Silicon and Hybrid Micro-Electronic Sensors

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SILICON AND

HYBRID

MICRO-ELECTRONIC

SENSORS

S.

MIDDELHOEK,

D.J.W. NOORLAGandG.K. STEENVOORDEN

DelftUniversityofTechnology,Delft, TheNetherlands (In finalformApril23, 1982)

The penetrationof micro-electronics into newmarkets is seriouslyhamperedbythe lack of efficient, low-cost sensors. Itisnot surprising therefore to find that thesedevicesare thesubjectof researchin many laboratories. This papergiws a reviewof the mostimportantsensors which arefabricated in silicon planar or hybrid tchnology. The review is prcdd by some general considerations with respecttomeasurementsystemsand signalconversion.

1. INTRODUCTION

Information-processing systemsconsistofacentral signalprocessorplusinputand output transducers which interface to the non-electronic environment(Figure

1).

In theinput

transducer -often calledsensor- ameasurandsuchas temperature, displacement,

concen-tration,switchposition,etc.,isconvertedintoan electronicsignal.

In

the signal processor often calledmodifier this electronicsignal isin some waymodified, e.g. amplified,

filtered or converted from ananalog to a digital signal. In theoutputtransducer either

the electronicsignalisagainconvertedintoasignal whichcanbe perceived byoneofour

senses or it is used to cause some action, e.g. to print a characteror toclose avalve.

Until recently the signal processor was more or less the only part of the

information-processingsystem thatwas the target of very intensiveand cosily research and develop-ment work.

As a result of thisactivity wehave at present at our disposala large numberofvery sophisticated micro-electronic building blocks suchas microprocessors, microcomputers,

A/D

converters,voltage/frequencyconverters, operational amplifiers,etc. All these

com-ponentsare characterizedby anunprecendented performance/priceratio.

However,

the neglect of the transducer side of the system has now caused an undesirable situationmarked by the fact that the furtherpenetrationof micro-electronics

into the many newly envisaged markets is notyet possible because of the lack of trans-ducers with performance]price ratios comparable to those of the micro-electronic

circuits2andwithoutproblemsininterfacingwiththeelectronicmodifier.

Understandably,the transducerfield iscurrentlyattracting moreandmore interest, so

thatinmany laboratoriesthroughout theworld newtransducer principles and structures are bieng studied.

The aimofthispaper is to give areviewof the mostimportantsensorsasdescribedin

the literature. The great progressin siliconintegratedcircuits during recent decades has beenmade possible by the invention of the siliconplanar technology

(photolithography,

diffusion, epitaxy and metallization). Not surprisingly attempts are being madeby the

electronicsindustryanduniversity laboratories to extentthe application of this successful technology to sensors as well. Also through thick- and thin-film technology a large number ofuseful andinnovative sensorshave been fabricatedinrecentdecades.One

sec-tion of this paper

(Section

3)

will be devoted to a discussion of the so-called silicon micro-transducers

(SMT’s)

and another

(Section

4)

tothe so-called hybridtransducers.

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218 S.

__

MIDDELHOEK, D.J.W. NOORLAG AND G.K. STEENVOORDEN

INPUT SIGNAL OUTPUT

TRANSDUCER PROCESSOR TRANSDUCER

FIGURE Block diagramof a measurement orcontrolsystem.

But first we will turn our attention, in Section2,tosomeconsiderationswithrespectto

thedifferentsignals which havetobedetected by the sensors.

2. SIGNALCONVERSION

Inan inputtransducerorsensor ameasurand suchastemperature,pressure or

pH

valueis

convertedintoanelectronicsignal.Becauseinformationflowisinconceivable withoutthe flow of energy, sensors areinfactdevicesthatconvertenergy fromoneformintoanother. Forinstrumentationpurposessixenergy and thussixsignal domainscanbe distinguished: radiant signals (light intensity, phase), mechanical signals (pressure, displacement), thermal signals(temperature,

heat),

electricalsignals(voltage, resistance),magneticsignals (fieldstrength,permeability)andchemicalsignals(concentration,toxicity).3

Figure 2 shows a diagram indicating the five signal conversions which canoccur in sensors. In a signal processor

(the

central building block ofan information-processing system) the signal remains in the electrical signal domain, whereasinthe output

trans-ducer anelectricalsignal isconverted backintooneofthe otherfivesignaldomains.

