pressure sensor case study
TRANSCRIPT
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1 M.S Ramaiah School of Advanced Studies - Bangalore
MEMS Pressure Sensor
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Market2002: 3.4 billion units will be sold in 2002, generating over US$ 17 billion(All Mems Devices).
Market forecasts : expect these devices to reach 10.4 billion units in 2006, delivering in excess of US$ 34 billion in revenue,
Source: "MEMS and MicroStructures Technology: an Application and Market Evaluation, Second Edition", from Venture Development Corporation (VDC).
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TOP Ten ProductsAs for areas of opportunity, VDC's market attractiveness index identifies the top 10 near term opportunities in the MEMS / MST market:
Micro-fluidic biochips for medical diagnostics and drug discovery
Glucose micro-fluidic monitoring sensors
Tire pressure sensors
Hard disk drive heads
Consumer print heads for inkjet printers
Over the counter micro-fluidic testing devices for detecting medical conditions
Large format print heads
Devices that enable advanced automotive functions
ABS accelerometers and gyroscopes
Automobile mass airflow sensors
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The Pressure Sensor
Fig.5. TOP VIEW : Silicon Membrane wafer
Bonding padsConductor Pattern
Piezoresistors
Silicon Cap Wafer
Silicon Substrate
Glass Plate for support
Silicon Membrane
Fig.3. Cross section of a typical sensor die
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MEMS Pressure SensorThese are based on the deflection of Silicon Membrane.
The sensing is of two typesCapacitive
Piezoresistive
Silicon Cap Wafer
Silicon Substrate
Glass Plate for support
Silicon Membrane
Cross section of a typical sensor die
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FabricationBulk micro machining in single crystal silicon and
Surface micromachining in polysilicon.
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Pressure Sensor Rangevacuum,
Low pressure (0.02 to 0.1 Atm),
Medium pressure (0.25 to 10 Atm),
High pressure (60 to more than 500 Atm).
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Capacitive pressure sensorshigh sensitivity
small dynamic range because the gap between the capacitor plates must be very small to obtain a large capacitance.
A thin silicon diaphragm is employed with a narrow capacitive gap and a vacuum cavity for reference pressure.
The silicon diaphragms have better mechanical properties, including freedom from creep, resulting in better repeatability than metal diaphragms.
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Use of a P++ (heavily doped boron) etch stop layer provides accurate control of diaphragm thickness.
Structure of a capacitive absolute pressure sensor
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The sensor is formed from two glass substrates and a silicon wafer.
The silicon wafer is sandwiched between the two glass wafers by anodic bonding, simultaneously forming a sealed reference cavity.
An alloy of Zn-V-Fe is used as a Non Evaporable Getter (NEG) to maintain the reference cavity at high vacuum. After bonding in vacuum, the NEG can absorb the remaining gas in the reference cavity.
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Pressure range 0-100mTorr
Can be extended to about 500 mtorr.
1. A Ultra-Sensitive, High-Vacuum Absolute Capacitive Pressure Sensor; Technical Digest of the 14th IEEE International Conference On Micro Electro Mechanical Systems (MEMS 2001), pp. 166-169, Interlaken, Switzerland, Jan. 21-25, 2001.
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Working of piezoresistive sensor
The sensing material in a piezoresistive pressure sensor is a diaphragm formed on a silicon substrate, which bends with applied pressure.
A deformation occurs in the crystal lattice of the diaphragm because of that bending. The membrane defection is typically less than 1 μm.
This deformation causes a change in the resistivity of the material. This change can be an increase or a decrease according to the orientation of the resistors.
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Working of piezoresistive sensor
The Piezoresistive sensor utilizes silicon strain gauges configured as a Wheatstone bridge in which one or more legs change value when strained. The output normalized to input pressure is known as sensitivity (mV/V/Pa), and is related to the piezoresistive coefficients.These sensors require an applied current and signal-conditioning electronics for operation. Due to the simple construction and their large output signal, Piezoresistive sensors take a primacy within pressure sensors. Now a days Piezoresistive pressure sensors are available for different nominal pressure ranges from 10mbar up to 1000 bar and can therefore be used for different applications.
