Low-Frequency Vibrational Energy Harvesting at
the Micro and Meso Scale
by
Haluk Akay
Bachelor of Science, Mechanical Engineering
Carnegie Mellon University (2016)
Submitted to the Department of Mechanical Engineering in partial
fulfillment of the requirements for the degree of Master of Science in
Mechanical Engineering at the Massachusetts Institute of Technology
June 2018
Massachusetts Institute of Technology 2018. All rights reserved.
Signature redacted
Author
Department o>echanical Engineering
May 19, 2018
Certified by Signature redacted
Sang-Gook Kim
Professor of Mechanical Engineering
Thesis Supervisor
Accepted by Signature redacted
Rohan Abeyaratne
Chair, Department Committee on Graduate Theses
MASSACHUSETS INSTITUTEOF TECHNOLOGY
JUN 2 5 2018
LIBRARIES
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Low-Frequency Vibrational Energy Harvesting at the Micro
and Meso Scale
by
Haluk Akay
Submitted to the Department of Mechanical Engineering on May 19, 2018
in partial fulfillment of the requirements for the degree of
Master of Science in Mechanical Engineering
Abstract
Energy harvesting from environmentally available vibrations is a solution to providing
electric power for remote and mobile applications, such as the Internet of Things. A
buckled beam-based MEMS device can harvest power from low frequency, low amplitude
vibrations which has never been achieved by a micro-scale device. Due to the poor
fabricated quality of the active piezoelectric material in the device, however, the generated
power has been an order of magnitude less than expected. It is important to harvest more
than 1 micro Watt at the device size smaller than a coin in order to implement this
technology for real world application. The buckled beam vibrational energy harvesting
device has been evaluated for its power generation performance and its specific
microfabrication recipe with respect to piezoelectric materials has been analyzed from a
process control standpoint to identify areas of improvement. The process has been
redesigned for a simpler and streamlined recipe. Complex residual stress control feedback
loops have been replaced with a simple post-fabrication assembly to induce controlled
buckling in active MEMS beams. Using a custom-designed fixture, the post-fabrication
buckling concept has been demonstrated to achieve accurately buckled beams.
With regard to mesoscale energy harvesting, a product has been designed that converts
regular human walking motion to electricity. The device harvests electric power using air
bulbs, distributed in the sole of a shoe to drive a series of micro-turbines connected to small
DC motors. The number and position of air bulbs is optimized to harvest the maximum
airflow from each foot-strike. The system is designed to continuously drive the micro-
turbines by utilizing both outflow and inflow from the air bulbs. A prototype combat boot
was fitted on the right foot of a 75kg test subject, and produced an average continuous
power on the order of lOs of mW over a 22Q load during walking at 3.0 mph. This combat
boot provides enough electric power to a passive GPS tracker that periodically relays
geographical coordinates to a smartphone via satellite.
Thesis Supervisor: Sang-Gook Kim
Title: Professor of Mechanical Engineering
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4
Acknowledgements
I would like to acknowledge and thank my advisor Professor Sang-Gook Kim for his
mentorship and encouragement. I also need to recognize and thank Dr. Ruize Xu, my
former group-mate who preceded me on this energy harvesting project for the knowledge
he lent me before graduating. I would like to thank the MTL staff, especially Dennis Ward,
for their help and exceeding any expectations one could have in providing advice and help
while I learned the microfabrication process. Finally, I would like to thank everyone in my
personal life who are constantly helping to push me ahead.
5
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Table of Contents
Chapter 1 Introduction........................................................................................ 12
1.1 Thesis Objective.................................................................................................. 12
1.2 Thesis Organization............................................................................................ 12
Chapter 2 Low-Frequency Vibration MEMS Energy Harvesting ........................ 14
2.1 Background ........................................................................................................... 14
2.2 Device Design and Dynamic Modeling of Low Frequency Energy Harvesting............ 18
2.3 Piezoelectric Performance of Previous Generation Device ........................................ 21
2.5 Chemical Vapor Deposition Process Control.......................................................... 30
2.5.1 CVD Process Outline ........................................................................................ 31
2.5.2 Experimental Design............................................................................................ 32
2.5.3 Testing and Experimental Results ........................................................................ 34
2.5.4 Data Analysis.................................................................................................. 36
Chapter 3 Post-Fabrication Buckling Design ...................................................... 39
3.1 M otivation............................................................................................................. 39
3.2 Design Goals ......................................................................................................... 41
3.3 M odeling...............................................................................................................45
3.3.1 Analytical Buckled Beam Model ....................................................................... 45
3.3.2 Experimental Buckled Beam Model.................................................................... 49
3.4 Design Paths..........................................................................................................51
3.4.1 Achieving Buckling without Frame Modification.................................................... 52
3.4.2 Achieving Buckling by Frame Modification .......................................................... 55
3.5 Proposed Designs ................................................................................................... 56
3.5.1 Experimental Frame Bending Fixture.................................................................... 56
3.5.2 Snap-Fit Corrective Frame Fixture Design and Process ............................................ 59
3 .6 T estin g .................................................................................................................. 6 3
3.6.1 Fixture Construction ........................................................................................ 63
3.6.2 R esults ............................................................................................................. 6 6
Chapter 4 Energy Harvesting Footwear ............................................................ 71
4.1 Introduction to W earable Energy Harvesting.......................................................... 71
4.2 M esoscale Energy Harvesting System Design ............................................................ 73
4.2.1 Air Bulb Design and Placement.......................................................................... 73
4.2.2 Turbine Enclosure Geometry .............................................................................. 76
4.3 Theoretical Calculations of Upper Bound Power .................................................... 77
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4.4 GPS - Equipped Combat Boot Prototype................................................................ 79
4 .5 R esu lts .................................................................................................................. 8 2
4.6 Discussion.............................................................................................................85
Chapter 5 Sum m ary........................................................................................... 87
5.1 Thesis Summary.....................................................................................................87
5.2 Future work ........................................................................................................... 88
Appendix A 90
A. 1 PECVD Process Capability Study Data ................................................................. 90
A.2 Electric Testing of Experimental Frame Bending Fixture......................................... 93
Bibliography 94
8
List of Figures
FIGURE 2 - 2 BUCKLED BEAM ENERGY HARVESTER SCHEMATIC [1].................................................... 17
FIGURE 2 - 3 MASS-SPRING-DAMPER M ODEL SCHEMATIC ................................................................... 19
FIGURE 2 - 4 CLAMPED-CLAMPED BUCKLED BEAM ENERGY HARVESTER............................................... 20
FIGURE 2 - 5 POLARIZATION - ELECTRIC FIELD CURVE (R. Xu 2018) [1] ............................................. 23
FIGURE 2 - 6 JEON RECIPE, CRACKED PZT ........................................................................................ 26
FIGURE 2 - 7 M ITSUBISHI RECIPE, CRACKED PZT ............................................................................. 26
FIGURE 2 - 8 XRD OF ANNEALED PZT (R. Xu BASELINE RECIPE) [1]................................................. 27
FIGURE 2 - 9 POLARIZATION CURVE FROM FABRICATION WITH NEW SOL-GEL ...................................... 28
FIGURE 2 - 10 POLARIZATION OF VARIOUS PZT FROM OUR RESEARCH GROUP..................................... 29
FIGURE 2 - 11 CVD PROCESS BLOCK DIAGRAM ............................................................................... 31
FIGURE 2 - 12 EXPERIMENTAL DESIGN GEOMETRY .......................................................................... 33
FIGURE 2 - 13 CVD M EASUREMENT SAMPLING ............................................................................... 35
FIGURE 2 - 14 FILM THICKNESS OVER THREE CVD RUNS WITH IDENTICAL PARAMETERS..................35
FIGURE 2 - 15 CVD AVERAGE SILICON DIOXIDE FILM THICKNESS ................................................... 36
FIGURE 2 - 16 AVERAGE THICKNESS AND VARIANCE FOR COLD-START RUNS..................................... 37
FIGURE 3 - 1 RESIDUAL STRESS CONTROL FEEDBACK LooP [1].......................................................... 40
FIGURE 3 - 2 CROSS-SECTIONAL VIEW OF BUCKLED BEAM DEVICE ..................................................... 42
FIGURE 3 - 3 CROSS-SECTIONAL VIEW OF SIMPLIFIED PIEZOELECTRIC ENERGY HARVESTER .................. 44
FIGURE 3 - 4 MONOLITHIC MEMS ENERGY HARVESTER DEVICE GEOMETRY ...................................... 45
FIGURE 3 - 5 CLAMPED-CLAMPED BEAM SCHEMATIC .......................................................................... 46
FIGURE 3 - 6 BUCKLED CLAMPED-CLAMPED BEAM SCHEMATIC........................................................... 46
FIGURE 3 - 7 BUCKLED CLAMPED-CLAMPED BEAM MODEL ................................................................... 47
FIGURE 3 - 8 1:35 SCALE MODEL OF BUCKLED BEAM DEFLECTION ..................................................... 50
FIGURE 3 - 9 TOPSIDE VIEW OF MEMS ENERGY HARVESTING DEVICE................................................... 51
FIGURE 3 - 10 FRAME BUCKLING VERSUS FRAME BENDING .............................................................. 52
FIGURE 3 - I 1 INNER FACE OF FRAME UNDER COMPRESSION ............................................................ 54
FIGURE 3 - 12 FRAME BENDING FIXTURE SCHEMATIC (SIDE VIEW) ................................................... 57
FIGURE 3 - 13 ALUMINUM BEAM - DEVICE FRAME INTERFACE (NOT TO SCALE)................................. 57
FIGURE 3 - 14 ISOMETRIC RENDERING OF EXPERIMENTAL FRAME BENDING FIXTURE........................... 58
FIGURE 3 - 15 SIDE RENDERING OF EXPERIMENTAL FRAME BENDING FIXTURE ................................. 58
FIGURE 3 - 16 SNAP-FIT SCHEMATIC ................................................................................................ 59
FIGURE 3 - 17 SNAP-FIT M ODEL FREE BODY DIAGRAM.......................................................................60
FIGURE 3 - 18 EXPLODED VIEW OF SNAP-FIT FIXTURE ....................................................................... 62
FIGURE 3 - 19 CROSS SECTIONAL VIEW OF SNAP-FIT FIXTURE .......................................................... 62
FIGURE 3 - 20 DETAIL CROSS-SECTION VIEW OF DEVICE / MOLD INTERFACE....................................... 63
FIGURE 3 - 21 FRAME BENDING FIXTURE.......................................................................................... 64
FIGURE 3 - 22 FRAME BENDING FIXTURE, SIDE VIEW ......................................................................... 64
FIGURE 3 - 23 Two POSITIONS OF BUCKLING OBSERVED ON FRAME BENDING FIXTURE ....................... 65
FIGURE 3 - 24 PROFILOMETER MEASUREMENT OF BEAM BUCKLING .................................................. 67
FIGURE 3 - 25 ACCELERATION OF PROOF MASS, FREQUENCY DOMAIN RESPONSE ............................. 68
FIGURE 3 - 26 DEVICE ELECTRIC FUNCTION VERIFICATION (OPEN CIRCUIT VOLTAGE)........................ 69
FIGURE 4 - 1 SCHEMATIC OF FOOTWEAR ENERGY HARVESTING SYSTEM ................................................ 74
FIGURE 4 - 2 PEAK POWER RELATIVE TO LENGTH ALONG SHOE.............................................................. 75
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FIGURE 4 - 3 RADIAL SYMMETRY OF DUAL TURBINE ENCLOSURE ALLOWS FOR BI-DIRECTIONAL FLOW TO
CONTINUOUSLY DRIVE GENERATORS............................................................................................ 76
FIGURE 4 - 4 PROTOTYPE COMBAT BOOT FITTED WITH ENERGY HARVESTING SYSTEM........................80
FIGURE 4 - 5 PEAK POWER OUTPUT VERSUS VALUES OF LOAD RESISTANCE ........................................... 82
FIGURE 4 - 6 VOLTAGE OUTPUT MEASURED FOR 0.5 Hz FOOTSTEPS ....................................................... 83
FIGURE 4 - 7 VOLTAGE OUTPUT MEASURED FOR 1.0 Hz FOOTSTEPS ..................................................... 83
FIGURE 4 - 8 VOLTAGE OUTPUT MEASURED FOR LUNGES.......................................................................84
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List of Tables
TABLE 2 - 2 PZT SPIN-COATING STANDARD OPERATING PROCEDURE ................................................... 24
TABLE 2 - 3 PZT RECIPE COM PARISON .................................................................................................... 25
TABLE 2 - 4 SUMMARY OF FACTORS AND LEVELS................................................................................. 34
TABLE 2 - 5 SIGN EFFECTS FOR 22 DESIGN [15].................................................................................. 34
TABLE 2 - 6 ANALYSIS OF VARIANCE FOR FACTORIAL EXPERIMENT...................................................... 38
TABLE 3 - 1 FUNCTIONAL LAYERS OF BUCKLED BEAM ENERGY HARVESTER ..................................... 43
TABLE 3 - 2 PREDICTED FRAME WIDTH DISPLACEMENT BASED ON EXPERIMENTAL MODEL ................... 50
TABLE 4 - 1 POWER GENERATED FOR VARIOUS MODES OF MOVEMENT ................................................ 84
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Chapter 1 Introduction
1.1 Thesis Objective
The objective of this thesis is to harvest energy at the MEMS scale from low-frequency
vibrations below 100 Hz. This operating range which has been identified as the key to
implementing microelectromechanical systems (MEMS) energy harvesters for commercial
application.
