RF-MEMS Devices Taxonomy

Transcription

1 RF- Devices Taxonomy Dr. Tejinder Pal Singh (T. P. Singh) A. P., Applied Sciences Department RPIIT Bastara, Karnal, Haryana (INDIA) Abstract The instrumentation and controls in the fields like defense, aerospace, electronics and communication continually needs for novel technologies and developments. Radio frequency microelectromechanical systems (RF- ) technology shows remarkable potential over the years. It has already made its charisma felt significantly by providing replacement in electronics and communication systems with quality factors and defined tunability. The RF- devices and components have emerged as potential candidates for modern applications in the different fields. The center theme of this paper is to classify the different RF- devices under the different domains. This research paper also highlights the fact that the limitations faced by the current RF devices can be overcome by the flexibility and better device performance characteristics of RF- components. This paper reviews the importance of RF-, their salient features, and possible applications. Keywords-RF- Devices; Reliability; Taxonomy; RF systems; Application I. INTRODUCTION At present, Micro-Electro Mechanical systems () are attaining much attractiveness from fields (such as electronics and communication, defense, and space) because of the benefits of size reduction without any reduction in the performance of a system. In the modern communication field, there is a need to have a technology that can push the operating frequencies higher, offer broader bandwidths, and manage multiple broad band signals, all within a single device. Today, radio frequency microelectromechanical system (RF-) technology is rapidly growing, and trying to solve these challenges. The key benefits include low loss, high isolation, near-perfect linearity, and instantaneous bandwidth that traditional semiconductor technologies cannot provide [1] [2]. The fundamental blocks of RF- systems are the micromechanical components like tunable capacitors, high quality inductors, micro-resonators, high performance filters and switches which have the potential to substitute the traditional distinct components. These devices are fabricated using structures like cantilevers, air bridges, membranes and inter-digital structures. This paper here briefly reviews the significance of RF- devices, their key features, and potential applications. RF- technology has the applications in the field like defense, communication, phased array radars, reconfigurable antennas, automotive and cellular phone. The potential applications of RF- devices are shown in Fig. 1. The RF- provide components with reduced size and weight, low loss, low power consumption, wide bandwidth, higher linearity, lower phase noise, better phase stability and high isolation. Wherever the application needs such features, can offer solutions to substitute either components or circuits or the subsystems using the components [3]. There are numerous RF- components which are either used directly for replacement or integrated to form a microsystem along with other semiconductor devices. POTENTIAL APPLICATIONS OF RF MASS APPLICATIONS MOBILES, GPS, RFID, WLAN, CONSUMER & IT TELECOM INFRASTRUCTURE BASE STATIONS MICROWAVE COM TEST RF INSTRUMENTATIO N RF AUTOMOTIVE ANTICOLLISI ON RADAR ROOF ANTENNA HIGH VALUE APPLICATIONS DEFENSE RADIO DEFENSE COMMUNICATI ON SYSTEM MISSILES SATELLITES Figure 1. Potential Applications of RF- devices. The components can also go along with silicon technology or GaAs technology and the components can be incorporated to give a system solution. The restrictions of the usual RF integrated devices can be conquered by the flexibility and improved device performance properties of RF- components, which finally propagate the device level advantages to the system to achieve the unprecedented levels of performance. The component level to system level growth of a characteristic communication system using RF- devices is shown in Fig. 2. Print-ISSN: e-issn: RG Education Society (INDIA)

2 SYSTEM (Phased Array Antenna, Switch Matrix, Cell Phone, Pager) CIRCUIT (Phase Shifter, Transceiver, Filter, Oscillator) using the high resistivity substrate like quartz. The tuning range, nominal capacitance, tuning voltage, quality factor, and self-resonant frequency are the factors to be considered during the design taking into account specific requirement. Different micromachined tunable capacitors are shown in Fig. 3. DEVICE (Switch, Inductor, Resonators, Varactor) Figure 2. RF-s Components and Subsystems. The possible substitutes that are better over the traditional components in terms of high quality, low power consumption and wider operational frequency range are the tunable capacitors, high Q inductors, high performance filters, and oscillators. These components can be made using pre-cmos or post- CMOS