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piezoresistive silicon Pressure Sensor
Mature processing technology.
Different pressure levels can be achieved according to the application.
Also, various sensitivities can be obtained.
Read-out circuitry can be either on-chip or discrete
Low-cost
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DiaphragmThe pressure sensitive diaphragm is formed by silicon back-end bulk micromachining.Silicon diaphragms are formed by Anisotropically etching the back of a silicon wafer. Usually a square membrane can be formed by wet etching in KOH or TMAH (TriMethyl Ammonium Hydroide) solution. The circular membranes can be obtained by dry etch process. The silicon diaphragms 5-50 microns ±1 micron and area 1- 100 square mm.The size and thickness of the finished diaphragm depend on the pressure range desired.
diaphragm
The SEM (Scanning Electron Microscope) view of the back-side of one of the sensor
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Typical Piezoresistive Pressure Sensor
The diffused resistors
Diffused resistors
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The piezoresistive elements (i.e., the diffused resistors) are located on an n-type epitaxial layer of typical thickness 2-10 micron. The epitaxial layer is deposited on a p-type substrate.
The aluminum conductors join the semiconductor resistors in a bridge configuration and are attached to the bond pads for circuit interconnection.
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The resistors are placed on the diaphragm such that two experience mechanical tension in parallel and the other two are perpendicular to the direction of current flow. Thus, the two pairs exhibit resistance changes opposite to each other. These pairs are located diagonally in the bridge such that applied pressure produces a bridge imbalance. Deformation by applied pressure causes high levels of mechanical tension at the edges of the diaphragm. Positioning the resistors in this area of highest tension increases sensitivity.
The pressure sensor chip
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Alumina substrates are used for the packaging of the sensor
Here the sensor is bonded on the substrate .The wire bonding is also done.
The alumina substrate has a hole at the middle. This is required for differential pressure measurements and the air pressure is always applied to the back side of the sensor via this hole.
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The packed pressure sensorA cap is made for the input pressure port. The electrical connections are covered with epoxy for electrical isolation.
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The Wheat Stone BridgeThe stress is a direct consequence of the membrane deflecting in response to an applied pressure differential across the front and backsides of the sensor.
The membrane deflection is a few microns.
Fig.1. The wheat stone bridge configuration.
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PiezoresistanceResistance of Silicon is dependent on the changes in length caused by stress. Resistive changes are not isotropic, and can be divided into two independent components, one parallel to the direction of stress, and the other perpendicular to it, in the form of:
Perpendicular and parallel components of stress
Perpendicular and parallel piezoresistive coefficients.
The coefficients are functions of temperature and doping concentration.
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Pressure Units Conversion1 atmosphere
1.01295 bar
1.01295 x 106 dynes/cm2
29.9213 in. Hg
406.86 in. water
1.03325 kg/cm2
1.01295 x 105 Pa or N/m2
14.696 PSI or lb/in2
760 torr or mm Hg
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Typical Pressure sensor Design Specification
Pressure Range : 12-18 psi(absolute)
Sensitivity = 2mV/V/psi (minimum)
Frequency Bandwidth: 25 kHz(minimum)
Pressure Resolution :0.001 psi
Maximum overpressure: 60 psi
Maximum surface undulation: 10 micron
Power and Ground Line current: 24 mA
Signal lines 1 mA
Accuracy: 0.01% FS
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The Pressure Sensor Membrane analysis
The Diaphragm Pressure Gauge uses the elastic deformation of a diaphragm (i.e. membrane)
A typical Diaphragm pressure gauge has a diaphragm One side of which is open to the external targeted pressure, PExt, and the other side is connected to a known pressure, PRef,. The pressure difference, PExt - PRef, mechanically deflects the diaphragm
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The pressure-loaded diaphragm can be considered as a rectangular plate subjected to a uniformly distributed loading.