The microfabrication process design involved in producing a bi-stable buckled beam-based
vibrational MEMS energy harvester is analyzed and simplified by removing nonfunctional
design elements included purely for residual stress balancing and moving buckling
operations post-process. Fixtures and methods of achieving post-fabrication modification
of MEMS devices have been designed, built, and tested for this purpose.
At the meso-scale, an energy harvesting system embeddable in footwear has been designed
to convert the impact of footsteps to usable electric power by way of airflow-driven
miniaturized turbines connected to small DC generators. A prototype system has been
installed in a combat boot and powers a GPS receiver that can provide information of the
user's location upon command.
1.2 Thesis Organization
This thesis is divided into two parts, based on separate efforts to harvest energy at low
frequencies. The first part addresses energy harvesting at the microscale using a bi-stable
buckled beam MEMS device developed by our group which targets vibration energy
harvesting at low frequencies and low amplitude. The second part presents a solution to
harvesting energy at the mesoscale from human footstep motion utilizing a turbine system
embedded in footwear, driven by airflow generated by the compressive force of the user's
12
foot inside the shoe. The first chapter provides a brief outline of energy harvesting and the
objective and organization of the thesis.
The second chapter analyzes the microfabrication process used to produce our group's
most recent MEMS device and identifies opportunities for improvement through analysis
of process capability and detailed analysis of the fabrication methodology. A higher quality
device from a piezoelectric standpoint is fabricated with material properties at par with
previous generation devices from our research group. The complexities of the stress control
feedback process are highlighted.
The third chapter presents a post-fabrication buckling fixture solution to achieve buckled
MEMS thin films after microfabrication. An analytical model of the MEMS frame buckling
is developed. A meso-scale model of the buckled MEMS membranes is used to predict
range of moment needed in the frame to induce buckling. Different methods of inducing
buckling post-process are evaluated, and an experimental frame bending fixture is
designed, built, and tested and validated for inducing the correct amount of buckling while
preserving electrical function. A snap-fit fixture design is proposed as a production
appropriate evolution of the frame bending concept.
The fourth chapter presents energy harvesting footwear. The design is optimized by
number and placement of air bulbs within the shoe sole. The turbine enclosure is designed
to accept airflow from both outlet and inlet such that the turbines are spinning nearly
continuously. A GPS receiver powering prototype is built to demonstrate the function of
the system.
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Chapter 2 Low-Frequency Vibration
MEMS Energy Harvesting
2.1 Background
Energy harvesting has played a pivotal role in economic development by enabling
agricultural and industrial processes to draw upon the natural environment for their energy
needs. Thousands of years ago, water wheels were used to grind wheat into flour. More
recently, geothermal heat energy has been used to drive generators to produce usable
electric power. With the development of microelectromechanical systems (MEMS), the
size of cutting-edge energy harvesting systems has been dramatically reduced to the point
where a fully packaged device can be about the dimensions of a large coin.
With the miniaturization of energy harvesting, the source of energy upon which these
devices draw for power has changed. Precision microfabrication techniques have enabled
the design of devices with critical feature dimensions on the order of nanometers. Such
devices can be fabricated from novel materials with piezoelectric, electrostatic,
ferromagnetic, and other properties which allow for energy harvesting of naturally
occurring vibrations.
The aim of vibrational energy harvesting is to scavenge usable electric energy from
naturally occurring vibrations in the ambient environment. One method of transforming
vibrational kinetic energy into electricity is to exploit the properties of piezoelectric
materials for this purpose. Piezoelectric materials not only deform mechanically when a
potential difference is applied across them, but also conversely develop surface electric
charge when strained [1]. Vibrational energy harvesting devices using piezoelectric
materials are designed to resonate in the frequency range of a targeted ambient vibration
such that repeated deformation of the piezoelectric material results in electric charge.
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Piezoelectric energy harvesting devices can be generally separated into two categories.
Bulk piezoelectric devices utilize piezoelectric materials on the mesoscale and scavenge
energy from large deformations caused by mechanical impact or other substantial impulses
[2]. Large mass of the active piezoelectric material translates to greater surface charge and
more electric power generated. Such devices do require significant kinetic energy input to
work, however.
The latter category of piezoelectric energy harvesting devices falls in to the family of
MEMS devices and utilizes thin-film membranes of piezoelectric materials with
thicknesses ranging on the orders from 101 to 10' nanometers. Depending on the thickness
and other device-specific geometric designs, these membranes resonate on a wide spectrum
of ambient vibration frequencies to produce electricity.
Piezoelectricity and the molecular structure of piezoelectric materials is an important topic
to understand in order to address the challenges of fabricating MEMS devices with active
piezoelectric components. The mechanical deformation to surface charge piezoelectric
effect is found in select materials with non-centrosymmetric atomic structures [3]. In a
specific temperature range, dipoles in piezoelectric materials align to create a net
polarization within smaller regions of the material known as Weiss domains [4]. In order
to truly produce a net polarization of the material, all the Weiss domains must be aligned
which can be executed by applying a high electric field at a high temperature point in a
process termed "poling."
An example of a piezoelectric material with the stated properties described above is lead
zirconate titanate, abbreviated as (PZT). PZT has a polycrystalline, ceramic perovskite
structure [3]. PZT has high piezoelectric coupling when compared to other piezoelectric
materials, and so is favored for energy harvester fabrication where small amounts of strain
must be translated to the largest electric polarization possible [6].
15
When considering potential applications for MEMS energy harvesting devices, both the
power requirements of the application and the "niche" benefit of using a MEMS device for
this purpose must be taken into account. The so-called "low-hanging fruit" among
applications when using these criteria are low-power sensors used to monitor variables
such as temperature or flow rate in remote locations. Not only are such sensors being
designed for increasingly low power consumption needs, but the need for such constant
monitoring often arises in remote locations such as oil pipelines or difficult-to-access
situations such as long stretches of piping in refineries which can extend for kilometers.
While evaluating the application of MEMS energy harvesters, it is also important to take a
survey of the available vibrations occurring in the potential use environment. The
frequency range of many vibrational energy harvesting opportunities falls below 100 Hz
for environments such as pipe flow, and motors. For this reason, the target operating
frequency of a MEMS energy harvesting device should fall in this range.
Our research group has been developing such thin-film piezoelectric resonators since 2005,
with the intention of designing a device that harvests energy from low frequency, low
amplitude vibrations on a large operating bandwidth. However, these past energy
harvesters operated at a frequency on the order of 104 Hz and at a high amplitude of 4 g
[7]. Various efforts using magnets integrated into mechanical assemblies [8] and utilizing
soft materials [9] have been reported to lower the operating frequency range of energy
harvesters. A method of note to lower operational frequency is to design a nonlinear
resonance-based energy harvester with two states of stability [10]. This concept was
demonstrated experimentally by way of a mesoscale piezoelectric film supported by a steel
sheet. None of these efforts to lower the operational frequency range had been applied to
MEMS scale design or microfabricated and tested until a buckled beam energy harvesting
MEMS device developed by Xu [1,13]. This device applied the concept of a bi-stable
buckled beam design at the MEMS scale to harvest energy from low frequency, low
amplitude vibrations. A precisely targeted deflection of 200pm due to buckling was
16
induced by balancing residual stresses in multiple microfabricated material layers in the
device. This enabled an array of buckled piezoelectric beams to snap between the two states
of stability during excitation. The fabricated monolithic device proved this concept and
exhibited the intended behavior during dynamic testing on a mechanical shaker test rig.
However, the power generated (85 nW) was an order of magnitude less than what was
theoretically predicted, indicating opportunity for improvement.
Figure 2 - 1 Buckled Beam Energy Harvester Schematic [1]
A MEMS device such as the one designed by Xu is fabricated in a clean-rom facility using
chemical vapor deposition or sol-gel spin-coating to create uniform material coats on a
silicon substrate, and pattern each layer iteratively using photolithography to build the
device layer by layer. For a sophisticated device such as the energy harvester designed by
Xu, there is added complexity in the microfabrication process because of multiple design
requirements that need to be achieved.
The key deliverable of the buckled beam energy harvesting device is the quantity of electric
power outputted for an input vibration below 100 Hz and at an acceleration of 1 g. The
target value for the RMS power output is on the order of micro-Watts, so that the device
can be integrated with a low-power sensor and transmit data through a nodal network
without an external power supply. In reality, when the initial batch of devices was
fabricated, a peak output power of just 85 nW was measured for testing at 70 Hz and 0.5
g.
17
By evaluating the performance of each of piezoelectric layer and dynamic performance of
the device, opportunities to increase the power output through better process control during
microfabrication can be identified.
2.2 Device Design and Dynamic Modeling of Low Frequency
Energy Harvesting
Vibrational energy harvesting is based on the principle of translating a vibration into usable
electric power. As discussed previously, materials with piezoelectric properties are able to
experience mechanical strain and create an electric potential difference. In order to induce
stain in the active piezoelectric material of the energy harvester, the device structure must
be coupled as directly as possible to the stated vibration. The resultant vibrational
displacement to the energy harvester can be described using the equation (1) below.
z(t) = zosin (ot) (1)
The model of a vibrational energy harvester can be considered as a mass-spring damper
system, the frame of which is directly coupled to the oscillations of the source of vibrations,
as shown in Figure 2-3 [1].
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k (Spring
constant)
m (Mass) -
b (Damping
constant)
Figure 2 - 2 Mass-Spring-Damper Model Schematic
For such a simple mass-spring-damper system described, the system can be
equation 2, with the input vibration z(t) described in equation 1.
m2 + mi + bi + kx = 0
modeled using
(2)
For such a system, the linear resonant damped frequency can be expressed using equation
3.
0D = -k bk b2 (3)
As shown in equation (3), the natural frequency of such an energy harvester is inversely
proportional to its oscillating mass. Early iterations of MEMS piezoelectric energy
harvesters featured micro-scale cantilevered beams [12], but these devices had operational
frequencies on the order of 104 Hz.
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x(t)
z(t)i..... t -
I F-W 4
The buckled beam energy harvester designed by Xu can be modeled as a doubly clamped
beam with proof mass mounted midway along its length, as shown in Figure 2-4. This
model has both linear and nonlinear stiffness components.
Piezoelectric layer
Interdigiated Electrodes
Figure 2 - 3 Clamped-Clamped Buckled Beam Energy Harvester
For a nonlinear system, the natural undamped frequency can be expressed as a function of
nonlinear stiffness and linear stiffness [14].
1 kL+keq 1 kL+4c2kN
(tN 2T m 27r M(4
In equation 4 ke,, is the equivalent linear stiffness, and 6 is the maximum deflection. The
nonlinear system can be modeled by the characteristic bi-stable Duffing equation 5 [11],
where kL and kN are the linear and nonlinear beam stiffness, cl. and CN are the linear and
nonlinear electromechanical coupling.
m2+m +b+ kLX + kNX3 +CNxVN + CLVL = 0 (5)
The expressions for both linear and nonlinear stiffness are expressed below, and are
functions of the beam geometry where W, H, and L are the width, height, and beam length,
cE is the elastic constant, and T is the residual stress [1,8]. The stiffness is a lumped sum
across i number of material layers in the device.
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kL = i=1CE,(HU 3 - H + [ Z~ t 1 Toii] (6)
kN 8 1=1 CE,iHi (7)
Linear stiffness k, is broken into two components for beam bending and compressive
residual stress in the layer, which has a negative sign and at large magnitudes will render
the entire k term negative, modeling the bi-stability of the system through equation (5).
When linear stiffness changes sign, the frequency response of the system becomes lower.
At low frequency responses, the model predicts a large amplitude shift between the two
regions of stability. This bi-stable nonlinear oscillation is the key dynamic response to
implementing low-frequency energy harvesting at the MEMS scale.
2.3 Piezoelectric Performance of Previous Generation Device
The piezoelectric performance is tied to the thickness and quality of the PZT deposited and
patterned on the wafer. Thicker coats of PZT are desirable as a more massive active
piezoelectric layer translates to larger charge accumulation under the levels of strain
experienced by the device during vibration testing. PZT is deposited on the wafer in the
form of a solution-gel, or sol-gel, and spin-coated to create a uniform film of 70 - 80 nm
thickness per coating. The film is pyrolyzed on a hot plate and then photo-lithography
technique is used to pattern the 2D surface profile desired for the PZT layer. The unwanted
areas are then etched away and the wafer is annealed at high temperature to complete the
process. The target number of layers is three, aiming for a total of 240 nm of PZT in the
device. However, the number of layers is process-limited. Residual compressive internal
stresses in the PZT are the highest for any layer in the device, at approximately 650 MPa.
