Rf Mems Report

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Johnson Taye

RF MEMS Switches

Study of RF MEMS capacitive switches: An Brief overview

Silchar, September 2012

2nd

year, Master of Technology

National Institute of Technology SilcharDepartment of Electronics and CommunicationEngineering

NITSNational Institute ofTechnology Silchar


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Overview

Micro-Electro-Mechanical Systems, or MEMS, is a technology that in its most generalform can be defined as miniaturized mechanical and electro-mechanical elements

(i.e., devices and structures) that are made using the techniques of microfabrication.

The critical physical dimensions of MEMS devices can vary from well below one

micron on the lower end of the dimensional spectrum, all the way to several

millimeters. Likewise, the types of MEMS devices can vary from relatively simple

structures having no moving elements, to extremely complex electromechanical

systems with multiple moving elements under the control of integrated

microelectronics. The one main criterion of MEMS is that there are at least some

elements having some sort of mechanical functionality whether or not these

elements can move. The term used to define MEMS varies in different parts of the

world. In the United States they are predominantly called MEMS; while in some

other parts of the world they are called Microsystems Technology or

micromachined devices.

While the functional elements of MEMS are miniaturized structures, sensors,

actuators, and microelectronics, the most notable (and perhaps most interesting)

elements are the microsensors and microactuators. Microsensors and

microactuators are appropriately categorized as transducers, which are defined as

devices that convert energy from one form to another. In the case of microsensors,

the device typically converts a measured mechanical signal into an electrical signal.

MEMS products range from physical sensors (e.g. pressure, inertial) to radio-

frequency (RF) devices and are useful for many different industrial and research

applications. They are already used in automotive, chemical, medical, optical, robotic

and sensing systems, for instance.


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INTRODUCTION: RF MEMS

History of RF MEMS

Microelectromechanical Systems (MEMS) have been developed since the 1970s for

pressure and temperature sensors, accelerometers, gas chromatographs, and other

sensor devices. MEMS switches for low-frequency applications have also been

demonstrated in the early 1980s but remained a laboratory curiosity for a long time.

They are essentially miniature devices that use a mechanical movement to achieve a

short circuit or an open circuit in a transmission line. But in 19901991, under the

support of DARPA (Defense Advanced Research Projects Agency), Dr. Larry Larson at

the Hughes Research Labs in Malibu, California, developed the first MEMS switch

(and varactor) that was specifically designed for microwave applications. However,

and as usual with any leap in technology, it was far from mature and had poor yield

and virtually no reliability. Still, it demonstrated excellent performance up to 50 GHz,

far better than anything that could be achieved with GaAs devices.

The initial results of Larson were so outstanding that they stirred the interestof several groups in the U.S. government; and by 1995, Rockwell Science Center and

Texas Instruments both had developed an outstanding RF MEMS switch. The

Rockwell switch was a metal-to-metal contact type, suitable for DC-60 GHz

applications, while the Texas Instruments switch was a capacitive contact switch,

suitable for 10120 GHz applications. The rest is history; by 1998, the University of

Michigan, The University of California, Berkeley, Northeastern University, MIT

Lincoln Labs, Columbia University, Analog Devices, Northrup Grumman, and several

other companies were actively pursuing RF MEMS devices. By 2001, there were

more than 30 companies working in this area, including the giants of consumer

electronics, such as Motorola, Analog Devices, Samsung, Omron, NEC, and ST-

Microelectronics.

RF MEMS has seen an amazing growth in the past many years due to its

immense commercial and defense potential. The reason is that while there were


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tremendous advances in GaAs HEMT devices (high-electron mobility transistor) and

in silicon CMOS (complementary metal-oxide-semiconductor) transistors; there was

barely an advance in semiconductor switching diodes from 1985 to 2000. The cut-off

frequency of pi-n diode switches is not advancing at the same rate as other

technologies. The cut-off frequency of a device is the frequency, at which the on and

off impedance becomes equal. For satellite and short-range communication

applications, the operating frequency is increasing and they are approaching to the

THz range. So a new switching technology is required to meet the demands for

higher cut-off frequencies. RF MEMS device can fulfill these demands at a lower loss.

