Magnetics Business & Technology - March/April 2021 - 18

RESEARCH & DEVELOPMENT

Fabrication of NdFeB Micro-Magnets at Institut NEEL Unlocks Enormous Potential for MEMS
By Nora Dempsey, Staff Researcher, Institut NEEL, Grenoble, France

The figure above shows: (a, b) Plan-view optical images of a 50 µm thick NdFeB film deposited on a pre-patterned 4-inch Si substrate together with (c) a cross-sectional scanning electron microscope image of a few micro-magnets and (d) a Magneto-Optical image of the stray
magnetic field produced at a scan height of 50 µm above the film.
The discovery of high performance NdFeB magnets, almost 40
years ago, revolutionized the design of electromagnetic motors,
generators and actuators. Today they are key components of
a range of devices including gearless wind turbines, electric
and hybrid-electric vehicles, electric bicycles, hard disk drives
and air-conditioners. Researchers at Institut NEEL (CNRS,
Grenoble, France) have mastered the fabrication of NdFeB
micro-magnets, unlocking enormous potential for the development of Micro-Electro-Mechanical Systems (MEMS) and other
micro-scale devices, with applications in areas as diverse as
consumer electronics and bio-technology.
The force of interaction between a magnet and another object
is proportional to the gradient of the stray magnetic field it produces. Scaling down the size of a magnet, while preserving its
extrinsic magnetic properties (remanence, coercivity), leads to
an increase of its stray magnetic field gradient. As a result, the
magnetic field gradient force per unit volume can be increased
by orders of magnitude in micro-scale systems compared to
macro-scale systems.
However, downscaling the size of NdFeB magnets is not trivial.
Machining of sintered bulk magnets has a lower limit of a few
hundred micrometres, due to surface degradation, and is material wasteful. Smaller magnets can be produced by polymer
bonding techniques, including 3D printing. However, the rema-

Magnetics Business & Technology * March/April 2021

nence of polymer bonded magnets is limited by dilution of the
magnetic powder.
The manipulation and integration of small free-standing sintered or bonded magnets can be very challenging. NdFeB hard
magnetic films can be produced by physical vapour deposition
techniques. To produce a stray magnetic field, such films must
be patterned at a length scale comparable to the film thickness,
because of the demagnetising field effect.
Researchers at Institut NEEL have produced high coercivity
(~ 2 T) and high remanence (~ 1.3 T) NdFeB films of thickness
up to 50 micrometres, by triode sputtering onto Si substrates.
These films have been patterned using standard clean-room
facilities, i.e. photo-lithography and deep reactive-ion etching of
the Si substrate prior to film deposition (see figure).
The size, shape and disposition of the micro-magnets can be
adapted to suit specific applications. A chemical mechanical
planarization process has been developed to remove material
deposited outside of the etched regions, if required. The fact
that the micro-magnets are substrate-mounted means they can
be integrated into complex micro-devices, and the use of large
substrates permits batch fabrication. Deposition is now made
on 4-inch substrates, and the sputtering technique used can be
upscaled to allow deposition on even larger substrates.
www.MagneticsMag.com

http://www.MagneticsMag.com

Magnetics Business & Technology - March/April 2021

Table of Contents for the Digital Edition of Magnetics Business & Technology - March/April 2021