Using plate theory: the deflection can be converted to a pressure loading.
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Plate TheoryIn general we are looking for a relationship of the form:
The governing differential equation for the deflection of the rectangular plate can be written asThis is the plates differential equationIt is a linear, 4th order, inhomogeneous, partial differential equation.
w(x,y)= is the lateral deflection due to the applied pressure P(x,y). P(x,y) can be taken as uniformly distributed applied pressure P.The parameter D is the flexural rigidity of the plate.
w(x,y)=F(p(x,y))
D
)1(12 2
3
v
EhD
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SolutionA simple solution valid for small deflection is given here.
Max(a/b) 1.0 1.2 1.4 1.6 1.8 2.0
0.0138 0.0188 0.0226 0.0251 0.0267 0.0277 0.0284
Where is
3
4
max
),min(
Eh
bapw
Poissons Ratio = 0.3
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The maximum stress occurs at the centre of the longer edges and is given as
Max(a/b) 1.0 1.2 1.4 1.6 1.8 2.0
0.3078 0.3834 0.4356 0.4680 0.4872 0.4974 0.5000
Where is :
2
2
max
),min(
h
bap
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SolutionFor a square diaphragm of side L : Origin (0,0) is at the centre of the diaphragm
)
2cos(1)
2cos(1
4),( max
L
y
L
xwyxw
34
42
max )1(6Eh
pLw
For silicon with E= 168Gpa, v=0.064, and
L =140 micron and H = 5 micron
micron 465.0)60(
micron 139.0)18(
micron 092.0)12(
max
max
max
psiw
psiw
psiw
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The Maximum Stress
ph
L
2
2
max
)1(6
For silicon with E= 168GPa, v=0.064, and
L =140 micron and H = 5 micron
MPapsi
MPapsi
MPapsi
196)60(
8.58)18(
2.39)12(
max
max
max
The value is much less than the Yield strength of Silicon :2800-6800 MPa
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Maximum StressThe stresses are proportional to pressure and to the the square of the ratio L/ H where H is the thickness of the membrane and L is the edge length.
The maximum stress developed in a square membrane of size ‘L’ and thickness ‘H’ at a pressure ‘P’ is given approximately as
2
3078.0
H
LP
For a pressure of 10MPa, A typical square diaphragm of 1000 micron length and 50 micron thickness will have maximum stress of ~1231 MPa
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Maximum Membrane Deflection
The maximum membrane deflection at the center of the diaphragm is given as
3
max 0138.0
H
L
E
LPw
If the Young’s modulus of Silicon membrane isE=190GPa, The maximum deflection is ~5.81 micron For the previous example.
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Piezoresistive strain gaugePiezoresistive effect is a material property where the bulk resistivity is influenced by the mechanical stresses applied to the material.
the resistance:R
L: length, A: area of cross section, ρ: resistivity
L
A
)21(
)21(
p
d
R
dR
L
dL
p
d
R
dR
A
dA
p
d
R
dRA
LR
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Piezoresistive effect Geometry change
Semiconductor (Si, Ge) hasmuch larger gauge factor
)21( eStraingaug
)21(
d
R
dRK
p
d
R
dR
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For a simple case resistivity is scalar; the electric field and the current density are related as:
For a three dimensional Anisotropic crystal
JE
3
2
1
345
426
561
3
2
1
J
J
J
E
E
E
The resistivity tensor has 6 components
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General formulation: Including the piezoresistive effects
J ) ( E Where
Ε electric field
ρ resistivity tensor
piezoresistive tensor [resistivity/stress]
σ stress tensor
J current density [current/area]
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For Si, Ge, there are only 3 independent piezoresistive coefficients coefficients
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Piezoresistive coefficientsPiezoresistive coefficients depend strongly on doping type and temperature. Typical values at room temperature for Si and Ge are:
Coefficients decrease non-linearly with temperature.
For higher doping levels (> 1019 cm-3), temperature dependence becomes smaller.
