sensors

AUTOMATION AND CONTROL INSTITUTETAMPERE UNIVERSITY OF TECHNOLOGY

HELSINKI UNIVERSITY OF TECHNOLOGY

Control Engineering Laboratory

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Microsensors




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Outline

BackgroundPressure sensorsAcceleration sensorsGyroscopesChemical sensorsBiosensors




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Why miniaturizing

Prize, size, weight

Efficient use of IC technology

Sensor arrays

Online measurements instead of laboratory measurements

...what is a µsensor...?




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Some application areas

Automobile industrysafety and environmental measurements

Medical applicationscontinuous measurement of chemical/physical parameters, home devices, remote microsurgery, minimum invasion therapy

Consumer electronics, home automatione.g. accelerometers in HDD, sensors in camcorders, game controllers, air quality sensors etc.

Environmental protectiondifferent concentration measurements

Food processingdetection of contaminants and impurities

Process industryRobotics




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Sensor markets

MST markets in 1996*:~14 billion $US (forecast for 2002: ~38 billion $US)…compare to the ~300 billion $US for world semiconductor markets…

Market growth (forecast, 2000…2004) ~20%

Sold µsensors (1996):44 % pressure sensors, (115 million units, 600 million US$)

38 % chemical sensors9% accelerometers(24 million units, 240 million US$)

2% gyros(6 million units, 150 million US$)

5% magnetoresistiveflow, force, temperature, bio

*NEXUS, 1998

MST markets (millions US$) for established product types1996 and 2002 (forecast)

34%

34%

8%

3%

9%

5%2%2%2%1%0%0%

36%

29%

11%

8%

6%

4%2%2%1%1%0%0% HDD heads

Inkjet printheads

Heart pacemakers

In vitro diagnostics

Hearing aids

Pressure sensors

Chemical sensors

Infrared imagers

Accelerometers

Gyroscopes

Magnetoresistive sensors

Microspectrometers




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Sensor classification




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Sensor related terms

Integrated sensorIntelligent (smart) sensor

NonlinearityHysteresisCreepAngle random walkScale factorBias driftZROSensitivity




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Piezoresistive pressure sensors

Measures material stressesProperties:

small sizegood linearity over wide dynamic rangemoderately high pressure sensitivityrelative freedom from hysteresis and creep

Resistors normally arranged in a Wheatstone bridge => the temperature coefficients cancel

pressure

substrate

Principle of piezoresistive pressure sensor.

membranepiezoresistor

Different types of sensors: gauge, differential and absolute




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sensitivity of piezoresistors decreases as temperature increases

nonlinearity of response to applied pressure depends upon the location of resistors in the strain field and the deflection of membraneas the deflection increases towards 10 % of the diaphragm thickness, the nonlinearity of the resulting strain increases

properly designed devices are capable of achieving nonlinearity less than 0.01 % of the full scale

The performance of the sensor varies over both temperature and pressure

NovaSensor:standard chip sizes start at1 mm x 0.7 mm x 0.175 mm

Entran: smallest diameter 1.27 mm




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Resistor placement:

Square silicon diaphragm => deflection results in symmetric stress distribution with maximum values at the center of each edge => piezoresistors diffused in those regions (full bridge sensitivity)

Rectangular diaphragm => stress varies from tensile to compressive along a path from the long edges towards the center => resistors placed perpendicular to the strain field (resistor placement in this type of sensor is less critical than in square diaphragm device)

Resistor placement in square diaphragm type of sensor.

Calculated stress distributions for device with diaphragm dimensions of 1 mm x 10 µm and applied

pressure of 300 mmHg.




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Example:Diaphragm 40 x 40 µm

200 nm thick Si3N4 + 1 µm thick plasma-SiN

Sacrificial etching using KOH solution – etch holes (diameter 8 µm) at the diaph. corners or edges => polysilicon sacrificial layer is removed and the silicon substrate just under the diaphragm is anisotropically etched through holes => pyramidical cavity as reference vacuum chamber

Conception for the microdiaphragm processing.




