Dimensions and mechanical parameters of the switches.
Radio frequency microelectromechanical system (RF-MEMS) switches have demonstrated superior electrical performance (lower loss and higher isolation) compared to semiconductor-based devices to implement reconfigurable microwave and millimeter (mm)-wave circuits. In this chapter, electrostatically actuated RF-MEMS switch configurations that can be easily integrated in uniplanar circuits are presented. The design procedure and fabrication process of RF-MEMS switch topologies able to control the propagating modes of multimodal uniplanar structures (those based on a combination of coplanar waveguide (CPW), coplanar stripline (CPS), and slotline) will be described in detail. Generalized electrical (multimodal) and mechanical models will be presented and applied to the switch design and simulation. The switch-simulated results are compared to measurements, confirming the expected performances. Using an integrated RF-MEMS surface micromachining process, high-performance multimodal reconfigurable circuits, such as phase switches and filters, are developed with the proposed switch configurations. The design and optimization of these circuits are discussed and the simulated results compared to measurements.
- RF-MEMS switches
- MEMS fabrication technology
- reconfigurable microwave circuits
- uniplanar multimodal circuits
- circuit simulation
- electromechanical simulation
- phase switches
Radio frequency microelectromechanical system (RF-MEMS) switches are aimed to perform the control function in tunable and reconfigurable RF/microwave and millimeter-wave (mm-wave) systems. Electrostatic actuation is often preferred to other actuation mechanisms like electrothermal [1, 2] and phase-change/phase-transition materials , 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, 5, 6, 7], fused-quartz and glass substrates [8, 9, 10, 11], or CMOS [12, 13, 14, 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 .
The mechanical and electrical design of the RF-MEMS switches has been comprehensively studied in the literature , and it highly depends on the circuit or transmission media in which it is to be integrated and the technology platform . A number of solutions can be found, including integration in microstrip transmission lines , coplanar waveguides (CPWs) , coplanar striplines (CPSs) and slotlines , planar structures embedded in rectangular waveguides , and micromachined waveguides for sub-mm-wave frequencies . 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  using BEOL in BiCMOS technology.
Series and parallel RF-MEMS switch topologies can be implemented, with either ohmic-contact  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 mm-wave frequencies by properly choosing the ON-state capacitance and the series inductance which depends on the membrane dimensions .
Mechanical topologies for RF-MEMS switches include bridge-type clamped-clamped or beam-suspension 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, 27, 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) . Mechanical transients may produce bouncing phenomena [30, 31, 32, 33, 34] which degrade the RF behavior of the switch and can be studied more efficiently with energy models .
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) . 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, 38, 39, 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, 10, 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 . 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.
2. Uniplanar lines and multimodal models for transitions and discontinuities
2.1. The slotline and the coplanar waveguide
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 voltages and currents (both for the total voltage and current
The CPW is a multimodal transmission line: it can propagate two fundamental quasi-TEM modes simultaneously (the even and odd modes) whose voltages and currents are defined as in Figure 3(a). The odd mode is often seen as spurious, and its propagation cut by means of air bridges (described subsequently). However, it can be used to design new kinds of compact uniplanar circuits. In a CPW section, the even and odd modes do not interact between them and therefore can be circuitally modeled as two independent ideal transmission lines, as shown in Figure 3(b), with equations analogous to those of the slotline for either of them.
2.2. Multimodal models for CPW circuits
The even and odd modes behave differently when they encounter any transition or asymmetry, and there they may also interact between them. A multimodal model is a circuit model that makes the behavior of the different modes at a transition or asymmetry explicit. As an example, some simple multimodal models are presented subsequently; more complex ones are described in [6, 9, 10, 11, 38, 39, 40, 58].
2.2.1. Symmetric CPW-to-slotline transition
The layout of this transition is shown in Figure 4(a) (the depicted voltages and currents are the total ones for each mode, computed at the transition plane). Its behavior is easy to understand intuitively. At the transition, the odd mode transforms into the slotline mode and vice versa due to their similarity of voltage and current orientations (caused by the similarity of their electromagnetic fields). The even mode, however, is left in open circuit when the slotline begins since its current in the CPW central strip can flow no more. Therefore, the multimodal circuit model for the symmetric CPW-to-slotline transition is that of Figure 4(b). As can be seen, a multimodal circuit model confines the contributions of each mode present in a transition into a different port.
2.2.2. Impedances connecting the two outer CPW strips and air bridges
Suppose an impedance connects the two outer CPW strips as shown in Figure 5(a). This circuit can model an air bridge (a conducting wire connecting the two outer CPW strips, with an impedance
2.2.3. Asymmetric shunt impedances in a CPW
In the two previous examples, the even and odd modes behaved in a different way at the analyzed transitions but did not interact between them due to the symmetry of the transitions. When the transitions are asymmetric, as it is the case for the asymmetric shunt impedances connecting the strips of the CPW shown in Figure 6(a), the modes interact between them. The behavior of this transition is not obvious, but it can be rigorously modeled by the circuit shown in Figure 6(b) . As can be seen, in this case, there is an energy balance between even and odd modes (there is a circuit connection between the even- and odd-mode ports), provided that the impedances
3. Fabrication technology for electrostatically actuated RF-MEMS switches
3.1. RF-MEMS technology platform
A flexible technology platform has been developed and optimized at the FBK Institute (Trento, Italy) for the fabrication of RF-MEMS. Basic components (like low-loss CPW, microstrip line and slotline, ohmic [41, 42] and capacitive [43, 44] switches, variable capacitors and inductors) can be integrated in complex reconfigurable RF circuits. Many kinds of devices were produced, mainly for space and communication application, like switching matrices [45, 46], tunable and switchable phase shifters , reconfigurable antennas, impedance matching networks , VCOs [49, 50], and tunable filters. Depending on the used substrate, high-resistivity silicon (<40 GHz), or fused quartz (>40 GHz), the working frequency range spans from sub-GHz up to more than 100 GHz.
