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CHAPTER-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 integrated circuit (IC) batch processing techniques and can

range in size from a few micrometers 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 manufacturing expertise from a wide and diverse range of technical

areas including integrated circuit fabrication technology, mechanical

engineering, materials science, electrical engineering, chemistry and

chemical engineering, as well as fluid engineering, optics, instrumentation

and packaging. The complexity of MEMS is also shown in the extensive

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 has been identified as one of the most promising technologies

for the 21st Century and has the potential to revolutionize both industrial and

consumer products by combining siliconbased microelectronics with

micromachining technology. Its techniques and microsystembased devices

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have the potential to dramatically affect of all of our lives and the way we

live. If semiconductor micro fabrication was seen to be the first micro

manufacturing revolution, MEMS is the second revolution.

The field of MEMS has been divided into four main sections. In the

first section, the reader is introduced to MEMS, its definitions, history,

current and potential applications, as well as the state of the MEMS market

and issues concerning miniaturization. The second section deals with the

fundamental fabrication methods of MEMS including photolithography,

bulk micromachining, surface micromachining and high-aspect-ratio

micromachining, assembly, system integration and packaging of MEMS

devices is also described here. The third section reviews the range of MEMS

sensors and actuators, the phenomena that can be sensed or acted upon with

MEMS devices, and a brief description of the basic sensing and actuation

mechanisms. The final section illustrates the challenges facing the MEMS

industry for the commercialisation and success of MEMS.

1.1 Basic components of MEMS:

In the most general form, MEMS consist of mechanical

microstructures, microsensors, microactuators and microelectronics, all

integrated onto the same silicon chip. The vision of MEMS whereby micro

sensors, micro actuators and micro electronics and other technologies, can

be integrated onto a single microchip is expected to be one of the most

important technological breakthroughs of the future.

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Figure 1.1 Components of MEMS

1.1.1 Micro sensors

A very small sensor with physical dimensions in the submicrometer to

millimeter range. Over the past several decades MEMS researchers and

developers have demonstrated an extremely large number of micro sensors

for almost every possible sensing modality including temperature, pressure,

inertial forces, chemical species, magnetic field, radiation, etc. Remarkably,

many of these micro machined sensors have demonstrated performances

exceeding those of their macro scale counterparts.

Sensors are an essential element in many measurement, process, and

control systems, with countless applications in the automotive, aerospace,

biomedical, telecommunications, environmental, agricultural, and other

industries. The stimulus to miniaturize sensors lies in the enormous cost

benefits that are gained by using semiconductor processing technology, and

in the fact that microsensors are generally able to offer a better sensitivity,

accuracy, dynamic range, and reliability, as well as lower power

consumption, than their larger counterparts. Mechanical microsensors form

perhaps the largest family of microsensors because of their widespread

availability. Microsensors have been produced to measure a wide range of

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mechanical properties, including force, pressure, displacement, acceleration,

rotation, and mass flow. Force sensors generally use a sensing element that

converts the applied force into the deformation of the elastic element.

Applications for chemical and biochemical microsensors are

environmental monitoring and medicine. Only a few such devices now exist.

Examples include a glucose biochemical microsensor and ion-selective

field-effect devices used to measure blood pH. The use of microsensors to

gather medical diagnostic information is an attractive proposition, and

eventually there may even be implanted microsensors to diagnose health

problems, using smell-sensitive array devices. In the case of micro sensor,

the device typically converts a measured mechanical signal into an electrical

signal.

1.1.2 Micro electronics

In microelectronics and MEMS industry you need to observe, analyze,

and identify the characteristics of your products. This is equally true in

process control and failure analysis. The knowledge of your device’s

physical structure is vital in understanding the process. Development of

microelectronics and design of mechanical microsystems (MEMS) rely on

efficient microscopes. Any number of functional features such as cracks,

voids or connections can be investigated. In the creation of miniaturized

mechanical devices, light microscopy serves as your tool for assessing

prototypes and monitoring results of precision machining. The increasing

integration of microelectronics into the nanometer area makes it necessary

for you to examine these structures with scanning electron microscopes.

SEM deliver resolutions in the nanometer range and large depth of field,

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resulting in brilliant images and precise circuit editing and analysis. Circuit

editing with FIB-SEM even allows to fabricate complex structures.

1.1.3 Micro structures

Microstructure is defined as the structure of a prepared surface or

thin foil of material as revealed by a microscope. The microstructure of a

material (which can be broadly classified into metallic, polymeric, ceramic

and composite) can strongly influence physical properties such as strength,

toughness, ductility, hardness, corrosion resistance, high/low temperature

behavior, wear resistance, and so on, which in turn govern the application of

these materials in industrial practice. Microstructure at scales smaller than

can be viewed with optical microscopes is often called ultrastructure or

nanostructure.

1.1.4 Micro actuators

Micro actuators are important building blocks for many MEMS

devices, which generate forces or displacements to realize scanning, tuning,

manipulating or delivering function.Micro electrostatic actuators are more

popular than others due to their high compatibility with mature micro

fabrication processes, low power consumption, simple structures and quick

response.

1.1.5 Actuators

An actuator is a type of motor that is responsible for moving or

controlling a mechanism or system. It is operated by a source of energy,

typically electric current, hydraulic fluid pressure, or pneumatic pressure,

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and converts that energy into motion. An actuator is the mechanism by

which a control system acts upon an environment. The control system can be

simple (a fixed mechanical or electronic system), software-based (e.g. a

printer driver, robot control system), a human, or any other input. Actuators

are a type of tool which is used to put something into automatic action. It is

used on a wide variety of sources, from humans putting something into

action to computers starting up a program.

There are several reasons why actuators may be used in an office

work area. Most of these will be used as the thing that positions or moves a

certain object. For example, many will be used to move or position valve

doors in certain systems. They may also be used to maneuver certain

mechanical devices that are used in a work area. By using actuators, it is

easier for one to work in their area, as well as easier to maneuver something

around in the certain area.

1.2 Materials for MEMS

1.2.1 Silicon

One of very few materials that can be economically manufactured in

single crystal Substrates. It is a Diamond lattice.

Figure 1.2 Diamond structure

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In silicon crystal orientation, each site is tetrahedrally coordinated

with four other sites in the other sublattice. It has properties such as extensive studies and documentation, Suitable for electronic, mechanical,

thermal, and optical integration.It can sustain harsh (mechanical) handling

conditions and mechanical properties are uniform across wafer lots. It has

structures like crystalline, polycrystalline-polysilicon, amorphous.

1.2.2 SiO2/Quartz/Glass

The stable oxide is one of the key elements for the success of silicon

in IC,Excellent thermal and electrical insulation, Sacrificial layers in surface

micromachining processes.

a) SiO2

Fused silica is a purer version of fused quartz that is made

synthetically from various Silicon gasses. It has 17 crystalline phases.

b) Quartz

It is a single crystal material, low impurity concentration. Fused

quartz is the amorphous form of quartz. It is made from natural crystalline

quartz, usually quartz sand that has been mined.

c) Glass

A glass is an amorphous solid, impurities, low melting temperature.

Like most glasses, the dominant component of Borosilicate glass is SiO2

with boron and various other elements added to give it its excellent qualities.

