11
CHAPTER 2
RF MEMS BASICS
This chapter provides the basic introduction to RF MEMS switches. RF MEMS have
in general seen a remarkable growth in the past two decades due to the immense potentials
in defense and commercial applications. The major part of this chapter is committed to the
comparison of the RF MEMS switches with state of the art solid state switches. RF MEMS
switches reported over last decade, applications and challenges related to the reliability of
the devices are also discussed.
2.1 Switches for Microwave Applications
Currently in microwave industry, mechanical and semiconductor switches are being
used. The main use of RF or microwave switches is in signal routing and impedance
matching [1]. Telecommunication applications cover a broad range of frequencies, ranging
from MHz to radio frequencies (GHz). This broad spectrum requires variety of RF switches
for different frequency bands. Also, personalized use of hand held electronic devices
emphasize more on the scalability or downsizing of devices with high performance. The
selection of the switch is largely governed by frequency and speed of the application under
consideration, e.g. in some applications high power handling is required, thus a mechanical
switch can be used for this type of requirement; in some areas very high speed is required
with moderate isolation and low insertion loss and power handling is not an issue, thus PIN
diodes or FETs can be used for this purpose; in some applications very high isolation with
very low insertion loss is required with moderate speed and hence MEMS switches are best
candidates to fulfill this criteria. Different types of microwave switches exist in the market,
everyone with its own set of pros and cons. In the case of silicon FETs, they can handle high
power at low frequencies, but the performance drops significantly with increase in
frequency. Whereas in the case of GaAs MESFETs and PIN diodes, the high-frequency
operation is quite well with small signal amplitudes or briefly it can be summed up that, at
high frequencies, the solid state switches have high insertion loss and poor isolation. Thus,
mechanical coaxial and waveguide switches offer the advantage of low insertion loss, high
12
isolation, large power handling and high linearity, but are heavy and slow. On the other hand
semiconductor switches provide faster switching speed and are smaller in size, but have
inferior performance than mechanical coaxial switches. For above problem, the best remedy
can be the use of MEMS technology for RF applications. MEMS technology has its own set
of advantages to be used for RF applications. MEMS switches provide the advantages of
both mechanical and semiconductor switches. They provide high isolation and low insertion
loss with almost zero DC power consumption with a small size and low weight as discussed
in further sections of this chapter.
2.2 RF MEMS
Micro-electro-mechanical systems (MEMS) have been developed since 1970s for
different applications, e.g. pressure sensor, accelerometers, temperature sensor & other
sensor devices. In view of the fact demonstrated by Peterson in 1979 [2], that a bulk
micromachined cantilever can be used as a switching element, the standard device of the RF
MEMS ‘the switch’, is the first and the most studied in this field. MEMS switches for low
frequency applications have been demonstrated in the early 1980s but remained a laboratory
curiosity for a long time. The first MEMS switch designed for RF applications was reported
in 1990 by Larson [3], and its results were so outstanding that afterwards several groups like
Texas Instruments, Rockwell Science Centre, Raytheon, LG, etc. start research in this field
[4]. RF MEMS devices which work as basic building blocks for any RF system are: RF
MEMS switches, high Q inductors, filters & resonators and tunable capacitors or varactors.
2.2.1 RF MEMS Switch
In a variety of applications, high frequency switches are essential components, e.g.
mobile phones, wireless local networks, radars and satellites etc. The thrust for RF MEMS
switch applications in communication has been mainly due to the highly linear
characteristics of the switches over a wide range of frequencies. The MEMS devices offer
better isolation (>30 dB) and low insertion loss (<0.15 dB) compared to the contemporary
solid state devices. With high levels of integration, negligible current, low power
consumption and improved overall performance, RF switches are preferred for space, air
borne and hand held communication applications. Phase shifters, switch matrices, receivers
13
and transmitter sections are some of the applications being developed using MEMS
switches. Like any other switch, a MEMS switch has two stable states, ‘ON’ and ‘OFF’.
