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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
3
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].
4
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.
5
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 .
8
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
9
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
10
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].
11
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.
14
(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.
15
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
18
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.