4A1.4
RF MEMS SWITCHES: STATUS OF THE TECHNOLOGY
Gabriel M. Rebeiz
EECS Department
The University of Michigan
Ann Arbor, MI 48109-2122
Tel: (734) 647-1793, Fax: (734) 647-2106, Email: rebeiz@umich.edu
ABSTRACT
Pull-down
electrode
This paper presents the latest accomplishments in
RF MEMS switches, and at the same time, an assessment of their potential applications in defense and
commercial systems. It is seen that RF MEMS devices offer spectacular performance at microwave frequencies, but suffer from reliability problems and the
potential of relatively high-cost hermetic packaging.
Still, this technology offers such tremendous advantages over GaAs and silicon switching devices that, in
the author’s opinion, it will find many applications in
satellite, base-station and defense applications, particularly at high microwave frequencies.
Cantilever
Anchor
Contact
areas
(a)
PROS AND CONS OF RF MEMS
SWITCHES
MEMS switches are surface-micromachined devices
which use a mechanical movement to achieve a short
circuit or an open circuit in the RF transmission-line
(Figs. 1-2). RF MEMS switches are the specific micromechanical switches which are designed to operate at
RF to mm-wave frequencies (0.1 to 100 GHz). The
advantages of MEMS switches over PIN diode or FET
switches are [1]:
Near-Zero Power Consumption: Electrostatic actuation requires 30-80 V, but does not consume any
current, leading to a very low power dissipation (10100 nJ per switching cycles). On the other hand, thermal/magnetic switches consume a lot of current unless
they are made to latch in the down-state position once
actuated.
Very High Isolation: RF MEMS metal-contact
switches are fabricated with air gaps, and therefore,
have very low off-state capacitances (2-4 fF) resulting
in excellent isolation at 0.1-60 GHz. Also, capacitive
switches with a capacitance ratio of 60-160 provide
excellent isolation from 8-100 GHz.
Very Low Insertion Loss: RF MEMS metal-contact
and capacitive switches have an insertion loss of 0.1 dB
up to 100 GHz.
Linearity and Intermodulation Products: MEMS
switches are extremely linear devices and therefore re-
(b)
Fig. 1. The metal-contact Analog Devices (a) and Rockwell Scientific switches (b).
sult in very low intermodulation products in switching
and tuning operations. Their performance is 30-50 dB
better than PIN or FET switches.
Potential for Low Cost: RF MEMS switches are
fabricated using surface micromachining techniques
and can be built on quartz, Pyrex, LTCC, mechanicalgrade high-resistivity silicon or GaAs substrates.
RF MEMS switches also have their share of problems, and these are:
Relatively Low Speeds: The switching speed of
most electrostatic MEMS switches is 2-40 µs, and
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Anchor
Contact
area
45
µm
Pull-down
electrode
(a)
(b)
Fig. 2.
The capacitive-contact Lincoln Labs (a), and
Raytheon switches (b).
thermal/magnetic switches are 200-3, 000 µs. Certain
communication and radar systems require much faster
switches.
High Voltage or High Current Drive: Electrostatic
MEMS switches require 30-80 V for reliable operation,
and this requires a voltage up-converter chip when
used in portable telecommunication systems. Thermal/magnetic switches can be actuated using 2-5 V,
but require 10-100 mA of actuation current.
Power Handling: Most MEMS switches cannot
handle more than 200 mW although some switches
have shown up to 500 mW power handling (Terravicta and Raytheon). MEMS switches that handle 1-10
W with high reliability simply do not exist today.
Reliability: The reliability of mature MEMS
switches is 0.1-40 Billion cycles. However, many sys-
tems require switches with 20-200 Billion cycles. Also,
the long term reliability (years) has not yet been addressed. It is now well known that the capacitive
switches are limited by the dielectric charging which
occurs in the actuation electrode, while the metalcontact switches are limited by the interface problems
between the contact metals, which could be severe
under low contact forces (in electrostatic designs, the
contact forces are around 40-100 µN per contact).
It is important to note that the reliability and packaging issues have been the limiting factors to the quick
deployment of RF MEMS switches, and they are currently under intense investigations. DARPA has initiated two programs in 2002 and 2003 to address these
problems, the RF MEMS Improvement program (Dr.
Larry Corey), and the HERMIT program (Dr. Clark
Nguyen), and it is expected that some of these problems will be solved in the coming 2-3 years.