approaches to integrate the components on the same wafer. The chief benefit given by is the exact tunability and the tuning ratio which the traditional components cannot offer [4]. This provides the ability to fine tune the circuits to the best optimum level and achieve the highest performance. Re-configurability is another required feature used to reconfigure the same antennas for different frequencies without shifting from one antenna to another. This helps in faster scanning and better directivity of the antennas. The switches, which are characterized by low loss, low actuation voltages and better reliability, are employed for reconfiguring the antennas, routing networks, tunable filters, etc. Due to high linearity, RF- devices are brilliant candidates for broad band communication systems. II. RF- DEVICES A. Tunable Capacitors The IC technology gives a fixed capacitance with a sandwiched dielectric layer between two conductive electrodes. Here, the parasitic capacitance and the series resistance will cause losses and reduced quality factor. A chip area is also needed for high capacitance. The micromachined tunable capacitors meet the requirements to a great extent. The tunable capacitors can be realized either by surface micromachining or by bulk micromachining. But, the surface micromachined capacitors are straightforward and implemented by having a bottom electrode on the substrate and a suspended electrode on the top with an air gap [5] [6]. (a) Three-plate gap-tuning variable capacitor [18]. (b) Gap-tuning variable capacitor using an electrothermal actuator[19]. Figure 3. Top view of micromachined tunable capacitors. B. RF- Switches The RF- switches have huge potential in consumer electronics, defense, communication, and aerospace. The applications include phase shifters, switchable capacitors, tunable filters, switch matrix for routing and phased array antennas[7][8][9]. Ideally, switches are used to perform signal routing without causing any power loss to signal. The desirable parameters in RF switches are low insertion loss and return loss in the closed state, high isolation in the open state, high linearity, high power handling capability, low operating voltages, high reliability, small size, and low cost. The tunability is achieved by the displacement of the top membrane by applying an electrostatic force between the two plates. For high capacitance values, a suitable dielectric with high dielectric constant can be employed in the place of air between the two plates. The parasitic capacitance can be reduced and Q can be increased by 32

3 DIELECTRIC PULL-DOWN ELECTRODE MEMBRANE ANCHOR (a) Z 0 Z 0 C (a) Suspended Spiral type of Inductor [21]. L Top conductor R S Magnetic core SWITCH IMPEDANCE Z s = R s + jωl + 1 / jωc RESONANT FREQUENCY = f 0 = (1/2T 2 ) * (1/ (LC) 1/2 ) Pad (b) Figure 4. (a) Top view the RF switch and (b) equivalent circuit [20]. There exist various configurations in RF- switches depending on the application. Usually a series ohmic switch is used while transmitting the RF signals in the frequencies dc to few GHz. For higher frequencies, a shunt switch is used which provides good isolation [10]. The key parameters considered while designing a switch are the losses, switching speeds, actuation voltages, RF power handling, and life cycles. The widely used actuation mechanism in switches is the electrostatic actuation where the potential is applied between the two electrodes to bring in the displacement of the free beam. Top view and its equivalent circuit are shown in the Fig. 4. C. Variable Inductors variable inductors find wide applications in the RF--based communication systems. In normal planar inductor, the resistive metal lines and the dielectric losses in the substrate, degrade the Q factor and cause parasitic capacitances. The micromachined inductors with suspended metal structures give high Q factor resulting in high frequency performance of the systems. Micromachined inductors are used in low-noise oscillators, high-gain amplifiers, and matching networks. These are characterized with inductances of few nh and the Q factors of using inductors. There are two configurations of inductors: solenoid type and suspended spiral type as shown in Fig. 5. The inductor consists of a planar spiral made in one layer of metal and a connection to the center of the spiral in another layer of metal [10]. D. Resonators (b) Solenoid type of Inductor [22]. Figure 5. Planar on-chip inductors. Q factor for normal electrical RLC circuits is limited up to 100 due to the parasitic resistive losses in the circuit. This also increases the insertion losses that cause unwanted signal attenuation. The micromachined resonators with Q factors much above 1000 over a wide range of tunable frequencies when used in combination with the integrated electronics will result in highly stable communication system. There are two types of resonators: beam resonator and comb drive resonator. Both these suffer from the limitation of operational frequency which can be extended only up to few hundreds of MHz only. The bulk cavity resonators exhibit a high Q exceeding 10,000. However, dimensions are especially in the low frequency range up to few hundreds of MHz. The micromachined resonators can be realized in dimensions that are order of magnitude smaller than cavity resonators, at the same time, meeting the high quality requirements in addition to low loss and high linearity[11] [12]. Both configurations cannot get higher frequencies. The film bulk acoustic resonators (FBAR) are better for applications requiring higher frequencies (up to 2 GHz) with Q >