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For a long narrow resistor with axis along principal axis of stress, The relative resistance change can be expressed as
•The general expressions for πl and πt are :
•Where (l1, m1, n1) and (l2, m2, n2) are the sets of direction cosines between the longitudinal resistor direction (subscript 1) and the crystal axis, and between the transverse resistor direction (subscript 2) and the crystal axis
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πl and πt in [110] direction in case of (100) silicon wafer are :
Piezoresistance coefficients πl and πt in (100) silicon for [110] direction(in 10-11 GPa-1)
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Strain GaugeThe strain gauge can be used in different geometries for measuring pressure.
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Piezoresistive Pressure SensorAll piezoresistors are aligned with 110 directions i.e. along an axis of the principal stress
tl
1 l
Longitudinal stress can be considered as one of the principal stress along (110) direction Longitudinal and transverse stresses are related via the Poisson Ratio.
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0242
0131
)1(
and )1(
define weIf
RRR
RRR
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Piezoresistive pressure Sensor
small. very are , Since
1065.642)1(2
)2(
)1()1(
21
1121
21
21
221
22
21
4321
4231
ls
o
s
o
XV
V
RRRR
RRRR
V
V
Pa.in
107.61
106.67
Where
112
111
l
l
l
X
X
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For silicon with E= 168GPa, v=0.064, and
L =140 micron and
H = 5 micronMPapsi
MPapsi
MPapsi
MPapsi
196)60(
33.65)20(
8.58)18(
2.39)12(
max
max
max
max
psi.per 1011.2
psi 1For
1065.64
3
11
XV
V
XV
V
s
o
ls
o
psi.per 1011.2
psi 1For
1065.64
3
11
XV
V
XV
V
s
o
ls
o
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Resistor specification:1 mA current: 5 V source :The resistor should be about 5 Kohms.
Electrical Resistivity: 0.84 Ohm cm
L= 30 micron W= 2 micron t = .25 micronA
LR
Pressure Resolution: 0.001 psiSensitivity = 2mV per volt per psi :
5 V source : 10 mV per psi = 10 microvolt per 0.001 psi.
An instrumentation amplifier INA166UA with a gain of 2000 and bandwidth of 450 kHz may be employed for the amplification of the signal.
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The pressure sensor membrane
The structure of the pressure sensor can be designed in the MEMSPRO and analyzed using ANSYS .
Preliminary simulation of pressure sensor membrane have been performed using ANSYS software.
These simulation have also been performed for a typical pressure sensor geometry.
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The membrane:Meshed Structure
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The deflection analysisMaximum Deflection: 5.9 micron
Maximum Deflection: 5.9 micron
In Meters
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Stress Analysis
Maximum Stress:1030 MPa
In Pa
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The pressure sensor Model in MEMSPRO
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Meshed Model
Meshed in Hypermesh 5.0
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The deflection analysis
In micron
Maximum Deflection:3.5 micron
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Stress analysis
Maximum Stress: 424 MPaIn MPa
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The Schematic of Piezoresistive Pressure sensor
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Voltage Sensitivity Simulation
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The ¼ th model of the pressure sensor can also be made in ANSYS and simulation can be performed without much error.
Use Orthotropic model :SOLID45
The structure is made as sum of three solids with origin at (00) Two corners(x,y,z)
The membrane 2 micron thick. Solid1: (0, 0, 18) (100,100,20)
Other parts: Solid2: (0,70,0) and (70,100,18)
Solid3: (70,0,0) and (70,70,18)
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Material ParameterYoungs Modulus:
Ex= 169.5 GpaEx= 169.5 GpaEx= 169.5 Gpa
Poisson’s Ratio:Vxy= vyx=0.0551Vyz= vzy =0.3559Vzx=vxz =0.2730
Shear ModulusGxy= 80.32 GpaGyz= 62.50GPaGzx= 51.06 GPa
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The 5 micron top layer containing membrane is meshed as 50X50X10(element size being2X2X0.2 micron) and
Other parts as 50X15X9 and 35X15X9( Element size being 2X2X2 micron)
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¼ Model
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Meshed Model
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Full Model:TOP View
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Full Model Bottom View
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Deflection
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Stress Sx
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Stress Sy
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Stress Sz
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Stress S1
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Stress S2
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Stress S3
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Von Mises Stress
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The stress values are obtained along the central line and the region of maximum stress is obtained by taking the average of the stresses on all the nodes intended for the resistors.
