jueves, 11 de febrero de 2010

RF MEMS Switches

MEMS INDUSTRY ROADMAP

The development of MEMS (Microelectromechanical
Systems) technology makes it possible to fabricate
electromechanical and electronics components in
a single microscale device.  Broadening the
microworld beyond transistor-based technologies
provides a fundamental paradigm shift in
microsystem design, integrating sensors,
actuators, and other moveable parts with
microelectronics components on a single chip. 

At this time, a handful of drivers dominates
MEMS technology development. 
The defense and aerospace industries
have thus far been the primary catalysts
because they are seeking performance
advantages over existing technologies
without the development of high-volume
production methods.  However, it is
precisely large-volume production that will
unleash MEMS as a high market share
contender.  The automotive and IT industries
have led the way in implementing MEMS
devices into commercial systems, with
accelerometer triggers for airbag deployment
and ink jet printer nozzles, respectively. 
Other emerging applications include fuel
nozzles, pressure sensors, drug delivery systems,
chemical diagnostic devices, genomics,
bioengineering, data storage, and RF devices.



RF MEMS switches, with their low loss, high
isolation, high linearity, and low power consumption,
will bring significant improvements to RF systems. 
The most immediate application will be in
aerospace, defense, and high-cost commercial
applications, as their performance advantages
are required and the high packaging cost
isn't prohibitive in these low-volume applications
areas.  Once low-cost production and packaging
methods have been established, RF MEMS
stand to assume a sizeable portion of the RF
device market.

NASA ROADMAP

The advantages outlined above make the
infusion of RF MEMS switches into NASA
spacecraft an exciting leap toward faster, more
powerful communications and radiometric
systems.  For example, currently available
commercial RF transceiver solutions are
nearing fully integrated on-chip systems, but
the switches (and resonators) remain
off-chip components. MEMS switches
(and MEMS resonators) can help the industry
reach fully integrated communications
subsystems.  In the case of the transceiver,
lower insertion loss in the switches between the
antenna and the first low-noise amplifier (LNA)
leads to lower noise figures and higher sensitivity
of the receiver.  This effect is magnified when
more than one antenna is used, which requires
additional switches in the antenna-LNA path. 
Half duplex systems utilize RF switches to switch
between transmit and receive modes.  Redundant
systems use RF switches to change between
redundant segments.  Multiple antenna systems,
such as diversity receivers or ping-pong mode
radar systems use RF switches for antenna
switching.  Virtually all spacecraft with
communications or radiometric systems employ
such configurations and their performance
currently suffers from high insertion loss/low
isolation in solid-state RF switches.

NASA planning committees have recognized the
emergence and necessity of miniaturization in
the next generation of spacecraft.  For example,
as shown in Figure 2, 3 out of 5 NASA strategic
technology areas are in fields directly dependent
upon microsystem development[i], with potential
implementation of MEMS-based systems in a
broad array of future missions.  Specifically, as
mentioned above, MEMS RF switches are a strong
candidate for high bandwidth and spacecraft
network communications systems because of their
performance advantages over solid-state technology.



Description of rf mems technology


MEMS – vs. –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 waveguide 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
(see Table 1).  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 o
utside 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). 
Table 2 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.


1.1.1.1.1.1             Parameter
Switch A
Switch B
Switch C
1.1.1.1.2                   Insertion Loss
1.7 dB
0.9 dB
1.2 dB
Isolation
45 dB
38 dB
24 dB
Intercept Point
43 dBm
41 dBm

Power Handling (CW)
23 dBm
23 dBm
23 dBm
Bandwidth
dc – 20 GHz
dc – 3 GHz
74 GHz – 80 GHz
Switching Speed
6 ns
20 ns
2 ns
Power Consumption
5V, 10ma typical
5V, 90ma
1.25 V, 10 mA
Cost per
~$30
$1.63
$5.98
Size
1.3 mm ´ 0.85 mm (chip)
3.1 mm ´ 3.1 mm (packaged)
1.6 mm ´ 0.73 mm (chip)
Table 1Typical parameters from 3 commercially
available RF solid-state switches.

