This paper reviews the recent progress in MEMS for radio frequency (RF) applications from a device perspective. RF MEMS devices reviewed include switches and relays, tunable capacitors, integrated inductors, mechanical resonators and filters, and some representative microwave and millimetre-wave components. Important device parameters are highlighted, as they have significant contributions to the performance of the final products in which the devices are used. The challenges and statuses of these RF MEMS devices are outlined and discussed. The intent of this topical review is to provide perspective to newcomers in the field, and empower potential end-users with an overall device picture, current status, and a vision of their ultimate performance capabilities.

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RF MEMS from a device perspective

An electrophoretic display has a substantially two-dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of particles that visually contrast with the dielectric liquid and also exhibit surface charges; a pair of electrodes, at least one of which is visually transparent, disposed on and covering opposite sides of the microcapsule arrangement; and means for creating a potential difference between the two electrodes, the potential difference causing the particles to migrate toward one of the electrodes. The display may be powered by one or more piezoelectric elements, which are also suitable for powering other types of nonemissive displays.

Nonemissive displays and piezoelectric power supplies therefor

A display includes an encapsulated display media, a rear electrode, and a movable electrode. The encapsulated display media comprises a plurality of capsules, each capsule comprising a plurality of particles dispersed in a fluid. The display media has a first surface and a second surface. The rear electrode is disposed adjacent the second surface of the display media. The movable electrode and the rear electrode apply an electric field across the display media.

Microencapsulated electrophoretic electrostatically-addressed media for drawing device applications

A full color, reflective display having superior saturation and brightness is achieved with a novel display element comprising multichromatic elements. In one embodiment a capsule includes more than three species of particles which differ visually. One embodiment of the display employs three sub-pixels, each sub-pixel comprising a capsule including three species of particles which differ visually. Another embodiment of the display employs color filters to provide different visual states to the user. The display element presents a visual display in response to the application of an electrical signal to at least one of the capsules.

Full color reflective display with multichromatic sub-pixels

The damping characteristics of beam-shaped microactuators that oscillate in the transverse direction are analytically evaluated to establish a design method that minimizes energy consumption and increases dynamic performance. The damping force due to airflow is calculated using the Navier-Stokes equation, the accuracy of which is verified by comparing the calculated damping with experimental results. The contributions to the damping due to squeeze force, internal friction, and support loss are calculated by using the Reynolds equation, structural damping theory, and a two-dimensional theory of elasticity, respectively. The final formulae, obtained in simple and closed forms for easy use in the actual design process, are then used to evaluate the relationships between the beam size and the damping ratio of silicon cantilevers, Permalloy cantilevers, and Permalloy spiral springs. Finally, the step response of a Permalloy cantilever is calculated and the relationship between beam size and settling time is determined.

Damping characteristics of beam-shaped micro-oscillators

An electromagnetic microrelay is proposed as a practical application of microelectrochemical system (MEMS) technology and the basic performance of its key components is evaluated. The contact resistance and breakdown voltage of contact electrodes are measured for very small loads and gaps. The static and dynamic deflection of the contact springs is measured for very small movements. Based on these experimental results, design guidelines for key components are proposed. Two prototypes that partly use MEMS technology are produced and their switching performance is demonstrated. One prototype uses a flat microspring with a length and width of 0.3 mm; its switching speed is more than ten times that of a conventional relay. The other is a 2×2 matrix relay, with four self-latching switches; each switch has two stable positions, open and closed, and is driven by a current pulse. This component study and prototype development verify the suitability of MEMS technology for mechanical relays.

Electromagnetic microrelays: Concepts and fundamental characteristics

A planar microtransformer for use in microswitching converters of portable equipment is described. This microtransformer, consisting of planar coils and CoZrRe amorphous magnetic layers on a Si substrate, is fabricated by a dry process. It is about 0.3-mm thick, and 3*4 mm/sup 2/. Its coils provide a relatively high inductance of 33 nH/mm/sup 2/. This microtransformer is implemented in a forward converter, which operates well at 32 MHz. >

A new planar microtransformer for use in micro-switching converters

A microtransformer that is monolithically integrated with rectifier diodes is developed as the first step towards developing a monolithic microswitching converter that can be integrated with semiconductor devices and magnetic components. This microtransformer consists of planar coils and CoZrRe amorphous magnetic layers on Si substrate. Two Schottky barrier diodes are formed on the Si substrate and directly connected to the secondary coil. This microtransformer chip is shown to perform both conversion and rectifying functions.

Planar microtransformer with monolithically-integrated rectifier diodes for micro-switching converters

We have developed a compact buck-converter using a thin-film inductor that is suitable for portable electronic equipment. The inductor consists of planar Cu coils and CoZrTa magnetic layers on a Si substrate; it is fabricated by a dry process. The converter is 3 mm thick and 10/spl times/6 mm in area; it operates at 1.6 MHz and has an efficiency of up to 85%.

A compact buck-converter using a thin-film inductor

We discuss photolithographic fabrication techniques and experimental results for a prototype electromagnetically driven microvalve. The valve is constructed on a silicon substrate, using a magnetic suspension spring with a valve cap, a valve plate with a 30 μm diameter bore, and an external coil for driving the valve cap. The external electromagnetic drive approach was chosen for its ease of use and practicality in controlling the valve actuator. The valve cap, made of soft magnetic material (NiFe) and supported by the spring, moves vertically as a result of the magnetic field applied by the external coil. To precisely adjust both the valve cap and the valve plate bore and to minimize fluid leakage, a new self-alignment process was developed. The valve is controlled by a 0.1–100 Hz rectangular magnetic field applied by the external coil. The resulting minimum gas flow rate can be controlled to within the neighborhood of 3 × 10−5 torr·l/s.

Keiichi Yanagisawa

Papers

Electromagnetic microrelays: Concepts and fundamental characteristics

A new planar microtransformer for use in micro-switching converters

Planar microtransformer with monolithically-integrated rectifier diodes for micro-switching converters

A compact buck-converter using a thin-film inductor

Electromagetically driven microvalve