RF MEMS and Their Applications / Edition 1 available in Hardcover
Microelectromechanical systems (MEMS) refer to a collection ofmicro-sensors and actuators, which can react to environmentalchange under micro- circuit control. The integration of MEMS intotraditional Radio Frequency (RF) circuits has resulted in systemswith superior performance levels and lower manufacturing costs. Theincorporation of MEMS based fabrication technologies into micro andmillimeter wave systems offers viable routes to ICs with MEMSactuators, antennas, switches and transmission lines. The resultantsystems operate with an increased bandwidth and increased radiationefficiency and have considerable scope for implementation withinthe expanding area of wireless personal communication devices. Thistext provides leading edge coverage of this increasingly importantarea and highlights the overlapping information requirements of theRF and MEMS research and development communities.* Provides an introduction to micromachining techniques and theiruse in the fabrication of micro switches, capacitors andinductors* Includes coverage of MEMS devices for wireless and Bluetoothenabled systemsEssential reading for RF Circuit design practitioners andresearchers requiring an introduction to MEMS technologies, as wellas practitioners and researchers in MEMS and silicon technologyrequiring an introduction to RF circuit design.
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RF MEMS and Their Applications
By V. K. Varadan K. J. Vinoy K. A. Jose
John Wiley & SonsISBN: 0-470-84308-X
Chapter OneMicroelectromechanical systems (MEMS) and radio frequency MEMS
During the past decade, several new fabrication techniques have evolved which helped popularize microelectromechanical systems (MEMS), and numerous novel devices have been reported in diverse areas of engineering and science. One such area is microwave and millimeter wave systems. MEMS technology for microwave applications should solve many intriguing problems of high-frequency technology for wireless communications. The recent and dramatic developments of personal communication devices forced the market to acquire miniaturized efficient devices, which is possible only by the development of radio frequency (RF) MEMS.
The term RF MEMS refers to the design and fabrication of MEMS for RF integrated circuits. It should not be interpreted as the traditional MEMS devices operating at RF frequencies. MEMS devices in RF MEMS are used for actuation or adjustment of a separate RF device or component, such as variable capacitors, switches, and filters. Traditional MEMS can be divided into two classes: MEMS actuators and MEMS sensors. The first one is a kind of moving mechanism activated by an electrical signal like Micromotor. Micro sensors are currently available for a large number of applications. Historically, owing to their ease of fabrication, these were the earliestmicrosystems. Another reason for the actuators not becoming popular is that the amount of energy generated by such tiny systems does not cause much impact in the associated systems. However, it can be seen later, for microwave and millimeter wave systems, these forces are sufficient to change the properties of overall systems. Passive devices include bulk micromachined transmission lines, filters and couplers. Active MEMS devices include switches, tuners and variable capacitors. The electromotive force used to move the structures on the wafer surface is typically electrostatic attraction, although magnetic, thermal or even gas-based microactuator structures have been developed.
Following the classical review paper by Brown (1998), the RF MEMS development to date can be classified into the following categories based on whether one takes an RF or MEMS view point: (1) RF extrinsic in which the MEMS structure is located outside the RF circuit and actuates or controls other devices in the RF circuit. In this class, one would consider the example of a tunable microstrip transmission line and associated phased shifters and arrays. Microstrip lines are extensively used to interconnect high-speed circuits and components because they can be fabricated by easy automated techniques. (2) RF intrinsic in which the MEMS structure is located inside the RF circuit and has both the actuation and RF-circuit function. In this class, one could consider traditional cantilever and diaphragm type MEMS which can be used as electrostatic microswitch and comb-type capacitors (Brown, 1998). With the invention of electroactive polymers (EAPs), multifunctional smart polymers and microstereo lithography, these types of RF MEMS can be easily conceived with polymer-based systems. They are also flexible, stable and long lasting. Moreover, they can be integrated with the organic thin film transistor. (3) RF reactive in which the MEMS structure is located inside, where it has an RF function that is coupled to the attenuation. In this class, capacitively coupled tunable filters and resonators provide the necessary RF function in the circuit. Microwave and millimeter wave planar filters on thin dielectric membrane show low loss, and are suitable for low-cost, compact, high-performance mm-wave one-chip integrated circuits.