In addition, multiple coatings may prevent healthy formation of the PZT in the perovskite
crystalline structure, as described in section 2.1, needed to attain the desirable piezoelectric
coefficient.
21
One method of quantifying the piezoelectric properties of a fabricated energy harvesting
device is observing the remnant polarization and saturation polarization while applying a
large electric field at high temperature in a process known as "poling." This aligns the
Weiss Domains and primes the PZT for peak piezoelectric performance.
Remnant polarization Pr has units of charge per unit of square area and is a measure of the
orientation of Weiss Domain dipoles that are still aligned even after the large electric field
applied during poling is removed. Saturation polarization P, has the same units of
polarization and indicates the maximum level of polarization reached at the highest electric
field.
As stated before, the most recent energy harvester fabricated by R. Xu (2018) with the
buckled beam design was not able to generate more than 85 nW of peak power, despite
showing the targeted buckling deflection in the piezoelectric beams and desired nonlinear
resonance under 100 Hz. Upon further evaluation it was identified that the PZT quality was
not optimal. This was evident from the Polarization-Electric Field curve obtained when
poling the device, shown in Figure 2-5. The remnant polarization was measured to be just
3.75 pC / cm 2 [1].
22
30
20-
C4
10-
0-0
-10-
-20
-30
-1500 -1000 -500 0 500 1000 1500
Electric Field (kV/cm)
Figure 2 - 4 Polarization - Electric Field Curve (R. Xu 2018) [1]
Previous work with piezoelectric materials, specifically PZT has yielded better
performance in the past from our research group. In 2004, Jeon reported remnant
polarization of 20 pC / cm 2 [cite] and others have replicated this quality of PZT on the
same order of magnitude, as compared in Figure 2-10.
In order to support the work of R. Xu to successfully fabricate the buckled-beam energy
harvester design such that it delivered sufficient target power on the order of micro-Watts,
the PZT baseline coating recipe parameters were compared with other recipes for
validation. The parameters of interest were specifically related to the coating of PZT on the
substrate. The generalized procedure for spin-coating PZT is shown below in Table 2-2.
Spin-coating and subsequent patterning of the PZT is done in the Photolithography room
of the Technology Research Laboratory (TRL) at MIT's Microsystems Technology
Laboratories (MTL).
23
Table 2 - 1 PZT Spin-Coating Standard Operating Procedure
Step Machine Name Operation Parameters
1 Spin-Coater Statically dispense PZT Ramp speed as 500 rpm
sol-gel on substrate, for 5 sec, 3500 rpm for
followed by spin- 30 sec
coating
2 Hot Plates Pyrolyze PZT on hot Ramp temperature as
plates 180'C for 1 min, 390-C
for 5 min, 180'C for
1min
3 HMDS, Coater Coat with photoresist N/A
4 EVJ Expose N/A
(Photolithography)
5 Acid-Hood Etch PZT N/A
6 RTA-Annealing Anneal PZT 700'C for 60 seconds
While steps 3-5 are relatively well-defined processes of photolithography, the coating,
pyrolyzing, and annealing of PZT is less understood as a process. Therefore, three recipes
were compared with the goal of performing X-ray crystallography (XRD) on the three
results in order to observe crystalline structure and intensity peaks that correlate to correctly
crystallized PZT. The first recipe used the baseline parameters detailed in Table 2-2. The
second recipe was that used by Jeon to create PZT which had reported remnant polarization
of 20 pC / cm 2. The third recipe was that recommended by the manufacturer of the PZT
sol-gel, Mitsubishi Materials. These three recipes all called for different process parameters
and even different material layer stacks following deposition of silicon oxide by chemical
vapor deposition. For example, the Jeon Recipe excludes PT from the material stack. Table
2-3, below, details the parameters for each of the three tested recipes.
24
<I
Table 2 - 2 PZT Recipe Comparison
Recipe Step/ Xu (Baseline) [11 Jeon (2004) [121 Mitsubishi Materials
Parameter
Post - CVD n/a Anneal in furnace at Anneal in furnace at
Annealing 750'C, 30 min 7500 C, 30 min
ZrO2 Pyrolysis Bake in furnace at Bake on hotplate at Bake on hotplate at
100*C for 1min, on 350'C for 1 min, in 350C for 1 min, in
hotplate at 300'C furnace (add oxygen) furnace (add oxygen)
for 5 min, on ramp to 700C for 15 ramp to 700C for 15
hotplate at I 000C min, ramp down to min, ramp down to
for 1 min 100 0C 1000C
PT Coating Coat 2 Layers PT n/a Coat 2 Layers PT, as
per baseline
PZT Coating 500 RPM for 3 sec 500 RPM for 3 sec 500 RPM for 3 sec
(3 Coats) 3500 RPM for 30 1500 RPM for 30 sec 3000 RPM for 30 sec
sec
PZTPyrolysis 180C for I min 180C for 1 min 180C for 1 min
390C for 5 min 350C for 5 min 350C for 5 min
180'C for I min 180C for 1 min 180C for 1 min
PZT Annealing RTA - 700C for 1 Furnace - 700C for RTA - 700*C for I min
min 15 min (add oxygen) (add oxygen)
Each recipe was followed for processing a total of three wafers with the intention of
performing X-ray crystallography. None of the wafers was patterned using
Photolithography because X-ray crystallography involves destructively imaging samples
in a way that the wafer could not be used to produce devices later on. This experiment was
purely to investigate the PZT crystal structure of each recipe.
The results of the experiment clearly pointed to Xu's baseline recipe as the optimal recipe
for depositing this PZT sol-gel. Neither the Jeon recipe nor Mitsubishi Materials recipe
were able to produce an imageable wafer. Both experienced failure modes of cracking.
Illustrated in Figure 2-6, the Jeon wafer cracked after annealing the second coat of PZT.
As illustrated in Figure 2-7, the Mitsubishi wafer cracked after annealing the third coat of
PZT into large pieces.
25
'I
Figure 2 - 5 Jeon Recipe, Cracked PZT
Figure 2 - 6 Mitsubishi Recipe, Cracked PZT
The baseline wafer, however, yielded an acceptable surface upon visual observation. As a
result, X-ray crystallography was performed on the baseline wafer despite not having other
recipes to compare with. Xu performed the XRD imaging, and the resultant plot is shown
in Figure 2-8.
26
-4
150
100
8
50
20 30
Position [291
Figure 2 - 7 XRD of Annealed PZT (R. Xu Baseline Recipe) [1]
X-ray crystallography demonstrated a peak at 1-1-0 indicating promising orientation of the
PZT crystal structure. Based on this result, the process recipe was not identified as a risk
or a direct cause of the poor remnant polarization of previous buckled-beam devices. Upon
further evaluation of the process, it was discovered that the PZT sol-gel being used for
processing was several months old. Despite being preserved in a refrigerated environment
in a sealed container, degradation of the PZT sol-gel quality was identified as a risk factor.
As a result, a new container of PZT sol-gel was ordered from Mitsubishi materials, and the
process was replicated with the intention of producing better quality PZT. In addition to
using a new sol-gel, other process improvements were implemented, as listed below:
* New PZT Sol-gel from Mitsubishi Materials (opened October 2017;
opened December 2016)
* Pre-baking substrates for 5 minutes at 75'C prior to all sol-gel
operations to dehumidify wafer
* Pre-baking substrate for 5 minutes at 1000C prior to all annealing
dehumidify wafer
previous was
spin coating
operations to
Two wafer's worth of buckled beam energy harvesting devices were fabricated from the
new PZT sol-gel. A selected device was poled at 250'C at an electric field of 250 kV/cm
27
PZT (110) U ZrO2
o A Pb (Zr.52 Ti0.48) O
ZrO2
PZT (100)
PZT (111) PZT (200) A0A4:3
0A 4
so 6040
for a time of 30 minutes, and then the Polarization-Voltage (P-V) curve was measured
using a probe station. The resultant curve is shown in Figure 2-9 below.
4W
Voltage [V]
Figure 2 - 8 Polarization Curve from Fabrication with new Sol-Gel
The remnant polarization of the PZT is Pr is 23.5 pC/CM2, much more significant then
when compared to the polarization curve of the previous device which had remnant
polarization of just Pr of 3.75 ptC/CM2 in Figure 2-5. In fact, this device's PZT quality is
comparable to previous work from other projects involving PZT either for energy
harvesting (Jeon, 2004 [7], and Hajati, 2011 [9]) or ultrasonic medical imaging (Smyth,
2016 [2]). A comparison figure of various reported polarization levels from our research
group is shown in Figure 2-10.
28
90
80
70
60
50
0
-j 40
$ 30
20
10
0
Y.B. Jeon et. al
(2004)
11
A. Hajati (2011) K. Smyth (2016) R. Xu (2018) Current Iteration*
(2018)
n Remnant Polarization (Pr) U Saturation Polarization (Ps)
Figure 2 - 9 Polarization of Various PZT from our Research Group
Based on the promising results of the remnant polarization Pr in the most recent fabrication
effort with new sol-gel and selected process improvements, a higher output power of the
device is expected, on the order of micro-watts.
29
2.5 Chemical Vapor Deposition Process Control
Chemical vapor deposition (CVD) is used in the microfabrication process of the buckled
beam energy harvester to deposit material for three separate layers. These three layers are
the 300 nm thick silicon oxide below the piezoelectric active layers, the 800 nm thick
silicon nitride above the piezoelectric active layers, and the 400 nm thick silicon oxide
layer which is the topmost on the device.
The key inputs selected when running the CVD process for an intended material thickness
are the deposition rate, measured in units of nanometers per minute, and the time of
deposition. Other process inputs such as deposition frequency and material are also detailed
in the recipe. Theoretically the thickness of the deposited layer is predicted by the following
relation in equation 8.
Theoretical thickness [nm] = Deposition rate x Deposition time [mn] (8)
In practice, however, the measured thickness cannot be accurately predicted solely by
multiplying these inputs as suggested in equation 8. The specific machine used to run the
CVD process is a Surface Technology Systems CVD (STS-CVD) model installed in the
Technology Research Laboratory (TRL) of MIT's Microsystems Technology Laboratories
(MTL). After repetitive use of this machine, the user becomes familiar with the
unpredictability of the deposition process and it is laboratory custom procedure to have to
run at least one or two calibration runs with dummy silicon pieces to determine which input
parameters will give the desired deposited film thickness. As a part of comprehensively
evaluating the microfabrication process for the buckled beam device, the CVD process was
examined in detail.
30
2.5.1 CVD Process Outline
In order to thoroughly design a process control study on the STS-CVD machine, a full
understanding of its operation is necessary. The STS-CVD operates with a "black-box"
model where the user does not interact with the sample at all during processing, which is
advantageous in the fact operator-related error is minimized.
For processing a 4" silicon wafer (which is the correct substrate used in buckled beam
energy harvesting device fabrication), a step-by-step walk through of the procedure is
summarized as follows, and illustrated as a block diagram in Figure 2-11. After logging
into the system's digital interface using user credentials, the user vents the loading
chamber. Once this chamber reaches atmospheric pressure, it opens and the user manually
places the 4" wafer into one of two loading trays. This is one of the few opportunities for
operator error, should the wafer top surface contact anything during loading and become
scratched or deformed.
Operator Tasks
User Loads Wafer into
Loading Chamber
User unloads wafer +-
Loading Chamber
Wafer automatically
loaded into Reaction
Chamber
Pumped and mapped
Wafer automatically
unloaded into
Reaction Chamber
venting
4-1
Reaction Chamber
Purged and pumped
out
Gas stabilization
Processing
Figure 2 - 10 CVD Process Block Diagram
31
Purged and pumped
out
Once the wafer is in the loading chamber, the chamber is pumped and "mapped" so that
the machine knows which tray contains the substrate. After the recipe is edited, saved, and
selected, the wafer is loaded automatically into the reaction chamber. The reaction chamber
is then purged (10 seconds), pumped out (10 seconds), and then experiences 90 seconds of
gas stabilization before the process commences according to the loaded recipe. Following
completion of the process, the reaction chamber is pumped out (20 seconds), purged (10
seconds), and pumped out again (10 seconds) before the wafer is automatically unloaded
into the loading chamber. After venting the loading chamber, the user can remove the
processed wafer and the run is complete.
The film thickness can be measured by a nano-spectrometer using the known refractive
index of the deposited material. Before processing the actual experimental wafer,
calibration runs on dummy cleaved wafer pieces are necessary to determine which input
parameters will give the desired thickness of deposition.
2.5.2 Experimental Design
For a comprehensive analysis of the STS-CVD, experimental design is required to identify
running which types of tests will give the best understanding of process capability. From a
high level, the first element of the process to be investigated is the "cold start" state, by
which the machine is used after being idle for an extended duration of time. The aim of
investigating this is to determine whether it is necessary to run an empty run purely for the
purpose of "warming up" the machine. This was tested by booking the machine on the
earliest Monday morning slot available after a weekend without use and running a 50
nm/min x 3 minute process with 10 replicates consecutively three times. Additionally, tests
were run for different deposition rates all at a constant 3 minute deposition time, and
conversely runs with different deposition times all at a constant 50 nm/min deposition rate
were also run.
