MEMS BASED VIBRATION ENERGY HARVESTING

THESIS REPORT

Submitted in partial fulfillment ofthe requirements for the award of M.Tech Degree in

Electronics and Communication Engineering (Applied Electronics andInstrumentation)

of the University of Kerala

Submitted by

VISAKH. V

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERINGCOLLEGE OF ENGINEERING

TRIVANDRUM

2013

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERINGCOLLEGE OF ENGINEERING

TRIVANDRUM

CERTIFICATE

This is to certify that this thesis report entitled MEMS BASED VIBRATION EN-ERGY HARVESTING is a bonafide record of the work done by Visakh. V, underour guidance towards partial fulfillment of the requirements for the award of Mas-ter of Technology Degree in Electronics and Communication Engineering (Signalprocessing), of the University of Kerala during the year 2013.

Dr. M.R. BaijuProfessor,Department of ECE,College Of Engineering, Trivandrum.(GUIDE)

Dr. Jiji C.V.Professor,Department of ECE,College Of Engineering, Trivandrum.(Stream Head and P.G. Coordinator)

Dr.Vrinda V.NairProfessor,Department of ECE,College Of Engineering, Trivandrum.(Head of the department)

ACKNOWLEDGEMENTS

I would like to express my sincere gratitude and heartful indebtedness to my

guide, Dr. M.R. Baiju , Professor,Department of Electronics and Communication En-

gineering, CET for his valuable guidance and encouragement given to me throughout

my thesis work.

I am thankful to Dr. Vrinda V.Nair, Head of the Department, Dr. Jiji C.V.

P. G. Coordinator and Thesis Coordinator, Department of Electronics and Communica-

tion Engineering for their help and support.

I also acknowledge other members of faculty in the Department of Electronics

and Communication Engineering and all my friends for their whole hearted cooperation

and encouragement.

Above all I am thankful to the Almighty.

Visakh. V

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ABSTRACT

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TABLE OF CONTENTS

1 INTRODUCTION TOMEMS 1

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 History of Mems . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 MEMS Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3.1 Basic MEMS Microfabrication Processes . . . . . . . . . . 3

1.3.2 Basic MEMS Manufacturing Technologies . . . . . . . . . 12

1.4 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2 Conclusion 17

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LIST OF FIGURES

1.1 Contact Photolithography . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Photolithography using Negative and Positive Photoresists . . . . . 6

1.3 Wet Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.4 Under Cut during Wet etching . . . . . . . . . . . . . . . . . . . . 10

1.5 Crystallographic Planes in Silicon . . . . . . . . . . . . . . . . . . 11

1.6 Reactive Ion Etching(RIE) . . . . . . . . . . . . . . . . . . . . . . 11

1.7 Deep Reactive Ion Etching(DRIE) . . . . . . . . . . . . . . . . . . 12

1.8 Major fabrication steps in the LIGA process . . . . . . . . . . . . . 14

1.9 Major steps in the LIGA process . . . . . . . . . . . . . . . . . . . 15

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LIST OF TABLES

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CHAPTER 1

INTRODUCTION TOMEMS

1.1 Introduction

MEMS is a process technology used to create tiny integrated devices or systems

that combine mechanical and electrical components. They are fabricated using inte-

grated circuit (IC) batch processing techniques and can range in size from a few mi-

crometers to millimetres. These devices (or systems) have the ability to sense, control

and actuate on the micro scale, and generate effects on the macro scale.

The interdisciplinary nature of MEMS utilizes design, engineering and manufac-

turing expertise from a wide and diverse range of technical areas including integrated

circuit fabrication technology, mechanical engineering, materials science, electrical en-

gineering, chemistry and chemical engineering, as well as fluid engineering, optics,

instrumentation and packaging. The complexity of MEMS is also shown in the exten-

sive range of markets and applications that incorporate MEMS devices. MEMS can

be found in systems ranging across automotive, medical, electronic, communication

and defence applications. Current MEMS devices include accelerometers for airbag

sensors, inkjet printer heads, computer disk drive read/write heads, projection display

chips, blood pressure sensors, optical switches, microvalves, biosensors and many other

products that are all manufactured and shipped in high commercial volumes.