Packaging: MEMS switches need to be packaged
in inert atmospheres (Nitrogen, Argon, etc..) and
in very low humidity, resulting in hermetic or nearhermetic seals. Hermetic packaging costs are currently relatively high, and the packaging technique itself may adversely affect the reliability of the MEMS
switch. Microassembly (Fig. 3) and Analog Devices
have both developed excellent packages for RF MEMS
switches. The Microassembly package is based on
gold-to-gold thermo-compression at 250◦ C while the
Analog Devices package is based on glass-to-glass seal
at 400−450◦ C. Other companies which have packaged
switches are Terravicta (ceramic package) and Omron
(glass-to-glass).
Cost: While MEMS switches have the potential of
very low cost manufacturing, one must add the cost
of the packaging and the high-voltage drive chip. It is
therefore hard to beat a $0.3-0.6 single-pole doublethrow 3 V PIN or FET switch, tested, packaged and
delivered. It is for this reason that Prof. Rebeiz believes that RF MEMS switches will be first used in
defense and high-value commercial applications and
not in cellular phones.
DETAILED DISCUSSION OF MEMS
SWITCHES
Actuation Mechanisms: The actuation forces required for the mechanical movement can be obtained
using electrostatic, magneto-static, piezoelectric or
thermal designs. To date, only electrostatic-type
switches have been demonstrated at 0.1-100 GHz with
high reliability at low RF powers for metal contact
and medium power levels for capacitive contacts (100
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Million to 50 Billion cycles depending on the manufacturer) and wafer-scale manufacturing techniques.
Other switches which have demonstrated excellent
performance are the Microlab Latching switch (up to
100 Million cycles) using magnetic actuation, and the
thermal switches developed independently by Cronos
Microsystems and the Univ. of California, Davis. It
is hard to test thermal switches for long cycle times
due to their slow switching response (1-3 ms).
used.
Circuit and Substrate Configurations: As is the
case with all two-terminal devices, the switches can
be placed in series or in shunt across a transmission
line. Typically, capacitive switches have been used in
a shunt configuration, while DC-contact switches are
placed in series. The reason is that it is easier to get
a good isolation with a limited impedance ratio (such
as the capacitive switch) in a shunt-circuit than in a
series circuit. Also, MEMS switches are compatible
with both microstrip and CPW lines on glass, silicon
and GaAs substrates, and have been used in these
configurations all the way to 100 GHz. For low loss
applications at microwave frequencies, it is important
to use high-resistivity substrates.
CIRCUITS WITH RF MEMS SWITCHES
Fig. 3. An SPDT switch packaged using a gold-to-gold
seal ring (courtesy of Microassembly, Inc.). The top
cover is taken off so as to show the seal ring [2].
Switching Time: Electrostatic switches can be
made small and with a very fast switching time
(2-30 µs) while thermal/magnetic actuation requires
around 100-2, 000 µs of switching time. An excellent
metal-contact switch developed by LETI using thermal actuation but with an electrostatic hold, thereby
requiring very little switching energy and virtually
zero hold-down power. However, its switching time
is still relatively slow (300 µs). The LETI switch has
been tested to more than 100 million cycles.
Contact Type: There are two different contacts in
RF MEMS switches, a capacitive contact and a metalto-metal (or DC) contact. The capacitive contact is
characterized by the capacitance ratio between the
up-state (open circuit) and down-state (short-circuit)
positions, and this is typically 80-160 depending on
the design. The down-state capacitance is typically
2-3 pF, and is suitable for 8-100 GHz applications. In
general, it is hard to obtain a large down-state capacitance using nitride or oxide layers, and this limits
the low-frequency operation of the device. On the
other hand, DC-contact switches with small up-state
capacitances (open circuit) can operate from 0.01 to
40 GHz, and in some cases, to 60 GHz (for example,
the Rockwell Scientific switch has an up-state capacitance of only 1.75 fF and an isolation of 23 dB at
60 GHz). In the down-state position (short-circuit),
the DC-contact switch becomes a series resistor with
a resistance of 0.5-2 Ω, depending on the contact metal
The near-ideal electrical response of RF MEMS
switches (both metal-contact and capacitive) have allowed many designers to build state-of-the-art switching circuits from 0.1 GHz all the way to 120 GHz.