The cut-off frequency of a typical RF MEMS switch can go up to 40 THz, which is trulyoutstanding. RF MEMS are nowadays becoming very popular due to their very low

insertion loss, high isolation and high operating frequency. The bias circuitry is very

simple compared with that of p-i-n diodes and FET switches. The power consumption

of the RF MEMS switch is in the micro watt range which is very low. This will make

them very suitable for portable device applications, like mobile handsets and

satellite communication.

RF MEMS research

There are several RF MEMS areas where research is going on at present. The

research areas are briefly mentioned below:

1) RF MEMS switches, varactors and inductors are nowadays mature enough for use

in practical application. They can operate from DC to several tens of GHz with very

good RF performances. In the RF MEMS switch, the suspended beam can move up to

several micrometers during actuation without any problem.

2) Acoustic vibrations can be used in FBAR (thin film bulk acoustic resonator) and

filters. They have very excellent performance with very high quality factor (Q) up to

several GHz. They are widely used for wireless applications, particularly in cellular


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telephones. The Surface Acoustic Wave (SAW) filters are usually large at low

frequencies but they can be miniaturized at higher frequency.

3) Lots of research is taking place in the field of mechanical resonators. The

mechanical movement of the beam is very small, in the range of nanometers. The

structure can be a bridge, a cantilever and a radial disk. The mechanical resonance

frequency of the bridge or cantilever depends on the dimensions and material of the

beam. These resonant elements can be used in filters, oscillators (for reference

clock), etc. Usually they have very high Q. Both bulk and surface micromachining

technique can be used for the processing of such devices. The resonance frequency

is not very high at present. As the process technology is advancing, smaller devicescan be fabricated with better control in resolution and higher resonance frequency.

RF MEMS Switches

RF MEMS switches are devices that use mechanical movement to achieve a short

circuit oran open circuit in the RF transmission line. RF MEMS switches are the specific

micromechanical switches that are designed to operate at RF-to-millimeter-wave

frequencies (0.1 to 100 GHz). There are two distinct parts to an RF MEMS switch or

varactor: the actuation (mechanical) section and the electrical section. The forces

required for the mechanical movement can be obtained using electrostatic,

magnetostatic, piezoelectric or thermal designs. To date, electrostatic-type thermal

switches and magnetostatic switches have been demonstrated at 0.1100 GHz with

high reliability (100 million to 60 billion cycles) and wafer-scale manufacturing

techniques. As for the electrical part, a MEMS switch can be placed in either series or

shunt configurations and can be a metal-to-metal contact or a capacitive contact

switch.


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The physical structure of the electrostatic-type MEMS switching device is shown.

Here a thin metal membrane of thickness t is suspended a short distance g above a

conductor. When a DC potential is applied between the two conductors, charges are

induced on the metal which tend to attract the two electrodes. Above a certain threshold

voltage, the force of attraction is sufficient to overcome mechanical stresses in the material,

and the membrane snaps down to the closed position shown on the right of Figure 1.

Figure 1: Physical structure and operation of electrostatic type RF MEMS airbridgeswitch (top) and cantilever switch (bottom).

RF MEMS switches are suitable for low-loss microwave systems, which do not

require high switching rates, such as: cellular telephone, airborne, automotive and

satellite communication.

The advantages of RF MEMS switches over their solid-state counterparts, such as

FETs or PIN diodes, are:

1. Low Power Consumption: Electrostatic actuated switches operate by a dc voltage

but do not consume any current resulting in a very low power consumption.

2. Very Low ON-state Insertion Loss.


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3. Very High Off-state Isolation.

4. High Intermodulation Products: Due to a very high linearity of the RF MEMS

switches (nop-njunction) they provide negligible intermodulation distortion.

5. Very Low Fabrication Cost: RF MEMS switches can be fabricated on different

substrates using surface micromachining techniques and do not require high cost

processes.

However, RF MEMS switches have also some disadvantageous, such as:

1. Relatively Low Switching Time: The switching time of most RF MEMS switches is

between 1 s and 200 s. This relatively low switching speed does not satisfy the

requirements for some communication and radar systems. However, etching holes in

the movable membrane allows faster operation by reducing the damping effect.

2. Low Power Handling: Most RF MEMS switches can handle a power of 20-50 mW,

but not more.

3. High Actuation-Voltage: Conventional electrostatic RF MEMS switches require 20-

80 V for a reliable operation, a rather high voltage. Therefore, a lot of work has been

done to reduce the actuation-voltage values.4. Low Reliability: The lifetime of mature RF MEMS switches is higher than 50 billion

cycles. But, a lot of microwave systems require switches with lifetime of more than

200 billion cycles.