1.2.3 Polymers

Spin coated with varying thickness; few nm – hundreds of microns. It

is used in sensing of chemical gases and humidity. It is used as Photoresists.

Epoxy based photoresist can form layers up to 100 µm. It is of low cost.

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1.3 MEMS Fabrication process

a) Photolithography steps

• Photoresist spinnning, 1-10 µm spin coating.

• Optical exposure through a photomask.

• Developing to dissolve exposed resist.

• Bake to drive off solvents.

• Remove using solvents (acetone) or O2 plasma.

b) Photoresist application

Spin-casting photoresist

• Polymer resin, sensitizer, carrier solvent.

• Positive and negative photoresist.

1.4 Micromachining

1.4.1 Bulk micromachining

Bulk micromachining involves the removal of part of the bulk

substrate. It is a subtractive process that uses wet anisotropic etching or a dry

etching method such as reactive ion etching (RIE), to create large pits,

grooves and channels. Materials typically used for wet etching include

silicon and quartz, while dry etching is typically used with silicon, metals,

plastics and ceramics.

a)Wet etching

Wet etching describes the removal of material through the immersion

of a material (typically a silicon wafer) in a liquid bath of a chemical

etchant. These etchants can be isotropic or anisotropic. Isotropic etchants

etch the material at the same rate in all directions, and consequently remove

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

material and this is known as undercutting.

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b)Dry etching

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 power

to drive the chemical reaction. Energetic ions are accelerated towards the

material to be etched within a plasma phase supplying the additional energy

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

temperatures than those usually needed.

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CHAPTER 2

LITERATURE SURVEY

2.1 “Carbon nanotube sensors for exhaled breath components”,

(Oleksandr Kuzmych, Brett L Allen and Alexander Star Published 22

August 2007)

In this project the detection of nitric oxide (NO) in gas phase is based

on a combination of acidic gas scrubbing, oxidation, and conductivity

measurements using a chemically functionalized carbon nanotube field-

effect transistor (NTFET) device. Gas mixtures containing NO are passed

through an Ascarite scrubber and then an oxidizing material (CrO3) which

converts NO into NO2. The latter is delivered to the surface of the NTFET

sensor coated with poly ethylene imine(PEI) polymer. The wide range of

NO gas concentrations from about 2 ppb up to 5 ppm was tested. The

primary reason for this interest involves the role NO molecules play in

biological function .

This proposed syetem used NTFET devices comprised of a network of

single-walled carbon nanotubes (SWNTs) contacted by Au/Ti metal

electrodes representing the source (S) and drain (D) with a Si back gate

separated by SiO2 in a FET configured circuit. S–D conductance versus gate

voltage (G–VG) plots are obtained, the transfer characteristic of a typical

NTFET device used in this paper. Devices exhibit p-type characteristics in

ambient conditions with positive threshold voltage values and low on/off

ratios, which are characteristic to network NTFET devices with 5 μm

separations . Presence of analytes or chemical modification of NTFETs

results in a change of conductance which is evident in the n-type NTFET

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character after PEI coating (vide infra).Here, PEI increases the sensitivity to

converted NO, and RH and CO2 affect the sensor response. This sensor

poses the advantages of real time measurements, along with low costs and

high sensitivity.

2.2 “MEMS based gas sensor for monitoring process chambers in

semiconductor manufacturing”, (Ijaz H. Jafria, Frank DiMeo Jr. b, Jeffrey

Neunerb, Sue DiMascio b, and James Marchettia IntelliSense Corporation

MA 01887b Advanced Technology Materials, Inc., 7 Commerce Drive,

Danbury, CT 06810 Published in 2000)

In this project a commercial software package IntelliSuiteTM was

used to speed the design and optimization of gas sensors. This is used to

model the coupled thermo-electro-mechanical responses of devices known

as microhotplates. Models were built based on the GDSII formatted mask

layout, process sequences, and layer thicknesses.The modeling results from

this study predict that the first three resonant frequency modes for these

devices are in the 612 to1530 kHz range for an all pinned device, and 134 to

676 kHz for a springboard arrangement. This process also involves the FEA

model creation and Microhotplate generation. This analysis consisted of the

creation of the 3D finite element mesh, verification of the consistency of the

model mesh size. In this work the finite element modeling is used to predict

the coupled thermo-electro-mechanical properties of CMOS based

microhotplates. The measured thickness of the thin film layers as inputs to

the model, and calculated the resonant frequency as a function of absorbed

mass. The analyses predict that the first three modes of natural frequency are

612, 1522, and 1530 kHz for the four post device, and 134, 382, and 676

kHz for a two post, or springboard arrangement. The simulation results agree

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very well with the experimental data at low power. At higher power,

however, the model and experimental results begin to diverge and are

currently working on improving the model by incorporating these effects.

2.3 “Single walled and multi walled carbon nanotube structure,

synthesis and applications”, (E.N. Ganesh, This project appear in the

International Journal of Innovative Technology and Exploring Engineering

(IJITEE) ISSN: 2278-3075, Volume-2, Issue-4, March 2013)

Carbon Nanotubes have exceptional mechanical and electrical

properties. Various methods have been thoroughly investigated for the

growth of CNTs. The best and the most commonly used method is Chemical

Vapour Deposition (CVD). The structural uniformity of carbon nanotubes

produced by plasma enhanced Chemical Vapour Deposition gives uniform

height and diameter. This paper discussed about Comparison of different

methods about synthesis of carbon nano tube, Structure of SWNT, DWNT

and MWNT carbon nano tube, Simulation of carbon nano tubes with

different distortions and the properties of carbon nano tube for Energy

storage and Medical applications. Carbon nanotubes (CNTs) are hollow

cylinders of carbon atoms . There are two types of nanotubes: single-walled

nanotubes (SWNTs) and multiwalled nanotubes (MWNTs), which differ in

the arrangement of their graphene cylinders. Here, The single layer and

multi layer Carbon nano tube using nano explorer tool was simulated.The

simulated properties of CNT would be used for energy storage purpose as

well for transmission of electrical energy. Though it is known that CNT’s

have high aspect ratio, Young’s modulus over one terra pascal, Tensile

strength of 200 Giga pascal. It describes about the advantage of plasma

enhanced CVD technique and the simulation of CNT structure also

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described. So, that the structural parameters of types of CNTs was calculated

and also various tube distortion in CNT and calculated energy per atom with

respect to radius also mentioned.