Switching between these two states can be achieved through the movement of a free moving
armature, moved using different types of actuation mechanisms; e.g, electrostatic,
piezoelectric, thermal or magnetic actuation. Electrostatic actuation is the most popular
because of its low power consumption, small electrode area and relatively short switching
time [1]. The other advantages of using electrostatic actuation are low fabrication
complexity, possibility of biasing the switch using high resistance bias lines and easy
integration with existing fabrication technology, with coplanar waveguide (CPW) &
microstrip lines. In MEMS switches, there is a mechanical armature, whose movement
decides the working of the switch. This movement can be vertical or lateral. In a
RF Switch
Series
Shunt
Capacitive type
Ohmic type
Capacitive type
Ohmic type
Lateral Movement
Vertical Movement
Lateral Movement
Vertical Movement
Lateral Movement
Vertical Movement
Lateral Movement
Vertical Movement
Electrostatic ActuationMagnetostaic ActuationPiezoelectric ActuationThermal actuation
Electrostatic ActuationMagnetostaic ActuationPiezoelectric ActuationThermal actuationElectrostatic ActuationMagnetostaic ActuationPiezoelectric ActuationThermal actuationElectrostatic ActuationMagnetostaic ActuationPiezoelectric ActuationThermal actuationElectrostatic ActuationMagnetostaic ActuationPiezoelectric ActuationThermal actuation
Electrostatic ActuationMagnetostaic ActuationPiezoelectric ActuationThermal actuationElectrostatic ActuationMagnetostaic ActuationPiezoelectric ActuationThermal actuation
Electrostatic ActuationMagnetostaic ActuationPiezoelectric ActuationThermal actuation
2x2x2x4=32
Fig. 2.1: Configuration tree for RF MEMS switches. RF MEMS switch can be implemented in
32 different configurations.
14
transmission line a MEMS switch can be placed in series or in shunt configuration. Also,
MEMS switches can be classified according to the contact type between the armature and
the transmission line. The contact can be a metal to metal type (Ohmic switches) or
capacitive type (Capacitive switches). Ohmic switches are good at low frequencies (<10
GHz), whereas at high frequencies capacitive switches show better performance. Overall
there are 32 different configurations of RF MEMS switches depending upon the actuation
mechanism, contact type, armature movement and circuit implementations. Fig. 2.1 shows
the configuration tree for RF MEMS switches. The main performance characteristics of RF
MEMS switches are high isolation in off-state, low insertion loss in on-state, return loss in
G
S D
18 nm
ON-state OFF-state
RON ≈ 5 Ω COFF ≈ 45ff
Physical
Gap of 3 µm
ON-state OFF-state
RON ≈ 0.5 Ω COFF ≈ 0.25ff
G
S
D
S
D
S
D
OUT
IN
OUT
IN
OUT
IN
IN OUT
Semiconductor Switch MEMS Switch
Isd
VsdLinearity
Icon
VconLinearity
Fig. 2.2: Comparison between semiconductor and MEMS switches.
15
both states, power handling capability, low power consumption and linearity. Fig. 2.2 shows
the comparison between semiconductor switches and MEMS switching devices. In on-state,
semiconductor switches have significant resistance (5 Ω) between source and drain, leading
to high insertion loss; whereas, in MEMS switches there is a direct metal to metal contact
resulting in minimum resistance (<0.5 Ω), leading to very low insertion loss [4]. The low
insertion loss in MEMS switches thus eliminates the requirement for amplifiers, etc. to
compensate for signal loss due to switching elements. In solid-state switches, off state is
poorer either because of the leakage current or the large parasitic capacitance between the
source and drain. In the case of MEMS switches, higher isolation can be achieved because
of the large physical gap between the bridge and the transmission line. As compared to the
state of the art semiconductor switches and PIN diodes or FETs, MEMS switches have many
advantages, such as [4]:
1. Nearly zero power consumption: Electrostatic actuation does not consume any
current leading to very low power dissipation (10-100nj per switching cycle).
2. Very low insertion loss: RF MEMS switches have insertion loss of -0.1dB up-to 40
GHz.
3. Very high isolation: RF MEMS switches are fabricated with air gaps, and hence they
Parameter MEMS Switches PIN FET
Voltage (V) 10 - 80 3 - 5 3 – 5
Current (mA) 0 3 - 20 0
Power Consumption (mW) 0.05 – 0.1 5 - 100 0.05 – 0.1
Switching Time 1 – 300 µs 1 – 100 ns 1 – 100 ns
Isolation (1-10 GHz) Very High High Medium
Isolation (10 – 60 GHz) Very High Medium Low
Isolation (60 – 100 GHz) High Medium None
Insertion Loss (1 – 100 GHz) (dB) 0.05 – 0.2 0.3 – 1.2 0.4 – 2.5
Power Handling (W) < 1 < 10 < 10
Cost (US$) 8 - 20 0.9 - 8 0.45 - 5
Life Time (cycles) > 109 > 109 > 109
Table 2.1: Comparison of MEMS switches with solid state devices.
16
have very low off state capacitance providing excellent isolation (capacitive
switches).
4. Very low cost: RF MEMS switches are fabricated using surface micromachining
techniques and batch fabrication techniques and can be built on LTCC, Si, GaAs or
quartz substrates.
5. Low intermodulation products: MEMS switches are very linear devices, and
therefore results in very low intermodulation distortion and no measurable
harmonics.
