FACTA UNIVERSITATIS 

Series: Electronics and Energetics Vol. 34, No 3, September 2021, pp. 333-366 

https://doi.org/10.2298/FUEE2103333T 

© 2021 by University of Niš, Serbia | Creative Commons License: CC BY-NC-ND 

Original scientific paper  

A COMPREHENSIVE OVERVIEW OF RECENT 

DEVELOPMENTS IN RF-MEMS TECHNOLOGY-BASED  

HIGH-PERFORMANCE PASSIVE COMPONENTS FOR 

APPLICATIONS IN THE 5G AND FUTURE 

TELECOMMUNICATIONS SCENARIOS* 

Girolamo Tagliapietra, Jacopo Iannacci 

Center for Sensors and Devices (SD) Fondazione Bruno Kessler (FBK),  

Trento-38123, Italy  

Abstract. The goal of this work is to provide an overview about the current development 

of radio-frequency microelectromechanical systems technology, with special attention 

towards those passive components bearing significant application potential in the currently 

developing 5G paradigm. Due to the required capabilities of such communication standard 

in terms of high data rates, extended allocated spectrum, use of massive MIMO (Multiple-

Input-Multiple-Output) systems, beam steering and beam forming, the focus will be on 

devices like switches, phase shifters, attenuators, filters, and their packaging/integration. 

For each of the previous topics, several valuable contributions appeared in the last decade, 

underlining the improvements produced in the state of the art and the chance for RF-

MEMS technology to play a prominent role in the actual implementation of the 5G 

infrastructure. 

Key words: RF-MEMS, switches, packaging, phase shifters, attenuators, filters 

1. INTRODUCTION  

Since the first discussion of RF-MEMS technology in late `90s, research and 

expectations concerning this topic have followed a fluctuating path, alternating the 

sensation of a general maturity and the need of some improvement to allow its practical 

utilization. After an initial enthusiasm towards RF passives based on MEMS, triggered by 

their outstanding performances, a subsequent phase of research was needed to overcome 

practical limits of these electro-mechanical systems, such as reliability and the possibility 

to package and integrate the devices with other existing technologies. Once these issues 

 
Received March 08, 2021; received in revised form April 05, 2021 
Corresponding author: Girolamo Tagliapietra 

Center for Sensors and Devices (SD) Fondazione Bruno Kessler (FBK), Trento-38123, Italy 

E-mail: gtagliapietra@fbk.eu 
* An earlier version of this paper was presented at the 1st International Conference on Micro/Nanoelectronics Devices, 

Circuits and Systems (MNDCS 2021), 29 - 31 January, 2021, in Silchar, Assam, India. [1].     



334 G. TAGLIAPIETRA, J. IANNACCI 

were addressed and RF-MEMS reached a renewed solidity, their supposed wide-scale 

market absorption in RF and millimeter wave communications was opposed by a scenario 

already dominated by cheaper and more consolidated technologies. In fact, advantages 

and commendable performances of RF-MEMS were not essential for the implementation 

of communication standards at the time [2]. Nevertheless, the last half decade portrays a 

more encouraging scenario: an increasing presence of RF-MEMS on the market, 

determined by a general maturity of this technology. 

Nowadays, components based on RF-MEMS are commonly available [3], so that to 

be employed in base stations [4] and mobile devices [5]. Moreover, RF-MEMS are 

predicted to play a prominent role in the deployment of the emerging 5G mobile 

communication standard, capitalising on their remarkable features of reconfigurability, 

low power consumption and RF performances along wide portions of frequency spectra 

[6]. Differently from previous opportunities, in the current scenario, the increasing 

interest towards RF-MEMS is actually driven by the needs of market. 

The use of femtocells, picocells and microcells to achieve high data rates imposes the 

employment of reliable, fast and wideband devices, capable to operate along frequency 

bands below 6 GHz and up to (24.25-29.5) GHz, (37-43.5) GHz, and (64-71) GHz bands, 

depending on the specific country [7][8]. From a more “macroscopic” point of view, the 

use of highly reconfigurable MIMO antennas, together with proper beam forming and 

beam steering of the basic radiation pattern is introduced to maximize the bandwidth available 

to users [7]. As mentioned in [9], in order to properly implement a network characterized by 

such variety of access points, various classes of powerful RF passives are required, including: 

▪ Very-wideband switches and switching networks (e.g. MPMT - Multiple Pole 
Multiple Throw), characterized by low losses, high Isolation and extremely low 

cross-talk between adjacent channels along the aforementioned frequency bands. 

▪ Reconfigurable filters with substantial rejection along stopband and minimal 
attenuation along the passband. 

▪ Very-wideband impedance tuners with multiple states. 
▪ Programmable attenuators with multiple configurations and constantly at attenuation 

along the considered portions of spectrum. 

▪ Very-wideband digital/analog phase shifters with multiple achievable values of 
phase shift. 

▪ Hybrid devices, comprising both attenuation and phase shifting capabilities into a 
single device. 

▪ Miniaturized antennas and antenna arrays permitting monolithic integration of the 
radiating element with abovementioned components. 

Thus, depending on the specific desired functionalities, RF-MEMS devices that could be 

adopted for implementation of 5G standard should satisfy different specific constraints. 

However, despite the variety of factors that may influence the requirements for a particular 

component, it is possible to quantitatively outline a set of reference performance to be 

pursued, which is reported in Table 1. As reported in [10][11], in the framework of MEMS 
devices, this set of reference performance must be matched with the separate and underlying 

issue of reliability. 

The goal of the present work is to provide a general overview about the state of the art 

of some RF-MEMS devices, which are considered as key components of RF front ends in 

the abovementioned 5G scenario. Switches are taken into account in Section 2 as one of 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 335 

the first components to be implemented by this technology, and for their massive presence in 

almost every complex RF-MEMS and non-RF-MEMS device. The section is complemented 

by a set of common guidelines for an optimal design of RF-MEMS switches. Packaging is 

considered in Section 3 as a critical step to enhance reliability, and to guarantee the practical 

integration of such devices on common platforms. Since beam forming techniques are 

basically performed by attenuating and phase-shifting the signal feeding a radiating element, 

phase shifters are discussed in Section 4, including their most common architectures. For the 

same reason, attenuators are treated in Section 5. As critical components in every RF front 

end, filters are treated in Section 6. These different topics are presented by explaining their 

basic working principles, and examining several research contributions available in the 

scientific literature. 

Table 1 Desirable characteristic features for devices operating in 5G network. 

Feature Desired Value Motivation 

Frequency range <6   & (24.25-71) GHz The diversification of the infrastructure imposes such 

wide range. Mobile terminals will have to operate 

correctly when connected to base stations or cells. 

Isolation > -30 or -40 dB A crucial feature in reconfigurable switch- based passives, 

it should be as high as possible. 

Loss < -1 dB Given the multiple components housed in common RF 

front ends, single losses should be minimized along the 

widest achievable frequency span. 

Cross-talk < -50 or -60 dB Presence of MIMO systems and closely spaced 

components prescribes maximum isolation between 

adjacent channels. 

Switching time < 200 or 300 µs The low latency required by the diversified network 

requires fast commutation of switching devices. 

Control voltage < 2 V Compliance with typical bias voltages of circuits and 

avoidance of ad hoc additional circuitry in mobile 

devices. 

2. SWITCHES 

RF-MEMS based switches represent one of the early and principal components in this 

field of research. They have been investigated since the second half of 1990s and they are 

part of almost all complex networks and devices implemented in such a technology. 

In order to play a prominent role in the current 5G paradigm, switches have to comply 

with the specific requirements reported in Table 1, together with a satisfying reliability, 

which should be greater than one billion cycles in some applications. Many valuable 

research solutions addressed the above-mentioned requirements in the last years, although 

trade-offs are generally needed between electrical and mechanical performances. 

From the point of view of classification, the movable membrane allowing or avoiding the 

electrical connection of the signal line to the output port, or to RF ground, may realize a ohmic 

(metal-metal) or capacitive (metal-dielectric) contact; on the basis of the contact, the switch 

will be ohmic or capacitive. The target of the connection (signal line or ground) determines 

the series or shunt nature of the device; hybrid series-shunt implementations are also 

achievable by particular arrangements [12]. The membrane may move vertically or 



336 G. TAGLIAPIETRA, J. IANNACCI 

horizontally, while its structure may be based on a cantilever or a clamped-clamped type, 

employing different shapes of beams and membranes. Actuation may take place by 

piezoelectric, electrothermal, electromagnetic or electrostatic transduction mechanism, with 

the last one being the most widely employed. 

From a practical point of view, Isolation performances of ohmic switches are quite 

broadband, limited only by the capacitance between the movable membrane and the 

signal line, when the membrane is not actuated. This capacitance determines a decreased 

impedance as frequency increases, affecting overall isolation of the device. In case of 

capacitive switches, the contact point is covered by a dielectric layer. This determines a low 

capacitance in non-actuation “OFF” state, and a high capacitance when the membrane is 

actuated in “ON” state. Since this limits the Isolation performances at both high and low 

frequencies, capacitive switches can be considered more narrow-band. Due to this dynamics 

of their capacitance values, capacitive switches are usually evaluated on the basis of their 

“ON/OFF” capacitance ratio, which should be as high as possible. Additional details 

concerning advantages or vulnerabilities of such devices in terms of structure, performances, 

frequency range of usability are investigated in depth in [11]. 