5. Packaging: The packaging technique itself can lead to significant degradation of

the switch reliability.

6. High Total Cost: Costs of packaging have a high share of total costs in switch

fabrication. Additionally, the high-voltage drive has to be considered, while

fabrication costs of the switch itself are rather low.


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Classification of RF MEMS Switches

RF MEMS swit ch es ar e genera lly classifi ed based on:

1. Contact type2. Actuation Mechanism3. Construction4. Circuit configuration

Two different contact mechanisms can characterize RF MEMS switch designs: a

capacitive contact (metal-insulator-metal) and a resistive (metal-to-metal) contact.

The capacitive contact switch is characterized by the capacitance ratio between the

up-state (ON-state) and the down-state (OFF-state) positions. This ratio typically

ranges from 80 to 200 and depends on the switch design. The OFF-state capacitance

amounts to 2-4 pF, and is suitable for 5-100 GHz applications. The conventional

dielectric films used in RF MEMS switches are silicon oxide or silicon nitride. They do

not allow to obtain a large down-state capacitance due to their low dielectric

constant that limits the low-frequency operation of the switches. This problem can

be solved by using high dielectric constant materials such as strontium titanate

(STO), barium strontium titanate (BST) or titanium oxide. But these dielectric

materials require a special technology and are not standard materials for CMOS

technology. Moreover, MEMS capacitive switches with high relative dielectric

constant materials are very sensitive to the capacitance degradation caused by non-

planar metal membranes and surface roughness of the dielectric layer. A method to

eliminate the effect of the surface roughness is the deposition of a thin metal filmdirectly on the dielectric layer; by doing so a metal-insulator-metal (MIM) structure

on the center line is formed. In the dielectric on ground (DOG) approach has been

introduced to increase the down-state capacitance and, in such a way, to improve

the switch isolation at lower frequencies. Resistive contact switches have small up-

state capacitances (ON-state) and operate from 0.01 GHz up to 60 GHz. The main


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disadvantage of the dc-contact switch, compared to the capacitive coupled one, is a

contact metal degradation leading to a short lifetime.

Four most frequently used actuation methods for RF MEMS switches are based on

electrostatic, magnetostatic, piezoelectric or thermal techniques. In the

magnetostatic MEMS switches the mechanical actuation is based on the Lorentz

force, when the pulling force is induced by a magnetic field. Motion force of

piezoelectric MEMS switches is based on the ability of certain materials to change

their geometrical dimensions in response to the applied electrical potential. A

change in a linear dimension due to thermal expansion of any solid underlies the

functioning of the thermal MEMS switches. But the majority of the RF MEMS

switches are electrostatic in nature. Their motion is actuated by the attractive

Coulombs force existing between charges of opposite polarity. The electrostatically

actuated MEMS switch has a great potential for microwave applications due to its

extremely high power handling capability and compatibility with other state-of-the-

art fabrication technologies for high level integrated circuits or systems. Moreover,

the fabrication of these switches is relatively simple and requires only a minimum ofcommonly available fabrication equipment. A drawback is the high actuation-

voltage, which is in the range of 20 V to 80 V. The high actuation-voltage can cause

the stiction between a movable membrane and a bottom electrode. It has been

investigated and published that the failure mechanism is due to a charge

accumulation in the dielectric material. This charge builds up in the dielectric layer

and is strictly related to the actuation-voltage. It has been observed that a switch

lifetime is increased by one order of magnitude for every 5-7 V reduction of the

applied voltage.

Two different constructions of the RF MEMS switches are commonly used: cantilever

orfixed-fixed beam. The cantilever type has an important advantage in comparison

to the suspension bridge, a lower actuation-voltage. But these switches are very

sensitive to residual mechanical stress in the beam material. The fabrication process

of the fixed-fixed beam configuration is easier and does not require special


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processing compared to dielectric beams or thick low-stress electroplated

cantilevers.

RF MEMS switches can also be classified according to the configuration of the circuit.

Generally, there are two configurations: shuntand series configurations depending

on the position across a transmission line. A shunt configuration is typically used for

capacitive switches, while a series configuration is major used for resistive-contact

switches. The reason is that it is easier to get a good isolation with a limited

impedance ratio (such as the capacitive switch) in a shunt-circuit than in a series

circuit. Both configurations of RF MEMS switches are implemented either for

microstrip or coplanar waveguide (CPW) lines on glass, quartz, silicon, GaAs, and

other substrates, and have been used in these implementations up to 100 GHz. High-

resistivity materials must be used as substrate in order to eliminate dielectric losses

in the substrate for low-loss microwave applications.

