sensor principles and microsensors part 1a

7/25/2019 Sensor Principles and Microsensors Part 1a

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Introduction to BioMEMS & Medical Microdevices

Sensor Princip les and Microsensors Part 1Companion lecture to the textbook: Fundamentals of BioMEMS and Medical Microdevices, byProf. Steven S. Saliterman, www.tc.umn.edu/~drsteve


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Steven S. Saliterman, MD, FACP

What is a sensor?

A sensor converts one form of energy toanother, and in so doing detects andconveys information about some

physical, chemical or biologicalphenomena.

More specifically, a sensor is atransducer that converts the measurand(a quantity or a parameter) into a signalthat carries information.


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Features of an ideal sensor: Continuous operation without effecting the

measurand.

Appropriate sensitivity and selectivity. Fast and predictable response.

Reversible behavior.

High signal to noise ratio.

Compact Immunity to environment.

Easy to calibrate.


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Examples of Sensor Methods

Piezoelectric Sensors Direct Piezoelectric Effect

Acoustic Wave Propagation

Quart crystal microbalance MEMS Structures

Thermal Sensing

Thermal and Non-Thermal Flow Sensing

Electrochemical Sensors Ion Selective Field Effect Transistors

Optical Sensors


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Piezoelectric Sensors

Direct transduction from mechanical to electricaldomains and vice versa. May be used as sensors oractuators.

The reversible and linear piezoelectric effect manifests

as the production of a charge (voltage) uponapplication of stress (direct effect) and/or as theproduction of strain (stress) upon application of anelectric field (converse effect).

Three modes of operation depending on how thepiezoelectric material is cut: transverse, longitudinaland shear.

Amplifiers are needed to detect the small voltage.

Tadigadapa, S., and K. Mateti. 2009. Piezoelectric MEMS sensors: state-of-the-art and perspectives. Measurement Science & Technolog y 20, no. 9:092001.


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SEM image of an etched feature in PZT ceramic

substrate with feature dimensions of 3 x 15 m.


Example: Lead Zirconate Titanate (PZT)


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Piezoelectric Materials

Crystals Quart SiO2 Berlinite AlPO4 Gallium

Orthophosphate GaPO4 Tourmaline (complex chemical structure)

Ceramics Barium titanate BaTiO3 Lead zirconate titanate PZT, Pb [ZrxTi1-x] O3 ; x = 0,52

Other Materials Zinc oxide ZnO

Aluminum nitride AlN

Polyvinylidene fluoride PVDF

Adopted from Piezomaterials.com


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Direct and Converse Piezoelectric Effects

Adopted from bme240.eng.uci.edu

Con verse Piezoelectr ic Effect - Application of an electrical field

creates mechanical deformation in the crystal.

Direct Piezoelectr ic Effect - When a mechanical stress (compressive or

tensile) is applied a voltage is generated across the material.

.

Poll ing- Random

domains are aligned

in a strong electric

field at an elevated

temperature.


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Typical Piezoelectric Circuit

1

T FOut

T P

Q RV

C C R

TOut

F

QVC

Tadigadapa, S., and K. Mateti. 2009. Piezoelectric MEMS sensors: state-of-the-art and perspectives. Measurement Science & Technology 20, no. 9:092001.


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Piezoelectric sensors maybe configured asdirect mechanical transducers or as resonators.

The observed resonance frequency andamplitude are determined by the physicaldimensions, material and mechanical andinterfacial inputs to the device.

Tadigadapa, S., and K. Mateti. 2009. Piezoelectric MEMS sensors: state-of-the-art and perspectives. Measurement Science & Technology 20, no. 9:092001.

Configurations


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Two Modes of Operation



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Approaches to Fabrication

There are essentially three approaches

to realizing piezoelectric MEMS devices:

1. Deposition of piezoelectric thin films on

silicon substrates with appropriateinsulating and conducting layers followed

by surface or silicon bulk micromachining

to realize the micromachined transducer

(additive approach).



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2. Direct bulk micromachining of single

crystal or polycrystalline piezoelectrics and

piezoceramics (subtractive approach).

3. Integrate micromachined structures insilicon via bonding techniques onto bulk

piezoelectric substrates (integrative

approach).



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Approaches to Fabrication



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Piezoelectric Thin Films



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Dry Etching Characteristics



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Illustration of Surface Micromachining

(a) Substrate silicon wafer.(b) Silicon substrate surface is thermally

oxidized.

(c) Bottom electrode such as a (1 1 1) platinum

film is deposited.

(d) The piezoelectric thin film is deposited and

annealed.

(e) Top electrode metal such as Cr/Au is

deposited.

(f) The entire piezoelectric, electrodes and

passive layer stack is patterned and etch to

expose the substrate silicon.

(g) Substrate silicon is etched from the front

side using anisotropic wet etchant orisotropic vapor phase XeF2etchant while

protecting the transducer stack.