Considering the large amount of literature in this field, some interesting proposals of the 

last half decade are considered. To this end, Table 2 is provided for a better comparison of the 

discussed research items in terms of their characteristic parameters. 

2.1. Ohmic Switches 

Considering ohmic switches, [13] presents a buckle-beam structure, whose actuation 

follows a vertical movement to connect the two interrupted sections of the signal line, 

thanks to a buried electrode. The beam structure is based on a Chevron thermal actuator, 

in which the provided voltage causes heating and deformation of the structure, with 

consequent horizontal displacement and de-actuation. Its working principle is represented 

in Figure 1a. Besides its remarkable electrical performances within the measured (0-20) 

GHz range, the device based on silicon substrate has been thermally characterized in the 

range from -20 to 140°C to test possible detrimental effects of common packaging 

processes on the original structure. The results demonstrate minimal variations of pull-in 

(actuation) voltage and losses. 

With the target of a device operating at D band ((110-170) GHz), [14] proposes an 

SPDT (Single Pole Double Throw) switch integrated in BiCMOS (Bipolar CMOS) 

technology, based on an initial T-junction connected to two terminal SPST (Single Pole 

Single Throw) switches. Each SPST switch is encompassed in a cavity containing two 

actuation electrodes on both sides of elevated signal line, an overhead membrane, and a 

metallic plate for protection on top of the cavity, as reported in Figure 1b. The actuation 

of the membrane will connect the signal line to ground, isolating the corresponding 

output port. By choosing an actuation voltage of 60 V, the authors reached switch-on and 

switch-off time smaller than 10 μs. 

Another substantial contribution comes from [15], in which the traditional series ohmic 

cantilever switch is provided with two additional cantilevers allowing a shunt connection to 

ground of the CPW (Coplanar WaveGuide), as shown in Figure 1c. Actuation of additional 

shunt connections has a dual purpose: decreasing the voltage swing on the main series 

contact and increasing Isolation of the output port. This protection mechanism has proven to 

extend reliability of this 400x300 μm2 device up to 500 million cycles with 1 W RF power. 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 337 

Beside electrical performances, the switching time is remarkable too, being 30.4 μs and 

39.8 μs for switch-on and switch-off time, respectively. 

Fig. 1 (a) Movable membrane reported in [13], (b) cross section of switch cavity reported in 

[14], (c) surface current distribution on switch realized in [15] when lateral cantilevers 

are actuated to ensure maximum isolation. 

In [16] a traditional series ohmic switch based on GaAs MMIC (Monolithic Microwave 

Integrated Circuit on Gallium Arsenide) technology is discussed, with a series of etched 

holes on the surface of the membrane, in order to decrease the pull-in voltage. Power 

handling characterization demonstrated acceptable performances up to 36 dBm, while 

reliability of 2.4x107 cycles has been verified. 

A more innovative design is reported in [17], where a laterally moving switch is 

proposed in order to connect the two terminations of the signal line. The movable section is 

always in touch with the input sections, and it comes in touch with the output section by 

electrostatic actuation. Triangular beams in the central part of the membrane structure 

provide mechanical restoring force, to move the section back to its rest position. A voltage 

of 90 V must be applied to ensure proper contact between the signal lines, with a 38 and 57 

μs for switch-on and switch-off time, respectively. Performances for an application in 5G 

scenario has been considered at 3.5 GHz and 28 GHz. More complex networks have been 

implemented by this basic switch, including SPDT, SP3T (Single Pole 3 Throws), and 

SP6T (Single Pole 6 Throws), showing to be operational beyond 1 billion cycles with 1 W 

RF power. Footprint of these devices based on alumina substrate does not exceed 0.7 mm2. 

  
(a) (b) 

 
(c) 



338 G. TAGLIAPIETRA, J. IANNACCI 

Another design has been simulated in [18], comprising: a cantilever vertical switch, 

with serpentine shaped beams, a center membrane and two free ends (contacts) provided 

with holes. The presence of holes on membrane and free ends, a 2.4 μm air gap, the 

choice of material and serpentine shaped beams allowed authors to reach a simulated low 

actuation voltage and commendable switch-on time of 55 μs. Moreover, the presence of 

two contact points is intended to increase both reliability and current-carrying capabilities 

of the device, decreasing the occurrence of detrimental surface nonidealities on the entire 

contact area. 

2.2. Capacitive Switches 

Considering capacitive switches, significant contributions have been proposed by 

[19][20][21], covering different aspects. The first one concerns a 4-beams cantilever 

structure: each beam is placed on a corner of a square area, each one provided with its 

shared tether and two springs. Stability of the structure against stress gradient is 

guaranteed by a central joint. Underlying dimples are employed to prevent a collapse of 

the structure and instability of capacitance value. Fabricated on quartz substrate, this 

400x400 μm2 device has a switch-on time of 55 μs. Besides the original design, depicted 

in Figure 2a, such work is remarkable in terms of power handling capabilities: the device 

has been characterized in its pull-in and de-actuation voltage, showing acceptable 

variations within (1-12) W. Hot switching conditions are also evaluated, demonstrating 

little sensitivity of the capacitance to temperature variations within 25-125°C. 

Focused on Ka-band, and on a design minimally affected by hard fabrication 

temperatures, [20] proposes a folded-leg structure characterized after packaging process. 

Both actuation voltage and up-state capacitance exhibited minimal changes after 200°C 

thermal treatment, demonstrating a stable behavior of such quartz-based device during 

the fabrication process. 

Equally focusing on Ka-band, [21] offers a device provided with an actuation 

electrode on both sides of signal line. Vertical movable membrane actuates in just 15 μs, 

and it features large perforated areas between surfaces devoted to actuation and 

capacitive coupling, connected to CPW ground by meandered beams. As indicated by 

Table 2, measured electrical parameters are remarkable, and this is also due to the high 

capacitance ratio of this roughly 1 mm2 device, fabricated on high resistivity silicon. 
The implementation proposed in [24] is marked by remarkable performances as well, 

since it exhibits minimal Insertion Loss along a wide frequency range, with a reduced 
actuation voltage (3.5 V). In fact, when all its membranes are actuated, an Insertion Loss 
smaller than 0.1 dB is achievable in the (0-25) GHz range. The simulated 640x820 μm2 
structure is based on silicon substrate and it is embodied in a CPW configuration with 
four cantilever membranes, each one provided with a single serpentine shaped beam. 
Activation of a single or multiple membrane determines different configurations, in 
which Return Loss is smaller than -20 dB along intervals varying from 1.6 to 16.1 GHz. 

Targeting K-band satellite applications, [22] introduces a simple and effective design 
on silicon nitride, simultaneously achieving low actuation voltage (4.7 V) and Insertion 
Loss (- 0.3 dB) up to 45 GHz. Capitalising on meandered beams, a traditional rectangular 
perforated membrane with stepped profile, and 1.7 μm air gap, a structure with 175 μs 
switch-on time and high capacitance ratio (132) is obtained. 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 339 

  
(a) (b) 

Fig. 2 (a) Top view of the concept described in [19], (b) Conceptual representation of 

capacitive switch realized in [23]. 

A more complex device is suggested by [23], combining thermal actuation and 

electrostatic holding: the first for the actual downward or upward movement, and the second 

for holding the actuated membrane. In this way, low actuation voltage is achieved, still 

involving low power consumption. As visible in Figure 2c, the moving membrane is 

composed by two metal layers: the first one for upward (C and D electrodes) and the second 

for downward actuation (A and B electrodes), with an interposed dielectric layer. The holding 

electrode (E) completes the device, attracting the first layer and keeping the whole membrane 

actuated. This double electrothermal mechanism is employed also in case of stiction, in order 

to provide additional restoring force when the membrane is stuck because of micro-welding or 

charges trapped in the dielectric. In fact, its measured electrical properties remained constant 

up to 21 million cycles, while no stiction took place even after 80 million cycles. With an 

actuation and holding voltage of 0.3 and 15.4 V, this 160x300 μm2 device demonstrates high 

Isolation up to 20 GHz. 

Table 2 Comparison between the different discussed realizations, including ohmic and 

capacitive switches. Reported values of Return Loss (S11), Insertion Loss (S12 

when CLOSE), or Isolation (S12 when OPEN) refer to worst value along the 

measured interval. 

