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CHAPTER 1

INTRODUCTION

1.1 INTRODUCTION TO MEMS PRESSURE SENSOR

The spectrum of capacitance based pressure sensor applications has

increased due to its advantages like high accuracy, free from temperature effects and

long-term stability. It finds broad application in the areas of harsh environmental

conditions, where these sensor characteristics are vital.

Micromachined Micro Electro Mechanical Systems (MEMS) pressure

sensor, finds wide applications in aerospace, medical, analytical instrumentation and

commercial. MEMS pressure sensor has more advantages than the conventional

pressure sensor because of its low weight, low cost, reliablity, smart function and

occupies less space [1]. Capacitive pressure sensors provide high sensitivity to

pressure, low power consumption, low noise, a large dynamic range and low thermal

sensitivity than the piezoresistive pressure sensors [2].

MEMS have been one of the key enabling technologies in the field of

microelectronics. They have successfully replaced all bulk sensing systems with

miniature scale sensors and are found to be suitable for many commercial and industrial

applications as well. Following this trend, MEMS have now matured to a point, where

they will be applied in biological and chemical applications and could successfully

replace the sensing systems that are currently being used [3].

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Silicon based devices are also attractive due to possibility of integrating

electronics next to the MEMS devices on the same substrate. Even now, most of the

devices are fabricated in silicon, because of its well known electrical and mechanical

properties [4]. A large variety of bulk micromachined and surface micromachined

pressure sensors have been developed for industrial, biomedical and automotive

applications [5].

In addition to silicon, alternative substrates such as metal, glass / quartz,

ceramics, plastic and polymer materials are gaining popularity. The driving factors for

this change are to produce devices which are bio-compatible, low material cost and

easy to fabricate.

1.1.1 MEMS Piezoresistive Pressure Sensor

A MEMS pressure sensor consists of a diaphragm that responds to a

mechanical input of pressure and outputs an electrical signal. In a piezoresistive

pressure sensor, elements are formed at the edges of the diaphragm. Electrical

resistance of the diaphragm changes, when it is mechanically stressed by the deflection

of the diaphragm caused by the applied pressure. By forming a bridge using these

piezoresistors, an electrical output is obtained.

Many commercialized MEMS pressure sensors [2], [6-10] are based on

piezoresistive transduction mechanism. They measure pressure variation into change in

resistance. Piezoresistive pressure sensors make use of the change in the resistance due

to the change in their physical dimensions and carrier mobility, when it is subjected to

strain. It has advantages such as simple to fabricate and no electronic circuit is required.

It has a high gauge factor but it has 0.27% per °C of temperature coefficient of

piezoresistivity [8]. This limits the operating temperature and requires temperature

compensation circuit.

Most researchers preferred piezoresistive technique because, the properties

of silicon material were well established and the facilities of existing silicon foundry

can be used for fabrication in batch production. Micromachined pressure sensors are

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fabricated using bulk and surface micromachining techniques [1], [6], [10]. High aspect

ratio structure is fabricated using bulk micromachining and surface micromachining is

preferred to a larger surface area with for a few micrometer depth.

1.1.2 MEMS Capacitive Pressure Sensor

Capacitive Pressure Sensor measures changes in pressure by the deflection

of a conductive diaphragm due to the measurand pressure. Parallel plate differential

pressure sensors typically have spacer (dielectric separator) between the two electrodes

and the deflection in the diaphragm due to change in pressure that produces a change in

capacitance [11]. In the proposed differential pressure sensor, the ambient pressure

(1013 mbar ) acts as reference pressure and the external pressure (measurand) acts as

the source. This technique reduces the package cost and eliminates the need of vacuum

sealing of the diaphragm. Moreover, it has high sensitivity for static and dynamic

pressure measurements.

1.2 EXISTING PITOT STATIC SYSTEM

A pitot static system is a system of pressure sensitive instruments that is

used in aviation, to determine the aircraft's altitude, airspeed and rate of climb and is

shown in the Figure 1.1. A pitot static system generally consists of a pitot tube, a static

port and pitot static instruments such as airspeed indicator, vertical speed indicator and

altimeter [12].