1.1.1.1.3                   PARAMETER
1.1.1.1.4                   RF MEMS
1.1.1.1.5                   PIN-DIODE
1.1.1.1.6                   FET
Voltage
20 – 80
± 3 – 5
3 – 5
Current (mA)
0
0 – 20
0
Power Consumption (mW)
0.5 – 1
5 – 100
-.5 – 0.1
Switching
1 – 300 ms
1 – 100 ns
1 – 100 ns
Cup (series) (fF)
1 – 6
40 – 80
70 – 140
Rs (series) (W)
0.5 – 2
2 – 4
4 – 6
Capacitance Ratio
40 – 500
10
n/a
Cutoff Freq. (THz)
20 – 80
1 – 4
0.5 – 2
Isolation (1 – 10 GHz)
Very high
High
Medium
Isolation (10 – 40 GHz)
Very high
Medium
Low
Isolation (60 – 10 GHz)
High
Medium
None
Loss (1 – 100 GHz) (dB)
0.05 – 0.2
0.3 – 1.2
0.4 – 2.5
Power Handling (W)
<1
<10
<10
3rd order Int. (dBm)
+66 – 80
+27 – 45
+27 - 45
Table 2  Performance comparison of
FETs, PIN Diode and RF MEMS Electrostatic Switches.

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 3.  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[1]
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 Figure 4.





RF Shunt Capacitive Switch


A circuit representation of a capacitive shunt
switch is shown in Figure 5.  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 line (see Figure 6).  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/(wC)].  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[ii]
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 Figure 6).  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)[iii], whereas
the contact switch can be used from dc levels. 


State of Technology


Maturity


Technology maturity is defined relative to the
performance requirements[iv].  Current
switch designs can perform within the
specifications needed for some commercial
communications applications,
and the first commercial MEMS switch
became available recently[v] for this
purpose and full commercialization is
expected in the next 2-3 years.  However,
for military and space applications,
the necessary testing has not been done
to determine if current MEMS RF switches
can meet the high reliability and low
risk requirements.  In terms of TRL,
Level 4 (Component and/or breadboard
validation in laboratory environment)
has been demonstrated.  Making the jump to
TRL 5 (Component and/or breadboard
validation in relevant environment.) is
currently being addressed by programs
such as PICOSAT, a small satellite
architecture providing cost-effective,
frequent space flight opportunities.
Support for programs like PICOSAT is
crucial for MEMS technology insertion in

spacecraft systems.
While many advances have been made
in the last decade, the overall maturity
level in RF MEMS switch technology
remains low.  Development is currently
in a gray region:  the design fundamentals
have been worked out, but the funding jump
necessary to proceed to an cost-effective,
packaged, highly reliable product hasn't
materialized.  Further financial investments
are needed to identify and characterize failure
mechanisms, design device modifications to
prevent them, and develop low-cost, reliable
packaging schemes.  Military agencies are
the principle investors at this time, but only
for limited improvements, not for a large-scale
technology insertion.  When a highly
profitable application is identified to overcome
the inertia of low-cost solid-state options,
the commercial industry will step in and advance
RF MEMS switches to full commercial
maturity.  In the meantime, it is advisable
that the aerospace community test available
RF MEMS devices under space mission
environments.  This testing will not be
done by the commercial sector and is
necessary for evaluating technology readiness
and providing developers with the necessary
feedback to ensure , space-flight

Current Commercial Efforts

1.1.1.1.1.1.1.1                   Company
1.1.1.1.1.1.1.1.1               Switching Time (ms)
Minimum Proven Lifetime (Billions of Cycles)
Motorola
4-6
>60
Analog Devices
3-6
>60
Omron
300
>1
Cronos
10,000
>1
Rockwell Scientific
8-10
>1
Samsung
100
>0.5
HRL
30-40
>0.1
Lincoln Labs
<1
>0.1
ST-Microelectronics
300
>0.1
Microlab
500
>0.1
NEC
30-40
 robust
Table 3Metal-to-Metal RF MEMS switch
comercial development efforts worldwide.

