lecture 14: magnetic sensors & actuators - engineeringmech466/mech466-lecture-14.pdf ·...

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MECH 466Microelectromechanical Systems

University of VictoriaDept. of Mechanical Engineering

Lecture 14:Magnetic Sensors & Actuators

© N. Dechev, University of Victoria

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Magnetic Force and Torque

Fabrication of Magnetic MEMS

Example of Magnetic Micromotor

Example of Magnetic Self-Assembly

Example of Micro-Coil Research Application

Overview


Consider the forces that develop on magnetic objects within magnetic fields, based on the diagrams:

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Behavior of Magnetic Dipoles in Magnetic Fields

© N. Dechev, University of Victoria[Fig. 8.5 from ‘Foundations of MEMS’, Chang Liu]

A concentrated force acting on each ‘pole’ of the dipole (i.e. each end of the magnetic bar), can be defined as:

and magnetization M is defined as:

Note that there are always ‘two’ poles on a dipole, and each pole has an opposite polarity.

If the magnetic field H is uniform across both poles, the ‘net force’ across the magnetic dipole (or magnetic bar) is zero, as shown in part (b) of the previous diagram.

4© N. Dechev, University of Victoria

Force on Magnetic Dipoles in Magnetic Fields

If the magnetic field H is non-uniform across both poles, the ‘net force’ across the magnetic dipole (or magnetic bar) can be expressed as:

where we can define the magnetic field gradient as:

Note that this corresponds to part (c) of the previous diagram.


Force on Magnetic Dipoles in Magnetic Fields

To make magnetic micro-devices, appropriate thin-film process technologies are required.

One of the important processes for depositing magnetic materials is electroplating, as shown below:

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Fabrication of Magnetic MEMS


[Fig. 8.6 from ‘Foundations of MEMS’, Chang Liu]

Micro-inductors represent a class of ‘passive devices’ that are of great interest.

They have application to a wide area of microelectronic and MEMS applications, including inductors, communications coils (RF or other), transformers, and magnetic actuators.

Presently, micro-inductors are fabricated as ‘spirals’ or ‘square spirals’ in 2-D layouts, as shown below:

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Fabrication of Micro-inductors



2-D micro-inductors, such as the one shown in Fig. 8.9(a) are simple to fabricate. However, they are not too effective due to their spiral shape, and the relatively low magnetic permeability of air or free space.

To improve their performance in creating a strong magnetic field, the magnetic cores have been fabricated at their center, as shown below:

However, the resulting improvements are marginal.

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2D Spiral Micro-inductors



An even bigger problem with ‘in-plane’ micro-inductors is their poor efficiency, or ‘quality factor’ since they are fabricated in-plane with the substrate.

By being in-plane with the substrate, the magnetic flux must pass through the silicon substrate, which is conductive, yet has a high resistance. Eddy currents will form in the substrate, and degrade the performance of the device.

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2D Spiral Micro-inductors


In order to avoid the problems with 2D in-plane micro-inductors, researchers are attempting to construct 3D micro-inductors with magnetic cores, and without cores.

The primary motivation is to minimize the amount of magnetic flux that must pass through the substrate.

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3D Micro-inductors


The micro-inductors shown below are fabricated using surface micromachining, or similar thin-film fabrication technologies.

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Monolithic Fabrication of 3D Micro-inductors


Micromachined, 3D micro-inductors [Microfabrica.com] HARM fabricated, 3D micro-inductors [KAIST]

CASE 8.2. Magnetic Beam Actuation: (Self Assembly of Magnetic Beams)

Here, a powerful electromagnet is used to create an intense magnetic field to ‘lift’ or ‘bend out-of plane’ magnetic beams.

Essentially, this is a magnetic-based ‘self assembly’ process.

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Case Study 8.2: Self-Assembly of 3D Micro-inductors


This magnetic-based assembly process can be used to create out-of-plane micro-inductors, such as the ones shown below:

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Self-Assembly of 3D Micro-inductors


PDMA, Plastic Deformation Magnetic Assembly [Jun Zou, Jack Chen, Chang Liu, and Jóse E. Schutt-Ainé]

ZOU et al.: PDMA OF OUT-OF-PLANE MICROSTRUCTURES: TECHNOLOGY AND APPLICATION 307

(a) (b)

Fig. 9. The application of PDMA to realize vertical planar spiral inductor. (a) Scanning electron micrograph of a planar spiral inductor fabricated on the substratesurface before the PDMA assembly; (b) Scanning electron micrograph of the same inductor after the PDMA assembly.

(a) (b)

Fig. 10. The measured parameter (0–1.1 GHz) of a planar spiral inductor before and after the PDMA assembly: (a) magnitude and (b) phase.

inductor. Although planar spiral inductors can be integrated

with other circuits using current standard integrated circuit (IC)

fabrication process, their performance is still unsatisfactory.

