mems switches seminar report

MEMS Switches Seminar Report ‘03

INTRODUCTION

Compound solid state switches such as GaAs MESFETs and PIN

diodes are widely used in microwave and millimeter wave integrated

circuits (MMICs) for telecommunications applications including signal

routing, impedance matching networks, and adjustable gain amplifiers.

However, these solid-state switches have a large insertion loss (typically

1 dB) in the on state and poor electrical isolation in the off state. The

recent developments of micro-electro-mechanical systems (MEMS) have

been continuously providing new and improved paradigms in the field of

microwave applications. Different configured micro machined miniature

switches have been reported. Among these switches, capacitive

membrane microwave switching devices present lower insertion loss,

higher isolation, better nonlinearity and zero static power consumption.

In this presentation, we describe the design, fabrication and performance

of a surface micro machined capacitive microwave switch on glass

substrate using electroplating techniques.

Dept. of ECE MESCE Kuttippuram-1-


SWITCH DESIGN AND OPERATION

The geometry of a capacitive MEMS switch is shown in Fig. 1.

The switch consists of a lower electrode fabricated on the surface of the

glass wafer and a thin aluminum membrane suspended over the electrode.

The membrane is connected directly to grounds on either side of the

electrode while a thin dielectric layer covers the lower electrode. The air

gap between the two conductors determines the switch off-capacitance.

With no applied actuation potential, the residual tensile stress of the

membrane keeps it suspended above the RF path. Application of a DC

electrostatic field to the lower electrode causes the formation of positive

and negative charges on the electrode and membrane conductor surfaces.

These charges exhibit an attractive force, which, when strong enough,

causes the suspended metal membrane to snap down onto the lower

electrode and dielectric surface, forming a low impedance RF path to

ground.



The switch is built on coplanar wave-guide (CPW) transmission

lines, which have an impedance of 50 that matches the impedance of the

system. The width of the transmission line is 160 m and the gap

between the ground line and signal line is 30 m. The insertion loss is

dominated by the resistive loss of the signal line and the coupling

between the signal line and the membrane when the membrane is in the

up position. To minimize the resistive loss, a thick layer of metal needs

be used to build the transmission line. The thicker metal layer result in a

bigger gap that reduces the coupling between signal and ground yet also

requires higher voltage to actuate the switch. To achieve a reasonable

actuation voltage, a 4-m-thick copper is used as the transmission line.

The glass wafer is chosen for the RF switch over a semi-conductive

silicon substrate since typical silicon wafer is too lossy for RF signal.



When the membrane is in the down position, the electrical isolation of the

switch mainly depends on the capacitive coupling between the signal line

and ground lines. The dielectric layer plays a key role for the electrical

isolation. The smaller the thickness and the smoother the surface of the

dielectric layer, the better isolation of the switch is. But there is another

trade-off here. When the membrane is pulled down, the biased voltage is

directly applied across the dielectric layer. Since this layer is very thin,

the electric field within the dielectric layer is very high. The thickness of

the dielectric layer should be chosen such that the electric field will never

exceed the breakdown electric field of the dielectric material. The silicon

nitride film has breakdown electric field as high as several mega-volts per

centimeter and can be utilized as dc block dielectric layer. In this project,

the thickness of the silicon nitride layer is chosen as 0.2 m to

accomplish the dc block and RF coupling purpose.

The switches were fabricated by surface micro-machining

techniques with a total of four masking level. No critical overlay

alignment was required. Fig. 2 shows the essential process steps:

1. Ti/Cu seed layer deposition: The starting substrate was a 2-

inch glass wafer. A layer of titanium (0.05 m) and copper (0.15m)

was sputtered on the substrate as seed layer for electroplating.

2. Silicon nitride deposition: A layer of silicon nitride (0.2m)

was deposited and patterned as DC block by using PECVD and reactive

ion etch (RIE).

3. Copper electroplating: A photo resist layer was spin coated and

patterned to define the electroplating area. Then, a 4-m-thick copper



layer was electroplated to define the coplanar wave-guide and the posts

for the membranes.

4. Aluminum deposition: A layer of aluminum (0.4m) was

deposited by using electron beam evaporation and patterned to form the

top electrode in the actuation capacitor structure.

5. Release: The photo resist sacrificial layer was removed to

finalize the switch structure.