In the present example the resistor may be placed 18 microns from the edge to 48 microns.between 24357 and 24342 nodes.
However to maximize the stress and voltage sensitivity its advisable to split the resistor into two parts and place between24347 and 24353. The average is 19.8 MPa
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THE FOLLOWING X,Y,Z VALUES ARE IN GLOBAL COORDINATES
NODE SX SY SZ SXY SYZ SXZ24317 -13.538 -13.698 -0.13369 1.59E-02 5.15E-03 7.67E-0324318 -13.49 -13.642 -0.13368 3.18E-02 5.12E-03 1.54E-0224319 -13.408 -13.55 -0.13366 4.75E-02 5.08E-03 2.31E-0224320 -13.293 -13.42 -0.13362 6.31E-02 5.02E-03 3.09E-0224321 -13.142 -13.255 -0.13358 7.85E-02 4.94E-03 3.89E-0224322 -12.955 -13.053 -0.13353 9.36E-02 4.84E-03 4.70E-0224323 -12.73 -12.816 -0.13347 0.10839 4.72E-03 5.52E-0224324 -12.464 -12.545 -0.1334 0.1228 4.59E-03 6.36E-0224325 -12.154 -12.239 -0.13332 0.13677 4.44E-03 7.22E-0224326 -11.799 -11.901 -0.13322 0.15023 4.27E-03 8.11E-0224327 -11.394 -11.531 -0.13312 0.16312 4.08E-03 9.02E-0224328 -10.936 -11.131 -0.133 0.17537 3.87E-03 9.97E-0224329 -10.421 -10.701 -0.13287 0.18691 3.65E-03 0.1094224330 -9.8435 -10.243 -0.13272 0.19766 3.40E-03 0.1195224331 -9.2001 -9.7589 -0.13256 0.20754 3.13E-03 0.1299924332 -8.4849 -9.2505 -0.13238 0.21647 2.85E-03 0.1408624333 -7.6923 -8.7196 -0.13219 0.22437 2.54E-03 0.1521624334 -6.8164 -8.1684 -0.13197 0.23113 2.21E-03 0.1639124335 -5.8506 -7.5993 -0.13174 0.23667 1.87E-03 0.1761424336 -4.788 -7.0147 -0.13149 0.24088 1.50E-03 0.1888824337 -3.6212 -6.4174 -0.13122 0.24366 1.10E-03 0.2021424338 -2.3425 -5.8104 -0.13093 0.24489 6.89E-04 0.2159724339 -0.94361 -5.1968 -0.13061 0.24446 2.52E-04 0.2303824340 0.58426 -4.5801 -0.13028 0.24225 -2.07E-04 0.245424341 2.2503 -3.9639 -0.1299 0.23813 -6.90E-04 0.2610524342 4.0642 -3.3523 -0.12954 0.23197 -1.20E-03 0.2773624343 6.0363 -2.7492 -0.12907 0.22366 -1.72E-03 0.2943924344 8.1769 -2.1594 -0.12876 0.21304 -2.27E-03 0.3120424345 10.496 -1.5876 -0.12838 0.2 -2.85E-03 0.3301724346 13.003 -1.0386 -0.12766 0.1844 -3.44E-03 0.349124347 15.732 -0.51535 -0.12319 0.16615 -4.04E-03 0.3734924348 18.767 -1.99E-02 -0.11726 1.45E-01 -4.68E-03 0.4099524349 22.113 0.42942 -0.14577 0.12144 -5.37E-03 0.4259824350 24.81 0.74101 -0.30298 9.55E-02 -6.00E-03 0.2312224351 24.316 0.82744 -0.43241 6.94E-02 -5.79E-03 -0.3081924352 19.529 0.75046 -0.20375 4.73E-02 -4.18E-03 -0.7303624353 13.177 0.52635 -3.77E-02 3.17E-02 -2.41E-03 -0.6828124354 8.2251 0.29812 -3.56E-02 2.19E-02 -1.31E-03 -0.4472724355 5.1752 0.14592 -7.68E-02 1.59E-02 -7.62E-04 -0.2629924356 3.3907 5.99E-02 -0.10452 1.19E-02 -4.81E-04 -0.1581824357 2.2998 1.10E-02 -0.1192 9.22E-03 -3.23E-04 -0.1009724358 1.5888 -1.86E-02 -0.12668 7.29E-03 -2.26E-04 -6.84E-0224359 1.0974 -3.82E-02 -0.13087 5.87E-03 -1.61E-04 -4.86E-0224360 0.74129 -5.23E-02 -0.13316 4.79E-03 -1.17E-04 -3.60E-0224361 0.47409 -6.34E-02 -0.13442 3.95E-03 -8.53E-05 -2.73E-0224362 0.27133 -7.27E-02 -0.13496 3.26E-03 -6.50E-05 -2.06E-0224363 0.12238 -8.09E-02 -0.13497 2.64E-03 -5.60E-05 -1.44E-0224364 2.86E-02 -8.81E-02 -0.13442 2.00E-03 -6.16E-05 -7.60E-0324365 -6.53E-03 -9.53E-02 -0.13533 1.19E-03 -1.03E-04 -1.06E-03