32
The second element of the process to be investigated is the process sensitivity with respect
to changes in deposition rate versus changes in deposition time. For this part of the
experimental design, the two factors are chosen to be deposition rate, and deposition time.
The response variable for this experiment is the film thickness. The factor levels were
selected as 50 (-) and 100 (+) nm / min for deposition rates and 2 (-) and 4 (+) minutes for
deposition times, with 5 replicates each. This is a 22 factorial design testing the two factors
at two levels each, illustrated in Figure 2-12.
High (+ -----------------------------
b ab
B - Deposition Time
(1) a
Low (-) -----------------------------
Low (-) High (+)
A - Deposition Rate
Figure 2 - 11 Experimental Design Geometry
The corresponding factors and levels are summarized in Table 2-4. The recipe was not to
be changed between runs to eliminate deterministic effects. The chosen recipe was Low
Frequency Silicon Dioxide, which is used for two of the three STS-CVD processes in the
buckled beam energy harvester fabrication process.
33
Table 2 - 3 Summary of Factors and Levels
Factor Symbol Level Symbol
50 nm / min
Deposition A
Rate 100 nm min +
2 minutes
Deposition B
Time 4 minutes +
The factorial design of this 22 experiment can be summarized run-by-run with table 2-5.
Table 2 - 4
Run
Sign Effects for 22 Design [15]
Factorial Effect
I A B AB
1 (1) + - - +
2 a + + - -
3 b + - + -
4 ab + + + +
2.5.3 Testing and Experimental Results
Results from experiment are expressed in plots below. Full data can be found in Appendix.
Each thickness was measured in two different locations on each lcm2 silicon sample. At
each location, two measurements were taken, as illustrated in Figure 2-13.
34
I
3mm 7mm
3mm
7mm
10 mm
10 mm
Figure 2 - 12 CVD Measurement Sampling
The results from the cold start measurement are below in Figure 2-14. After the machine
being in disuse for a period of 48 hours, the same runs were performed at a deposition rate
of 50 nm/min for a time period of 3 minutes, with ten replicates in each run.
Run 1
-e - -L -
---- ------
"Cold Start" Experiment
- Run 2
- - -.--
A S
* A S U
A
~ S
- A
- Run 3
IL
A
- -L
S- - -
- -
0 5 10 15 20 25
A Locatio 1, Measurement I N Locatio 1, Measurement2 Sample Number Average
* Locatlio 2, Measurement 1 * Locatim 2, Measument 2 - I Standard Deviation
Figure 2 - 13 Film Thickness over Three CVD Runs with Identical Parameters
35
270
265
260
255
250
245
240
235
230
22M
30
The results of the factorial experiment are summarized in the figure below, which took the
average value from each run and created a ruled surface.
ni
U'
H
400
350
300
250
200
150
100
50
0
100 E
2
50
-0
U'
4
Deposition Time (min)
HO-50 M 50-100 0100-150 a 150-200 0200-250 M 250-300 N 300-350
Figure 2 - 14 CVD Average Silicon Dioxide Film Thickness
2.5.4 Data Analysis
When analyzing the results of the "cold start" experiment, we can inspect how the average
thickness and variance change from run to run.
36
E 350400
-1.
254
252
250
o248
246
~244
242
240
4 238
236
234
- m - - - -
Run 2
Experimental Run
- - Average - Variance
Figure 2 - 15 Average Thickness and Variance for Cold-Start Runs
The average thickness was found by averaging all the measurements taken on all samples
for each run. The variance displayed in Figure 2-16 is between the average thicknesses for
each sample of each run. This figure suggests that running the STS-CVD multiple times
after disuse depends on the process requirements of the microfabricated device in
particular. Between Run 2 and Run 3, the average thickness increases by more than 10 nm
despite. all input parameters being held constant. For the buckled beam energy harvester,
this is a nontrivial change (3%) in material thickness of PECVD Silicon Dioxide that could
alter the residual stresses in the device.
When analyzing the results of the factorial experiment, an analysis of variance (ANOVA)
can be performed to understand the relative significance of deposition rate versus
deposition time and observe any two-way interactions between the factors. The ANOVA
table was populated using Factorial Regression on Minitab.
37
35
30
IF
Run I
5
0
Run 3
25
20
15
10
----
Table 2 - 5 Analysis of Variance for Factorial Experiment
Source Degrees of Sum of Mean Square F-Value P-Value
Freedom Squares
Model 3 175813 58604 1870.47
Linear 2 175620 87810 2802.64
Deposition 1 1523 1523 48.59
Rate (A)
Deposition 1 174098 174098 5556.68
Time (B)
2-Way 1 192 192 6.13 0.025
Interactions
(A*B)
Error 16 501 31 0.025
Total 19 176314
Regression can be performed using the data to create a model for the process based on the
inputs of deposition rate and time. The adjusted R2 for this model is 99.56%. The model is
expressed below.
Deposited Thickness [nm] = 93.3 time [min] + 0.35 deposition rate - 54.0
Based on this model, we can observe that the largest effect on deposition thickness given
the data taken is by varying deposition time as opposed to rate. For practical
implementation, recipe adjustment can be done most effectively by changes in deposition
time according to this analysis.
38
-1
Chapter 3 Post-Fabrication Buckling Design
3.1 Motivation
The microfabrication process involved with creating each batch of buckled beam energy
harvester devices has high costs associated with it due to the number of fabrication steps
involved. The largest factor in terms of design complexity is accurately balancing residual
stresses as the fabrication process progresses. Iterative stress measurements for each
additionally deposited layer are fed into an analytical model of lumped residual stresses so
that the subsequent process parameters can be adjusted accordingly to fabricate a buckled
MEMS film. This may become a key roadblock when addressing the mass production of
buckled beam energy harvesters.
The buckled beam energy harvester design's monolithic structure relies on precisely
balanced residual stresses in each of the eight layers of measurable thickness. The
challenge in achieving the desired net residual stress and subsequent target buckling
deflection is two-fold. Significant analytical modeling is required take as inputs material
properties and determine the necessary thickness of each layer to induce controlled
buckling in the thin film layers. Secondly, precisely fabricating these layers is nearly
impossible, such that a stress control feedback loop is necessary. As fabrication stages
progress, each additional material layer must be measured for thickness which is then re-
submitted to the analytical model in order to adjust the target thickness for subsequent
layers deposited.
39
Thin film
deposition on
dummy samples
Characterize the
thin films
(deposition rate,
residual stress)
Design thicknesses
of the rest control .
layers
Deposit the next Plug in the
control layer on measured data to
real samples the lumped model
Measure the
thickness and
residual stress
End
(deposition
Figure 3 - 1 Residual Stress Control Feedback Loop [1]
As outlined previously, there are a total of 56 fabrication steps, each with a dedicated
machine, included in the clean room approved process plan for producing these devices.
There are a number of secondary challenges resultant from the design of this long,
complicated process, summarized as follows:
* Long production lead-time
" High fabrication costs (both material and machine usage)
* Low throughput yield due to batch-to-batch process variations
Finally, the large number of steps in the fabrication process include several high-risk stages
such as spin-coating substrates with sol-gel or extracting individual devices from wafers,
which individually have low percentage yield due to poor process control and together
40
compound to a low overall throughput where yield percentages over 40% for working
devices in a given batch is typically not achievable given the existing fabrication process.
The need to accurately balance residual stresses significantly complexifies the fabrication
process which is already long and costly due to the device design. The motivation for
developing an alternative method to buckle the MEMS membranes after microfabrication
has concluded is to simply the production process in this regard.
3.2 Design Goals
The most direct method of simplifying the microfabrication process is to remove the
unpredictable and cost-intensive residual stress balancing scheme. This not only helps to
loosen tolerances on material thickness for each layer, but also helps to eliminate
unnecessary layers whose function is purely to counterbalance residual stresses. A cross-
sectional view of the device's material stack is shown in Figure 3-2, illustrating the nine
layers of material deposited on the silicon substrate. The intricate material stack presents
an opportunity to simplify the device recipe.
41
Silicon Oxide (400nm)
Electrodes
PT (1nnm)
Silicon Oxide (300nm)
Silicon Nitride (750nmi)
Thermal Oxide (1 000nm)
Silic01 SUbstrate (5_30pmn)
Figure 3 - 2 Cross-sectional View of Buckled Beam Device
In the cross-sectional view of the device, the silicon substrate layer (530prm) is only present
along the frame and the proof mass mid-beam section. Otherwise, the MEMS membrane
only contains the layers from Thermal Oxide to Silicon Oxide. The figure is not to scale.
In order to simply the device design so as to leave the buckling operation to post-fabrication
and merely fabricate functional material layers, the device needed to be redesigned with
fewer layers and therefore fewer fabrication steps. Table 3-1 shows a functional break-
down of the device on the layer level detailing the key design parameters and purpose of
each material included in the stack.
42
Functional Layers of Buckled Beam Energy Harvester
Layer Material Thickness Stress Function
0 Silicon 530 nm N / A Substrate,
Proof Mass
1 Thermal Oxide 1000 nm -300 MPa Structural
2 LPCVD Nitride 750 nm 170 MPa Structural
3 PECVD Oxide 300 nm -250 MPa Structural
4 Zirconium Dioxide 70 nm 370 MPa Active Layer
5 PT 10 nm 400 MPa Active Layer
6 PZT 240 nm 650 MPa Piezoelectric
Active Layer
7 Electrodes N IA N/A Interdigitated
Electrodes
8 LPCVD Nitride 800 nm -200 MPa Passivation
9 PECVD Oxide 400 nm -300 MPa Passivation
As illustrated in Table 3-1, the function of each layer can be generally categorized as either
supporting the electric function of the device in terms of transmitting electric potential from
the active piezoelectric layer through the interdigitated electrodes to externally facing
contact pads or contributing to the dynamic perfonnance in terms of producing a buckled
beam structure for the device. As shown in Table 3-1, every material layer except for the
gold electrodes and silicon proof mass and frame is under some form of internal stress,
whether compressive or tensile. The baseline design diligently accounts for the stresses in
each layer and balances the effective internal stress in the beams to achieve the desired
buckled deflection target of 200ptm of the proof mass from the frame.
There are advantages and drawbacks to the baseline design of incorporating multiple layers
of materials under different internal stress to achieve a specific amount of buckling. The
key positive aspect of this design is that the microfabrication process results in a monolithic
device in a natural state of buckling that is ready to be installed on a ceramic pin grid array
package for testing or use. However, this comes at the cost of a lengthy fabrication process
that requires progressive feedback control in order to correctly balance tensile and
compressive stresses. Following the fabrication of each additional layer on the device, the
stresses are measured and compared to target values. Due to process variation mainly with
43
Table 3 - I
respect to layer thickness, the internal stresses are not necessarily balanced as desired at
any given point during the fabrication process. Therefore, recipe parameters must be
adjusted to account for the inaccuracies and maintain the stress balance necessary to
achieve targeted buckling.
The original design goal was to eliminate non-functional layers and address buckling the
membrane post-fabrication. This can be achieved by eliminating the material layers
included in the recipe solely for balancing residual stresses. The cross section of such a
simplified device design is shown in Figure 3-3.
Silicon Nitride (800nm11)
Electrodes
Silicon Nitride (750nm)
SIliCOnI Substrate (530pmn)
Figure 3 - 3 Cross-sectional View of Simplified Piezoelectric Energy Harvester
By simplifying the material stack, the device becomes a piezoelectrically active array of
doubly-clamped micro-beams that are not in any sort of buckling by design. This reduces
the number of steps in the microfabrication process from 56 to 40, but also necessitates
post-fabrication modification of the device to achieve buckling in the beams, which is the
aim of this design. More specifically, the goal of this design was to induce a buckled state
of the device's beams after microfabrication is completed and achieve a buckled deflection
of 200pLm, identical to the baseline value of buckling.
44
3.3 Modeling
3.3.1 Analytical Buckled Beam Model
When designing a post-processing method of inducing a buckled deflection on the micro-
scale beam array, it is helpful to develop a simplified structural model of the beams to
analytically characterize what modifications will achieve the desired buckling.
As introduced previously, the monolithic MEMS geometry features an 18 mm by 18 mm
silicon frame bounding 28 pairs of beams that support a centrally-suspended silicon proof
mass, as shown in Figure 3-4.
28x pairs of beams
18 mm
Figure 3 - 4 Monolithic MEMS Energy Harvester Device Geometry
The MEMS device structure can be modeled as clamped-clamped beams of length 1/2 with
a centrally suspended proof mass m. The frame spacing across which the mass is suspended
is length L. A diagram of this model is shown in Figure 3-5.
45
L
1/2
L
Figure 3 - 5 Clamped-Clamped Beam Schematic
In the original baseline monolithic structure, each beam is under residual internal stress
such that when the devices are released, there is a net compressive stress, which, when
greater than the critical buckling load of the beams, induces buckling in the beams without
any external loads to the device. In order to achieve the same buckling, but with the
simplified device structure that has no designed residual stresses, the beams must be axially
compressed such that they buckle accordingly. This effect can be created by offsetting the
frame width to decrease length L, as shown in Figure 3-6.
