RF MEMS electrostatically actuated tunable capacitors and their applications: a review

This paper reviews the recent developments of micro-electromechanical system (MEMS) based electrostatically actuated tunable capacitors. MEMS based tunable capacitors (MBTCs) are important building blocks in advanced radio frequency communication systems and portable electronics. This is due to their excellent performance compared to solid state counterpart. Different designs, tuning mechanisms, and performance parameters of MBTCs are discussed, compared, and summarized. Several quantitative comparisons in terms of tuning range, quality factor (Q factor), and electrodes configurations are presented, which provide deep insight into different design studies, assists in selecting designs, and layouts that best suit various applications. We also highlight recent modern applications of tunable capacitors, such as mobile handsets, internet of things, communication sensors, and 5G antennas. Finally, the paper discusses different design approaches and proposes guidelines for performance improvement.


Introduction
With the rapid global advancement of technology, there is an increasing demand for reconfigurable radio frequency (RF) components. Frequency tuning, RF tunable filtering, and tunable matching impedances can be promising solutions for handling numerous telecommunication standards, and eliminating the need for complex hardware. Nowadays, * Author to whom any correspondence should be addressed.
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there is an urgent demand for integrated RF systems of low cost, low power, reconfigurable frequency, and operation in multiband due to the requirements of advanced technologies, such as, internet of things (IOT) applications, 5G communications, and fast data communications.
Since inception, microelectromechanical systems (MEMS) have shown great potential to replace RF solid-state devices with superior RF MEMS devices. MEMS based tunable capacitors (MBTCs) exhibit much higher Q factor, and tunability as compared to semiconductor diodes tunable capacitors. MEMS tunable capacitors are employed in today's wireless transceiver's RF front-ends [1], as shown in figure 1. They are used in voltage controlled oscillators (VCOs), low noise amplifiers (LNAs), power amplifier, tunable filters, re-configurable phase shifters, and impedance matching  networks [2]. Reconfigurable passive components such as reconfigurable LC tanks, and tunable filters, not only improve the performance of the systems but also widen the reconfigurability of the systems, besides reducing the size and power consumption [3].
There are several approaches for the actuation of MBTCs, such as electrostatic, electro-thermal, magnetic, and piezoelectric. Among them, electrostatic is the most attractive and commonly employed actuation method for tunable capacitors due to the fast response, low power consumption, and compactness. However, it may require high voltages (10-50 V) [2]. Traditionally, there are three methods for tuning MBTC: the gap tuning [4][5][6], area tuning [7], and dielectric displacement tuning [8].
A typical implementation of a MEMS tunable capacitor is based on two parallel plate electrodes, one of the electrode is fixed and the second is movable, as shown in figure 2(a) and electrical series equivalent model is shown in figure 2(b). The capacitance is tuned by varying the air gap, by changing the electrode overlap area using electrostatic actuation, or by modifying the dielectric property of the gap between the parallel plates.
In this paper, a thorough review of electrostatically actuated tunable capacitor is presented. Advanced applications of tunable capacitors are presented in section 2. The performance figure of merits are evaluated in section 3. Different types of electrostatically tunable capacitors designs, development, and performance comparisons are presented in section 4. Finally conclusions are presented in section 5.

