FY03 NEPP TRO -- RF MEMS Switches

*Dr. Joanne Wellman -- Jet Propulsion Laboratory*

The following document presents the state of RF MEMS switch technology
as of January, 2003 and evaluates its readiness for insertion into space
flight systems.[^1]

# MEMS INDUSTRY ROADMAP

The development of MEMS
([M]{.underline}icro[e]{.underline}lectro[m]{.underline}echanical
[S]{.underline}ystems) 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.

![](media/image1.wmf){width="5.995833333333334in"
height="3.1347222222222224in"}

Figure 1. Commercialization times (from beginning of product evolution
to full commercial production) for various MEMS technologies. (Courtesy
of MANCEF[^2] )

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[^3], 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.

![](media/image2.wmf){width="5.0in" height="3.75in"}

Figure 2. Alignment of potential MEMS technology capabilities with NASA
needs and technology development focus areas.

# 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
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[^4].

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.

+-----------------+----------------+-----------------+----------------+
| ###### Para     | **Switch A**   | **Switch B**    | **Switch C**   |
| meter {#paramet |                |                 |                |
| er .unnumbered} |                |                 |                |
+-----------------+----------------+-----------------+----------------+
| #####           | 1.7 dB         | 0.9 dB          | 1.2 dB         |
|  Insertion Loss |                |                 |                |
|  {#insertion-lo |                |                 |                |
| ss .unnumbered} |                |                 |                |
+-----------------+----------------+-----------------+----------------+
| **Isolation**   | 45 dB          | 38 dB           | 24 dB          |
+-----------------+----------------+-----------------+----------------+
| **Intercept     | 43 dBm         | 41 dBm          |                |
| Point**         |                |                 |                |
+-----------------+----------------+-----------------+----------------+
| **Power         | 23 dBm         | 23 dBm          | 23 dBm         |
| Handling (CW)** |                |                 |                |
+-----------------+----------------+-----------------+----------------+
| **Bandwidth**   | dc -- 20 GHz   | dc -- 3 GHz     | 74 GHz -- 80   |
|                 |                |                 | GHz            |
+-----------------+----------------+-----------------+----------------+
| **Switching     | 6 ns           | 20 ns           | 2 ns           |
| Speed**         |                |                 |                |
+-----------------+----------------+-----------------+----------------+
| **Power         | 5V, 10μa       | 5V, 90μa        | 1.25 V, 10 mA  |
| Consumption**   | typical        |                 |                |
+-----------------+----------------+-----------------+----------------+
| **Cost per**    | \~\$30         | \$1.63          | \$5.98         |
+-----------------+----------------+-----------------+----------------+
| **Size**        | 1.3 mm × 0.85  | 3.1 mm × 3.1 mm | 1.6 mm × 0.73  |
|                 | mm (chip)      | (packaged)      | mm (chip)      |
+-----------------+----------------+-----------------+----------------+

Table 1. Typical parameters from 3 commercially available RF solid-state
switches.[^5]