The base process requires eight lithography masks but, depending on the requirements, it can easily be expanded to deposit and pattern metal on the wafer backside to realize microstrip lines or antennas and to obtain devices suspended over thin membranes by locally removing the substrate. A wafer-to-wafer or a cap-to-die-bonding module is also available to encapsulate the delicate MEMS moving parts .
RF signal lines and ground area are made of thick electroplated gold to reduce insertion losses while actuation electrodes and DC-bias signal lines are made of a high-resistivity polysilicon to minimize coupling with adjacent RF lines. The movable and suspended structures of the electrostatically actuated switches, which can be either cantilevers or clamped-clamped beams, are made by gold deposited over a sacrificial photoresist layer having the thickness of the required air gap, while switch underpass lines and other conductors are made of a thin Al film. On ohmic-contact switches, the gold-to-gold contact area is defined by underneath polysilicon protruding dimples to ensure a repeatable contact force and a uniform and reproducible low contact resistance. On capacitive-contact switches, the contact capacitor is made by depositing a thin silicon oxide dielectric and an upper floating metal (FLOMET) electrode over the metal underpass line, obtaining a very well-defined and reproducible metal insulator metal (MIM) capacitor. In this way, when the switch-movable membrane is in an up position, the capacitance, due mainly to the air gap, is small while when it is actuated, the membrane contacts the top floating metal electrode, and the capacitance is defined by the MIM capacitor and not by the membrane itself. In this way, the switch is much more repeatable than the usual configuration, where the movable membrane directly touches the dielectric and the capacitance is strongly influenced by both the membrane deformations and surface roughness leading to a capacitance value much lower than the designed one.
For all the switch configurations, the actuation electrodes are separated from the contact area. This makes it possible to optimize them independently to sustain the high actuation voltage (up to 100 V) and reducing the charging phenomena. It is possible either to use a thicker dielectric over polysilicon to limit the electric field or better to use a dielectric-free configuration removing all the dielectric and using a matrix of mechanical stoppers to prevent short circuits. The height of the stoppers has to be designed in order to obtain an air gap between movable bridges and electrodes which is thick enough for isolation at the bias voltage used.
3.2. Fabrication process
The basic fabrication process for silicon substrate is reported in [20, 52] and illustrated in Figure 7, where a schematic cross section of an ohmic switch is represented. For high-frequency devices, the losses of the silicon are too high and quartz (fused silica) is preferred. Only minor adjustments are required to process transparent substrates.
The fabrication process starts with the oxidation of the high-resistivity (>5000 Ω·cm) 150-mm diameter silicon wafers in order to obtain a 1-μm-thick silicon oxide isolation layer. A 630-nm thick layer of polysilicon is then deposited by low-pressure chemical vapor deposition (LPCVD) and doped by ion implantation to obtain a sheet resistance of about 1600 Ω/sq. The polysilicon structures are defined by lithography and dry etching using chlorine-based gas plasma, and the residual photoresist is removed by an oxygen plasma (Figure 7(a)). An annealing at 925°C for 1 h in nitrogen atmosphere is required to diffuse and to electrically activate the B ions. To electrically isolate the polysilicon, 300 nm of silicon dioxide is deposited by LPCVD at 718°C (TEOS). When a backside conductive layer is required for microstrip lines or devices like phased array antennas, an aluminum film is sputtered and patterned on the wafer backside. The process continues on the front side with a lithography and dry etching to open holes in the TEOS layer for contacting the underneath polysilicon (Figure 7(b)). To realize connection lines, a conductive metal consisting of Ti/TiN diffusion barrier and Al1%Si is sputtered and patterned by dry etching (Figure 7(c)). The total thickness is the same as that of polysilicon to minimize distortion of the switch bridges crossing over both metal underpass and polysilicon actuation electrodes. A 100-nm thick SiO2 deposited by PECVD is used as dielectric for capacitive contacts as well as for metal isolation. Holes in the oxide (vias) are realized by lithography and dry etching to contact the underneath metal and for the dielectric-free actuation electrodes (Figure 7(d)).