Advanced applications
RF MEMS tunable capacitors, and switches are ideal for reconfigurable circuits and systems. These systems include matching networks, RF receivers, RF filters, phase shifters, antennas, and oscillators/resonators. They have very low insertion loss, and high Q factor and can be integrated on low dielectric-constant substrates, which is important for highperformance tunable filters, high-efficiency antennas, and lowloss matching networks [2,9,10]. In addition, RF MEMS devices have very low intermodulation products/harmonics, which are critical for LNAs. MBTC can be used for designing high performance RF filters. Tunable filters play important role in the communication industry, and are key part in almost every RF electronics and RF military equipment. Tunable filters are used for selection or rejection of desired channel from a wide band spectrum of RF signals. A compact tunable filter using tunable capacitor with improved stopband rejection has been presented in [11]. The 1.4−2.1 GHz filter has an insertion loss <2 dB with 1 dB fractional bandwidth of 12%-13%. This low loss is due to the high performance tunable capacitor implemented in modern communication systems. RF-MEMS tunable capacitor based tunable filters provide excellent performance in terms of tuning range and quality factor. Many other papers also reported utilizing MBTC for designing high performance RF filters [11][12][13][14][15][16][17][18][19][20][21][22].
MBTCs are also used in the development of RF phase shifters. Nowadays, for changing the radiation pattern, electronically scanned arrays have been utilized to change the phase of radiating part of the antenna, thereby guiding the radiation of the beams in space communications. The device used for creating phase differentiation among output terminal and input terminal is known as phase shifter. MEMS tunable capacitor based phase shifters have advantages over solid state counterpart due to small insertion loss, high linearity, low power, and high tunability. Several works have been reported in [23] for the development of phase shifters using MBTCs. In [24], an RF MBTC WS1042 designed by WiSpry has been used to steer the beam of an antenna using an innovative phase shifter. Simulations results confirm that considering 70 • and 105 • phase shift can cover more than 80 • and 100 • beam steering, respectively with a maximum gain of 7 dBi. Similarly, more recent works have been reported in [23, 25-30] utilizing RF MEMS tunable capacitor for the development of phase shifters.
Another common application of tunable capacitor is in matching networks. Matching networks are very critical for the design of LNA, power amplifier, VCO [31], etc. Normally, the frequency of operation in these devices remains fixed, but the power amplifier output impedance changes with time and one must tune the output matching network to get highest system efficiency. MEMS tunable capacitor based impedance matching networks are electronically tunable, provide low insertion loss, low power consumption, and good linearity. Therefore, several works have been reported [32-37], which utilize the MBTC as a basic component of the matching network.
Recently, MBTCs are used in mobile phones antennas tuning [23,38,39] and IOTs sensor communications [40,41]. Tunable antennas have a great potential for enhancing the antenna bandwidth while keeping a low profile. MBTCs provides this important feature of tunability to the mobile phone antenna, 4/5G communication antennas, IOTs antenna with low insertion losses, and high quality factor [38, 42,43]. The evolution of (4G-LTE) in smartphones resulted in the degradation of voice and data signal quality due to the integration of many electronic components with the antenna [44]. RF MEMS tunable capacitors integrated in impedance matching tuners enable impedance matching between the smart phone antennas and RF front ends, which reduces the degradation of receiving signals. The IOTs are emerging technology through which 5/6G mobiles communicate with electronic machines, electronic sensors, robots, smart cars, and aerial drones independently. This requires high operating frequency in mmW spectrum for Gigabit (Gb) communications, large scale multiple inputs and multiple output units, and large frequency re-configurability while reducing hardware redundancy. RF MEMS technology is one of the promising candidate for IOTs and 5G technology. However, further efforts are needed in the development of RF MEMS devices operating at high frequency.
MBTCs are also utilized in tuning and impedance matching of multi-nuclear magnetic resonance imaging (MRI) coils for diagnosis, screening of bio-samples, and diseases due to their high quality and tunability [45,46]. In addition, they have been recently utilized in tuning microwave circuits, such as half mode substrate integrated waveguide-complementary split ring resonator [47]. The resonance frequencies of 3.9 GHz and 7.35 GHz are tuned to 3.65 GHz and 6.6 GHz, respectively. The tuning range of capacitor is only 29%. Thus the resonance frequencies tuning ranges can be improved by using higher tuning range capacitors. Table 1 shows the comparison summary of the electrostatically actuated tunable capacitors used in RF communication industry, electronics, and biomedical industry. It is clear from the table that the performance of electronic devices have been significantly improved in terms of low losses, high tunability, efficiency, and quality factor. Nonetheless, considerable research is still under way to integrate RF MBTCs in power electronics [48], sensors, and advance electronics.
Due to the evolution of modern communication technology and the advent of 5G, 6G and terahertz communication, the operating frequency of the communication systems have increasing trends [49]. Therefore, high frequency MBTC is growing in demand not only in research and development but also in the consumer markets due to clear advantages of MBTC over solid state capacitors.

Quality factor
Quality factor (Q factor) is one of the most important figure of merit for tunable capacitors. It is defined as a measure of the loss in the microwave circuit, and mathematically is expressed as: The tunable capacitor Q factor can be derived as: where C is the capacitance of the tunable capacitor, Im and Re refers respectively to the imaginary and real part of the impedance Z parameter, ω is the resonant frequency, and R s is the series equivalent resistance. A large Q factor ensures a high selectivity in passive filters, and better performance in terms of low losses. The Q factor at microwave frequency can be measured using scattering parameters (S-parameter) obtained from a vector network analyzer. The Q factor of tunable capacitor at microwave frequency can be written as: where S 11 is the input port voltage reflection coefficient.

