Micro Sensors

8/7/2019 Micro Sensors

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Microsensors


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Outline

s Sensor & microsensor

s Force and pressure microsensors

s

Position and speed microsensorss Acceleration microsensors

s Chemical microsensors

s Biosensors

s Temperature sensors


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Sensor

sDevice that converts a non-electrical physical

or chemical quantity into an electrical signal

Sensor Processor DisplayOutput signalMeasurand


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Classification of sensors

s In the course bookclassification is based on

the function that the sensor

performs

3 pressure3 position

3 acceleration

3 etc.


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Sensor market

s Pressure 40 %

s Temperature 25 %

s

Acceleration 13 %s Flow 9 %

s Force 5 %


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Trends in sensor technology

s Miniaturization

s Integration (sensor, signal processing and actuator)3 sensor with signal processing circuits for linearising sensor output, etc.

3 sensor with built-in actuator for automatic calibration, change of

sensitivity etc.

s Sensor arrays

3 one-function units (to improve reliability)3multiple-function units


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Microsensors

s 1995 global sensor market 6 billion $US, 25 % from

MEMS based devices

s Annual increase in the market volume 20%

s Why microsensors3 lower manufacturing cost (mass-production, less materials)

3wider exploitation of IC technology (integration)

3wider applicability to sensor arrays

3 lower weight (greater portability)


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Applications ...

s Environmental applications3 determination of concentration of substances (carbon monoxide, heavy

metals, etc.)

s Food industry3 contaminants and impurities

s Process industry

s Robotics3 distance, acceleration, force, pressure, temperature


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Pressure sensors

s First microsensors developed and used by industry3 piezoresistive pressure sensor to reduce fuel consumption by a tight

control of the ratio between air and fuel

3disposable blood-pressure sensor to monitor the status of the patientduring operation

s Low production costs, high sensitivity and low

hysteresis

s Commercial products are usually either piezoresistive

or capacitive


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Pressure sensors: examples of

operation principles

s Membrane sensors3 deflection of the membrane

3 change in the resonance frequency

s Planar comb structures

s Optical methods (Mach-Zehnder interferometer)


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Piezoresistive pressure sensor

s Piezoresistors integrated in the membranes Pressure deflects the membrane

s Resistance changes proportional to deflection and thus to pressure

s Resistance change measured with Wheatstone bridge


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Capacitive membrane

pressure sensor

s Membrane deflects when pressure is applied

Distance between the electrodes changes

Capacitance changes

s Capacitive sensors have3 no hysteresis

3 better long-term stability and

3 higher sensitivity but

3 higher production costs

Chip dimensions: 8.4 mm x 6.2 mm

Fabrication: anisotropic etching


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Capacitive pressure sensor, based on

comb structure

s Utilizes parallel combstructure

s Force is applied parallel to the

sensor surfaces Force is transformed into

displacement => change in

capacitance

s On one side capacitance

increases and on the other

side decreases => higher

linearity and sensitivity21

21

CC

CCVVIO

+

=

Fabrication: anisotropic etching


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Mach-Zehnder interferometer

s Laser light brought into the sensorby optical fiber

s Light is split to two beams

s One light beam crosses a

micromembrane which isdeformed by pressure

s The deformation changes light

properties

s The beams are combined andbrought a photodiode

s Different propagation speeds

result in phase shiftChip size: 0.3 mm x 5 mmOutput: 14 V/mbar


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Position and speed microsensors

sApplications3Automobiles

3Robots

3Medical instruments

s Contact-free optical and magnetic methods are

the most significant for MST


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Magnetic sensor to measure angular

displacement

s Measurement of joint angle inrobotics

s Hall sensor based measurement of

angular displacement

3 Rotor with a row of teeth3 Stator contains Hall sensors

3 Permanent magnet located under the

sensors

3 Teeth passing by a Hall sensor change

magnetic field

Length: 4 mm

Resolution: 0.028 degrees


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Capacitive angular speed sensor

s The fork arrangement is used as aresonator

s The resonator starts to oscillate

when magnetic field and

alternating current are applied(Lorentz force)

s The amplitude of the swing angle

is detected by the capacitance

change between movable andfixed electrodes

Size: 20 mm x 20 mm

Sensitivity: 0.5 mV s/deg


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Acceleration microsensors

s Have mostly been used in automotive industry

s Usually detected with capacitive and piezoresistive methods

s An elastic cantilever where a mass is attached is mostly used

s Under acceleration mass displaces the cantilever

s Deflection of the cantilever is detected

s By increasing the mass sensitivity can be increased


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Examples of piezoresistive and