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Figure 1.1 Pitot static systems [13]

The pitot pressure is obtained from the pitot tube. The pitot pressure is a

measure of ram air pressure, created by air ramming into the tube, which is equal to

pressure. The static pressure is obtained through a static port. It is a flush mounted hole

on the fuselage of an aircraft, where it can access the air flow in a relatively

undisturbed area. Usually one or more static ports are located on each side of the

fuselage.

The pitot static system of instruments uses the principle of air pressure

gradient. It works by measuring pressures or pressure differences and uses these values

to assess the speed and altitude. These pressures can be measured either from the static

port (static pressure) or the pitot tube (pitot pressure). The static pressure is used in all

measurements, while the pitot pressure is only used to determine airspeed.

The pressure altimeter also known as the barometric altimeter which is used

to determine the changes in air pressure that occurs when the aircraft's altitude changes

and it is shown in the Figure 1.2.

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Figure 1.2 Barometric altimeter [13]

Pressure altimeters must be calibrated prior to flight in order to register the

pressure at an altitude above the sea level. The instrument case of the altimeter is

airtight and has a vent to communicate pressure from the static port. Inside the

instrument, there is a sealed aneroid barometer. As pressure in the case decreases, the

internal barometer expands, which is mechanically translated into a determination of

altitude. The reverse is true when descending from higher to lower altitudes.

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1.3 EXISTING MICRO PRESSURE SENSING TECHNOLOGY

Pressure sensors are categorized into two types. They are force collector

type and density properties [14].

1.3.1 Force Collector Type

Force collector mechanism such as diaphragm, piston, bourdon tube and

bellows are used to measure strain or deflection due to applied force (pressure) over an

area.Transduction mechanism is used to convert the pressure into electrical signal and

any of the transduction mechanism can be used from the following principle, such as

piezoresistive, capacitive, electromagnetic, piezoelectric, optical and potentiometric

[14]. Sensors of this type are shown in the Figure 1.3.

Figure 1.3 Force collector type pressure sensors: (a) simple diaphragm (b) corrugated

diaphragm (c) capsule (d) capacitive sensor (e) bellows (f) Bourdon tube

(g) Straight tube (adapted from [15]).

1.3.2 Density Properties

Pressure sensors use density properties to infer pressure of a gas or liquid.

Resonance, thermal and ionization are the transduction mechanism used to convert the

density parameter into electrical signal.

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1.4 RESEARCH MOTIVATION

The motivation of this research is to model a high sensitive capacitive

principle differential pressure sensor for aircraft altimeter. The lecture series of the

conference titled “Future MEMS application in Military Aircraft” organized by North

Atlantic Treaty Organization (NATO) addresses, the potential areas where MEMS

technology can be used to replace present technology in aerospace application [16].

Piezoresistive pressure sensing techniques is widely adopted in the present

pressure sensor. Aircraft altimeter has to be operated in wide temperature variation.

Piezoresistive techniques are highly sensitive to temperature variation and hence needs

temperature compensatory circuit. From literature, it is observed that the temperature

effect on capacitive sensitivity is negligible. Therefore capacitive transduction

mechanism was adopted to model.

1.5 RESEARCH OBJECTIVES

MEMS based single chip realization of altimeter as a replacement for

conventional altimeter offers advantages in terms of size, weight and cost. Capacitive

transduction mechanism is ideally free from temperature effect over piezoresistive

transduction mechanism.

Therefore, the problem was identified to design, model and analyze an

MEMS based Capacitive Differential Pressure Sensor (CDPS) for the pressure

range from -56 mbar to 900 mbar equivalent to the aircraft flying altitude of

50000 feet above the sea level.

MEMS based Capacitive Differential Pressure Sensor can be employed

in air data instrumentation used to measure the differential pressure ranging from

56 mbar56 mbar to 900 mbar .

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The research objectives were to:

(a) Propose a suitable sensor structure and diaphragm material for modeling.

(b) Conduct performance study and characterization of the best model.

(c) Develop mathematical formulation to estimate the micro device parameter such

as deformed diaphragm surface length, surface area, volume, center deflection

and capacitive characteristics of the design.

(d) Propose a simple fabrication flow with reduced fabrication complexity and

verify the fabrication process flow in Intellisuite MEMS design tool – IntelliFab

v8.6.