2.4 “Ultra sensitive and highly selective gas sensors using three-

dimentional tungsten oxide nanowire networks”, (Andrea Ponzoni, Yong

Ding published in 15 May 2006)

Three-dimensional 3D tungsten oxide nanowire networks have been

demonstrated as a high-surface area material for building ultrasensitive and

highly selective gas sensors. Utilizing the 3D hierarchical structure of the

networks, high sensitivity has been obtained towards NO2, revealing the

capability of the material to detect concentration as low as 50 ppb (parts per

billion). The results highlight that the nanobelts, nanowires technology can

be adopted for the development of gas.The performance of a solid-state gas

sensor is characterized by its sensitivity, stability and selectivity. The

working principle relies on modulation of electrical conductivity due to

surface oxidation ,reduction caused by gas exposure. Because, only the

surface layer is affected by such reactions, the sensitivity is strongly

dependent on the surface-to-volume ratio of the material used. This purpose

has been pursued by synthesizing layers with a porous morphology to

enhance the material surface area. Porosity is enhanced by means of the

thick film synthesis approach typically adopted in the gas sensing field. Such

high porosity is not easy to achieve by thin film approach. Another approach

largely used in the field is the rheotaxial growth and its thermal oxidation

RGTO method, which allows synthesizing a porous thin film monolayer

composed of nano sized grains. As far as grain size is concerned, it has to be

kept in the order of or lower than the debye length, which is the depth into

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the surface that the chemisorption process can effectively modify the

electrical properties of the metal oxide material. The proposed system

describes gas sensing performances of three-dimensional WO3−x nanowire

networks. This material can be applied in gas sensing field to develop NO2

sensors with performances suitable for practical application.

2.5 Summary

Asthma is a chronic disease which will interfere with a

person’s daily functioning. This literature review has following disadvantage

like complexity, large size and accuracy. The COMSOL Software didn’t

provide fast response. Its suits only for mechanical applications. The main

disadvantages selectivity and sensitivity. So to overcome the selectivity

problem the CNT gas sensor is designed and the size is also reduced. That

will be explained in upcoming chapters.

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CHAPTER 3

EXISTING SYSTEM

3.1 Introduction

This project reports on a novel miniaturized MEMS-based

amperometric nitric oxide sensor that is suitable for a point of care testing

device for asthma. The novelty lies in the combination of a high surface area

microporous structured electrode, nano-structured Nafion that is coated on

the side walls of the micropores, and liquid electrolyte. This combination

allows detection of very low concentration gas, has a high sensitivity of and

has both a response and a recovery time of 6 s. The sensor is integrated with

a PCB potentiostat to form a complete measuring module. The limit of

detection of this sensor was estimated to be 0.3ppb. An ideal sensor for

breath analysis should have low detection limit (<1 ppb), fast response, must

work in varying humidity, should have repeatability and should be batch

manufacturable. All these features are required for manufacturing a portable

gas monitoring equipment that can be used for in situ measurement of nitric

oxide gas.

Sensors using electrochemical methods have previously been

developed to meet the demand of very low concentration detection. However

for measurements of eNO, they lack important features such as fast

response, small size and high sensitivity. Although conventional

electrochemical sensors can detect down to single ppb gas concentration

they suffer from a long response time, typically in the order of 60-100 s.

consequently they require complicated flow handling and buffering of the

exhaled breath sample. Most of the miniaturized NO sensors are realized

using metal oxide as the sensing material and have limited detection range of

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1-100 ppm. Amperometric detection techniques using only solid polymer

electrolyte (SPE) like Nafion has been shown to detect NO but the detection

limit of 5 ppm is not useful for asthma detection . Amperometric sensors

with liquid electrolyte have modified electrodes of carbon or platinum to

have high sensitivity, but the sensor cannot be batch manufactured using

standard processing technology. In this project, a novel MEMS-based

amperometric nitric oxide sensor that has been designed to meet the required

criteria for a portable asthma detector.

3.2 Concept

The designed sensor works on the principle of amperometric detection

of the analyte. The oxidation of the analyte at the working electrode results

in a current that is detected by using a potentiostat that consists of a

transimpedance amplifier. It is also used for maintaining a constant potential

between the working and the reference electrode. The working electrode is

fabricated by etching silicon to form microporous structure. It is covered

with Al2O3 that is deposited by using atomic layer deposition, which is

followed by the deposition of platinum. The Al2O3 acts as an insulator

between the silicon and the platinum. The microporous structure thus

prepared is then covered with a thin layer of Nafion. The Nafion layer is the

diffusion layer that permits the gas, electrode and liquid interaction. This

interaction leads to the oxidation of the nitric oxide molecule causing a

current flow between the working and counter electrode. This current is

measured by using a PCB based potentiostat.

The counter electrode enables a current to flow through the sensor

cell. The working potential, electrolyte and electrode materials are so chosen

that the gas being measured is oxidized at the working electrode. As the

oxidization takes place at the working electrode, oxygen is normally reduced

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to water at the counter electrode. The resulting current which flows through

the sensor is directly proportional to the gas concentration.A micromachined

working electrode is fabricated and mounted together with the reference and

counter electrode.

3.3 Fabrication

The working electrode is fabricated using a double side polished

100mm, 300 μm thick silicon wafer. The silicon wafer is spin coated with 6

μm thick photoresist (AZ 9260). The wafer is then soft baked on a hot plate

for 2 min and is exposed with UV intensity of 300 mW/cm2 for 15 s. The

photoresist is developed using developer 2401 for 3 min in order to define a

pattern. The pattern defines the working electrode area. The grid pattern is 6

mm in diameter and has triangular micropores of 120 μm. The grid width is

approximately 20 μm. The grid pattern is then etched using deep reactive ion

etching for 1.5 hrs to form a microporous structure .

Figure 3.1.a Photoresist patterning and silicon grid etching

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Figure 3.1.b Photoresist removal and Al2O3 deposition followed by Pt

deposition using atomic layer deposition(ALD)

The etched silicon wafer is then transferred to an atomic layer

deposition chamber. Here a 10 nm layer of Al2O3 is deposited on the wafer

followed by a 10 nm thick platinum layer. The Al2O3 acts as an insulator

between the platinum (Pt) and the silicon. The platinum layer is the sensing

electrode for the gas sensor. The wafer is then diced into 1 cm x 1 cm sized

chips.

Figure 3.1.c The wafer is diced in 1cm×1cm chips followed by

Nafion coating on individual chips

Figure 3.1.d SEM picture showing a top view of a grid membrane

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Figure 3.2 SEM image showing the cross section of Nafion coated

grid

These chips are then individually dipped in 5% Nafion solution and

desiccated in a low pressure chamber. This low pressure treatment helps in

removing the air trapped in the micropores and thus allowing nafion to enter

the micropores. The Nafion coated chip is then removed from the low

pressure chamber and dried in air for 2 hrs. The Nafion coated grid

constitutes a membrane electrode assembly (MEA) which is used as the

working electrode in the sensor.

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Figure 3.3 NO sensor on a polycarbonate substrate and silver on

counter and reference electrode

The fabrication of the counter and reference electrode is carried out on

a 2 mm thick polycarbonate (PC) substrate. Silver of thickness 500 nm is

deposited on one side of the PC substrate using e-beam evaporation. The

silver is then patterned using a milling machine to define counter and

reference electrode. The reference electrode is oxidized to AgO2. This is

done by applying a voltage of 1.0 V on the silver electrode, which is the

anode by using a platinum electrode as the cathode. The stability of this

electrode is tested in H2SO4 solution. Nitrogen and air is bubbled alternately

into the solution and the open circuit potential of the silver electrode with

respect to a standard calomel electrode is measured.

The Open Circuit Potential (OCP) at this electrode was observed to

be stable. The oxidized silver electrode is used as the reference electrode in

the set-up. To compensate for the evaporation of the electrolyte through the

working electrode, a 1 mm wide and 1 mm deep channel is milled on the

back side of the PC substrate and this channel is sealed with another 1 mm

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thick polycarbonate cover. The channel is used as a connection between the

electrolyte filling port and the electrolyte chamber. This ensures a

continuous supply of electrolyte to the electrolyte chamber.