TEST RESULTS AND DISCUSSIONS

The probe station and network analyzer (HP 8510C) were used to

characterize the capacitive MEMS switch. Fig. 3 shows the micrograph

of a switch under test. When the switch is unactuated and the membrane

is on the up position, the switch is called in off-state. When the switch is

actuated and the membrane is pulled down, the switch is called in on-

state. The major characteristics of the switch are the insertion loss when

the signals pass through and the isolation when signals are rejected. In the

off-state the RF signal passes underneath the membrane without much

loss. In the on-state, between the central signal line and coplanar wave-

guide grounds exists a low impedance path through the bended

membrane. The switch will reflect the RF signal.

As shown in Fig. 4, in the off-state the switch has insertion loss of

approximately 0.1 dB at 10 GHz and 0.4 dB at 25 dB. Compared with

typical FET or PIN diode switches, which have about 1 dB insertion loss,

the MEMS switches have considerable advantages. For a multiswitch

system, the total loss is significantly lower when mechanical switches are

utilized. The return loss is better than 20 dB up to 25 GHz, which means

the MEMS switch has an excellent impedance match to 50.

The isolation and return loss of the switch in the on-state is shown

in Fig. 5. Due to the geometry of the capacitive switch, the signal cannot

be coupled to ground perfectly at the low frequency. As the frequency

becomes high, the coupling between the signal line and ground lines



makes the isolation of the switch approximately 15 dB at 20 GHz, which

is sufficient for switching RF signals.

The resonant frequency of 23.4 GHz was observed when the

membrane was in the down position. This means that the switch can be

equivalently modeled as a capacitor, inductor and resistor connected in

series between the signal and ground lines. Since the switch has a better

isolation around the resonant frequency, it can be designed such that the

desired frequency overlaps with the resonant frequency by adjusting the

geometry of the switch, i.e. the width of the membrane and the gap

between the membrane and the lower electrode.

The actuation voltage of the MEMS switch is about 50V.

The spring constant of the membrane and the distance between the

membrane and the bottom electrode determines the actuation voltage of

the switch. The spring constant of the membrane is mainly determined by

the membrane material properties, the membrane geometry, and the

residual stress in the membrane.



RF MEMS TECHNOLOGY

Currently, both series and shunt RF MEMS switch

configurations are under development, the most common being series

contact switches and capacitive shunt switches.

RF SERIES CONTACT SWITCH

An RF series switch operates by creating an open or short in the

transmission line, as shown in Figure 6. The basic structure of a MEMS

contact series switch consists of a conductive beam suspended over a

break in the transmission line. Application of dc bias induces an

electrostatic force on the beam, which lowers the beam across the gap,

shorting together the open ends of the transmission line . Upon removal

of the dc bias, the mechanical spring restoring force in the beam returns it

to its suspended (up) position. Closed-circuit losses are low (dielectric

and I2R losses in the transmission line and dc contacts) and the open-

circuit isolation from the ~100 μm gap is very high through 40 GHz.

Because it is a direct contact switch, it can be used in low-frequency

applications without compromising performance. An example of a series

MEMS contact switch, the Rockwell Science Center MEMS relay, is

shown in Error! Reference source not found.7.



Figure 6. Circuit equivalent of RF MEMS series contact switch.

Anchor

RF line RF line

Drive capacitor

Contact shunt

Spring

Figure7. Structure and operation of MEMS dc series switch .

RF SHUNT CAPACITIVE SWITCH

A circuit representation of a capacitive shunt switch is shown in

Figure8. In this case, the RF signal is shorted to ground by a variable

capacitor. Specifically, for RF MEMS capacitive shunt switches, a

grounded beam is suspended over a dielectric pad on the transmission


Biased - ON

Unbiased - OFF


line (see Figure9). When the beam is in the up position, the capacitance

of the line-dielectric-air-beam configuration is on the order of ~50 fF,

which translates to a high impedance path to ground through the beam

[IC=1/(C)]. However, when a dc voltage is applied between the

transmission line and the electrode, the induced electrostatic force pulls

the beam down to be coplanar with the dielectric pad, lowering the

capacitance to pF levels, reducing the impedance of the path through the

beam for high frequency (RF) signal and shorting the RF to ground .

Therefore, opposite to the operation of the series contact switch, the beam

in the up position corresponds to a low-loss RF path to the output load,

while the beam in the down position results in RF shunted to ground and

no RF signal at the output load (see Figure9). While the shunt

configuration allows hot-switching and gives better linearity, lower

insertion loss than the MEMS series contact switch, the frequency

dependence of the capacitive reactance restricts high quality performance

to high RF signal frequencies (5-100 GHz) , whereas the contact switch

can be used from dc levels.

