miércoles, 10 de febrero de 2010
Radant MEMS, Incorporated
An electrostatically actuated broadband microswitch has
been developed that has applications from DC through
the microwave region. The microswitch is a 3-terminal
device based on a cantilever beam and is fabricated
using an all-metal, surface micromachining process.
It operates in a hermetic environment obtained through
a wafer-bonding process. Characteristics of the
wafer-level packaged switch include DC on-resistance
of less than 1 Ohm with an actuation voltage of 80 V,
lifetime of greater than 1010 cycles with on-resistance
variation of less than 0.2 Ohm and current handling
capability of 1 Ampere. Key RF characteristics at 2 GHz
include an insertion loss of 0.3 dB and isolation of 30 dB.
Preliminary measurements at higher microwave frequencies
are extremely promising with full characterization and
planned product improvements underway.
MEMS microswitches are receiving increasing attention,
particularly in the RF community. Low power consumption,
large ratio of off-impedance to on-impedance and the ability
to be integrated with other electronics; makes micro
switches an attractive alternative to other mechanical and
solid-state switches. MEMS switches can be used in a
variety of RF applications including cell phones, phase
shifters and smart antennas and also in lower-frequency
applications such as Automatic Test Equipment
(ATE) and industrial and medical instrumentation.
We have developed an electrostatically actuated MEMS
microswitch for both DC and RF applications. The
microswitch is a 3-terminal device that employs a
cantilever beam and is fabricated using an all-metal,
surface micromachining process on high-resistivity silicon.
In operation, the beam is deflected by applying a voltage
between the gate and source electrodes, so that the free
end of the beam contacts the drain and completes an
electrical path between the drain and the source.
The contact material is a thin layer of a Platinum group
metal deposited on the underside of the beam and
on the drain. The device operates in a hermetic
environment obtained through a waferlevel capping process.
Since 1999 we have employed a commercial MEMS
foundry for switch fabrication and have obtained
fabrication yields in excess of 90%.
Ongoing activities include manufacture of prototypes for
use in a X-band Electronically Steerable Antenna (ESA)
under an US Air Force contract, process and design
modifications to optimize RF performance and contact
resistance, and development of a 4-terminal relay for better
isolation between the actuation and signal paths.
Low Frequency Performance and Applications
Important operational characteristics are as follows.
A gate voltage between 40 and 120 V, depending on
the specific design, actuates the microswitch. Devices
are usually operated at a contact force of 200 μN, and
have a single contact resistance of 3 Ω. Switches typically
have 8 contacts in parallel to yield a total on-resistance,
including interconnects, of less than 1 Ω at DC and low
Device lifetime is generally measured at a current of
10 mA or less with the current applied only during switch
closure (to avoid hot "breaks" and "makes", i.e.
"cold-switched"). Under these conditions, switch lifetime
exceeds 10 cycles . Measurements at higher
currents, up to 500 mA per switch contact, show lifetime
greater than 10 cycles. Currently, lifetime measurements
are limited by test time, rather than any failure mechanism.
A relatively small number of hot-switching lifetime
measurements with 1 V indicates a life of about 107 cycles
with the lifetime limited by erosion of the drain metal.
The on-response time is approximately 5 μs, and is
limited by squeeze-film damping of the cantilever beam.
The off-response time is much smaller, and is limited by
parasitics in our measurement circuit.
Another characteristic, which is important for some
low-frequency applications, is the "standoff" voltage,
or the maximum voltage that can be applied across an
open switch without damaging it. A voltage across the
open microswitch sets up a field in the gap between the
drain and the end of the beam. This can cause the switch
to fail in two ways, either by causing the beam to deflect
and closing the switch, or by causing an electrical breakdown
of the gap. In our current designs, the failure is due to beam
deflection, and occurs at about 150 V.
Microswitches are interesting candidates as replacements
for low-power mechanical relays (generally reed relays),
particularly in applications utilizing a large matrix of relays,
often for multiplexing. Examples of such applications are
pin electronics in Automated Test Equipment, transducer
arrays in medical ultrasound, and sensor and data acquisition
applications. Microswitches can bring significant savings in
size and power consumption in such applications, and may
also provide longer life. For example, a 8x8 array of reed
relays each 2 cm x1 cm in size would occupy 250 cm2, the
same array using microswitches would consume 1 cm2.
Some applications may require an improvement of the
stand-off voltage limit, and the hotswitched power handling
capability. Certain applications may require a true 4-terminal
relay, in which the signal path does not share a terminal with
the actuation circuit. Prototypes of such devices have been
RF Performance and Applications
The benefits and applications for MEMS RF switches as
fundamental building blocks, supplanting PIN diode and
FET RF switches are numerous because MEMS switches
combine the best features of both, having the low control
power requirements of FETs, but having "on" resistances
(and RF insertion losses) lower than PIN diodes. Furthermore,
MEMS switches have lower off-state capacitance and,
as a result, better off-state RF isolation than either
FETs or PIN diodes, and, in addition, have inherently high
RF linearity. Intended applications include microwave
switches that replace PIN diode and FET switches, while
providing lower insertion loss, higher isolation, higher
linearity, higher radiation resistance, superior tolerance for
high temperature environments, and lower prime power
consumption. Examples of such applications include T/R
switches in a variety of products such as cellular handsets
and base stations, phase shifters for Electronically Steerable
Antennas, tunable filters and reconfigurable antennas, to
name a few.
One application of current interest is the cellular telephone
market. Here, the frequency of interest is approximately
2 GHz. Measured performance of our wafer-level capped
MEMS switches are very good at this frequency, with
an insertion loss of 0.3 dB and isolation of 30 dB for a
single element, series, SPST switch in a 50 mil sq. capped
die package similar to that of Figure 3.
This RF insertion loss is several tenths of a dB better than
most current PIN diode, MESFET, and PHEMT switches
with comparable isolation at this frequency. By combining
series and shunt MEMS switches within the same capped
package, substantially higher isolation should be attainable
in the future, with little degradation in insertion loss. Future
plans also include capped SP2T, SP4T, and transfer switches.
This reflective loss will be mostly eliminated in the future
by reducing switch inductance. Efforts are currently underway
to increase the off-state switch isolation by modifying the switch
geometry and to decrease the on-state insertion loss by identifying
and minimizing the various loss mechanisms.
We have developed a surface micromachined microswitch which
can be used in applications from DC through microwave.
We have also developed a wafer scale package, and reported
results in this package up to 4 GHz. Key characteristics for a
single element, series, SPST switch are; at 4 GHz, insertion loss
of 0.4 dB and isolation of 27 dB.
Results from DC to low frequencies include a total
on-resistance, including interconnects, of less than 1 Ω,
cold switch lifetimes exceeding 1010 cycles, and hot
switch lifetimes exceeding 107 cycles. Additional testing is
underway along with efforts to further increase the lifetime
of these devices. Applications for our microswitch include
T/R switches in a variety of products such as cellular
handsets and base stations, phase shifters for Electronically
Steerable Antennas, tunable filters and reconfigurable
antennas, pin electronics for Automated Test Equipment,
transducer arrays in medical ultrasound, and sensor and
data acquisition applications.
Realizado: Franco A Rivera C.
Publicado por Tecnología en Telecomunicaciones - conocimientos.com.ve en 12:48
Etiquetas: Franco A Rivera C.