4 E. Resonators A phase shifter is a two port network, in which the phase difference between output and input signal can be managed by a control (dc bias) signal. Phase shifters with low cost, insertion loss, drive power, and continuous tunability result in light weight phased array antennas. The phase shifters find application in telecommunication and military radars. The phase shifters are extensively used in phased array antennas in defense for applications like smart munitions, missile communication, drones, missile radar seekers, aircraft and helicopters etc. There are two configurations of the phase shifters: one with switched line and the other with distributed transmission line (DMTL). The phase shifter with switched line approach has switches in the configuration which when selectively actuated results in desired phase shift while the DMTL phase shifter can be optimized to get low insertion loss over a broad frequency band. The phase shifters based on RF- switches causes reduction in losses than their solid state counter parts. The decrease in losses reduces the need for the MW power and so reduces the size but give better performance [13] [14]. switches in phase shifter design result in lower loss phase shifters at any frequency particularly from GHz. Figure 6 shows a three dimensional view of DMTL phase shifter at RCI. Figure 6. A three dimensional view of designed DMTL phase shifter at RCI [13]. F. RF- Transceivers These find applications in wireless systems. The need of portable wireless systems with more functionality and rapid growth in the technology create novel devices with greater functional density and with more integratability into the system [15]. RF- have made their influence with their potential in transceivers associated with low power consumption, high quality factor, and size reduction. In fact, applications like personal communication systems (PCS), wireless networking, and radars are being completely exploited by to decrease the parts count, power consumption, and lighter weight but with superior RF performance. III. TAXONOMY- VIEWPOINT RF- and microwave industry is reaping the benefits of technology. The continuous advance in technology attracted researchers towards the development of devices for RF applications. RF- devices have a wide range of potential applications in wireless communication, navigation, sensor systems. They could be used in switches, phase shifters, signal routings, impedance matching networks, exciters, transmitters, filters, RF receivers. RF- devices can be grouped as active devices and passive devices. Active devices: switches, varactors, and tuners. Passive devices: bulk micromachined transmission lines, filters, couplers. However, it is still premature for a taxonomy of RF- devices, yet the progress till date tends to put them into different classes depending on whether one takes an RF or viewpoint. From the RF viewpoint, the devices are simply grouped by the RF-circuit component they consists of, be it reactive elements, switches, filters, or something else. From the viewpoint, these are put into three separate classes based on where and how the actuation is carried out relative to the RF circuit. The three classes are mentioned below: A. RF Interinic These are the devices in which the structure is positioned inside the RF circuit and has the dual roles of both the actuation and RF-circuit function. In this group, one may regard as conventional cantilever and diaphragm type that can be employed as electrostatic microswitch and comb-type capacitors. With the disco of electro-active polymers, multifunctional elegant polymers and micro-stereo lithography, these RF- can be easily used with polymer based polymers. These are stable, flexible, and lifelong. In addition, these can be integrated with the organic thin film transistor. Shunt electrostatic micro switch, inductors and comb capacitors are the examples that are put in the RF-intrinsic class. B. RF Reactive In this group of RF- devices, the structure is positioned inside, where it has role of RF function that is attached to the attenuation. The examples of this class are capacitively coupled tunable filters and micromechanical resonators. These devices facilitate the required RF functions in the associated circuit. Millimeter wave and microwave planar filters on thin dielectric membrane exhibit low losses, and are suitable for low price, compact, high performance millimeter wave onechip integrated circuits. A collection of these devices is shown in the RF- technology diagram of figure 7. The richest class is clearly the RF-intrinsic, which already boasts three promising devices. Here, we have tunable capacitors and inductors that are expected to operate up to at least a few GHz in frequency, and we have RF-embedded switches that operate well from a few GHz up to at least 100 GHz. 34