Tuning range
The tuning range is another key parameter for designing tunable capacitors. Mathematically, it is expressed as: where C max is the maximum capacitance and C min is the minimum capacitance. A wide tunable range for the capacitance enables a correspondingly large re-configurability of the functional block that employs it. Therefore, most of the broadband communication systems require wide tuning range.

Reliability
The performance of tunable capacitors may change over time, particularly if placed in harsh environment. In most cases, the reliability of tunable capacitors are considered better than MEMS switches. This is because unlike MEMS switches, the tunable capacitors plates do not come in contact and similarly, there is no dielectric layer between the capacitor plates, hence no issue of charging of the dielectric layers. The issue of dielectric layer charging is very common in MEMS based switches [53]. This means that the pull-in voltage is not altered by the dielectric charging of the capacitor, which is a critical issue in MEMS based switches. Similarly, MEMS tunable capacitors do not need to be hermetically sealed since they do not suffer much from charging or surface-contact problems resulting from humidity [2].

Linearity
Parallel-plate capacitors are known to have high nonlinearity governing their voltage and capacitance parameters (C-V curves). This non-linearity greatly limits the applications where accurate capacitance is crucial. In addition, it is difficult to design a controller circuit for accurate capacitance in the range in which a dramatic change occurs. For example, the phase noise in the frequency tuning of VCOs, and the bandwidth variation in phase locked loops are also caused by the non-linearity of tunable capacitors [54,55]. Several solutions have been reported to overcome the issue of nonlinearity of parallel-plate capacitors. Lateral comb tunable capacitors were introduced in the early work of tunable capacitors for improved linearity. However, the tuning ranges were limited due to designs imperfections [56,57]. Shavezipur et al [58][59][60] introduced a linear tunable capacitor with a linearity factor of up to 99.7% along with a large tuning ratio of 68%. Similarly, parallel plate system with unconventional geometric shapes have also been investigated to improve the linearity between capacitance and voltage curve. However, the tuning range was low due to the sharp change in the capacitance values while approaching the pull-in point. Han et al [24] remarked that by increasing the distance between parallel plate capacitance instead of traditionally closing the gap between tunable capacitor, linearity of C-V response can be improved. They reported linearity factor of 99.1% and a large tuning ratio of 178%. Similarly, Zhuhao et al [61] reported a nonplanar upper plate and a side leverage electrode structure for large tuning range and high linearity. The results showed high linearity factor 99.92% and large tuning ratio of 309%. However, the size of the tunable capacitor is increased compared to traditional parallel plate capacitor.

MBTCs development
Next, we classify tunable capacitors into two main categories; in-plane and out-of-plane. We discuss their main features and then present a comprehensive comparison between them in section 4.3, and discuss their fabrication techniques in section 4.4. In addition, new techniques for improved tunability based on the two main categories are presented in sections 4.5 and 4.6.

In-plane laterally actuated tunable capacitors
Unlike, transverse actuated tunable capacitor, laterally actuated tunable capacitor takes the advantage of capacitance generated from the side walls. They involve interdigitated fingers (IDT) to increase the edge coupling length. The IDTs are shaped like teeth of a comb, as shown in figure 3. In comb drive systems, two set of electrodes are placed in the same plane. One set of electrode fingers is fixed, the stator, while the second set, rotor is suspended and free to move.
In 1998, Yao et al [7] reported a comb drive based tunable capacitor with a tuning range of 200% and a Q factor of 34 at 500 MHz. The electrical self-resonance frequency of the comb drive tunable capacitor is 5 GHz. The structure is fabricated on silicon on insulator (SOI) substrate, as shown in figure 4(a). When an electrostatic voltage is applied across the fixed and movable comb drive, it results in a change in the overlapping area. The parameters limiting the tuning range in such comb drive system are the supporting spring design and the length of comb's fingers. Similarly, Borwick et al [17] proposed similar variable comb drive tunable capacitor with a high tuning ratio of 8.4-1, and Q factor values of more than 100 in the 200-400 MHz, as shown in figure 4(b). The left side comb drive system was used as an actuator and the right side was used as a tunable capacitor. The high tuning ratio is due to double side metallization of silicon structure layer, which reduces the out of plane stresses and bending of the tunable capacitor. Furthermore, when implementing the tunable capacitor into a two-pole UHF filter, tuning over a 225-400 MHz range was achieved with a loss below 6.2 dB. Another comb drive tunable capacitor was presented by Zhejiang [62]. They proposed a comb drive tunable capacitor with a tuning range of 242%. The comb drive system was used for actuation, whereas a parallelplate capacitor was used for sensing, as shown in figure 4(c). Bumpers have been fabricated on the suspended springs for extending the tuning range. The capacitance increases parabolically before electrode contact with bumpers, and after contact the capacitance increases linearly.
Nguyen et al [63] proposed an electrostatic vertical comb drive actuator to extend the tuning range. The vertical comb drive tunable capacitor creates a large offset in comb fingers by small rotation of actuator's angle. A high tuning ratio of more than 31:1, and a maximum Q factor of 273 at 1 GHz have been reported. This is the maximum tuning range ever reported in literature. The SEM image of the vertical comb drive actuator is shown in figure 4(d).
Similarly, a new MEMS based variable capacitor using electrostatic vertical comb drive has been proposed by Saeid et al [64]. The proposed design was simulated in COMSOL, and Intellisuite software. In the design, an electrostatic vertical comb drive was introduced to extend the tuning range. According to the simulation results, the achieved tuning range was 285%. A detailed fabrication process was also proposed in the same paper but the design was not fabricated and the proposed fabrication process is complex.