Table 2.1 summarizes the performance comparison of the MEMS switches with PIN
diodes and FETs. Semiconductor switches provide the desired performance in terms of
switching speed and have low cost, but present power constraints and significant loss at high
frequencies makes them less preferable than MEMS switches.
2.2.2 Figure of Merit
In semiconductor switches, with direct current, off-state to on-state resistance ratio
can be used to characterize a switch. At microwave frequencies the off-state is determined
by the capacitance and ratio of impedance is given be [5]
The product of C and R is termed as FOM. A small FOM is better as it means a small on
state impedance as compared to the off-state. In capacitive type of switches, performance
characterization can be done on the basis of off-on capacitance ratio. For capacitive MEMS
switch, FOM is given by
where, εr is the dielectric constant of the dielectric material, go is the air gap and tdiel is the
dielectric layer thickness between the capacitors. For capacitive type of switches a high
FOM is better as it means down-state capacitance should be as high as possible and up-state
capacitance should be as small as possible. FOM for capacitive type of switches can be
improved by choosing high-k dielectric material and scaling down the thickness of the
dielectric layer as discussed in Chapter 4.
17
2.2.3 RF MEMS Switch Contact Configurations
As shown in Fig. 2.1, RF MEMS switch can be broadly classified on the basis of
contact mechanism (e.g. ohmic and capacitive contact), armature movement (e.g. vertical or
lateral), placement of armature with respect to transmission line (e.g. series or shunt) and
actuation mechanisms (e.g. electrostatic, piezoelectric, magnetic or thermal) [6, 7]. In ohmic
contact type switches, also known as metal to metal contact switches, there is a direct
contact (resistive contact) between the movable membrane and the transmission line (Fig.
2.3). Such type of switches are useful from DC to 8 GHz. The reliability of ohmic contact
type switches is largely determined by the metal to metal contact. Fig. 2.4 illustrates a
capacitive type of RF MEMS switch. The performance of a capacitive contact type switch
depends on the thickness and roughness of the dielectric layer and the gap between the
membrane and the transmission line. The capacitance change between the up-state and
down-state decides the transmission of the signal. The capacitance ratio (Cdown/Cup) is the
key parameter as discussed. A high capacitance ratio is always desirable. Because of the
coupling nature, MEMS capacitive switches are not suitable for low frequency applications.
Anchor
Pull down electrode
Switch contacts
RF-in RF-out
Front viewSide view
Signal line
Anchor
Dielectric
Gap
Signal Line
Anchor
Dielectric
CPW Ground CPW Ground CPW Ground CPW Ground
Up-state Down-state
Metal Bridge
Metal Bridge
Fig. 2.4: RF MEMS capacitive contact type switch in up-state and down-state. In up-state
signal passes from input port to output port, whereas in down state, signal couples to the
ground through the capacitive contact between the transmission line and the metal bridge.
Fig. 2.3: Electrostatically actuated metal to metal contact type MEMS switch.
18
Compared to the ohmic type switches, lifetime is not an issue due to capacitive contact,
however, the reliability is undermined due to the dielectric charging.
2.2.3.1 Series and Shunt implementation
From the application perspective, the MEMS switches are further classified as series
or shunt switches. The series and shunt configuration is determined by the position of the
metal armature with respect to the transmission line. Both the ohmic contact type and
capacitive contact type switches can be used in series or shunt configurations. But, generally
ohmic switches are used in series and capacitive switches are used in shunt configurations.
Switches can further be classified as in-line and broadside switches. In an inline switch, the
armature is an integral part of the transmission line (Fig. 2.5 (a)), whereas in broadside
switches the armature is placed perpendicular to the transmission line and is connected to
the CPW ground planes (Fig. 2.5 (b)). A basic capacitive shunt switch consists of a movable
metal bridge anchored to the ground plane of the CPW as shown in Fig. 2.5 (b). When zero
bias voltage is applied at the actuation electrodes, the membrane is at a gap from the
transmission line and the RF signal transmits through the signal line. In this stage, the small
overlap capacitance (femto-farads) results in signal loss termed as transmission loss and is
given by S21. When actuation voltage is applied at the inner electrodes, due to electrostatic
force developed between the membrane and the actuation electrode, the movable bridge
moves down and make a capacitive contact with the dielectric over the transmission line.
CPW
GND
CPW
GND
Membrane
Signal
Line
Membrane
Signal LineActuation Electrode Actuation Electrode
Dielectric
ground ground
(a) (b)
Fig. 2.5: (a) Top view of in-line RF MEMS switch, (b) Cross-sectional view of broadside
RF MEMS switch.
19
The high down state capacitance couples the RF signal to the ground and thus isolating the
output port from the input port. In on or off state the switch can be modeled as shown in the
Fig. 2.6 (a) and Fig. 2.6 (b) respectively.
