i

A

SEMINAR REPORT

ON

“MEMS TECHNOLOGY”Submitted by

RIYAJ MIRZA

In partial fulfillment for the award of the degree

Of

BACHELOR OF TECHNOLOGY

(RAJASTHAN TECHNICAL UNIVERSITY, KOTA)

IN

ELECTRONICS AND COMMUNICATION ENGINEERING

UNDER THE GUIDANCE OF

PROF. HEENA JOSHI

AT

JODHPUR INSTITUTE OF ENGINEERING AND TECHNOLOGY

MOGRA, N.H. 65, PALI ROAD,

JODHPUR-342802

MARCH&2016

ii

Candidate’s Declaration

I hereby declare that the work, which is being presented in the Seminar, entitled “MEMS

TECHNOLOGY” in partial fulfillment for the award of Degree of “Bachelor of

Technology” in Dept Of Electronics & communication Engineering with Specialization

in Electronics and communication and submitted to the Department of Electronics

and Communication Engineering, Jodhpur Institute of Engineering and

Technology, Rajasthan Technical University is a record of my own WORK carried

under the Guidance of Prof.HEENA JOSHI Department of Electronics and

Communication Engineering, Jodhpur Institute of Engineering and Technology .

I have not submitted the matter presented in this Seminar anywhere for the award of any

other Degree.

NAME-RIYAJ MIRZA

Enrolment No.:12EJIEC096

Counter Signed by

Name of Guide Name of Incharge

PROF.HEENA JOSHI PROF.SHWETA BHATI

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ACKNOWLEDGEMENT

We deeply express our sincere thanks to our Head of seminar coordinator Prof.

SHWETA BHATI for encouraging and allowing us to present the project on the topic

“MEMS TECHNOLOGY”.

We would like to make a number of acknowledgements to those who have helped us to

prepare this Seminar.

We are highly grateful to Prof. O. P. VYAS, Dean (Engineering), JIET for proving us

this opportunity to carry out independent study on this topic.

The divine support given by our guide Prof. HEENA JOSHI and Prof. K. K. ARORA,

HOD (M. Tech) & Prof. (Dr.) HEMANT PUROHIT, HOD (B. Tech) Department of

Electronics and Communication Engineering, J.I.E.T, Jodhpur, without them the work

would not be possible.

We take this opportunity to thank all ours teacher who have directly or indirectly helped

our project. We pay our respects and love to our parents and all other family members

and friends for their love and encouragement throughout our career. Last but not the least

we express our thanks to our friends for their cooperation and support.

NAME:-RIYAJ MIRZA

ROLL NO:-12EJIEC096

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ABSTRACT

Micromechanical systems can be combined with microelectronics, photonics or wireless

capabilities new generation of Microsystems can be developed which will offer far

reaching efficiency regarding space, accuracy, precision and so forth. Micromechanical

systems (MEMS) technology can be used fabricate both application specific device.

The associated micro packaging systems that will allow for the integration of devices or

circuits, made with non-compatible technologies, with a System-on-Chip environment.

The MEMS technology can be used for permanent, semipermanent or temporary

interconnection of sub modules in a System-on-Chip implementation. The

interconnection of devices using MEMS technology is described with the help of a

hearing instrument application and related.

MEMS technology has enabled us to realize advanced micro devices by using processes

similar to VLSI technology. When MEMS devices are combined with other technologies

new generation of innovative technology will created. This will offer outstanding

Functionality. Such technologies will have wide scale applications in fields ranging from

automotive, aerodynamics, and hydrodynamics, biomedical and so forth. The main

challenge is to integrate all these potentially non-compatible technologies into a single

working Microsystem that will offer outstanding functionality.

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TABLE OF FIGURE

FIGURE FIGURE NAME PAGE NO.NO.