Work Type Measured Ret. Loss Ins. Loss Isolation Actuation 
  @(GHz) (dB) (dB) (dB) (V) 

 [13] Ohm 0-20 25 0.45 20 90 
 [14] Ohm 140 - 1.42 54.5 60 
 [15] Ohm 0-40 - 0.48 36 60 
 [16] Ohm 1-40 20.09 0.25 22.5 17.5 
 [17] Ohm 3.5&28 21&11.4 0.28&2.82 28 90 
 [18] Ohm 1-40 15 0.6 19 3.1 
 [19] Cap 20 8 1.1 12 55 
 [20] Cap 35 18 0.3 35 25 
 [21] Cap 35 27 0.29 20.5 18.3 
 [24] Cap 5 28 2 49 3.5 
 [22] Cap 27 13 0.3 35 4.7 
 [23] Cap 2.4 - 0.28 31 15.4 



340 G. TAGLIAPIETRA, J. IANNACCI 

2.3. Set of Reference Guidelines in Design and Optimization of Multiphysics 

Design of RF-MEMS 

All the previously mentioned realizations exhibit respectable performances, covering 

different portions on the spectrum allocated to the current 5G paradigm. Bearing in mind 

the target requirements, some of the mentioned solutions are more favorable in terms of 

electrical performances, while others may be more convenient for their reliability, and 

others, again, for the possibility to be easily integrated with other circuits. Further solutions, 

instead, score viable trade-offs between all the mentioned characteristics. In any case, the 

existing vulnerabilities of a certain design concept can be properly addressed, depending on 

their nature. To this end, as pointed out in [11], there are different guidelines that may help a 

designer involved in the optimization of a particular device. 
Starting from the electromechanical behavior of RF-MEMS, a shorter switching time can 

be achieved by acting on the actuation principle, as well as on the structural or electrical 
features of the device at stake. Piezoelectric actuation generally guarantees a faster actuation, 
but it is prone to unwanted parasitic actuations [25]; in particular, mismatches of lattice or 
thermal expansion coefficients of the different layers will determine the presence of residual 
stress in the structure, leading to cracks and bends [26]. From the structural point of view, 
multiple and large holes on the movable membranes are a wise option, since they increase the 
speed of actuation by reducing the damping of air. This is one of the criteria behind the design 
of the membrane reported in [22]. Another option consists in modulating the amplitude of the 
biasing waveform, as discussed in [27]. While a smaller voltage (as compared to pull-in 
voltage) can be adopted to maintain the membrane actuated, a short and relatively high 
voltage peak of voltage can accelerate the movement of the membrane. 

An easier integration on common wireless or mobile devices usually requires low 
actuation voltages, compatible with typical supply voltages of CMOS technology (roughly 
from 1.5 to 6 V). Such low voltages avoid the presence of converters, saving costs and 
space on chips. Another beneficial effect is an enhanced reliability, provided by the reduction 
of mechanical shock on the movable membrane. Within the framework of electrostatic 
actuation, there are three main structural strategies to achieve such low voltages: decreasing 
the spring constant, maximizing the actuation area, and decreasing the air gap. 

The spring constant of the movable structure can be lowered by adopting folded or 
meandered beams with reduced thickness, as in [20][22]. The negative consequence of a 
reduced spring constant of a structure is an attenuation of its restoring force, which, in turn, 
determines an increased risk of stiction (missed release of the actuated membrane after the 
zeroing of the actuation voltage). This issue can be counteracted by inserting additional 
electrodes, which could help the proper release of the membrane; but sometimes this solution 
is not affordable: beside the significant cost in terms of complexity of the design, in some 
cases, the housing of electrodes above the membrane may excessively complicate the 
fabrication process. In those cases, a viable option is represented by a proper exploitation of 
packaging, as discussed in Section 3. Maximizing the actuation area consists in maximizing 
the overlapping area between the movable structure and the fixed electrodes; for this purpose, 
designers would prefer membranes with relatively large areas devoted to actuation, and large 
electrodes, as in [21]. The decrease of the air gap is a viable option, but it should be carefully 
managed in case of capacitive switches. In that case, a small gap between the non-actuated 
membrane and the underlying contact layer would significantly increase the OFF-state 
capacitance. However, the corresponding limitation in the achievable Isolation can be 
compensated by matching circuits that mitigate the parasitic capacitance [28][29]. 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 341 

Better electrical performances of switching devices can be obtained by a wise selection of 

the structure and materials, or by introducing additional electrical components. In particular, 

structural modifications may involve the deployment of additional shunt membranes on a 

common series ohmic SPST switch. This choice is the one adopted in [15], demonstrating 

beneficial effects on Isolation. Another strategy regarding series ohmic switches is reported in 

[30], in which a triangular cantilever membrane acts as a tapered section of the transmission 

line, reducing the Return Loss of the device. In case of capacitive switches, the achievable 

Isolation in mainly limited by the up-state capacitance, while Insertion Loss is bounded by 

down-state capacitance [31]. For this reason, a common practice is to maximize the 

capacitance ratio. The ratio can be maximized at structural level by adopting bending 

structures or by introducing floating metal layers. Bending structures, such as warped beams 

or plates, represent a viable option, but they may lead to an increased actuation voltage. The 

addition of a floating layer generally represents a better solution: the metal plate on top of the 

upper dielectric layer does not affect the up-state capacitance, but it acts as a wider upper plate 

of a capacitor when the membrane is actuated. This leads to a higher down-state capacitance. 

The solution of a floating metal layer should be preferred also because of a practical issue: the 

surface roughness of the dielectric is generally not accounted in simulations. Actuated 

membranes with a non-uniform adhesion to the underlying dielectric layer, or surface 

roughness, will determine an interposed layer of air, decreasing the achievable down-state 

capacitance [32][33]. The adoption of the floating layer allows to overcome these practical 

issues. 

Regarding the substrate and the dielectric, the choice of materials and their thickness 

is an important subject. Good electrical performances are usually obtained by using 

substrates with high resistivity, such as quartz, glass, high-resistivity silicon, gallium 

arsenide (GaAs) or silicon-on-glass (SiOG) [34][35][36]. In case of capacitive switches, 

dielectric materials with a high permittivity and arranged in very thin layer are preferable, 

since they ensure a high capacitance when the membrane is actuated. The main 

candidates are piezoelectric leade zirconate titanate (PZT), strontium titanate oxide 

(SrTiO3), tantalum pentoxide (Ta2O5) and hafnium dioxide (HfO2) [37][38][39]. However, 

while some of them are relatively expensive, the thickness of the dielectric must be 

chosen accordingly to the available fabrication process, and it must ensure to sustain the 

pull-in voltage without a breakdown. 

The introduction of additional electrical components generally involves the deployment of 

shunt inductors or sections of transmission lines. In fact, the beams supporting the 

movable structure introduce some parasitic inductance, affecting the Isolation of the 

device. This inductance is generally compensated by introducing matching sections or 

reactive components [29]. 

Reliability is an important issue, investigated in depth in the framework of RF-MEMS 

devices, thus different strategies have been proposed so far. The occurrence of stiction can be 

mitigated or counteracted by some of the already mentioned strategies, such as the insertion of 

additional electrodes to enforce the release, the decrease of pull-in voltage, the modification of 

voltage actuation waveform or the adoption of additional shunt membranes. Beside these 

techniques, other expedients regarding the structure and employed materials can be listed. 

From a structural point of view, the most common are the use of frames with reinforcing 

bars on the movable structure, and stopping pillars or thin posts under the membrane 

[40]. The underlying idea is to prevent deformations of the membrane and its complete 

adhesion to the dielectric layer, in which residual trapped charges may be present. Other 



342 G. TAGLIAPIETRA, J. IANNACCI 

possibilities to overcome the problem of trapped charge is the adoption of lateral actuation 

mechanisms, as in [17], or the more "radical" option of a dielectric-less design [41]. 

An increased lifetime of the device can be achieved also by a proper selection of 

materials. In case of ohmic switches, stiction is mainly caused by increase of resistance, 

welding, fusion or transfer of metal contacts. This is generally prevented by the adoption 

of harder metals, such as molybdenum (Mo), palladium (Pd), tungsten (W) and rhodium 

(Re), often organized in multilayers [42][43][44]. In case of capacitive switches, besides 

charge injection and trapping, reliability is threatened by mechanical shock and the 

breakdown of the dielectric layer. These problems can be mitigated by using layers that 

may rapidly release charges, such as aluminium nitride (AlN) and PZT [45], by removing 

surface roughness and nonidealities, or by adopting robust structures with double 

dielectric layers. 

3. PACKAGING 

When talking about reliability of MEMS components, it is not possible to omit the closely 

related field of packaging. Besides enclosing MEMS structures in a “sealed” space, less 

sensitive to changes of temperature or humidity, packaging avoids contamination with 

harmful agents and external mechanical shocks. However, this indispensable interface to outer 

world introduces: additional costs of production, increased volume and impairment on 

performances of the original (naked) MEMS device. The two main choices for packaging at 

technology level are chip-scale and wafer-level package. The latter is the most widely 

employed for its lower cost, efficiency in packaging process, reduced size and decreased 

impairment of performances. Such a technique can rely on two fundamental approaches: 

application of a cap and thin-film packaging. 

The former approach adopts silicon-based materials as a cap covering the device, as 

depicted in Figure 3a. Typical materials for the realization of the cap are represented by 

Benzocyclobutene (BCB), co-fired ceramic (LTCC), silicon (Si), and quartz (SiO2). 

Depending on the presence (or absence) of a sealing material, interposed between the cap and 

the device wafer, intermediate bonding (or direct bonding) techniques are employed to seal 

the cap. Intermediate bonding is generally more convenient because of its relatively low 

temperatures ((200-400)°C), since direct bonding techniques may require up to 1000°C (in 

case of fusion bonding, as reported in [46]). The cap is placed on MEMS device by optical 

alignment and typically sealed by gold (Au), SiO2, BCB, or SU-8. 