MEMS, an acronym that originated in the United States, is also referred to as

Microsystems Technology (MST) in Europe and Micromachines in Japan. Regard-

less of terminology, the uniting factor of a MEMS device is in the way it is made.

While the device electronics are fabricated using computer chip IC technology, the

micromechanical components are fabricated by sophisticated manipulations of silicon

and other substrates using micromachining processes. Processes such as bulk and sur-

face micromachining, as well as high-aspect-ratio micromachining (HARM) selectively

remove parts of the silicon or add additional structural layers to form the mechanical

and electromechanical components. While integrated circuits are designed to exploit

the electrical properties of silicon, MEMS takes advantage of either silicons mechani-

cal properties or both its electrical and mechanical properties.

In the most general form, MEMS consist of mechanical microstructures, mi-

crosensors, microactuators and microelectronics, all integrated onto the same silicon

chip.Microsensors detect changes in the systems environment by measuring mechan-

ical, thermal, magnetic, chemical or electromagnetic information or phenomena. Mi-

croelectronics process this information and signal the microactuators to react and create

some form of changes to the environment.

1.2 History of Mems

The historical progress of Mems is shown below [?]

1958: Silicon strain gauges commercially available

1961: First silicon pressure sensor demonstrated

1967: Invention of surface micromachining. Westinghouse creates the Resonant Gate

Field Effect Transistor, (RGT). Description of use of sacrificial material

to free micromechanical devices from the silicon substrate

1970: First silicon accelerometer demonstrated

1979 First micromachined inkjet nozzle

1982: Disposable blood pressure transducer

1982: Silicon as a Mechanical Material Instrumental paper to entice the scientific

community - reference for material properties and etching data for silicon.

1982: LIGA Process

1988: First MEMS conference

1992: MCNC starts the Multi-User MEMS Process (MUMPS) sponsored by

Defense Advanced Research Projects Agency (DARPA)

1992: First micromachined hinge

1993: First surface micromachined accelerometer sold (Analog Devices, ADXL50)

1994: Deep Reactive Ion Etching is patented

1995: BioMEMS rapidly develops

2000: MEMS optical-networking components become big business

1.3 MEMS Fabrication

MEMS fall into three general classifications; bulk micromachining, surface micro-

machining and high-aspect-ratio micromachining (HARM), which includes technology

such as LIGA (a German acronym from Lithographie, Galvanoformung, Abformung

2

translated as lithography, electroforming and moulding).

Conventional macroscale manufacturing techniques e.g. injection moulding, turn-

ing, drilling etc, are good for producing three dimensional (3D) shapes and objects, but

can be limited in terms of low complexity for small size applications. MEMS fabri-

cation, by comparison, uses high volume IC style batch processing that involves the

addition or subtraction of two dimensional layers on a substrate (usually silicon) based

on photolithography and chemical etching. As a result, the 3D aspect of MEMS de-

vices is due to patterning and interaction of the 2D layers. Additional layers can be

added using a variety of thin-film and bonding techniques as well as by etching through

sacrificial spacer layers.

1.3.1 Basic MEMS Microfabrication Processes

i Deposition Processes

Depositing thin films over the surface of substrates and other MEMS components

is a common practice in micromachining.Deposition adds thin films instead of consum-

ing the substrates unlike diffusion and thermal oxidation processes.

There are generally two types of deposition processes in micromachining. These

are (a) chemical vapor deposition (CVD) and (b) physical vapor deposition (PVD).

PVD involves the direct impingement of particles on the hot substrate surfaces. CVD,

on the other hand, involves convective heat and mass transfer as well as diffusion with

chemical reactions at the substrate surfaces.

a)Chemical Vapor Deposition

Chemical Vapor Deposition (CVD) involves the flow of a gas with diffused reac-

tants over a hot substrate surface. While the carrier gas flows over the hot solid surface,

the energy supplied by the temperature causes chemical reactions of the reactants.The

reaction of these species produces a solid phase, which is absorbed to the surface. The

by-products of the chemical reactions are removed. Continuous reaction causes a layer

of material to be built on the wafer surface.