Chapter -11.1 Electromechanical device 31.2 Electromechanical system 41.3 Surface mount 5

Chapter -22.1 Hot wall LPCVD reactor 72.2 Setup of electro deposition 92.3 Cold wall vapor phase epitaxial reactor 10

Chapter -33.1 System for e-beam evaporation material 123.2 RF sputtering system 133.3 Spin casting process used for photo resist 13

Chapter-44.1 Transfer of a pattern 144.2 Pattern definition of positive & -ve resist 154.3 Pattern transfer under layering by etching 16

Chapter-55.1 Difference b/w anisotropic & Isotropic 18

Chapter-77.1 IPod Touch 217.2 Inertial sensor 22

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TABLE OF CONTENTS

Title Page i

Candidate’s Declaration ii

Acknowledgement iii

Abstract iv

List of Figures v

List of Tables vi

Introduction ________________________________________________________1

Literature survey____________________________________________________2

CHAPTERS

1. MEMS TECHNOLOGY OVERVIEW_______________________________3

1.1 What is MEMS technology _______________________________________3

1.2 What are microsystem ___________________________________________4

1.3 Silicon_______________________________________________________6

1.4 Polymers_____________________________________________________6

1.5 Metals_______________________________________________________6

2. CHEMICAL VAPOR DEPOSITION________________________________7

2.1 Introduction__________________________________________________7

2.2 When do want to use CVD ? ____________________________________8

2.3 Electro Deposition_____________________________________________8

2.4 Use Electro Deposition _________________________________________8

2.5 Epitaxy______________________________________________________9

2.6 Thermal Oxidation____________________________________________10

3. PHYSICAL VAPOR DEPOSITION ________________________________11

3.1 When do we want to use PVD____________________________________11

3.2 Evaporation__________________________________________________11

3.3 Sputtering____________________________________________________12

3.4 When do we want to use casting ?_________________________________13

4. PATTERN TRANSFER____________________________________________14

4.1 Introduction___________________________________________________14

5. ETCHING PROCESS______________________________________________17

5.1 Wet Etching___________________________________________________17

5.2 When do we want to use Wet Etching ? _____________________________18

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6. FABRICATION TECHNOLOGIES__________________________________19

6.1 Introduction___________________________________________________19

6.2 IC Fabrication_________________________________________________20

7. APPLICATION___________________________________________________21

7.1 Pressure Sensors_______________________________________________21

7.2 Accelerometers ________________________________________________21

7.3 Inertial Sensor _________________________________________________22

8. ADVANTAGE & DISADVANTAGE OF MEMS________________________23

8.1 Advantage of MEMS_____________________________________________23

8.2 Disadvantage of MEMS___________________________________________23

CONCLUSION______________________________________________________24

REFERENCE________________________________________________________25

APPENDIX_________________________________________________________26

1

INTRODUCTION

Micro electromechanical systems (MEMS) are small integrated devices or systems that combine

electrical and mechanical components. They range in size from the sub micro meter level to the

milli meter level and there can be any number, from a few to millions, in a particular system.

MEMS extend the fabrication techniques developed for the integrated circuit industry to add

mechanical elements such as beams, gears, diaphragms, and springs to devices.

Examples of MEMS device applications include inkjet-printer cartridges, accelerometer, miniature

robots, micro engines, locks inertial sensors micro transmissions, micro mirrors, micro actuator

(Mechanisms for activating process control equipment by use of pneumatic, hydraulic, or

electronic signals) optical scanners, fluid pumps, transducer, pressure and flow sensors. New

applications are emerging as the existing technology is applied to the miniaturization and

integration of conventional devices.

These systems can sense, control, and activate mechanical processes on the micro scale, and

function individually or in arrays to generate effects on the macro scale. The micro fabrication

technology enables fabrication of large arrays of devices, which individually perform simple tasks,

but in combination can accomplish complicated functions. MEMS are not about any one

application or device, nor are they defined by a single fabrication process or limited to a few

materials. They are a fabrication approach that conveys the advantages of miniaturization, multiple

components, and microelectronics to the design and construction of integrated electromechanical

systems. MEMS are not only about miniaturization of mechanical systems; they are also a new

paradigm for designing mechanical devices and systems. The MEMS industry has an estimated

$10 billion market, and with a projected 10-20% annual growth rate, it is estimated to have a $34

billion market in 2002. Because of the significant impact that MEMS can have on the commercial

2

LITERATURE SURVEY

I. We referred following paper for writing this seminar report,

1. “Implementation of AHB Interface as SDR-SDRAM Controller’s CPU

Interface”, proceedings of 2nd national conference, Sapna Gupta, Arti Noor,

Shruti Sabharwal. INDIACom-2008, BVICAM-2008, Delhi held on 8-9 Feb

2008.