  
(a) (b) 

Fig. 3 (a) General practical implementation of capping reported in [47], (b) detail of 

quartz capped SP4T (Single Pole 4 Throw) switch, reported in [48]. 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 343 

Examples are reported in [48][49][50][51] showing switches encapsulated by intermediate 

bonding techniques at temperatures in the range (200-250)°C, which have not been 

particularly impaired by packaging procedures. In particular [48] presents various devices, 

including the SP4T switch represented in Figure 3b, protected by a quartz cap fixed on 

substrate by epoxy polymer SU-8. Depending on the substrate flatness, two fabrication 

methods are proposed: one for substrates with cavities and holes, and one for flat substrates. 

Experimental results of electrical parameters show minimal degradation of Insertion Loss (an 

increase of 0.1-0.2 dB) up to 30 GHz, providing a capped device not exceeding -0.7 dB up to 

40 GHz. Return Loss is slightly impaired by the impedance mismatch introduced by the cap, 

reaching values up to 10 dB in the range up to 20 GHz. In the remaining portion of the 

measured spectrum, the Return Loss of the capped device complies with the one of uncapped 

version along the measured range. Encapsulation did not drastically affect electro-mechanical 

properties. While for a shunt capacitive switch there was no change in capacitance and 

actuation voltage was just slightly increased, a measured SP4T switch showed a minimal 

change in series resistance and a decrease of actuation voltage. 

A more recent contribution comes from [52], in which a silicon cap bonded by spin-

coated BCB is accurately modeled and added to different switch devices. The different 

mounted caps have been tested in terms of resistance to shear stress, by applying 

horizontal force only to the cap while keeping the wafer fixed. Packages exhibited a high 

average break force (force needed to detach the cap) of 21 N. The increase of Insertion 

Loss due to the proposed package is around 0.2 dB, with an average increase of actuation 

voltage of 4.2 V among the nine measured switches. All capped switches show remarkably 

low Insertion Loss (<-0.44 dB), and reasonable Return Loss (<-20.9 dB) all over the 40 GHz 

measured interval, operating for more than 10.2 billion cycles. 

The approach of thin film packaging generally involves a sacrificial layer placed on 

movable parts of a MEMS device, covered with a thin film layer. Such layer will be 

perforated to remove the sacrificial layer, and then covered with a proper layer for final 

sealing, as shown in Figure 4a. In this case, gold [53] and silicon-based materials are 

commonly employed solutions for the thin film, as observed in [47]. Such materials, besides 

various polymers, can be adopted also as sealing layer, generally patterned by deposition, 

evaporation and electroplating, spin-coating or plasma-enhanced chemical vapor deposition 

(PECVD). Examples of such technology are reported in [53][54][55][56], showing 

encapsulated devices still maintaining remarkable RF performances after the packaging 

process.  

A more recent proposal comes from [57], in which a protective dielectric shell 

comprises an electrode, biased to ensure maximum vertical upward displacement of the 

movable membrane of a capacitive switch. This is beneficial both for the capacitance 

ratio and reliability aspects. Possible configurations of this device are represented in 

Figure 4b. Contact holes and release channels are etched on sides of the shell to remove 

the sacrificial layer; a Si3N4 layer is then deposited by PECVD to hermetically seal the 

structure. Measurements over hours of operations with square bipolar biasing voltages 

show that both actuation voltage and capacitance suffer from minimal variations, leading 

to an increase of just 0.2 dB in Insertion Loss. 

 



344 G. TAGLIAPIETRA, J. IANNACCI 

Fig. 4 (a) General practical implementation of thin-film packaging reported in [47], (b) 

cross section of the device reported in [57] in both its states. 

An interesting contribution is represented by [58], in which an investigation regarding 

the amount of perforated area on the thin film for the proper release of encapsulated 

devices is reported. In this work, thin film layer is perforated with holes of different 

diameters (7 and 8 μm), and different realizations of perforated films are obtained, each 

one with a different percentage of perforated area. After the identification of the most 

appropriate percentage (at least 57%), different combinations of power and time are 

considered for the proper release of the considered devices. Complete 3D FEM (Finite 

Element Method) models of the considered realizations on gold CPW have been 

implemented and simulated, showing no significant variations of Return or Insertion Loss 

introduced by the packaging phase. 

 
(a) 

 

(b) 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 345 

Table 3 Comparison between the different discussed packaging implementations, 

including both cap and thin film solutions. The provided dimensions in the order 

of mm, and tens or hundreds of µm, refer to dimensions of the whole packaged 

device. 

Work Type Cap or Film/ 
sealing 

Dimensions Ins. Loss Isolation Temp. 

  Materials (µm) (dB/@GHz) (dB/@GHz)   °C 

[49] Cap LTCC/Au 1.4x0.9x0.8 0.5/5 20/5 250 

[50] Cap BCB/BCB 2mm2x28 0.7/30 18/30 250 

[51] Cap Si 5000x4000x950 0.5/6 24/2.5 - 

[48] Cap SiO2/SU-8 - 0.7/30 - 200 

[52] Cap Si/BCB - 0.4/35 35/35 250 

[53] Film Au/SiO2 - - 27/2 <120 

[54] Film Si3N4/Au 75x80x 1.2 - 30/2 - 

[56] Film Na/SiO2 - x - x4 0.45/142 51.6/142 200 

[55] Film SiO2/ 

ALX2010 

<600x600x - 0.25/12 35/12 - 

[57] Film Si3N4 70x50x - 2/25 8/25 - 

4. PHASE SHIFTERS 

Being vital components in the architecture of Phased Arrays and RF front ends, phase 

shifters are abundantly employed in radar systems (L, S, C, X bands), as well as for 

mobile and satellite communications (Ku, K, Ka bands). Current developments in terms 

of mobile communications, with the adoption of millimeter wave frequencies, determined 

an accentuated interest towards MEMS-based phase shifters. Consequently, the literature 

regarding this field is quite populated. 

The first implementations of phase shifters based on RF-MEMS were proposed in late 

`90s, both concerning digital [59] and analog [60][61] realizations. Shortly after, the main 

topologies concerning the architecture of the proposed phase shifters were defined, 

according to the following classification: switched-line (SL), loaded-line (LL), distributed 

MEMS transmission line (DMTL), and reflection-type (RT) phase shifters. 

As reported by more comprehensive books and overviews, such as [62][63][64], the 

working principle of switched-line devices is the switching between signal paths of 

different electrical length, in order to achieve the desired phase delay. However, when a 

wide phase shifting range or multiple small phase steps are required, such strategy may 

lead to bulky arrangements. Conceptually similar, the basic idea of loaded-line phase 

shifters is to load a transmission line with two reactive impedance networks. Often realized 

with stubs, capacitive or inductive loads will cause phase lags or leads respectively, while the 

transmission line section will ensure impedance matching. As in the case of switched-line 

devices, extended shifting performances will lead to bulky phase shifters. 

In case of DMTL, the transmission line of the signal path is periodically loaded with 

varactors, changing the distributed capacitance of the line and, in turn, the line phase constant 

and, finally, the resulting phase delay at the output port. When wide shifting or precision 

performances are required, the presence of multiple MEMS capacitive membranes determines 

arrangements with pronounced area, high aspect ratio and prone to reliability issues. 



346 G. TAGLIAPIETRA, J. IANNACCI 

RT phase shifters typically employ quadrature couplers with equal and tunable reflective 

loads connected to coupled ports, controlling the phase delay of the reflected signal. Although 

this approach may seem less troublesome, it is characterized by underlying trade-off between 

the achievable shift range and the Insertion Loss. Such trade-off is due to the need of effective 

modeling, design and matching of the coupler and of the reflective load. 

As highlighted in [65], although phase shifters based on RF-MEMS represented a 

powerful solution to simplify the architecture of phased arrays, some substantial 

improvements were mandatory at that time. The main requirement was a general reduction of 

the physical footprint. Other concerns were mostly related to the employed switching 

units within the phase shifters, the most relevant ones being limited reliability, high 

switching time and limited power handling capabilities. These factors confined early RF-

MEMS phase shifters to arrays with relatively slow scanning and radiated power, while 

the physical size prevented the integration in case low microwave frequencies were at 

stake (<20 GHz). 

Many research solutions of the last decade remarkably faced the challenge of 

increasing the shifting range and phase precision, and considerable steps ahead have been 

made in the direction of miniaturization, still lowering insertion loss. Tables 4 and 5 are 

provided along the present section for a better comparison of the discussed research items 

in terms of their characteristics. 

4.1. Switched-Line 

Concerning switched-line topologies of RF-MEMS phase shifters, some relevant 

contributions are reported in [66][67][68]. In particular, the device presented in [66] and 

shown in Figure 5a has a wide shifting range and remarkable power handling capabilities 

(5 W), at the expense of losses (-3.1 dB Insertion Loss) and footprint (7.5x6 mm2). The 

employed SP2T and SP4T switches demonstrated very limited switching time (5 μs) and 

high reliability (100 M cycles), despite a rather high actuation voltage of 90 V. Since 

both [67][68] propose 1-bit 180° phase shifters, their intrinsic simplicity allows more 

appreciable losses and area (9 and 15 mm2). 

Additional suggestions are offered by [69][70][71][72]. In particular, [69] suggests a 

4-bit phase shifter for LTE base station, fabricated on common PCB substrate. The path 

from the input to the output port is composed by two sections, each one provided with 

four paths of different electric length, as depicted in the block scheme of Figure 5b. The 

switching is performed by two SPDT on both sides of a section, actuated by 90 V signal 

in 10 μs. 