The working electrode chip is glued on top of the counter and the

reference electrode by using acid resistant silicone epoxy. A gap of 500 μm

is maintained between the working electrode and the counter/reference

electrode by using a spacer frame made of oxidized silicon. The epoxy is

cured in air for about 1 hr. The assembly is then submerged into the

electrolyte solution and put in a vacuum desiccator so that the electrolyte

fills the channels and the chamber between the working and the

counter/reference electrode. The electrolyte now ionically connects the

working, counter and the reference electrodes.

3.4 Advantages

A micromachined nitric oxide sensor has a fast response, is

highly sensitive and stable.

This sensor is potentially suitable for use in point of care testing

in asthma diagnostics.

The NO sensor has high sensitivity in detection of Nitric Oxide.

This also allows fast response and recovery time of 6 s.

This work demonstrates the potential for miniaturized MEMS-

based electrochemical gas sensors with very high fast response

time, enabling sensing in portable devices.

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3.5 Disadvantages

The micro machined nitric oxide sensor has selectivity

problem,because it accurate result for particular gas only.

The nitric oxide gas sensor has very low sensitivity compared

to other sensors.

This sensor has very low concentration and repeatability.

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CHAPTER-4

PROPOSED WORK

In the existing system it has been developed to meet the demand of

very low concentration detection. Platinum is used as a sensing electrode for

detecting the gases.It detects only nitric oxide gas, however for measurement

of gases, they lack important features such as performance, sensitivity and

selectivity.The best result was achieved by using CNT-FET array instead of

platinum electrode. The CNT-FET sensor exhibits excellent sensing and

recovery properties in detecting the poisonous gases. The design of the gas

sensor is implemented using Intellisuite software.

4.1 Carbon nano tube

Recent development of nanotechnology has created a huge potential

to build highly sensitive, low cost, portable sensor with low power

consumption. The extreamly high surface to volume ratio and hollow

structure nanomaterial such as CNT is used for gas molecule adsorption and

storage. Carbon nanotubes has unique geometry, morphology and properties.

4.1.1 Types of CNT

There are two types of nanotubes: Single-walled carbon nanotubes

(SWCNTs) and Multiwalled carbon nanotubes (MWCNTs). An SWCNT

can be considered as a one-atomthick layer of graphite rolled up into a

seamless cylinder with a diameter of several nanometers, and length on the

order of 1–100 microns. MWCNTs consist of multiple layers of graphite

wrapped up together to form a tube shape, sharing the same central axis.

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4.1.2 Properties of CNT

The structure of CNT provides them with unique electrical,physical

and chemical properties. Mechanically, CNTs are the strongest and stiffest

fibers that are known currently due to the C–C bond. Thermally, CNTs have

high thermal stability in both vacuum and air. In terms of electrical

properties, CNTs can be either metallic or semiconducting, depending upon

the tube diameter and the chirality (the direction in which the graphite sheet

is rolled to form the tube). The chirality is usually represented by an integer

pair (n,m). Nanotubes with n − m = 3j ( j being a nonzero integer) are

metallic while all the others are semiconducting. The dielectric property of

nanotubes is highly an isotropic due to their nearly one-dimensional

structures, which may enable nanotubes to carry high currents with

negligible heating effect.

4.1.3 Techniques to prepare CNT

Three main techniques to prepare CNTs are as follows: (i) arc-

discharge technique; (ii) laser ablation technique; (iii) chemical vapour

deposition (CVD) Technique. The carbon arc-discharge method is the first

technique that was used to grow CNTs. The process is carried out in a

vacuum chamber with two carbon electrodes as carbon source. Inert gas

(typically helium) is supplied to increase the speed of carbon deposition.

When high dc voltage is applied between the carbon anode and cathode,

plasma of the inert gas is generated to evaporate the carbon atoms. The

ejected carbon atoms are then deposited on the negative electrode to form

CNTs. Both SWNTs and MWNTs can be grown by this method, while the

growth of SWNTs and other porous structured materials such as porous

silicon requires catalysts. It is the principal method to produce high quality

CNTs with nearly perfect structures. In the laser ablation technique, a carbon

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target is ablated by intense laser pulses in a furnace in the presence of an

inert gas and a catalyst. CNTs are formed and collected on a cold

substrate.Both the arc-discharge and laser-ablation methods require high

growth temperature, which is about 3000–4000◦C for the evaporation of

carbon atoms from solid carbon source.

In a CVD system, a gas hydrocarbon source (usually methane,

acetylene or ethylene) flows into the reaction chamber. The hydrocarbon

molecules are broken into reactive species at the temperature range of 550–

1000◦C. The reactive species react in the presence of catalysts (usually metal

particles such as Ni, Fe or Co) that are coated on the substrate, leading to the

formation of CNTs. Compared with the first two techniques,CNTs can be

synthesized at relatively low temperature using CVD method. Therefore, this

technique is more efficient and allows scaleup growth of SWCNTs. By

modification and calculated control of the growth parameters, vertically

aligned MWCNTs growth can be achieved by CVD technique. This

enhances CNTs electronic properties in different applications. High-quality

SWCNTs can also be obtained by the optimization of the catalysts.

However, one of the main disadvantages of CVD technique is the relatively

high defect densities in MWCNTs,which can be attributed to the lack of

sufficient thermal energy.

In all of these three growth methods, CNTs come with a number of

impurities, which may have negative effects on CNTs’ inherent properties.

In order to make CNT-based devices more efficient and consistent,

purification is an important process to obtain high-quality nanotubes.

Generally,the most commonly observed impurities are carbonaceous

impurities and metallic impurities. The carbonaceous components are the

byproducts of the reaction process while the metallic impurities are residual

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catalysts. To eliminate the carbonaceous impurities, the main method is

oxidation. Two approaches are developed: (i) gas phase purification, which

involves high temperature and has low yield production; (ii) liquid phase

purification, where nanotubes are washed in acidic solutions. Nitric acid,

sulphuric acid, and their mixture are all commonly used for liquid

purification . Metallic impurities can be removed by heating the samples up

to the evaporation temperature of the impurity. With the development and

modification of different purification procedure, so far CNTs samples with a

purity of up to 99.6% can be obtained. The quality of as-grown CNTs and

the purification can be inspected by microscopic techniques such as atomic

force and scanning tunneling microscopy, or spectroscopic techniques such

as Raman spectroscopy.

4.2 CNT-FET array

CNT-FET based sensor is fabricated using SWCNT, because

the fabrication of SWCNT is easy when compared to MWCNT. The

fabrication of CNT-FET array is composed of interdigited Pt electrodes with

CNT chain using Chemical Vapour Deposition method. This device is

composed by 12 CNT-FET and shows very high sensitivity for polluting

gases like NO2, NH3,SO2,CO2 etc. This microsystem consists in an

embedded heating layer placed between the substrate and dielectric

layer(Si3N4).This heater could be used to reduce the recovery time,

desorbing more rapidly the gas molecules.