2. Sharma, R., Chakravarty, T., and Bhattacharyya, A. B., “Analytical model for

optimum signal integrity in PCB interconnects using ground tracks”, IEEE

Transactions on Electromagnetic Compatibility, Vol. 51 (1), pp. 67-77,

2009. [[Impact Factor : 1.083, Indexed in SCOPUS]

3. Sharma, R. Chakravarty, T., and Bhattacharyya, A. B., “Transient Analysis ofMicrostrip-Like Interconnections Guarded by Ground Tracks,” Progress in

Electromagnetic Research, vol. PIER 82, pp.189-202, 2008.

II. We referred following book writing this seminar report.

1. Chauhan, T. & Bhagabati, C.D. & Kumar, V., 2011. Era of Energy

Harvesting: µ -Energy Scavengers using Microsystems (MEMS) Technology.

2. Shikha Sharma, Nidhi Gupta and Sudha Srivastava “Modulating Electron

Transfer Properties of Gold Nanoparticles for Efficient Biosensing” Current

Nanoscience (communicated).

3. Role of vibrational modes in structural relaxation in a supercooled

liquid”, Shankar P. Das and Sudha Srivastava in “Slow Dynamics in Complex

System” – Edited by Michio Tokuyama and Irwin Oppenheim, 1999 American

Institute of Physics (AIP).

III. We referred following content from Internet

1. Application of MEMS technology

2. Future scope of MEMS technology

3. Advantage and Disadvantage of MEMS/MICRO system

3

CHAPTER-1

“MEMS TECHNOLOGY OVERVIEW”

1.1 WHAT IS MEMS TECHNOLOGY?

Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors,

actuators, and electronics on a common silicon substrate through micro fabrication technology.

While the electronics are fabricated using integrated circuit (IC) process sequences, the

micromechanical components are fabricated using compatible "micromachining" processes that

selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical

and electromechanical devices.

Microelectronic integrated circuits can be thought of as the "brains" of a system and MEMS

augments this decision-making capability with "eyes" and "arms", to allow microsystems to sense

and control the environment. Sensors gather information from the environment through measuring

mechanical, thermal, biological, chemical, optical, and magnetic phenomena. The electronics then

process the information derived from the sensors and through some decision making capability

direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby

controlling the environment for some desired outcome or purpose. Because

Fig.1.1: electromechanical devices

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1.2 WHAT ARE MICROSYSTEMS?

MEMS is an abbreviation for Micro Electro Mechanical Systems. This is a rapidly emerging

technology combining electrical, electronic, mechanical, optical, material, chemical, and fluids

engineering disciplines. As the smallest commercially produced "machines", MEMS devices are

similar to traditional sensors and actuators although much, much smaller. E.g. Complete systems

are typically a few millimeters across, with individual features devices of the order of 1-100

micrometers across

Fig.1.2: Electro Mechanical Systems

MEMS devices are manufactured either using processes based on Integrated Circuit fabrication

techniques and materials, or using new emerging fabrication technologies such as micro injection

molding. These former processes involve building the device up layer by layer, involving several

material depositions and etch steps. A typical MEMS fabrication technology may have a 5 step

process. Due to the limitations of this "traditional IC" manufacturing process MEMS devices are

substantially planar, having very low aspect ratios (typically 5 -10 micro meters thick). MEMS

devices are typically fabricated onto a substrate (chip) that may also contain the electronics

5

required to interact with the MEMS device. Due to the small size and mass of the devices, MEMS

components can be actuated electrostatically (piezoelectric and bimetallic effects can also be used).

The position of MEMS components can also be sensed capacitive. Hence the MEMS electronics

include electrostatic drive power supplies, capacitance charge comparators, and signal

conditioning circuitry. Connection with the macroscopic world is via wire bonding and

encapsulation into familiar BGA, MCM, surface mount, or leaded IC packages.

Fig.1.3: Surface mount

A common MEMS actuator is the "linear comb drive" (shown above) which consists of rows of

interlocking teeth; half of the teeth are attached to a fixed "beam", the other half attach to a movable

beam assembly. Both assemblies are electrically insulated. By applying the same polarity voltage

to both parts the resultant electrostatic force repels the movable beam away from the fixed.

Conversely, by applying opposite polarity the parts are attracted. In this manner the comb drive

can be moved "in" or "out" and either DC or AC voltages can be applied. The small size of the

parts (low inertial mass) means that the drive has a very fast response time compared to its

macroscopic counterpart. The magnitude of electrostatic force is multiplied by the voltage or more

commonly the surface area and number of teeth. Commercial comb drives have several thousand

teeth, each tooth approximately 10 micro meters long. Drive voltages are CMOS levels.