A 5-bit architecture focused on Ku band is described in [70]. As reported in Figure 5c, 

out of the three sections in which the device is divided, two are characterized by four 

paths with different electrical length selected by a SP4T switch on each side. The 

remaining section contains just two paths, selected by a couple of SP2T switches. By 

applying 53 V pull-in voltage, switch-on time is 28 μs for both kinds of micro-relays. The 

overall area of the device is 3.19x5.17 mm2. In case of both cold and hot switching 

conditions, the device has been demonstrated with more than 10 million cycles from 0.1 

to 1 W of RF power. 

The 3-bit device reported in [71] has been characterized in the Ka band, and it is 

composed by two SP8T switches selecting one output of the available eight signal paths. 

With a total area of 5.9 mm2, this phase shifter based on Alumina substrate employs 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 347 

switches with actuation at 65 V. Reliability measurements at room temperature reported a 

satisfactory behavior beyond 100 million cycles for the range of power (0.1-1) W, and beyond 

30 million cycles for 2 W of RF power. Possible improvements for the two previous works 

are reported in [72], together with a DMTL solution for a K band phase shifter by the same 

authors. 

Another recent proposal is provided by [73], with two realizations of 4-bit phase 

shifters for K and Ka frequency bands, in both cases composed by 4 sections comprising 

a reference and a shifting path. A couple of monolithically integrated MEMS SPST 

switches control the activation of each path by 45 V signal, allowing 16 phase states. 

Fabricated variants for K band show a footprint of 10.5x7.5 mm2. 

 

Fig. 5 (a) Switched-line phase shifter described in [66], (b) block scheme of the device 

reported in [69], (c) top view of architecture proposed in [70]. 

4.2. Loaded-Line 

Regarding loaded-line architecture, noteworthy results are obtained in [74][75]. The 

first one describes a device composed by a transmission line periodically loaded by a 

total number of eight 4-bit tunable capacitive loads. The control of capacitors is achieved 

by (lateral) electrostatic and (vertical) electrothermal actuation. The overall 16 states 

 

 
(a) (b) 

 
(c) 



348 G. TAGLIAPIETRA, J. IANNACCI 

allow to tune digital capacitors from 15.5 to 111.6 fF, changing phase velocity of the line 

and determining a maximum phase shift of 337.5°. While electrostatic actuation requires 

20 V, its electrothermal requirement is just 4.8 V. The proposed phase shifter has been 

designed to be monolithically integrated with conventional CMOS, and involves an 

overall footprint of 4.32x0.5 mm2, although the aspect ratio is still relatively high. 

Another digital phase shifter is offered by [75], i.e. a 3-bit device allowing a total 315° of 

shift. In a more classic design, quarter wavelength microstrip sections are periodically loaded 

with different susceptances, sequentially composing 180°, 90° and 45° shifting sections, 

within an area of 20.5x6.5 mm2. 

Table 4 Comparison between the different discussed realizations of phase shifters, 

comprehending Switched-Line and Loaded-Line topologies. For each realization, the 

reported Return Loss, Insertion Loss, and phase Error referred to worst case among 

the various observed states are reported. 

Work Type Measured 

@(GHz) 

Ret. Loss 

(dB) 

Ins. Loss 

(dB) 

Min 

Shift (°) 

Max 

Shift (°) 

Error 

(°) 

[66] SL 1 15 3.1 5.625 337.5 - 

[67] SL 30 - 1.5 0 187 7 

[68] SL 24 19.2 1.65 0 184.6 4.6 

[69] SL 2.7 13.4 0.89 5 75 2.05 

[70] SL 17 22 3.72 11.25 348.75 1.14 

[71] SL 35 14 5.07 45 315 1.78 

[73] SL 19.5 15 6.8 22.5 180 4.5 

[74] LL 32 15 3.9 45 337.5 - 

[75] LL 2.5 8.5 4 45 315 7.9 

4.3. Distributed MEMS Transmission Line 

Concerning DMTL topology, multiple suggestions have been provided even only in 

the last half decade. Different examples are reported, covering a certain variety of possible 

arrangements and improvements. The most classic examples of DMTL design are 

reported in [76][77][78]. 

Being a DMTL phase shifter basically composed by a transmission line loaded with 

MEMS bridges, the work in [77] is the study of a simple one-bit DMTL cell aimed at the 

improvement of a basic switched-line 1-bit phase shifter in the range 1-6 GHz. Such 

study reports that adding a MEMS bridge on the shifting signal path introduces an 

improvement of 40% and 25% as compared to initial size and losses, respectively. 

Despite the traditional design, an innovative proposal is discussed in [76]. In this 

work, the signal line is loaded with 26 capacitive bridges, with 7 possible lengths. Since 

its longer bridges need lower actuation voltage (10 V) as compared to shorter bridges (40 

V), each buried electrode controlling a bridge is connected to the same DC voltage pad. 

In this manner, a single signal will actuate all bridges characterized by the same length, 

determining a precise phase shift. Such design avoids the presence of multiple control 

pads, reducing the footprint of this 7-states device to 7x0.8 mm2. 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 349 

Fig. 6 (a) Schematic section of DMTL phase shifter described in [83], (b) top view of 4-

bit device offered by [84], (c) schematic of the arrangement proposed in [85]. 

Following the traditional “straight loaded line" approach, [78] emphasizes the 

importance of miniaturization for movable components. In particular, the fabrication of 

bridges with a scaled area allowed the authors to design a basic 30° delay cell with 

1x0.66 mm2 area, reduced actuation voltage (14.6 V), and low switching time (0.6 μs). 

Promising performances of the cell thus permitted to cascade six of them, obtaining a 

180° phase shifter with 4x1 mm2 on quartz substrate. 

An improvement of the basic bridge cell loading the transmission line is provided by 

[79]. Besides the standard actuation electrode, two additional electrodes are introduced: 

each one on a side of the signal line, on a higher pillar. In this manner, besides non-

actuation state, each basic cell may achieve two different capacitive states, extending its 

reconfigurability. By cascading 32 of such cells, the Authors propose a 6-bit phase shifter 

with a maximum shift of 180°, by 5.625° steps. Characterized by reasonably low 

actuation voltages (6.8 and 3.4 V), the device exhibits an overall length of 12.8 mm. 

Additional remarkable works are represented by [80][81][82], comprising DMTL 

configurations working in the part of spectrum from X band to 60 GHz, with considerably 

small losses and phase error, investigated in depth also in terms of reliability. 

  

(a) (b) 

 
(c) 



350 G. TAGLIAPIETRA, J. IANNACCI 

A more compact and innovative design is proposed in [83], with two horizontal 

parallel fixed plates: the lower is the base for the dielectric of the CPW, while the upper 

is the support to which a piezoelectric transducer is attached. As it is shown in Figure 6a, 

on the lower side of the transducer, a slab with high dielectric constant is stuck. The 35 

μm air gap separating the slab from the underlying CPW is varied by the transducer, 

inducing broad changes of load (and thus phase delay) on the signal line. In order to 

extend the achievable phase shift, a grating and a serpentine shaped CPW line have been 

investigated, besides standard straight CPW configuration. With an overall length of 3 

mm, the device is fabricated on simple high-resistance silicon. 

As compared to previous designs, an arrangement that permits to drastically reduce 

the aspect ratio of the overall device is presented in [85]. In this case, a 5-bit phase shifter 

is implemented by a spiral-shaped CPW configuration of the signal line and loaded with 

31 capacitive MEMS switches, as depicted in Figure 6c. The actuation of switches takes 

place by groups. In details, the switches are divided into five groups, involving shifts of 

11.25°, 22.5°, 45°, 90° and 180°. The device has a remarkable footprint of 2.6x2.6 mm2. 

One last proposal to be mentioned about DMTL phase shifters is [84]. In this work, two 

compact devices (3 and 4-bit) are proposed, both exploiting the advantages of combining 

switched-line and DMTL topologies. In case of the 3-bit device, two bits control the SP4T 

switches (selecting one of the four delay lines), while one bit controls capacitive bridges 

loading the lines. The same principle applies to the 4-bit version displayed in Figure 6b. 

Both devices are designed to operate in Ka band, at 35 GHz, showing valuable maximum 

Return Loss (<-12 dB), contained phase error (<3.8°), and compact sizes of 2.24x1.91 mm2 

and 2.78x2.69 mm2, respectively. Moreover, both devices exhibit a low aspect ratio. 

Devices have been evaluated also in terms of reliability, sustaining more than 1 billion 

cycles with 0.1 W of RF power in cold switching conditions, and more than 400 million 

cycles with 0.5 W in hot switching conditions (85°C). In addition, the inclusion of the two 

phase shifters within a low cost package has been accurately characterized. 

4.4. Reflection Type 

Regarding the Reflection Type architecture, research contributions in the last decade 

mainly focused on the improvement of both quadrature coupler and reflective loads. 

Some noteworthy contributions are represented by [86][87][88]. In the first one, a 2-bit 

device composed by two directional couplers (3dB branch line) and four loads is presented. 

Activation of loads connected to the output ports of a coupler by MEMS switches 

determines a 45°, 180° or 225° shift. Implemented on a Sapphire substrate, the whole circuit 

covers an area of 7.9x4 mm2. This device, which has been conceived for the Ka band, was 

characterized also in terms of power handling capabilities, showing a commendable 

behavior up to 32 dBm input power. 