1.1.1.1.6.1.1.2                   Company
Switching Time (ms)
Minimum Proven Lifetime (Billions of Cycles)
Raytheon
4-20
>25
Lincoln Labs
20
>60
Northrup Grumman
4-8
>10
Daimler Benz
10-20

Bosch
10-20

IMEC (Belgium)
10-20

LG (Korea)
30-40
>0.1
Table 4.  Capacitive RF MEMS switch

commercial development effort worldwide.

 

Current University Efforts


UNIVERSITY – LOCATION
SWITCH TYPE
The University of Michigan – Ann Arbor
Metal-to-Metal Contact , Capacitive
Northeastern University
Metal-to-Metal Contact
University of California – Berkeley
Metal-to-Metal Contact
University of Illinois – Urbana Champaign
Metal-to-Metal Contact
University of Colorado – Boulder
Metal-to-Metal Contact
University of Limoges – France
Metal-to-Metal Contact , Capacitive
University of California – Davis
Metal-to-Metal Contact
Korea Advanced Institute of Technology – Korea
Metal-to-Metal Contact
Seoul National University – Korea
Metal-to-Metal Contact , Capacitive
National Taiwan University – Taiwan
Metal-to-Metal Contact , Capacitive
University of California – Santa Barbara
Capacitive
Nanyang Technological University – Singapore
Capacitive
Table 5Leading RF MEMS switch
efforts at academic institutions.

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 6.0, 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 – Teravicta10) 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.  Pitting and hardening can be
reduced by decreasing the contact force
during actuation.  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[vi].  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[vii], 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 microwelding 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[viii].  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[ix].  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 outgassing 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 outgassing
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 6.0
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.

Environmental Stresses


The following environmental effects are the
major contributors to device degradation:

Atomic Oxygen erosion


Hermetic packaging will mitigate this concern.
If the RF MEMS switch were exposed (leak in
hermetic seal), the formation of insulating
compounds at the compound surfaces may
occur and increase power loss.  Hermetic
seal tests during screening would reveal a leak
and ensure no failures due to atomic oxygen
exposure.

Radiation effects (SEU, TID, FLASH X-RAY)


Total dose (gamma ray) effects have already
been demonstrated in series contact switches
(dielectric charge separation can affect pull-down
voltage in some switch geometries) and will
likely accelerate dielectric failures in capacitive
shunt switches.  Further total dose testing
(both gamma and proton) and flash x-ray testing
has not been done but is recommended to
fully understand the effect of the radiation
environment on MEMS RF switches.

Thermal cycling


Unpowered thermal cycling should have no
effect on the device itself, other than induced
stresses from package thermal mismatches.
Operational thermal cycling (powered)
could have serious performance repercussions:
mechanical deformations could cause pull-down
voltage to vary significantly with temperature,
metal contact material parameters could
become non-ideal at high temperatures, and the
effects of temperature variations on contact
and restoring forces (and hence contact resistances)
are unknown.  Full thermal cycling characterization
has not been done and is required for insertion into
spacecraft systems.

Contamination, Outgassing


Clean hermetic packaging will mitigate potential
environmental contaminants.  Hermetic seal tests
during screening would reveal a leak and ensure no
failures due to contamination.

Vibration


The extremely small inertial mass of MEMS RF
switches makes vibration effects highly unlikely,
especially during downtime.  However, the effects
of random vibration on a powered device have
not yet been established and should be
investigated before insertion into spacecraft systems.

Extreme High –or Low – T


As described in 7.2.3, the effect of temperature
extremes on switch performance have not
been fully explored.

