Magnetic MEMS

The use of magnetic materials in MEMS is a recent development in which particular emphasis is given on ferromagnetic materials. These magnetic materials could be soft or hard. The design of ferromagnetic MEMS and the methods of integrating both soft and hard magnetic materials with it are a current research and development field. Soft ferromagnetic materials have found their usefulness in microsensors, microactuators, and microsystems. However, hard magnetic materials have unique advantages that are driving their integration into other applications.

Patterns of hard magnetic materials in the micron scale are of interest for the novel design of magnetic recording media. Hard magnetic films with a thickness of several microns are grown by the sputtering technique, practically a difficult process. In order to overcome the problem, electrodeposited Co80Pt20 alloys are grown up to a several micron thickness while maintaining their hard magnetic properties. The batch fabrication process for micromachining thick films of highperformance hard magnetic materials is improving. Subtractive etching processes are needed to define the patterns of the hard materials. Micron size patterns can also be obtained by optical lithography, thus allowing a better control of the magnetic properties. Many researchers are currently trying to achieve a submicron size for the production of patterned media. Principles of magnetic MEMS are described below.

Anisotropic magnetoresistive (AMR) sensors allow for detecting the strength and direction of magnetic fields. They are used for the measurement of distance, proximity, position, angle and rotational speed. The AMR sensors undergo a change in resistance in response to an applied magnetic field vector generated by passing the current in a coil. When a magnetic field is applied, the magnetisation rotates toward the field. The variation in resistance depends on the rotation of magnetisation relative to the direction of current flow. The change in resistance of the material used in the AMR sensor is highest if the magnetisation is parallel to the current and lowest if it is perpendicular to the current. The sensitivity to the direction of the magnetic field allows for the measurement of angle also. The development of micromachined magnetic devices has relied primarily on the use of nickel-iron permalloy. Permalloy has a low magnetic anisotropy. The anisotropy field for permalloy is between 3 and 5 oersted. Permalloy is used in a number of applications since it has good soft magnetic properties, high permeability, high magnetoresistive effect, low magnetostriction, stable high frequency operation, and excellent mechanical properties. In hard disk magnetic recording heads, permalloy is widely used for magnetoresistive sensors and flux guiding elements. Devices such as magnetic separators, micropumps, magnetic micromotors, inductors, switches, and microrelays have also been fabricated using permalloy as the magnetic material as well as in moving members (Taylor 1999).

Permanent magnets are used for sensors and actuator applications that can provide the desired constant magnetic field without the consumption of electrical energy. Another important characteristic is that they do not generate heat. Furthermore, the energy stored in a permanent magnet does not deteriorate when the magnet is properly handled and micromachined. This feature is due to the fact that it does no net work on its surroundings. Moreover, permanent magnets can achieve relatively high energy density in microstructures, as compared to other energy storage devices (Cho 2002). This is why there has been a growing interest in the realisation of permanent magnets in MEMS devices recently. Apparently, magnetic MEMS devices have several advantages over electrostatic types. One of the important advantages is the generation of long-range force and deflection with low driving voltage in harsh environments. MEMS devices with a permanent magnet have benefits of low power consumption, favorable scaling and simple electronic circuitry. Lagorce et al. introduced screen-printing technology to integrate a permanent magnet in a microactuator. Actuators are capable of generating large bi-directional forces with long working lengths. These permanent magnets were screen printed on a copper cantilever beam using a magnetic paste composed of epoxy resin and strontium ferrite particles. Coercivity of 350 kA/m and residual induction of 65 mT have been reported in a disk type permanent magnet with a millimeter range diameter and 100 micron in thickness.

A number of micromachined magnetic actuators require materials with desirable magnetic properties such as high permeability and a thickness in the range of several micrometers. Investigations on the use of rare-earth magnetic powders in microsystems are in progress. The powders are essentially deposited by screen printing or other methods to fabricate strong permanent magnets with wafer level processes. Deflections of over 900 micron have been reported with surface micromachined cantilevers, magnets and pancake coils. Magnets with typical volumes measuring in the range of 0.11 mm3 can produce forces in the mN range. A typical magnetically actuated microcantilever incorporating a screen printed magnet measures 2000x1000x100 microns. Deflection at the tip of the cantilever is about 1000 micron, subjected to a driving current of 200 mA which is passed through a 40 turn pancake coil. Large deflection causes nonlinear performance similar to electrostatic systems, however, small deflection shows a better linear current-deflection relationship.