Among the different advances in the field of RF-MEMS phase shifters listed in [87], a 

couple of them are interesting from the point of view of the design strategy. In particular, 

for the design of devices working in K and Ka band, a hybrid distributed-reflective 

architecture is adopted. When small shift (22.5° or 45°) is required, only the input 

microstrip lines of the coupler are loaded with capacitive radial stubs by MEMS switches, 

without actuating any load on the other ports of the coupler. Differently, when a higher shift 

is required (from 90° to 270°), capacitive loads are activated on the output lines of the 

coupler by switches. Two packaged realizations of such architecture have been measured in 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 351 

their 16 phase states, one working at 15 GHz and shown in Figure 7a, while the other at 

21 GHz. The packaged devices proved to be able to sustain over 250 billion switch actuations. 

A robust implementation of a phase shifter mounted on a base station for electronically 

scanned antennas, is reported in [88]. It is composed by two stages: one introducing 0°, 

11.25°, 22.5° or 33.75°, while the other introducing 0°, 45° or 90° shift. Each stage comprises 

a quadrature coupler and LC loads, both realized by microstrip. Loads are selected by 

commercial Omron SPDT MEMS switches. The proposed device is realized on common 

RO4003 substrate. The employed switches have been extensively characterized in terms of 

reliability: at 2 GHz RF power and hot switching conditions, switches operated beyond 100 M 

cycles (in (0.1-0.3) mW range) and slightly more than 5 M cycles (in the (0.5-1) W range). 

 

 

(a) (b) 

Fig. 7 (a) Realization of Reflection Type phase shifter working at 15 GHz, described in 

[87], (b) top view of the architecture proposed by [89]. 

A significant suggestion in terms of reconfigurability and shifting range comes from 

[90]. After a detailed characterization of a proposed reconfigurable quadrature hybrid 

coupler based on MEMS varactors, this one is adopted in the implementation of a 360° 

phase shifter. In this case, each output branch of the reconfigurable coupler is connected 

to a dual-resonant LC load, realized by two parallel branches connected to ground, each 

one including an inductor and a 5-bit varactor, controllable by 35 V actuation. This kind 

of load allows a 360° phase shift, without the need of including another stage. By acting 

on the 10 bits is then possible to achieve 1024 phase states, although some of them will 

be quite similar for what concerns the resulting shift. 

A device operating at K band, with more classic design, is proposed in [91]. A Lange 

coupler and two reflective paths, each one provided with three switches along the lines, 

are enclosed in the footprint of 2.45x1.77 mm2. The length of short-ended lines can be 

decreased by actuating one of the shunt switches. While the total length of the load line 

corresponds to a 135° shift, actuation may introduce 90°, 45° or 0°. 

The same concept is exploited in [89], where a coupler made by folded broad-side 

coupled lines is presented, after a detailed theoretical analysis of the qualities to be 

pursued in the design of both the coupler and the reflective loads. Its ultrawideband 

performances comprehend: reflection coefficients of all ports smaller than -25 dB and 

isolation greater than -28 dB, over the whole (70-86) GHz interval. Eight switches are 

comprised in such 3-bit device depicted in Figure 7b, for a total footprint of 2.68x0.70 

mm2, considering the device and different pads. 

The most recent analog implementation is offered by [92], in which output ports of 

the Lange coupler are connected to a LC reflective load (three inductive line sections 

spaced out by couples of shunt varactors) in a CPW configuration. The capacitance of 



352 G. TAGLIAPIETRA, J. IANNACCI 

contactless varactors loading the lines is simultaneously tuned from 85 to 200 fF by the 

control voltage applied to the pads of a Chevron electrothermal actuator, thus determining the 

output phase shift of this 4x2.6 mm2 device. The continuous tuning range covers 120° shift, 

achievable by 8 V control signal. 

Table 5 Comparison between the different discussed realizations, comprehending Distributed 

MEMS Transmission Line and Reflection Type topologies. For each realization, 

Return Loss, Insertion Loss, and phase Error referred to the worst case among the 

various configurations are reported. 

Work Type Measured 

@(GHz) 

Ret. Loss 

(dB) 

Ins. Loss 

(dB) 

Min 

Shift (°) 

Max 

Shift (°) 

Error 

(°) 

[76] DMTL 30 13 1.7 - 106 - 

[78] DMTL 22 12 3 30 180 - 

[79] DMTL 30 16 0.8 5.625 180 <1 

[83] DMTL 30 12 2.55 - 170 - 

[85] DMTL 40 9.9 15.1 11.25 348.75 3.72 

[84] DMTL 35 14-12 3.8-5.4 45-22.5 315-337.5 2.3-3.8 

[86] RT 26 7 2.5 45 225 - 

[87] RT 15-21 10-12 2.2-2.2 22-22 337-337 7-5.3 

[88] RT 2 21 0.93 11.25 123.75 5 

[90] RT 2 25 2.4 - 360 11.25 

[91] RT 20 13.61 1.61 45 135 1.15 

[89] RT 74 18 4.9 22.5 195 8.6 

[92] RT 28 15 6.35 - 120 - 

5. ATTENUATORS 

Differently from the previous two kinds of device, RF-MEMS based attenuators are not 

characterized by a dense presence of scientific literature, remaining a relatively niche scenario. 

Nonetheless they also represent key components in the evolution towards the IoT and 5G 

paradigms, together with impedance tuners, filters and resonators. 

Among the remarkable characteristics typical of RF MEMS, in case of attenuators the 

most valuable are represented by reconfigurability, broadband behavior and good linearity. 

Desirable attenuators should exhibit multiple configurations of attenuation, each one with a 

flat and stable value of impedance along a wide range of frequencies. Table 6 is provided at 

the end of the present section for a better comparison of the discussed proposals in terms of 

their characteristic parameters. 

From a general point of view, devices operating by both digital or analog control are 

present in the literature. Since the first decade of the new millennium, the first one is the most 

widespread, as it is possible to observe by looking at [93][94]. 

In [93] a 6-bit attenuator is presented: within the 8.77 mm2 device encompassed in CPW 

configuration, two SPDT switches and two branches loaded with six series resistors and 

corresponding bypass cantilever membranes are comprised. While switches are meant to 

select one branch or another by ~15 V actuation, cantilevers will shorten or load resistors on 

the selected branch. With 64 possible impedance states, the device demonstrated remarkable 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 353 

linear performances in terms of Insertion and Return Loss. Conceptually similar, [94] 

proposes a CPW-based structure in which signal line crosses an initial section with 3 series 

resistors, and then separates into two parallel lines: each loaded with 3 resistors and their 

movable membrane. The two branches eventually meet to form the output line, as shown in 

Figure 8a. By an actuation voltage between 70 and 80 V, and by 7-bit sequences is then 

possible to control 128 possible impedance states. Also in this case the performances are 

stable, demonstrated over a wider interval. 

Fig. 8 (a) Complete architecture described in [94], (b) schematic of device proposed in [96]. 

Substantially new contributions appeared only in the last half decade with [95][96], both 

proposing 3-bit attenuators enclosed in CPW configuration and demonstrated up to 20 GHz. 

With a reduced footprint of 3.2 mm2, [96] offers a structure composed by three equal sub-

units in series on the main signal line, determining an attenuation of 5, 10, 20 dB respectively. 

As depicted by the schematic of Figure 8b, each sub-unit is composed by a T-shaped resistive 

network and two cantilever switches: one switch rules shunt connection to ground, the other 

allows to bypass the two series resistive loads. The proposed design achieves 8 different states 

with a limited attenuation error, at the price of 70 V actuation. 

In a similar way, [95] proposes a device composed by three cascaded sub-units of 10, 

20, 40 dB attenuation. Each sub-unit comprises a T-junction power splitter, leading to an 

 

(a) 

 

(b) 



354 G. TAGLIAPIETRA, J. IANNACCI 

unloaded branch or to a branch loaded with π-shaped resistive network by its SPST 

switch. Specular SPST switches and T-junction complete the sub-unit. Such design allows 

to reach wide attenuation (70 dB), with a reduced pull-in voltage (30 V) and footprint 

(2.45x4.34 mm2). 

Previous research items are generally demonstrated over a limited range of frequencies, 

with a limited number of possible states (except [93][94]). 

The first offer for an attenuator covering a wider spectrum came with [97]. In this work, a 

8-bit device is presented, measured in the interval (0.01-110) GHz. Enclosed within CPW 

configuration and with an overall footprint of 3x1.95 mm2, the attenuator is substantially 

composed by eight cascaded modules of ohmic switches along the main RF signal line, 

actuated by 45 V signals. Depending on the type of module, each switch will contribute to 

shorten (when actuated) the resistor in series on the signal line or to connect (when actuated) 

signal line to a shunt resistor to ground. The latter module thus may operate or avoid shunt 

resistive path to ground, while the former module may shorten the series resistor, and so on, 

alternately. Such asymmetric network is measured under a wide variety of configurations, 

considering VWSR (Voltage Wave Standing Ratio), Insertion and Return Loss, and 

identifying the most advisable configurations and causes of the unadvisable ones. 