A comparison of RF MEMS switches, FETs and PIN Diode is shown in the table 1

below.

Table 1: Performance Comparison of FETs, PIN Diode, and RF MEMS Electrostatic

Switches


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State-of-the-art

During the last years, the MEMS market showed a rapid roughly exponential growth.

It is expected to reach the $8.3 billion dollars by 2012, up from $5.6 billion in 2006.This is mostly due to the incorporation of MEMS devices into accelerometers,

microphones, gyroscopes, pressure devices and more recently, the application of RF

MEMS devices in mainstream consumer products. More than 120 research centers,

laboratories and universities in Europe are involved in MEMS technologies,

developing and investigating a large variety of modern processes and device

concepts and providing considerable tests. A huge amount of RF MEMS switches and

RF MEMS based devices have been introduced, developed, tested and presented in

open sources. The choice of the type of RF MEMS switch depends on the

performance requirements, on the application and on the manufacturing facilities. In

Figure 2 several examples of these switches are shown.


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Figure 2: Photomicrographs of MEMS switches: (a) low-voltage MEMS shunt-capacitiveswitch of the University of Michigan, (b) direct contact MEMS switch of the Ewha WomansUniversity, (c) shunt-capacitive switch of Electronic Laboratory (EPFL), (d) the Radant MEMSseries switch, (e) the Rockwell Scientific MEMS series switch, and (f) the Raytheon MEMS

shunt-capacitive switch.

CAPACITIVE SWITCHES

A dielectric is deposited on top of the bottom electrode of the capacitive contact

switch. When the suspended beam is in the upstate the capacitance is in the range

of fF. When a DC actuation voltage is applied between the actuation electrode and

the suspended beam, the suspended beam will move downward and collapse on the

bottom electrode. This will increase the capacitance in the range of pF, 20-100 times

higher than the upstate capacitance. The upstate capacitance depends mainly on the

initial gap. The downstate capacitance depends on the dielectric thickness, dielectric

constant etc. SiO2, Si3N4, TiO2 can be used as dielectric for RF MEMS switches. The

capacitive contact switch is suitable for high frequency application. At low frequency(DC), the impedance always will be very high for a capacitive switch, whatever the

capacitance value is. A Capacitive contact switch is easier to fabricate than a DC

contact switch since it requires fewer process steps. Capacitive switches with their

large contact area can handle more RF power than metal-to-metal contact switches

and are therefore the preferred switch for applications requiring 100500 mW of RF

power. However, most capacitive switches operate at 6 GHz and above due to their

relatively small down-state capacitance (25 pF), and DC-contact switches are the

only switch of choice at 0.16 GHz.

Two types:

1. Shunt2. Series

Figure 3 below shows a capacitive contact shunt bridge switch. The structure is that

of a fixed-fixed bridge.


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Figure 3: A capacitive contact shunt bridge

The capacitive shunt RF MEMS switch comprises of a dielectric between the bridge

and the transmission line. During the OFF state, the bridge is suspended over the

transmission line. The RF signal passes through the transmission line freely. The

switch capacitance is usually around 10-100 fF, which translates to a high impedance

path to ground through the beam. In the ON state, a DC actuation voltage is exerted.

The DC biased force pulls the bridge towards the transmission line, resulting in a high

switch capacitance. The high switch capacitance, usually around 1-10 pF, causes a

low impedance path from the transmission to the ground plane.

Figure 4: The schematic of a capacitive shunt switch configuration.


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Figure 5: A capacitive contact series cantilever switch

The figure 5 above shows a RF MEMS capacitive series cantilever switch. It uses a

cantilever beam for the transmission purpose. The switch will stay ON when the

beam is in downstate and OFF when the beam is in upstate. When the electrode and

the beam is actuated with a biased voltage, dielectric gets charged and pulls the

suspended end of the beam towards it, thus turning the switch ON.

Self-Actuation of Capacitive Switches: Mechanical Model

Capacitive switches are sensitive to the RF power on the t-line because the overlap

area between the beam and the t-line is quite large (around 100 _ 100 mm2). On the

other hand, the overlap area of metal-contact switches is around 20 _ 20 mm2;

therefore, metal-contact switches are quite insensitive to the RF power.