1.6 REVIEW OF PREVIOUS WORK

Literature review on the characteristics of diaphragm material

Silicon is used as diaphragm material [17-18], [19], [20] for high pressure

application, for the pressure range (0-414) kPa for a square diaphragm of dimension

600 µm x 600 µm with 5 µm thickness. Silicon material is also used for ultra low

pressure application for the pressure range (66-106) kPa for a square diaphragm of

dimension 1000 µm x 1000 µm and 3 µm thickness [20]. Further, silicon or polysilicon

is extensively used as diaphragm membrane material with square, circular and

rectangular dimensions [18], [21-23].

Silicon carbide (SiC) is used for high temperature application for a circular

diaphragm of 800 µm diameter and 0.5 µm thicknesses, characterized at 400°C for the

pressure range of (0-333) kPa [24]. SiC diaphragm for high pressure application is

characterized at 300°C for the dimension 4880 µm x 2800 µm with 20 µm thickness for

the pressure range of (0-344) kPa [25].

Young and Ko developed a touch mode single crystal 3 silicon carbide

capacitive pressure sensor for high temperature application [26] and is shown in the

Figure 1.4. Silicon substrate was used as base material and silicon carbide as a

diaphragm for a circular dimension. Diaphragm was bonded to base cavity at vacuum

pressure of 48 kPa. This was tested for the temperature range up to 400°C for the

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pressure range from 0 to 333 kPa. The observation is that the effect of temperature on

capacitive sensitivity is negligible.

Figure 1.4 Cross section view of silicon carbide diaphragm pressure sensor [26]

Sung-Pil Chang and Mark G. Allen have developed capacitive pressure

sensor with stainless steel diaphragm material. The circular diaphragm of 1000 µm

diameter and 2.7 µm thickness was characterized for the pressure range of (0-180) kPa

[27], [28].

Liquid crystal polymer (LCP) exhibits good dimensional stability, good

material flexibility, extremely low moisture absorption and high chemical resistance

and therefore is suitable for high sensitive capacitive pressure sensors application.

Jithendra N. Palasagaram and Ramesh Ramadoss fabricated circular

diaphragm capacitive pressure sensor using printed circuit processing techniques of

3000 µm diameter with 50 µm thickness and characterized it for the pressure range of

(0-170) kPa [29-30].

Parylene material is used as diaphragm material for intraocular pressure

(IOP) monitoring in glaucoma patients. The sensor is monolithically microfabricated by

exploiting parylene as a biocompatible diaphragm material and suitable for minimally

invasive intraocular implantation. The fabricated parylene diaphragm had dimensions

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of 4000 µm x 2000 µm of 8 µm thickness and was characterized for the differential

pressure range of (0-4) kPa [31].

Polyimide material exhibits an excellent balance of physical, chemical, and

electrical properties over a wide temperature range with superior dimensional stability

particularly at high temperatures and also has good adhesion characteristics [32].

Jeahyeong Han and Mark A. Shannon developed smooth contact capacitive

pressure sensors with single and double parabolic cavity operated in touch- and

peeling-mode. Sandwich polyimide diaphragm fabricated for the dimension

18000 µm x 16000 µm and 6 µm thickness was characterized for the differential

pressure range of (0-35) kPa [33].

Sung-Pil Chang and Mark G. Allen also developed capacitive pressure

sensor with polymide diaphragm material for circular diaphragm of 1000 µm diameter

and 12.7 µm thickness and characterized for the pressure range of (0-180) kPa [27],

[28].

Min Xin Zhou et al. proposed a modeling of triple layered absolute

capacitive pressure sensor [21] which is shown in the Figure 1.5.

Figure 1.5 Simplified cross section structure description of the capacitive pressure

sensor with a triple-layered composite-membrane [21].

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This sensor was developed for barometric application in micro weather

station. It was designed to operate in the pressure range from 50 to 110 kPa. The

change in the capacitance for a single layer was 6 pF and for a triple layer 36 pF.

Multilayer diaphragm membrane design using polymer material was observed in the

design.

This work helps to develop a composite layered diaphragm using

polymer material.