L-d d
Figure 3 - 6 Buckled Clamped-Clamped Beam Schematic
By narrowing the dimension L by an offset x, the beam is placed under a compressive stress
and buckling occurs, raising or lowering the proof mass by a distance S out of the neutral
plane.
46
.1 Or
1/2
zzzzZor4Z
................
W(X) P
A--_------------ ---
L-d
Figure 3 - 7 Buckled Clamped-Clamped Beam Model
This buckling can be analytically modeled for a clamped-clamped beam loaded with an
axial compressive force P shown in Figure 3-7, resulting in an internal moment M inside
the beam. The moment can be simply expressed using the force P and beam deflection w(x)
which is a function of distance along the beam, in equation 9.
M = -Pw(x) (9)
For the buckling case, the moment can also be related to Euler-Bernoulli beam theory for
the second derivative of deflection as a function of distance long the beam with the
following equation (10).
M = )( 2W) (10)
Combining equations (9) and (10) gives a differential equation (11)
Pw + EI d 2 = 0 (11)
47
The second-order differential equation can be solved using the Euler solution for buckling,
equation (12).
w = eAX (12)
Substituting equation (12) into a re-arranged equation (11) isolating the second-order term
gives us equation (13).
-eAX + t2ellx = 0 (13)
El
Where the solution for k can be expressed in equation (14).
2 P (14)
El
This gives a general solution to equation (11) as the expression below for deflection in the
buckled beam model.
w(x) = A sin x + Bcos x + Cx + D (15)
For the clamped - clamped boundary conditions of this model, we can solve the deflection
w(x) as the following relation for the deformed shape of the buckled beam. Because the
beam is clamped at both ends, we know that the displacement of the beam w(O) and w(L)
at either end must both be equal to 0. From the clamped-clamped boundary condition we
also similarly know that the slope of the modeled beam must be zero, such that w'(0) and
w'L) are also zero. Therefore, the constants A, B, C, and D can be determined to give a
model of the deflection of the beam along its length in equation 16.
w(x) = ci sin ( L ) + c 2 cos (2L ) (16)
48
Using equation 16, the force P required to compress the device frame such that the beams
buckle 200pim out of the plane can be calculated. By modelling the beams' cross-sectional
geometry as a rectangular prism, the moment of inertia I can be approximated, and using
material properties of the beam layer stack, an effective Young's Modulus can be
approximated to predict an appropriate value for the compressive force needed to be
applied to theoretically achieve buckling of 200ptm.
3.3.2 Experimental Buckled Beam Model
The analytical buckled beam model derived in the previous section is helpful in
understanding the theoretical structural mechanics of the modeled clamped-clamped
buckled beam case and predicting the approximate force needed to compress the fabricated
device frame to produce the desired buckling, but the model is based on several
assumptions including but not limited to a simplified geometry as well as lumped material
parameters. While these assumptions accurately reflect the buckling case of the actual
device, a higher degree of granularity can be achieved by way of a more precise
experimental model.
Due to the high time and monetary cost of fabricating the actual energy harvesting MEMS
devices of interest, a mesoscale scale model was designed to mimic the buckling and
determine a relationship between horizontal compression distance and change in vertical
buckling height of the device out of the neutral frame plane.
The mesoscale model was constructed at a scale of 1:35. The beam was modeled with a
sheet of stainless steel with an aluminum proof mass of 30g. The desired buckled deflection
for the scale model was 7 mm to be scaled with the actual target of 200 tm for the MEMS
device. Deflections from 0 to 14 mm were investigated with the experimental model. The
results of the model, seen in Figure 3-8, can be scaled down to predict the amount of
displacement required in the actual MEMS device, shown in Table 3-2.
49
Experimental Model Buckled Deflection
/ 
____00
________ 
I
0.5 2 2.5 3
Horizontal Displacement of Free End (mm)
Figure 3 - 8
Table 3 - 2
1:35 Scale Model of Buckled Beam Deflection
Predicted Frame Width Displacement based on Experimental Model
Desired Buckling Change in Frame Length
o Pm 0 pm
100 Pm 19.7 pm
200 pm* 38.9 ptm
300 pm 68.9 pm
400 pm 87.1 pm
*targeted buckling deflection
Based on the results of the experimental beam model, in order to achieve buckling of the
piezoelectric beams of a targeted 200 pim, the silicon frame length will have to be narrowed
approximately 40 pm, or 0.2 % of the frame length.
50
16
12
U
S100
8
S4
S2
0 W0 1.5 3.5I
3.4 Design Paths
Based on the analytical and experimental modeling of buckling in the MEMS device, the
key functional requirement of any method that would induce such buckling is the ability to
accurately adjust the width of the device frame to a fine tolerance. The question resultant
from this requirement is how to achieve this given the current device geometry. As
discussed previously, the MEMS device in question is bound by an 18 mm by 18 mm
continuous silicon frame on all four sides, as illustrated in the topside view of the device
in Figure 3-9.
Figure 3 - 9 Topside View of MEMS Energy Harvesting Device
As depicted in Figure 3-9, the MEMS beams are constrained at their base position by the
silicon device frame. In terms of developing a method of post-fabrication buckling, there
are two paths forward. The first option involves designing a fixture or process to induce
buckling by narrowing the frame width without requiring changes to the original MEMS
device design, i.e. taking the current simplified baseline device. The advantage to this
option is that no tool or mask changes are required for the microfabrication process, so
51
minimal adjustments needed for the process design. The key disadvantage is overcoming
challenges that the rigid frame will pose in terms of designing a fixture to narrow its width.
The second option in terms of paths forward to develop methods of post-fabrication
buckling involves modifying the geometry of the device frame to allow for re-positioning
after fabrication. A main advantage of this option is the constraints of the rigid continuous
frame are lifted allowing for different forms of positioning of the frame. The disadvantage
of modifying the microfabrication process is the costs of verifying this process change and
any possible effects it may have on other steps of fabrication. Both design paths were
investigated in parallel in order to thoroughly examine their respective feasibility.
3.4.1 Achieving Buckling without Frame Modification
When considering a method to narrow the intact silicon frame width, two options are
apparent, illustrated in Figure 3-10. Compressive forces can'be applied to either end of the
frame to deform it without reaching the critical buckling load in an attempt to reduce its
width. The other option involves bending the frame sides without plastically deforming the
frame with the aim of bringing the opposite two sides closer together.
P
M
Buckling Bending
ttt
P M
Figure 3 - 10 Frame Buckling versus Frame Bending
52
The feasibility of compressing the frame to bring the opposite ends closer together can be
assessed analytically. If we model the frame edges as bars with uniform cross-sections,
then the force needed to compress the frame to produce an inward deflection of 6 is shown
in equation 17, where A is the cross-sectional area of the frame edge, L is the length of the
frame, and E is the elastic modulus of silicon.
P AE (17)
Assuming an elastic modulus of approximately 190 GPa for the silicon substrate, a length
of 18 mm, and a cross-sectional area of 0.40 mm 2, it would require maintaining a
compressive force of approximately 6.0 N (assuming 2 frame edges) to achieve the
projected compression required, on the order of 40 tm, as predicted with the experimental
model in section 3.3.2.
The value for P estimated using equation 17 should be compared to the critical buckling
load defined by equation 18 for a clamped-clamped beam. While the intention of applying
force P to the frame is to compress, buckling in the frame itself is not desirable.
4 iT 2 EI (8Pcritical = 4 (18)
Assuming the same geometric and material properties stated when calculating the
compressive force, the critical buckling load is approximately 220 N from equation 18.
Therefore, the compressive force P (6.0 N) is well below the critical buckling load.
Similarly, the feasibility of bending the frame inwards to bring the opposite ends closer
together can be assessed analytically. In order to do so, the frame must be considered in
detail. Upon bending the silicon frame, the length along the neutral axis of the frame will
theoretically neither elongate nor shrink in length. However, the inner face of the frame
radially closer to the curvature resultant from bending will be under compression and will
53
decrease in length. Conversely, the outer face of the frame is under tension and will
elongate, as illustrated in Figure 3-11.
radius
of curvature
M M
Figure 3 - I I Inner Face of Frame under Compression
If the frame edges are modeled as beams subjected to a moment M, as shown in Figure 3-
11, then the curvature of the frame K can be approximated as a function of that moment as
well as the elastic modulus of silicon Es; and the moment of inertia of the frame Jf, in
equation 19.
M
K= (19)
Esi Ifr
By taking the inverse of the curvature found using equation 19, the radius of curvature of
the bent frame can be estimated. Given the frame thickness is known, the change in length
of the inner face can be approximated as well.
Based on the experimental model, a change in length of the inner face required is
approximately 40 pm. The radius of curvature needed in the bent frame for such a length
54
change can be calculated by finding the difference between the extended frame in equation
(20) and bent frame, as shown in equation (21). The extended length and desired bent frame
distance are both known, so these two equations can be solved simultaneously to find the
radius of curvature.
Lextended = 0 27T rcurvature (20)
Lbent = 2 rcurvature sin ( (21)
Solving these equations simultaneously gives a radius of curvature of 77.5 mm, or a
curvature of 12.9 m-. Based on this calculation, the moment found using equation 19 is
determined to be approximately 0.025 N-m.
3.4.2 Achieving Buckling by Frame Modification
If the constraint of developing a post-fabrication buckling process for the continuous
silicon frame of the baseline device is lifted, then there is considerably more design
freedom to work with. When designing a new frame geometry, however, the functional
requirements of the frame still need to be kept in mind. As illustrated in Figure 3-9, the
current frame not only provides structural support to the beams suspending the proof mass
but also houses the electrically active contact pads from which power is transmitted when
the device is actively harvesting energy. A new frame design must still house these contact
pads suitably and also provide the same structural support for the device. Not only does the
frame act as a support to the device during use and testing but also the frame is often times
a fixture point which is used to transfer individual devices during the fabrication process
without touching the delicate piezoelectric beams. The functional requirements for the
redesigned frame can be listed as follows:
55
* Provide ample surface area for electrical contact pads
" Structurally support the micro-scale beam array
" Facilitate adjustability with respect to frame width
Based on these defined functional requirements, an optional fourth requirement can be
added which splits potential designs into two general categories. The frame width
adjustment can either have discrete positions or have a continuous range of adjustability.
3.5 Proposed Designs
Based on the idea exploration throughout various design spaces given different constraints,
two parallel paths were chosen to pursue. First, a fixture was conceived to bend the device
frame to induce buckling in the piezoelectric beams. Second, a novel frame geometry was
designed to allow a snap-fit, one-time frame length modification. Both of these designs are
elaborated upon in this section.
3.5.1 Experimental Frame Bending Fixture
The following fixture design was developed for concept proof and experimentation. The
concept driving the development of the frame bending fixture was that the frame length
could be made shorter by applying a moment on either end of the frame to compress the
face on the inner side of the resultant curvature. The fixture design presented here utilizes
controlled bending of a cantilevered aluminum beam upon which the device is fastened,
shown in Figure 3-12. The dotted silhouette of the beam and MEMS device subassembly
represents the unbent mode.
56
- --U11_
MEMS Device
Aluminum Beam
Steel Wedge
Clamp Fixture
UO'"0,00
Figure 3 - 12 Frame Bending Fixture Schematic (Side View)
When the steel wedge is driven under the aluminum beam, the beam lifts with the
increasing height of the wedge. This bends the entire cantilevered aluminum beam. Figure
3-13 shows a close-up view of the interface between the MEMS device frame and the
aluminum beam. With a properly mated surface interface, the device frame will bend with
the same curvature as the aluminum beam, inducing buckling in the piezoelectric beams.
MEMS Device Frame
Aluminum Beam
* ~*uuSamoa**%*5S
'S..
Figure 3 - 13 Aluminum Beam - Device Frame Interface (not to scale)
57
M 
.................... 
. ...................................... 
X
NIN9991"WwwrMp . ...................................... * 7Z
00
0000
0000,0 
40 00
Figure 3 - 14 Isometric Rendering of Experimental Frame Bending Fixture
Figure 3 - 15 Side Rendering of Experimental Frame Bending Fixture
58
3.5.2 Snap-Fit Corrective Frame Fixture Design and Process
While the experimental frame bending fixture was effective in proving the concept of frame
bending translating to buckling in the MEMS membranes of the energy harvesting device
in that it had a large range of buckling made possible by the adjustable wedge, it was geared
towards an experimental application. Then subsequent iteration of design focusing on
achieving buckling post-fabrication is a scalable process utilizing a monolithic corrective
frame modification process.
Based on analytical and experimental modeling results of the device frame bending, a
known curvature i can be identified within a certain tolerance as the benchmark curvature
needed to induce desired buckling in the MEMS beams. The experimental fixture design
utilizes a strong bond between a bent cantilevered beam and the device to couple bending
in the fixture to the silicon frame. This can be achieved more reproducibly and precisely
by a fixture rigidly cast in the geometric form of the curvature.