5 TABLE I. TAXONOMY OF RF- DEVICES [16] Figure 7. Three different RF- device categories [23]. IV. TAXONOMY-RELIABILITY VIEWPOINT The taxonomy of RF- devices [16] [17] is recently becomes a hot issue in such a way that it has tendency to include any device which is made with at least one step of micro-machining technology. So, it has become necessary to make a division of various RF- devices in such a way that it becomes significant for studies regarding reliability. This is important to make some common criteria for the accelerated tests and ageing models. Three different classes of reliability of RF- devices are briefed in the table 1. This taxonomy of RF- devices has been done in accordance with the level of mechanical complexity and boundary conditions. The class-i has all the passive components that have been designed for diminished losses through micromachining fabrication. Mechanical movements of any part of the structure of this class of RF- devices are not required during the functioning and working. However, some deformations might take place during various processes involved in fabrication. Reliability and stability of this class of RF- devices in the long term do not alter significantly from those of conventional RF passive components. Stability problems of the structures of these devices might take place to thin dielectric membranes. Often, these dielectric membranes are used for high quality factor passive components fabricated by using micromachining. When heat diffusion of the bulk material is not good, then the membranes also have tendency to expose thermal problems. In addition to these problems, the devices which are under repeated temperature cycles during assembly and packaging process can be prone to structural deformations that cause failure of the device. The second class of RF- devices demands mechanical movement of some part during the working of the devices. This class of RF- devices consists of devices having micro-machined structure and moveable parts. Notable examples of this class are high quality factor micro-electro- mechanical resonators and continuously tuning capacitors. Due to repeated mechanical movements and vibrations, novel stress mechanisms are introduced on the constituted parts of these devices. Class I II III Micromachined Yes Yes Yes Structures Movable Parts No Yes Yes Impact No No Yes Examples of RF- Devices High-Q Suspended Inductors: spiral, self assembled coils; low-loss RF- Membranes; RF-CMOS substrate removal postprocessing Very High-Q micro-electromechanical resonators; continuously tuning capacitors. Ohmic contacts RF relays; switched capacitors; capacitive coupling RF- switches and multiplexers. Plastic deformations, mechanical relaxations, fatigue, creep etc can disturb the stability of electro-mechanical behavior of these devices. All these failure and degradation mechanisms cause the mechanical failure of second class of RF- devices. In addition to this, oxidation and absorption like surface effects can cause stresses in moving and oscillating part. As a result complex stability problems are introduced that help in the failure of device. The third class of RF- devices comprises of all the devices demanding two distinguished mechanical moveable parts to attain and keep contact during a definite time of cycle of the operation. Novel problems related to reliability are caused due to the presence of mechanical contact between the moving parts of device. These reliability problems may be of mechanical type and electrical type. The major effect that diminishes the working of devices is the stiction of mechanical parts that keep the mechanical contact. Due to stiction of mechanical parts restoration of resting position becomes almost impossible even after the removal of actuation force. The stiction can happen due to many factors like redistribution and accumulation of electric charge in dielectric slabs, capillary effects due to humid environment, micro welding of metals due DC or RF power etc. Examples of this class of devices are ohmic-contact RF- relays, switched capacitor, capacitive coupling RF- switches and multiplexers. Electrical ohmic contacts between two metallic surfaces may be affected from stability problems that arise due to number of cycles, variation in resistance of ohmic contacts, transfer and erosion of material, surface contaminations and other surface effects like absorption and oxidation. V. TAXONOMY-APPLICATION VIEWPOINT RF- include several distinct types of devices, such as RF- switches and relays, tunable inductors, resonators, varactors (variable capacitors), antennas, transceivers and phase shifters. Applications of RF- include all types of wireless communications, radar, satellites, Missiles, instrumentation, WLAN, GPS, RFID and test equipment. Compared to conventional RF 35

