1. Introduction

The extraordinary evolution and the knowledge built-in the radio-frequency field were noticed in various applications such as militaries, medicine, and telecommunication. At the system level, one trend in the field of wireless telecommunications is the design of multiband and multimode devices, with an ever-increasing number of features, leading to the so pursued reconfigurable systems.

in airbag triggering), entertainment (motion detection in a video game), and optics (micro

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MEMS devices have found application in many different fields. As shown in Figure 1, the

• Sensors: miniaturized systems, made from microtechnologies used for sensor applications in measurement and instrumentation fields, such as pressure sensors and capacitive

• MOEMS: this type of MEMS can be used in optical technologies, such as micro mirrors,

• Bio-MEMS: miniaturized systems, made from micro- and nanotechnologies derived from microelectronics (integrated circuits) which is intended to carry out experiments in biol-

• RF MEMS: in the field of microwaves, they improve the performance of tunable devices to various functions, such as variable passive components, resonators, filters, and antennas. In this way, RF MEMS is usually related with the application of MEMS technologies to develop systems that contribute to the RF system development. They can be used to increase the performance or to implement characteristics not achievable by other solutions, even if performance is slightly degraded. Despite a few drawbacks, the emergence of RF MEMS represents a revolution in the development of new radio-frequency systems. In fact, these elements should compete or replace certain semiconductor components in microwave applications. They are very compact (typically a few hundred square micrometers) and can be up to 50% smaller than

ogy/chemistry, such as DNA chip, microchemical reactor, and micro valves.

mirrors), to mention just a few applications.

optical switches, and optical cavities.

MEMS components can be classified into four main families [6]:

semiconductor components performing the same function [7].

2.1. What is RF MEMS?

accelerometer.

Figure 1. MEMS families.

At present, reconfigurable systems have become very promising in a wide range of applications, including future services of wireless communication systems. However, the wide spread of wireless communication systems and the emergence of new wireless communication standards have introduced new challenges in the hardware design for transmitters and receivers. To tackle this problem, nowadays telecommunication systems need to use a significant number of tunable components, where the performance is degraded when compared to their equivalents at fixed frequencies.

A telecommunication system is said to be tunable or reconfigurable, when some of its characteristics (central frequency, bandwidth, polarization, etc.) can be modified by an external control signal (electrical, mechanical, thermal, etc.). Despite tunable components can be realized using many different designs, mainly, two approaches exist for tunability:


The RF MEMS devices feature low-power consumption, high linearity, wide bandwidth, and high dynamic range, which are among the most important requirements that each component must meet in order to achieve high-performance wireless systems.

This chapter will present the development of frequency and phase reconfigurable components, based on capacitive tunable RF MEMS.

## 2. RF MEMS technologies

RF MEMS has its origin in the MEMS systems, which are miniature electronic and/or mechanical systems designed to perform specific tasks. They consist of motors, gears, levers, electrical devices, or tiny sensors. These devices are used in many applications and their size range from a few micrometers to a few millimeters. By the late 1960s, MEMS systems were used as precise sensors of hydraulic pressure in aircraft. Today, these systems play an important and everincreasing role in the fields of medicine (detection of organic cells), automotive (accelerometer in airbag triggering), entertainment (motion detection in a video game), and optics (micro mirrors), to mention just a few applications.

#### 2.1. What is RF MEMS?

1. Introduction

94 MEMS Sensors - Design and Application

reconfigurable systems.

equivalents at fixed frequencies.

tric [1] and ferromagnetic [2]).

based on capacitive tunable RF MEMS.

2. RF MEMS technologies

components.

The extraordinary evolution and the knowledge built-in the radio-frequency field were noticed in various applications such as militaries, medicine, and telecommunication. At the system level, one trend in the field of wireless telecommunications is the design of multiband and multimode devices, with an ever-increasing number of features, leading to the so pursued

At present, reconfigurable systems have become very promising in a wide range of applications, including future services of wireless communication systems. However, the wide spread of wireless communication systems and the emergence of new wireless communication standards have introduced new challenges in the hardware design for transmitters and receivers. To tackle this problem, nowadays telecommunication systems need to use a significant number of tunable components, where the performance is degraded when compared to their

A telecommunication system is said to be tunable or reconfigurable, when some of its characteristics (central frequency, bandwidth, polarization, etc.) can be modified by an external control signal (electrical, mechanical, thermal, etc.). Despite tunable components can be real-

• The first way is achieved by the possibility to change the substrate permittivity (ferroelec-

• The second way consists in a change of the capacitive or the inductive load by the addition of tunable radio-frequency integrated circuits (RFIC). This method relies on semiconductor devices (diode [3] and transistors [4]) or mechanical (RF MEMS [5])

The RF MEMS devices feature low-power consumption, high linearity, wide bandwidth, and high dynamic range, which are among the most important requirements that each component

This chapter will present the development of frequency and phase reconfigurable components,

RF MEMS has its origin in the MEMS systems, which are miniature electronic and/or mechanical systems designed to perform specific tasks. They consist of motors, gears, levers, electrical devices, or tiny sensors. These devices are used in many applications and their size range from a few micrometers to a few millimeters. By the late 1960s, MEMS systems were used as precise sensors of hydraulic pressure in aircraft. Today, these systems play an important and everincreasing role in the fields of medicine (detection of organic cells), automotive (accelerometer

ized using many different designs, mainly, two approaches exist for tunability:

must meet in order to achieve high-performance wireless systems.