A schematic of the proposed "snap-fit" fixture is shown below in Figure 3-16. The
illustration is not to scale, and the curvature is exaggerated in order to demonstrate
conceptual function.
P (dist.)
Figure 3 - 16 Snap-Fit Schematic
59
-- '-I
Such a design utilizes the same model of curvature presented in 3.4.1. In this case, however,
the parameter of interest is the appropriate force applied on the frame against the mold
fixture. The frame can be modeled as beam, where the desired deflection due to bending is
known and the applied force is of interest. The boundary conditions of the beam can be
considered as free supports that just provide vertical resistance to the applied force. A free
body diagram detailing the mechanical deformation of the frame is shown below in Figure
3-17. The distributed force P is lumped as a point load mid-way along the beam.
L
....................................................................
PI
t
F
t
F
Figure 3 - 17 Snap-Fit Model Free Body Diagram
By summing forces in the z direction, we can find that the reaction force at each support is
half that of P. Given the desired curvature of K, we can determine the necessary deflection
and respective force needed. At the center of the beam, the moment is PL/4. From the
equation 22 below, curvature is related to moment through the elastic modulus and beam's
moment of inertia.
M= KE I (22)
60
Therefore, the necessary force required lumped at the middle of the frame model can be
found by solving for P.
P= 4KEI (23)
L
Knowing the frame length is 18 mm, curvature is 12.9 m-, using an elastic modulus of
silicon as 190 GPa, the necessary force P can be estimated as 5.51 N.
This frame modification process is a kind of dry molding technique to elastically deform
the frame. The mold against which the device is deformed and which defines the radius of
curvature is in the form of a half-pipe. Along with the snap-fit fixture design, a standard
operating procedure (SOP) must been developed to detail the order of operations for how
such a MEMS device can be modified at production scale. This SOP will supplement the
fixture design as a defined method of executing the post-fabrication beam bending
successfully and take into account operator technique and other variables to maximize
efficiency of frame bending. Multiple renderings of the fixture design are displayed in
Figures 3-18-20.
61
Exploded View of Snap-Fit Fixture
Figure 3 - 19 Cross Sectional View of Snap-Fit Fixture
62
I 1 I
Clamp
U~
k
K ~
2
evice
Mold
Figure 3 - 18
CMold
Figure 3 - 20 Detail Cross-Section View of Device / Mold Interface
3.6 Testing
Of the proposed designs, the frame bending fixture was selected for construction and
testing as it did not require major changes to the microfabrication process, so pre-fabricated
MEMS devices could be used for testing the effectiveness of inducing buckling.
Additionally, the frame bending fixture design allowed for a continuous range of induced
buckling deflection which was desirable from a testing standpoint.
3.6.1 Fixture Construction
The frame bending fixture was constructed according to the design proposed in section
3.5.1. An aluminum beam of thickness 1.6 mm was waterjet cut to a width of 22 mm with
a square aperture of dimensions 12.75 mm by 12.75 mm in the center such that the beam
did not interfere with the motion of the piezoelectric beams and proof mass during dynamic
testing. The MEMS device was adhered in position to the aluminum beam with epoxy
63
adhesive. One end of the aluminum beam was clamped to a steel base and the other end
remained free, as shown in Figure 3-21 and Figure 3-22 Steel sheets of thickness 0.25 mm
were incrementally inserted beneath the free end to raise its vertical position and actuate
the bending in the device.
Aluminum Beam
MEMS Device
- - Steel Base
Figure 3 - 21 Frame Bending Fixture
& Steel Wedge
Aluminum Beam
MEMS Device
Steel Base
Figure 3 - 22 Frame Bending Fixture, Side View
The MEMS energy harvesting device used for testing did not have any measurable
buckling in its immediate post-fabrication state. Due to process issues, the residual stresses
64
of the batch from which this device was released were not balanced correctly resulting in
a device that was ideal for testing on the fixture presented in this section.
A closer visual of the mounted device on the fixture qualitatively indicated that as a result
of the bent frame, the piezoelectric beams were buckled. Two distinct modes of buckling
were observed, shown in Figure 3-23, correlating to the two positions of stability in the bi-
stable buckled beam device. By imparting an impulse on the fixture with an action such as
a tap of tweezers on the aluminum beam, the beams snapped back and forth between
positions of stability.
Figure 3 - 23 Two Positions of Buckling Observed on Frame Bending Fixture
65
3.6.2 Results
The frame bending fixture was tested statically and dynamically. Statically, the actual
buckling deflection of the proof mass experienced was measured. Dynamically, the entire
fixture was mounted on an electromagnetic shaker and the acceleration of the proof mass
was measured while varying the excitation frequency, to verify the resonance of the MEMS
device was in the desired range.
Buckling deflection was measured using an optical surface profilometer. Profile
measurements of the aluminum beam, the device frame, the piezoelectric beams, and the
proof mass were taken. The proof mass and piezoelectric beams' profiles were measured
in both positions of stability in order to understand the buckling deflection on either side
of the neutral plane. As illustrated in Figure 3-24, the proof mass deflected downwards at
an average 207 pm, and deflected upwards at an average 246 pm, for a total range of
vertical motion of 453 pm, within 13% of the desired 200 pm.
66
1400
1200
1000
800
600
400
200
0
0 2 4 6 8 10 12
Distance along Frame (mm)
14 16 18 20
Figure 3 - 24 Profilometer Measurement of Beam Buckling
Due to the nature of the fixture setup, the cantilevered mode of fixing the aluminum beam
meant that raising the free end resulted in the profiles appearing exaggeratedly inclined
when viewed at a high level of granularity in z - height.
Dynamic testing was executed by mounting the frame bending fixture assembly on an
electromagnetic shaker. The acceleration of the shaker was measured using an
accelerometer, and a signal generator controlled the shaker's vibration frequency and
amplitude. The movement of the proof mass relative to the device frame was measured
using a laser vibrometer. Two different types of dynamic testing were conducted. First, the
electromagnetic shaker swept through a range of frequencies from 1 Hz to 1000 Hz at 1 G
in order to identify the resonant peak of the device by measuring the proof mass
acceleration in the frequency domain. The results of this test are shown in Figure 3-25.
67
Profile of Buckled MEMS Device
Proof Mass Device Frame
- --- -Aluminum Beam
Proof Mass
- I -
- - .1
a
Based on the frequency sweep, the device resonance has a relatively wide bandwidth in the
range of 0 to 100 Hz.
3.5
. - --
F3 ur 3-25AceertinofPro Mss reuec DmanRepos
F.5w g e frqunc sweemesureent teerifni the diw
N
stb1t was 1isal apar9 t Figure4 3-26 sh3 w the S64 ag measurement in4he6time
d Figufre a three-secdextation af Pro Hzasd F.5qG.nTheDopein Rsponpsenta
difestene ordesrtoinfy ehathe firameibnig obseraie, and ot oaparnts thth ectonact
and earecingrslsnre voltage spikvbrtngte hae ate aonat The reuen -f this tests
acceptable electric function of the device while in the frame bending fixture assembly.
68
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
3Hz Excitation - Time Domain Voltage Measurement
0 0.5 1 1.5
Time (seconds)
2 2.5 3
Figure 3 - 26 Device Electric Function Verification (open circuit voltage)
Based on the promising results of the electric function and the resonance at excitations
below 100 Hz, the device mounted in the frame bending fixture assembly was tested at
frequencies from 10 Hz to 100 Hz for open circuit voltage and proof mass displacement at
intervals of 10 Hz, all at I g. Electric function was verified at these operating frequencies.
Data has been plotted and is attached in Appendix A.2.
69
-- --- - - -I-
4
70
Chapter 4 Energy Harvesting Footwear
4.1 Introduction to Wearable Energy Harvesting
The human body maintains a diverse portfolio of physical activity. Certain human
processes, such as the blood coursing through veins and arteries, run continuously in the
background, while others, such as sprinting, occur as acutely executed functions of motion.
Regardless of the nature of each activity, every physical motion of the human body presents
an opportunity to harvest energy and power a portable device without relying on a battery
in remote areas [16]. Of these activities, the impact of the human foot on the ground during
walking and running presents the greatest occasion to harvest energy in terms of energy
expenditure [17]. While bipedal movement is not a continuous activity, its occurrence is
correlated with the need for electric power. This raises the challenge of converting foot-
falls into usable electric power.
Regular walking motion at 1.3m/s (3.0mph) typically results in each foot striking the
ground at a frequency of approx. 1 Hz for an adult. According to previous investigations,
the dynamical vertical force can be up to 130% of the person's body weight [21]. Assuming
each foot is lifted just 1cm from the ground, there is a potential energy of approx. 9.6 J for
a 75kg adult dissipated with every footstep considering only the vertical motion of the foot.
In order to prevent the discomfort of walking on a sponge or swamp, the maximum power
harvested should not be more than 10% of the foot energy. The challenge presented by this
opportunity is to harvest the foot strike energy as efficiently as possible up to 100 mW (for
1Hz walking), despite unpredictable nature of human motion [18].
The early and most intuitive method of scavenging energy from footsteps utilized
piezoelectric materials embedded in areas of footwear that experience the most strain.
Shenck and Paradiso demonstrated a method of harnessing footstrike energy
piezoelectrically by inserting a PZT foil into the heel area of a shoe [19]. An average power
71
of 8.4 mW for the PZT device was reported for a foot strike frequency of 0.9 Hz. The
benefit of such a design is its unobtrusiveness with respect to the user's ergonomic
experience with the product. However, the magnitude of energy that can be harnessed is
limited by the use of piezoelectric devices which rely on mechanical strain created by the
heel inside the shoe to create a potential difference. Moro et al. also demonstrated an energy
harvesting shoe design with a system of piezoelectric vibrating cantilevers, but power
generated was restricted by the low frequency of foot impact [20]. Above all, the major
limitation to piezoelectric energy harvesting from walking is that the total foot energy
cannot be converted to the strain of the piezoelectric material without complex additional
mechanisms.
We previously developed an air pump-turbine-based system to solve the difficulties of
harvesting energy from foot strikes by absorbing the foot strike energy with deformation
of air bulbs and utilizing an airflow speed amplification mechanism to drive a micro-
turbine [18]. The turbine was activated by airflow and operated with a high frequency,
enabling the foot-strike energy to be harvested effectively. The turbine casing was designed
specifically to enable the device to operate continuously with airflow from both directions
[21]. A prototype was fabricated and then tested under different situations. A 6 mW peak
power output was obtained with a 4.9 Q load for each foot strike. Considering that a
maximum of 100 mW could be potentially harvested as described above, there was a room
for improvement.
This work presents optimized components and system design to solve these challenges by
using footsteps to compress nylon lungs embedded in a shoe sole to pneumatically drive
miniature turbines and power small generators for electric power. A combat boot has been
developed to provide electric power to a passive GPS tracker that can periodically ping
geographical coordinates to a satellite relay this information to a command center.
72
4.2 Mesoscale Energy Harvesting System Design
The system for harvesting energy presented in this chapter converts able-bodied bipedal
locomotion to usable electricity by way of unobtrusive hardware installed in footwear to
generate airflow from the compressive forces applied by feet inside shoes during walking
[24]. A diagram of this energy harvesting system embeddable in different forms of
footwear is displayed in Figure 4-1. Air bulbs are positioned in the sole, optimized relative
to the length of the foot. Outflow from the bulbs is directed towards a two-stage turbine
enclosure which drives two DC motors in series to generate electrical power. This energy
is stored in a supercapacitor and used to charge a battery which powers a GPS receiver.
4.2.1 Air Bulb Design and Placement
The key design parameters considered for the air bulbs were their effective stiffness and
time period of regaining original shape after being compressed. The bulbs' effective
stiffness was bound by the requirements that the bulbs not only compress under the user's
weight but also self-reset to their original decompressed form and do so fully before the
user takes another step. The time available to reset depends on the frequency of foot-strikes
during motion which varies with gait and speed of walking or running. The resultant time
allowance for the air bulb to regain its initial shape can be as little as 0.3 seconds [22] when
stepping with higher frequency. The actual air bulbs selected for testing were large nylon
hand pumps used in sphygmomanometers, or blood pressure meters. The volume of each
air bulb is 110 cubic centimeters, with length of 7.5 cm and diameter of 4 cm. When
installed in the boot prototype, the bulbs were positioned with a tight tolerance with respect
to each other and the supportive structure of the shoe interior, but still retained their shape
when sitting neutrally in the shoe. Three bulbs were used for prototyping. This bulb type
was only used for the proof-of-concept. Future design iterations will include embedded
bulbs in the insole or outsole to minimize the discomfort to the user.