6 components, RF- offer significant benefits, like lower power consumption, lower insertion loss, and lower cost. Another possible application of RF- is their implementation in transceivers in wireless systems. The table 2 shows the taxonomy of RF- devices as per the application viewpoint. The technology has the potential of replacing many traditional Radio Frequency (RF) components used in now-a-days mobile, communication and satellite systems. In many cases, such RF- components would not only reduce substantially the size, weight and power consumption but also promise superior performance in comparison with current technologies. These days and RF- can be found in many different applications across multiple markets. RF- experts believe market forces are enabling a second wave of applications, limited to some selected but industries in which components have clear advantages over traditional electronic components. In particular, the telecommunications industry is ripe for technology. RF- are mainly used in the fields such as automotive, electronics, space, defense, medical and communications. Sr. No. TABLE II. Devices Requiremen t 1 Switches TAXONOMY OF RF- DEVICES AS PER APPLICATION DOMAIN Wireless WLA N GPS Phase shifters Inductors Tunable Capacitors Resonators VCOs Memtenna Sr. Devices Instrumentatio RFID Rada No Requiremen r. t n 1 Switches Phase shifters Missile s Inductors Tunable Capacitors Resonators 6 VCOs 7 MEMTENN A ---- mediu m ---- mediu m CONCLUSION The RF- technology is on the edge of bringing out a noteworthy change in the fields of communication radar communication, instrumentation and aerospace in the coming years. An incredible amount of work is going on the earth due to their potential for tactical applications in defense field. In this paper, a brief summary of RF- devices (micromechanical resonators, variable inductors, RF- switch, tunable capacitors, phase shifters, and transceivers) is given. This paper reviews the importance of RF-, their salient features, and possible applications in brief. This paper also classifies the RF- devices on the basis of different viewpoints such as reliability, application, and. Three different classes of reliability of RF- devices are briefed in the table 1.This taxonomy of RF- devices has been done in accordance with the level of mechanical complexity and boundary conditions. From the viewpoint; these are put into three separate classes based on where and how the actuation is carried out relative to the RF circuit. The three classes are RF-extrinsic, RF-intrinsic, and RF-reactive. REFERENCES [1] H J De Los Santos. Introduction to microelectromechanical (MEM) microwave systems. Artech House, Norwood, MA, [2] J J Yao. RF from a device perspective. J. Micromech. Microeng. 2000; 10, R9-R38. [3] S Lucyszyn. Review of radio frequency microelectromechanical systems technology. In: Proceedings of IEEE Sci. Meas. Techn. 2004, 151(2), p [4] A Dec and K Suyama. Micromachined electromechanically tunable capacitors and their application to ICs. IEEE Trans. Microwave Theo. Techn. 1998; 46(12), [5] D J Young and B E Boser. A micromachined variable capacitor for monolithic low Noise VCOs. In: Proceedings of Solid-state Sensors and Actuator Workshop, Technical Digest, Hilton Head, NC. 1996, p [6] J J Yao, S Park and J Denatale. High tuning ratio based tunable capacitors for RF communications applications. In: Proceedings of Solid-state Sensors and Actuator Workshop, Technical Digest, Hilton Head, SC. 1998, p [7] E R Brown. RF- switches for reconfigurable integrated circuits. IEEE Trans. Microwave Theo. Techn. 1998; 46 (11), [8] J Danson, C Plett and N Tait. Design and characterization of a capacitive switch for improved RF amplifier circuits. In: Proceedings of IEEE Custom Integrated circuits. 2005, p [9] B Lacroix et al. Sub-microsecond RF switched capacitors. IEEE Trans. Microwave Theo. Techn. 2007; 55(6), [10] M Maluf and K Williams. An introduction to microelectromechanical systems engineering. Ed. 2. Artech House, Inc, [11] K Wang, A C Wong and C T C Nguyen. VHF free-free beam high-q micromechanical resonators. J. Microelectromechanical Sys. 2000; 9(3), [12] W T Hsu and C T C Nguyen. Stiff compensated temperature insensitive micromechanical resonators. In: Proceedings of 15 th 36

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