The first proposals of analog attenuators came only during the last couple of years. In [98], 

Authors present a design based on CPW meandered configuration. A membrane with low-

resistivity silicon slab is deployed over the signal line, with a highly magnetized permanent 

magnet on the upper side facing an overhead planar spiral coil. The direction of the current 

determines the attraction (or repulsion) of the underlying magnet and membrane, while the 

amount of current determines the variation of the gap between the slab and the signal line and, 

in turn, the quantity of signal power absorbed by the silicon slab. This 2x5 mm2 device 

exhibits low Insertion Loss, for an attenuation range of 16.4 dB.  

A radically different design is proposed in [99]. This work is substantially based on 

two quadrature hybrid coupler and two varactors. While “Isolated” ports of the couplers 

are terminated with a matched load, the corresponding “Through” and “Coupled” ports of 

couplers are connected by separate RF lines, each loaded with a shunt varactor. The 

plates of a varactor are on the two sides of the RF line and are moved by an 

electrothermal Chevron-type actuator, driven by a control signal with maximum value of 

9.8 V. Beside actuation voltage, the area of the device is moderate (3.8x3.1 mm2), as 

well. Despite the limited attenuation and frequency range, the previous two proposals 

represent remarkable steps ahead in the direction of further low-cost and low-power 

reliable RF-MEMS attenuators. 

Table 6 Comparison between the different discussed realizations of attenuators. 

Work Range 

(GHz) 

Measured 

@(GHz) 

Ret. Loss 

(dB) 

Ins. Loss 

(dB) 

Max Att. 

(dB) 

Step 

(dB) 

Error 

(dB) 

[93] 0.05-13 8 15 1.8 19.7 - - 

[94] 0.1-40 20 - 2.5 38.5 - - 

[95] 0.1-20 0.1-20 11.9 0.73 70 10 <3% 

[96] 0-20 0-20 12 2 35 5 <5% 

[97] 0.01-110 20 - 5 45 - - 

[98] 21.5-26.5 23.5 18 1.2 17.6 - <0.2% 

[99] 58-62 60 20 10 25 - - 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 355 

6. FILTERS 

Filters play a crucial role in every RF front-end, enabling a suitable signal-to-noise 

ratio for the desired signal contributions and rejecting the out-of-band noise within the 

desired frequency bands. Moreover, by considering the growing overpopulation of the 

limited available spectrum in recent years, it is possible to understand why the role has 

gained an increasing relevance [100]. Given the additional portions of spectrum provided 

by 5G paradigm and the required compliance with different communication standards, 

the presence of agile reconfigurable filters represents a stringent need for radio front-ends 

of future mobile devices [101].  

In addition to these technical considerations, the requirements of the market must be 

taken into account as well; this determines a constant demand of devices characterized by 

low cost and power consumption, small size, light weight, wide tunability and high 

performances. These motivations make the design of reconfigurable filters a particularly 

challenging task, under a constant attention by researchers. 

The generic structure of a reconfigurable filter includes a resonator, made by various 

types of transmission lines, lumped elements, cavities or dielectric solids, whose resonant 

frequency, bandwidth or filtering nature is modified by tuning elements [102]. 

From the point of view of classification, these devices can be categorized on the basis of 

their resonators. Bearing this in mind, the three main categories consist in 3D, planar, or 

lumped LC structures. The first one includes resonators made by waveguides (e.g. substrate 

integrated waveguides - SIW), cavities (e.g. evanescent-mode resonators - EVA) and 

dielectric resonators (DR). For the sake of brevity, resonators exploiting acoustic or 

ferromagnetic principles are not discussed in the present treatise. The second category is 

composed by resonant structures made with CPW, microstrip and striplines, generally 

arranged according to different layouts, such as comb-line, Hairpin or edge-coupled 

configurations. LC structures are usually employed at lower frequencies (<1 GHz), as the 

wavelength of signals would make inconvenient the use of planar structures [103]. 

The reconfigurability of these devices is provided by tuning elements, which are 

generally represented by banks of capacitors, varactors, switches to select reactive loads, 

or actuation mechanisms that modify the dimensional (and thus electrical) features of the 

resonator. 

RF-MEMS technology applied as a valuable candidate for widening reconfigurability 

in filters since the early 2000s [104], capitalizing on its intrinsic high quality factor (Q), 

negligible losses and power consumption, and simple biasing of its switches and lumped 

components. For the sake of completeness, semiconductors represent another common 

choice as tuning element, but they are employed only when a short switching time is a 

primary concern, due to their inconvenient losses and quality factor [105]. Filters can be 

evaluated on the basis of power handling and consumption, linearity and tuning speed, 

but features like Q, tuning range, Insertion and Return Loss, size and integrability with 

common electronic supports, remain the most accepted figures of merit. 

Considering the vast amount of literature in this field, some interesting research 

solutions of the last half decade are reported in the next paragraphs. Tables 7 and 8 are 

provided along the present section, for a better comparison of the different research items 

in terms of their features. 



356 G. TAGLIAPIETRA, J. IANNACCI 

6.1. Three-dimensional 

Regarding resonators based on waveguides, K band application and a high Q factor 

are the target of the bandpass filters reported in [106]. Among the different discussed 

realizations, an interesting realization is the one adopting virtual movable walls. In this 

approach, the resonator is obtained by inserting spaced metallic septa along the E-plane 

(the plane along which electric field propagates) in the center of the waveguide. The 

lateral walls of the waveguide facing the septa present a recess, and six dies are arranged 

on the wall. Each die is composed by a quartz substrate on which a conductive path can 

be allowed or avoided by an ohmic switch. The actuation of the switches allows to 

establish a conductive path that will electrically "hide" the lateral recess of the guide, 

varying the electrical effective width of the resonant cavity represented by the space 

between the central septa and lateral walls. This configuration allowed a 725 MHz shift 

from the basic resonant frequency (22.17 GHz). Although its Q factor and electrical 

performances are remarkable, the device is characterized by the typically huge dimensions 

and aspect ratio of waveguide-based devices. In fact, the overall length (48 mm) makes this 

device more suitable for fixed infrastructures. 

Another bandpass implementation featuring waveguide resonant cavities is provided by 

[107], in which an alternation of inverters and cavities composes the whole structure. The 

three stages of inverters consist in dielectric substrate laminates on which RF-MEMS switches 

are integrated, whose activation allows a 2,4 GHz tuning range for the resonance of the two 

cavities, up to 14.6 GHz, by three possible intermediate steps. Also in the case of this device 

for Ku band applications, a remarkable Q factor is achieved for the three states, at the 

expenses of volume and aspect ratio. 

Regarding resonators based on EVA cavities, K and Ka are the bands selected by the 

Authors of [108][109], for their bandpass filter. The structure of the resonator is traditional: a 

gold-sputtered cavity on silicon substrate, encompassing two posts, and covered by a 

diaphragm. The two areas of the diaphragm near the posts are provided with electrodes, for a 

variable deflection of the two areas toward the underlying posts. In this configuration, the 

resonant frequency (20 GHz) of the cavity can be shifted through the deflection, which 

modifies the capacitance between the posts and the corresponding areas of the diaphragm. 

This architecture enabled a high Q factor and an outstanding continuous tuning range of 20 

GHz, still maintaining good electrical performances. The price to pay for such achievements 

corresponds to the typical drawbacks of micromachined 3D resonators: increased complexity 

of fabrication and large actuation voltages. 

The same arrangement is adopted for the development of the tunable filter presented in 

[110], operating in V band. The two-pole filter demonstrated a fairly wide tuning range ((57.7-

63) GHz) and limited Insertion Loss, featuring a CPW-microstrip transition for a simpler 

integration on planar layout technologies. The integration is simplified also by virtue of a 

further advance in the miniaturization of such device, whose size does not exceed 5x8 mm2. 

A similar arrangement is proposed for Ku band operations in [111], with a copper cavity 

encompassing two internal posts and fed by CPW line. In this case two implementations are 

offered, one using clamped-clamped RF-MEMS structures, and the other using disk-shaped 

patches of Vanadium Dioxide (VO2) as tuning mechanism controlling the areas of the 

diaphragm facing the posts. The second approach involves a thermal activation of such 

material to trigger an abrupt change of conductivity (metal-to-insulator transition) of the 

material, determining in turn a variation of the parasitic capacitance between the disks and the 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 357 

upper part of the posts. This strategy allowed to achieve 1 GHz tuning range from the original 

resonant frequency (14.7 GHz), still providing a satisfying Q factor and reasonable electrical 

performances. 

Table 7 Comparison between the different discussed realizations of filters. Reported values 

of Return Loss and Insertion Loss refer to the worst assumed value. 

Work Type 

(BP/BS) 

Tuning Range  

(%) 

Q 

unloaded 

Ret. Loss 

(dB) 

Ins. Loss 

(dB) 

Actuation 

(V) 

[106] BP 3.2 1450 -13 -1.3 70 

[107] BP 18 1700 -15 -0.7 100 

[108] BP 100 540 -15 -3.09 180 

[110] BP 9 330 - -2.9 140 

[111] BP 7 1198 -14 -2.26 - 

  [112] BP 200 64 -10.5 -3.5 0.5 

[113] BP 3 3850 - -0.44 - 

[114] BP 35 92 -10 -4.29 - 

[115] BP 19 22 -15 -0.2  

Another analog filter based on metallic cavity and inner post is provided by [112]. This 

implementation involves a sort of inverted arrangement: the post arises in the vertical 

direction from the upper side, facing the movable membrane on the bottom side of the cavity. 