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1.3 SILICON

The economies of scale, ready availability of cheap high-quality materials and ability to

incorporate electronic functionality make silicon attractive for a wide variety of MEMS

applications. Silicon also has significant advantages engendered through its material properties. In

single crystal form, silicon is an almost perfect Hookean material, meaning that when it is flexed

there is virtually no hysteresis and hence almost no energy dissipation. The basic techniques for

producing all silicon based MEMS devices are deposition of material layers, patterning of these

layers by photolithography and then etching to produce the required shapes.

1.4 POLYMERS

Even though the electronics industry provides an economy of scale for the silicon industry,

crystalline silicon is still a complex and relatively expensive material to produce. Polymers on the

other hand can be produced in huge volumes, with a great variety of material characteristics.

MEMS devices can be made from polymers by processes such as injection molding, embossing or

stereo lithography and are especially well suited to microfluidic applications such as disposable

blood testing cartridges.

1.5 METALS

Metals can also be used to create MEMS elements. While metals do not have some of the

advantages displayed by silicon in terms of mechanical properties, when used within their

limitations, metals can exhibit very high degrees of reliability. Metals can be deposited by

electroplating, evaporation, and sputtering processes. Commonly used metals include gold, nickel,

aluminum, chromium, titanium, tungsten, platinum, and silver.

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CHAPTER-2

‘‘CHEMICAL VAPOR DEPOSITION’’

2.1 INTRODUCTION

In this process, the substrate is placed inside a reactor to which a number of gases are supplied.

The fundamental principle of the process is that a chemical reaction takes place between the source

gases. The product of that reaction is a solid material with condenses on all surfaces inside the

reactor.

The two most important CVD technologies in MEMS are the Low Pressure CVD (LPCVD) and

Plasma Enhanced CVD (PECVD). The LPCVD process produces layers with excellent uniformity

of thickness and material characteristics. The main problems with the process are the high

deposition temperature (higher than 600°C) and the relatively slow deposition rate. The PECVD

process can operate at lower temperatures (down to 300° C) thanks to the extra energy supplied to

the gas molecules by the plasma in the reactor. However, the quality of the films tend to be inferior

to processes running at higher temperatures. Secondly, most PECVD deposition systems can only

deposit the material on one side of the wafers on 1 to 4 wafers at a time. LPCVD systems deposit

Fig.2.1: Typical hot-wall LPCVD reactor.

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2.2 WHEN DO WE WANT TO USE CVD?

CVD processes are ideal to use when you want a thin film with good step coverage. A variety of

materials can be deposited with this technology; however, some of them are less popular with fabs

because of hazardous by-products formed during processing. The quality of the material varies

from process to process, however a good rule of thumb is that higher process temperature yields a

material with higher quality and less defects.

2.3 ELECTRO DEPOSITION

This process is also known as "electroplating" and is typically restricted to electrically conductive

materials. There are basically two technologies for plating: Electroplating and Electro less plating.

In the electroplating process the substrate is placed in a liquid solution (electrolyte). When an

electrical potential is applied between a conducting area on the substrate and a counter electrode

(usually platinum) in the liquid, a chemical redox process takes place resulting in the formation of

a layer of material on the substrate and usually some gas generation at the counter electrode. In the

electro less plating process a more complex chemical solution is used, in which deposition happens

spontaneously on any surface which forms a sufficiently high electrochemical potential with the

solution. This process is desirable since it does not require any external electrical potential and

contact to the substrate.

2.4 WHEN DO WE WANT TO USE ELECTRO DEPOSITION?

The electro deposition process is well suited to make films of metals such as copper, gold and

nickel. The films can be made in any thickness from ~1µm to >100µm. The deposition is best

controlled when used with an external electrical potential, however, it requires electrical contact

to the substrate when immersed in the liquid bath. In any process, the surface of the substrate must

have an electrically conducting coating before the deposition can be done.

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Fig.2.2: Typical setup for electro deposition.

2.5 EPITAXY

This technology is quite similar to what happens in CVD processes, however, if the substrate is an

ordered semiconductor crystal (i.e. silicon, gallium arsenide), it is possible with this process to

continue building on the substrate with the same crystallographic orientation with the substrate

acting as a seed for the deposition. If an amorphous/polycrystalline substrate surface is used, the

film will also be amorphous or polycrystalline.