Literature review on sensor structure and diaphragm modeling

Simple parallel plate capacitive sensor structure

Qiang Wang and Ko developed an absolute pressure sensor using simple

parallel plate capacitive sensor structure [18-19]. Han and Shannon proposed, touch

and peel mode capacitive pressure sensor with polyimide diaphragm membrane.

Polymer diaphragm membrane with metal deposition was proposed and developed in

his work [27], [28-31]. Young et al. has proposed capacitive differential pressure sensor

with silicon carbide diaphragm material to measure pressure at high temperature

environment [24].

Qiang Wang et al. modeled the touch mode capacitive pressure sensor

diaphragm [18-19], [34], shown in the Figure 1.6. Silicon was used as structure and

diaphragm membrane material. In his work, the diaphragm deflection characteristic

was studied for square, rectangle and circular diaphragm. The stress, due to the applied

pressure was analyzed. Rectangle diaphragm showed higher stress than square and

circular diaphragm. Capacitive sensitivity reported as 4.35 fF / kPa for circular

diaphragm of 5 µm thickness for the pressure range from 345 to 690 kPa.

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(a)

(b)

Figure 1.6 Schematic diagram of principle of touch mode pressure sensor

(a) Normal mode operation (b) Touch mode operation [17]

Wen H. Ko was the first author, who proposed the MEMS capacitive

principle sensor. Silicon material was used as diaphragm membrane to measure the

narrow pressure range targeted for tire pressure measurement.

From the above reference, a simple MEMS capacitive sensor structure

was been taken.

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Simple parallel plate capacitive sensor structure having center bossed diaphragm

Capacitive pressure sensor produces quadratic variation in capacitance for

the working pressure range. Nonlinearity in capacitive sensitivity is observed, which is

due to diaphragm edge built in. To reduce the non linearity in capacitive sensitivity,

improvement is made in the diaphragm modeling by introducing small boss at the

center of diaphragm.

Abhijeet V. Chavan and Kensall D. Wise developed a vacuum sealed

capacitive pressure sensor with bossed polysilicon circular diaphragm membrane [35]

that is shown in the Figure 1.7. His diaphragm modeling for absolute capacitive

pressure sensor reduces capacitance nonlinearity but increases the structure fabrication

complexity.

This sensor was developed for barometric pressure application for the

pressure range of (67-107) kPa. Capacitance sensitivity 0.203 fF / kPa were reported

for the pressure resolution of 4.93 kPa. As the capacitive sensitivity is not linear over

the operating range of pressure, the center boss on diaphragm membrane was

introduced which helps to reduce non linearity.

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(a)

(b)

Figure 1.7 Cross sections of the capacitive sensor using a single transfer lead from the

sealed cavity. (a) The cut is shown through the lead transferring the glass electrode out

of the cavity. A tab used to contact the wafer bulk during bonding is also shown

(b) The cut is shown along a device diagonal. The inner ring forms the vacuum seal; the

outer ring provides a permanent contact to the silicon electrode [35]

From this work, it is observed that the introduction of center boss on

the diaphragm membrane of capacitive sensor will help to linear the capacitive

sensitivity.

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Complex parallel plate capacitive sensor structure

Zhang et al. proposes a parallel plate high sensitive MEMS capacitive

absolute pressure sensor [22]. In this model, polysilicon ultra thin diaphragm is used to

sense the pressure. It has fixed top plate and movable bottom plate of capacitive sensor.

Movable bottom plate is attached to the center of diaphragm with small die separator.

The fabrication process of the sensor structure is more complex.

Complex comb drive finger parallel plate capacitive sensor structure

Duck-Bong Seo and Robin Shandas [36] proposed a capacitance sensor

having a comb drive finger plates focused to solve non linear capacitive sensitivity in

the membrane type capacitive pressure sensors. He developed a comb drive capacitor

design to solve the problem.

Table 1.1 discusses the literature review on capacitive pressure sensor.