73
GPS Transmitter
Power Management Module
Turbine Enclosure
Air Bulbs (3x)
Insole
Rubber Tubing
Boot
Figure 4 - 1 Schematic of Footwear Energy Harvesting System
Optimal placement of the air bulbs along the shoe was determined by two design
requirements. The first design requirement was that chosen array of air bulbs produce the
maximum air outflow with each step. The second design requirement was that the air bulbs
be positioned such that time period during which air outflow was generated was maximized
during each step cycle. Furthermore, each additional air bulb added to the system increases
the effective stiffness of the entire bulb array. Therefore, the design decision to add each
additional air bulb to the system was based on the justification that the marginal
contribution of airflow to the turbine enclosure was greater than the resultant decrease in
airflow from the array when compared with the original configuration. Testing, detailed
below and summarized in Figure 4-2, found three bulbs to be optimal given the stiffness
and volume of the off-the-shelf nylon air bulbs selected for this purpose.
Positioning the air bulbs with respect to maximizing air outflow was maximized by
identifying high pressure regions of the foot during walking. Although the morphology of
human feet varies greatly among individuals, clinical research has shown that the pattern
of weight distribution on the foot of an able-bodied human generally has maximums in the
heel, ball, and toe regions of the foot [23]. In order to verify this pattern was generally
accurate for the test user of the prototype, the test user stood on a sheet of pressure
74
indicating sensor film which confirmed same three regions as bearing the weight of the
body.
In order to quantify how varying air bulb position along the length of the shoe directly
affected energy harvesting, a single air bulb-turbine-generator assembly was developed
specifically for this setup. The relative change in airflow outputted by the air bulb
depending on location in the shoe was measured by observing the peak open-circuit voltage
from the generator. The test setup was incrementally shifted along the length of the shoe
and compressed by the weight of the human test subject at each location. The results of this
test are presented in Figure 4-2. In order to maximize the time period of air outflow during
each step cycle, spacing between groups of air bulbs was introduced to create sequential
compression of the air bulbs based on the gait of human walking motion. Based on these
design requirements, two air bulbs were positioned in the heel area of the sole, and one in
the ball area.
Optimal Air Bulb Placement
Heel
i/ Ball
L -t 4 i.oIIrbe
0 10 20 30 40 50 60 70
Percentage of Length along Shoe Sole
80 90 100
Figure 4 - 2 Peak Power relative to Length along Shoe
75
6K,
500
~400
0~
200
100
n
4.2.2 Turbine Enclosure Geometry
The turbine enclosure was designed to accept flow from either the inlet (exhalation of the
air bulbs during compression), or the outlet (inhalation of the bulbs during decompression),
while maintaining revolution of the turbines in the same rotational direction regardless of
the flow direction. Figure 4-3 depicts a drawing of the turbine enclosure with the top
enclosure translucent to illustrate how the symmetric positioning of the two turbines allows
conversion of airflow to rotational kinetic energy regardless of which inlet the air enters.
The previous iteration of turbine enclosure design also geometrically enabled flow from
opposite directions to rotate a single turbine in the same anti-clockwise direction, but this
feature was not used to harvest energy during air bulb inhalation on the off-step. The air
bulbs used in the previous iteration were off-the-shelf pumps for inflating the sleeve of a
blood-pressure meter and had embedded check valves ensuring that a dedicated outlet only
allowed airflow outwards. For the prototype presented in this chapter, the check valves
were removed, and the bulb inlet was plugged to force the air bulbs to re-inflate after
compression by drawing air back through the turbine enclosure on the off-step.
1.0 cm
2.4 cm
3.5 cm
Figure 4 - 3 Radial Symmetry of Dual Turbine Enclosure allows for Bi-directional
Flow to continuously drive Generators
76
The turbines themselves were designed for efficiency of airflow harvesting by optimizing
blade number and internal diameter using computational fluid dynamics (CFD) [21]. The
turbine blades were offset by 8.5mm from the housing to prevent contact during use while
maintaining efficiency.
The entire energy harvesting system including air bulbs, tubing, connectors, turbine
components, and electronics has a mass of 130g. Each combat boot used for testing has a
mass of 770g, so the additional weight experienced by the user is 17% of the original
baseline shoe weight. Subsequent design iterations will eliminate the majority of mass from
PCBs and other meso-structures when the prototype is developed for mass-application
products. Since the bulbs will be embedded in cavities in the insole of the shoe, we target
a net mass increase on the order of approximately 20 grams.
4.3 Theoretical Calculations of Upper Bound Power
In our group's previous work [18], the theoretical upper limit power by the air bulb
compression and mini-turbine energy harvesting was modeled. We assumed the air bulb
displacement to be 10 mm, and an amplification factor resulting from the ratio of the
sectional area of the pump to that of the rubber tube inlet to be 6 = 500, then the velocity
of air entering the turbine can be estimated as 10 m/s for walking at 3.0mph.
The kinetic energy for the airflow entering the turbine at relatively high velocity can be
calculated from equation 24.
E = mav2 (24)
2
The mass of air is represented by ma and the velocity of airflow is v. The time period of
each step was measured to be approximately 0.2 sec for walking at 3.0 mph. In Figure 4-7
we observed the voltage signature output with respect to time to show working duration to
77
be approximately 1.0 seconds, which includes the inhalation time after the foot step up.
The calculation in equation 25 counts only the down strike of the foot on an individual air
bulb.
The energy from each step can be estimated as:
E = ma (6 vO)2 = 1(10-4 X 1.29)(500 x 0.2)2 = 0.645J (25)2 2
Therefore, the upper limit for the power to be theoretically generated from 1.0 Hz foot
strikes is approximately 645 mW.
78
4.4 GPS - Equipped Combat Boot Prototype
A product prototype was built with the intent of applying the energy harvesting concept
outlined in this chapter to a real-life opportunity. The prototype presented here uses the
harvested energy from walking to power a GPS module that relays geographic coordinates
when requested by an SMS message from a cell phone. The motivation for selecting GPS
functionality as the main application of this prototype was due to the expected use of such
a product in remote environments. Potential users of energy harvesting footwear will likely
be in an environment where marginal electric power can be valuable, meaning use of this
product would take place far from human infrastructure capable of delivering electric
power. Such users may include military personnel, emergency first-responders, and
outdoor recreational explorers.
A prototype energy harvesting system was built and installed in a combat boot, shown in
figure 4-4. Three air bulbs were sourced from off-the-shelf latex blood pressure
measurement pumps and embedded in the shoe sole at the optimized locations. The air
bulbs are transparently overlaid in figure 4-4; two bulbs are positioned in the heel while
one bulb is positioned in the ball region. Rubber tubing was used to direct air from the
bulbs to the turbine enclosure. The turbine enclosure and the turbines themselves were 3D
printed. The turbine diameter was 1Oimm, with a height of 5 mm. Each turbine was directly
connected to the shaft of a 1.5V DC motor used as generators. The turbine enclosure was
mounted externally to the outer side of the combat boot.
79
GPS Transmitter
Power Management Module
Turbine Enclosure
Air Bulbs (3x)
Figure 4 - 4 Prototype Combat Boot Fitted with Energy Harvesting System
For experimentally measuring power output, the motors were connected in series to a
resistor, and power was estimated by measuring potential difference across the resistor. For
functional demonstration, the motors were connected to a boost converter and
supercapacitor made by Advanced Linear Devices (EH4295), and the output power was
regulated by a power management circuit (EH300). The supercapacitor was used to charge
a Li-Po battery which powered an off-the-shelf GPS receiver. Power measurements were
conducted while a 75 kg test subject walked at different controlled speeds on a treadmill
while wearing the combat boot.
The off-the-shelf GPS receiver used for this prototype communicated on the 3G network.
The architecture of the GPS module used for the prototype can be broken down into two
functions: receiving a positioning signal from a satellite (to obtain geographic coordinates),
and transmission to the GSM network (to communicate the coordinates to a cellphone).
The off-the-shelf location tracker receives a low-powered microwave GPS signal from at
least three or four satellites (for 2D and 3D positioning respectively). The module hardware
amplifies the signal to increase gain over noise, and mixes, filters, and processes the signal
in order to convert to measurements of position and time. The GPS receiver took an average
80
of 12.6 seconds to communicate to the satellite, requiring 5.80 J of energy. Communicating
location in the form of an SMS message to a cellphone took 13.2 seconds and required 6.25
J of energy. This measurement was conducted by connecting the GPS module to an
oscilloscope and measuring current and voltage consumed during transmissions. The
values were averaged over 8 runs.
This high power-consuming 3G GPS is actually not necessary for the basic functions
provided by this prototype but was used for the ease of using off-the-shelf components for
testing and demonstration in certain parts of Asia where 2G is no longer available. Using
a custom-built 2G GPS unit with minimal energy needs in the future could reduce footsteps
needed per signal by an order of magnitude.
Potential strategies for reducing the power consumption of the GPS module include
transmitting the geographic coordinates for longitude and latitude to fewer decimal places
to reduce energy costs during transmission via GSM [25]. Additionally, separating the
receiving and transmitting processes into two charging cycles to create independency
between the two processes will add to the robustness of the design. An initial stage would
charge a power bank and releases enough energy to power the GPS receiver and log the
geolocational coordinate string for a microprocessor to store the data. The second stage
would allow the stored position coordinates to be transmitted. Furthermore, increasing the
efficiency of the module by customizing the digital signal processing algorithms and
running the microcontroller at a lower speed would reduce the power-consumption of the
hardware architecture, and allow for more frequent transmissions based on the same rate
of walking.
81
4.5 Results
The device was tested at different walking styles and speeds and voltage was measured
over a 22Q resistor. This resistance value was optimized by measuring peak power output
for a footstep cycle for various load resistances, shown in Figure 4-5.
E
41)
0
0~
a)
0~
1000
800
600
400
200
0
Optimizing Resistance of Dual Turbine System
0
Figure 4 - 5
5 10 15 20 25
Load Resistance ( Q)
Peak Power Output versus Values of Load
The human factor involved with harvesting energy from footsteps meant that such a device
would naturally be subjected to a variety of types of movement by users with varying
weights. Due to experimental constraints, a single user of weight 75 kg tested the prototype.
The prototype was tested on a treadmill at two different walking speeds (1.5mph and
3.0mph) which corresponded to foot strikes at frequencies of 0.5Hz and 1.0Hz respectively.
Voltage measured across a 22Q resistor for these cases is shown in Figures 4-6 and 4-7.
Additionally, power measurements were taken while the test subject lunged, attempting to
simulate maximum impact to the shoe. These voltage measurements are shown in Figure
4-8.
82
30
Resistance
I
Walking at 1.5mph
1 2
Time (s)
3 4
Figure 4 - 6
5
4
3
2
1
0
Voltage Output measured for 0.5 Hz Footsteps
Walking at 3.0mph
---
0.5 1 1.5 2 2.5 3 3.5 4 4.5
Time (s)
Figure 4 - 7 Voltage Output measured for 1.0 Hz Footsteps
83
5
4
3
1
0
0
54
3
0)
CM
1
0
-I~~
Lunging
).5 1 1.5 2 2.5 3 3.5 4
Time (s)
Figure 4 - 8 Voltage Output measured for Lunges
. 5
The power measurements in Figures 4-6-8 are based on measurements for voltage over the
known resistor load and estimated using the equation expressed in equation 26 relating
electric power P to potential difference V and load resistance R.
P =
R (26)
The peak power for these tests can be estimated by relating peak voltage and known load
resistance, 22K in these cases. The average continuous power was found by calculating the
total electrical energy generated over a six-second interval of use and dividing by time.
These values are displayed in Table 4-1.
Table 4 - 1 Power Generated for Various Modes of Movement
Movement Type Average Power (mW) Peak Power (mW)
0.5 Hz Footsteps 29.9 161
1.0 Hz Footsteps 86.4 516
Lunges 106 679
84
.....  ......
.1'
While power measurements were conducted while the test subject moved in controlled
motions on a treadmill, the functional demonstration of the GPS capabilities of the
prototype was conducted in a remote area on uneven terrain, allowing the user to create
random motions similar to those experienced during routine use. The off-the-shelf GPS
receiver used for testing required 15 minutes of activity in the combat boot before enough
energy was produced to send a text message with geographical coordinates.
From equation 25, the upper theoretical limit for power output by the turbine was 645 mW
at footstep frequency of 1.0 Hz. Based on the average power measurement in table 4-1, this
prototype has achieved capturing 13.4% of the foot strike energy. This value is comparable
to the initial goal of capturing 10% of the foot strike energy as a goal to balance maximizing
harvested energy and minimizing side effects on the user's experience while wearing the
shoe during use.
4.6 Discussion
This chapter presented an energy harvesting system designed to convert footsteps into
usable electric power. The device used an array of air bulbs which sequentially compressed
and decompressed during the user's walking motion, to continuously drive a series of
miniature turbines to generate energy. The turbine enclosure was designed specifically to
accept flow from opposite directions while ensuring the turbines themselves always turned
in the same rotational direction. As a result, at a moderate walking speed of 3.0 mph
(footstep frequency of approximately 1 Hz), there was no measurable down-time when the
turbines were not rotating and producing electric power.
Because the energy harvesting device depends on human movement, it is difficult to
quantify its performance with respect to a wide range of users. The prototype presented in
this chapter was tested on a 75 kg subject and generated an average continuous power
between 30 mW and 80 mW depending on the gait and speed of the subject's movement.