There are several technologies for creating the conditions inside a reactor needed to support

epitaxial growth, of which the most important is Vapor Phase Epitaxy (VPE). In this process, a

number of gases are introduced in an induction heated reactor where only the substrate is heated.

The temperature of the substrate typically must be at least 50% of the melting point of the material

to be deposited.

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An advantage of epitaxy is the high growth rate of material, which allows the formation of films

with considerable thickness (>100µm). Epitaxy is a widely used technology for producing silicon

on insulator (SOI) substrates. The technology is primarily used for deposition of silicon. A

schematic diagram of a typical vapour phase epitaxial reactor is shown in the figure below.

Fig.2.3: Typical cold-wall vapour phase epitaxial reactor.

2.6 THERMAL OXIDATION

This is one of the most basic deposition technologies. It is simply oxidation of the substrate surface

in an oxygen rich atmosphere. The temperature is raised to 800° C-1100° C to speed up the process.

This is also the only deposition technology which actually consumes some of the substrate as it

proceeds. The growth of the film is spurned by diffusion of oxygen into the substrate, which means

the film growth is actually downwards into the substrate. As the thickness of the oxidized layer

increases, the diffusion of oxygen to the substrate becomes more difficult leading to a parabolic

relationship between film thickness and oxidation time for films thicker than ~100nm. This process

is naturally limited to materials that can be oxidized, and it can only form films that are oxides of

that material.

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CHAPTER-3

‘‘PHYSICAL VAPOR DEPOSITION’’

PVD covers a number of deposition technologies in which material is released from a source and

transferred to the substrate. The two most important technologies are evaporation and sputtering.

3.1 WHEN DO WE WANT TO USE PVD?

PVD comprises the standard technologies for deposition of metals. It is far more common than

CVD for metals since it can be performed at lower process risk and cheaper in regards to materials

cost. The quality of the films are inferior to CVD, which for metals means higher resistivity and

for insulators more defects and traps. The step coverage is also not as good as CVD. The choice

of deposition method (i.e. evaporation vs. sputtering) may in many cases be arbitrary, and may

depend more on what technology is available for the specific material at the time.

3.2 EVAPORATION

In evaporation the substrate is placed inside a vacuum chamber, in which a block (source) of the

material to be deposited is also located. The source material is then heated to the point where it

starts to boil and evaporate. The vacuum is required to allow the molecules to evaporate freely in

the chamber, and they subsequently condense on all surfaces. This principle is the same for all

evaporation technologies, only the method used to the heat (evaporate) the source material differs.

There are two popular evaporation technologies, which are e-beam evaporation and resistive

evaporation each referring to the heating method. In e-beam evaporation, an electron beam is

aimed at the source material causing local heating and evaporation. In resistive evaporation, a

tungsten boat, containing the source material, is heated electrically with a high current to make the

material evaporate. Many materials are restrictive in terms of what evaporation method can be

used (i.e. aluminium is quite difficult to evaporate using resistive heating), which typically relates

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to the phase transition properties of that material. A schematic diagram of a typical system for e-

beam evaporation is shown in the figure below.

Fig.3.1: Typical system for e-beam evaporation of materials

3.3 SPUTTERING

Sputtering is a technology in which the material is released from the source at much lower

temperature than evaporation. The substrate is placed in a vacuum chamber with the source

material, named a target, and an inert gas (such as argon) is introduced at low pressure. Gas plasma

is struck using an RF power source, causing the gas to become ionized. The ions are accelerated

towards the surface of the target, causing atoms of the source material to break off from the target

in vapour form and condense on all surfaces including the substrate. As for evaporation, the basic

principle of sputtering is the same for all sputtering technologies. The differences typically relate

to the manner in which the ion bombardment of the target is realized. A schematic diagram of a

typical RF sputtering system is shown in the figure below.

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Fig.3.2: Typical RF sputtering system.

3.4 WHEN DO WE WANT TO USE CASTING?

Casting is a simple technology which can be used for a variety of materials (mostly polymers).

The control on film thickness depends on exact conditions, but can be sustained within +/-10% in

a wide range. If you are planning to use photolithography you will be using casting, which is an

integral part of that technology. There are also other interesting materials such as polyimide and

spin-on glass which can be applied by casting.