The CNT-FET arrays have been fabricated using n-doped Silicon

substrate covered with thermally grown SiO2.The linearly joined CNTs can

be used in micro electronic devices like FET. These devices are covered by

27

specific polymers to enhance sensitivity to a specific gas as well as

selectivity.The CNT-FET sensors could detect a concentration of

100ppb.The main advantages of CNT-FET is they are ultra

compact,effective at room temperature, very low power consumption and a

very fast response time,finally relatively low cost can be reached.

28

CHAPTER-5

SOFTWARE REQUIREMENT

Computer simulation has become an essential part of science and

engineering. Digital analysis of components is important when developing

new products or optimizing designs. A computer simulation environment is

simply a translation of real-world physical laws into their virtual form. The

simplification in the translation process helps to determine the accuracy of

the resulting model. Hence it is ideal to have a simulation environment for

which INTELLISUITE is used here.

5.1 Intellisuite

IntelliSense ignited the MEMS industry in the early 1990’s with its

IntelliSuite family of innovative CAD tools. IntelliSuite is the industry-

leading tool set for MEMS layout design, advanced process simulation,

FEA, parametric analysis, system simulation, packaging analysis and more.

Covering all aspects of the MEMS design cycle, IntelliSuite provides a

groundbreaking, end-to-end software solution for MEMS professionals.

IntelliSense also provides a global One Stop MEMS Solution for

universities, blue-chip companies and start-ups with its custom design,

consulting and fabrication services.

IntelliSuite is a tightly integrated design environment that will link

your entire MEMS organization together. Built to scale from a point tool to

an organization-wide tool, IntelliSuite unifies various engineering and

manufacturing tasks into a single living design environment. With the design

around collaboration, IntelliSuite allows the design, process packaging and

system teams collaborate on MEMS devices that can be prototyped and

29

manufactured with fewer costly iterations. IntelliSuite contains a wide range

of closely integrated tools to seamlessly go from schematic capture and

optimization to design verification and tape out. A flexible design flow

allows you to start your design at schematic, layout or 3D level.

IntelliSuite consists of a number of advanced tools that work together.

For instance, Synple allows you to capture your MEMS at a schematic level,

much like SPICE for electrical circuits. Your design can then be quickly

iterated and optimized at different granularities. Sophisticated synthesis

algorithms can automatically convert your schematic into mask layout, 3D

or better yet a meshed structure for full multiphysics analysis.

Blueprint, is a physical design tool that incorporates advanced layout,

design rule check, cross section exploration, and automated mask to hex

mesh capabilities. Clean Room process suite allows you to create and debug

your process flow and your mask set before you enter the clean room. It

allows you to make virtual prototypes to save costly fabrication mistakes.

Advanced Fastfield Multiphysics tools feature fully coupled electrostatic,

mechanical, fluidic, and electromagnetic engines. Advanced Model Order

Reduction based Extraction techniques capture electromechanical, fluidic

and damping behavior into compact models.

Figure 5.1 Seamless design flow

30

5.2 Advantages of intellisuite

The most significant advantage of INTELLISUITE is their major

updates for all software modules and the addition of a new tool, parametric

designer.

Design for manufacturability

1. Fabrication database

2. Thin-film materials engineering

3. Virtual prototyping

Ease of use

1. Consistent user interface

2. Communication with existing tools

Accuracy

1. MEMS-specific meshing and analysis engines

2. In-house code development

3. Validated by in-house MEMS designers

4. ISO certified for quality

31

5.3 Blueprint

Blueprint is the first layout tool designed specifically for the MEMS

community. The goal with Blueprint was to revisit the workflow of the

layout engineer and to create the design that would be effortless to use.

The Blueprint layout editor some new drawing future including an

image import option that let users add JPG, PNG and BMP images into their

current layout. Blueprint is specially designed for constructing and editing

MEMS device masks. It supports layout drawing, VB script layout input,

block hierarchy, design rule check (DRC), and 2D cross-section viewing.

There are five ways to start Blueprint:

Click on IntelliSuite ... Edit Layout in Blueprint from

IntelliFab;

Click on Layout ... Create/Edit ... Top Mask/Bottom Mask from

AnisE;

Click on the Blueprint icon from the Windows Start menu;

Double click on the Blueprint application program;

Double click on any mask file (*.vec, *.gds, *.msk). The .vec,

.gds and .msk are the Blueprint binary file format, industry

standard GDSII file format and InstlliMask text-basedfile.

Figure 5.2 Process in blueprint

32

5.4 Exposure

Exposure is a new addition to IntelliSense’s strong suite of process

simulation tools. This deep resist/SU8 simulator can accurately model

lithography processes like exposure, post-exposure bake, and development.

The dynamic cellular automata simulator and included material databases

ensure robust and speedy lithography simulations.

5.5 Intellifab

IntelliFAB allows designers to takes a different approach. Process

flow forms the basis of creating your devices. This comprehensive process

simulation modules incorporate deposition, etching, bonding, doping,

electroplating, liftoff, and other process steps common in MEMS design.

Other process induced effects, such as micro-assembly, are also addressed to

generate accurate geometric modelsfor the complete range of MEMS

devices.

IntelliFAB, a process simulation module in IntelliSuite, is specially

designed for MEMS process modeling and virtual fabrication. Being a

virtual cleanroom, IntelliFAB offers the capabilities of simulating MEMS

processes that may not be simulated in traditional software packages. With

IntelliFAB, a user can build a process traveller, simulate the process and

visualize the fabricated 3-dimensional structures at the end of each step.

When building a process traveller, the designer uses a standard

process design template and creates a process table on it by selecting such

process steps as wafer or mask definition, thin-film deposition, etching, and

wafer bonding from a database of over 70 process steps. IntelliFAB also

33

features state-of-the-art auto-meshing capabilities to directly mesh solid

models created from the process flow. During the post-fabrication

simulation, a better understanding of the device performance can be

achieved due to incorporation of the processing effects and thin-film

material properties into the device simulation.To open IntelliFAB, select the

following from the start menu of Windows.

Click Start … Programs … IntelliSuite … IntelliFab

Figure 5.3 IntelliFAB main window

34

5.6 Intellietch

The industry’s most powerful wet etch simulator can now simulate

anisotropic etching of quartz in addition to silicon. Features that IntelliEtch

is known for, like etching of high-order planes and multi-masking, are made

easier to use in a huge interface update. The new GPU-based simulator can

run etch simulations over 100 times faster than the previous CPU-based

version.

5.7 3D Builder/hexpresso

IntelliSuite’s state-of-the-art automeshing tools are again updated with

cutting-edge advancements. Material properties and boundary conditions

can be automatically applied when a 3D meshed model is generated. New

adaptive meshing and mesh refinement settings allow users to have full

control over the automated meshing process. Meshing is now faster and

more robust than ever before.

Figure 5.4 Layout of 3D builder

35

5.8 Thermo electro mechanical analysis module

The original MEMS multiphysics simulator has received some powerful

updates in IntelliSuite v8.7. New coupled package-device simulation

capability can be used to analyze packaging effects on a MEMS device.