Approximately 15 minutes of varied forms of movement was required to generate enough
85
energy to send a GPS transmission detailing geographical coordinates, sent as a text
message to a cell phone.
If the airflow generation and air bulb compression is further optimized within the sole of
the boot, the power output can be improved to the theoretical prediction of the previous
section. Considering the GPS receiver used in this prototype required very high power for
transmission to satellites, this combat boot can be optimized to provide more power at each
step to a lower power consuming GPS in order to deliver transmissions more frequently.
Future work on this device will involve optimizing the circuit design by developing a
custom designed power management system and low-power GPS receiver to reduce the
number of footsteps required to operate as well as to improve the performance of the air
bulb-turbine efficiency. In addition, ergonomic factors not considered in this design such
as the effect of prolonged, repetitive use on the user's feet need to be taken into account
and incorporated into the final product-oriented design.
86
Chapter 5 Summary
5.1 Thesis Summary
This thesis has contributed to the objective of achieving low frequency vibrational energy
harvesting at the microscale and mesoscale. At the microscale, the microfabrication process
for producing piezoelectric bi-stable buckled beam-based energy harvesters was analyzed
and quality of processed piezoelectrically active material (PZT) was identified as an
opportunity for improvement. New batches of devices were fabricated with remnant
polarization of the PZT layer an order of magnitude larger than the previously reported
value measured for this device. The process capability of specific microfabrication steps
was also examined from a statistical process view point. The cumbersome method of
balancing residual stresses in each material layer of the device to achieve film buckling
was identified as a process of significant complexity. This process was moved to post-
fabrication, thereby allowing the microfabricated material stack to include just functional
layers. A post-fabrication buckling process by inducing bending in the device frame was
modeled and a fixture enabling this procedure was designed. An experimental iteration of
the fixture was built and tested with actual buckled beam energy harvesters in order to
prove the concept and validate its dynamic performance. Electrical function of the device
was preserved.
At the mesoscale, an energy harvesting method to convert energy dissipated through
walking to usable electricity for commercial application has been optimized and applied to
the purpose of powering a GPS receiver embedded in a combat boot. Air bulbs were placed
in the sole of the shoe to generate airflow from walking, which was used to drive a turbine
assembly on both the down strike and uplift of the foot's motion during walking and
jogging. The power generated was stored in a supercapacitor and used to charge a li-po
battery that powered a GPS receiver capable of geographic coordinates upon demand at
intervals of 15 minutes.
87
5.2 Future work
Future work on the MEMS vibrational energy harvesting device will focus on achieving
power generation on the order of micro-watts for frequencies below 100 Hz at amplitudes
below 1 g. Armed with the assets of high PZT material quality and a method of enabling
bi-stable buckled beams through post-process adjustment fixtures, this objective is now
possible to achieve with the next batch of fabricated devices. With a MEMS device capable
of delivering power at this level, commercial application is within reach. The energy
harvester can be integrated with a low-power sensor and transceivers to create a nodal
sensory network powered by vibrations from the ambient environment. Other Internet of
Things applications are also open to exploration with this novel energy harvesting
opportunity available. The longevity and durability of this MEMS energy harvester with
respect to fatigue over high cycle use must also be investigated further.
Future work on the energy harvesting footwear design involves developing a unified shoe
design with cavities and other embedded features to reduce the mass of the added
components from the energy harvesting system. Furthermore, studying the ergonomics of
the energy harvesting shoe is important in understanding any side effects marginal damping
may have on human walking. From an electronics stand point, an application-specific GPS
receiver with more efficient power electronics and communication algorithms must be
developed. Most electronics components used for the prototype demonstration were off-
the-shelf products. Optimization of a customized voltage boosting, charge storing, and GPS
receiving system can reduce the number of steps required to relay geographic coordinates.
Other applications beyond GPS must also be investigated including device charging and
variable shoe stiffness among others.
88
89
Appendix A
A.1 PECVD Process Capability Study Data
PECVD process capability study data
Run 1
Deposition Deposition Sample Location 1, Location 1, Location 2, Location 2,
Rate [nm / Time Measurement Measurement Measurement Measurement
min] [min] I [nm] 2 [nm] I [nm] 2 [nm]
50 3 1 231 234 246 245
50 3 2 240 243 247 250
50 3 3 232 233 260 260
50 3 4 235 237 232 234
50 3 5 247 251 239 234
50 3 6 236 242 267 263
50 3 7 236 235 232 234
50 3 8 233 230 246 252
50 3 9 240 242 236 233
50 3 10 233 230 237 243
Run 2
Deposition Deposition Sample Location 1, Location 1, Location 2, Location 2,
Rate [nm / Time Measurement Measurement Measurement Measurement
min] [min] I [nm] 2 [nm] I [nm] 2 [nm]
50 3 1 247 241 249 251
50 3 2 243 240 260 259
50 3 3 239 238 240 237
50 3 4 238 243 239 243
50 35 241 237 243 234
50 3 6 238 238 245 241
50 3 7 236 238 234 240
50 3 8 237 238 251 244
50 3 9 238 234 239 233
50 3 10 243 239 239 246
90
Run 3
Deposition Deposition Sample Location 1, Location 1, Location 2, Location 2,
Rate [nm / Time Measurement Measurement Measurement Measurement
min] [min] I [nm] 2 [nm] 1 [nm] 2 [nm]
50 3 1 253 253 256 256
50 3 2 253 255 251 249
50 3 3 251 249 255 251
50 3 4 254 251 255 260
50 3 5 263 262 266 247
50 3 6 254 256 261 260
50 3 7 252 251 243 246
50 3 8 242 237 241 243
50 3 9 245 244 248 248
50 3 10 246 238 257 262
Run 4
Deposition Deposition Sample Location 1, Location 1, Location 2, Location 2,
Rate [nm / Time Measurement Measurement Measurement Measurement
mi] [min] n 2 [nm] I [nm] 2 [nn]
50 2 1 152 155 155 150
50 2 2 154 155 159 156
50 2 3 140 142 141 144
50 2 4 156 151 153 156
50 2 5 163 160 159 163
Run 5
Deposition Deposition Sample Location 1, Location 1, Location 2, Location 2,
Rate [nm / Time Measurement Measurement Measurement Measurement
min] [min] I [nm] 2 [tim] I [nm] 2 [nm]
50 4 1 333 335 333 333
50 4 2 339 338 342 339
50 4 3 338 336 334 335
50 4 4 333 330 331 331
50 4 5 330 330 327 325
91
Run 6
Deposition Deposition Sample Location 1, Location 1, Location 2, Location 2,
Rate [nm / Time Measurement Measurement Measurement Measurement
min] [min] I [nm] 2 [nm m] 2 [nm]
100 2 1 165 162 170 172
100 2 2 164 162 169 162
100 2 3 159 163 159 161
100 2 4 159 169 163 167
100 2 5 166 167 162 168
Run 7
Deposition Deposition Sample Location 1, Location 1, Location 2, Location 2,
Rate [nm / Time Measurement Measurement Measurement Measurement
min] [min] 1 [nm] 2 [nm] 1 n 2 [nm]
100 4 1 351 349 348 344
100 4 2 353 355 357 355
100 4 3 364 363 364 364
100 4 4 354 354 356 355
100 4 5 364 366 364 365
92
A.2 Electric Testing of Experimental Frame Bending Fixture
Open circuit electric testing of buckled beam MEMS device mounted on experimental
frame bending fixture.
Voltage Measurement
10 20 30 40 50 60
Frequency (Hz)
70 80 90 100
Figure A.2-1 Open Circuit Peak Voltage at varied operating Frequency
Proof Mass Displacement
0.6
0.5
E 0.4
E
-. 3
0 0.3E
U
CL 0.2
0.1
0
10 20 30 40 50 60
Frequency (Hz)
70 80 90 100
Figure A.2-2 Proof Mass Displacement at varied operating Frequency
93
bCu
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
A L
'SN
V
ee-100,
0
Bibliography
1. Ray Xu. Low-Frequency, Low Amplitude MEMS Vibration Energy Harvesting. PhD
thesis, Massachusetts Institute of Technology, 2010.
2. Katherine Marie Smyth. Piezoelectric Micro-machined Ultrasonic Transducers for
Medical Imaging. PhD thesis, Massachusetts Institute of Technology, 2017.
3. Stephen D. Senturia. MicrosYstem Design. Kluwer Academic Publishers, 2001.
4. Jaffe, Bernard. Piezoelectric ceramics. Vol. 3. Elsevier, 2012.
5. Kholkin, A. L., N. A. Pertsev, and A. V. Goltsev. "Piezoelectricity and crystal
symmetry." Piezoelectric and Acoustic Materials for Transducer Applications.
Springer, Boston, MA, 2008. 17-38.
6. Beeby, S. P., M. J. Tudor, and N. M. White. "Energy harvesting vibration sources
for microsystems applications." Measurement science and technology 17.12 (2006):
R175.
7. A. Hajati and S.-G. Kim, "Ultra-wide bandwidth piezoelectric energy harvesting,"
Appl. Phys. Lett., vol. 99, no. 8, p. 83105, 2011.
8. Pillatsch, Pit, Eric M. Yeatman, and Andrew S. Holmes. "A piezoelectric frequency
up-converting energy harvester with rotating proof mass for human body
applications." Sensors and Actuators A: Physical 206 (2014): 178-185.
9. Edamoto, M., et al. "Low-resonant-frequency micro electret generator for energy
harvesting application." Micro Electro Mechanical Systems, 2009. MEMS 2009.
IEEE 22nd International Conference on. IEEE, 2009.
10. F. Cottone, et al., "Piezoelectric Buckled Beams for Random Vibration Energy
Harvesting," Smart Materials and Structures, 21(3):035021, 2012.
11. R. Xu, and S.-G. Kim, "Modeling and Experimental Validation of Bi-Stable Beam
Based Piezoelectric Energy Harvester." Energy Harvesting and Systems 3.4 (2016):
313- 321.
12. Y B Jeon, R Sood, J Jeong, and S Kim. MEMS power generator with transverse
mode thin film PZT. Sensors and Actuators A: Physical, 122(1):16-22, jul 2005.
94
13. R. Xu, H. Akay, S-G. Kim, Micro Buckled Beam Based Ultra-Low Frequency
Vibration Energy Harvester. Hilton Head Solid-State Sensors, Actuators and
Microsystems Workshop. 2018
14. Arman Hajati. Ultra Wide-Bandwidth Micro Energy Harvester. PhD thesis,
Massachusetts Institute of Technology, 2010.
15. Montgomery, Douglas C. Statistical qualit control. Vol. 7. New York: Wiley,
2009.
16. Yeatman, E.; Mitcheson, P. Energy Harvesting and Power Delivery. In Body Sensor
Networks; Yang, G.-Z., Ed.; Springer: London, UK, 2014; pp. 237-272.
17. Paradiso, J.A.; Starner, T. Energy scavenging for mobile and wireless electronics.
IEEE Pervasive Comput. 2005, 4, 18-27.
18. Fu, H.; Cao, K.; Xu, R.; Bhouri, M.; Martinez-Botas, R.; Kim, S.G.; Yeatinan, E.
Footstep Energy Harvesting Using Strike-Induced Airflow for Human Activity
Sensing. In Proceedings of the 2016 IEEE 13th International Conference on
Wearable and Implantable Body Sensor Networks, San Francisco, CA, USA, 14-17
June 2016.
19. Fu, H.; Xu, R.; Seto, K.; Yeatman, E.M.; Kim, S.G. Energy Harvesting from Human
Motion Using Footstep-Induced Airflow. 1. Phys. Conf Ser. 2015, 660, 012060.
20. Shenck, N.S.; Paradiso, J.A. Energy scavenging with shoe-mounted piezoelectrics.
IEEE Micro 2001, 21, 30-42.
21. Moro, L.; Benasciutti, D. Harvested power and sensitivity analysis of vibrating shoe-
mounted piezoelectric cantilevers. Smart Mater. Struct. 2010, 19, 115011.
22. Akay, H.; XLi, R.; Seto, K.; Kim, S.-G. Energy Harvesting Footwear. U.S. Patent
No.62/449208, filed October 2017.
23. Grieve, D.W.; Gear, R.J. The Relationships between Length of Stride, Step
Frequency, Time of Swing and Speed of Walking for Children and Adults.
Ergonomics 2007, 9, 379-399, doi: 10.1080/00140136608964399.
24. Hughes, J.; Clark, P.; Jagoe, R.R.; Gerber, C.; Klenerman, L. The pattern of pressure
distribution under the weightbearing forefoot. Foot 1991, 1, 117-124.
95
25. Perrucci, G.P.; Fitzek, F.H.P.; Widmer, J. Survey on Energy Consumption Entities
on the Smartphone Platform. In Proceedings of the 2011 IEEE 73rd Vehicular
Technology Conference (VTC Spring), Yokohama, Japan, 15-18 May 2011; pp. 1-
6, doi:10.1 109/VETECS.2011.5956528.
96