contact terminals are closed via the bridging contacts located at each end and
both sides of the beam. Two switching pairs are closed simultaneously. This is shown in Figure 4.
5. DESIGN PROCEDURE
By looking at existing seesaw designs available, it was discovered that an improvement could
be achieved by adding additional contacts. The SPST Seesaw switch [1] [4] could be improved to a
DPDT switch by adding a set of upper and lower contacts to each side of the seesaw.
One of the important design requirements for the seesaw switch is the pivot, as it is necessary
for the switch to be used in three dimensions. In order to allow the beam to pivot in any orientation,
consideration was given to the effect of gravity, even though this is relatively weak at the micro-scale.
To keep in control of the movement, it is vital that the pivot is attached to the beam, for elastic
recovery.
Standard materials (silicon bulk general, copper bulk general and aluminium bulk general)
were used from the Intellisense materials database. A number of database materials were tried and
tested. Silicon bulk general, copper bulk general and aluminium bulk general were selected as ideal
materials for use in MEMS fabrication. Silicon bulk general was used for all of the substrates and
aluminium bulk general was used for the contacts and the electrostatic plates, as shown in Table II.
For the pivot, two copper bulk general material thicknesses were considered, in order to evaluate
their properties. The elastic recovery of the materials is an important property of the pivot to enable
the ‘off-state’ of the switching to occur while providing flexibility for contact.
Table II: Materials and beam specifications
Materials
Substrate (Si) Silicon Bulk General
Pivot width 0.048 μm (Cu) Copper Bulk General
Pivot width 0.053 μm
(Cu) Copper Bulk General
and
(Al) Aluminium Bulk General
Beam (Al) Aluminium Bulk General
Contacts (Al) Aluminium Bulk General
Electrostatic Plates (Al) Aluminium Bulk General

6. METHODOLOGY OF TESTING THE PARAMETERS
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The technique of testing was by using Intellisense simulation software. The parameters of the
simulation provides the electrostatic plates with controlled pulse-widths. If the beam did not respond
to the pulse, the amplitude was then increased in increments, until the correct response in
displacement was observed.
The results were displayed graphically, by the simulation package, and the associated data
exported into a Microsoft Excel spread-sheet for numerical analysis. This empirical approach to the
research improved each area of the RF MEMS seesaw switch via multiple iterations, which yielded
lower voltages and von mises stress levels.
The waveforms have a mark-space ratio of 1:1 (equivalent to a duty cycle of 50%), which
enables each side of the seesaw to be switched in an alternating fashion.
7. EXPERIMENTAL ANALYSIS
Using Thermo Electro-Mechanical analysis, a set of dynamic results were produced (Figures
6-8). This uses time, stress and displacement data to provide the seesaw switch with a time based
movement.
Figure 5 shows that the displacement of the beam reaches 1 μm (thus enabling the contacts to
close) when a voltage of 14 volts was applied across the electrostatic plates, for copper, using static
simulation for a ranged voltage analysis.
Figure 5: Static analysis of displacement vs. Voltage for copper with a width of 0.048 μm
This displacement is also shown in Figure 6 and Figure 7 as a function of time with pivot
widths of 0.048 μm and 0.053 μm, respectively. The 0.053 μm thick pivot does not allow sufficient
displacement due to its stiffness and prevents the contacts from closing.

Figure 6: Dynamic analysis of displacement vs. Time using a copper pivot with a width of 0.048 μm
Figure 7: Dynamic analysis of displacement vs. Time using a copper pivot with a width of 0.053 μm
81

Figure 8 also shows a graph of displacement of the beam as a function of time, using
aluminium. Although aluminium has the same pivot thickness of 0.053 μm as the copper pivot
shown in Figure 7, it provided a displacement which reached its target destination due to the innate
flexibility of the material.
Figure 8: Dynamic analysis of displacement vs. Time using an Aluminium pivot with a width of
0.053 μm

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A dynamic simulation was carried out using aluminium with a pivot thickness of 0.048 μm,
however, the software reported that the aluminium pivot material exceeded its boundary conditions
and failed, therefore no graph was produced.
A separate experiment was conducted for the thickness of the pivot. Figures 9 and 10 show
von mises stress and displacement vs the thickness of the aluminium and copper materials
respectively, with a pulling voltage of 3 V.
The experiments show aluminium to provide higher flexibility over copper at a thickness of
0.05μm, but compromises von mises stress that goes beyond its ultimate yield strength, which would
lead to fracture failure.
Copper provides an optimum displacement at the same thickness, with a von mises stress
under the yield strength, which guarantees no deformity of the material during the electrostatic
pulling force. Therefore, copper bulk general was selected as the pivot material, because of its
intrinsic properties.
Figure 9: Von Mises Stress and Displacement vs Thickness of Aluminium pivot at 3V
Figure 10: Von Mises Stress and Displacement vs Thickness of Copper pivot at 3V

