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Microelectromechanical Systems (MEMS) and Radio Frequency MEMS

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Microelectromechanical Systems (MEMS) and Radio Frequency MEMS 1 Microelectromechanical systems (MEMS) and radio frequency MEMS 1.1 INTRODUCTION 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 fre- quencies. 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 earliest microsystems. 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 2 MEMS AND RF MEMS 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 real- ization of microwave switches. These possess very high linearity, low dc standby power and low insertion loss (Larson, 1999). These switches are based on electrostatic attrac- tion 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 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. Sev- eral 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. 1.2 MEMS 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 electrome- chanical structures for desired sensing and actuating functions. The system also may need micropower supply, micro relay and microsignal processing units. Microcompo- nents 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 inte- grated 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 MEMS 3 the commercialization of some less-integrated MEMS devices, such as microaccelerome- ters, 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 poten- tial application market. As of the end of the 1990s, most MEMS devices with various sensing or actuating mechanisms were fabricated using silicon bulk micromachining, sur- face micromachining and LIGA1 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 micro- electromechanical 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 process- ing technologies or stereolithography on the polymeric multifunctional structures. Stere- olithography is a poor man’s LIGA for fabricating high aspect ratio MEMS devices in UV-curable semi-conducting polymers. With proper doping, a semiconducting poly- mer 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 fab- ricated. 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,
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