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For example , deposition of Silicon Nitride on Silicon substrates is done by the fol-

lowing chemical reaction.

3SiH4 + 4NH3 Si3N4 + 12H2 (700 to 900 C)

Apart from CVD at atmospheric pressure(APCVD), there are two variants of

CVD. They are (1) Low pressure CVD (LPCVD) and (2) Plasma-enhanced CVD (PECVD).

In LPCVD , the reaction takes place at very low pressures of the order of 1 to 8 torr.

The reduction of gas pressure will increase the rate of deposition. PECVD utilizes the

radio-frequency (RF) plasma to transfer energy into the reactant gases which allows

the substrates to remain at lower temperature.

b) Physical Vapor Deposition

Sputtering is a process that is often used to deposit thin metallic films of the

order of hundreds of angstroms. Plasma is made of positively charged gas ion. The

positive ions of the metal in an inert argon gas carrier bombard the surface of the target

at high velocity that the momentum transfer on impingement causes the metal ions to

evaporate.The metal vapor is then led to the substrate surface and is deposited after

condensation.

ii Pattern Transfer

Integrated circuits and microfabricated MEMS devices are formed by defining pat-

terns in the various layers created by wafer-level process steps [?] . Pattern transfer

consists of two parts: a photo-process, whereby the desired pattern is photographi-

cally transferred from an optical plate to a photosensitive film coating the wafer, and a

chemical or physical process of either removing materials to create the pattern. Most

processes remove unwanted material by etching away chemically.

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Photolithography

Optical lithography is very much like the photographic process of producing a

print from a negative. The enabling materials of optical lithography are photoresists,

polymeric optically-sensitive materials that are deposited onto the wafer surface by

spin casting. Following spinning, the resists are prebaked at low temperature to re-

move solvent, but are not fully hardened. Completion of the hardening process occurs

after optical exposure.

Figure 1.1 illustrates the lithographic process analogous to contact printing.A

photomask contains the pattern to be transferred as a set of opaque and transparent re-

gions. It is brought into contact with an oxidized silicon wafer coated with photoresist.

Ultraviolet light is directed through the mask onto the wafer, exposing the unprotected

portions of the resist, which change their chemical properties as a result of the light

exposure.The photochemical processes in the photoresist are relatively high in contrast,

and develop sharp boundaries between exposed and protected regions. Contact lithog-

raphy is one of the standard processes used in MEMS manufacture.

Figure 1.1: Contact Photolithography

There are two types of photolithography as illustrated in figure 1.2a Negative pho-

toresist functions much like the photographic printing process. The regions of the pho-

toresist that are exposed to the ultraviolet light become cross-linked and insoluble in the

developer, while the protected regions remain soluble. After immersion in the developer

or exposure to a continuous spray of developer, the soluble portions are removed.

5

(a) Negative Photoresist

(b) Positive Photoresist

Figure 1.2: Photolithography using Negative and Positive Photoresists

The net result is a transfer of pattern into the photoresist so that after etching, the

6

opaque regions of the mask become regions cleared of photoresist.

To transfer the pattern into the oxide, the resist must first be hardened by baking to

make it more chemically inert. After this postbake, the silicon dioxide can be removed

by an etching process.Following the etching, the photoresist is removed, leaving the

mask pattern transferred into the oxide layer.

Positive photoresist works oppositely to negative photoresist. The chemistry of the

photoresist is different. Regions exposed to the UV light become more soluble in the

developer than the protected regions. After development and postbake, the protected

regions of resist remain on the wafer so that after etching, the opaque regions of the

mask remain as oxide and the clear regions are removed.