+-------------------+-----------------+---------------+---------------+
| ##### PA          | #####           | #             | ##            |
| RAMETER {#paramet | RF MEMS {#rf-me | #### PIN-DIOD | ### FET {#fet |
| er-1 .unnumbered} | ms .unnumbered} | E {#pin-diode |  .unnumbered} |
|                   |                 |  .unnumbered} |               |
+-------------------+-----------------+---------------+---------------+
| Voltage           | 20 -- 80        | ± 3 -- 5      | 3 -- 5        |
+-------------------+-----------------+---------------+---------------+
| Current (mA)      | 0               | 0 -- 20       | 0             |
+-------------------+-----------------+---------------+---------------+
| Power Consumption | 0.5 -- 1        | 5 -- 100      | -.5 -- 0.1    |
| (mW)              |                 |               |               |
+-------------------+-----------------+---------------+---------------+
| Switching         | 1 -- 300 μs     | 1 -- 100 ns   | 1 -- 100 ns   |
+-------------------+-----------------+---------------+---------------+
| C~up~ (series)    | 1 -- 6          | 40 -- 80      | 70 -- 140     |
| (fF)              |                 |               |               |
+-------------------+-----------------+---------------+---------------+
| R~s~ (series) (Ω) | 0.5 -- 2        | 2 -- 4        | 4 -- 6        |
+-------------------+-----------------+---------------+---------------+
| Capacitance Ratio | 40 -- 500       | 10            | n/a           |
+-------------------+-----------------+---------------+---------------+
| Cutoff Freq.      | 20 -- 80        | 1 -- 4        | 0.5 -- 2      |
| (THz)             |                 |               |               |
+-------------------+-----------------+---------------+---------------+
| Isolation (1 --   | Very high       | High          | Medium        |
| 10 GHz)           |                 |               |               |
+-------------------+-----------------+---------------+---------------+
| Isolation (10 --  | Very high       | Medium        | Low           |
| 40 GHz)           |                 |               |               |
+-------------------+-----------------+---------------+---------------+
| Isolation (60 --  | High            | Medium        | None          |
| 10 GHz)           |                 |               |               |
+-------------------+-----------------+---------------+---------------+
| Loss (1 -- 100    | 0.05 -- 0.2     | 0.3 -- 1.2    | 0.4 -- 2.5    |
| GHz) (dB)         |                 |               |               |
+-------------------+-----------------+---------------+---------------+
| Power Handling    | \<1             | \<10          | \<10          |
| (W)               |                 |               |               |
+-------------------+-----------------+---------------+---------------+
| 3^rd^ order Int.  | +66 -- 80       | +27 -- 45     | +27 - 45      |
| (dBm)             |                 |               |               |
+-------------------+-----------------+---------------+---------------+

Table 2 Performance comparison of FETs, PIN Diode and RF MEMS
Electrostatic Switches[^6]

### 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 (see 4.1.2 and 4.1.3).

#### 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[^7]. 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 I^2^R 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.

![](media/image3.png){width="2.35in" height="2.0034722222222223in"}

Figure 3. Circuit equivalent of RF MEMS series contact switch.

![](media/image5.wmf){width="2.9305555555555554in"
height="2.4993055555555554in"}

Figure 4. Structure and operation of MEMS dc series switch[^8]**.**

#### 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 \[I~C~=1/(ωC)\]. 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[^9]. 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)[^10], whereas the contact switch can be used from dc levels.

![](media/image6.png){width="2.79375in" height="2.0034722222222223in"}

Figure 5. Circuit equivalent of RF MEMS series contact switch.

![](media/image7.jpeg){width="4.502083333333333in"
height="2.4090277777777778in"}![](media/image8.jpeg){width="4.496527777777778in"
height="1.5944444444444446in"}

Figure 6. Top and cross-sectional view of the Raytheon capacitive RF
MEMS switchError: Reference source not found.

# State of Technology

### Maturity

Technology maturity is defined relative to the performance
requirements[^11]. Current switch designs can perform within the
specifications needed for some commercial communications applications,
and the first commercial MEMS switch became available recently[^12] 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 robust, space-flight
quality devices.

### Current Commercial Efforts

+----------------------+------------------------+---------------------+
| ######## Company {#  | ######### Switchin     | > **Minimum Proven  |
| company .unnumbered} | g Time (μs) {#switchin | > Lifetime          |
|                      | g-time-μs .unnumbered} | > (Billions of      |
|                      |                        | > Cycles)**         |
+----------------------+------------------------+---------------------+
| Motorola             | 4-6                    | > \>60              |
+----------------------+------------------------+---------------------+
| Analog Devices       | 3-6                    | > \>60              |
+----------------------+------------------------+---------------------+
| > Omron              | 300                    | > \>1               |
+----------------------+------------------------+---------------------+
| > Cronos             | 10,000                 | > \>1               |
+----------------------+------------------------+---------------------+
| > Rockwell           | 8-10                   | > \>1               |
| > Scientific         |                        |                     |
+----------------------+------------------------+---------------------+
| > Samsung            | 100                    | > \>0.5             |
+----------------------+------------------------+---------------------+
| > HRL                | 30-40                  | > \>0.1             |
+----------------------+------------------------+---------------------+
| > Lincoln Labs       | \<1                    | > \>0.1             |
+----------------------+------------------------+---------------------+
| >                    | 300                    | > \>0.1             |
|  ST-Microelectronics |                        |                     |
+----------------------+------------------------+---------------------+
| > Microlab           | 500                    | > \>0.1             |
+----------------------+------------------------+---------------------+
| > NEC                | 30-40                  |                     |
+----------------------+------------------------+---------------------+