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4 polysilicon piezoresistors are formed in the diaphragm1 µm thick SiN layer deposited on top of the surface to seal the etch holes using plasma CVD7 masks required for the fabrication of the sensor

Pressure inside the cavity less than 0.3 torrPressure sensitivity ~2µV/V/kPaNonlinearity less than 1 % of full scaleTemperature coefficient of pressure sensitivity –0.13 % /Celsius (T: -50...+150 Celsius)

Piezoresistive pressure sensor - a) schematic cross-sectional structure, b) scanning electron microscope

photomicrograph.




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Capacitive pressure sensors

Measures average deflection

Properties (compared to piezoresistive counterparts):

higher pressure sensitivity

lower temperature sensitivity

more nonlinear

require larger die area and more sophisticated sensing circuitry

no hysteresis

better long-term stability

higher production costs

pressure

reference capacitors

sensing capacitors

Principle of a capacitive pressure sensor.




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Capacitive pressure sensors

The change of capacitance is not linear with respect to deformation or pressure (but the relationship is reproducible)

The structure of the sensor is relative simple and the fabrication can be done using conventional micromachining techniques

Disadvantage: small capacitance (generally 1...3 pF) => measurement circuit has to be integrated on the chip or specially designed to null the stray capacitance

To achieve high sensitivity and wide operating range => use of ultra thin diaphragm with center boss




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Resonant pressure sensor

Measures resonant frequencies

The resonator is exited e.g. thermally, electromagnetically or electrostatically

Frequency or frequency difference is measured

The resonant element can be sealed in a vacuum to get better accuracy

Resonant pressure sensor.




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Other types of pressure sensors

FISO Technologies:fiber optic in-vivo pressure transducer,diameter 0.5 mm




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Pressure sensors

Some future trends:

Cost reductionDie size reductionIncreased integrationDevelopment of sensors that operate in higher temperaturesExtended operating range of existing sensorsImproved accuracy and resolutionImproved packaging




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Acceleration sensors

Mass produced sensors usually silicon based

Most (all) current accelerometers consist of a proof or a seismic mass supported by a suspension frame

Under the influence of acceleration, the proof mass generates a force => displacement or strain in the suspension element

The means by which the force is measured are varied, depending on issues of performance and cost

Entran: Smallest diameter 3.43 mm




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Capacitive accelerometers

One of the two electrodes is the suspended proof mass, which deflects with respect to the fixed electrode when accelerated

Capacitor is used in a capacitive bridge to transduce the displacement to voltage

No temperature dependence

High-precision silicon accelerometers will almost certainly be capacitive

Capacitive cantilever microsensor

Capacitive measurement of acceleration.




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Fabrication:A silicon wafer is etched on the back side (or both sides) to form the suspension and proof massBulk micromachined accelerometers are typically a few millimeters on each side – anodic bonding can be used to form the fixed electrodes and to provide overrange protection

In surface-micromachined sensors a low-pressure chemical vapor deposition (LPCVD) or electroplating is used to deposit and define the proof mass and its suspensions on the front side of the silicon wafer




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Fabrication continues....

A sacrificial spacing layer, typically LPCVD oxide, is removed after the structural film is deposited to release the structure from the substrate – no anodic bonding is required

The sensors are typically a few hundred microns on each side, with the thickness of the mechanical and spacer layers determined by the deposition times of those layers

Performance of the device depends critically on the uniformity and mechanical properties of the deposited layers

Interface electronics are often integrated on the same chip




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Since micromachined accelerometers are used in wide range of applications, their required specifications are also application dependent and cover a rather broad spectrum, e.g.

microgravity measurements: range of operation greater than +-0.1 g, resolution less than 1 µg, frequency range 0...1 Hzballistic and impact sensing applications: range of over 10000 g, resolution less than 1 g in 50 kHz bandwidth is required

Table: Typical specifications of accelerometersfor automotive and inertial navigation applications.

Parameter Automotive Navigation Range ±50 g (airbag)

±2 g (vehicle stability) ±1 g

Frequency range 0 – 400 Hz 0 – 100 Hz Resolution < 100 mg (airbag)

< 10 mg (vehicle stability) < 4 µg

Nonlinearity < 2 % < 0.1 % Price 10 $ (airbag)

15 $ (vehicle stability)




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Piezoresistive accelerometers

Oil damping...