Because direct contact between the wafer and the mask can eventually cause damage

to the mask, a variant of the contact lithography is to leave a small air gap between the

mask and the photoresist-covered wafer. This is called proximity lithography [?]. The

achievable resolution is somewhat less than with contact lithography, because diffrac-

tion can occur at the edges of the opaque regions.When using contact lithography, the

mask must be the same size as the wafer, and every feature to be transferred must be

placed on the mask at its exact final size.

Electron Beam Lithography

Electron beam lithography ( e-beam lithography) is the practice of scanning a

beam of electrons in a patterned fashion across a surface covered with a film ( resist), [?]

("exposing" the resist) and of selectively removing either exposed or non-exposed re-

gions of the resist ("developing"). The purpose, as with photolithography, is to create

very small structures in the resist that can subsequently be transferred to the substrate

material, often by etching. It was developed for manufacturing integrated circuits, and

is also used for creating nanotechnology architectures.

The primary advantage of electron beam lithography is that it is one of the ways

to beat the diffraction limit of light and make features in the nanometer regime. This

form of maskless lithography has found wide usage in photomask-making used in pho-

7

tolithography, low-volume production of semiconductor components, and research and

development.

The key limitation of electron beam lithography is throughput, i.e., the very long

time it takes to expose an entire silicon wafer or glass substrate. A long exposure

time leaves the user vulnerable to beam drift or instability which may occur during the

exposure.

Track Technology

Ion track technology is a deep cutting tool with a resolution limit around 8 nm

applicable to radiation resistant minerals, glasses and polymers. It is capable to generate

holes in thin films without any development process. Structural depth can be defined

either by ion range or by material thickness. Aspect ratios up to several 104 can be

reached. The technique can shape and texture materials at a defined inclination angle.

Random pattern, single-ion track structures and aimed pattern consisting of individual

single tracks can be generated.

X-ray Lithography

X-ray lithography, is a process used in electronic industry to selectively remove

parts of a thin film. It uses X-rays to transfer a geometric pattern from a mask to a

light-sensitive chemical photoresist, or simply "resist," on the substrate. A series of

chemical treatments then engraves the produced pattern into the material underneath

the photoresist.

iii Etching

Etching is one of the most important processes in microfabrication.It involves the

removal of materials in desired areas by physical or chemical means. It establishes per-

manent patterns developed at the substrate by photolithography.There are two types of

etching techniques. They are (1) Chemical or Wet Etching and (2)Physical or Dry

Etching .

In plasma etching, high energy plasma containing gas molecules, free electrons, and

gas ions bombards the surface of the target substrate and knock off the substrate mate-

8

rial from its surface.

1)Wet Etching

Wet etching involves using solutions with diluted chemicals to dissolve substrates.

For example, diluted hydrofluoric (HF) solution is used to dissolve SiO2, Si3N4 etc.

whereas potassium peroxide (KOH) is used to etch the silicon substrates as described

in section (1.3.1)

In wet etching, the part of the substrate that is not covered by the protective mask

id dissolved in the etchants and removed.The etching can undercut the part that is im-

mediately under the protective mask after a lengthy period of time.

(a) Substrate in wet etching (b) Partially etched substrate

Figure 1.3: Wet Etching

Isotropic Wet Etching

Isotropic etchants etch the material at the same rate in all directions, and conse-

quently remove material under the etch masks at the same rate as they etch through the

material; this is known as undercutting . The most common form of isotropic silicon

etch is HNA,which comprises a mixture of hydrofluoric acid (HF), nitric acid (HNO3)

and acetic acid(CH3COOH). Isotropic etchants are limited by the geometry of the struc-

ture to be etched. Etch rates can slow down and in some cases (for example, in deep and

narrow channels) they can stop due to diffusion limiting factors. However, this effect

can be minimized by agitation of the etchant, resulting in structures with near perfect

and rounded surfaces.