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8. CONCLUSION

After numerous iterations of the design, the electrostatic supply voltage was reduced
significantly from typical values exceeding 40 volts to 14 volts for copper bulk general. This was
achieved using empirical analysis to observe each area of the structure for improvements.
The maximum pulling force was achieved by making use of the surface area of the
electrostatic plates on the beam and the base of the seesaw. Also, an improvement was made by
optimising the thickness of the pivot to operate under von mises stress and applying the minimum
voltage for increased reliability.
A working simulation was achieved without compromising the ‘Air-Gap’ between the
contacts, which retained isolation when the switch was open-circuited, with alternating pulses. By
using dynamic analysis on Intellisuite software, the seesaw action was enabled.
The seesaw RF MEMS switch is an improved concept over existing designs, which are
limited to Single-Pole Single-Throw (SPST) switching [1]
[4]. Additional contacts, in the improved design, achieve DPDT switching, and required lower
actuation voltages, due to the reduction in size.
The seesaw switch may be configured into three switching modes (SPST, SPDT and DPDT) making
it a versatile component for use within integrated circuits for mobile communication devices.
9. ACKNOWLEDGMENT
The authors’ acknowledgment is given to Jianhua Mao from Intellisense for his software
support and valuable advice on Intellisuite modelling and simulation software tools.
10. REFERENCES
[1] J. M CABRAL A.S. HOLMES, (2006). “A novel seesaw-type RF MEMS switch.”
Electrotechnical Conference, 2006. MELECON 2006, IEEE Mediterranean 2006, pp. 288-
292.
[2] K.HYOUK, C. DONG-JUNE, P.AE-HYOUNG, L. HEE-CHUL, P. YONG-HEE, K. YONG-DAE,
N. HYO-JIN, J. YOUNG-CHANG B. JONG-UK (2007). “Contact materials and
reliability for high power RF-MEMS switches.” Micro Electro Mechanical Systems, 2007.
MEMS. IEEE 20th International Conference on 2007, pp. 231-234.
[3] H. JAAFAR, N. FONG LI N.A.M. YUNUS (2011). “Design and simulation of high
performance RF MEMS series switch.” Micro and Nanoelectronics (RSM), 2011 IEEE
Regional Symposium on 2011, pp. 349-353.
[4] K.JONGSEOK, K. SANGWOOK, Y. HONG, J. HEEMOON L. SANGHOON (2007).
“Variable pivot seesaw actuated RF MEMS switch for reconfigurable system application.”
Micro Electro Mechanical Systems, 2007. MEMS. IEEE 20th International Conference on
2007, pp. 775-778.
[5] A.M. PASHA M.A. SAQIB (2009). “Design optimization for low voltage DC contact RF
MEMS shunt switch.” Electrical Engineering, 2009. ICEE '09. Third International
Conference on 2009, pp. 1-6.
[6] H. R. Shea (2006), “Reliability of MEMS for space applications” Proc. of SPIE Reliability,
Packaging, Testing, and Characterization of MEMS/MOEMS V on 2006, vol. Vol. 6111,
61110A.
[7] J. HWANG, 2007. “Reliability of Electrostatically Actuated RF MEMS Switches” Radio-
Frequency Integration Technology, 2007. RFIT 007. IEEE International Workshop on 2007,
pp. 168-171.

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[8] S. PRANONSATIT, G. HONG, A.S. HOLMES and S. LUCYSZYN, 2006. “Rotary RF
MEMS Switch Based on the Wobble Motor Principle” Micro Electro Mechanical Systems,
2006. MEMS 2006 Istanbul. 19th IEEE International Conference on 2006, pp. 886-889.
[9] Anesh K Sharma, Ashu K Gautam, CG Balaji, Asudeb Dutta and SG Singh, 2012. “Shunt Rf
Mems Switch with Low Potential and Low Losson Quartz for Reconfigurable Circuit
Applications”, International Journal of Electronics and Communication Engineering
Technology (IJECET), Volume 3, Issue 2, pp. 497 - 510, ISSN Print: 0976- 6464,
ISSN Online: 0976 –6472.
11. ABBREVIATIONS
2G Second Generation
3G Third Generation
4G Fourth Generation
BIO MEMS Biological Micro Electro-
Mechanical Systems
DC Direct Current
DPDT Double-Pole-Double-Throw
GHz Giga-Hertz
GPS Global Positioning System
GSM Global System for Mobile
Communications
MEMS Micro Electro-Mechanical
Systems
MOMEMS Micro-Opto Electro-
Mechanical Systems
ms Milliseconds
SPDT Single-Pole Double-Throw
SPST Single-Pole Single-Throw
SP8T Single-Pole Eight-Throw
RX Receive
RF MEMS Radio Frequency Micro
Electro-Mechanical Systems
TX Transmit
μm Micrometre
Wi-Fi Wireless Fidelity
WiMAX Worldwide Interoperability for
Microwave Access

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