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Figure 6: (a, b) Capacitive switches in shunt and series configurations. (c) Rectifying

effect of the V2 component in the force equation.

For an incident wave of power P, the RF voltage on the t-line is

V+ = Vpksin(wt) = (2PZo)1/2 sin(wt)

where Zo is the characteristic impedance of the t-line. For the shunt switch, it is

assumed that the up-state capacitance of the switch (Cu) is small enough that it does

not result in a large reflection coefficient (V-


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where Vdc-eq is the equivalent DC voltage due to the RF power on the line. The EMS

bridge cannot respond to the sin(wt) component because it is much higher than its

mechanical resonant frequency. However, it will respond to the DC component, and

a power P results in

For k =10 N/m and A = 100 X 100 m

2

, a power of 0.11 W results in Vdc-eq = 2.27.1V and a deflection of 0.0120.12 m. The deflection is much larger for low-k

switches and is 0.060.6 m for k = 2 N/m. For a large enough RF power resulting in

Vdc-eq Vp, the MEMS switch will self-actuate to the down-state position. A shunt

capacitive switch can therefore be used as a power limiter on a t-line with the

maximum power given by

For a pull-down voltage of 130 V, the RF power required for self-actuation in a 50-

W system is 0.0218 W (shunt case). Therefore, low-spring-constant, low voltage

MEMS bridges can be used as protective circuitry in front of sensitive amplifiers and

electronics.

For the case of a series capacitive switch, the open-circuit voltage on the t-line is (V- V+ or S11 1)

Vsw = 2Vpk sin(wt) (series switch)

and this voltage is applied between the bridge and the pull-down electrode. The

equivalent DC voltage becomes


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Vdc-eq = Vpk = 2 (series switch)

and the series capacitive switch can handle one-fourth of the RF power as the shuntswitch (for the same spring constant and the same electrode area).

Once the switch is self-actuated due to the RF voltage, it results in a large

capacitance to ground which acts as a short circuit, and the RF voltage across the

switch immediately drops to nearly zero. The electrostatic force is very low, and the

switch will tend to rise back to its original position. However, once it starts to rise, an

air gap is obtained above the electrode this results in a rapid drop in the switch

capacitance, and the creation of an RF voltage between the switch and the t-line.This, in turn, will result in an electrostatic force which tends to pull the switch back

to the down-state position. An equilibrium height will be achieved, which balances

the mechanical pull-up force and the pull-down force due to the RF voltage at the

switch (g0).


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MEMS Capacitive Switch: Electromagnetic Modeling

MEMS CAPACITIVE SHUNT SWITCHES: Physical Description

Figure 7: Illustration of a typical MEMS shunt switch shown in cross section and plan

view. The equivalent circuit is also shown

A MEMS shunt capacitive switch is shown in figure 7 above. The switch is suspended

at a height g above the dielectric layer on the t-line, and the dielectric thickness is td

with a dielectric constant r. The switch is L m long, w m wide, t m thick. The

width of the t-line is W m. The substrate can be silicon, GaAs, alumina, LTCC, or aquartz dielectric.

The MEMS shunt switch can be integrated in a coplanar-waveguide (CPW) or

in a microstrip topology. In a CPW configuration, the anchors of the MEMS switch

are connected to the CPW ground planes. In a microstrip configuration, one anchor

is connected to quarter-wave open stub that results in a short circuit at the bridge.


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The second anchor of the bridge is left unconnected or, is connected to the bias

resistor.

A DC voltage is applied between the MEMS Bridge and the microwave line.

This results in an electrostatic force that causes the MEMS Bridge to collapse on the

dielectric layer, largely increasing the bridge capacitance by a factor of 30100. This

capacitance connects the t-line to the ground and acts a short circuit at microwave

frequencies, resulting in a reflective switch. When the bias voltage is removed, the

MEMS switch returns back to its original position due to the restoring spring forces

of the bridge.

Up-State Capacitance

The parallel-plate capacitance of the MEMS shunt switch is

The second term in the denominator is due to the finite thickness of the dielectric.

For a dielectric thickness of 1500 , a relative dielectric constant of 7.6 (SixNy), and if

the capacitance is calculated as C = 0A/g, then the error is 0.5% and 1.3% for a

bridge height of 4 m and 1.5 m, respectively, and results in an accurate

approximation.