2.2.3.2 Ohmic Series switch
As shown in Fig. 2.3 front view, in ohmic series switch, the signal is interrupted by a break
in the t-line. The movable armature could be a metallic membrane or a dielectric membrane
with metallic contacts. Under no bias condition, the armature is in up position and there is a
break in the transmission line; the switch is in off-state. When actuation voltage is applied at
the actuation electrode the armature snaps down due to electrostatic force and the signal line
gets connected through armature. This is the on-state of the switch. To keep the insertion
loss as low as possible the on-state resistance of contacts should be as small as possible.
2.2.4 Other RF MEMS Components
2.2.4.1 MEMS Capacitors
There are many broadband applications with specific design requirements in which
the capacitor controls the critical electrical parameters. They include low-noise amplifiers,
harmonic frequency generators and frequency controllers [8, 9]. Many of the modern
wireless system constraints requirement of high quality, low phase noise, stable operation
with wide tuning range voltage controlled oscillators (VCOs). The tuning range of these
VCOs must be large enough to cover the entire frequency band of interest. These tunable
capacitors or varactors are electronically controlled for the desired operation. Semiconductor
Rs
Cu
L
Z0 Z0
Cd
Z0 Z0
(a) (b)
Fig. 2.6:Equivalent circuit of RF MEMS capacitive switch in (a) on-state and (b) off-state.
20
on-chip varactors or MOS capacitors suffers from excessive series resistance and non-
linearity [10]. RF MEMS varactors on the other hand use highly conducting thick metal
layers, with air as a dielectric, thus offering substantial improvement over conventional on-
chip varactor diodes in terms of power loss. In addition, the RF MEMS capacitors have
excellent linearity, wide tuning range and ability to separate the control circuitry from the
signal circuit, which greatly simplifies the overall design. The tuning of the capacitance can
be achieved by three different ways; (a) tuning the dielectric constant (b) by tuning the air
gap and (c) by tuning the overlap area. The first method can be implemented by considering
the dielectric material, whose dielectric constant changes with the change in boundary
conditions e.g. dielectric constant of BST changes with change in temperature. In the later
two cases the air gap between the two plates or overlap area can be changed by electrostatic
forces and capacitance can be linearly changed with the application of applied voltage.
The principle of gap tuning of the variable capacitors and varactors is similar to the
RF MEMS capacitive switches as shown in Fig.2.5 (b). The plate or membrane is suspended
with the anchors to the CPW ground plane. When actuation voltage is applied at the
actuation electrodes, the membrane starts moving down due to electrostatic force developed
between the two plates. This operation can be linearly controlled by changing the
dimensions of the membrane. The down state capacitance is determined by the dielectric
layer over the transmission line and the overlap area. As the substrate is high resistivity
(a) (c)
(b)
(d)
Fig. 2.7: (a) Top view of variable capacitor, suspended at four anchor points. (b) & (c)
electrical equivalent of single electrode capacitor and variable capacitor with electrodes
on top and bottom of the suspended membrane, (d) schematic of MEMS inductor coil
suspended at a height of 5 µm from ground.
21
substrate and metal lines are fabricated over that dielectric substrate, the effect of parasitic
capacitances is almost negligible. Fig. 2.7 (a) shows the top view of variable capacitor
suspended over the actuation electrode. Fig. 2.7 (b) and (c) shows the electrical equivalent
of a variable capacitor with single side actuation electrode and wide-range variable capacitor
with actuation electrodes on both bottom and top side of the movable membrane.
2.2.4.2 MEMS Inductors
RF inductors are needed in any wireless front-end circuitry; the performance of both
transceivers and receivers depend heavily on this component. The key parameters that
characterize the performance of inductors are the quality factor Q, inductance L and self
resonance frequency. The Q-factor is an important characteristic for inductors and
determines the energy dissipation in the inductors; high Q implies low energy dissipation.
The quality factor for planar spiral inductors and junction diode capacitors are only of the
order of low 10s at higher frequencies and hence alternative off-chip technologies including
inductors and tunable capacitors are often used for high Q applications. High-Q inductors
reduce the phase noise and the power consumption of Voltage Controlled Oscillators
(VCO's) and amplifiers and reduce the return loss of matching networks and filters. The
quality factor of inductors can be increased by using a thick metal layer and by isolating the
inductor from the substrate. To isolate the inductor, bulk micromachining or self-assembly
can be used. Further, tunable inductors allow for performance optimization of RF front-end
circuits. Most of the reported MEMS inductors are static fixed value inductors. Few
published papers report use of MEMS switches as variable inductors [11]. However, it
provides only discrete values of inductance depending on the ON/OFF configuration of the
switches. A tunable inductor using self-assembly technique has been reported [12]. The
main disadvantage of using these type of inductors is that, they are suspended on the
substrate, thus becoming prone to the electromagnetic interference in the transceiver system.