Long Lifetime – materials issues


If operated within recommended parameter space
and packaged in a clean hermetic environment,
MEMS RF switch lifetimes have been shown to
exceed 60 billion cycles.

 

Storage (survival conditions)


For devices sealed in a clean hermetic package,
no effect is expected.  Hermetic seal screening
would ensure successful storage.

 

Human life issues – extremely low risk restrictions


RF MEMS technology is too immature to be
considered in situations where failure of the device
could result in loss of human life.  Only after all of
the aforementioned issues have been addressed
should they be considered appropriate for such
low-risk applications.

(The high-stress environment testing mentioned
above has not been done because it is not n
ecessary for most commercial applications, the
automotive industry excepted)

Mission Scenarios


Below each of the following mission categories
is a flight readiness recommendation for use of
current RF MEMS switch technology in that s
cenario and a supplemental table listing
potential environmental stresses and their
associated effects on switch performance.

1.1.2        LEO, ISS, Shuttle

·         RF MEMS switches do not
currently meet the reliability
requirements for this scenario.
·         Issues to be addressed: 
wafer-level hermetic packaging,
thermal cycle performance
characterization, extremely low risk
statistical testing, proton total dose
radiation testing.

Environment/condition
1.1.2.1.1              Expected Effect on Switch Performance
Vibration (launch)
None
Outgassing (vacuum effect)
None if hermetically sealed
Contamination
None if hermetically sealed
Thermal Cycling
Unknown
Atomic oxygen
None if hermetically sealed
Human Life issues
Very high reliability requirements
Radiation Effects (proton) – unshielded component
Unknown

1.1.3        Aeronautics

RF MEMS switches do not currently
meet the reliability requirements for
this scenario.
Issues to be addressed:  wafer-level
hermetic packaging, high temperature
performance characterization,
extremely low risk statistical testing.

Environment/condition
1.1.3.1.1              Expected Effect on Switch Performance
Vibration (launch)
None
High Temperature (inside nose cone)
Unknown
Contamination
None if hermetically sealed
Human Life issues
Very high reliability requirements

1.1.4        MEO, GEO

RF MEMS switches do not
currently meet the reliability
requirements for this scenario.
Issues to be addressed:  wafer-level
hermetic packaging, temperature
cycling performance characterization,
total dose/single event radiation
effects testing, identify effects of
plasma charging/discharging.

Environment/condition
1.1.4.1.1              Expected Effect on Switch Performance
Vibration (launch)
None
Outgassing (vacuum effect)
None if hermetically sealed
Contamination
None if hermetically sealed
Thermal Cycling
Unknown
GEO - Radiation Effects TID (electrons)
Possible pull-down voltage shifts due to charge separation in dielectric and other effects
MEO – Radiation Effects TID (protons and electrons)
Possible pull-down voltage shifts due to charge separation in dielectric and other effects (electrons)
Unknown (protons)
Plasma effects – spacecraft charging/discharging
Unknown

1.1.5        Mars surface

RF MEMS switches do not currently
meet the reliability requirements for
this scenario.
Issues to be addressed:  wafer-level
hermetic packaging, temperature
cycling performance characterization,
Low temperature performance
characterization, total dose/single
event radiation effects testing.

Environment/condition
1.1.5.1.1              Expected Effect on Switch Performance
Vibration (launch)
None
Outgassing (vacuum effect)
None if hermetically sealed
Contamination
None if hermetically sealed
Thermal Cycling
Unknown

Low Temperatures
Unknown
Radiation Effects TID (electrons and protons)
Possible pull-down voltage shifts due to charge separation in dielectric and other effects (electrons)
Unknown (protons)

1.1.6        Jovian system, outer planets

RF MEMS switches do not currently
meet the reliability requirements for
this scenario.
Issues to be addressed:  wafer-level
hermetic packaging, Low temperature
performance characterization, plasma
effects testing, reconfigurability testing,
accelerated life testing., total dose
radiation testing