Table 3. Metal-to-Metal RF MEMS switch commercial development efforts
worldwideError: Reference source not found.

+-----------------------+-----------------------+-----------------------+
| ######## Company {#c  | **Switching Time      | **Minimum Proven      |
| ompany-1 .unnumbered} | (μs)**                | 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
worldwideError: Reference source not found.

### Current University Efforts

+---------------------------------------+------------------------------+
| **UNIVERSITY -- LOCATION**            | > **SWITCH TYPE**            |
+---------------------------------------+------------------------------+
| > The University of Michigan -- Ann   | > Metal-to-Metal Contact ,   |
| > Arbor                               | > Capacitive                 |
+---------------------------------------+------------------------------+
| > Northeastern University             | > Metal-to-Metal Contact     |
+---------------------------------------+------------------------------+
| > University of California --         | > Metal-to-Metal Contact     |
| > Berkeley                            |                              |
+---------------------------------------+------------------------------+
| > University of Illinois -- Urbana    | > Metal-to-Metal Contact     |
| > Champaign                           |                              |
+---------------------------------------+------------------------------+
| > 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         | > Metal-to-Metal Contact     |
| > Technology -- Korea                 |                              |
+---------------------------------------+------------------------------+
| > Seoul National University -- Korea  | > Metal-to-Metal Contact ,   |
|                                       | > Capacitive                 |
+---------------------------------------+------------------------------+
| > National Taiwan University --       | > Metal-to-Metal Contact ,   |
| > Taiwan                              | > Capacitive                 |
+---------------------------------------+------------------------------+
| > University of California -- Santa   | > Capacitive                 |
| > Barbara                             |                              |
+---------------------------------------+------------------------------+
| > Nanyang Technological University -- | > Capacitive                 |
| > Singapore                           |                              |
+---------------------------------------+------------------------------+

Table 5. Leading RF MEMS switch efforts at academic institutionsError:
Reference source not found.

# 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 -- TeravictaError: Reference source not
found) 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[^13]. 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[^14],
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[^15]. 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
[^16].

### 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[^17]. 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[^18][^19].

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 cyclesError: Reference source not found.

### 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 necessary 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
scenario and a supplemental table listing potential environmental
stresses and their associated effects on switch performance.

### LEO, ISS, Shuttle

-   RF MEMS switches do not currently meet the reliability requirements
    for this scenario.

-   [Issues to be addressed:]{.underline} wafer-level hermetic
    packaging, thermal cycle performance characterization, extremely low
    risk statistical testing, proton total dose radiation testing.

+-----------------------------------+-----------------------------------+
| > **Environment/condition**       | ##### Expected Effect on Switc    |
|                                   | h Performance {#expected-effect-o |
|                                   | n-switch-performance .unnumbered} |
+-----------------------------------+-----------------------------------+
| > 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) --   | > Unknown                         |
| > unshielded component            |                                   |
+-----------------------------------+-----------------------------------+

### Aeronautics

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

+-----------------------------------+-----------------------------------+
| > **Environment/condition**       | ##### Expected Effect on Switch   |
|                                   | Performance {#expected-effect-on- |
|                                   | switch-performance-1 .unnumbered} |
+-----------------------------------+-----------------------------------+
| > Vibration (launch)              | > None                            |
+-----------------------------------+-----------------------------------+
| > High Temperature (inside nose   | > Unknown                         |
| > cone)                           |                                   |
+-----------------------------------+-----------------------------------+
| > Contamination                   | > None if hermetically sealed     |
+-----------------------------------+-----------------------------------+
| > Human Life issues               | > Very high reliability           |
|                                   | > requirements                    |
+-----------------------------------+-----------------------------------+