Down-State Capacitance and Capacitance Ratio

The MEMS switch capacitance in the down-state position can be easily calculatedusing Cd =0rA/td. In this case, the thickness of the dielectric is so small that thefringing capacitance can be neglected. The up-state/down-state capacitance ratio is


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For a capacitance area of 80 X100 m, a dielectric thickness of 1000 (1500) , r =

7.6, and a height of 1.5 m and 4 m, the capacitance ratio is 60 (38):1 and 120

(80):1, respectively.

The down-state capacitance can be degraded if the MEMS bridge layer or

dielectric layer is not perfectly flat. This can be due to the deposition parameters of

the nitride or the fabrication process of the MEMS bridge. The downstate

capacitance is

where A is the capacitive area of the bridge, d1 is the amplitude of the roughness, d2

is the dielectric thickness, and r is the relative dielectric constant of the dielectric

layer.

Figure 8 shows the reduction in the down-state capacitance assuming a

perfectly flat dielectric layer, and a roughness 0500 in the MEMS bridge. The

contact area is assumed to be 50% of the total bridge area. The dielectric layer is

10001500 thick with r =7.6. It is seen that a capacitance degradation of 3035 %

occurs for a roughness of 100 (10 nm). If the roughness is large and results in a

contact area of only 20 %, then the downstate capacitance will be around 20 % of

the parallel-plate value for a roughness of 200 .

Figure 8: Simulated down-state capacitance degradation versus roughness in theoverlying bridge layer for low (a) and high (b) dielectric constant materials.


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MEMS Capacitive Series Switch

(a) (b)

Figure 9: (a) cross section of a capacitive inline switch (b) general circuit model , The

actuation voltage is applied using metal or resistive bias lines that are connected to the t-lineand the pull-down electrodes

The figure 9 above shows a capacitive MEMS series switch. It is a cantilever type

structure with a beam.

Up-Capacitance

The up-state capacitance is composed of (a) a series capacitance (C s) between the t-line and the switch metal and (b) a parasitic capacitance (Cp) between the open ends

of the t-line. The total up-state capacitance of the series switch is the same for a DC-

contact switch or a capacitive series switch, and it is

for a cantilever switch with two contact areas, and is

Cu = Cs + Cp

for an inline switch with one contact area.

Down-State Resistance

The series resistance of the MEMS switch is that of a short section of t-line together

with a DC contact resistance. The contact resistance depends on the size of the


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contact area, the mechanical force applied, and the quality of the metal-to-metal

contact. Hyman measured a gold-to-gold contact resistance of 0.1 for an applied

force of 100500 N, a contact area of 20 m2, and a current of 0.110 mA. The

values above are given for very clean gold layers, and practical contact resistances

are around 0.51 per contact area.

The MEMS series switch resistance between the contact areas is dependent

on its length and width. The switch resistance is obtained by calculating the loss

Figure 10: Equivalent circuit for the series switch in the down-state position

of a t-line of the same dimensions of the MEMS switch. The total switch resistance is

Rs = 2Rc + 2Rs1 + Rl

for a cantilever switch with two contact areas,

Rs = Rc + 2Rs1 + Rl

for an inline switch with one contact area, and

Rs = 2Rs1 + Rl

for a capacitive contact switch.

For DC-contact switches, the series resistance is dominated by Rc. The total

switch resistance is 12 , depending on the contact area and applied force.


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PARAMETERS OF RF MEMS SWITCHES

1. Insertion Loss: RF MEMS Switches should have very low insertion loss(0)while in ON state, i.e the switch should be loss less

2. Isolation: The RF MEMS Switch should have very high isolation, i.e upto therange of infinity while in the OFF state. The higher the isolation of the switch

in OFF state is, the better the switch will perform.

3. Actuation voltage: Usually, the actuation voltage of RF MEMS Switches arevery high w.r.t MOSFETs and PIN diode switches. They are in the range of 30-

100 volts. Lots of research is being done in order to make switches with low

actuation voltage. The lower the actuation voltage is, the faster will the

switch work.

4. Capacitance ratio: The capacitance ratio, i.e the ratio of the down-statecapacitance with that of the up-state capacitance, should be very high for the

RF MEMS switch. This is so, as the down-state capacitance of the actuated

switch is very large compared to the up-state capacitance of the unbiased

switch.

