pressure sensor case study

1 M.S Ramaiah School of Advanced Studies - Bangalore

MEMS Pressure Sensor


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).


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


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


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


FabricationBulk micro machining in single crystal silicon and

Surface micromachining in polysilicon.


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).


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.


Use of a P++ (heavily doped boron) etch stop layer provides accurate control of diaphragm thickness.

Structure of a capacitive absolute pressure sensor


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.


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.


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.


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.


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


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


Typical Piezoresistive Pressure Sensor

The diffused resistors

Diffused resistors


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.


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


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.


The packed pressure sensorA cap is made for the input pressure port. The electrical connections are covered with epoxy for electrical isolation.


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.


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.


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


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


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


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.


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


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


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


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


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


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


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.


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


Piezoresistive effect Geometry change

Semiconductor (Si, Ge) hasmuch larger gauge factor

)21( eStraingaug

)21(

d

R

dRK

p

d

R

dR


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


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]


For Si, Ge, there are only 3 independent piezoresistive coefficients coefficients


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.


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


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)


Strain GaugeThe strain gauge can be used in different geometries for measuring pressure.


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.


0242

0131

)1(

and )1(

define weIf

RRR

RRR


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


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


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.


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.


The membrane:Meshed Structure


The deflection analysisMaximum Deflection: 5.9 micron

Maximum Deflection: 5.9 micron

In Meters


Stress Analysis

Maximum Stress:1030 MPa

In Pa


The pressure sensor Model in MEMSPRO


Meshed Model

Meshed in Hypermesh 5.0


The deflection analysis

In micron

Maximum Deflection:3.5 micron


Stress analysis

Maximum Stress: 424 MPaIn MPa


The Schematic of Piezoresistive Pressure sensor


Voltage Sensitivity Simulation


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)


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


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)


¼ Model


Meshed Model


Full Model:TOP View


Full Model Bottom View


Deflection


Stress Sx


Stress Sy


Stress Sz


Stress S1


Stress S2


Stress S3


Von Mises Stress


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


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


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


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

XXXX

psiV

mVX

V

mVX

V

V

s

o

.10704.01028.14 33


2 Micron Membrane Deflection


2 micronMembrane Stress


Voltage Sensitivity

52

51

1011176

1012245

X

X

psiV

mVX

V

mVX

V

V

s

o

.1085.51005.117 33


Resistor ArrangementLow pressure Geometry High pressure Geometry


Thank you


Applications of Piezoresistor sensors

Blood pressure measurements

Monitoring the suction and brake pressure in automobiles.


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.


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


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.


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


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.


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


Signal conditioning circuitsPressure sensor

Temperature sensor

Conductivity measurement

Temperature compensation

The required signal conditioning electronics may be placed over the ceramic substrate.


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.


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.

pressure sensor case study

Documents

reference pressure

high pressure

applied pressure

low pressure

medium pressure

pressure sensor rangevacuum

silicon wafer

mems pressure sensorthese