9

Figure 1.4: Under Cut during Wet etching

Anisotropic Wet Etching

Anisotropic etchants etch faster in a preferred direction. Potassium hydroxide (KOH)

is the most common anisotropic etchant as it is relatively safe to use. Structures formed

in the substrate are dependent on the crystal orientation of the substrate or wafer. Most

such anisotropic etchants progress rapidly in the crystal direction perpendicular to the

(110) plane and less rapidly in the direction perpendicular to the (100) plane. The di-

rection perpendicular to the (111) plane etches very slowly if at all. Figures 19c and

19d shows examples of anisotropic etching in (100) and (110) silicon. Silicon wafers,

originally cut from a large ingot of silicon grown from single seed silicon, are cut ac-

cording to the crystallographic plane. They can be supplied in terms of the orientation

of the surface plane.

2)Dry Etching

10

Figure 1.5: Crystallographic Planes in Silicon

Dry etching relies on vapour phase or plasma-based methods of etching using

suitably reactive gases or vapours usually at high temperatures. The most common

form for MEMS is Reactive Ion Etching (RIE) which utilizes additional energy in the

form of radio frequency (RF) power to drive the chemical reaction. Energetic ions are

accelerated towards the material to be etched within a plasma phase supplying the ad-

ditional energy needed for the reaction; as a result the etching can occur at much lower

temperatures (typically 150 - 250 C) ,sometimes room temperature) than those usually

needed (above 1000 C). RIE is not limited by the crystal planes in the silicon, and as a

result, deep trenches and pits, or arbitrary shapes with vertical walls can be etched .

Figure 1.6: Reactive Ion Etching(RIE)

Deep Reactive Ion Etching (DRIE) is a much higher-aspect-ratio etching method

that involves an alternating process of high-density plasma etching (as in RIE) and

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protective polymer deposition to achieve greater aspect ratios.

Figure 1.7: Deep Reactive Ion Etching(DRIE)

Chemically reactive vapors are some times used as etchants.This technique is

called Vapor Etching.There is one vapor etchant that has become commercially impor-

tant in micromachining processes. The gas xenon diflouride,XeF2 is a highly selective

vapor etchant for silicon, with virtually no attack of metals, silicon dioxide, or other

materials [?]. As a result, it is ideal for the dry release of surface micromachined struc-

tures in which polysilicon is used as the sacrificial layer. This process is used in the

manufacture of the electrostatically actuated projection display chip .

1.3.2 Basic MEMS Manufacturing Technologies

i Bulk Micromachining

Bulk micromachining is an important class of MEMS process. In bulk microma-

chining processes, a portion of the substrate(bulk) is removed in order to create free-

standing mechanical structures (beams and membranes) or unique three-dimensional

features (such as cavities , through-wafer holes, and mesas).Bulk micromachining can

be applied to silicon, glass, gallium arsenide and other materials of interests.

There are two major categories of processes for bulk silicon etching. They are Wet

12

Etching and Dry Etching.Wet silicon etching processes use liquid chemical solutions

in contact with silicon as described in page 9. Dry etching processes use plasma (high

energy gas containing ionized radicals) or vapor-phase etchants to remove materials

as described in page 10. So the various microfabrication processes involved in bulk

micromachining technique are isotropic wet etching, anisotropic wet etching,Reactive

Ion Etching (RIE), Deep Reactive Ion Etching etc.

ii Surface Micromachining

Unlike Bulk micromachining, where a silicon substrate (wafer) is selectively

etched to produce structures, surface micromachining builds microstructures by de-

position and etching of different structural layers on top of the substrate [?]. Gener-

ally polysilicon is commonly used as one of the layers and silicon dioxide is used as

a sacrificial layer which is removed or etched out to create the necessary void in the

thickness direction. Added layers are generally very thin with their size varying from

2-5 Micro metres. The main advantage of this machining process is the possibility of

realizing monolithic microsystems in which the electronic and the mechanical compo-

nents(functions) are built in on the same substrate. The surface micromachined compo-

nents are smaller compared to their counterparts, the bulk micromachined ones.