One of the earliest reported applications of silicon-based RF MEMS technology for microwave applications is in the area of surface micromachined actuators for the realization of microwave switches. These possess very high linearity, low dc standby power and low insertion loss (Larson, 1999). These switches are based on electrostatic attraction counterbalanced by suitable mechanical forces on the beam to pull the switch into the right position. This switch can be designed to present nearly 50[ohm] impedance across a broad range of frequencies when closed, and nearly an open circuit when there is no connection. This property makes this an attractive choice for microwave applications. Several new switch architectures have also been reported, including the air-bridge structure (Goldsmith, Eshelman and Dennston, 1998). This structure utilizes very high capacitance variation to achieve the switching action. This scheme, however, suffers from relatively high switching voltage requirements.
MEMs technology is also used for RF applications in the area of variable capacitors, as a replacement for varactor diodes as tuners (Wu et al., 1998). Here, either a lateral or a parallel plate capacitance variation can be obtained with suitable fabrication approaches. The capacitance variation in the parallel plate version is over 3 : 1 making them attractive for wide-band tuning of monolithic voltage-controlled oscillators (VCOs). However their range is often limited by the low-frequency mechanical resonance of the structure.
The term MEMS refers to a collection of microsensors and actuators which can sense its environment and have the ability to react to changes in that environment with the use of a microcircuit control. They include, in addition to the conventional microelectronics packaging, integrating antenna structures for command signals into micro electromechanical structures for desired sensing and actuating functions. The system also may need micropower supply, micro relay and microsignal processing units. Microcomponents make the system faster, more reliable, cheaper and capable of incorporating more complex functions.
In the beginning of the 1990s, MEMS emerged with the aid of the development of integrated circuit (IC) fabrication processes, where sensors, actuators and control functions are co-fabricated in silicon. Since then, remarkable research progresses have been achieved in MEMS under strong capital promotions from both government and industry. In addition to the commercialization of some less-integrated MEMS devices, such as microaccelerometers, inkjet printer heads, micro mirrors for projection, etc., the concepts and feasibility of more complex MEMS devices have been proposed and demonstrated for the applications in such varied fields as microfluidics, aerospace, biomedicine, chemical analysis, wireless communications, data storage, display, optics, etc. (Fujita, 1996, 1998). Some branches of MEMS, such as micro-opto-electromechanical systems (MOEMS), micro total analysis systems (µTAS), etc., have attracted a great deal of research interest since their potential application market. As of the end of the 1990s, most MEMS devices with various sensing or actuating mechanisms were fabricated using silicon bulk micromachining, surface micromachining and LIGA processes (Bustillo, Howe and Muller, 1998; Guckel, 1998; Kovacs, Maluf and Petersen, 1998). Three dimensional microfabrication processes incorporating more materials were presented for MEMS recently when some specific application requirements (e.g. biomedical devices) and microactuators with higher output power were called for in MEMS (Fujita, 1996; Guckel, 1998; Ikuta and Hirowatari, 1993; Takagi and Nakajima, 1993; Taylor et al., 1994; Thornell and Johansson, 1998; Varadan and Varadan, 1996; Xia and Whitesides, 1998).
Micromachining has become the fundamental technology for the fabrication of microelectromechanical devices and, in particular, miniaturized sensors and actuators. Silicon micromachining is the most mature of the micromachining technologies and it allows for the fabrication of MEMS that have dimensions in the submillimeter range. It refers to fashioning microscopic mechanical parts out of silicon substrate or on a silicon substrate, making the structures three dimensional and bringing new principles to the designers. Employing materials such as crystalline silicon, polycrystalline silicon and silicon nitride, etc., a variety of mechanical microstructures including beams, diaphragms, grooves, ori- fices, springs, gears, suspensions and a great diversity of other complex mechanical structures has been conceived (Bryzek, Peterson and McCulley, 1994; Fan, Tai and Muller, 1987; Middelhoek and Audet, 1989; Peterson, 1982; Varadan, Jiang and Varadan, 2001).
Sometimes many microdevices can also be fabricated using semiconductor processing technologies or stereolithography on the polymeric multifunctional structures. Stereolithography is a poor man's LIGA for fabricating high aspect ratio MEMS devices in UV-curable semi-conducting polymers. With proper doping, a semiconducting polymer structure can be synthesized and using stereo lithography it is now possible to make three-dimensional microstructures of high aspect ratio. Ikuta and Hirowatari (1993) demonstrated that a three-dimensional microstructure of polymers and metal is feasible using a process named IH Process (integrated hardened polymer stereolithography). Using a UV light source, XYZ-stage, shutter, lens and microcomputer, they have shown that microdevices such as springs, venous valves and electrostatic microactuators can be fabricated. In case of difficulty on the polymeric materials, some of these devices can be micromachined in silicon and the system architecture can be obtained by photoforming or hybrid processing (Ikuta and Hirowatari, 1993; Takagi and Nakajima, 1993; Tani and Esashi, 1995; Varadan, 1995; Varadan and Varadan, 1995, 1996). The photoforming or photo fabrication is an optical method such as the stereolithography, photo mask layering process and IH process which involves solidification of photochemical resin by light exposure. Takagi and Nakajima (1993) proposed new concepts of 'combined architecture' and 'glue mechanism' using the photoforming process to fabricate complicated structures by combining components, each of them made by its best fabrication process. Batch processing of such hybrid silicon and polymer devices thus seems feasible.
