Active Magnetic MNEMS Overview

Introduction

For micro and nano electro-mechanical systems (MNEMS), there are two physical principles lending themselves particularly well for being executed in thin-film technology: the electrostatic and the electromagnetic principle (plus, strictly speaking, the electrodynamic one). Compared to electrostatic devices, electromagnetic ones are more robust, capable of achieving higher forces, but also more complex and less applicable for extreme miniaturization. There are two types of magnetic forces that may be employed: (1) Maxwell forces applied in a variable reluctance approach (the length of an air gap between two poles is minimized) and (2) Lorentz forces, with a current carrying conductor being exposed to a magnetic B-field. While there are a variety of research activities in various fields of application, there is still a substantial lack of commercialization, as we will see.

Optical Applications

The Karlsruhe Institute of Technology (KIT), Germany, developed a thin-film actuator taking advantage of a ferromagnetic shape memory alloy (FSMA). It allows controlling a bulk micromirror with two degrees of freedom. The actuation principle is based on a ferromagnetic and martensitic (which is non-magnetic) transformation. The system is intended for use as an optical scanner. The Leibniz Universitaet Hannover (LUH), Germany, developed a ferrofluidic microactuator proposed for the electromagnetic controlling of an adaptive micro-optical system. The latter consists of an array of microcoils for manipulating the position of a ferrofluidic plug in a microchannel. By displacing the ferrofluid plug, an optically active fluid is moved and forms a liquid lens with an adjustable focal length.

Microfluidic and Biomedical Applications

The Ajou University in Suwon, South Korea, developed a microactuator with three diaphragms that can be driven individually. The actuator consists of parylene diaphragms, spiral copper coils, and permanent magnets. Another example of a micropump is a microsystem developed by the Hsinchu University in Taiwan intended for pneumatic applications. It features an elastomer membrane actuator with a semi-embedded coil.

An attractive approach for transporting biological compounds is conjugating them to magnetic beads. The Nagoya University in Japan developed a system for the magnetic bead agitation, combining a multi-layer flat coils in flex cable technology with a permanent magnet. The LUH developed a microsystem for magnetic force-enhanced gene delivery. Conjugating genes with magnetic nanoparticles allows for the magnetic manipulation of these complexes. The magnetic microactuator features a row of individually excitable poles for magnetically actuating these complexes.

The Arizona State University in Tempe, Arizona, developed a microspeaker MEMS for hearing applications. The result is a fully integrated, electromagnetically actuated microspeaker with a discrete, wax bonded, Nd-Fe-B particles magnet. The LUH designed a microactuator serving as an implantable hearing aid to overcome ambylacousia. The actuator consists of a mechanical system containing a plunger, a boss, and a membrane as well as an electromagnetic device containing a coil system, soft magnetic flux guides, and a flux closure located underneath the mechanical system. Both the physiological restrictions in the middle ear and the cochlea defined the maximal size of the microactuator. The mechanical system was foremost designed with respect to the resonant frequency.

Communications and Computer Technology

There are various approaches to conduct research MEMS type RF switches (i.e. microrelays for RF frequency applications. The East China Normal University in Shanghai, PRC, developed a bistable electromagnetic microactuator. Its footprint is 2 mm x 2.2 mm. Te required current pulse is 50 mA; the switching time is 20 µs. The University in Orono, Maine, developed a Bidirectional electromagnetic actuator. It is fabricated on a single wafer, with an Au microcoil, NiFe membrane with supported legs and a Co-Pt permanent magnet integrated in the membrane. Photoresist serves as a sacrificial layer to allow for the top part to actuate after the completion of the fabrication. Other entities conducting research on MEMS type relays are Shanghai Jiao Tong University, PRC, UC Berkeley, University of Minnesota in Minneapolis, both USA, the Beijing University of Technology, PRC, and the Louisiana State University in Baton Rouge, USA.

For achieving an optimal read/write performance in Hard Disc Drives (HDD), a recording head is aligned perfectly with the data track to be written or read. To achieve a perfect track registration, the use of dual-stage actuators (DSA) has been suggested for quite a while. Taking advantage of Micro Electro-mechanical System (MEMS) technology, the LUH developed a Slider with an Integrated Microactuator (SLIM) for use in Hard Disc Drives (HDD). By placing the read/write element on a small chiplet rather on the trailing edge of a slider, the design promises to be cost competitive.

Automotive Application

Although gyroscope systems for yaw measurement are sensors, they do require an actuation. There are also alternative physical principles applied, the two most important ones are a disc and a tuning fork. The national Cheng Kung University in Tainan, Taiwan, developed a MEMS type gyro with a reciprocally rotating disc with electromagnetic drive. Bosch, Germany developed an electromagnetically driven, tuning fork type gyroscope for automotive applications. The start of production was in 1998. This seems to be the only electromagnetically actuated MEMS device, which saw large-scale production.

Conclusion

In research, there is a considerable interest in electromagnetic and electrodynamic MNEMS devices. Numerous research entities have been looking in various applications, coming up with interesting solutions in the areas of optical, biomedical, communication and computer technology, as well as automotive technology. However, automotive seems to be the only area, which saw commercialization of magnetic micro actuation devices, interestingly enough in the field of sensing.

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