As the structures are built on top of the substrate and not inside it, the substrates prop-

erties are not as important as in bulk micromachining, and the expensive silicon wafers

can be replaced by cheaper substrates, such as glass or plastic. The size of the substrates

can also be much larger than a silicon wafer, and surface micromachining is used to pro-

duce TFTs on large area glass substrates for flat panel displays. This technology can

also be used for the manufacture of thin film solar cells, which can be deposited on

glass, but also on PET substrates or other non-rigid materials.

iii High Aspect Ratio(HAR) Micromachining

HAR Micromachining is used to increase the aspect ratio of structures. The main

HAR process used now are LIGA and SLIGA processes.

LIGA process

The LIGA process for manufacturing MEMS and microsystems does not have the major

short comings of surface micromachining and bulk micromachining. These major short

comings are 1) low geometric aspect ratio and 2) the use of silicon-based materials.This

13

process offers a a great potential for manufacturing non-silicon-based microstructures.

The single most important feature of this process is that it can produce thick microstruc-

tures that have extremely flat and parallel surfaces such as microgear trains, motors and

generators.

The term LIGA is an acronym for the German terms Lithography (Lithographie),

electroforming (Galvanoformung), and molding(Abformung).

Figure 1.8: Major fabrication steps in the LIGA process

As shown in figure 1.8 , the LIGA process begins with deep x-ray lithography that

sets the desired patterns on a thick film of photoresist. X-rays are used as the light

source in photolithography because of their short wavelength, which provides higher

penetration power into the photoresist materials This high penetration power is neces-

sary for high resolution in lithography and for a high aspect ratio in the depth The short

wavelength of x-ray allows an aspect ratio of more than 100:1 to be achieved.

The LIGA process outlined in figure may be demonstrated bya a specific example as

illustrated in figure 1.9. The desired product in this example is a microthin-wall metal

tube of square cross-section.The process begins by depositing a thick film of photresist

material on the surface of a substrate as shown in figure 1.9 (b).A popular photoresist

material that is sensitive to x-ray is polymethylmethacrylate(PMMA). Masks are used

in the x-ray lithography. Most masking materials are transparent to x-rays , so it is

necessary to apply a thin film of gold to the area that will block x-ray transmission. The

thin mask used for this purpose is silicon nitride with a thickness varying from 1-1.5m.

14

Figure 1.9: Major steps in the LIGA process

The deep x-ray lithography will cause the exposed area to be dissolved in the sub-

sequent development of the resist material figure 1.9 (c). The PMMA photoresist after

the development will have the outline of the product, i.e. the outside profile of the

tube.This is followed by electroplating of the PMMA photoresist with a desired metal,

usually nickel, to produce the tubular product of the required wall thickness figure 1.9

(d). The desired tubular product is produced after the removal of the photoresist materi-

als(PMMA in this case)by oxygen plasma or chemical solvents.For most applicatioons

the desired product is metal molds for subsequent injection molding of microplastic

products as shown in figure 1.8.

SLIGA process

From figures 1.8 and 1.9, the finished product, whether it is a microstructure or

a metal mold, is attached to the substrate, or base plate.The attachment to the elec-

trically conductive substrate is necessary for the electroplating process. However, this

attachment is considered as a redundancy in the LIGA process. For instance, the hollow

15

square tube produced in the LIGA process as described in figure 1.9 would not be sep-

arated after electroplating of metal film on the inner walls . A modified process called

sacrificial LIGA (SLIGA) has been developed to solve this problem. The principle of

SLIGA is to introduce a sacrificial layer between the PMMA resist and the substrate

thereby to allow the separation of the finished mold from the substrate after the electro-

plating. The separation is achieved by the removal of the sacrificial layer by etching.

Polyimide with a metal-film coating is used as a common sacrificial layer material for

that purpose.

1.4 Applications

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CHAPTER 2

Conclusion

INTRODUCTION TO MEMSIntroductionHistory of MemsMEMS Fabrication Basic MEMS Microfabrication ProcessesBasic MEMS Manufacturing Technologies

Applications

Conclusion