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National University of Singapore

Faculty of Engineering

Department of Mechanical Engineering

SEMESTER 6

ME3281 Term Paper

MEMS ENERGY HARVESTERS

CHOO HAN LIN

A0086988Y

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Introduction

MEMS energy harvesters cover a broad spectrum of inventions designed specifically for the

usage of collecting ambient waste energy from the surroundings and converting them into

useful (primarily electrical) energy. The power output of these harvesters may vary greatly

and as such, harvesters continue to be a field of research with significant room for

development.

The waste energy collected by these

energy harvesters may vary greatly in

source, such as residual heat, kinetic

energy by vibration, solar power, etc.

The purpose of use tends to be the feedback

of the generated electrical power into

powering components of the source system,

and is accomplished through several designs

which will be discussed.

Figure 1: d33 piezoelectric energy harvester

(Park, Park, & Lee, 2010)

Figure 2: Pyroelectric energy harvester

(Hunter, et al., 2011)

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Operation Principles

The following section covers the operational principles of two selected energy harvesters,

namely the d33-Mode vibration energy harvester as well as the Oak Ridge thermal energy

harvester concept.

Vibration Energy Harvester (Piezoelectricity)

Vibration energy has considerable potential for micropower energy

harvesting, because it provides a higher power density than other

systems, has an infinite lifetime, requires no physical connection to the

outside of the system, and is reliable in harsh environments.

(Park, Park, & Lee, 2010)

The vibration energy harvester makes use of the bending of micro cantilever beams when

experiencing vibrations as well as the piezoelectric effect to cause an overall change in

capacitance and hence a generation of current.

Figure 3a) and 3b) comparing the d31 and d33 designs

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The d33-Mode harvester makes use of a modified version of the existing d31 energy harvester,

and operates on a principle of a sequence of interdigital shaped electrodes in Figure 3b),

rather than the parallel plates seen in Figure 3a) to generate a voltage, and thus power through

the bending of a cantilever beam. This design can be seen in Figure 1 in the previous page.

The displacement is expressed as y(t) and is a function as

Because of the interdigital fingers, Park considers the design to be superior to that of the d31

mode harvester due to its ability to generate a larger voltage with lower displacements of the

cantilever. The stress experienced by the d33-mode is given by

( )

( )

Where g is the gap of the interdigital shaped electrode, I is the effective inertia, h is the

thickness of the silicon cantilever, w is the width of the comb electrode and m is the weight of

proof mass.

( )

( )

Park considers his design superior to that of the d31 because the harvester stress is

independent of the l, the length of the cantilever beam or x, the distance from the fixed area,

which allows a homogeneous distribution of stress between the electrodes and thus a more

stable current which is less prone to current crowding losses.

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Thermal Energy Harvesters (Pyroelectric)

In the U.S. around 55% of the energy generated from all sources in 2009 was lost

as waste heat. A technology to recover or convert this low-grade waste heat to

usable electricity could save industrial sectors tens of billions of dollars annually

through increased process efficiencies

(Hunter, et al., 2011)

In contrast to the piezoelectric effect seen in the vibration energy harvester, the pyroelectric

harvesters, while also making use of the bending of cantilever beams as seen in figure 4

below, draw their name from the use of material thermal properties to harvest energy.

The basic principles of thermal energy harvesters are founded in the fact that materials such

as triglycine sulphate (TGS) demonstrate a pyroelectric effect whereby it exhibits a

spontaneous temperature-dependent polarization (Webster, 1998).

This effect leads to the temporary

generation of a current but would not

otherwise generate electrical power if

temperatures remain constant.

Figure 4: Beam bending in pyroelectric harvester

Figure 5: Charge/field relationship of

pyroelectric capcitor

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The harvester makes use of a bimorph metal plate to trigger the bending of the harvester.

The cantilever structure is heated through the anchor as an initial stage, bending towards the

heat sink cold surface as demonstrated in Figure 4, where it loses heat through contact of the

proof mass with the surface and bends back towards the hot surface.

Subsequent continual transfers between the proof masses and the surfaces allow the

pyroelectric material to undergo a constantly fluctuating temperature environment.

This triggers an indefinitely repeating process

which would produce power through the

pyroelectric material as long as the heat source

surface temperature maintains a steady

difference from that of the heat sink.

Figure 6: Schematic layout of thermal energy harvester

Figure 7: Temperature and power graph

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Fabrication

The fabrication processes of the discussed MEMS energy harvesters are discussed in the

following sections.

Fabrication of Vibration Energy Harvester

Parks d33-Mode harvester is

fabricated using the following

process, and is also detailed by

Figure 8 on the right.

The harvester makes use of simple

manufacturing techniques and is

straightforward and simple.

