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

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31

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