Out of plane transverse tunable capacitors
These tunable capacitors are comprised of parallel plates with their broad side parallel to each other. In a typical capacitor, one of its plate is fixed and the other is movable. The movable plate is suspended or attached to a suspended structure, which acts as a spring, as shown in figure 2(a). Electric field lines are parallel to each other and perpendicular to plate surfaces in the overlapped region. When a DC bias is applied between the two plates, an attractive electrostatic force is induced, which deflects the movable plate toward the fixed one, and thus increases the capacitance across the plates. By increasing the applied voltage, one can further increase the displacement of the movable plate. However, this is limited to one-third of the initial air gap d/3 (i.e. one-third of the initial air gap), and the phenomenon pull-in, at which the electrostatic force dominates the restoring mechanical force, collapsing the suspended plate into the fixed plate. The electrostatic force Fe and the voltage at pull-in are given by where A is the cross-sectional area of the plates, d is the gap between plates, C is the capacitance across plates, K m is the stiffness, and C o is the initial capacitance across the plates. The theoretical tuning range of the parallel plate capacitor is limited to 50% due to pull-in effect. However, the practical tuning range of parallel plate capacitor is much lower than the theoretical limit due to the parasitic effect [67].

Non-conventional designs of transverse tunable capacitors
To improve the tuning range, and linearity several nonconventional designs have been reported in the literature. These designs greatly improve the tuning range, quality factor and linearity of the tunable capacitors. Zou et al [66] proposed a new design of tunable capacitors with a tuning range of 69.8%. This range exceeds the theoretical maximum range of 50%. This is due to the higher gap between driving electrode than the gap between capacitor plates. The tunable capacitor consists of one suspended top plate and two fixed bottom plates. One of the two fixed plates, E2, and the top plate E1 form a variable capacitor, whereas the other fixed plate E3 and the top plate, E1 are used to provide electrostatic actuation for capacitance tuning, as shown in figure 5(a). The surface micromachining technique has been used in the fabrication on a pyrex glass wafer. The Q factor of 30 at 5 GHz, and self-resonance frequency beyond 10 GHz have been achieved [65]. However, this tuning range is still not enough for broad band communications as most of the broad band communication systems require high tuning range. Therefore, Dec et al [68] used two plates and three plates structure with tunability of 50% and 87%, respectively. In addition to this, the Qfactor of 20 is obtained at 1 GHz. The control voltage of only 4.4 V has been used, which is compatible with CMOS electronic devices. The design is shown in figure 5(b), and it is implemented in the standard PolyMUMPs process, which has three structure layers and two sacrificial layers for the prototype design.
Similarly, another parallel plate MEMS tunable capacitor has been fabricated in a thin film technology with a high tuning range [69]. A capacitance tuning ratio of up to 17 has been demonstrated with the voltage requirement of 20 V. The high tuning ratio is due to different gaps with actuator and capacitors electrodes, which assists in the utilization of greater displacement of movable electrode in the tunable capacitor. This is the second highest tuning range in parallel plate capacitors ever reported. A Q-factor of 150-500 has been demonstrated in the frequency range of 1-6 GHz. This high Q factor is due to the high resistance substrate wafer. To avoid pull-in of the tunable capacitor, special stoppers are designed at the edges of the structure.
Besides tuning range and Q factor, linearity of capacitance vs voltage curve is also crucial in some applications. Bakri-Kassem et al [12] developed a parallel-plate tunable capacitor with curled moving plate, which exhibits linear tunability of 115%. Shavezipur et al [58] introduced a parallel-plate capacitor with structural nonlinearity to get high tunability with a controlled plate displacement to get linear tunability of 100%. Addition of extra stiffness to capacitor's structure and design optimization increases tunability and linearity of C-V curve by creating discontinuity in the curve. In the butterfly-shape design, the moving electrode is divided into two trapezoidal segments, each one has two nodal displacements and therefore two degrees of freedom, as shown in figure 5(c). In another work, Shavezipur et al [60] introduced triangular shaped parallel capacitor with a maximum tunability of 71% and linearity factor of 0.943.