(h) Alternatively, the substrate silicon is

anisotropically etched from backside to

release the transducer structure.



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Piezoelectric Effects

The piezoelectric effect is a linear phenomenon wheredeformation is proportional to an electric field:

is the mechanical strain,

is the piezoelectric coefficient,

is the electric field,

is the displacment (or charge density) linearly, and

is the stress.

Where

S

d

E

D

T

S dE and D dT

These equations are known as the conversepiezoelectric effectand the direct piezoelectric effectrespectively.


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Piezoelectric Constitutive Equations

, , 1 to 6; and , , 1 to 3,

is the strain,

is the dielectric displacement (or charge density),

is the electric field

Eji ij kl k

Tm nl lm ln

Where

i j m k l n

S

D

E

S s T d E

D d T E

(The superscripts and refer to measurement at a constant field and stress. )

,

is the stress, and

, and are the elastic compliances.E Tij kl ln

E T

T

s d


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Surface Acoustic Waves

Generation of surface acoustic waves (SAW)in quartz by interdigitated transducers:

Gardner, JW, VK Varadan and OO Awadelkarim, Microsensors, MEMS andSmart Devices. John Wiley & Sons, Ltd. W. Sussex (2001).


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Delay-line SAW

Typically with a sensing film such polyimide deposited on thesurface in the area between the interdigitated transducers:

Gardner, JW, VK Varadan and OO Awadelkarim, Microsensors, MEMS and

Smart Devices. John Wiley & Sons, Ltd. W. Sussex (2001).


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Two Port Delay Line and Resonator



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The change in SAW velocity is related to the mass of athin loss-less film on the sensor surface (left):

1 2

1 2

is the SAW velocity,

and are the substrate material constants,

is the SAW frequency,

is the height of the layer, and

is the density of the thin film lay

( )

R

R

R

Where

V

k k

f

h

Vk k fh

V

er.

0

0

0 0

is the frequency change,

is the intial SAW frequency,

is the phase shift, and

is the total degrees of phase in the sensor delay path

(as measured be

R

Where

f

f

fVV f

tween the centers of the IDTs)

The change in SAW velocity can be determined

experimentally by measuring the phase shift or thefrequency shift (right).

Calculating SAW Velocity


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Quartz Crystal Microbalance (QCM)

Mass-sensitive devices suitable for detecting a variety ofanalytes. Thin AT-cut quartz wafer with a diameter of 0.25-1.0 inches, sandwiched between two metal electrodeswhich are used to establish an electric field acrossthecrystal:

Cahayaalone.blogspot.comChimique.usherbrooke.ca


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Sauerbrey Equation

Mass changes on the QCM surface result in afrequency change according to the Sauerbreyequation:

0

20

is the change in frequency,

is the resonant frequency of the quartz resonator,

is the mass change,

is the active vibrating area

is the is the shear mod

2

Q

Q Q

Where

f

f

m

A

f mf

A

ulus of the quartz, and

is the the density of quartzQ


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MEMS Structures

Construction:a) Cantilever beam,

b) Bridge structure,

c) Diagram ormembrane.

Detection Methods: Electrical,

Magnetic,

Optical,

Acoustic.




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Cantilever Beam

The displacementxof the beam is related to

the applied force and length of the beam:3

is Young's modulus,

is the second moment of inertia,

i

or ( is the spring constant)3

m

m

x

x x m mm m

Where

E

I

F

lx F F k x k

E I

s the force or point load, and

is the length.l




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Bridge Structure

The sinusoidal solution for displacementxof a bridgestructure is:

2

2

is a constant,

is Young's modulus,

sin and (the buckling force)

m

y m mCritical

m m

Where

A

E

F E Ix A F

E I y l

is the second moment of inertia,

is the force andis the length.

m

y

I

Fl


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MEMS Sensor Piezoelectric Pressure

Piezoresistive pressure sensor

with reference pressure cavityinside chip.

Esashi, Masayoshi. 2012. Revolution of Sensors in Micro-Electromechanical

Systems. Japanese Journal of App l ied Physics 51, no. 8:080001.


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MEMS Sensor Capacitive Pressure

Integrated capacitive

pressure sensor fabricated

by wafer level packaging.




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Principle and photograph of SAW passive wireless

pressure sensor and example of measurement

(change in time converted into phase).

MEMS Sensor SAW Pressure




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Common two-wire tactile sensor network that

sequentially selects sensors.

MEMS Sensor Tactile Sensor




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Schematic of event-

driven (interrupt)

tactile sensor

network, example of

operation, and

photographs of

prototype IC.

MEMS Sensor Tactile Sensor


Systems. Japanese Journal of Applied Physics 51, no. 8:080001.


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Poly-Si surface

micromachined integrated

accelerometer (cross

sectional structure and

photographs of two-axis

accelerometer).

MEMS Sensor - Accelerometer




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Schematic ofaccelerometer with

thick epitaxial poly-Si

layer and photograph

of resonant gyroscope.

