MEMS magnetic actuator

The three major operations in MEMS are:

• Sensing: measuring a mechanical input by converting it to an electrical signal, e.g. a MEMS accelerometer or a pressure sensor (could also measure electrical signals as in the case of current sensors)
• Actuation: using an electrical signal to cause the displacement (or rotation) of a mechanical structure, e.g. a synthetic jet actuator.
• Power generation: generates power from a mechanical input, e.g. MEMS energy harvesters

These three operations require some form of transduction schemes, the most popular ones being: piezoelectric, electrostatic, piezoresistive, electrodynamic, magnetic and magnetostrictive. The MEMS magnetic actuators use the last three schemes for their operation.

A typical MEMS actuator is shown on the right. For a single turn of circular coil, the equations that govern its operation are:[3]

• The H-field from a circular conductor:

${\displaystyle H(z)={\frac {Ir^{2}}{2(r^{2}+z^{2})^{3/2}}}}$

• The force produced by the interaction of the flux densities:

${\displaystyle F_{z}=B_{I}A_{mag}\int _{z}^{z+h_{mag}}{\frac {dHz}{dz}}dz}$

The deflection of a mechanical structure for actuation depends on certain parameters of the device. For actuation, there has to be an applied force and a restoring force. The applied force is the force represented by the equation above, while the restoring force is fixed by the spring constant of the moving structure.

The applied force depends on both the field from the coils and the magnet. The remanence value of the magnet,[4] its volume and position from the coils all contribute to its effect on the applied Force. Whereas the number of turns of coil, its size (radius) and the amount of current passing through it determines its effect on the Applied Force. The spring constant depends on the Young's Modulus of the moving structure, and its length, width and thickness.

There are several ways by which the magnet could be fabricated on a MEMS structure:

Sputtering

• Sputtering: Argon ion bombardment of the material release particles of the material. Mainly for depositing rare earth magnets. Deposition rate and film surface area depend on sputtering tool and target size

Pulsed Layer Deposition

• Pulsed Layer Deposition: a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited
• Electroplating
• Screen printing
• Wax/Parylene bonding[13][14][15]

• High-power dissipation. This is a major problem for magnetic MEMS, but work is underway to circumvent this.[16]
• Fabrication of the coil
• Integration of the micromagnet into the MEMS device
• Process-material compatibility
• Integratability into the overall microfabrication process (maintain cost and throughput)
• So that preexisting processes in the fabrication of the MEMS device will not be tampered with, deposition temperatures and post-deposition treatment/conditions must be tolerable. Also, the micromagnet must be able to withstand any chemical treatment that will come after its deposition
• Issues with magnetization (One may want to have more than one direction of magnetization; this creates a problem)[17]

Each of these challenges can be mitigated or lessened by the right choice of material, choice of molding or fabrication method, and the type of device that is to be constructed. Applications of the magnetic actuator include: the synthetic jet actuator, micro-pumps and micro-relays.