To realize the bottom part of the gold-to-gold contacts of ohmic switches as well as an electrically floating metal layer for capacitive-contact switches, a 5-nm Cr adhesion layer and 150-nm Au are deposited by an electron beam gun, patterned and wet etched (Figure 7(d)). A photoresist sacrificial layer (spacer) is lithographically defined under movable structures and suspended air bridges because later it can be easily removed by oxygen plasma to form an air gap (Figure 7(e)). To make the RF structures, a conductive seed layer of 2.5 nm of Cr and 25 nm of Au is deposited by e-beam, patterned using thick AZ 4562 positive resist, and a 1.8-μm-thick first gold layer (bridge) having a slightly tensile residual stress is selectively grown by electroplating (Figure 7(f)). A second 3.5-μm-thick gold layer (CPW) is then defined by AZ 4562 and electroplated. The thinner Au bridge layer is used to make the suspended and movable structure while both layers are superimposed to obtain thicker low resistance signal lines and ground areas. To better control the deformation of the movable parts of the switch, it is possible to use the bridge layer for deformable suspension legs and bridge plus CPW layer for a stiffer main body that moves rigidly, almost without deformations. This concept is applied in the fabricated devices, described in Section 5. To complete the fabrication, the seed layer is removed by wet etching, and the suspended structures are released by removing the spacer underneath by an oxygen plasma (Figure 7(g)).
4. Electromechanical models for electrostatically actuated RF-MEMS switches: energy considerations
Mechanical design plays an important role in the RF behavior of MEMS switches because it couples important parameters such as the required actuation voltage (also called pull-in voltage,
Another trend to achieve fast-switching is to use actuation voltages beyond pull-in. The accumulated electrostatic energy will generate mechanical energy that will be released in the form of mechanical oscillations (bouncing) of the switch membrane [30, 31, 32, 33, 34, 35]. To accurately approach RF-MEMS mechanical design, the dynamic behavior of RF-MEMS switches should be considered rather than only static behavior.
The analysis of the electromechanical exchange of energy in the RF-MEMS is an analytical tool that can provide inside knowledge on the required minimum
Figure 8 shows the schematic of the one-dimensional (1D) lumped-mass model that, combined with classical Newtonian mechanics, can be used to predict the behavior under applied electrostatic forces of an RF-MEMS switch (either a membrane or a cantilever). If air damping is considered the only non-conservative force, then the equation of the motion for the 1D model shown in Figure 8 is
The study of the energy exchange of the mechanical system not only provides the position of the contact point of the MEMS switch but also provides deep insight to the required
Figure 9 shows the evolution of the total energy
From Eq. (2), the actuation voltage can be obtained since at the point of instability (or switching) . In case of nonlinear mechanical behavior, the resulting expression of
Eq. (3) is used in the simulations of the fabricated switches, discussed in the next Section.
5. Electrostatically actuated RF-MEMS switch configurations for reconfigurable uniplanar circuits
This section presents electrostatically actuated switch configurations which can easily be integrated in reconfigurable uniplanar circuits. All the considered devices were fabricated using the eight-mask surface micromachining process from FBK explained in Section 3. The structures are composed of a 1.8-μm-thick gold layer and reinforced with a 3.5-μm-thick superimposed gold frame to increase the rigidity of the cantilever or the bridge. The switches were designed taking into account the mechanical analysis described in Section 4.
To reduce the initial deformation of the switch membrane, different authors have reported on the effective stiffness of common suspensions types [26, 27, 28]. A coupled-field 3D finite element analysis (FEA) with ANSYS® Workbench™ can be used to model the mechanical structure and tune the measured initial deformation according to the residual stress produced by the fabrication process. The robustness of the design to manufacturing stresses can also be studied with this software.
The switch designs presented use either a clamped-clamped suspension or alternative suspension techniques such as straight-beam, curved-beam, or folded-beam which reduce the initial stress and the actuation voltage. Some FEA results are shown to assess the ability of the proposed suspensions to absorb the initial stress. Hysteresis measurements are also presented to show the featured pull-in and pull-out voltages. At the end of the section, Table 1 summarizes the switch dimensions and the main mechanical parameters for the different switches.
|Parameter||Ohmic-cantilever straight/semicircle||Capacitive, clamped/folded||Ohmic-series||Ohmic-parallel (SAB)|
|Supporting beam radius (μm)||−/16||—||—||—|
|Meander length, ||—||−/30||—||30|
|Supporting beam width (μm)||16||−/10||—||10|
|Supporting beam length, ||40||−/75||—||75|
|Supporting beam length, ||—||−/45||—||77.5|
|Membrane width, ||90||90||100||90|
|Window width, ||—||—||—||60|
|Membrane length, ||170||580/230||580||165|
|Bottom electrodes area (μm2)||13,500||2 × 22,800/2 × 7650||2 × 22,800||2 × 7650|
|Contact area (μm2)||3 × 10 × 30||100 × 90*/50 × 90*||2 × 10 × 90||2 × 10 × 12|
|Spring constant, ||2.3/1.6||104.5/32.5||104.5||15.4|
|Pull-in voltage, ||10.6/8.8||60/14||49||14|
|Resonant frequency (KHz)||8‡/6.6‡||16.25 ‡/25.3‡||16.25†,‡||17.1‡|
The RF behavior of the switches, including equivalent circuits in ON and OFF states, is discussed in detail in Section 6.