Among the different realizations, the most interesting one is represented by a 45° top-angled 

post facing the gold plate tuned by thermally driven actuators. The need for a precise and 

constant control over the position of the membrane determines the presence of a sensing 

capacitance and a polysilicon thermometer under the membrane, enabling a closed-loop 

control of the power supplied to the actuators. The simulated performances of such 

arrangement demonstrated a wide continuous tuning range, concerning C, X, and Ku bands in 

the (6-18) GHz interval and limited Insertion Loss. In addition, the device requires less than 

0.5 V for a remarkably fast actuation (1 μs). 

A more traditional filter based on cavity and posts is described in [113], whose tuning 

principle is utilized in commercial Omron MEMS switches; switches may establish or 

avoid the connection to ground for a couple of striplines inserted in the cavity, fed by 

common SubMiniature-version-A (SMA) probes. The absence of lumped components 

allowed the Authors to avoid undesired degradation of the high Q factor for this 2-bit 

device, greater than 2000 for all the tuning states. Designed for C band applications, the 

filter demonstrated minimal Insertion Loss (<-0.44 dB) within a relatively limited 530 

MHz tuning range in the vicinity of the designed frequency of 7 GHz. 

Regarding resonators based on SIW, [114] proposes a couple of realizations, employing 

folded-ridged quarter-mode SIW resonators, and operating at L and S bands. In both the 

bandpass realizations, the tuning mechanism comprises a couple of commercial SP4T RF-

MEMS switches, selecting lumped reactive loads, while the two-pole resonator is represented 

by two folded-ridged SIW cavities coupled by a iris opening. A quarter-mode resonator SIW 

is obtained by cutting a SIW waveguide along a plane of symmetry of the resonance 

associated with its fundamental mode. This strategy enabled a substantial miniaturization of a 

single SIW-based resonator, featuring a λ/16 transverse width as compared to the reported 

state of the art regarding such devices. In terms of electrical performance, the filter equipped 



358 G. TAGLIAPIETRA, J. IANNACCI 

with capacitive loads exhibits reasonable Insertion and Return Loss along its eight tuning 

states, providing a 305 MHz tuning range in the vicinity of the center operational frequency 

(865 MHz). 

Another interesting implementation is described in [115], involving a half-mode SIW 

resonator loaded by a complementary split ring resonator (CSRR) and a MEMS movable 

membrane. In this arrangement, the CSRR in engraved on top of the 6x6 mm2 half mode 

SIW, and an overhead metallic movable membrane realizes the tuning, acting as variable 

capacitance. This configuration determined a satisfying 500 MHz tuning range, starting 

from the basic resonant frequency (3.15 GHz). In addition, simulations show a 

remarkably small Insertion Loss (<-0.2 dB) and appreciable Return Loss (>-15 dB) for 

the reported tuning states. 

Fig. 9 (a) Explanatory cross section of the EVA cavities described in [108] and [109], (b) 

3D model of the device reported in [115], (c) Scanning Electron Microscope 

(SEM) photography of the layout realized in [116]. 

6.2. Planar Structures 

Regarding filters based on planar structures, the first proposed device [116] is a 

tunable bandstop filter for Ka-band applications, realized in CPW configuration. The 

resonance is achieved by arranging a CSRR both on the signal line and ground sides. The 

default resonance frequency (38.8 GHz) can be shifted to 35.32 GHz by the activation of 

three movable membranes on top of the resonator engraved on the signal line, letting 

 

(a) 

  

(b) (c) 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 359 

them act as varactors. The novelty of this research contribution is represented by the fact 

of being the first proposal for CPW filter based on CSRR, compatible with CMOS 

technology. The device, featuring a gold metallization on a silicon substrate is remarkable 

also in terms of dimensions, not exceeding 1.46x5 mm2. 

The same frequency band is targeted in [117], in which a bandstop filter based on shape 

defected ground (SDG) is presented. More specifically, the device consists in a CPW 

configuration, periodically loaded by four recesses with spiral-shaped metal engravings, on 

both its ground planes. The resonant frequency (26 GHz) of this arrangement is controlled by 

a SPST RF- MEMS switch on each spiral, whose actuation increases the overall length of the 

spiral itself. The reported simulations show an encouraging 3 GHz tuning range and a 

remarkable stopband attenuation. 

An interesting contribution comes from [118], combining Spurline and DMTL 

approaches to obtain a tunable bandstop filter working at Ku band. In Spurline-based 

filters, the resonance is achieved by short-ended stubs along the signal line of a CPW. In 

this case, reconfigurability is enforced by shunt capacitive MEMS switches, whose 

actuation determines a reduced length of the stubs and thus a shift in the resonant frequency 

(18 GHz). The whole 1.2x10 mm2 device is composed by three sections: two sections, each 

one loaded by a couple of stubs, and an interposed section. In the latter, the signal line is 

periodically equipped with three overhead movable MEMS membrane, realizing a typical 

DMTL configuration. The actuation of such membranes is intended in order to tune the 

bandwidth of the filter, thus realizing a reconfigurable device both in terms of frequency 

and bandwidth. From a quantitative point of view, the filter shows good performance within 

its 2.5 GHz tuning range; the filter also provides minimal Insertion Loss (<-1 dB) while 

non-operational, and remarkably large attenuation level (>-70 dB) for all its tuning states. A 

similar approach is adopted in [119], with two cascaded sections of CPW loaded in DMTL 

fashion, coupled by series capacitive RF-MEMS switches. 

Targeting C band and high attenuation along the stopband, [120] proposes a three-

pole bandpass filter encompassing commercial components. In fact, the three cascaded 

half-wavelength resonators are realized by microstrip on Roger RO4003C substrate, 

while each digital variable capacitor loading a resonator is a commercial product by 

Cavendish Kinetics. The resulting device is quite compact, providing a wide tuning range 

by means of 5 states covering the (5.3-7) GHz interval. In each state, the attenuation 

along the stopband is greater than -30 dB up to 10 GHz, still maintaining limited losses. 

A comparable architecture for L band applications is described in [121], comprising a 

three-pole resonator made by coupled quarter-wavelength sections of microstrip in 

combline configuration. Commercial RF-MEMS varactors are employed for tuning the basic 

resonant frequency (2.1 GHz) and for the control over the capacitive coupling between the 

two microstrip resonators. Also in this case, the layout is quite compact, since the size does 

not exceed 9x5 mm2; the attenuation in the stopband is comparable as well. With the proposed 

device, a remarkable 700 MHz tuning range can be achieved by 21 capacitance states, 

with appreciable electrical performances.  



360 G. TAGLIAPIETRA, J. IANNACCI 

Table 8 Comparison between the different discussed realizations of filters. Reported 

values of Return Loss and Insertion Loss refer to the worst assumed value. 

Work Type 

(BP/BS) 

Tuning Range  

(%) 

Q 

unloaded 

Ret. Loss 

(dB) 

Ins. Loss 

(dB) 

Actuation 

(V) 

[116] BS 10 - -2 -31 25.65 

[117] BS 10 - -3 -42 - 

[118] BS 16 - -10 -75 - 

[120] BP 32 200 -17.5 -5.3 - 

[121] BP 50 253 -20 -2 30 

[122] BP 650 - -12.7 -6.8 40 

A tunable bandpass filter covering almost the entire UHF band is presented in [122]. 

The basic structure is represented by a half-wavelength and a quarter-wavelength 

microstrip resonator in concentric/nested arrangement. A couple of RF-MEMS switches 

control the activation of each resonator, so that the activation of one, of the other, or of 

both resonators, realizes a first coarse tuning of the resonant frequency. The fine tuning is 

then performed by a bank of switched capacitors connected to each resonator. The choice 

of the nested arrangement allowed the Authors to achieve fairly limited dimensions 

(26x36 mm2), for a device working at relatively low frequencies, fabricated on common 

RT/Duroid substrate and equipped with commercial lumped components. In terms of 

capabilities, the wide tuning range ((0.4-3) GHz) of such device is inevitably 

counterbalanced by contained electrical performances. 

7. CONCLUSIONS 

The abovementioned examples of RF-MEMS devices portray the current state of the art 

of a technology that is now mature to face current challenges posed by the 5G applications. 

Besides the intrinsic reconfigurability, considerable wideband performances and low power 

consumption, research on RF MEMS has provided components characterized by an 

increasing miniaturization and reliability, manageable control signals, and compatibility 

with existing technologies. Among the different proposals, while some may seem more 

appropriate for fixed infrastructures, other may apply to mobile low power devices for their 

limited size and control voltage. Many devices proved a satisfying reliability under extensive 

power and temperature measurements, reinforced by low cost and effective packaging 

strategies. The range of frequencies covered by the mentioned examples comprehend both 

portions below 6 GHz and higher ones, involving all those bands engaged in the realization of 

5G infrastructure. In addition to the previous considerations, the electrical performances of 

the reported RF MEMS devices proved themselves to be a valuable plus of this technology, 

in view of a potentially significant role in 5G scenario. 

Acknowledgement: This paper is published thanks to the support of the project “Wafer Level 

Micropackaging di RF MEMS switch per applicazioni spaziali” (F36C18000400005) funded by the 

Agenzia Spaziale Italiana (ASI). 



 A Comprehensive Overview of Recent Developments in RF-MEMS Technology 361 

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