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Table 1.1 Summary of literature review on capacitive pressure sensor

* Silicon diaphragm material

† Polymer diaphragm material

Authors References Sensor Dimension in µm and diaphragm

material

Pressure range Units

Qiang Wang et al. [17] 1500 x 480 x 5*

(rectangular) 0 -60 psi

Wen H Ko et al. [18] 600 x 600 x 5* (square) 0 -60 psi

1500 x 457 x 5* (rectangular)

0.1-1000 psi

Yong S Lee et al. [19] 2100 x 2100 x 24*

(square) 0-350 mmHg

Min-Xin Zhar et al. [21] 200 x 200 x 1* (square) 0.01- 0.12 MPa

Yixian Ge et al. [22] 1650 x 50* (circular) 0-3 MPa

Darrin J young et al. [26] 400 x 0.5* (circular) 1100-1760 Torr

Sung- pil chang et al. [33] 100 x 100 x 50†(square) 0-200 kPa

Abhijit V et al. [35] 1000 x1000 x3*

(square) 500-800 Torr

Albert K Henning et al. [37] 762 x 1* (circular) 0-20 psi

Hussam Eldin et al. [38] 1200 x 1200 x15*

(square) 0-120 mmHg

Patel Hardik et al. [39] 2400 x 2400 x 50†

(square) 10-1000 mbar

Orhan Akbar et al. [40] 2600 x 1600 x 1.2*

(rectangular) 0-50 mmHg

Sippola et al. [41] 4800 x 2800 x 64*

(rectangular) 0-50 psi

Kerstin E. Babbit et al. [42] 500 x 1* (circular) 0-3 psi

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Literature discussion on the analytical modeling

Classical plate theory has been used to analyze the center deflection and

stress of the circular and square micro diaphragm with clamped edges. Finite Element

Method (FEM) tool results were compared with classical plate theory result and the

accuracy was evaluated.

Timoshenko S and Woinowsky Kriger has given fourth order differential

equation to get the center deflection, stress and strain components imposed on the

square, rectangular and circular plates with various boundary conditions [43].

The FEM methodology is used in various areas of engineering, in which the

problems are modeled for the partial differential equations given in classical plate

theory. The results obtained with FEM tool is regarded as relatively accurate and

versatile numerical tool for solving differential equations that model micromachined

pressure sensor [44-45].

M. Arad et al. adopted biharmonic equation, to study the deflection of

rectangular loaded plates. The advantage of the suggested scheme is demonstrated for

solving problems of the deflection of rectangular plates for cases of different boundary

conditions, such as a simply supported plate and a plate with built in edges. The

numerical results are compared with exact solutions, which show sixth-order accuracy

of the method [45].

Fouad Kerrour and Farida Hobar proposed Galerkin method to evaluate the

deflection of thin silicon square plate. Polynomial model and trignometrical model

were used in his work. The developed algorithm is simple, easy to implement and has

good stability. His results reveal fourth order and reduce computation time [46].

Ali Ergun and Nahit Kumbasar propose a new approach of improved finite

difference scheme for a thin plate deflection analysis. Improved finite difference

scheme algorithm uses Lagrange interpolation polynomial and Betti reciprocal

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theorem, which converge rapidly to the exact solution with high accuracy and having

good agreement with other numerical methods [47].

C. Erdem Imraketal gave an exact solution of the governing equation of an

isotropic rectangular plate for clamped edges. A numerical method for clamped

isotropic rectangular plate under distributed loads and an exact solution of the

governing equation in terms of trigonometric and hyperbolic functions were given. He

compared his centre deflection results with the previously reported work, which shows

sixth degree accuracy [48].

Gong et al. provided a new analytical solution for centre deflection. In their

work, clamped silicon plate was considered. The deflection of simply supported

structure and the edges with the boundary condition was estimated and by

superposition, the centre deflection was evaluated [49].

V.M.A. Leitao proposed a meshless method for the analysis of Kirchoff

plate bending [50]. He used radial basic function. His results showed fourth degree

accuracy.

Classical double cosine series expansion and Sherman Morrison Woodbury

formula was used by R.L.Taylor and S. Govindjee [51]. His results converged with

additional degree accuracy on comparing with classical method [43].

Clark and Wise solved differential equation governing the deflection of thin

square diaphragm membrane with clamped edges [52]. He approximated the derivative

functions into coefficient of the finite difference equations. His results were close to

the classical method.