5.1. Cantilever-type switches for switchable asymmetric shunt impedances
Figure 10 shows a cantilever-type ohmic-contact switch, able to synthesize asymmetric shunt impedances in a CPW to control the CPW even mode as explained in Section 2.2.3. The cantilever is placed above a rectangular notch in the upper lateral ground plane to which is anchored using a suspension composed of two 16-μm-wide beams, providing a low spring constant value and a low pull-in voltage. The beams may be either straight-shaped (Figure 10(a)) or semi-circular-shaped (Figure 10(b)). The latter is used to reduce the initial deformation of the switch due to the residual stress  produced by the fabrication process [20, 52]. To compute the initial deflection, a 3D FEA using ANSYS® was performed. The initial stress values used for the simulation were
5.2. Bridge-type switches
Bridge-type switches can be used to perform both ohmic contacts and capacitive contacts in uniplanar circuits for multiple applications. In contrast to the cantilever-type switches, the bridge-types are symmetric structures and therefore (as discussed in Section 2.2), when actuated, they are able to control one of the fundamental CPW modes (either even or odd), leaving the other mode ideally unaffected. Some minor effects such as a small even-mode parasitic capacitance (in case of the ohmic-contact parallel switch discussed in Section 5.2.3) occur but with a very limited impact on the circuit behavior.
5.2.1. Capacitive-contact parallel switch
Figure 11 shows two fabricated capacitive-contact parallel switches. The change between ON and OFF states is performed by moving the suspended membrane, which can be actuated through bias pads connected to two symmetrical polysilicon electrodes placed under the membrane. A floating metal (FLOMET) strip is placed on top of the dielectric under the membrane. The overlapping area between FLOMET and a multi-metal layer under the bridge (CPW center conductor) defines a MIM capacitor, as described in Section 3. When the actuation voltage is equal or higher than the pull-in voltage
The capacitive switch shown in Figure 11(a) uses a clamped-clamped membrane suspension. The device was fabricated on a quartz substrate with a thickness
5.2.2. Ohmic-contact series switch
A photograph of an ohmic-contact series switch is shown in Figure 13(a). It was fabricated on a quartz substrate with a thickness
For this switch, the switching and release times were key parameters in radiometric applications, as discussed in Section 7. The measured switching and release times are 100 μs and 15 μs, respectively. To more accurately assess the switch behavior after the membrane release (evolving from ON state to OFF state), the energy model discussed in Section 4 was applied as follows. The evolution in time of the switch transmission coefficient magnitude (isolation) at a given RF frequency (
5.2.3. Ohmic-contact parallel switch (switchable air bridge)
Figure 14(a) shows a switchable air bridge (SAB) that can be used in CPW reconfigurable multimodal circuits for a selective use of the CPW odd mode. The device was fabricated on a quartz substrate with a thickness
The air gap of ohmic contacts placed on the top and bottom electrodes could be affected by stress gradients during fabrication. As with the previous switches, the deformation of the bridge was simulated using ANSYS® 3D FEA. The structure can handle positive- and negative-stress gradients without compromising on the function of the switch. Figure 15(a) shows the deformation of the bilayer membrane. For this case, the simulated maximum initial deflection is smaller than 0.16 μm.
The measured topography of the device just after fabrication (Figure 15(b)) shows a very good agreement with the 3D FEA results, thus validating this analysis. This model can then be used to extract the nonlinear stiffness values that in turn can modify the potential energy curve shown in Figure 9.
Table 1 shows the dimensions and the main mechanical parameters of the switches presented in this section. The membrane/cantilever height is 1.6 μm in all cases except for the capacitive-contact parallel switch with the clamped-clamped membrane and the ohmic-contact series switch, both featuring a membrane height of 2.7 μm. While the pull-in voltages are measured, the spring constant and the mechanical resonant frequency are computed using the 3D FEA method presented in Section 4. For the ohmic-contact series switch, the mechanical resonant frequency was also measured using the method reported in .
6. RF-MEMS switch electrical modeling and characterization
6.1. Parallel and series switch models
6.1.1. Capacitive-contact parallel switch
The configuration of a capacitive-contact parallel switch is shown in Figure 11. It is able to control the propagation of the CPW even mode and simultaneously suppress the CPW odd mode because the lateral metal planes are permanently connected through the switch membrane. When the switch is in its “up” (OFF) state, the capacitance between the elevated membrane and the RF line underneath (
Figure 16 shows an equivalent circuit of the switch, which applies to both states. Capacitance
6.1.2. Parallel and series ohmic-contact switches
The configuration of an ohmic-contact parallel switch (switchable air bridge or SAB) is shown in Figure 14. This switch is used to efficiently control the CPW odd-mode propagation. When the switch is in its “up” (OFF) state, the capacitance between the elevated membrane and the two RF ohmic contacts at either lateral metal plane is negligible (
Figure 17 shows an equivalent circuit of the switch for the OFF state (Figure 17(a)) and ON state (Figure 17(b)). The same parasitic inductance
To control the CPW even-mode propagation using ohmic-contact switches, the cantilever-type switch configuration shown in Figure 10 can be used. It is observed that the odd mode is suppressed using air bridges. Therefore, the equivalent circuit for this kind of switches is the same as the one shown in Figure 17, but changing the CPW line characteristic impedances
The CPW even mode can be controlled without suppressing the odd mode by using the bridge-type ohmic-contact series switch configuration shown in Figure 13. In this case, the equivalent circuit for the even mode is shown in Figure 18. In the OFF state, the switch behaves as a small series capacitance (
6.2. Experimental characterization
Figures 19 and 20 compare the measured to simulated results of the switches presented in Section 5 and modeled in Section 6.1. The switch insertion loss (ON state) and isolation (OFF state) are plotted as a function of frequency.