A magnetic actuator consists of a magnetic field source and a magnetically susceptible unit. The actuation force is proportional to the magnetic field intensity, the magnetic susceptibility, and the mass of magnetic susceptible material (Son, http://sonicmems.ece.cornell.edu/Papers/Sunny_paper_mmb7.pdf). A large-force, fully integrated, electromagnetic actuator for microrelay applications is reported by Wright (Wright 1997) (Fig. 2.6). The actuator has a footprint of less than 8 mm2 and its fabrication is potentially compatible with CMOS processing technology. It is designed for high efficiency actuation applications. The actuator integrates a cantilever beam and planar electromagnetic coil into a low-reluctance magnetic circuit using a combined surface and bulk micromachining process. Test results show that a coil current of 80 mA generates a 200 ^.N actuation force. Theoretical extrapolation of the data, however, indicates that a coil current of 800 mA can produce an actuation force in the millinewton range.

Fig. 2.6. Top view of a magnetic actuator developed by Write, et.al

An example of a bi-directional magnetic actuator used for optical scanning applications can be given. It is composed of a silicon cantilever beam and an electromagnet. At the tip of the cantilever beam, a permanent magnet array is electroplated in order to achieve the bi-directional actuation. Below the cantilever beam, the permanent magnet array is placed along the axis of the electromagnet (Cho 2002). For a large bi-directional deflection and dynamic scanning capability, Cho has designed an optical scanner supported by two serpentine torsion bars. A schematic diagram of the scanner is illustrated in Fig. 2.7. The scanner is designed to have a silicon mirror supported by two serpentine torsion bars (Fig. 2.8), carrying a permanent magnet made by bumper filling at its tip on the opposite side.

Silicon cantilever beam A'

Silicon cantilever beam A'

Permanent magnet array

Fig. 2.7. Cantilever beam magnetic microactuator. (a) Schematic view and (b) Another view (Source: Cho 2002)

Permanent magnet array

Fig. 2.7. Cantilever beam magnetic microactuator. (a) Schematic view and (b) Another view (Source: Cho 2002)

Fig. 2.8. Magnetically driven optical scanner (Source: Cho, 2002)

Son and Lal (Son and Lal 1999, 2000, 2001) reported a novel magnetic actuator that allows remote magnetic actuation with piezoresistive feedback for microsurgery applications. The actuator consists of an electromagnet, a ferromagnetic mass, and a cantilever. Conventional actuator fabrication lithography combined with electroplating is required to make high aspect ratio structures. However, high aspect ratio columns with a suspension of ferromagnetic nanoparticles and epoxy using magnetic extrusion were fabricated. The fabrication process did not require lithography. The remote magnetic actuator with feedback consists of two basic units. They are the actuation unit and feedback unit. The ferromagnetic mass is first placed on the tip of the cantilever. The electromagnet is placed above the ferromagnetic mass. If AC current is applied to the electromagnet, the magnet repeats pulling and releasing the cantilever. If the cantilever is released, the bent cantilever returns to the initial position by its own spring force. One implied factor is that if the frequency of current applied to the electromagnet equals the resonance frequency of the cantilever, the actuation is amplified by the Q factor of the mechanical resonance. The resonant frequency fr of the cantilever with mass can be varied due to the change of its mechanical boundary conditions. The feedback unit consists of three sub-units, namely the strain gauge, amplifier and voltage controlled oscillator (VCO). When the cantilever vibrates at different frequencies, the value at the strain gauge decreases. The drop in the value of the strain gauge triggers the VCO to adjust the frequency using linear feedback to find the new fr (Son and Lal) (Fig. 2.9).

Fig. 2.9. Remotely actuated magnetic actuator (Source: Son and Lal 1999, 2000, 2001)

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