Fig.3.3: The spin casting process as used for photoresist in photolithography

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CHAPTER-4

“PATTERN TRANSFER”

4.1. INTRODUCTION

Lithography in the MEMS context is typically the transfer of a pattern to a photosensitive material

by selective exposure to a radiation source such as light. A photosensitive material is a material

that experiences a change in its physical properties when exposed to a radiation source. If we

selectively expose a photosensitive material to radiation (e.g. by masking some of the radiation)

the pattern of the radiation on the material is transferred to the material exposed, as the properties

of the exposed and unexposed regions differs (as shown in figure 1).

Fig.4.1: Transfer of a pattern to a photosensitive material.

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In lithography for micromachining, the photosensitive material used is typically a photoresist (also

called resist, other photosensitive polymers are also used). When resist is exposed to a radiation

source of a specific a wavelength, the chemical resistance of the resist to developer solution

changes. If the resist is placed in a developer solution after selective exposure to a light source, it

will etch away one of the two regions (exposed or unexposed). If the exposed material is etched

away by the developer and the unexposed region is resilient, the material is considered to be a

positive resist (shown in figure 2a). If the exposed material is resilient to the developer and the

unexposed region is etched away, it is considered to be a negative resist (shown in figure 2b).

Fig.4.2 :a) Pattern definition in positive resist, b) Pattern definition in negative resist.

Lithography is the principal mechanism for pattern definition in micromachining. Photosensitive

compounds are primarily organic, and do not encompass the spectrum of materials properties of

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interest to micro-machinists. However, as the technique is capable of producing fine features in an

economic fashion, a photosensitive layer is often used as a temporary mask when etching an

underlying layer, so that the pattern may be transferred to the underlying layer (shown in figure

3a). Photoresist may also be used as a template for patterning material deposited after lithography

(shown in figure 3b). The resist is subsequently etched away, and the material deposited on the

resist is "lifted off". The deposition template (lift-off) approach for transferring a pattern from

resist to another layer is less common than using the resist pattern as an etch mask. The reason for

this is that resist is incompatible with most MEMS deposition processes, usually because it cannot

withstand high temperatures and may act as a source of contamination.

Fig.4.3: a) Pattern transfer from patterned photoresist to underlying layer by etching, b) Pattern

transfer from patterned photoresist to overlying layer by lift-off.

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CHAPTER-5

“ETCHING PROCESSES”

In order to form a functional MEMS structure on a substrate, it is necessary to etch the thin films

previously deposited and/or the substrate itself. In general, there are two classes of etching

processes:

1. Wet etching where the material is dissolved when immersed in a chemical solution

2. Dry etching where the material is sputtered or dissolved using reactive ions or a vapor

phase etchant.

5.1 WET ETCHING

This is the simplest etching technology. All it requires is a container with a liquid solution that will

dissolve the material in question. Unfortunately, there are complications since usually a mask is

desired to selectively etch the material. One must find a mask that will not dissolve or at least

etches much slower than the material to be patterned. Secondly, some single crystal materials, such

as silicon, exhibit anisotropic etching in certain chemicals. Anisotropic etching in contrast to

isotropic etching means different etches rates in different directions in the material. The classic

example of this is the <111> crystal plane sidewalls that appear when etching a hole in a <100>

silicon wafer in a chemical such as potassium hydroxide (KOH). The result is a pyramid shaped

hole instead of a hole with rounded sidewalls with an isotropic etchant. The principle of anisotropic

and isotropic wet etching is illustrated in the figure below.

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5.2 WHEN DO WE WANT TO USE WET ETCHING?

This is a simple technology, which will give good results if you can find the combination of etchant

and mask material to suit your application. Wet etching works very well for etching thin films on

substrates, and can also be used to etch the substrate itself. The problem with substrate etching is

that isotropic processes will cause undercutting of the mask layer by the same distance as the etch

depth. Anisotropic processes allow the etching to stop on certain crystal planes in the substrate,

but still results in a loss of space, since these planes cannot be vertical to the surface when etching

holes or cavities. If this is a limitation for you, you should consider dry etching of the substrate

instead. However, keep in mind that the cost per wafer will be 1-2 orders of magnitude higher to

perform the dry etching

If you are making very small features in thin films (comparable to the film thickness), you may

also encounter problems with isotropic wet etching, since the undercutting will be at least equal to

the film thickness. With dry etching it is possible etch almost straight down without undercutting,

which provides much higher resolution.