Dynamic contact analysis can be used to simulate models with multiple

points of contact. IntelliSuite’s powerful reduced-order macromodel

extractor can now generate piezoelectric as well as multi-contact

macromodels. In addition, the finite element solver has been updated to

ensure greater stability and simulation accuracy. It Create Finite Element

Models from masks layouts and process flows, or directlyelement by

element using interactive builder.It includes properties like Piezoelectric

Analysis (Contact, Magnetic, Electromagnetic), Parametric variations and

Sub modeling analysis, Mechanical Deformation with a time varying

voltage signal, Micro assembly and latching modeling.Some of the many

other new features in IntelliSuite v8.7 include:

New MEMS-specific drawing capabilities in the Blueprint™ layout

editor

A new Job Manager™ utility which allows users to queue and run

multiple simulations in a row

New library elements to make design exploration and optimization

easier in the SYNPLE™ schematic-based simulator

A new Mesh Manipulator™ utility which can create curved mesh

surfaces and import/export a variety of file formats.

Independent electrostatic and mechanical meshing based on exposed

face mesh algorithm and support multi-bank exposed face meshing.

36

5.9 Intellimask

IntelliMask is specially designed for constructing and editing MEMS

device level masks. IntelliMask incorporates MEMS specific design features

unavailable in standard electronics and mechanical layout tools.

The tools in IntelliMask are Arcs, Splines, Wires and Curves which

are termed as Standard features. There are five ways to start IntelliMask:

Selecting Construct…Layout…Create Mask from IntelliFab.

Selecting a Create/Edit menu item in the AnisE main user

interface.

Selecting the IntelliMask icon from the Windows Start menu.

Double clicking on the IntelliMask application, or double clicking

on any mask file (“*.msk”).

Figure 5.4 IntelliMask main window

37

CHAPTER-6

MODELING INSTRUCTION

6.1 Steps to design gas sensor

1. Definition Si.

2. Deposition SiO2.

3. Deposition PRS1800.

4. Definition UV.

5. Partially etch photoresist.

6. Partially etch SiO2.


8. Deposition Si3N4.

9. Deposition photoresist.

10. Definition UV.

11. Etch through photoresist.

12. Etch through Si3N4.


14. Deposition Au.

15. Deposition ZnO.

16. Deposition photoresist.

17. Definition UV.

18. Etch through photoresist.

19. Etch through ZnO.


21. Etch Si3N4.

22. Deposition Ti.

38

6.2 Process flow

1. Open the gas sensor process file (ProcessFlow.fab) in IntelliFAB

using the default process database. This can be done by following the steps

below.

2. Launch IntelliFAB.

Click Start …Programs …IntelliSuite …IntelliFab.

3. Click File … Open fab file.

Figure 6.1 Setup window

Click OK.

39

6.3 Fabrication steps

Figure 6.2 The process flow steps in IntelliFAB

The user can explore the process table by double clicking on each of the

process steps and looking at the process parameters.

40

4. Launch FABViewer and visualize the process.

Click Visualize.

Figure 6.3 Definition of silicon

Figure 6.4 Deposition of photoresist

41

Figure 6.5 Exposure of UV

Figure 6.6 Etching of photoresist

42

Figure 6.7 Deposition of Si3N4

Figure 6.8 Deposition of ZnO

43

Figure 6.9 Deposition photoresist

figure 6.10 Deposition of titanium

44

6.4 Thermo-electro-mechanical (TEM) analysis

6.4.1 Export to TEM

1. Click IntelliSuite … TEM.

Figure 6.11 Hexpresso mesh methods

2. Click Mesh Structure.

3. Save the .save file.

Click Save.

Figure 6.12 Mesh parameters

45

4. Click Mesh.

Figure 6.13 Meshing progress

When the meshing is complete, IntelliSuite will automatically create a finite

element model and open it in TEM (Figure 6.14).

Figure 6.14 3D FEA model in TEM

46

6.4.2 View settings

TEM allows the user to independently set the X, Y and Z zoom

factors.

1.The user can use the mouse to move the device in the 3D

space.

2. The user can use the View … Zoom … Define to specify the

view settings in the X, Y and Z directions.

6.4.3 Mesh refinement

1. If the current mesh size is too big, the user can refine the mesh.

Click Mesh … Auto.

2. Enter the maximum mesh size in the dialog box.

IntelliSuite allows the user to apply a variety of loads to the

device. This ranges from force, pressure, Coriolis force, temperature

and other loads. The loads can be constant, time varying or frequency

varying.

3.Start simulation

Click Analysis … Start Static Analysis.

4.View results

Click Result …Displacement…Norm.

47

CHAPTER-7

COMPONENT DESCRIPTION

7.1 Elements of gas sensor

7.1.1 Nafion

Nafion is a sulfonated tetrafluoroethylene based fluoropolymer-

copolymer discovered in the late 1960s by Walther Grot of DuPont. It is the

first of a class of synthetic polymers with ionic properties which are called

ionomers.

7.1.1.1 Preparation

Nafion derivatives are first synthesized by the copolymerization of

tetrafluoroethylene (TFE) and perfluoro (alkyl vinyl ether) with sulfonyl

acid fluoride. The latter reagent can be prepared by the pyrolysis of its

respective oxide or carboxylic acid to give the olefinated structure.

The resulting product is an -SO2F-containing thermoplastic that is

extruded into films. Hot aqueous NaOH converts these sulfonyl fluoride (-

SO2F) groups into sulfonate groups (-SO3−Na+). This form of Nafion,

referred to as the neutral or salt form, is finally converted to the acid form

containing the sulfonic acid (-SO3H) groups. Nafion can be cast into thin

films by heating in aqueous alcohol at 250 °C in an autoclave. By this

process, Nafion can be used to generate composite films, coat electrodes, or

repair damaged membranes.

48

7.1.1.2 Properties

It is highly conductive to cations.

It resists chemical attack. According to DuPont and alkali metals.

The Teflon backbone interlaced with the ionic sulfonate groups gives

Nafion a high operating temperature.

It is selectively and highly permeable to water.

7.1.1.3 Applications

Nafion's superior properties make it suitable for a broad range of

applications. Nafion has found use in fuel cells, electrochemical devices,

chlor-alkali production, metal-ion recovery, water electrolysis, plating,

surface treatment of metals, batteries, sensors, Donnan dialysis cells, drug

release, gas drying or humidifaction, and superacid catalysis for the

production of fine chemicals. It is also often cited for theoretical potential

(i.e., thus far untested) in a number of fields. With consideration of Nafion's

wide functionality, only the most significant will be discussed below.

7.1.1.4 Chlor-alkali production cell membrane

Chlorine and sodium/potassium hydroxide are among the most

produced commodity chemicals in the world. Modern production methods

produce Cl2 and NaOH/KOH from the electrolysis of brine using a Nafion

membrane between half-cells. Before the use of Nafion, industries used

mercury containing sodium amalgam to separate sodium metal from cells or

asbestos diaphragms to allow for transfer of sodium ions between half cells;

both technologies were developed in the latter half of the 19th century. The

disadvantages of these systems is worker safety and environmental concerns

associated with mercury and asbesto

played a part. Nafion was the direct result of the chlor

addressing these concerns; Nafion could tolerate the high temperatures, high

electrical currents, and corrosive environment of the electrolytic cel

Figure 7.1

The figure to the right shows a chlor

functions as a membrane between half cells. The membrane allows sodium

ions to transfer from one cell to the other with minimal electrical resistance.