First micromachined and one of the first commercialized µaccelerometers were piezoresistive

Main advantages: the simplicity of the structure and fabrication process as well as their readout circuitry

Disadvantages: large temperature sensitivity, small overall sensitivity (compared to their capacitive counterparts)

Fabrication using bulk-micromachining and wafer-bonding can be very similar with capacitive sensors

Piezoresistive acceleration sensor




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Force-balanced accelerometers

The suspended proof mass deflects in response to the acceleration

The signal is sensed and fed back to counteract the displacement

Structure virtually stationary

Good dynamic range, ultra sensitive

Commonly used in navigational applications

Capacitive sensing

Fabrication of the sensing element same as for open-loop accelerometers

Feedback-control requires sophisticated electronics

Cost increase reduced by integrating the electronics on the same chip with the sensing element

Analog Devices: 2.5 x 2.5 mm (with readout electronics)




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Tunneling accelerometers

The high sensitivity of electron tunneling is utilized

Used e.g. as sensitive acoustic sensors

Proof mass is displaced due to acceleration, the read-out circuit responds to the change of current, adjusts the bottom deflection voltage to move the proof mass back to its original position, maintaining the tunneling current constant

Acceleration measured reading the bottom deflection voltage

First introduced by Jet Propulsion Laboratory, Pasadena




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Other types of accelerometers

Thermal devicesthermal transduction

– temperature flux from heater to a heat sink is inversely proportional to their separation

– change in distance between heater and heat-sink plates is measured

free convection heat transfer– hot air bubble which heat distribution changes in the presence of acceleration– the heat profile is sensed

Resonant devicesOpticalElectromagneticPiezoelectric




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Accelerometers

Some future trends:

Cost reduction

Long-term stability

Better temperature sensitivity

Packaging

Interface circuit with low drift read-out/control circuit having high sensitivity, low noise level and long dynamic range




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Gyroscopes

Vibrating mechanical elements to sense rotation

No rotating parts => easy to miniaturize and fabricate using micromachining techniques

Silicon micromachining process for fabrication of vibratory gyroscopes fall into four categories:

1) silicon bulk micromachining and wafer bonding

2) polysilicon surface micromachining

3) metal electroforming and LIGA

4) combined bulk-surface micromachining




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Gyroscopes

All vibratory gyroscopes are based on the transfer energy between two vibration modes of structure caused by Coriolis acceleration

Principle of gyroscope.




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Gyroscopes

Gyroscopes can be classified into three different categories:

inertial-grade devices (optical gyroscopes, ring laser gyroscopes)tactical-grade devices (fiber optical gyroscopes)rate-grade devices

The research has mainly concentrated on silicon micromachined rate-grade sensors (automobile industry)

Table: Performance requirements for different classes of gyroscopes.




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Gyroscopes

Vibrating gyros:

Tuning forks2 tines connected to a junction bar are differentially resonated to a fixed amplitude

when rotated => Coriolis effect => differential sinusoidal forcedevelops on the individual tines

force is detected as differential bending of the tuning fork tines or torsional vibration of the tuning fork stem

Vibrating beamsvibrating part is beam (or membrane)

Vibrating shells (vibrating ring)




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Gyroscopes

Mechanisms to drive the vibrating structure into resonance are primarily:

electrostatic

electromagnetic

piezoelectric

Detection mechanisms used to sense the Coriolis-indused vibration in the second mode are:

capacitive

piezoresistive

piezoelectric

x-y gyro, 8mmx8mm,capacitive sensing




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Gyroscopes

Example I:

Vibrating ring gyroscope

Two identical flexural modes having equal natural frequenciesRing is electrostatically vibratedRotation => Coriolis => energy transferred from primary mode to secondary flexural mode Rotation monitored capacitivelyLess temperature sensitive

Temperature range –40…+85 CelsiusZero bias drift < 10 degrees/sResolution ~0.5 degrees/s

Structure of vibrating ring gyroscope.