Fig.5.1: Difference between anisotropic and isotropic wet etching.

19

CHAPTER-6

“FABRICATION TECHNOLOGIES”

6.1 INTRODUCTION

The three characteristic features of MEMS fabrication technologies are miniaturization,

multiplicity, and microelectronics. Miniaturization enables the production of compact, quick-

response devices. Multiplicity refers to the batch fabrication inherent in semiconductor processing,

which allows thousands or millions of components to be easily and concurrently fabricated.

Microelectronics provides the intelligence to MEMS and allows the monolithic merger of sensors,

actuators, and logic to build closed-loop feedback components and systems. The successful

miniaturization and multiplicity of traditional electronics systems would not have been possible

without IC fabrication technology. Therefore, IC fabrication technology, or micro fabrication, has

so far been the primary enabling technology for the development of MEMS. Micro fabrication

provides a powerful tool for batch processing and miniaturization of mechanical systems into a

dimensional domain not accessible by conventional techniques. Furthermore, micro fabrication

provides an opportunity for integration of mechanical systems with electronics to develop high-

performance closed-loop-controlled MEMS.

Advances in IC technology in the last decade have brought about corresponding progress in

MEMS fabrication processes. Manufacturing processes allow for the monolithic integration of

micro electromechanical structures with driving, controlling, and signal-processing electronics.

This integration promises to improve the performance of micromechanical devices as well as

reduce the cost of manufacturing, packaging, and instrumenting these devices.

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6.2 IC FABRICATION

Any discussion of MEMS requires a basic understanding of IC fabrication technology, or micro

fabrication, the primary enabling technology for the development of MEMS. The major steps in

IC fabrication technology are:

Film growth: Usually, a polished Si wafer is used as the substrate, on which a thin film is

grown. The film, which may be epitaxial Si, SiO2, silicon nitride (Si3N4), polycrystalline

Si, or metal, is used to build both active or passive components and interconnections

between circuits.

Doping: To modulate the properties of the device layer, a low and controllable level of an

atomic impurity may be introduced into the layer by thermal diffusion or ion implantation.

Lithography: A pattern on a mask is then transferred to the film by means of a

photosensitive (i.e., light sensitive) chemical known as a photoresist. The process of pattern

generation and transfer is called photolithography. A typical mask consists of a glass plate

coated with a patterned chromium (Cr) film.

Etching: Next is the selective removal of unwanted regions of a film or substrate for pattern

delineation. Wet chemical etching or dry etching may be used. Etch-mask materials are

used at various stages in the removal process to selectively prevent those portions of the

material from being etched. These materials include SiO2, Si3N4, and hard-baked

photoresist.

Dicing: The finished wafer is sawed or machined into small squares, or dice, from which

electronic components can be made.

Packaging: The individual sections are then packaged, a process that involves physically

locating, connecting, and protecting a device or component. MEMS design is strongly

coupled to the packaging requirements, which in turn are dictated by the application

environment.

21

CHAPTER-7

APPLICATIONS

7.1PRESSURE SENSORS

MEMS pressure micro sensors typically have a flexible diaphragm that deforms in the presence of

a pressure difference. The deformation is converted to an electrical signal appearing at the sensor

output. A pressure sensor can be used to sense the absolute air pressure within the intake manifold

of an automobile engine, so that the amount of fuel required for each engine cylinder can be

computed.

7.2ACCELEROMETERS

Accelerometers are acceleration sensors. An inertial mass suspended by springs is acted upon by

acceleration forces that cause the mass to be deflected from its initial position. This deflection is

converted to an electrical signal, which appears at the sensor output. The application of MEMS

technology to accelerometers is a relatively new development. Accelerometers in consumer

electronics devices such as game controllers (Nintendo Wii), personal media players / cell phones

(Apple iPhone ) and a number of Digital Cameras (various Canon Digital IXUS models).

Fig.7.1: iPod Touch

22

The consumer market has been a key driver for MEMS technology success. For example, in a

mobile phone, MP3/MP4 player or PDA, these sensors offer a new intuitive motion-based

approach to navigation within and between pages. In game controllers, MEMS sensors allow the

player to play just moving the controller/pad; the sensor determines the motion.