Figure 8. Circuit equivalent of RF MEMS series contact switch.



Figure 9. Top and cross-sectional view of the Raytheon capacitive RF

MEMS switch



PRODUCTION AND MANUFACTURING ISSUES

PACKAGING

The primary production issue at this time is the lack of low-cost

packaging options. As will be discussed in section Error: Reference

source not found, the hermeticity requirement for RF MEMS switch

packaging leaves only high-cost, military- or space-grade traditional

packaging methods as appropriate for high reliability assurance.

Expensive packaging precludes the large-scale production needed for

extensive reliability testing and the low risk statistics for widespread

commercial sales.

AVAILABLE VENDORS

Significant manufacturing hurdles have the following

repercussions for spacecraft systems MEMS technology insertion. First,

there are few available vendors (currently one – Teravicta) and limited

in-stock product. Second, and most importantly, much reliability testing

remains to be completed and what has been done isn’t widely available

due to commercial proprietary concerns. For space flight applications,

this means that if one can find switches to purchase, the knowledge of

their physics of failure and, consequently, the ability to predict what

conditions may trigger them, is severely compromised. In-house

performance characterization and reliability testing, and the resulting

database of MEMS RF switch failure mechanisms, will enable

accelerated MEMS technology insertion.



GENERAL RELIABILITY CONCERNS

Because RF MEMS switches are at such a low maturity level, there

are reliability concerns at all levels – design, fabrication, post-

production/packaging, and system insertion/harsh environments. Before

addressing the failure modes, it is useful to point out that RF MEMS

switches are not subject to structural mechanical failure of the beam: the

beams don’t crack or break even after billions of cycles.

For low - medium power operation (<100 mW) the primary design

failures are based in materials choice and placement, increased resistance

at the metal contact in series switches and dielectric charging in shunt

switches.

METAL CONTACT RESISTANCE (Series Contact Switches)

Series contact switches tend to fail in the open circuit state with

wear. Even though the bridge is collapsing and making contact with the

transmission line, the conductivity of the contact metallization area

decreases until unacceptable levels of power loss are achieved. These

out-of-spec increases in resistivity of the metal contact layer over cycling

time may be attributed to frictional wear, pitting, hardening, non-

conductive skin formation, and/or contamination of the metal.

Decreasing the contact force during actuation can reduce pitting and

hardening. But tailoring the design to minimize the effect involves



balancing operational conditions (contact force, current, and

temperature), plastic deformation properties, metal deposition method,

and switch mechanical design. In other cases, the resistivity of the

contact increases with use due to the formation of a thin dielectric layer

on the surface of the metal. While this has been documented, the

underlying physical mechanisms are not currently well understood. As

the RF power level is raised above 100 mW, the aforementioned failures

are exacerbated by the increased temperature at the contact area and,

under hot-switching conditions, arcing and micro welding between the

metal layers.

DIELECTRIC BREAKDOWN (Shunt Capacitive Switches)

Shunt capacitive switches often fail due to charge trapping, both at

the surface and in the bulk states of the dielectric. Surface charge transfer

from the beam to the dielectric surface results in the bridge getting stuck

in the up position (increased actuation voltage). Bulk charge trapping, on

the other hand, creates image charges in the bridge metallization and

increases the holding force of the bridge to a value above its spring

restoring force. There are several actions that can be taken to mitigate

dielectric charging in the design phase, including choosing better

dielectric material and designing peripheral pull-down electrodes to

decouple the actuation from the dielectric behavior at the contact. Unlike

series contact switches, capacitive shunt switches do not experience hard

failures at RF power levels > 100 mW, as long as the bridge contact

metallization is thick enough to handle the high current densities.

However, RF power may be limited in some cases by a recoverable

failure, self-actuation. While not yet fully understood, it has been



observed that a capacitive shunt switch will self-actuate at 4W of RF

power (cold-switching failure) and experience latch-up (stuck in down

position) in hot-switching mode at 500 mW. Even though these

“failures” are recoverable – the switch operates normally if the RF power

is decreased below the latch-up value of 500 mW – they still illustrate a

lifetime consideration for high power applications.