The demand for fully integrated planar inductors and capacitors for the realization of MEMS
and monolithic microwave integrated circuits (MMICs) is growing steadily. Conventional
inductive components are inherently three-dimensional (3D) in nature and the
implementation of these components in planar shape is quite challenging. Small size and
weight, low power consumption, mass production, reliability and reproducibility are some of
22
the numerous advantages of integration of MICs with MEMS [13]. MEMS technology
improves the on-chip inductor performance by etching away the sacrificial layer (spacer or
lossy substrate) underneath the spiral inductor, resulting in a hanging membrane or
suspended inductor coil. Fig. 2.7 (d) shows the schematic of spiral MEMS inductor,
suspended at a height of 5 µm from the ground plane.
2.2.5 General Fabrication Process for RF MEMS Capacitive Switches
In general RF MEMS capacitive switches are fabricated using surface
micromachining techniques. Though designing and fabricating a RF MEMS switch with
microstrip configuration results in smaller size, the CPW configuration results in easy
fabrication process. The choice of materials and fabrication process design depends on the
specifications of the device. Most of the reported switches, are fabricated using a five mask
level process, excluding packaging. During the material selection for the fabrication, the
overall thermal budget and the material etching at different phases are the general
> 5 kΩ Thermal Oxide
Signal Line
Actuation Electrodes
GroundGround
(a)
(b)
(c)
(d)
(e)
(f)
Dielectric
Spacer
Electroplated Bridge
Photoresist mould
Final released switch
Fig. 2.8: (a) - (f): General process steps for RF MEMS capacitive switch.
-m
23
compatibility issues for process design. For ohmic contact type switches, the contact
material and the thickness of the contacts are the key parameters which will decide the
lifetime of the switch. Below are the summarized, general fabrication steps for RF MEMS
capacitive switch fabrication (Fig. 2.8):
a. For RF MEMS switches, a high resistivity substrate (> 5kΩ-m) is the starting
material. A thermal oxide layer is generally preferred over the Si substrate [3, 14,
15].
b. The actuation electrodes, transmission line and ground planes of CPW are fabricated
in metal layers and patterned accordingly.
c. A thin dielectric layer is deposited and patterned. This layer acts as a dielectric layer
for the capacitive type of switch.
d. A sacrificial layer or spacer layer, which could be a metal layer or a photo-resist is
deposited and patterned to obtain the gap between the transmission line and the
suspended membrane.
e. After spacer patterning, a thin seed layer is generally deposited followed by a
photoresist mould formation using thick photoresist lithography for suspended
membrane formation.
f. Suspended membrane is fabricated by electroplating process, followed by a release
process. The release process can be dry plasma ashing or wet etching process,
depending upon the spacer or sacrificial layer.
2.3 Existing RF MEMS Switches (MEMS Switch Library)
Following section includes some RF MEMS switches, which were developed by
industry, university and government laboratories. As discussed in former section, almost all
of the MEMS switches are fabricated using surface micromachining techniques on a high
resistivity substrate, which can be Si, GaAs or quartz [5].
2.3.1. Raytheon Capacitive MEMS Shunt Switch
The Raytheon shunt switch also known as Texas Instruments Switch, was developed
by Chuck Goldsmith and his co-workers in 1995 - 2000 [14, 16] . The device is a capacitive
type of MEMS switch with 1000Å of Si3N4 as a dielectric layer. The bridge membrane is
composed of 0.5 µm of aluminum that is suspended 3 - 5µm above the transmission line.
24
The device length and width are 270 - 350 µm and 50 - 200 µm respectively. Thick
polyimide used as sacrificial layer is released using a plasma etching technique. A
capacitance ratio of 80 - 120 has been achieved for actuation voltages of 30 - 50 volts. The
switching time is 3µs/5µs (Down/Up) providing an isolation and insertion loss of -35 dB (at
30 GHz) and -0.07 dB (10 - 40 GHz) respectively.
2.3.2. University of Michigan Capacitive MEMS Shunt Switches
The university of Michigan has developed a novel low-voltage MEMS capacitive
shunt switch with Nickel suspended membrane [17]. The membrane is suspended using
meander type support structures, which results in a low spring constant and hence low
actuation voltage for the switch. For the length of 500 - 700 µm, width of 200 - 250 µm and
a thickness of 2 - 2.5 µm, actuation voltage between 6 - 20 Volts can be achieved for a gap
height of 4 - 5 µm. The dielectric layer used is Si3N4 having a thickness of 1000 - 1500Å.
