**1. Introduction**

Radio frequency microelectromechanical system (RF-MEMS) switches are aimed to perform the control function in tunable and reconfigurable RF/microwave and millimeter-wave (mmwave) systems. Electrostatic actuation is often preferred to other actuation mechanisms like electrothermal [1, 2] and phase-change/phase-transition materials [3], due to its negligible current consumption, no requirement for external heating sources and integration capability with well-established technologies such as high-resistivity silicon [4–7], fused-quartz and glass substrates [8–11], or CMOS [12–15] and SiGe BiCMOS [16, 17] processes. The latter can provide totally integrated, efficient systems containing sensors, control electronics, and MEMS-reconfigurable RF communication circuits [18].

of the switch can be obtained and integrated in the multimodal models. In this way, efficient and compact reconfigurable circuits for communication systems at microwave and mm-wave

RF-MEMS Switches Designed for High-Performance Uniplanar Microwave and mm-Wave Circuits

http://dx.doi.org/10.5772/intechopen.76445

119

In this chapter, a detailed study of RF-MEMS switches to be used in multimodal uniplanar circuits is presented. The switch electromechanical design considerations are explained in detail, and a number of switch configurations proposed, simulated mechanically, and fabricated using the FBK flexible technology platform [20]. The fabricated switches are measured, and the experimental results successfully compared to simulations, thus validating the design approach. An estimation of the RF behavior of the switches is obtained from 2.5 D electromagnetic simulation. The RF behavior after fabrication is assessed by measuring the switch transmission coefficient for both (ON/OFF) states. Equivalent circuit topologies are also proposed and the value of the circuit elements computed by fitting the simulated results to the measurements. The switch transmission coefficient is also used for the measurement of the switch hysteresis. The proposed switches are integrated into the microwave and mm-wave multimodal reconfigurable circuits to validate the multimodal design approach. Some examples of fabricated multimodal reconfigurable filters and phase switches using RF-MEMS switches with various mechanical topologies (bridge-type featuring ohmic contact and capacitive contact, and cantilever-type featuring ohmic contact) are presented.

This chapter is organized as follows. After this introduction, the multimodal circuits and models for uniplanar transitions and discontinuities are explained in Section 2. The RF-MEMS fabrication technology platform is described in Section 3. The electromechanical analysis derived from the energy approach is studied in Section 4. The fabricated switches are described in Section 5. The RF equivalent circuit for the switches is analyzed in Section 6. The reconfigurable multimodal microwave and mm-wave circuits are described in Section 7. The chapter

The slotline and the CPW are uniplanar transmission lines. The slotline consists of two conductor strips on a dielectric substrate (**Figure 1(a)**). The CPW consists of three conductor strips on a dielectric substrate (**Figure 1(b)**). The slotline is a monomodal transmission line: it propagates only one fundamental quasi-transversal electromagnetic (TEM) mode, whose

(*z*), and *Vs*

**Figure 2(a)** and can be circuitally modeled as an ideal transmission line (**Figure 2(b)**), with

+(*z*) = *VS*

*<sup>I</sup>*

− (*z*), *I s* −

<sup>+</sup> *e* <sup>−</sup>*jβ<sup>S</sup>*

+(*z*)/*Z*0*<sup>S</sup> I*

*<sup>z</sup> VS*

*S*

(*z*) and *I s*

<sup>−</sup>(*z*) = *VS*

<sup>−</sup>(*z*) = −*VS*

(*z*), and for the forward

(*z*), respectively) are defined as in

its phase constant.

<sup>−</sup> *e* <sup>+</sup>*jβ<sup>S</sup> z*

<sup>−</sup>(*z*)/*Z*0*<sup>S</sup>*

**2. Uniplanar lines and multimodal models for transitions and** 

frequencies can be designed [6, 9–11].

ends with some conclusions.