RADIATION AND OTHER EFFECTS

There are some areas of RF MEMS reliability research that have not

been investigated in detail and are in need of immediate attention. For

example, RF MEMS series contact switches were thought to be immune

to radiation effects until JPL’s total dose gamma irradiation experiments

on the RSC MEMS contact switch showed design-dependent charge

separation effects in the pull-down electrode dielectric material, which

noticeably decreased the actuation voltage of the device. This

immediately begs the question of how radiation effects will accelerate the

dielectric material failure mechanisms of capacitive switches, which have

known dielectric failure mechanisms, or other series switches that utilize

dielectric material in their electrode structures. These and other issues,

such as reconfiguration (does a switch recover from long-duration

continuous actuation?) and long lifetime ruggedness must be investigated

in detail to ensure robust and reliable design of RF MEMS devices.

PACKAGING

Beyond the design and production phases, reliability concerns can

be introduced in post-production (such as release stiction fails) and, most



importantly, in packaging. Several factors must be considered before

choosing a package for RF MEMS switches. First and foremost, RF

MEMS performance will quickly degrade in the presence of

contaminants and humidity. Therefore, the initial package criterion is

hermeticity.

A traditional approach would involve dicing the wafer, releasing the

device, attaching the substrate to the package base, and attaching the lid

with a hermetic seal, incorporating baking and vacuum conditions as

necessary to ensure no out gassing after seal. With the many options

available for microelectronics packaging, a suitable hermetic package can

be found that minimizes thermal-mismatch induced stresses and provides

low-loss RF electrical connections. Although it is possible to

successfully package MEMS RF switches in this manner, it is impractical

for two reasons: it’s prohibitively expensive for large-scale production

and manipulating released devices is tedious. In response to these

difficulties, the current trend is toward wafer-level packaging, which

reduces cost and mitigates the structural fragility by bonding the package

around the released switch in the production phase, before dicing and

subsequent handling. Wafer-level packaging for RF MEMS is a topic of

intense study. Work is currently underway to find a suitable bonding

method that provides adequate hermetic seal without out gassing

contaminants into the body of the package or thermally damaging the

delicate MEMS structures.



RELIABILITY AND RADIATION TALL TENT POLES

The failure mechanisms outlined in section Error: Reference source

not found determine the specific reliability concerns for each mission

scenario. In general, one must address both the operational and

environmental stresses imposed on the device throughout the lifetime of

the mission. The only operational stress addressed here will be high RF

power, since RF MEMS technology is not yet mature enough to consider

system-level behaviors.

OPERATIONAL

Power Handling

As outlined above, reliable operation of RF MEMS switches at

power levels above 500 mW cannot be guaranteed at this time.

Capacitive shunt switches experience recoverable failures at this level,

while series contact switches may permanently fail in the short circuit

configuration if hot switched above 100 mW. Hot-switching series

contact switches at any power is not recommended. Thermal dissipation

precautions in packaging are unnecessary, as RF MEMS do not generate

sufficient thermal energy.



TECHNOLOGY EVOLUTION IN NEAR TERM

The fundamental architecture for RF MEMS switches, both contact

series and capacitive shunt, is stable and likely to persist through

commercial insertion. Design subtleties will be adjusted to optimize

performance (i.e. more robust metal contact) and increase reliability, but

will likely be considered revisions rather than a new design. Since there

is no set packaging method, the end product has yet to be fully realized.



COMPARISION OF MEMS SWITCHES WITH SOLID

STATE SWITCHES

RF switches are used in a wide array of commercial, aerospace, and

defense application areas, including satellite communications systems,

wireless communications systems, instrumentation, and radar systems. In

order to choose an appropriate RF switch for each of the above scenarios,

one must first consider the required performance specifications, such as

frequency bandwidth, linearity, power handling, power consumption,

switching speed, signal level, and allowable losses.

Traditional electromechanical switches, such as wave-guide and

coaxial switches, show low insertion loss, high isolation, and good power

handling capabilities but are power-hungry, slow, and unreliable for long-

life applications. Current solid-state RF technologies (PIN diode- and

FET- based) are utilized for their high switching speeds, commercial

availability, low cost, and ruggedness. Their inherited technology

maturity ensures a broad base of expertise across the industry, spanning

device design, fabrication, packaging, applications/system insertion and,

consequently, high reliability and well-characterized performance

assurance. Some parameters, such as isolation, insertion loss, and power

handling, can be adjusted via device design to suit many application

needs, but at a performance cost elsewhere. For example, some

commercially available RF switches can support high power handling,

but require large, massive packages and high power consumption.