The down state switching time is between 20 - 40 µs with a capacitance ration of 30 - 50.
The switch show an isolation of -25 dB (at 30 GHz)in off-state and an insertion loss of -0.1
dB (1 - 40 GHz). Due to the low spring constant, the switch is vulnerable to external
mechanical forces such as acceleration and vibrations. This problem can be solved by
incorporating a second electrode (pull-up) above the suspended membrane. This pull-up
electrode will hold the membrane in up-state when the switch is not actuated, thus reducing
the switch sensitivity to mechanical shocks and vibrations. Also, the up-state time of low
spring constant switches can be improved by incorporating the electrode on top of the
suspended membrane.
2.3.3. University of Michigan Capacitive MEMS Shunt Switches
As discussed in the above example, to decrease the spring constant, spring type
anchors can be used, and to reduce the devices vulnerability to shocks and vibration another
electrode can be fabricated on the top of the suspended membrane. But, fabricating another
electrode on top of the suspended membrane is a complex fabrication process, which
requires need of special and costly releasing equipments and techniques. In view of this the
University of Michigan also developed, a low height switch with Ti/Au membrane, having
high spring constant [18, 19, 20, 21]. The switch is based on a 0.8 - 1.0 µm thick Ti/Au
membrane, suspended at height of 1.5 - 2.2 µm above the transmission line. A low-gap
25
height results in a low pull-down voltage of 12 - 24 Volts while still maintaining a high
spring constant for the membrane. The low-height switch therefore has a relatively high
mechanical resonant frequency and a fast switching time. Also it is not sensitive to
vibrations, with a compromise in capacitance ratio (20 - 40). An isolation of -30 dB (at 30
GHz) with an insertion loss of -0.03 dB (at 10 GHz) and -0.05 dB (at 30 GHz) has been
achieved with this switch.
2.3.4. The LG-Korea Capacitive Shunt Switch
As discussed above, with small gaps, one has to compromise with the capacitance
ratio. Park and team presented very high capacitance ratio MEMS capacitive switches by
using high-k dielectric materials [22, 23]. The switch design is based on the fixed-fixed
beam capacitive shunt switches with strontium-titanate-oxide (SrTiO3) as a dielectric layer.
The relative dielectric constant of SrTiO3 is 30 - 120, depending on the deposition
temperature with very low leakage current. The reported capacitance ratio of fabricated
devices is 600 with a down state capacitance of 60 pF. The membrane is suspended through
the low spring constant springs, resulting in actuation voltages of 8 - 15 Volts. The switch
isolation is better than -40 dB (at 3-5 GHz) and -30 dB (at 10 GHz) with an insertion loss of
-0.1 dB (at 1-10 GHz).
2.3.5. DTIP low actuation voltage switch
This switch is quite long with four actuation electrodes and a low actuation voltage
of 7.5 Volts [24]. The structure consists of a large membrane supported over three pillars. At
rest state, the membrane is at a nominal height over the transmission line, which is the off-
state of the switch. Odd-states can be achieved by applying the actuation voltage at any of
the odd or even electrodes. When actuation voltage is applied at the outer electrodes, a large
up -state deflection can be obtained. The switch shows an isolation of -30 dB (at 24 GHz)
and an insertion loss of 0.65 dB (at 24 GHz). As the membrane is only supported over the
pillars and is not anchored to any plane, the switch is highly vulnerable to mechanical
shocks and vibrations.
26
2.4 Challenges in RF Switches
Most of the challenges in RF MEMS development are interrelated or a trade off with
other parameters. As an example, many of the fabricated devices have actuation voltages
above 20 Volts and hence need up-converters in order to be integrated with other semi-
conductor based systems with standard voltage sources. Such a system consumes more
space and is expensive. Also, high actuation voltage switches have low reliability in terms of
dielectric charging and stiction. In some applications, high switching speed is needed, which
is a major problem area in MEMS switches. To decrease the switching time, the spring
constant of the membrane should be increased, which in turn results in increase of actuation
voltage, which is not required. Also, power handling is one of the major issues in MEMS
switches. RF MEMS metal to metal contact switches with life time up-to a few billion
cycles can handle only (0.5 - 5 mW) of RF power. For high RF power handling, the
reliability of MEMS switches reduces drastically. The failure mechanisms depend on the RF
power used and can be due to thermal stress, dielectric breakdown, self-actuation and
current density issues. Capacitive switches with their large contact area can handle more RF
power than metal-to-metal contact switches and therefore are preferred for applications
requiring 30-300 mW of RF power.