The membrane was also reinforced with additional membranes to prevent

gas product mixing and minimize back transfer of Cl

side positive and cathode side negative terminal is applied.

7.1.1.5 Proton exchange membrane (PEM) for fuel cells

Although fuel cells have been used since the 1960s as power supplies

for satellites, recently they have received renewed attention for their

potential to efficiently produce clean energy from hydrogen. Nafion was

found effective as a membrane for

cells by permitting hydrogen ion transport while preventing electron

49

associated with mercury and asbestos, although economical factors also

played a part. Nafion was the direct result of the chlor-alkali industry


electrical currents, and corrosive environment of the electrolytic cel

Figure 7.1 A chlor-alkali cell

The figure to the right shows a chlor-alkali cell where Nafion



membrane was also reinforced with additional membranes to prevent

gas product mixing and minimize back transfer of Cl− and −OH ions. Anode

side positive and cathode side negative terminal is applied.

roton exchange membrane (PEM) for fuel cells

though fuel cells have been used since the 1960s as power supplies



found effective as a membrane for proton exchange membrane (PEM)

by permitting hydrogen ion transport while preventing electron

s, although economical factors also

alkali industry


electrical currents, and corrosive environment of the electrolytic cells.

alkali cell where Nafion



membrane was also reinforced with additional membranes to prevent

OH ions. Anode

though fuel cells have been used since the 1960s as power supplies



(PEM) fuel

by permitting hydrogen ion transport while preventing electron

50

conduction. Solid Polymer Electrolytes, which are made by connecting or

depositing electrodes (usually noble metal) to both sides of the membrane,

conduct the electrons through an energy requiring process and rejoin the

hydrogen ions to react with oxygen and produce water. Fuel cells are

expected to find strong use in the transportation industry.

7.1.1.6 Superacid catalyst for fine chemical production

Nafion, as a superacid, has potential as a catalyst for organic

synthesis. Studies have demonstrated catalytic properties in alkylation,

isomerization, oligomerization, acylation, ketalization, esterification,

hydrolysis of sugars and ethers, and oxidation. New applications are

constantly being discovered. These processes, however, have not yet found

strong commercial use.

7.1.1.7 Several examples are shown below

a)Alkylation with alkyl halides

Nafion-H gives efficient conversion whereas the alternative method,

which employs Friedel-Crafts synthesis, can promote polyalkylation:

b)Acylation

The amount of Nafion

with aroyl chloride is 10–

c)Catalysis

Nafion-H increases

o-trialkylsilation of alcohols, phenol, and carboxy

d)Isomerization

Nafion can catalyze a

enzymes within the Nafion by enlarging pores with

maintains a structure and pH to provide a stable environment for the

enzymes.

Applications include catalytic oxid

51

The amount of Nafion-H needed to catalyze the acylation of benzene

–30% less than the Friedel-Crafts catalyst:

H increases reaction rates of protection via dihydropyran or

trialkylsilation of alcohols, phenol, and carboxylic acids.

Nafion can catalyze a 1,2-hydride shift. It is possible to immobilize

within the Nafion by enlarging pores with lipophilic salts. It


Applications include catalytic oxidation of adenine dinucleotides.

H needed to catalyze the acylation of benzene

Crafts catalyst:

via dihydropyran or

. It is possible to immobilize

salts. It


.

52

7.1.1.8 Sensors

Nafion has found use in the production of sensors, which with

application in ion-selective, metallized, optical, and biosensors. What makes

Nafion especially interesting is its demonstration in biocompatibility. It has

been shown to be stable in cell cultures as well as the human body, and there

is considerable research towards the production of higher sensitivity glucose

sensors.

7.1.1.9 Modified nafion for PEM fuel cells

Normal Nafion will dehydrate (thus lose proton conductivity) when

temperature is above ~80 °C. This limitation troubles the design of fuel

cells, because higher temperatures are desirable for a better efficiency and

CO tolerance of the platinum catalyst. Silica and zirconium phosphate can

be incorporated into Nafion water channels through in situ chemical

reactions to increase the working temperature to above 100 °C.

53

7.1.2 Platinum

Platinum black is widely used as a thin film covering solid platinum

metal, forming platinum electrodes for applications in electrochemistry. The

process of covering platinum electrodes with such a layer of platinum black

is called "platinization of platinum". The platinized platinum has a true

surface area much higher than the geometrical surface area of the electrode

and, therefore, exhibits action superior to that of shiny platinum.

Platinum black powder is used as a catalyst in proton exchange

membrane fuel cells. In common practice, the platinum black is either

sprayed using an ultrasonic nozzle or hot pressed onto the membrane or gas

diffusion layer. A suspension of platinum black and carbon powder in

ethanol-water solutions serves to optimize the uniformity of the coating,

electrical conductivity, and in the case of application to the membrane, to

prevent dehydration of the membrane during the application. Historically

many "self-lighting" gas lamps, ovens, and stove burners used platinum

black to catalyze the oxidation of a small amount of gas, lighting the device

without a match or spark. This works particularly well for producer gas,

town gas, and wood gas which contain a substantial fraction of hydrogen gas

(H2) which is particularly well catalyzed by platinum black.

Platinum black powder can be manufactured from ammonium

chloroplatinate by heating at 500 °C in molten sodium nitrate for 30

minutes, followed by pouring the melt into water, boiling, washing, and

reduction of the brown powder (believed to be platinum dioxide) with

gaseous hydrogen to platinum black. Before platinization, the platinum

surface is cleaned by immersion in aqua regia (50% solution, i.e., 3 volumes

54

of 12 mol/kg of HCl, 1 volume of 16 mol/kg HNO3, 4 volumes of water).

Platinization is often conducted from water solution of 0.072 mol/kg of

chloroplatinic acid and 0.00013 mol/kg of lead acetate, at a current density

of 30 mA/cm2 for up to 10 minutes. The process evolves chlorine at the

anode; the interaction of the chlorine with the cathode is prevented by

employing a suitable separation (e.g. a glass frit). Electroplating with the

current density of 5 mA/cm2 while reversing the polarity. After platinization,

the electrode should be rinsed and stored in distilled water. The electrode

loses its catalytic properties on prolonged exposure to air. Platinum sponge

is yet another form of platinum metal with a developed surface area, distinct

from platinum black and platinized platinum. Platinum sponge can be

obtained by strongly heating ammonium chloroplatinate. It has a gray to

black colour, while its catalytic properties vary depending on the specifics of

the manufacturing. Hydrogen saturated hydrochloric acid, the shiny

platinum electrode is observed to assume positive potential versus that of

platinum black at zero net current (+ 340 mV at room temperature). With the

temperature increasing to 70 °C, the difference in potentials dropped to zero.

The reason for this is not perfectly clear, although several explanations have

been proposed.

7.1.2.1 Choice of platinum

Inertness of platinum (it does not corrode).

The capability of platinum to catalyze the reaction of proton

reduction.

A high intrinsic exchange current density for proton reduction on

platinum.

Excellent reproducibility of the potential.