In Table 2, the values of the equivalent circuit elements obtained to fit measurement are listed. The capacitive switch features a high capacitance ratio (
|Parameter||Capacitive: direct clamp/folded||Ohmic: parallel||Ohmic: cantilever||Ohmic: series|
|28 / 7||28 / 7||180||180||100||100||0||230|
|1 / 30.5||3800 / 1530||0.9||−||5.1||−||3.35||−|
|0.27 / 0.1||0.27 / 0.1||0.3||3.2||0||1||0||2.4|
|76.1 / 78.6||76.1 / 78.6||100||100||45.3||45.3||97||97|
|118.7 / 148.5||118.7 / 148.5||195||195||63.5||63.5||152||152|
7. Design of multimodal reconfigurable microwave and mm-wave circuits
This section presents two applications of the RF-MEMS presented in Section 5 when combined with the multimodal circuits presented in Section 2: 180° phase switches and bandpass filters. All of them were fabricated using the process described in Section 3.
7.1. Phase switches
A 180° phase switch is a circuit that shifts the phase between 0 and 180°. This is a multi-purpose element required in the microwave and millimeter-wave applications, such as high-sensitivity radiometers or electronic beam steering in phased-array antennas. The specifications for these systems are, in most cases, a broadband operation, a low power consumption, and small size. They can be implemented monolithically using HBT/HEMT-based MMICs or using MEMS-based solutions as shown subsequently.
Figure 21(a) shows a compact, uniplanar 180° phase switch fabricated on a quartz substrate . It is based on two different back-to-back (BTB) CPW-to-slotline transitions  (symmetric and antisymmetric transitions, respectively), creating two phase paths with a relative transmission phase shift between them of 180°. Each path is selected using two single-pole-double-throw (SPDT) switches. The SPDT consists of two ohmic-contact MEMS series switches (described in Section 5.2.2) and a power divider. Figure 21(b) shows the measured performance of the circuit, featuring a 180° ± 5° phase shift between both states in a bandwidth of 35% (14–20 GHz), with an insertion loss smaller than 2 dB in both paths.
Figure 22(a) shows the photograph of a second compact, uniplanar 180° phase switch fabricated on a quartz substrate . In this case, it is based on an air-bridged CPW cross . The two CPW arms of the cross are loaded with capacitive-contact MEMS switches (described in Section 5.2.1). The two phase-switch states (0°/180°) are obtained by actuating the MEMS switches in opposite states (ON/OFF and OFF/ON), resulting in a multimodal interaction between the two CPW modes (even and odd) at the air-bridged cross. The CPW-to-slotline transition  at the input port and the CPW taper at the output port are included in order to enable the measurement of the circuit with a probe station. Figure 22(b) shows the measured results of phase shift between both states and insertion loss, featuring 180° +1.8°/−1° and ± 0.1 dB insertion-loss unbalance, respectively, in a very wide bandwidth (5–25 GHz). The measured insertion loss is better than 2 dB in 10–20-GHz frequency band.
7.2. Uniplanar bandpass filters
Figure 23(a) shows a second-order bandwidth-reconfigurable bandpass filter, which was fabricated on a quartz substrate (
Figure 24(a) shows a second-order bandpass filter fabricated on a 5-KΩ-cm high-resistivity silicon substrate (εr = 11.9,
In this chapter, the design considerations of RF-MEMS switches aimed to be integrated in uniplanar multimodal reconfigurable circuits for operation in the microwave and mm-wave bands have been presented. Different configurations of series and parallel switches featuring ohmic and capacitive contact have been discussed in detail, including the analysis of mechanical topologies which minimize the initial deformation due to residual stress. The fabrication process has been described, which provides the required flexibility to integrate the MEMS switches into higher-level RF communication systems. The switch RF behavior has been modeled using suitable equivalent circuits for both (ON/OFF) switch states. The models can easily be embedded into complex multimodal environments which enable multipurpose, compact designs. The fabricated switches have been experimentally characterized in terms of hysteresis, RF isolation (OFF state), and RF insertion loss (ON state), demonstrating an excellent behavior. As practical examples of the application into communication systems, some of the proposed switches have successfully been integrated into 0°/180° phase switches and into reconfigurable filters with either center frequency or fractional-bandwidth reconfiguration, showing excellent performances.
This work was supported by the Spanish MEC under Projects TEC2013-48102-C2-1/2-P, TEC2016-78028-C3-1-P and grant BES-2011-051305, the Mexican CONACYT under fellowship 410742, and the Unidad de Excelencia Maria de Maeztu MDM-2016-0600 financed by the AEI, Spain.