capacitive principles


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A capacitive accelerometer

by Analog Devices

s A mass-produced capacitive accelerometer was presented in1991

s Microelectronic circuits for signal preamplification,

temperature compensation and system self-test were integratedinto the sensor

s One of the first successful commercial accelerometers

s Currently used in airbag systems

s Range 5 g, resolution 0.005 g


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Capacitive cantilever microsensor

s Sensor consists of cantileversacting as one electrode, an

electrode strip and a contact strip

s Sawtooth voltage applied to

gradually increase theelectrostatic force

s Finally cantilever touches the

contact strip

s Acceleration affects the magnitudeof the voltage that is required for

contactCantilever length: 120 - 500 mSensitivity: 0.6 - 100 mV/g

Fabrication: dry etching


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Piezoresistive microsensor

with oil damping

s Sensor consists of cantilever beams, a seismic mass and oil.s Oil dampens the resonance of the suspended mass

Cantilever length: 480 m

Seismic mass: 2 mg

Fabrication: wet etching


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Chemical sensors ...

s Conventional measurement methods are often verycomplicated and expensive, require laboratory conditions, etc.

s Objectives of microsensors:3

small and inexpensive3 mass-produced

3 accurate and robust

3 use only small amount of reagents

3short response times


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Chemical sensors ...

s Research trends (in addition to the development of sensorunits):3 integration of sensors into measurement systems (signal processing)

3 integration of several types of sensors (to test n concentrations)

3 microsystems with several identical sensors (local analysis of a substance,

distribution of a parameter over a certain domain)

s Sensor principles3 potentiometer principle in connection with FET

3 acoustic sensors

3 optical sensors


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Structure of a chemical

sensor system

s A sensitive layer is in contact with the substance

s Chemical reaction occurson the sensitive layer

s Due to the reaction physical, optical, acoustic or dielectric properties are

changed

s Transducer transforms the signal into electrical form


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Operation principle of interdigital

transducer sensors

s Interdigital transducers usingcapacitive measurement are often

used in chemical sensors

s The capacitance can be adjusted

by changing the dielectricproperties of the sensitive layer

s E.g. resistance of SnO2 sensitive

layer changes when it interacts

with certain substances

2

2

1

=d

UeAF

N


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Optical sensor principle

s Optical sensors are inexpensive,easy to sterilize, can handle small

samples and are highly sensitive

s Coupling grid detector

3 substance to be analyzed is in directcontact with the waveguide

3 depending on the concentration of the

substance its index of refraction varies

=> amount of light striking the sensor

depends on the concentration


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Ion sensitive FET sensor

s For continuous measurement ofpH value and gases in blood (O2,

CO2)

s A device for external use to make

on-line diagnosis of a patients Consists of a sensor, a blood

sampling and processing part

s Uses ion sensitive FET: gate

potential is proportional to gasconcentration

Chip size: 10 mm x 10 mm


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Biosensors

s Measurement principle is similar as with chemicalsensors

s Sensitive layer is biologically sensitive, containing

e.g. enzymes or antibodies

s Interaction between the molecules of the bioelement

and the molecules of the substance changes a physical

or chemical parameter

s Parameter change is converted into electrical signal

s

Signal represents concentration to be measured


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Applications of biosensors

s Biological and nutritional research3 to detect e.g. heavy metals or allergens

s Medical applications3

patient data recording for correct and quick diagnosis during surgery

s Integration of biosensors with microfluidic components

=> very small analyzerss Difficulties

3 immobilization of proteins

3 proteins are not stable for a very long time


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Metabolism sensors

s Uses biosensitive enzymes tocatalyze a chemical reaction

s Phosphate measurement3

enzyme NP detects phosphate andtriggers chemical reaction

3 one product of the reaction HX is

transformed into XO in another

chemical reaction after consuming

oxygen3 amount of oxygen can be measured

using a chemical sensor

3 phosphate concentration is

proportional to the amount of

consumed oxygen

Nucleoside phosphorylase (NP)

hypoxanthine (XP)

xanthine oxidase (XO)


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Immuno-sensors

s Antibody is the biosensitiveelement

s Immobilized antibody molecules

bond with antigen molecules in

the substance (lock and key)s The concentration of antigens can

be measured using for example

interferometric method (light

intensity changes)


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Temperature sensors

s Important role in monitoring systems3 process industry

3 medicine

3environmental protection

s Heating and air conditioning systems

s Indirect measurement of other parameters, e.g. in

flow sensorss Error compensation for temperature dependent

sensors and actuators

Micro Sensors

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