MEMS devices have found application in many different fields. As shown in Figure 1, the MEMS components can be classified into four main families [6]:


In this way, RF MEMS is usually related with the application of MEMS technologies to develop systems that contribute to the RF system development. They can be used to increase the performance or to implement characteristics not achievable by other solutions, even if performance is slightly degraded. Despite a few drawbacks, the emergence of RF MEMS represents a revolution in the development of new radio-frequency systems. In fact, these elements should compete or replace certain semiconductor components in microwave applications. They are very compact (typically a few hundred square micrometers) and can be up to 50% smaller than semiconductor components performing the same function [7].

Figure 1. MEMS families.

#### 2.2. RF MEMS switch as a building block

The microelectromechanical components enable the reconfiguration of electronic devices using mechanical movements. Using this feature, one building block widely used to enable a device's tunability is the MEMS switch, and we call it a MEMS microswitch. The microelectromechanical part of the microswitch, or varactor components, has the form of a mobile beam suspended and anchored to one of its ends. The beam can be built-in, or double-embedded. The main idea behind a tunable device is the fact that when a MEMS switch moves, besides switching from on and off states, it may exhibit a different RF load. And controlling such RF load, it is possible to tune different RF devices.

Ohmic Capacitive

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Table 2. Classification of RF MEMS switches: cantilever and fixed beam, capacitive and ohmic, series, and shunt

Cantilever OFF

Fixed-fixed beam OFF

ON

ON

electrical model of MEMS switches.

#### 2.3. RF MEMS switch control mechanism

The mechanical movement of the beam is obtained by applying an actuating force. This actuating force is generally of an electrostatic nature [8, 9], but it can be thermal [10], piezoelectric [11], or magnetic [12]. Table 1 is showing the comparative study of the different types of actuation [8, 13].

Electrostatic actuators are the most used components because they consume very little, occupy a very small volume, and has a short switching time. In this chapter, the electrostatically actuated RF MEMS will be explored as a solution for different applications.

#### 2.4. RF MEMS switches topologies

RF MEMS microswitches are components intended to perform an electrical function through the control of a movable or mechanically deformable structure. There are two main types of RF MEMS components: the capacitive touch switch (contact: metal-dielectric-metal) and the resistive or ohmic contact switch (contact: metal-metal). In both cases, it is necessary to apply a force to the movable part of the component to move the MEMS beam. In this work, we will only be interested in RF MEMS capacitive shunt switch based on electrostatic actuators.

In order to summarize the previous points for RF MEMS, Table 2 makes a comparison between the two types, ohmic and capacitive, and their configurations [14–17].


Table 1. Comparison of the different types of actuation.

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2.2. RF MEMS switch as a building block

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load, it is possible to tune different RF devices.

2.3. RF MEMS switch control mechanism

2.4. RF MEMS switches topologies

Table 1. Comparison of the different types of actuation.

of actuation [8, 13].

actuators.

The microelectromechanical components enable the reconfiguration of electronic devices using mechanical movements. Using this feature, one building block widely used to enable a device's tunability is the MEMS switch, and we call it a MEMS microswitch. The microelectromechanical part of the microswitch, or varactor components, has the form of a mobile beam suspended and anchored to one of its ends. The beam can be built-in, or double-embedded. The main idea behind a tunable device is the fact that when a MEMS switch moves, besides switching from on and off states, it may exhibit a different RF load. And controlling such RF

The mechanical movement of the beam is obtained by applying an actuating force. This actuating force is generally of an electrostatic nature [8, 9], but it can be thermal [10], piezoelectric [11], or magnetic [12]. Table 1 is showing the comparative study of the different types

Electrostatic actuators are the most used components because they consume very little, occupy a very small volume, and has a short switching time. In this chapter, the electrostatically

RF MEMS microswitches are components intended to perform an electrical function through the control of a movable or mechanically deformable structure. There are two main types of RF MEMS components: the capacitive touch switch (contact: metal-dielectric-metal) and the resistive or ohmic contact switch (contact: metal-metal). In both cases, it is necessary to apply a force to the movable part of the component to move the MEMS beam. In this work, we will only be interested in RF MEMS capacitive shunt switch based on electrostatic

In order to summarize the previous points for RF MEMS, Table 2 makes a comparison

Type of switching Switching speed (μs) Switch size Consumption (mW)

between the two types, ohmic and capacitive, and their configurations [14–17].

Electrostatic 0.05–200 Small ˜0 Thermal 50–200 Medium <100 Piezoelectric 1–200 Medium ˜0 Magnetic 500–4000 Large <200

actuated RF MEMS will be explored as a solution for different applications.

Table 2. Classification of RF MEMS switches: cantilever and fixed beam, capacitive and ohmic, series, and shunt electrical model of MEMS switches.