### MEO, GEO

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

+-----------------------------------+-----------------------------------+
| > **Environment/condition**       | ##### Expected Effect on Switch   |
|                                   | Performance {#expected-effect-on- |
|                                   | switch-performance-2 .unnumbered} |
+-----------------------------------+-----------------------------------+
| > Vibration (launch)              | > None                            |
+-----------------------------------+-----------------------------------+
| > Outgassing (vacuum effect)      | > None if hermetically sealed     |
+-----------------------------------+-----------------------------------+
| > Contamination                   | > None if hermetically sealed     |
+-----------------------------------+-----------------------------------+
| > Thermal Cycling                 | > Unknown                         |
+-----------------------------------+-----------------------------------+
| > GEO - Radiation Effects TID     | > Possible pull-down voltage      |
| > (electrons)                     | > shifts due to charge separation |
|                                   | > in dielectric and other effects |
+-----------------------------------+-----------------------------------+
| > MEO -- Radiation Effects TID    | > Possible pull-down voltage      |
| > (protons and electrons)         | > shifts due to charge separation |
|                                   | > in dielectric and other effects |
|                                   | > (electrons)                     |
|                                   | >                                 |
|                                   | > Unknown (protons)               |
+-----------------------------------+-----------------------------------+
| > Plasma effects -- spacecraft    | > Unknown                         |
| > charging/discharging            |                                   |
+-----------------------------------+-----------------------------------+

### Mars surface

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

+-----------------------------------+-----------------------------------+
| > **Environment/condition**       | ##### Expected Effect on Switch   |
|                                   | Performance {#expected-effect-on- |
|                                   | switch-performance-3 .unnumbered} |
+-----------------------------------+-----------------------------------+
| > Vibration (launch)              | > None                            |
+-----------------------------------+-----------------------------------+
| > Outgassing (vacuum effect)      | > None if hermetically sealed     |
+-----------------------------------+-----------------------------------+
| > Contamination                   | > None if hermetically sealed     |
+-----------------------------------+-----------------------------------+
| > Thermal Cycling                 | > Unknown                         |
+-----------------------------------+-----------------------------------+
| > Low Temperatures                | > Unknown                         |
+-----------------------------------+-----------------------------------+
| > Radiation Effects TID           | > Possible pull-down voltage      |
| > (electrons and protons)         | > shifts due to charge separation |
|                                   | > in dielectric and other effects |
|                                   | > (electrons)                     |
|                                   | >                                 |
|                                   | > Unknown (protons)               |
+-----------------------------------+-----------------------------------+

### Jovian system, outer planets

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

+-----------------------------------+-----------------------------------+
| > **Environment/condition**       | ##### Expected Effect on Switch   |
|                                   | Performance {#expected-effect-on- |
|                                   | switch-performance-4 .unnumbered} |
+-----------------------------------+-----------------------------------+
| > 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  | > Possible pull-down voltage      |
| > energy electrons)               | > shifts due to charge separation |
|                                   | > in dielectric and other effects |
+-----------------------------------+-----------------------------------+

### Outside solar system, very long life missions

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

+-----------------------------------+-----------------------------------+
| > **Environment/condition**       | ##### Expected Effect on Switch   |
|                                   | Performance {#expected-effect-on- |
|                                   | switch-performance-5 .unnumbered} |
+-----------------------------------+-----------------------------------+
| > 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        | > Unknown                         |
| > (interstellar radiation         |                                   |
| > protons)                        |                                   |
+-----------------------------------+-----------------------------------+

# 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.

# Acknowledgements

The author would like to acknowledge Prof. Gabriel Rebeiz (The
University of Michigan), Prof. Linda Katehi (Purdue University), Dr.
Robert Mihailovich (RSC) and Dr. Steven Lewis (Analog Devices) for their
valuable input. This work was carried out at the Jet Propulsion
Laboratory, California Institute of Technology, under contract with the
National Aeronautics and Space Administration, Code AE, under the NASA
Electronics Parts and Packaging Program (NEPP).