7.3INERTIAL SENSORS

Inertial sensors are a type of accelerometer and are one of the principal commercial products

that utilize surface micromachining. They are used as airbag-deployment sensors in

automobiles, and as tilt or shock sensors. The application of these accelerometers to inertial

measurement units is limited by the need to manually align and assemble them into three-axis

systems, and by the resulting alignment tolerances, their lack of in- chip analog-to-digital

conversion circuitry, and their lower limit of sensitivity.

Fig.7.2: Inertial sensors

23

CHAPTER-8

ADVANTAGES & DISADVANTAGES OF MEMS

8.1 ADVANTAGES OF MEMS

Minimize energy and materials used in manufacturing

Cost/performance advantages

Improved reproducibility

Improved accuracy and reliability

Increased selectivity and sensitivity

8.2 DISADVANTAGES OF MEMS

Farm establishment requires huge investments

Micro-components are Costly compare to macro-components

Design includes very much complex procedures

Prior knowledge is needed to integrate MEMS devices.

24

CONCLUSION

The automotive industry, motivated by the need for more efficient safety systems and the desire

for enhanced performance, is the largest consumer of MEMS-based technology. In addition to

accelerometers and gyroscopes, micro-sized tire pressure systems are now standard issues in new

vehicles, putting MEMS pressure sensors in high demand. Such micro-sized pressure sensors can

be used by physicians and surgeons in a telemetry system to measure blood pressure at a stet,

allowing early detection of hypertension and restenosis. Alternatively, the detection of bio

molecules can benefit most from MEMS-based biosensors. Medical applications include the

detection of DNA sequences and metabolites. MEMS biosensors can also monitor several

chemicals simultaneously, making them perfect for detecting toxins in the environment.

Lastly, the dynamic range of MEMS based silicon ultrasonic sensors have many advantages over

existing piezoelectric sensors in non-destructive evaluation, proximity sensing and gas flow

measurement. Silicon ultrasonic sensors are also very effective immersion sensors and provide

improved performance in the areas of medical imaging and liquid level detection.

The medical, wireless technology, biotechnology, computer, automotive and aerospace

industries are only a few that will benefit greatly from MEMS.

This enabling technology allowing the development of smart products, augmenting the

computational ability of microelectronics with the perception and control capabilities of

micro sensors and micro actuators and expanding the space of possible designs and

applications.

MEMS devices are manufactured for unprecedented levels of functionality, reliability, and

sophistication can be placed on a small silicon chip at a relatively low cost.

MEMS promises to revolutionize nearly every product category by bringing together

silicon-based microelectronics with micromachining technology, making possible the

realization of complete systems-on-a-chip.

25

REFERENCES

1. http://www-bsac.eecs.berkeley.edu/

2. http://www.darpa.mil/mto/

3. IEEE Explore http://ieeexplore.ieee.org/Xplore/DynWel.jsp

4. Introduction to Micro engineering http://www.dbanks.demon.co.uk/ueng/

5. MEMS Clearinghouse http://www.memsnet.org/

6. MEMS Exchange http://www.mems-exchange.org/

7. MEMS Industry Group http://www.memsindustrygroup.org/

8. MOSIS http://www.mosis.org/

9. MUMPS http://www.memscap.com/memsrus/crmumps.html

10. Stanford Centre for Integrated Systems http://www-cis.stanford.edu/

11. USPTO http://www.uspto.gov/

12. Trimmer http://www.trimmer.net/

13. Yole Development http://www.yole.fr/pagesAn/accueil.asp

26

APPENDIX

Each of the three basic microsystems technology processes we have seen, bulk micromachining,

sacrificial surface micromachining, and micro molding/LIGA, employs a different set of capital

and intellectual resources. MEMS manufacturing firms must choose which specific microsystems

manufacturing techniques to invest in.

MEMS technology has the potential to change our daily lives as much as the computer has.

However, the material needs of the MEMS field are at a preliminary stage. A thorough

understanding of the properties of existing MEMS materials is just as important as the

development of new MEMS materials.

Future MEMS applications will be driven by processes enabling greater functionality through

higher levels of electronic-mechanical integration and greater numbers of mechanical components

working alone or together to enable a complex action. Future MEMS products will demand higher

levels of electrical-mechanical integration and more intimate interaction with the physical world.

The high up-front investment costs for large-volume commercialization of MEMS will likely limit

the initial involvement to larger companies in the IC industry. Advancing from their success as

sensors, MEMS products will be embedded in larger non-MEMS systems, such as printers,

automobiles, and biomedical diagnostic equipment, and will enable new and improved systems.