Another major issue in RF MEMS switches is the packaging of devices. The
operation of the device is highly affected by the presence of water vapors, contaminations
and the hydrocarbons present in the atmosphere. Packaging contributes to almost 80% of the
total cost of the device manufacturing cost and its performance and reliability highly
depends on the packaging. Thus, in order to meet the cost, performance and reliability
MEMS packaging tends to be customized to specific application. To avoid failure of RF
MEMS switches proper hermetic packaging is required. Hermetic packaging is a complex
technology, which costs almost 10 – 15 times higher for the MEMS devices as compared to
the semiconductor devices. But, hermetic bonding process requires very high temperatures
for achieving a good seal contact. For released structures or suspended membranes high
temperature processing can bow the membranes by several microns, thus damaging the
switch or deteriorating the reliability of switch. Other packaging issues and different
packaging technologies and packaging levels are discussed in Chapter – 5 of this thesis.
27
2.5 Symmetric Toggle Switch
As discussed in former sections, low actuation voltage switches with low switching
time and high reliabilty against mechanical shocks and vibrations are required. Few of the
existing capacitive switches have high capacitance ratio, but they have high switching time
and actuation voltages. Few have low actuation voltages, but are highly prone to mechanical
shocks and vibrations. In the meander based switch design the reliabilty against self biasing
and external mechanical shocks can be improved by incorporating an additional electrode on
the top of the suspended membrane. This increases the process complexity and also adds the
parasitic capacitances. To obviate the above problems, this thesis focusses on a novel switch
topology called “Symmetric Toggle Switch” (STS) [25]. STS is implemented using standard
50 Ω CPW configuration (Fig. 2.9). The device consists of a pair of micro-torsion actuators
placed symmetrically around the transmission line. They are suspended at a gap of 3µm
above the transmission line, and anchored to CPW ground planes using euler beams called
as 'spring'. They are connected to each other with connecting levers and an overlap area. The
transmission line is divided into three parts. The input and output ports are in thick gold;
whereas, the area under the bridge capacitive area is fabricated in multi-metal layer
(Ti/TiN/Al:Si/Ti/TiN) to provide the smooth capacitive contact. There are four actuation
electrodes beneath the micro torsion actuators for pull-in and pull-out.
transmission line
contact area
micro-torsion
actuator
connecting lever
spring
pull-in electrode
pull-out electrode
anchor
underpass area
Underpass Area with
Floating metal layer
Fig. 2.9: 3 -D model of Symmetric Toggle Switch.
28
Additional electrodes (on the same plane), to clamp the beam in up-state makes the
device impervious to self biasing and vibrations. The use of micro-torsion springs also
improves the travel range. The device can also be configured as MEMS varactor with a wide
capacitance range for a given gap and voltage, not achievable with conventional MEMS
varactor design. Another, outstanding feature of the device as a RF MEMS switch is its
tunability over a wide frequency range. The present configuration reduces the in-built stress
related deformation, though devices become longer as compared to other similar topologies.
Due to the presence of four actuation electrodes, inner two for pull-in and outer two for
achieving minimum insertion loss and getting rid of external shocks and vibrations in on
state, the switch dimensions are very large. Also, long size result in low spring constant
implying low resonance frequency and therefore increase in up-state time. Thus, the outer
two electrodes can be used to reduce the up-state time. Chapter 3 explains the design &
modeling, fabrication and characterization of STS. Dimensional optimization of actuator
area and the capacitive area in view of the required mechanical and electrical performance
has been done by incorporating high-k dielectric material (hafnium oxide) further explained
in Chapter 4.
29
References:
[1]. H. A. C. Tilmans, “MEMS components for wireless communications”, in
Eurosensors XVI, 16th European Conference on Solid State Transducers, Prague,
CZ, September 15-18, 2002, pp. 1-34.
[2]. K. E. Peterson, “Micromachined Membrane Switches on Silicon”, IBM J. Res.
Dev., vol. 23, 1979, pp. 376-385.
[3]. L. E. Larson, R. H. Hackett, M. A. Melendes and R. f. Loher, “Micromachined
Microwave actuator (MIMAC) technology – a new tuning approach for microwave
integrated Circuits”, in Microwave and Millimeter wave Monolithic Circuits
Symposium digest, Boston, MA, USA, June, 1991, pp. 27-30.
[4]. Gabriel M. Rebeiz, “RF MEMS Theory, Design and Technology”, 1st ed. Wiley-
Interscience, 2003.
[5]. P. D. Grant, M. W. Denhoff, “A comparison between RF MEMS switches and
semiconductor switches”, Proceedings of the 2003 International conference on
MEMS, NANO and Smart Systems (ICMENS'04), 2004.
[6]. Elliot R. Brown, “RF-MEMS switches for reconfigurable integrated circuits”, IEEE
Transactions on Microwave theory and Techniques, vol. 46, no. 11, November 1998,
pp 1868 - 1880.