Majid et al [70] presented a parallel plate tunable capacitor in which both plates move to a distance of d/6, and total displacement of d/3, when electrostatic force is applied across the plates. The main advantage of the design is that it inhibits the pull-in of the tunable capacitor and enables good control on the capacitance change. Secondly, it helps in structure's stress and strain reduction, thus increases the lifetime of the tunable capacitor. Thirdly, it offers the linear behavior of the capacitance change in the one-third of gap between two plates. Simulation results shows that the design can get 59% tuning at 7 V. The Q factor is 91 at 11 GHz, which is excellent for high frequency applications. However, the design is not fabricated and measurements results are not available.
Han et al [24] reported a novel method to achieve highly linear capacitance, and a large tuning range in a parallel-plate MEMS tunable capacitor by using leverage structure in the design to drive the movable plate to the increasing gap direction. Liu et al [71] proposed a similar new structure for improving the linearity of capacitance-voltage curve as shown in figure 5(d). The non-planar top electrode offers small gap for high initial maximum capacitance. When the downward electrostatic force is applied at the two drive electrodes, it moves up the middle capacitor plate to control the capacitance by deforming the torsional beams. This increases the gap between the top and bottom electrode. The tunable capacitor has a stable performance from 1 to 10 GHz and a tuning ratio greater than 10.  [68], (c) butterfly shaped tunable capacitor for high linearity [58], (d) leverage structure tunable shunt capacitor for high tunability [71]. © 2000 IEEE. Reprinted, with permission, from [66]. © 1998 IEEE. Reprinted, with permission, from [68]. Reprinted from [58], Copyright (2008), with permission from Elsevier. © 2019 IEEE. Reprinted, with permission, from [71].

Performance comparison of lateral comb drive and transverse parallel plate tunable capacitors
Parallel plate and lateral comb drive actuated, and sensed tunable capacitors are the common electrostatic techniques for MEMS capacitor tuning. Both techniques have their advantages and disadvantages. However, the main difference between them is that in transverse tunable capacitor, there is a non-linear relation between the electrostatic force and the voltage across the plates, and the capacitance produced is inversely proportional to the gap between the plates. The capacitance of the parallel plate changes sharply with the change in separation gap. On the other hand, in lateral comb drive capacitor the relation between generated force and voltage is linear, similarly, the relation between capacitance change and overlap length change is linear.
It is clear from table 2 that laterally comb drive actuated tunable capacitors have high tuning ranges compared to parallel plate capacitors. The tuning range in lateral comb drive system depends upon the overlapping area of comb fingers and the mechanical design of spring suspension. However, they have large footprint area as compared to parallel plate tunable capacitors. On the other hand, parallel plate tunable capacitors are more attractive due to their small footprint, and fast response. The tuning range has been increased by using different techniques to suppress the pull-in effect of the parallel-plate capacitors as reported in [4,5,66,72]. However, the Q factor of parallel plate actuated tunable capacitors are higher than the comb drive ones. In addition to this, comb drive tunable capacitors require high driving voltages, and lower electrical resonance frequency. The maximum tuning ratio in parallel plate capacitor is 22:1. However, the tuning voltage is at the high side (30-55 V). Similarly, the maximum tuning range in lateral comb drive capacitor is 31:1 with voltage requirement of 40 V. The Q factor is also higher than the parallel plate capacitor operated at 1 GHz.