In the past 4 years, these applications concentrated
on the replacement of GaAs phase shifters which are
commonly used in phased arrays by the thousands
of units. A comparison between 3-bit GaAs phase
shifters and MEMS phase shifters is shown in Table
I and it is seen that MEMS switches provide an immense performance benefit especially at Ka-Band to
W-band applications.
TABLE I
Average on-wafer loss for RF MEMS and GaAs-FET
3-bit phase shifters.
Freq. (GHz)
X-Band (10)
Ku-Band(20)
Ka-Band (35)
V-Band (60)
W-Band (94)
Loss RF MEMS (dB)
0.3/bit
0.45/bit
0.6/bit
0.8/bit
0.9/bit
Loss GaAs FET (dB)
1.2/bit
1.6/bit
2.3/bit
2.8/bit
3.3/bit
Fig. 4 presents a 4-bit miniature RF MEMS phase
shifter developed jointly by the Univ. of Michigan and
Rockwell Scientific. It is based on the Rockwell metalcontact switch and on CLC delay lines for miniaturization. The phase shifter results in an average loss of
1.4 dB at 10 GHz, a ±3◦ phase error, and is matched to
−13 dB at the input and output ports from 6-16 GHz.
This phase shifter represents the smaller design using
RF MEMS to-date, and with excellent response.
Fig. 5 presents an 885-986 MHz 5-pole tunable
filter using switched MEMS capacitors developed
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85Ω
45o-bit
90o-bit
feed 4 switches 8 switches
180o-bit
16 switches
85Ω
feed
1.92mm
Reference
Plane
5.04mm
Reference
Plane
Fig. 6. The 3-bit true-time delay distributed MEMS phase
shifter at 77-100 GHz. The size is 1.9 × 5 mm2 .
Fig. 4. The 4-bit miniature X-band phase shifter developed
by the Univ. of Michigan and Rockwell Scientific. The
size is 3.2 × 2.1 mm2 .
Fig. 5.
An 885-986 MHz 5-pole tunable filter using
switched MEMS capacitors developed by Raytheon
Systems Co. The size is 3.5 × 14 mm2 .
by Raytheon Systems Co. In this case, capacitive switches are used to switch fixed-value metalinsulator-metal capacitors in the transmission line.
The filter employs 18 switches and is a very complicated circuit with variable resonators and impedance
inverters. Its measured response is nearly ideal, with
excellent frequency tuning capabilities, very high linearity (in terms of measured IIP3) and a loss of 56 dB due to the finite Q of the planar inductors used
(Q = 30 at 0.9 GHz).
Fig. 6 presents a W-band 3-bit phase shifter developed at the Univ. of Michigan using MEMS capacitive switches [3]. This is the highest frequency MEMS
phase shifter to-date and results in an average loss of
2.7-2.9 dB at 77-94 GHz with an associated phase error of ±3◦ . The results are about 8 dB better than
GaAs designs.
Other circuits, which are not shown due to space
constraints, are very wideband SP4T switches, highisolation series/shunt switches covering 0.1-50 GHz,
double-pole double-throw transfer switches, and a
whole range of phase shifters from 8 GHz to 120 GHz.
Also, tunable filters covering 200 MHz to 23 GHz have
been developed by various groups. In general, RF
MEMS circuits outperform GaAs FET and PIN diode
circuits by a large margin at all frequencies of interest
to the RF and microwave communities. Most of the
circuits developed in the world can be found in [1].
THE FUTURE
It is now clear that we understand RF MEMS
switches well, both from the mechanical and electrical/electromagnetic point of view. We can design
complicated circuits using MEMS switches or varactors, and we can accurately predict their performance
all the way to 120 GHz. They are still not accepted
in the commercial and defense arena due to their
need of a hermetic package, and their reliability under
medium to high-power conditions. There is currently
an intense effort to solve these problems, and the author believes that RF MEMS switches and varactors
will play an essential role in future high-value commercial and defense systems.
References
[1]
[2]
[3]
Gabriel M. Rebeiz, RF MEMS: THEORY, DESIGN, AND
TECHNOLOGY, Wiley, January 2003.
Dr. Michael Cohn, www.microassembly.com.
J.J. Hung, L. Dussopt and G.M. Rebeiz, “A Low-Loss Distributed 2-Bit W-Band MEMS Phase Shifter,“ submitted
for the publication to the 2003 European Microwave Conference.
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