SEM image




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Gyroscopes

Example II:

Tuning fork gyroscope

Combs are exited so that electrostatic forces are generated which do not depend on the lateral position of the mass

Simple(r) fabrication

Easy integration with on-chip electronics

Gyro dimension 0.7 x 0.7 mm

Low cost sensorSchematic drawing of the comb-drive tuning

fork gyro.




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Mechanical µsensorsSome µsensor manufacturers:

NovaSensorVTI TechnologiesAnalog DevicesTriton TechnologiesEG&GDelcoCSEMTEMIC (Daimler-Benz)BoschMotorolaHoneywellSensonorSamsungGeneral Motors




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Chemical µsensors

Detect the concentration

Qualitative or quantitative measurements

~60 % of sensors are gas sensors

Structure of chemical µsensor.

Sen

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Chemical reaction Change of

physical state

Transformationinto electrical

signal Evaluation

Sensorsignal

Structure:chemical reaction in sensitive layer and transformation into electric signal




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Chemical µsensors

Application areas:

Environment

Automobiles

Medicine

Nutrition

Process control

Laboratory measurements




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Chemical µsensors

Conventional methods:

achieve good results, but

complicated and expensive measuring methods

amount of reagents needed

measurements done in laboratory

slow




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Chemical µsensors

The goal is to develop µsensors (µsystems) which:are easy to manufacture

are accurate and robust

use only small amounts of reagents

have short response times

have intelligent signal processing

sensor arrays/matrices

real time measurement

biocompatible and no cross-sensitivity




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Interdigital transducers

Chemical sensor based on interdigitaltransducer.

Electric resistance or dielectric constant variation of sensitive layer changes when interacting with certain substance

Capacitive measurement

Response (selectivity, sensitivity, response time) optimized by choosing the operating temperature

IDC and integrated heater (TU Berlin).




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Interdigital transducers

Chip dimensions: 14x10 mm2

Sensitive area: 4x7 mm2

Substrate material: quartz glass

Operating temperature: 380 Celsius

Maximum temperature deviation: less than 2.8 %

Heating power: 3.5 W

Sensitive layer: NiCr/AuSensor with on-chip heating.

TU Berlin




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Pellistors

Porous alumina bead containing suitable catalyst surrounds a thin platinum wire which acts both as heater and resistor thermometer – membranes (thin film technology)Increase of temperature is measured (resistance change, in suitable bridge circuit)When gas is burnt, specific activation energies are released

Not very selective

Pellistor principle.

Sensor array – thin film membranes, membrane size 1.25x1.25 mm2, 250 Celsius achieved with 100 mWheating power.

TU Berlin, Uni Magdeburg




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Optical sensors

Coupling grid structureFilm or fiber made of high refractive index material embedded in/between lower index materials Optical grating couples the incoming light (He-Ne, laser) into the waveguideSubstance to be analyzed changes the refraction index of the waveguide The amount of light striking the detector (e.g. photodiode) is proportional to concentration of the substanceOptical fibers (interferometers)

Optical waveguide.

refract = taittaa




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Optical sensors

Waveguide material (SOL-GEL) Si xTi(1-x)O2 , (x~0.25±0.05)

Waveguide film refractive index (nF)=1.77±0.03thickness (dF)=170-220 nm

Substrate glass slideL=48 mm, w=16 mm, H=0.5 mmrefractive index (nS)~1.53

Gratingarea 2x16 mm, depth ~20 nm, grating periodicity 2400 lines/mm (0.4166 µm)

OWLS instrument by Microvacuum

The light intensity is measured with the photo detectors as the angle of incidence of the laser light is varied from -6 ...+6 degrees. From the measured mode spectra, the covering medium's physical parameters (thickness, optical refractive index, density etc.) are calculated




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ISFET sensors

The substance to be measured changes Vgs => change in Ids => Ids kept constant (voltage to reference electrode adjusted) => Vgs proportional to pH

Free area of sensitive layer few µm2

Sensitive layer: Al2O3, Si3N4, Ta2O5, SiO2

Manufacturing process compatible with industrial CMOS production lines

Normal MOSFET vs. ISFET.