[7]. Gabriel M Rebeiz, Jeremy B Muldavin, “RF MEMS switches and switch circuits”,
IEEE microwave magazine, vol. 59, December 2001.
[8]. J Jason Yao, “RF MEMS from a device perspective”, J of Micromech. and
Microeng., vol. 10, April 2000.
[9]. S. Lucyszyn, “Review of radio frequency micromechanical systems technology”,
IEE Proc. Sci Meas. Technol., vol. 151, no. 2, March 2004.
[10]. M. Soyer, K. A. Jenkins, J. N. Burghartz and M. D. Hulvey, “A 3V 4GHz NMOS
voltage controlled Oscillator with integrated resonator”, in Proc. IMAPS 2001,
Baltimore, Maryland, USA, October 9 - 11 2001, pp. 471-476.
[11]. W-H. Chu, M. Mehregany, and R. L. Mullen, “Analysis of Tip Deflection and
Force of a Bimetallic Cantilever Micro-actuator”, J. Micromech. Microeng., vol. 3,
1993, pp. 4-7.
30
[12]. V. M. Lubecke, B. Barber, E. Chan, D. Lopez, M. E. Gross, and P. Gammel, “Self-
Assembling MEMS Variable and Fixed RF Inductors”, IEEE Trans. Microwave
Theory and Techniques, vol. 49, no. 11, November 2001, pp. 2093-2098.
[13]. Vijay K. Vardan, K. J Vinoy, K. A. Jose, “RF MEMS and Their Applications”, 1st
ed. Wiley-Interscience, 2003.
[14]. C.L. Goldsmith, Z. Yao Z, S. Eshelman and D. Denniston, “Performance of low
loss RF MEMS Capacitive Switches”, IEEE Microwave and Guided Wave Letters,
vol. 8, no. 8, August 1998, pp. 269-271.
[15]. C. Goldsmith,T. H. Lin, B. Powers, W-R. Wu and B. Norwell, “Micromechanical
Membrane switches for microwave application”, in Tech. Digest, IEEE Microwave
Theory and Technique Symp., 1995, pp. 91 - 94.
[16]. Z. J. Yao, S. Chen, S. Eshelman, D. Denniston, and C. L. Goldsmith,
“Micromachined low-loss microwave switches”, IEEE J. Microelectromechanical
Systems, vol. 8, June 1999, pp. 129-134.
[17]. D. Peroulis, S. Pacheco, and L.P.B Katehi, “MEMS devices for high isolation
switching and tunable filtering”, in IEEE MTT-S International Microwave
Symposium Digest, Boston, MA, June 2000, pp. 1217 - 1220.
[18]. J. B. Muldavin and G. m. Rebeiz, “High isolation MEMS shunt switches; Part 1:
Modeling”, IEEE Trans. Microwave Theory Tech., vol. 48, no. 6, June 2000, pp.
1053-1056.
[19]. J. B. Muldavin and G. M. Rebeiz, “High isolation MEMS shunt switches; Part 2:
Design”, IEEE Trans. Microwave Theory Tech., vol. 28, no. 6, June 2000, pp. 1053
– 1056.
[20]. N. S. Barker and G. M. Rebeiz, “Distributed MEMS true-time delay phase shifters
and wideband switches”, IEEE Trans. Microwave Theory Tech., vol. 46, no. 11,
November 1998, pp. 18881-1890.
[21]. J. B. Muldavin and G. M. Rebeiz, “Inline capacitive and DC contact MEMS shunt
switches”, IEEE Microwave Wireless Comp. Letters, vol. 11, no. 8, August 2001,
pp. 334 - 336.
31
[22]. J. Y. Park, G. H. kim, K. W. Chung, and J. U. Bong, “Fully integrated
micromachined capacitive switches for RF applications”, in IEEE MTT-S
International Microwave symposium Digest, Boston, MA, June 2000, pp. 283-286.
[23]. J. Y. Park, G. H. kim, K. W. Chung, and J. U. Bong, “Electroplated RF MEMS
capacitive switches”, in IEEE International conference Microelectromechanical
Systems, January 2000, pp. 639 - 644.
[24]. Renaud Robin, Salim Touati, Karim Segueni, Olivier Millet, Lionel Buchaillot, “A
new four states high deflection low actuation voltage electrostatic MEMS switch for
RF applications”, Proc. of MEMS and MOEMS, April 9 - April 11, 2008.
[25]. K. J. Rangra, “Electrostatic low actuation voltage RF MEMS switches for
Telecommunication”, Ph. D. Thesis, International Doctorate school in information
and communication technologies, DIT, University of Trento, Italy, February 2005.
32