Naigang Wang

Permanent Magnets for MEMS

This paper reviews the state of the art for the microfabrication of permanent magnets applicable to microelectromechanical systems (MEMS). Permanent magnets are a key building block for the realization of magnetically based MEMS sensors, actuators, and energy converters. In this paper, the basic theories and operational concepts for permanent magnets are first described. Then, different classes of permanent-magnet materials and associated performance tradeoffs are introduced. Challenges relating to the integration of permanent magnets into MEMS applications are then discussed. Last, a summary and review of previously reported fabrication strategies and material properties is provided.

Modeling of magnetic vibrational energy harvesters using equivalent circuit representations

This paper develops and analyzes an equivalent circuit model of magnetic energy harvesters using reduced-order lumped element modeling (LEM) methods. This model is intended to enhance the design and analysis of a magnetic energy harvesting system by enabling direct physical insight into the system dynamics and simple circuit analysis techniques to extract all relevant performance parameters. Moreover, the model provides the ability to use circuit simulation software (e.g. PSPICE) to model the entire system in conjunction with nonlinear and/or active power electronic circuits. The circuit model is experimentally validated through electrical and mechanical measurements on a prototypical electromagnetic energy harvester.

Wax-bonded NdFeB micromagnets for microelectromechanical systems applications

A microfabrication technique is reported for the fabrication of wax-bonded micromagnets in the 100μm–1mm range. This dimensional size is not typically achievable by thin-film deposition techniques or by conventional macroscale magnet processing, and is of interest for magnetic microelectromechanical systems applications. The magnet fabrication approach incorporates a wax powder binder to “lock” the magnetic microparticles together to improve the coercivity. In practice, a NbFeB powder and wax powder are packed into pre-etched trenches in a silicon wafer and subsequently heated, allowing the wax to melt and bond the magnetic powder. Using this method, 500×500×320μm3 magnets have been demonstrated with a coercivity of 737kA∕m and a maximum energy product of 16.6kJ∕m3.

Thick Electroplated Co-Rich Co-Pt Micromagnet Arrays for Magnetic MEMS

Photolithographically-defined patterned arrays of Co-rich Co-Pt permanent magnets with thicknesses up to 10 mu m were fabricated by aqueous electrodeposition. These micromagnets exhibited strong perpendicular magnetic performance while being deposited at low temperatures (65deg C) and without the need for post-deposition annealing. Co-Pt magnet arrays grown 10-mu m thick on textured Cu (111) seed layers on Si (110) substrates exhibited coercivities of 330 kA/m (4.1 kOe) and energy products 69 kJ/m3. Additionally, magnets deposited 8-mu m thick on untextured Cu seed layers on standard Si (100) substrates exhibited coercivities up to 260 kA/m (3.3 kOe) and energy products up to 27 kJ/m3. The high magnetic performance and process integrability of these permanent magnet arrays make them well suited for the development of magnetic microelectromechanical systems (MEMS).

Fabrication and characterization of parylene-bonded Nd―Fe―B powder micromagnets

This paper presents the fabrication and characterization of parylene-bonded isotropic Nd–Fe–B powder micromagnets with dimensions in the range of 100–1000 μm. The batch fabrication process involves dry-pressing of magnetic powders into microstructured cavities in a substrate followed by conformal vapor deposition of parylene C. The parylene coating penetrates the gaps between the magnetic particles and mechanically bonds the powder. Additionally, by mixing magnetic powders with different particle sizes, higher fill factors are achieved, thereby increasing the magnetic moment and energy product. An intrinsic coercivity of 720 kA/m, a remanence of 0.36 T, and a maximum energy product of 22 kJ/m3 are demonstrated for magnets with dimensions 700 × 700 × 220 μm. The room-temperature processing steps and chemically stable parylene coating facilitate the integration of these magnets with other microfabrication processing steps.

An electromagnetically actuated microspeaker with fully-integrated wax-bonded Nd-Fe-B micromagnets for hearing aid applications

We present a fully-integrated, electromagnetically actuated microspeaker intended for hearing aid applications. The microspeaker uses the Lorentz force to actuate multi-turn, multi-layer copper coils embedded in a 3-mm-diameter parylene diaphragm. Process-compatible micromachined permanent magnets are employed for size reduction and batch-manufacturability. The permanent magnets are composed of micro-size Nd-Fe-B particles in a wax binder. The microspeaker produces 0.64 µm peak displacement at 1 kHz for a drive current of 88 mArms (6 Ω coil impedance), corresponding to a 46 mW power consumption.

Batch-Fabricated Electrodynamic Microactuators With Integrated Micromagnets

This paper reports the fabrication and characterization of two fully batch-fabricated electrodynamic microactuators. The actuators utilize wax-bonded Nd-Fe-B powder magnets and electroplated copper coils for electrodynamic actuation out of plane. A cantilever-type microactuator achieves a 2.66 ¿m vertical deflection at a driving current of 5.5 mArms at 100 Hz. A piston-type actuator with elastomeric membrane obtained a 1.17 ¿m vertical displacement at a driving current of 13.5 mArms at 100 Hz. These microactuators demonstrate the integrability of wax-bonded Nd-Fe-B powder magnets into microscale electromechanical transducers.

Fabrication, Characterization, and Modeling of Fully-Batch-Fabricated Piston-Type Electrodynamic Microactuators

This paper presents the fabrication, characterization, and modeling of electrodynamic microactuators. The actuators are piston-type devices, each comprising of a circular flexible polydimethylsiloxane membrane, a multi-turn Cu coil, and an integrated powder-based NdFeB permanent magnet. The devices are fully batch-fabricated in a single wafer using only three masks. Ranging in diameter from 2.5 to 5.2 mm, three different device designs are quasi-statically and dynamically characterized for their electromechanical performance. The resonant frequencies of the three actuators range from 224 to 820 Hz. The maximum displacements span from 4 to 64 μm for an input power ranging from 250 to 525 mW. The experimental results are supported by a parametric lumped element model of the transducer.