Fabrication processes for in-plane and out of plane tunable capacitors
There are mainly two methods for the fabrication of electrostatically actuated tunable capacitors: bulk micromachining and surface micromachining. The bulk micromachining is mostly used for the fabrication of in-plane motion tunable capacitors, whereas surface micromachining is generally incorporated for the fabrication of out-of-plane tunable capacitors.
Bulk micromachining is mainly applied on silicon, glass, and gallium arsenide wafers. Etching in bulk micromachining is categorized into wet and dry etching. For dry etching, deep reactive ion etching (DRIE) is the common technique used for fabricating in-plane tunable capacitors. It offers deep and high vertical aspect ratio, vertical wall profile, and good material selectivity. The vertical wall anisotropy is controlled by adjusting several process parameters, which include process pressure, temperature, DC bias voltage, input power, and the chemical gases used in the process.
Tunable capacitors fabricated using bulk micromachining require fewer masks than surface micromachining as reported in [56,74]. In these works, the tunable capacitors are fabricated using silicon-glass bonding processes. The silicon wafer is etched using ICP deep etcher from bottom and top for patterning of comb fingers, and bonded to glass wafer using anodic bonded process. Another bulk micromachining siliconglass bonding process tunable capacitor with improved performance is reported in [17]. The tunable capacitor achieved high Q factor and large tuning ratio. The improved performance is due to double sided metallization of silicon layer for comb drive system to reduce the out-of-plane bending caused by Coefficient of thermal expansion (CTE) mismatch between silicon and metal layers as, shown in figure 4(b).
Similarly, another high performance tunable capacitor fabricated using DRIE etching having angular vertical comb drive system is reported in [63]. The tunable capacitor is fabricated on glass wafer to reduce the parasitic capacitance. A SOI wafer is bonded to a Borofloat glass wafer to create a silicon on glass wafer. A DRIE etching of silicon is performed to build device structure. Photosensitive cyclotene resist benzocyclobutene hinges are patterned to physically connect the horizontal anchors with the angular comb fingers, as shown in figure 4(d).
Another common process for fabricating in-plane tunable capacitor using bulk micromachining technique is given in [62]. The tunable capacitor is fabricated on oxidized silicon wafer and a SU-8 bonding has been utilized for bonding ultrathin silicon wafer on top, which is patterned using DRIE. The SU-8 bonding helps in replacing expensive SOI wafers and enables flexible designs.
The surface micromachining technique fabricate microstructures by adding material layer by layer on top of the substrate. The polycrystalline silicon (polysilicon) is a common material for the layers. Several two layers (two parallel plates) and three layers (three parallel plates) tunable capacitors are fabricated using surface micromachining technique in a standard PolyMUMPs process [83]. The process offers three layers of polysilicon and a gold layer on the top polysilicon layer. The tunable capacitors with three parallel plates achieved greater tunability than two plates parallel plate capacitor reported in [4,72]. Another design of tunable capacitor is reported in [73] using bottom and top polysilicon layers of the PolyMUMPs process. Similarly, some novel geometry tunable capacitor fabricated in PolyMUMPs standard process are reported by Shavezipur [58,60,84].
Several other prototypes have been fabricated with surface micromachining using aluminum or copper metal layers as structural layers [8,18,75]. The Young's Modulus of aluminum and copper is lower than silicon. Thus, these are more flexible and require low actuation voltage with better RF performance (low ac resistance). In addition, gold is also used as a structural layer for good RF performances reported in [6,71]. Similarly, some designs have been fabricated using polymide as a structural layer to achieve low actuation voltages [5,66]. The Young's modulus of polymide is only 3 GPa [85].