Bakri-Kassem M, Mansour RR. High power latching RF MEMS switches. IEEE Transaction on Microwave Theory and Techniques. 2015:222-232. DOI: 10.1109/TMTT.2014.2376932
Girbau D, Pradell L, Lázaro A, Nebot A. Electrothermally-actuated RF-MEMS switches suspended on a low-resistivity substrate. IEEE/ASME Journal of Microelectromechanical Systems. 2007:1061-1070. DOI: 10.1109/JMEMS.2007.904744
Mennai A, Bessaudou A, Cosset F, Guines C, Passerieux D, Blondy P, Crunteanu A. High Cut-off Frequency RF Switches integrating a Metal-Insulator Transition Material. In: IEEE MTT-S Int. Microwave Symposium; May 2015; Phoenix. New York: IEEE. pp. 1-3
DiNardo S, Farinelli P, Giacomozzi F, Mannocchi G, Marcelli R, Margesin B, Mezzanotte P, Mulloni V, Russer P, Sorrentino R, Vitulli F, Vietzorreck L. Broadband RF-MEMS based SPDT. In: 36th European Microwave Conference; September 2006; Manchester. New York: IEEE. pp. 1727-1730
Ocera A, Farinelli P, Cherubini F, Mezzanotte P, Sorrentino R, Margesin B, Giacomozzi F. A MEMS-Reconfigurable Power Divider on High Resistivity Silicon Substrate. In: IEEE/MTT-S International Microwave Symposium; May 2007; Honolulu. pp. 501-504
Contreras A, Casals-Terré J, Pradell L, Giacomozzi F, Iannacci J, Ribó M. A Ku-band RF-MEMS frequency-reconfigurable multimodal bandpass filter. International Journal of Microwave and Wireless Technologies. 2014:277-285. DOI: 10.1017/S1759078714000567
Cazzorla A, Sorrentino R, Farinelli P. Double-actuation extended tuning range RF MEMS Varactor. In: 45th European Microwave Conference; September 2015; Paris. pp. 937-940
Vähä-Heikkilä T, Varis J, Tuovinen J, Rebeiz GM. A reconfigurable 6-20 GHz RF MEMS impedance tuner. In: IEEE MTT-S International Microwave Symposium; June 2014; Fort Worth. New York: IEEE. pp. 729-732
Llamas MA, Girbau D, Ribó M, Pradell L, Lázaro A, Giacomozzi F, Margesin B. MEMS-based 180° phase switch for differential radiometers. IEEE Transaction on Microwave Theory and Techniques. 2010:1264-1272. DOI: 10.1109/TMTT.2010.2045558
Llamas MA, Girbau D, Ribó M, Pradell L, Giacomozzi F, Colpo S. RF-MEMS uniplanar 180° phase switch based on a multimodal air-bridged CPW cross. IEEE Transaction on Microwave Theory and Techniques. 2011:1769-1777. DOI: 10.1109/tmtt.2011.2140125
Contreras A, Ribó M, Pradell L, Casals-Terré J, Giacomozzi F, Iannacci J. K-band RF-MEMS uniplanar reconfigurable-bandwidth bandpass filter using multimodal immittance inverters. Electronics Letters. 2013:704-706. DOI: 10.1049/el.2013.0681
Mansour RR. RF MEMS-CMOS device integration: An overview of the potential for RF researchers. IEEE Microwave Magazine. 2013:39-56. DOI: 10.1109/MMM.2012.2226539
Bakri-Kassem M, Fouladi S, Mansour RR. Novel high-Q MEMS curled-plate variable capacitors fabricated in 0.35-μm CMOS technology. IEEE Transaction on Microwave Theory and Techniques. 2008:530-541. DOI: 10.1109/TMTT.2007.914657
Fouladi S, Mansour RR. Capacitive RF MEMS switches fabricated in standard 0.35-mm CMOS technology. IEEE Transaction on Microwave Theory and Techniques. 2010:478-486. DOI: 10.1109/TMTT.2009.2038446
Riverola M, Uranga A, Torres F, Barniol N, Marigó E, Soundara-Pandian M. A reliable fast miniaturized RF MEMS-on-CMOS switched capacitor with zero-level vacuum package. In: 2017 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications (IMWS-AMP); September 2017. pp. 1-3
Kaynak M, Ehwald KE, Drews J, Scholz R, Korndörfer F, Knoll D, Tillack B, Barth R, Birkholz M, Schulz K, Sun YM, Wolansky D, Leidich S, Kurth S, Gurbuz Y. BEOL embedded RF-MEMS switch for mm-wave applications. In: IEEE MTT-S International Electron Devices Meeting; December 2009; Baltimore/New York: IEEE; p. 1-4
Kaynak M, Wietstruck M, Zhang W, Drews J, Barth R, Knoll D, Korndörfer F, Scholz R, Schulz K, Wipf C, Tillack B, Kaletta K, Suchodoletz MV, Zoschke K, Wilke M, Ehrmann O, Ulusoy AC, Purtova T, Liu G, Schumacher H. Packaged BiCMOS embedded RF-MEMS switches with integrated inductive loads. In: IEEE MTT-S International Microwave Symposium; June 2012; Montreal. New York: IEEE; p. 1-3
Rynkiewicz P, Franc, A-L, Coccetti, F, Tolunay-Wipf, S, Wietstruck M, Kaynak M, Prigent G. Tunable dual-mode ring filter based on BiCMOS embedded MEMS in V-band. In: Asia Pacific Microwave Conference; November 2017; Kuala Lumpur/New York: IEEE; p. 124-127