In spite of this design flexibility, two major areas of concern with

solid-state switches persist: breakdown of linearity and frequency

bandwidth upper limits. When operating at high RF power, nonlinear

switch behavior leads to spectral regrowth, which smears the energy

outside of its allocated frequency band and causes adjacent channel

power violations (jamming) as well as signal to noise problems. The

other strong driving mechanism for pursuing new RF technologies is the

fundamental degradation of insertion loss and isolation at signal

frequencies above 1-2 GHz.

By utilizing electromechanical architecture on a miniature- (or

micro-) scale, MEMS RF switches combine the advantages of traditional

electromechanical switches (low insertion loss, high isolation, extremely

high linearity) with those of solid-state switches (low power

consumption, low mass, long lifetime). shows a comparison of MEMS,

PIN-diode and FET switch parameters. While improvements in insertion

loss (<0.2 dB), isolation (>40 dB), linearity (third order intercept

point>66 dBm), and frequency bandwidth (dc – 40 GHz) are remarkable,

RF MEMS switches are slower and have lower power handling

capabilities. All of these advantages, together with the potential for high

reliability long lifetime operation make RF MEMS switches a promising

solution to existing low-power RF technology limitations.



ADVANTAGES OF MEMS SWITCHES

(1) Small size :

Semiconductor manufacturing techniques used in the batch fabrication of

micro systems, these systems process sizes ranging from micro meters to

a few milli meters.

(2) Low Cost :

Mems technology allows complex electromechanical systems to be

manufactured using batch fabrication techniques, allowing cost of

switches to be put in party with that of integrated circuits. Much of labour

involved in packing and assembly of such a system would simply

disappear.

(3) Low power consumption :

The mems switches are power efficient. The power losses in data

transmission and also the time lag are eliminated because the switches are

made next to control circuitory in the same chip.

(4) High isolation :

Isolation of mems switches in the range 1-40GHz is very high than the

other switches.

(5) Ability to be integrated with other electronic devices with

excellent linearity.



CONCLUSION

MEMS capacitive switches of RF applications show low

insertion losses in the OFF state and high isolation in the ON state. The

micro machine switches have applications in phased antenna arrays, in

MEMS impedance matching networks, and in communication

applications.



REFERENCES

1. IEEE Instrumentation and Management magazine, March 2003,

“MEMS Switches” (Page No. 12).

2. Gopinath A and Rankain J B “GaAs FET RF Switches”, IEEE

Trans. On Electron Dev., Vol.ED-32, No.7,July 1985.

3. Cavery,R.H. “Distortion of Off-State Arsenide MESFET

Switches”, IEEE Trans. On Microwave Theory and Tech.

Vol.41,No.8, August 1993.



ABSTRACT

A surface micro machined capacitive switch has been designed and

fabricated on a glass substrate. The switch is constructed of a thin

metallic membrane crossing over an electroplated coplanar wave-guide

transmission line. The electrostatic actuation is utilized as the switching

mechanism. The actuation voltage is around 50V. The switch showed low

insertion loss of 0.1 dB at 10 GHz and 0.4 dB at 25 dB, and isolation of

15dB at 20GHz. This device offers a potential application in

telecommunication, phase antenna array system, etc.



ACKNOWLEDGEMENT

I extend my sincere thanks to Prof. P.V.Abdul Hameed, Head

of the Department for providing me with the guidance and facilities for

the Seminar.

I express my sincere gratitude to Seminar coordinator

Mr. Manoj K, Staff in charge, for his cooperation and guidance for

preparing and presenting this seminar.

I also extend my sincere thanks to all other faculty members of

Electronics and Communication Department and my friends for their

support and encouragement.

Renjith P.



CONTENTS

INTRODUCTION

SWITCH DESIGN AND OPERATION

TEST RESULTS AND DISCUSSIONS

RF MEMS TECHNOLOGY

PRODUCTION AND MANUFACTURING ISSUES

GENERAL RELIABILITY CONCERNS

TECHNOLOGY EVOLUTION IN NEAR TERM

COMPARISION OF MEMS SWITCHES WITH SOLID

STATE SWITCHES

ADVANTAGES OF MEMS SWITCHES

CONCLUSION

REFERENCES


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