Switchable tunable capacitor
Modern communication systems require high tuning range and fast response. Unfortunately, traditional parallel plate, and comb drive tunable capacitors are limited in tuning range whereas thermally actuated tunable capacitors are slow in tuning. Therefore, switchable tunable capacitors have gained increasing attention because high tunability can be achieved by selection from parallel connection of tunable capacitor through switches. The capacitance of the variable capacitor depends on the selection of actuated switches, as shown in the schematic of figure 6. However, the size of switched capacitor is relatively large due to the array of capacitors and switches integrated together on the same chip. These tunable capacitors also suffer from high parasitic effect due to the large size, and their operation is often limited to 1-10 GHz.
Goldsmith et al [18] proposed a bistable switchable tunable capacitor with a high tuning range of 22:1. The obtained tunable capacitance ranges from 1.5 pF to 33.2 pF. The controlled voltage is in the range of 30-55 V, and switching speed is less than 10 µS. The fast switching speed allows quick tuning of capacitor elements. The high switching speed is related to the high mechanical natural frequency of the switch. The membrane moves from the contact position to its mechanically neutral position when there is no electrostatic force exerted on it. By neglecting air damping, Van der Waals force, bouncing effects, and contact complications, this release time can be approximated by 1/4 period of the membrane free vibration. The switching speed is even higher when electrostatic force is applied. In another work [86], they fabricated 16 state variable capacitor together with fixed capacitors and demonstrated a tunable filter for Ultra-high frequency (UHF) and Very High Frequency (VHF) bands. To realize a tunable capacitor, a fixed capacitor is connected in series with a capacitive switch for a two state capacitance as shown in figure 6. The capacitive switch has maximum value when it is ON (down state) and minimum value when OFF (up state). The combinations of these two-state capacitance with a fixed capacitance allows construction of variable capacitor as shown in figure 7. A four bit switched capacitor can provide total of 16 capacitance values. Similarly, Belkadi et al fabricated switched capacitors on glass wafers. The MEMS capacitors are tuned by deflecting thin gold metal layers on dielectric layer, thus increases the capacitance 3 times to the initial capacitance, from 25 fF to 75 fF. Several high power tunable capacitor banks are reported in [87][88][89]. These high power capacitors are essential for impedance tuning of high power transmitters at the base station of mobile industries and satellite communications.

Dielectric tuning tunable capacitors
In this method, a dielectric material between the two fixed plates are changed to get high tunability and Q factor. Yoon et al [8] presented a movable dielectric tunable capacitor for attaining high Q factor of the capacitor. In the proposed design, the top and bottom plates are made stationary and the dielectric between the plates is made movable, as shown in figure 8(a). The total tunability of 40% and Q factor of 218 have been achieved at 1 GHz.
Similarly, another work is presented in [90] based on changing the dielectric medium using deionized (DI) water between the plates. DI water is injected into the SU 8 channel for changing the dielectric between capacitor plates. The initial capacitance when microfluidic channel is empty, is C min = 0.11 pF and when it is full, is C max = 5.76 pF. This allows a very wide tuning range with T r = 5136% at 4.5 GHz. Furthermore, the resonant frequency ranges from 5.67 GHz to 19.81 GHz. The maximum Q factor value Q max = 84.27 is achieved when the capacitor is empty and it reduces to Q min = 3.99, when it is filled with DI water at 4.5 GHz. Habbachi et al [91] reported the effect of dielectric liquid on the tunability of MEMS capacitor. A wide tuning range of around 7660% can be obtained by changing the fluid position in the channel between the electrodes, which is shown in figure 8(c). The Q factor varies from 51.9 when it is empty to 1.49 when it is fully filled.
Although the tunability of dielectric liquid tunable capacitors is high but they suffer from low Q factors when filled by dielectric medium. In addition, these tunable capacitors are also bulky and occupies large area. Further, dielectric liquid tunable capacitors require microfluidic channels, which adds fabrication complexity.

Electrically floating plate tunable capacitors for high quality factor
The traditional parallel plate capacitor, and lateral comb drive systems have the disadvantage of low Q-factor. This is due to the RF losses in the suspension beams of the tunable capacitors. These suspension beams are normally made long and thin to acquire low stiffness values for low actuation voltages [4,75]. Thus, the Q factor is low due to RF losses in the suspension beams. Lee et al [92] proposed a tunable capacitor actuated with an electrically floating plate. The electrically floating plate means that the suspended plate is not connected electrically and RF signal does not pass through the longs beams of suspended plate, which are made long to reduce the operating voltages. A SEM image of the proposed tunable capacitor with a thick electrically floating top plate, and a thin mechanical spring beams are shown in figure 9(a). The proposed design showed 200% increment in Q factor at 5 GHz compared to the conventional parallel plate capacitor. They achieved a tuning range of 41% and a Q factor of 34.9 at 5 GHz.
Yong et al [93] proposed an electrically floating movable dielectric fabricated in SOI-MUMPs. The tunable capacitor is actuated with lateral comb drive system and is sensed using parallel-plate capacitor, as shown in figure 9(b). A tunability of 172% was achieved with the voltage range of −120 V to 120 V. The Q factor of 0.35 was achieved at 1 GHz. This Q factor is very low due to the RF losses in the silicon material. Another electrostatically actuated, and electrically floating plate tunable capacitor is proposed by Khan et al in [82]. The device is fabricated in Metal-MUMPs process. To minimize the size of the device, the tunable capacitor is designed to be actuated using the same parallel plates of the capacitor, as shown in figure 9(c). The tunable capacitor achieved a wide tunable range of 225% with the help of a stopper and a Q factor of 1150 at 50 MHz.

