Integration of Micromagnetics
Introduction
Magnetic actuators are capable of generating large bi-directional forces with long working lengths. They are widely used in the macro world and are of growing interest to the micro world. Many papers have been published on the use of magnetics where their unique characteristics make them the actuation method of choice in specific situations. They have the advantages that they can be actuated with external fields, operate in a variety of contaminated or dirty environments, generate large forces over large distances and provide 'latching' action. One of the obstacles to widespread use of micro magnetic actuation is the inclusion or permanent magnetic materials in micro systems. With the exception of magnets that are assembled into micro systems micro fabricated magnets are much weaker on a per volume basis then traditionally fabricated magnets.
We are developing processes for the integration of hard magnetic materials into microsystems for actuation and sensing. Beginning with the study of micro magnet assembly for micro actuators we continued our investigations into wafer level processes using screen printing and novel electroplating processes.
Assembly
In a two stage assembly process small magnets measuring 550 by 220 by 110 microns were placed on the surface of a permalloy sheet with alternating polarity. A pusher then pushes the magnets together against alignment fixtures before they are glued to a small metallic keeper for assembly into MEMS systems. Successful modeling of the system is critical for reliable operation. This is demonstrated by the choice of a magnetically permeable substrate with a minimum permeability limit which prevents the magnets from rolling over or self assembling in a vertical fashion and a two stage assembly process which minimizes magnet interactions during the pick and place.
Figure 2: Sequential steps in the assembly of micro magnets for a linear electromagnetic actuator
Screen printing
We are investigating the use of rare-earth magnetic powders in microsystems. These powders, being the same as those used to create the strong NdFeB magnets found on the market today, are deposited with screen printing or other methods to fabricate strong permanent magnets with wafer level processes. Deflections of over 1000 micron were demonstrated with surface micro machined cantilevers, magnets and pancake coils. Magnets with volumes measuring in the range of 0.1 mm3 can produce forces in the mN range.
A magnetically actuated cantilever (see video) incorporates a screen printed magnet that measures 2000 by 1000 by 100 microns. Deflection at the tip of the cantilever is more than 1000 microns when 200 mA of current are passed through a 40 turn pancake coil. Large deflections in this system lead to snapdown performance similar to electrostatic systems, while small deflections (< +_150 microns) show a linear current-deflection relation.
Magnetic Composite Electrodeposition (MCE)
A new method for incorporating permanent magnets is under development. It relies on the incorporation of hard magnetic particles in a soft magnetic electrodeposited matrix. Initial results have demonstrated magnetic properties of the deposited films to increase by 600% to 2000% with particle loading of 10%-20%. We are currently working to increasing the particle loading of the MCE magnets to increase their strength and uniformity.