The combined architecture may also result in sheets of smart skin with integrated sensors and actuators at the µm to mm scale. For some applications (say airfoil surface), the smart skin substrate has to be flexible to conform to the airfoil shape and at the same time it has to be compatible with the IC processing for sensor and smart electronics integration. It has been proposed by Carraway (1991) that polyimide is an excellent material for use as the skin because of its flexibility and IC processing compatibility. The control loop between the sensors and actuators employs the multifunctional materials which provide electrical functionality at selected locations using conductive polymers and electrodes that are connected to on-site antennas communicating with a central antenna. A related and difficult problem, and one which has been largely unaddressed, is the method for telemetry of the data. In some applications, stresses and strains to which the structure is subjected may pose a problem for conventional cabling. In others, environmental effects may affect system performance. Advances in ultra flat antenna technology coupled with MEMS sensors/ actuators seems to be an efficient solution. The integration of micromachining and microelectronics on one chip results in so-called smart sensors. In smart sensors, small sensor signals are amplified, conditioned and transformed into a standard output format. They may include microcontroller, digital signal processor, application-specific integrated circuit (ASIC), self-test, self-calibration and bus interface circuits, simplifying their use and making them more accurate and reliable.
The basic MEMS utilize a diaphragm-based, a microbridge-based or a cantilever-based structure. Special processing steps commonly known as micromachining are needed to fabricate these membranes, cantilever beams, resonant structures, etc., which will be discussed later. For a given application, it may be necessary to have integrated MEMS employing one or more of the basic structures. These three structures provide some feasible designs for microsensors and actuators that eventually perform the desired task in most smart structures. However, the main issues with respect to implementing these structures are the choice of materials that are to be used in fabricating these devices and the micromachining technology that may be utilized. To address the first issue, we note that in all of the three structures proposed the sensing and actuation occur as a result of exciting a piezoelectric layer by the application of an electric field. This excitation brings about sensing and actuation in the form of expansion in the diaphragm, or in the free-standing beam in the microbridge structure, or in the cantilever beam. In the former two cases the expansion translates into upward curvature in the diaphragm or in the free-standing beam, hence resulting in a net vertical displacement from the unexcited equilibrium configuration. In the cantilever case, however, and upon the application of electric field, the actuation occurs by a vertical upward movement of the cantilever tip. Evidently, in all three designs the material system structure of the active part (diaphragm, free-standing beam, or cantilever beam) in the microactuator must comprise at least one piezoelectric layer as well as conducting electrodes for the application of electric field across this layer. Piezoelectric force is used for actuation for many of the applications mentioned above. Micromachining is employed to fabricate the membranes, cantilever beams and resonant structures.
Microsensors and actuators are fabricated using the well-known micromachining techniques in the microelectronics industry. Three-dimensional microactuators in polymer structures can be achieved using stereolithography on UV-curable backbone-type polymers (Ikuta and Hirowatari, 1993; Takagi and Nakajima, 1993; Tani and Esashi, 1995; Varadan, 1995; Varadan and Varadan, 1995, 1996). In the integrated MEMS device, we may use photoforming processing in achieving the combined sensor and actuator architecture as outlined by Takagi and Nakajima (1993). For large actuation, one could use a flex tensional transducer consisting of a piezoelectric diaphragm bridged into a cavity (Chin, Varadan and Varadan, 1994).
Silicon micromachining has been a key factor for the vast progress of MEMS in the past decade. This refers to the fashioning of microscopic mechanical parts out of silicon substrates and more recently other materials. It is used to fabricate such features as clamped beams, membranes, cantilevers, grooves, orifices, springs, gears, suspensions, etc. These can be assembled to create a variety of sensors.
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Table of Contents
Microelectromechanical Systems (MEMS) and Radio FrequencyMEMS.
MEMS Materials and Fabrication Techniques.
RF MEMS Switches and Micro Relays.
MEMS Inductors and Capacitors.
Micromachined RF Filters.
Micromachined Phase Shifters.
Micromachined Transmission Lines and Components.
Integration and Packaging for RF MEMS Devices.