High power handling tunable capacitors
In some communication systems, such as radio detection and ranging, long distance communication, and satellite communication, high power handling passive devices possess great importance due to their crucial role in the performance of the systems. Electrostatically actuated tunable capacitors are known for compact size, fast response, and good performance. However, they can handle only low power signal due to    [92], (b) thermally actuated transverse parallel plate comb drive system [93], (c) transverse actuated and sensed comb drive tunable capacitor [82]. © 2008 IEEE. Reprinted, with permission, from [92]. © 2009 IEEE. Reprinted, with permission, from [93]. Reprinted from [82], Copyright (2019), with permission from Elsevier. the well-known phenomenon of self-actuation. When a high power RF power is applied to tunable capacitors, electrostatic force generated due to the signal power is so large that can induce the pull-in effect, which is a great issue in electrostatic MEMS capacitors. To avoid this issue, Reines [94] reported RF shunt tunable capacitor with large spring constant. The tunable capacitor consists of separate RF and DC electrodes, which are fabricated underneath a circular diaphragm, as shown in figure 10. The design increases both the restoring force, and the RF self-actuation voltage, resulting in improved power-handling capabilities. The prototype requires a small pull-in voltage of only 24-28 V. A ring-shaped RF transmission line is routed underneath the higher spring constant portion of the circular beam to increase both the restoring force and the RF self-actuation voltage, as shown in figure 10. Separate DC bias electrodes are placed on the either side of the RF line to result in a relatively low pull-in voltage. Similarly, another technique is reported for the high power handling with relatively low voltage in [52]. The tunable capacitor is connected with two fixed and two MEMS tunable capacitors all connected in series. This quadruple series capacitor structure helps in reduction of the actuation voltage across the MEMS tunable capacitor. Thus the power-handling capability is greatly improved. For comparison purpose, a summary of the reported high power electrostatically actuated tunable capacitor is presented in table 3.

Commercial devices
Presently, there are few commercially available MEMS tunable capacitors manufactured by several companies such as Menlo micro, Cavendish Kinetics, ST Micro, WisSpry, and Nanusens. Recently in 2021, Nanusens manufactured RF   [99]. The switch's actuator is a parallel plate capacitor with a movable electrode. The switch is used for base station antennas in RF communications.
In 2020 Menlo micro, produced a highly reliable MEMS switch using electrostatic parallel plate actuation. The new device has super low resistance and leakage current, high linearity, and low power consumption. The switch is based on highly conductive and highly reliable alloy that can withstand billions of switching cycles. Currently, MEMS industry is working on the performance improvement of key parameters, such as reduction in parasitic capacitance, large tuning range, improved linearity, reliability, and quality factor.

Conclusions
MEMS tunable capacitors have become primary choice over CMOS tunable capacitors in RF industries due to their superior performance. In this paper, a review of electrostatically actuated tunable capacitors has been performed. Different designs have been investigated and their influence on the performance improvement of tunable capacitors has been studied. Besides, many applications of tunable capacitors, and their influence on the performance of applications have been summarized. Considerable future research is expected on high frequency ranges, terahertz (THz), and 5G ranges due to excellent performance of tunable capacitors in these ranges. In the last two decades, significant work has been reported on MEMS based electrostatically tunable capacitors. Different out-of-plane and in-plane designs with parallel-plate, lateral comb drive systems and a combination of both configurations have been reported using different types of materials and fabrication processes to maximize the tuning range, quality factor and linearity. Some of them also worked on improving the reliability of the tunable capacitors, however, the reliability is always high due to non-contact nature of the device.
The summarized performances with quantitative performance of tunable capacitors in tables 1 and 2 give guidelines for designing high performance tunable capacitors. The type of actuation, fabrications process, and design parameters, such as gaps between plates, and dimensions play very crucial role in the performance of the tunable capacitors. In addition, advanced application of the electrostatically actuated tunable capacitors have been summarized in table 3. The current state of the art RF communication industry demands high frequency, large re-configurability to cover different services, reduced power consumption and miniaturization. Therefore, MBTC has been exploited largely in modern communication industry.