MINIMUM VALUES NODE 24317 24317 24351 24365 24350 24352 VALUE -13.538 -13.698 -0.43241 0.11870E-02-0.60028E-02-0.73036
MAXIMUM VALUES NODE 24350 24351 24354 24338 24317 24349 VALUE 24.810 0.82744 -0.35618E-01 0.24489 0.51470E-02 0.42598
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THE FOLLOWING X,Y,Z VALUES ARE IN GLOBAL COORDINATES
NODE SX SY SZ SXY SYZ SXZ24341 2.2503 -3.9639 -0.1299 0.23813 -6.90E-04 0.2610524342 4.0642 -3.3523 -0.12954 0.23197 -1.20E-03 0.2773624343 6.0363 -2.7492 -0.12907 0.22366 -1.72E-03 0.2943924344 8.1769 -2.1594 -0.12876 0.21304 -2.27E-03 0.3120424345 10.496 -1.5876 -0.12838 0.2 -2.85E-03 0.3301724346 13.003 -1.0386 -0.12766 0.1844 -3.44E-03 0.349124347 15.732 -0.51535 -0.12319 0.16615 -4.04E-03 0.3734924348 18.767 -1.99E-02 -0.11726 1.45E-01 -4.68E-03 0.4099524349 22.113 0.42942 -0.14577 0.12144 -5.37E-03 0.4259824350 24.81 0.74101 -0.30298 9.55E-02 -6.00E-03 0.2312224351 24.316 0.82744 -0.43241 6.94E-02 -5.79E-03 -0.3081924352 19.529 0.75046 -0.20375 4.73E-02 -4.18E-03 -0.7303624353 13.177 0.52635 -3.77E-02 3.17E-02 -2.41E-03 -0.6828124354 8.2251 0.29812 -3.56E-02 2.19E-02 -1.31E-03 -0.4472724355 5.1752 0.14592 -7.68E-02 1.59E-02 -7.62E-04 -0.2629924356 3.3907 5.99E-02 -0.10452 1.19E-02 -4.81E-04 -0.1581824357 2.2998 1.10E-02 -0.1192 9.22E-03 -3.23E-04 -0.1009724358 1.5888 -1.86E-02 -0.12668 7.29E-03 -2.26E-04 -6.84E-0224359 1.0974 -3.82E-02 -0.13087 5.87E-03 -1.61E-04 -4.86E-0224360 0.74129 -5.23E-02 -0.13316 4.79E-03 -1.17E-04 -3.60E-0224361 0.47409 -6.34E-02 -0.13442 3.95E-03 -8.53E-05 -2.73E-0224362 0.27133 -7.27E-02 -0.13496 3.26E-03 -6.50E-05 -2.06E-0224363 0.12238 -8.09E-02 -0.13497 2.64E-03 -5.60E-05 -1.44E-0224364 2.86E-02 -8.81E-02 -0.13442 2.00E-03 -6.16E-05 -7.60E-0324365 -6.53E-03 -9.53E-02 -0.13533 1.19E-03 -1.03E-04 -1.06E-03
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This does not give the desired sensitivity. The sensitivity can be incresed by decreasing the membrane thickness. A 2 micron membrane gives the desired sensitivity.The maximum stress being 181.14 Mpa
Pa.in
1018.151056.24107.61 107.61
1038.13108.191067.6X 106.67561111
2
5611111
l
l
l
XXXXX
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psiV
mVX
V
mVX
V
V
s
o
.10704.01028.14 33
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2 Micron Membrane Deflection
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2 micronMembrane Stress
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Voltage Sensitivity
52
51
1011176
1012245
X
X
psiV
mVX
V
mVX
V
V
s
o
.1085.51005.117 33
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Resistor ArrangementLow pressure Geometry High pressure Geometry
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Thank you
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Applications of Piezoresistor sensors
Blood pressure measurements