**2.1. The slotline and the coplanar waveguide**

and backward propagating waves *Vs*

*S* (*z*) = *I S* +(*z*) + *I S* <sup>−</sup>(*z*) *I*

(*z*) = *VS*

voltages and currents (both for the total voltage and current *Vs*

+(*z*) + *VS*

+ (*z*), *I s* +

<sup>−</sup>(*z*) *VS*

*S* +(*z*) = *VS*

is the characteristic impedance of the slotline mode and *β<sup>s</sup>*

**discontinuities**

*VS*

where *Z*0*<sup>s</sup>*

The mechanical and electrical design of the RF-MEMS switches has been comprehensively studied in the literature [19], and it highly depends on the circuit or transmission media in which it is to be integrated and the technology platform [20]. A number of solutions can be found, including integration in microstrip transmission lines [21], coplanar waveguides (CPWs) [4], coplanar striplines (CPSs) and slotlines [11], planar structures embedded in rectangular waveguides [22], and micromachined waveguides for sub-mm-wave frequencies [23]. Depending on the specific designs and dimensions, they can operate in the microwave and the mm-wave bands, at frequencies as high as 240 GHz as reported in [24] using BEOL in BiCMOS technology.

Series and parallel RF-MEMS switch topologies can be implemented, with either ohmiccontact [22] or capacitive-contact [4, 25]. While ohmic switches can operate in a very wide frequency band from DC to mm-waves featuring excellent OFF-state isolation and very low ON-state insertion loss, capacitive switches are frequency selective (being the center frequency defined by a series LC-resonant circuit) but their operation can be extended well beyond mmwave frequencies by properly choosing the ON-state capacitance and the series inductance which depends on the membrane dimensions [24].

Mechanical topologies for RF-MEMS switches include bridge-type clamped-clamped or beamsuspension membranes and cantilever-type switches. Important switch parameters, such as the actuation voltage or the fabrication residual stress, are dependent on the particular selected topology [26–28]. Using three-dimensional (3D) mechanical simulation, the material physical properties are taken into account to a priori assess the behavior of the switch geometry (including the suspension type) in terms of initial membrane deformation, pull-in voltage, spring constant, mechanical resonant frequency, and transition times from OFF to ON states (and vice versa) [29]. Mechanical transients may produce bouncing phenomena [30–34] which degrade the RF behavior of the switch and can be studied more efficiently with energy models [35].

RF-MEMS switches featuring the above mechanical topologies are compatible with and can be conveniently integrated in uniplanar structures (CPW, CPS, and slotline) to perform a control function. In case of multimodal transmission lines like CPW, they can be used to selectively control the two CPW fundamental propagation modes (even and odd) [36]. To accurately analyze the interaction between modes in complex uniplanar structures (transitions, discontinuities), multimodal circuit models are derived from the application of the general multimodal theory [37–40]. Moreover, suitable equivalent circuits for both (ON/OFF) states of the switch can be obtained and integrated in the multimodal models. In this way, efficient and compact reconfigurable circuits for communication systems at microwave and mm-wave frequencies can be designed [6, 9–11].

In this chapter, a detailed study of RF-MEMS switches to be used in multimodal uniplanar circuits is presented. The switch electromechanical design considerations are explained in detail, and a number of switch configurations proposed, simulated mechanically, and fabricated using the FBK flexible technology platform [20]. The fabricated switches are measured, and the experimental results successfully compared to simulations, thus validating the design approach. An estimation of the RF behavior of the switches is obtained from 2.5 D electromagnetic simulation. The RF behavior after fabrication is assessed by measuring the switch transmission coefficient for both (ON/OFF) states. Equivalent circuit topologies are also proposed and the value of the circuit elements computed by fitting the simulated results to the measurements. The switch transmission coefficient is also used for the measurement of the switch hysteresis. The proposed switches are integrated into the microwave and mm-wave multimodal reconfigurable circuits to validate the multimodal design approach. Some examples of fabricated multimodal reconfigurable filters and phase switches using RF-MEMS switches with various mechanical topologies (bridge-type featuring ohmic contact and capacitive contact, and cantilever-type featuring ohmic contact) are presented.

This chapter is organized as follows. After this introduction, the multimodal circuits and models for uniplanar transitions and discontinuities are explained in Section 2. The RF-MEMS fabrication technology platform is described in Section 3. The electromechanical analysis derived from the energy approach is studied in Section 4. The fabricated switches are described in Section 5. The RF equivalent circuit for the switches is analyzed in Section 6. The reconfigurable multimodal microwave and mm-wave circuits are described in Section 7. The chapter ends with some conclusions.