Rebeiz GM. RF MEMS, Theory, Design and Technology. Hoboken: Wiley; 2003. 483p. DOI: 10.1002/0471225282
Giacomozzi F, Mulloni V, Colpo S, Iannacci J, Margesin B, Faes A. A flexible fabrication process for the fabrication of RF MEMS devices. In: International Semiconductor Conference; Octuber 2011; Sinaia. pp. 155-158
Benoit RR, Barker NS. Superconducting tunable microstrip gap resonators using low stress RF MEMS fabrication process. Journal of the Electron Devices Society. 2017:239-243. DOI: 10.1109/JEDS.2017.2706676
Pelliccia L, Farinelli P. Sorrentino R. High- tunable waveguide filters using Ohmic RF MEMS switches. IEEE Transaction. on Microwave Theory and Techniques. 2015:3381-3390. DOI: 10.1109/TMTT.2015.2459689
Shah U, Reck T, Frid H, Jung-Kubiak C, Chattopadhyay G, Mehdi I, Oberhammer J. A 500-750 GHz RF MEMS waveguide switch. IEEE Transactions on Terahertz Science and Technology. 2017:326-334. DOI: 10.1109/TTHZ.2017.2670259
Tolunay-Wipf S, Göritz A, Wipf C, Wietstruck M, Burak A, Türkmen E, Gürbüz Y, Kaynak M. 240 GHz RF-MEMS Switch in a 0.13 μm SiGe BiCMOS Technology. In: IEEE Bipolar/BiCMOS Circuits and Technology Meeting; October 2017; Miami. pp. 54-57
Nadaud K, Roubeau F, Pothier A, Blondy P, Zhang L-Y, Stefanini R. High Q zero level packaged RF-MEMS switched capacitor arrays. In: 11th European Microwave Integrated Circuits Conference; Octuber 2016; London/New York: IEEE; p. 448-451
Young WC, Budynas RG, Sadegh AM. Roak's Formulas for Stress and Strain. 8th ed. New York: McGraw-Hill; 2012
Wong WC, Azid IA, Majlis BY. Theoretical analysis of stiffness constant and effective mass for a round-folded beam in MEMS accelerometer. Journal of Mechanical Engineering. 2011; 57:517-525. DOI: 10.5545/sv-jme.2009.151
Fedder GK. Simulation of microelectromechanical systems [thesis]. Berkeley: Department of EECS University of California; 1994
Contreras A, Casals-Terre J, Pradell L, Ribó M, Heredia J, Giacomozzi F, Margesin B. RF-MEMS switches for a full control of the propagating modes in uniplanar microwave circuits and their application to reconfigurable multimodal microwave filters. Microsystem Technologies. 2017:5959-5975. DOI: 10.1007/s00542-017-3379-8
Llamas, MA, Girbau D, Pradell L, Lázaro A, Giacomozzi F, Colpo S. Characterization of Dynamics in RF-MEMS Switches. In: 10th International Symposium. RF MEMS and RF Microsystems (MEMSWAVE); 6-8 July 2009; Trento. pp. 117-120
Fargas-Marques A, Casals-Terre J, Shkel A. Resonant pull-in condition in parallel-plate electrostatic actuator. Journal of Microelectromechanical Systems. 2007:1044-1053. DOI: 10.1109/JMEMS.2007.900893
Casals-Terre J, Fargas-Marques A, Shkel A. Snap-action Bistable micromechanisms actuated by nonlinear resonance. Journal of Microelectromechanical Systems. 2008:1082-1093. DOI: 10.1109/JMEMS.2008.2003054
Elata D, Bamberger H. On the Dynamic Response of Electrostatic MEMS Switches. Journal of Microelectromechanical Systems. 2008:236-243. DOI: 10.1109/JMEMS.2007.908752
Leus V, Elata D. A new efficient method for simulating the dynamic response of electrostatic switches. In: 22nd International Conference on Micro Electro Mechanical Systems (MEMS); January 2009; Sorrento. New York: IEEE; p. 1115-1118
Casals-Terre J, Llamas MA, Girbau Pradell L, Lázaro A, Giacomozzi F, Colpo S. Analytical energy model for the dynamic behavior of RF MEMS switches under increased actuation voltage. Journal of Microelectromechanical Systems. 2014:1428-1439. DOI: 10.1109/JMEMS.2014.23147
Contreras A, Casals-Terré J, Pradell L, Giacomozzi F, Colpo S, Iannacci J, Ribó M. A RF-MEMS switchable CPW air-bridge. In: 7th European Microwave Integrated Circuits Conference; October 2012; Amsterdam/New York: IEEE; p. 441-444
Ribó M, Pradell L. Circuit model for mode conversion in coplanar waveguide asymmetric shunt impedances. Electronics Letters. 1999:713-715. DOI: 10.1049/el:19990507
Ribó M, de la Cruz J, Pradell L. Circuit model for slotline-to-coplanar waveguide asymmetrical transitions. Electronics Letters. 1999;1153-1155. DOI: 10.1049/el:19990779
Ribó M, Pradell L. Circuit model for coplanar-slotline tees. IEEE Microwave Guided Wave Letters. 2000:177-179. DOI: 10.1109/75.850369
Ribó M, Pradell L. Circuit model for a coplanar –Slotline cross. IEEE Microwave Guided Wave Letters. 2000:511-513. DOI: 10.1109/75.895085