Monitoring the suction and brake pressure in automobiles.
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SUMMARYSi is one of the most important piezoresistive material.
The piezoresistive sensors can be used fro measurement of hydrostatic as well as differential pressure.
Piezoresistive materials are very useful as pressure sensor.
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The temperature sensorPlatinum Resistance thermometer:
The resistance at low temperatures below 0C can be expressed as a third order polynomial
For the range 0 to 850°C, It can be expressed as a second-order polynomial
The coefficients are as follows:Ro : nominal value or nominal resistance
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Temperature CoefficientThe mean temperature coefficient between 0 °C and 100 °C is defined as
The typical value for a Pt100 thermometer is 0.00385055 per °C.
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The Temperature sensorTwo wire or Four wire measurement
Wheatstone bridge arrangement
The temperature sensing resistor may be placed on a ceramic substrate.
Fig.7. Bottom View: The Alumina substrate
Platinum resistor
Platinum electrodes
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Conductivity SensorThe conductivity measuring cell
Usually formed by two 1-cm square surfaces spaced 1-cm apart.
It is required to scale down the cell dimensions for microsensor.
The conductivity sensor is a four electrode cell with platinum electrodes.
The conductivity of a solution with a specific electrolyte concentration will change with change in temperature.
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Conductivity SensorTemperature compensated conductivity:
Conductivity at the reference temperature( 20oC or 25oC).
A useful algorithm for temperature correction is: C(T) = C(25) [1 + 0.021 (T - 25)]
Where
C(T) is the measured conductivity of a solution at sample temperature T in oC and
C(25) is the conductivity of the solution at 25oC
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Signal conditioning circuitsPressure sensor
Temperature sensor
Conductivity measurement
Temperature compensation
The required signal conditioning electronics may be placed over the ceramic substrate.
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The SensorThe sensor die is bonded to the alumina substrate and packaged.
The electrodes for conductivity measurement and the platinum resistors are deposited on the alumina substrate.
The contacts are brought on top through thruhole plating.
The Bottom side will be in contact with water whereas the topside will contain the electrical contacts.
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The DesignThe design involves:
Designing a suitable package for the device
the signal conditioning circuit
the pressure sensor membrane geometry:
Maximizing the sensitivity by optimizing the membrane dimensions.
The resistor geometry for the temperature measurement.
The conductivity cell/ electrodes geometry.