5. Operating frequency: The switch is designed either as a metal contact switchor a capacitive switch depending on the requirement of the wireless or RF

circuit. Usually, DC contact switch is used for low frequency applications,

while capacitive contact switch is for high frequency applications.

6. Switching time: This is an important aspect of the RF MEMS switch. Thefaster, the better. Generally, the desired ON-OFF switching time of RF MEMS

switch is 5 s.

7. Low power: RF MEMS switches are a good low power switching device as thepower consumption is 0 as very less or almost nil current is drawn by the

switch.


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DESIGN OF RF MEMS CAPACITIVE SWITCH

DESIGN OF CPW MEMS SHUNT CAPACITIVE SWITCHES

1. C-Band to X-Band Designs

Consider a MEMS shunt switch with capacitance Cu and Cd connected in shunt

across a t-line of impedance Z0. In the up-state position, the reflection coefficient is

and for S11 < -10 dB and CuZ0 2 we have

Figure 11: Calculated reflection coefficient for different up-state bridge capacitances.

and Figure above presents the up-state reflection coefficient for bridge capacitances.

The loss in the up-state position is dependent on the definition of the reference

planes and is less than -0.1 dB up to 50 GHz. In the down-state position, the LCR


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model results in a resonant frequency given by f0 = 1/(LCd)1/2. However, for resonant

frequencies above 25 GHz and for X-band operation, the inductance and series

resistance of the LCR model can be neglected and the insertion loss (isolation) is

and for S21 < -10 dB and CdZ0 >> 2:

The insertion loss can also be written as

This equation determines the isolation obtained knowing the capacitance ratio and

the allowable up-state reflection coefficient, and it is plotted in Figure below.

Figure 12: Calculated isolation versus allowable up-state reflection coefficient and capacitiveratio for a CPW shunt MEMS switch.


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It is universal for capacitive shunt switches with no inductance effects, and it is

independent of the technology used and number of switches employed.

A typical down-state capacitance using MEMS bridges is 6 pF for a 1000--

thick nitride layer, and it results in an isolation of around -20 dB at 10 GHz. This

capacitance is limited by the area of the MEMS bridge (around 160 X180 m2) and

the contact planarity between the dielectric and the MEMS bridge. If more isolation

is desired using shunt capacitive switches, then it is possible to put two or three

MEMS bridges close to each other. In this case, the isolation is improved by 6 dB for

the two bridge designs, and 9.5 dB with the third bridge (3 X capacitance). The

penalty paid is the increase in the up-state reflection coefficient. The insertion lossfor the two- and three-bridge circuits is minimal for Rs = 0.1 0.25 W.

Another solution is the use of very high dielectric constant materials for the

dielectric layer. In this case, a down-state capacitance of 3060 pF can be achieved,

resulting in -32 to -38 dB of isolation at 5 GHz using Eq. (8.4). The LC resonant

frequencies of these switches is around 612 GHz (Cd = 3060 pF, L = 612 pH),

which makes them ideal for 2- to 20-GHz operation.

2. Millimeter-Wave Designs

The MEMS shunt switch with a length of 250350 mm is ideal for mm-wave

operation. The up-state performance of the MEMS switch is given by Eq. (8.2), and

Cu = 3050 fF results in an S11 of -13 to -17 dB at 30 GHz. However, in the down-state

position, the inductance of the bridge plays an important role since the LC resonant

frequency of the MEMS switch is between 25 and 60 GHz. Therefore, Eq. (8.4) does

not apply at 2060 GHz. At the resonant frequency, the isolation is limited by the

series resistance of the switch:

The response of a typical 35-GHz MEMS switch with Cu = 35 fF, Cd = 2.8 pF, L 8 pH,

and Rs = 0.25 W is shown in Fig. 8.5. In this case, the downstate LC resonance is at 34


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GHz. The insertion loss at 3238 GHz is -40 dB and is limited by the bridge series

resistance. The up-state capacitance of Cu = 35 fF can be achieved with a 3-m gap

and a capacitive area of 95X100 m2. The bandwidth of operation in a CPW

implementation is limited by S11 = -12 to -10 dB at 5060 GHz.