Gaddi R, Bellei M. Gnudi A, Margesin B, Giacomozzi F. Low-Loss Ohmic RF-MEMS Switches with Interdigitated Electrode Topology. In: Symposium on design, test, Integration and Packaging of MEMS/MOEMS (DTIP 2004); 2004. pp. 161-166
Farinelli P, Margesin B, Giacomozzi F, Mannocchi G, Catoni S, Marcelli R, Mezzanotte P, Vietzorreck L, Vitulli F, Sorrentino R, Deborgies F. A low contact-resistance winged-bridge RF-MEMS series switch for wide-band applications. Journal of the European Microwave Association. 2007; 3:268-278
Calaza C, Margesin B, Giacomozzi F, Rangra K, Mulloni V. Electromechanical characterization of low actuation voltage RF MEMS capacitive switches based on DC CV measurements. Microelectronic Engineering. 2007:1358-1362. DOI: 10.1016/j.mee.2007.01.196
Mulloni V, Solazzi F, Resta G, Giacomozzi F, Margesin B. RF-MEMS switch design optimization for long-term reliability. Analog Integrated Circuits and Signal Processing. 2014:323-332. DOI: 10.1007/s10470-013-0220-x
De Angelis G, Lucibello A, Proietti E, Marcelli R, Bartolucci G, Casini F, Farinelli P, Mannocchi G, Di Nardo S, Pochesci D, Margesin B, Giacomozzi F, Vendier O, Kim T, Vietzorreck L. RF MEMS ohmic switches for matrix configurations. International Journal of Microwave and Wireless Technologies. 2012:421-433. DOI: 10.1017/S1759078712000074
Diaferia F, Deborgies F, Di Nardo S, Espana B, Farinelli P, Lucibello A, Marcelli R, Margesin B, Giacomozzi F, Vietzorreck L, Vitulli F. Compact 12×12 switch matrix integrating RF MEMS switches in LTCC hermetic packages. In: 44th IEEE European Microwave Conference; 6-9 October 2014. Rome/New York: IEEE; p. 199-202
Bastioli S, Di Maggio F, Farinelli P, Giacomozzi F, Margesin B, Ocera A, Pomona I, Russo M,Sorrentino R. Design and Manufacturing of a 5-bit MEMS Phase Shifter at K-band. In: European microwave In: Integrated Circuit Conference (EuMIC 2008); 27-30 October 2008. Amsterdam/New York: IEEE; p. 338-341
Bedani M, Carozza F, Gaddi R, Gnudi A, Margesin B, Giacomozzi F. A reconfigurable impedance matching network employing RF-MEMS switches. In: Symposium on Design, Test, Integration and Packaging of MEMS and MOEMS (DTIP); 25-27 April 2007; Stressa
Gaddi R, Gnudi A, Franchi E, Guermandi D, Tortori P, Margesin B, Giacomozzi F. Reconfigurable MEMS-enabled LC-tank for multi-band CMOS oscillator. In: IEEE MTT-S International Microwave Symposium; 17 June 2005; Long Beach. New York: IEEE. pp. 1353-1356
Cazzorla A, Farinelli P, Urbani L, Cacciamani F, Pelliccia L, Sorrentino R, Giacomozzi F, Margesin B. MEMS-based LC tank with extended tuning range for low phase-noise VCO. International Journal of Microwave Wireless Technologies. 2017:249-258. DOI: 10.1017/S1759078715001579
Giacomozzi F, Mulloni V, Colpo S, Faes A, Sordo G, Girardi S. RF-MEMS packaging by using quartz caps and epoxy polymers. Microsystem Technologies. 2014; 12:1941-1948. DOI: 10.1007/s00542-014-2256-y
Giacomozzi F, Iannacci J. RF MEMS technology for next-generation wireless communications. In: Handbook of Mems for Wireless and Mobile Applications. Woodhead Publishing; 2013. pp. 225-257. DOI: 10.1533/9780857098610.1.225
Alameh AA, Nabki F. A 0.13- μm CMOS dynamically reconfigurable charge pump for electrostatic MEMS actuation. IEEE Transaction on Very Large Scale Integration Systems (VLSI). 2017:1261-1270. DOI: 10.1109/ICECS.2014.7050074
Ismail Y, Lee H, Pamarti S, Yang C-KK. A 34V charge pump in 65nm bulk CMOS technology. In: IEEE International Solid-State Circuits Conference; February 2014; San Francisco. New York: IEEE. pp. 408-409
Sattler R, Plötz F, Fattinger G, Wachutka G. Modeling of an electrostatic torsional actuator: Demonstrated with an RF MEMS switch. Sensors Actuators A: Physical. 2002:337-346. DOI: 10.1016/S0924-4247(01)00852-4
Mercier D, Blondy P, Pothier A. Monitoring mechanical characteristics of MEMS switches with a microwave test bench. In: ESA-ESTEC 4th Round Table on Micro/Nano Technologies for Space; May 2003
Llamas MA, Ribó M, Girbau D, Pradell L. A rigorous multimodal analysis and design procedure of a uniplanar 180° hybrid. IEEE Transaction on Microwave Theory Techniques. June 2009; 16:1832-1839. DOI: 10.1109/TMTT.2009.2022881
Contreras A, Ribó M, Pradell L, Blondy P. Uniplanar bandpass filters based on multimodal immitance inverters and end-coupled slotline resonators. IEEE Transaction on Microwave Theory Techniques. 2013:77-88. DOI: 10.1109/TMTT.2012.2226743