Oftentimes, planar spiral inductors are directly fabricated onto

the dielectric layer on top of the conductive substrate, which

lowers the quality factors and degrades the performance by

introducing extra loss, noise and parasitic capacitance [22].

Another disadvantage of a planar spiral inductor lies in the fact

that it has a large footprint to achieve the required inductance

value [23]. As an example, Fig. 8 shows the microscopic

picture of a voltage controlled oscillator circuit chip with

two integrated planar spiral inductors. Each inductor has an

inductance value of only 7 nH. However, the two inductors

occupy most of the chip area.

In recent years, much effort has been made to reduce the

adverse effects from the substrate so as to improve the per-

formance of planar spiral inductors. The methods reported so

far include using high resistive substrate [24], coating poly-

imide to increase thickness of the dielectric layer underneath

the inductor [25], partially or completely removing the sub-

strate material underneath the inductor [26]. More recently, sur-

face micromachining technology has been applied in the fabri-

cation of planar spiral inductors to create an air gap between the

inductor and substrate [27]. These methods may involve cer-

tain materials or fabrication steps, which are not compatible

with current standard IC fabrication process. Moreover, none

of the methods mentioned above has solved the large footprint

problem of planar spiral inductors.

We have successfully applied the PDMA to assemble vertical

planar spiral inductors. The entire process is compatible with

present standard IC fabrication. The inductor can be monolithi-

cally integrated with other circuits even after the IC components

have been fabricated. First, the planar spiral inductors are fabri-

cated by using a standard surface micromachining process [see

Fig. 9(a)]. This process will be discussed in detail in future pub-

lications. Next, PDMA is implemented after all the fabrication

steps are finished [see Fig. 9(b)]. The geometric parameters of

the planar spiral inductor are specially designed so that the rest

angle of the inductor after the assembly is almost 90 . However,

the substrate loss and parasitics can still be reduced even if the

inductor stays at an angle smaller than 90 off the substrate.

The vertical inductors offer two major advantages over the

horizontal ones. First, the vertical inductors have almost zero

footprints. This will help to increase the achievable device den-

sity and lower the fabrication cost of a radio frequency (RF)

circuit chip. Second, the substrate effect is also greatly reduced.

Fig. 10(a) and (b) shows the measurement results of one

planar spiral inductor before and after the assembly. Although

the design and fabrication of the planar spiral inductor is not op-

timized, our measurement result has clearly shown that the in-

ductor has lower loss and better characteristic of inductance in

the vertical position after the assembly. We believe that this ben-

Another interesting type of micro-coil is created using bi-metallic structures, that deform due to the application of heat:

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Self-Assembly of 3D Micro-inductors


High-Q, Microcoils [PARC Research]

Micro-Inductors can also be assembled, by serial robot-based systems:

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Serial (Sequential) Assembly of 3D Micro-inductors


Example of Assembled 3D Micro-Transformer


Serial (Sequential) Assembly of 3D Micro-inductors

CASE 8.1. Magnetic Motor



Case Study 8.1: Magnetic Based Micromotor

Examination of Stator Pole operation:

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Case Study 8.1: Magnetic Based Micromotor



Magnetic MEMS can often be used with externally applied magnetic fields, in interesting new applications.

The system shown here is a ‘magnetic micro-robot’ that can be moved within the body, by carefully controlling the magnetic field. This alleviates complex ‘on-board propulsion and power issues.

If fitted with appropriate tools, the robot can perform various tasks.

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Magnetic Biomicro-robots


Bio-Micro-robots [B. Nelson, Institute of Robotics and Intelligent Systems, Swiss Federal Institute]

In the field of cell biology, it is sometimes useful to be able to isolate and immobilize individual cells for study.

One manner in which to capture free floating cells, and array them into an ordered pattern on a surface, is through the use of magnetic MEMS.

With the exception of blood cells, most biological cells are non-magnetic, and hence the first step is to magnetically ‘label’ the cells.

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Magnetic Single Cell Micro Array (MSCMA)


Desired Cell

Magnet Disc

‘Tetrameric antibody complexes’ are designed to (a) attach to target cell at one end and (b) attach to magnetic material at other end.

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Desired Cell


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Desired Cell


Desired Cell

Desired Cell

Desired Cell

Desired Cell

Desired Cell

Desired Cell

Desired Cell

Desired Cell

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Common magnetic microarray schemes, [William Liu]

A number of different methods may be used to capture the magnetically labelled cells at specific sites. These include:

(a) micro-electromagnets

(b) micro-posts

(c) micro-rails

(d) micro-pins

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By using an external magnetic field, we can create geometrical patterns that will ‘focus’ the magnetic field at specific points.

Illustration of (a) MSCSA Device top view,(b) close-up 3D view with permalloy (grey) and gold (orange). [William Liu]

Cell capture results with (a) actual test, (b) control test (no external field applied). [William Liu]

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