Environment/condition
1.1.6.1.1              Expected Effect on Switch Performance
Vibration (launch)
None
Outgassing (vacuum effect)
None if hermetically sealed
Contamination
None if hermetically sealed
Low Temperature
Unknown
Plasma effects
None if hermetically sealed
Long-life issues
Unknown, but believed to be on par with solid state technology if operated within the recommended specifications.
Radiation effects – TID (high energy electrons)
Possible pull-down voltage shifts due to charge separation in dielectric and other effects

1.1.7        Outside solar system, very long life missions

RF switches do not currently meet
the reliability requirements for this
scenario.
Issues to be addressed:  wafer-level
hermetic packaging, low temperature
performance characterization,
reconfigurability testing, accelerated
life testing, total dose radiation testing

Environment/condition
1.1.7.1.1              Expected Effect on Switch Performance
Vibration (launch)
None
Outgassing (vacuum effect)
None if hermetically sealed
Contamination
None if hermetically sealed
Low Temperature
Unknown
Long storage issues
Unknown, but believed to be minimal if hermetically sealed
Long-life issues
Unknown, but believed to be on par with solid state technology if operated within the recommended specifications.
Radiation effects – TID (interstellar radiation protons)
Unknown

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.

 

specification and/or requirements

 for technology items.


In section 7.2 above, suggestions were
made as to what tests need to be
completed before considering using RF
MEMS switches in spacecraft systems,
which could lead to the first stages of
an RF MEMS qualification guideline. 
Many of these tests can be performed
immediately on available designs and
would give a strong indication of the
robustness and reliability of RF MEMS
switches in spacecraft environments. 

It should be noted that compiling a complete
list of testing requirements for space
flight qualification is difficult to establish until
final materials and packaging choices have
been made and the devices become
available in quantity.  A single MEMS RF
switch design has only recently become
commercially available and a projected
timeframe for full RF MEMS switch
commercialization is on the order of 2-3
years, assuming the necessary funds are
invested in the near future. 

Conclusions


Virtually all spacecraft with communications
or radiometric systems employ RF
switching network configurations and their
performance currently suffers from high
insertion loss/low isolation in solid-state
switches.  RF MEMS technology offers the
necessary performance advantages over
existing solid-state options.  However,
development has not reached full maturity
(2-3 more years required) and technology
insertion hinges on focused investment,
particularly in the areas of reliability
assurance (risk mitigation) and wafer-level
hermetic packaging development.





[i] NASA Technology Plan, Section 4:  "Strategic Technology Areas," updated August, 2001.
[ii]Charles L. Goldsmith, Zhimin Yao, Susan Eshelman, and David Denniston, "Performance of Low-Loss RF MEMS Capacitive Switches," IEEE Microwave and Guided Wave Letters, Vol. 8, No. 8, pp. 269 – 271, Aug. 1998.
[iii] G. Rebiez, RF MEMS Theory, Design, and Technology, John Wiley and Sons Publications, Hoboken, New Jersey, 2003, p.87.
[iv] Newman, H.S "RF MEMS switches and applications," 40th Annual Reliability Physics Symposium Proceedings, pp. 111 –115 (2002).
[vi] G. Rebiez, RF MEMS Theory, Design, and Technology, John Wiley and Sons Publications, Hoboken, New Jersey, 2003, p.193 - 199.
[vii] Wellman, J. and Garcia, A., "High Power (>1W) Application RF MEMS Lifetime Performance Evaluation," http://nepp.nasa.gov.
[viii] Rizk, J.B.; Chaiban, E.; Rebeiz, G.M., "Steady state thermal analysis and high-power reliability considerations of RF MEMS capacitive switches," Microwave Symposium Digest, 2002 IEEE MTT-S International , vol. 1, pp. 39 –242  (2002).

Franco Rivera
CRF


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