Firstly, thermal oxidation of the polycrystalline silicon as well as layering of the PZT,

platinium and zirconium oxide through electrode deposition results in the structure of figure

8a. Park uses electrode patterning to obtain the interdigital structure of his design on the

platinium in 8b) before etching the pattern into the PZT in 8c).

The silicon beam and inertial mass was then obtained in 8d) and 8e) through the use of deep

reactive-ion etching (RIE) before finally releasing the SiO2 oxide via etching to obtain the

product in 8f)

Figure 8: Fabrication sequence of d33

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Fabrication of Thermal Energy Harvester

Due to the lack of literature on the fabrication of pyroelectric harvesters, the following section

is largely theoretical and does not reflect that of actual fabrication techniques.

Figure 9 on the left shows the

cantilever used in the pyroelectric

energy harvester. It is assumed that

the anchors, heat sinks, heat sources

and proof masses used are of

materials with low thermal

resistance but not electrically

conductive.

This leads to the assumption that with the exception of the main cantilever body, the rest of

the cantilever body is made of polycrystalline un-doped silicon to reduce electrical short

circuiting. The body of the cantilever is described as a thin metal layer of approximately 10-

50 nm, while the pyroelectric P(VDF-TrFE) and Ti bimorph metal layers are 2-10 m thick.

It can then be deduced that based on the structural shape, a similar process to that of Parks

vibration harvester is used. Figure 10 on the next page shows a possible fabrication process.

Figure 9. Pyroelectric Capacitative Cantilever

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Theoretical Fabrication of Pyroelectric Harvester

The silicon layers in blue are first set in place

with the yellow anchor piece first deposited. A

layer of silicon dioxide in red is then set to

separate the contact from the base silicon layer.

Selective photolithography through a mask transfers the pattern of the lower red portion while

deposition of the proof mass fills the remaining portion.

Thin layers are then deposited through electrodeposition to form the main cantilever, and the

electrical components are machined.

More photoresist is added to successfully add the upper contact of the beam before finally

adding the last portion of silicon. The photoresist and silicon dioxide would then be released

anisotropically to form the final structure.

Benefits of Fabrication

Based on Parks fabrication technique as well as the theoretical fabrication technique of the

pyroelectric harvester lead to the conclusion that both products are easily mass produced in a

large 2D array should the need call for it, which is ideal for both authors recommendations as

they recommend the devices be used in bulk.

Figure 10: Assumed fabrication process

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Applications of Devices

d33-Mode Energy Harvester

Parks design for the d33-Mode harvester is stated to vary from that of previous designs by

being able to successfully scavenge power from a much lower frequency and amplitude of

vibration, and also be able to produce a higher output voltage that its predecessor the d31 by

virtue of the changes in design. The design was tested and compared to Parks numerical

modelling and it was found that the maximum power was maximized at 528Hz and has a

power density of 7.3 mW.cm-3

.g-2

.

Park theorizes that the output is high enough to be useable in practical applications, especially

if connected in an array with more vibration harvesters. However, it is likely that the resonant

frequency can be adjusted by adjusting the weight of the proof mass as well as the length of

the cantilever beam in order to tune the harvester for a variety of applications.

Pyroelectric Harvester

While Hunters team also found success in the testing of their energy harvester, little

recommended use of the pyroelectric harvester is mentioned. The introduction mentions

possible opportunities for such active cooling and power generation for sensor systems, such

as that of on-chip active heat sinks in standalone computers and data processing sensors, as

the harvester has successful applications in environments where a sharp temperature gradient

is observed and the resultant power used to reduce the power consumption of the system.

However, there has been little research done on the heat dissipation efficiency of such

harvesters. Impeding the heat dissipation of these electrical systems may result in a lower

equipment lifespan and lead to an overall low rate of return on the harvesters instead.

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Conclusion

In this paper, two forms of energy harvesters meant to reduce the wastage of energy have

been discussed thoroughly through a description of the principles of operation as well as

actual and theoretical fabrication methods. The applications of each harvester have been

mentioned as well.

This term paper helps to further the knowledge of additional MEMS devices not covered in

the lecture as well as enhancing the information already covered, and is a meaningful way to

conclude the module.

Works Cited

Hunter, S. R., Lavrik, N. V., Bannuru, T., Mostafa, S., Rajic, S., & Datskos, P. G. (2011).

Development of MEMS based pyroelectric thermal energy harvesters. SPIE.

Park, J. C., Park, J. Y., & Lee, Y.-P. (2010). Modelling and Characterization of Piezoelectric

d33-Mode MEMS energy harvester. JOURNAL OF MICROELECTROMECHANICAL

SYSTEMS.

Webster, J. G. (1998). The Measurement, Instrumentation and Sensors Handbook. CRC

Press.