Sentron

ISFET = Ion Sensitive Field Effect Transistor

MOSFET = Metal Oxide Silicon Field Effect Transistor

IMEC (Siemens), IFT, MESA (Sentron), TIMA




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Chemical µsensors

Resonance quartz sensor

Chemical reaction in the sensitive layer changes the resonator mass => resonator frequency changes

Frequency measured

Bimetal sensor

Chemical reaction => heat => bending of bimetal cantilevers

The motion is measured

Resonating quartz sensor.




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SAW sensors

SAW = Surface Acoustic Wave

Detection mechanism of the sensor is a mechanical (acoustic) wavealternating voltage applied to interdigital transducer is electromagnetically transformed into acoustic wave

in receiving transducer acoustic signal is transformed back to electric signal

analyzed substance reacts on the sensitive layer => wave transmission changes (velocity and/or amplitude of the wave)

operation from 25 to 500 MHz

High sensitivity, good linearity, stability, versatility

piezoelectric substrate

input transducer output transducer




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SAW sensors

Fabricated on substrates such as SiO2, LiTaO3, LiNbO3, (ZnO, GaAs, SiC, LGS, AlN, PZT, PVdF)

Fabricated using photolithographic process

cleaning and polishing the piezoelectric substrate

metal (aluminum) deposited uniformly onto the substrate

device spin-coated with photoresist

device exposed to UV light through mask (and exposed areas removed with a developing solution)

remaining photoresist is removed => IDT

Performance of the sensor optimized by selecting the length, width, position, and thickness of the IDT




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Biosensors

Major application areas in:

Medicine

Environmental diagnostics

Food industries

Processin industries

Military

Biological research

Biological and chemical warfareMilitary

Bimolecular interactionsResearch & Development

Pesticides Agriculture & Related Industries

Fermentation monitoring and control

Bioprocess Monitoring

Fish freshness & SucroseFood and Drink Industry

Biochemical Oxygen DemandEnvironmental Monitoring

Blood glucose monitoring at homeRenal Failure monitoring

Healthcare Industry In vitro diagnostics In vivo diagnostics

ExamplesGeneral Area

renal = munuais-sucrose = sokeri, sakkaroosipesticide = tuolaistorjuta-aine




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Biosensors

Same measuring principles as for chemical µsensors

Structure:

biologically sensitive elements (enzymes, receptors and antibodies) integrated with the sensor (also nucleic acids, bacteria, cell organisms, or even whole tissues of higher organisms)

interaction between sensitive layer and molecules of the substance => modulation of physical or chemical parameters

modulation is converted into an electrical signal (electrochemical, optical, mass or thermal changes are most common)

Biosensors divided into two groups:

1) metabolism sensors

2) immuno-sensors

antibody = vasta-ainereceptor = reseptorienzyme = entsyymimetabolism = aineenvaihduntaimmuno- = combining form (indicating immunity)




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Biosensors

Very selective and sensitive measurements possible

Simplicity, speed

receptor and transducer are integrated into one single sensor

measurement of target analytes without using reagents (compare to conventional methods where many steps are used and each step may require a reagent to treat the sample)

Continuous monitoring capability

sensors can regenerate and reuse the immobilized biological recognition element




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Metabolism sensors

Used to detect chemically active molecules

Biosensitive enzymes as biocatalysts to detect molecules in a substance and catalyze a chemical reaction

analyzed substance chemically transformed

cource of reaction detectedand evaluated (chemical sensor)

Phosphate measurement with metabolism sensor.




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Immuno-sensors

Detect chemically inactive moleculesAntibodies as biosensitive elements

Lock and key principle as detection method

antigen molecule interacts withthe analysed substance => immobilized antibody molecules (lock) on the sensor surface bond with the antigen molecule (key) in the substanceonly one type of antigen can bond with certain type of antibody

Bonding process can be registered directly (transducer) or indirectly(antigen markers)

Immuno-sensing using an optical transducer.

antigen = antigeeni(a substance that stimulates the production of antibodies)

sensors

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