It is easy to design a low reflection switch at 60 GHz by decreasing the

upstate capacitance to 20 fF (S11 - 15 dB at 60 GHz). This can be achieved with an

area of 60 X 80 m2 for a height of 3 m, and an associated inductance of around 10

pH. For a capacitance ratio of 5060, the down-state capacitance is 1.01.2 pF and

the resonant frequency is 4650 GHz. This switch results in excellent performance up

to 70 GHz.

Figure 13: Simulated S-parameters for a K-band CPW MEMS shunt switch


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DESIGN OF MEMS CAPACITIVE SERIES SWITCHES

Capacitive series switches can be fabricated using the standard fixedfixed beam

capacitive switch, but with the anchors attached to the output t-line, or using a

cantilever and a capacitive contact area, as in the case of the Lincoln Laboratories

switch The design of MEMS capacitive series switches in the up-state position is

identical to MEMS DC-contact series switches. The only difference is that capacitive

series switches have an up-state capacitance of 825 fF and therefore result in a

much lower isolation than DC-contact series switches. In the down-state position,

the insertion loss is given by

and for 1/2CdZ0 the insertion loss is nearly equal to 0 dB. In fact,

Figure 14: Implementation of a capacitive series switch in a microstrip t-line using a fixedfixed beam design [top view (a)] and a cantilever design [side view (b)].

Figure 15: Equivalent circuits for inductive matching of capacitive series switches in the

down-state position.


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For Cd = 1 pF, S21 = -0.34 dB to -0.08 dB at 816 GHz. Therefore, capacitive series

switches with Cd = 12 pF are ideal for 6-GHz applications and above. Also, the

reduction in S21 is solely due to the series capacitance, and this can be tuned with a

series inductance to result in an isolation of 0 dB. The price paid is a reduction in the

insertion-loss bandwidth. The equations for the match condition for each of the

circuits shown in Figure 15 are (S11 = 0)

for the double t-line. For Zh

Z0

(Zh

= 80 ), simplified equation is:

The double t-line match, results in a perfect match as long as the following condition

is satisfied:


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SOME RF MEMS SWITCH ARCHITECTURES & SIMULATION

RESULTS

1. H-Shaped Low Actuation-Voltage RF MEMS Switches

Figure 16: H-shaped (a) capacitive switch (b) DC contact switch

Circuit Simulations

Figure 17: EM simulations for the designed shunt-capacitive and shunt-resistive RFMEMS switches: (a) in the ON-state and (b) in the OFF-state.


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Figure 18: EM and equivalent circuit model simulations for the designed shunt-resistive RF MEMS switch: (a) in the ON-state and (b) in the OFF-state.

Figure 19: EM simulation for the designed RF MEMS switches in the ON-state for differentvalues of the initial gap height g0: (a) shunt-capacitive switch and (b) shunt-resistive switch.


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2.Lateral contact Switch

Figure 20: lateral operated metal to metal contact shunt MEMS switch

Figure 21: Shunt electrode layout and its simulation; a) Schematic illustration, b) Simulatedcurrent density in actuated (switch-off) state, and c) Simulated S-parameters.


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Figure 22: Measured frequency response

3. RF MEMS Capacitive Switch with pull-up and pull-down electrodes

Figure 23: Cross section of the miniature MEMS capacitive switch with top and bottomelectrodes.

Circuit simulations


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Figure 24: Measured and fitted S-parameter of a two-port 2 X 2 array, in three states.


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SCOPE OF WORK

There is a strong drive towards development of RF MEMS Switches with low

actuation voltage, which is suitable for the integration with the existing CMOS

circuits. There are many scope of further work in the RF MEMS Switch. Different

structures can be simulated in order to lower the actuation voltage, increase

capacitance ratio, decrease switching time, increase the life cycles of the switch etc.

Some can be pointed out as:

1. Medium- to high-power (0.055 W) metal-to-metal contact switches, and high-power (0.55 W) capacitive contact switches are needed. This may be achieved at

the expense of size and switching speed of the device.

2. MEMS switches with a switching time of 100200 ns and an actuation voltage of

3050 V should be developed.

3. CMOS controllers and voltage up-converters should be integrated with MEMS

switches and varactors inside the hermetically sealed cavity.

4. Use of tethered cantilever MEMS switch may lead to minimize the dielectric

charging effect, thus increasing the life cycle of the switch.

5. Springless switch structures could be developed in order to minimize the effect of

spring constant on the switch.

6. A four way switch can be developed with two switches, one acting as a spare if the

other stops working.

Rf Mems Report

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