[^1]: Some of the information contained herein was culled from two RF
    MEMS overview publications. For a more detailed treatment, please
    see :

    [RF MEMS Theory, Design, and Technology]{.underline}, Gabriel M.
    Rebeiz, Hoboken, NJ, John Wiley & Sons, Inc., 2003.

    [RF MEMS and their applications]{.underline}, Vijay K. Varadan, K.J.
    Vinoy, K.A. Jose, Hoboken, NJ : John Wiley & Sons, Inc., 2003.

[^2]: Job Elders and Steven Walsh, Co-Chairs, *"International MEMS,
    MicroSystems, Top Down Nano Roadmap,"* Micro and Nanotechnology
    Commercialization Education Foundation (MANCEF), Albuquerque, New
    Mexico, 2002, p.53.

[^3]: NASA Technology Plan, Section 4: *"Strategic Technology Areas,"*
    updated August, 2001.

[^4]: Gupta, R.; Zaghloul, A.; Fassal, F.; Mahle, C. "*The impact of
    miniaturized RF technologies on payload architectures of future
    digital communications satellites,"* Tenth International Conference
    on Digital Satellite Communications, pp. 15-19 May 1995. Page(s):
    637 -645 vol.2

[^5]: [http://www.hittite.com](http://www.hittite.com/)

[^6]: G. Rebiez, [RF MEMS Theory, Design, and Technology]{.underline},
    John Wiley and Sons Publications, Hoboken, New Jersey, 2003.

[^7]: Other mechanisms for actuating the suspended beam, such as
    thermal, magnetic, and piezoelectric actuation, are discussed in
    reference 5. They are not presented here as they are not the most
    widely used in the leading RF MEMS development efforts, but could
    play a role in future successful commercial designs.

[^8]: R. E. Mihailovich, M. Kim, J. B. Hacker, E.A.Sovero, J. Studer, J.
    A. Higgins, and J. F. DeNatale, IEEE Microwave and Wireless
    Components Letters, vol. 11, no. 2, pp. 53-55, Feb. 2001.

[^9]: 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.

[^10]: G. Rebiez, [RF MEMS Theory, Design, and Technology]{.underline},
    John Wiley and Sons Publications, Hoboken, New Jersey, 2003, p.87.

[^11]: Newman, H.S "*RF MEMS switches and applications,"* 40th Annual
    Reliability Physics Symposium Proceedings, pp. 111 --115 (2002).

[^12]: [http://www.teravicta.com](http://www.terravicta.com/).

[^13]: G. Rebiez, [RF MEMS Theory, Design, and Technology]{.underline},
    John Wiley and Sons Publications, Hoboken, New Jersey, 2003, p.193 -
    199.

[^14]: Wellman, J. and Garcia, A., *"High Power (\>1W) Application RF
    MEMS Lifetime Performance Evaluation,"*
    [http://nepp.nasa.gov](http://nepp.nasa.gov/).

[^15]: 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).

[^16]: Pillans, B.; Kleber, J.; Goldsmith, C.; Eberly, M. ,"*RF power
    handling of capacitive RF MEMS devices,*"\
    Microwave Symposium Digest, 2002 IEEE MTT-S International Vol.1, pp.
    329 --332 (2002).

[^17]: S. McClure, L. Edmonds, A. Johnston, R. Mihailovich, J. Lehman,
    C. Yui, P. Alonzo, J. DeNatale, *"Radiation Effects in
    MicroElectroMechanical Systems (MEMS): RF Relays,"* IEEE
    Transactions on Nuclear Science, Vol. 49, to be published.

[^18]: The sufficiency of near-hermeticity is currently being
    investigated.

[^19]: Note that hermeticity implies a controlled environment, not
    vacuum (RF MEMS are packaged in controlled atmospheric pressure
    environments).