55

a)The surface of platinum is platinized to the following reasons

Increase total surface area. This improves reaction kinetics and

maximum possible current.

Use a surface material that adsorbs hydrogen well at its interface. This

also improves reaction kinetics.

Other metals can be used for building electrodes with a similar function such

as the palladium-hydrogen electrode. Because of the high adsorption activity

of the platinized platinum electrode, it's very important to protect electrode

surface and solution from the presence of organic substances as well as from

atmospheric oxygen. Inorganic ions that can reduce to a lower valency state

at the electrode also have to be avoided (e.g., Fe3+, CrO42−). A number of

organic substances are also reduced by hydrogen at a platinum surface, and

these also have to be avoided.

b)Cations that can reduce and deposit on the platinum can be source of

interference

silver, mercury, copper, lead, cadmium and thallium.

Substances that can inactivate ("poison") the catalytic sites include arsenic,

sulfides and other sulfur compounds, colloidal substances, alkaloids, and

material found in living systems.

56

7.1.3 Micro-hotplate

The concept of micro-hotplates on silicon appeared in the 80’s and

was first used for thermal and gas sensors. Micro-hotplates have been used

in gas sensors, integrated circuits, actuators and fluidics systems in fields

such as electronic noses, electronics, space, biomedical and micro total

analysis systems.

Depending on the applications, different fabrication techniques were

used to thermally isolate the heating element. The first versions were used

for polymer and metal-oxide gas sensors. Lately, micro-hotplates for drop-

coated and MOSFET gas sensors were designed. The processing of the

micro-hotplates having a silicon island was modified to thermally isolate

electronic devices. An array of 4 gas sensitive MOSFETs including a diode

used as temperature sensor and a heating resistor were implemented in the

silicon island suspended by a dielectric membrane.

The MOSFET have their gate replaced by catalytic metals and are

suitable for gas sensing operation at an operating temperature up to 200°C.

The low-thermal mass device allows new modes of operation for MOSFET

gas sensors such as a cycled temperature mode and combined field-effect

and the next generation of low-power MOSFET will be fabricated using

Silicon On Insulator (SOI) wafers to decrease the mass of the silicon island

in the aim of improving the power consumption and the thermal time

constant of the device.

Micro-hotplates are integrated in the system to lower the power

consumption and thermal constant time. Micro-hotplates can be used for

instance in micro-thruster for the control of nano satellites. They have been

also developed for chemical micro sensors for space application. Another

57

area of application is in the “lab on chip” systems for micro total analysis. At

IMT, the work on micro-hotplates has started on gas sensing applications.

The micro-hotplates are also of interest for pneumatic actuator

systems. Micro-transportation systems applicable to highly clean

environment are in great demand, and the practicability of pneumatic

conveyor suitable to IC-process and micro factory has previously been

demonstrated. Pneumatics actuators are spared from the problem of particle-

attraction and the version developed at IMT is valve free. They have shown

excellent switching behavior of the working gas by thermo-electrically-

triggered Coanda effect. Since many nozzles are used to move the slider in

the conveyor, micro-hotplates are integrated in the system to lower the

power consumption and thermal constant time.

In this communication, a review of the micro-hotplates design and

processing developed mainly at IMT will be presented for these different

applications.Micro-hotplates from IMT are based on a low stressPECVD or

LPCVD silicon nitride membrane. In the case of metal-oxide gas sensors,

the heater and the electrodes are made of platinum. The insulator between

them can be composed of silicon nitride or oxide thin films. The micro-

hotplates withstand a high annealing temperature of 700°C necessary for

drop-coated gas sensors. They can also be operated in a pulsed temperature

mode of operation to temperatures up to 700°C. This makes possible the

annealing on chip of the gas sensitive coating. A silicon island was added

underneath the membrane to improve the temperature distribution over the

sensing area.

Finally, upcoming activities on space and micro total analysis systems

will be presented. Micro-hotplates can be used for instance in micro-thruster

for the control of nanosatellites. They have been also developed for chemical

58

microsensors for space applications. Another area of application is in the

“lab on chip” systems for micro total analysis. Flow sensors and heaters are

needed to control the chemical reactions such as in Polymerase Chain

Reaction (PCR) where the temperature has to be cycled.

59

CHAPTER-8

RESULT AND DISCUSSION

8.1 Detection of gases

Figure 8.1 Detection of NO2 gas

Figure 8.2 Detection of NH3 gas

60

Figure 8.3 Detection of SO2 gas

Figure 8.4 Detection of CO2 gas

61

8.2 Lookup table

Table 1 Properties of gases

PROPERTIES / GASES

NO2 NH3 CO2 SO2

Molecular weight(g/mol)

46.006 17.03 44.01 64.064

Pressure(Mpa)

10.132 11.33 7.377 7.884

Temperature(oC)

158 132.25 30.98 157.49

Gas Density(kg/m3)

3.4 0.889 2.813 3.0571

Thermal conductivity(mW/mK)

12.961 22.916 14.674 8.4339

8.3 Plotting points

Table 2 Plotting values

S.NO PRESSURE(Mpa) SENSITIVITY

1 7.377 7.35895 e-035

2 7.884 8.14583 e-035

3 10.132 4.87843 e-035

4 11.33 6.50458 e-035

62

8.3 Sensitivity plot

Figure 8.5 Pressure Vs Sensitivity

0

0.01

0.02

0.03

0.04

0.05

0.06

7.3 7.8 10 11

Sen

siti

vity

Sensitivity Plot

Pressure(Mpa)

63

CHAPTER-9

CONCLUSION & FUTURE WORK

9.1 Conclusion

The MEMS gas sensor using CNT-FET array is presented has high

sensitivity, selectivity, fast response and recovery time. This sensor is used

as a point of care testing in asthma diagnosis. The design and fabrication of

suitable CNT-FET array, the stability and sensor response during long

period of time, humidity, etc. can be improved. Once this technology gets

commercialized it will be broadly implemented. Due to the enormous

application, it might be reasonable to hope that the large scale fabrication

method of CNT-FET would be developed resulting in the decrease of the

cost.

9.2 Future work

Future work will continue to focus on generating improved

component assembly and device fabrication strategies. For small or medium-

scale batch production, a hybrid design whereby component parts are created

separately for subsequent assembly into a complete system is an attractive

option, as it removes many of the restrictions imposed by the need for

process compatibility. Integrated gas sensor with a micro-hotplate (MHP)

demonstrates better sensitivity, faster response, and lower power

consumption than traditional thick film devices. For the detection of a target

gas, a gas-sensitive layer is applied on a suspended micro-hotplate that is

mounted on top of a silicon chip. Any gas-induced change of the gas-

sensitive layer’s surface potential is detected and digitally processed by the

integrated electronics.

64

CHAPTER-10

REFERENCES

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2. B. Gao, Y.F. Chen, M.S. Fuhrer, D.C. Glattli, A. Bachtold, (2005)

“Four-point resistance of individual single-wall carbon nanotubes”,

Phys. Rev. Lett. 95, 196802.

3. Charles P. Poole Jr , Frank J. Owens, (2012) “Introduction to

nanotechnology”, Wiley India Pvt.Ltd.

4. E.N.Ganesh, (March 2013) “International Journal of Innovative

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