MEMS Sensor - Accelerometer




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Automotive sensor

for yaw rate andacceleration.

MEMS Sensor Yaw & Acceleration




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MEMS Sensor - Gyroscope



Two-axis resonant

gyroscope used for image

stabilization (photograph

and schematic).


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Apollo 11 Gyroscope 1969

Gyroscope image courtesy of https://www.flickr.com/photos/blafetra/14524883649
https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwi295PchoLLAhXF5yYKHUfeAD0QjRwIBw&url=https://www.flickr.com/photos/blafetra/14524883649&psig=AFQjCNHgcqRL12hoF057586YtNGz1JnqyQ&ust=1455910317654040


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Sensor Fusion:Adafruit BN0055

Absolute Orientation(Euler Vector, 100Hz) Threeaxis orientation data based on a 360 sphere.

Absolute Orientation(Quaterion, 100Hz) Fourpoint quaternion output for more accurate datamanipulation.

Angular Velocity Vector(100Hz)Three axis of 'rotation speed' in rad/s.

Acceleration Vector(100Hz)Three axis of acceleration (gravity + linear motion)in m/s^2.

Magnetic Field Strength Vector(20Hz) Three axisof magnetic field sensing in micro Tesla (uT).

Linear Acceleration Vector(100Hz) Three axis oflinear acceleration data (acceleration minus gravity)in m/s^2.

Gravity Vector(100Hz) Three axis of gravitationalacceleration (minus any movement) in m/s^2.

Temperature(1Hz) Ambient temperature indegrees celsius.

Courtesy of Adafruit


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Thermosensors

Platinum resistor:

Linear, stable, reproducible.

Material property dependency on

temperature, Thermocouples (e.g.. Type K)

Thermistor: a semiconductor device

made of materials whose resistancevaries as a function of temperature.

Thermodiode and Thermotransistor.


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Thermocouple

Potentiometric devices fabricated by the joining of twodifferent metals forming a sensing junction: Based on the thermoelectric Seebeck effectin which a

temperature difference in a conductor or semiconductorcreates an electric voltage:

is the electrical voltage,

is the Seebeck coefficient expressed in volts/K , and

is the temperature difference ( - ).

s

S ref

s

WhereV

T T T

V T




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Thermodiode and Thermotransistor

When ap-ndiodeis operated in a constant current (IO)circuit, the forward voltage (Vout) is directly proportionalto the absolute temperature(PTAT).

is the Bolzman constant,

is temperature,

is the charge on an electron,

is the operating current and

is the saturation current.

ln 1out

b

S

B

S

Where

k

T

q

I

I

k T IVq I




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Thermal Flow Sensing

Hot wire or hot element anemometers.

Based on convective heat exchange takingplace when the fluid flow passes over the

sensing element (hot body). Operate in constant temperature mode or in

constant current mode.

Calorimetric sensors.

Based on the monitoring of the asymmetryof temperature profile around the hot bodywhich is modulated by the fluid flow.


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Thermal Flow Sensor

The heat transferred per unit time from a resistive wireheater to a moving liquid is monitored with athermocouple:




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In a steady state, the mass flow rate can bedetermined:

The volumetric flow rate is calculated asfollows:

1 2

2 1

is the mass flow rate,

is the heat transferred per unit time,is the specific heat capacity of the fluid and

, are temperature.

( )

m

h

m

hm

m

Where

Q

Pc

T T

PdmQ T T

dt c

is the volumetric flow rate,

is the mass flow rate and

is the density.

V

m

m

mV

m

Where

Q

Q

QdV

Q dt


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Thermal Flow Sensor with Thermopile

Silvestri, S. and E. Schena Micromachined Flow Sensors in Biomedical

Applications. Micromachines2012, 3, 225-243


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Non-Thermal Flow Sensors

Cantilever type flow sensors Measuring the drag-force on a cantilever beam.

Differential pressure-based flow sensors When a fluid flow passes through a duct, or over a surface, it produces a

pressure drop depending on the mean velocity of the fluid.

Electromagnetic

Laser Doppler flowmeter The phenomenon is due to the interaction between an electromagnetic or

acoustic wave and a moving object: the wave is reflected back showing afrequency different from the incident one.

Lift-force and drag flow sensors Based on the force acting on a body located in a fluid flow.

Microrotor Rotating turbine

Resonating flow sensors Temperature effects resonance frequency of a vibrating membrane.




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Cantilever Type Sensor



Able to Sense Direction


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Summary

A sensor is a transducer that converts themeasurand(a quantity or a parameter) into asignal that carries information.

Thepiezoelectric effectis a linearphenomenon where deformation isproportional to an electric field.

Mass changes on the QCM surface result in afrequency change according to the Sauerbreyequation.

MEMS Structures and Sensors Thermo Sensors Flow Sensors Magnetic Sensors

sensor principles and microsensors part 1a

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