Zaniar Tokmechi and David A. Pape discussed conventional study on

deflection of plates with simply loaded conditions. Double trigonometric series of

Navies’ solution was adopted [53-54].

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David A. Pape et al. analyzed the deflection with various boundary

conditions and his results showed excellent agreement with reference [43].

From this review on analytical modeling, it is observed that, the

physical parameter estimation of micro device parameters is not considered.

Hence a new formulation is carried out to estimate the physical parameters of

micro devices.

Literature discussion on the fabrication process

Han and Shannon fabricated capacitive pressure sensor using polyimide

material diaphragm [27]. In his work, the sensor was fabricated into two modules such

as, upper flexible electrode with Cr/Au/Cr deposition on polyimide material form top

plate of capacitive sensor and bottom electrode (die) fabricated using silicon substrate.

Epoxy adhesive is used to attach polyimide diaphragm to the silicon die.

Chang et al. fabricated robust capacitive pressure sensor using kapton

polyimide film diaphragm [27]. For making conducting plate, metallization done on

polyimide film, few nanometers of Ti/Cu/Ti is deposited on polyimide material.

Stainless steel is used as sensor base structure. Reactive ion etching (RIE) is used for

dry etching polyimide material. Epoxy adhesive is used to bond the diaphragm to the

sensor structure.

Palasaragam and Ramdoss fabricated capacitive pressure sensor using

liquid crystal polymer (LCP) [29] as diaphragm and structural material. Screen printing

technique is adopted for metallization of top and bottom electrode. Thermocompressive

bonding is used for sensor assembly. Chavan and Wise fabricated vacuum sealed

polysilicon diaphragm capacitive pressure sensor [35]. Polysilicon is deposited on

substrate using plasma enhanced CVD process.

From this review the polyimide material fabrication and assembly

technique are studied.

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1.7 ORGANIZATION OF THE THESIS

Chapter 1 discusses the significance and application domain of MEMS

pressure sensor. Literature review on diaphragm material, dimension, design, model of

sensor structure and analytical solution are described. MEMS capacitive principle

pressure sensing technique and fabrication process is reviewed. Motivation of the work

and objective of the research are also discussed.

Principle of MEMS Capacitive Differential Pressure Sensor (CDPS), CDPS

structure and diaphragm membrane material study are discussed in chapter 2. In this

chapter, sensor structures from simple to complex models are discussed. These models

will be characterized for center deflection and capacitive sensitivity.

Model 3-High sensitive CDPS structure for square and circular diaphragm

membrane with polyimide material will be dealt in chapter 3. Analysis on centre

deflection sensitivity, capacitive sensitivity, stress on diaphragm membrane, effect of

temperature on the deflection and capacitive sensitivity are discussed.

Chapter 4 discusses the formulation to estimate the microscale physical

parameter of MEMS CDPS structure derived by Finite Element Method (FEM) for

square and circular diaphragm membrane. Formulation to estimate the physical

parameter such as finite change in deformed diaphragm surface length, surface area and

volume are discussed. Further center deflection and capacitance characteristics were

also derived and the results obtained from the derived formulation are compared with

the results of Intellisuite MEMS design tool - TEM module v8.6.

Chapter 5 explains the simple fabrication process step and verified the

process flow in Intellisuite MEMS design tool – IntelliFab v8.6. Conclusion and further

direction will be discussed in Chapter 6.

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1.8 SUMMARY

The importance of MEMS technology and MEMS pressure sensor

applications are discussed. Among various MEMS pressure sensing technique

piezoresistive and capacitive sensing are widely used, merits and limitations of these

techniques are discussed.

The motivation for modeling and analysis of MEMS Capacitive differential

pressure sensor and objectives of the research work are discussed.

Pitot static systems along with the instruments are given and barometric

altimeter is explained. Various micro pressure sensing technologies used are discussed.

Literature review on MEMS capacitive pressure sensor modeling, design,

fabrication technique, structure materials and diaphragm dimensions were carried out.

Parameters for modeling and analysis are identified.

FEM modeling of square, rectangular and circular diaphragm modeling to

evaluate deflection characteristics was also reviewed. Finally, the organization of the

thesis was briefly discussed.