For

example, in case of an optical display the signal is converted into the radiant signal

domain.

T

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SILICON ANDHYBRIDMICRO-ELECTRONIC SENSORS 219

In order to convert signals an immense number of physical effectsare available and have been described in the literature on solid-state physics. For instance to convert a radiantsignal into anelectrical signal, such effectsas the photovoltaic effect, photocon-ductance,photodielectric effect,photomagneto-electric effect and theDembereffectcan be used.Twokinds of sensors, basedontheseeffects,canbe distinguished.Bearin mind

that some sensors generate an electricaloutputwithout an auxiliary energy sourceand others onlyfunctionwhen auxiliary energyissupplied.

By restrictingourselves to sensors in theradiant signaldomain,wecanusethe

photo-voltaic effect to make sensors of the formerkind.Without an auxiliary energysource a

light intensity is convertedinto a voltage. The solar cellisbased onthis effect. Sensors

operating without auxiliary sources arecalled self-generatingsensors.

Thesecond kind ofsensor isknownunder the name of modulatingsensor.When,for instance, a photoconductor or a photodiode isexposedto light, electron-hole pairs are

generated, sothat theresistanceof thedevicedecreases.Thisdecreasecanbedetected by

using an auxiliaryenergy source. In Figure3 somephysical effects whichcanbeused for self-generating and modulating sensors aretabulatedforthefive differentsignaldomains.

3. SILICON MICRO-TRANSDUCERS

(SMT’S)

The siliconplanar technology makesit possibletobatchfabricatelarge numbers of iden-tical circuits, whichthemselvesconsistofalarger number of components. The threemost

significant features of thetechnology are

(1)

that

SiO2

layers, simplyobtainedby oxidi-zing the wafer in an oxygen-containing atmosphere, canbe usedto mask the impurity

doping process,

(2)

that all structures are obtainedby means ofphotolithographic

tech-niquesand

(3)

thatall passiveandactivecomponents andtheirmetal interconnections are obtainedonly byprocessingthesiliconwaferononeside.

When the same siliconplanar technologyisappliedtosensors,sometimesafew

altera-tionshave to be appliedwith respectto wafer or chip size, chip thickness, metalization, bonding,etc.4 SIGNAL DOMAINS RADIANT MECHANICAL THER4AL MAGNETIC CHEMICAL PHYSICAL EFFECTS SELF GENERATING PHOTOVOLTAIC EFFECT PIEZOELECTRIC EFFECT SEEBECK EFFECT MAGNETOELECTRO EFFECT CHEMO VOLTAIC EFFECT MODULATING PHOTOCONDUCTANCE PIEZORESISTANCE THERMORESISTANCE MAGNETORESISTANCE IONCONDUCTANCE

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220

The most seriousproblems are encountered when the sensors haveto beusedina

non-encapsulated form. Sensors for chemical signals in particular show that this technical problemstill demandsample attention. The initialsteps for applyingsiliconplanar tech-nology tothe sensor fieldwere taken inabout 1970.Afew veryinterestingdeviceswere

the result ofthis effort. Nevertheless, only thosedeviceswhichweredesigned for sensor-ing optical signals were widely accepted on the electronics market. Because low-cost, sophisticated signal processors, e.g. microprocessors, did notyet exist,the development

ofsensors for the other signaldomains wasslowed downoreveninterrupted.

Today the situation is different. Low-cost microprocessors and microcomputersare

available on a large scale and muchinteresthas beenkindled inapplying these

compo-nents to new markets. Therefore the need today for equally low-cost sensors is very

acute.This is best illustratedby the automotive industry.

Many

sensor technologiescan

be adaptedinsuch away that they produce suitable sensors compatibleto microproces-sors. In the rest of this section anumberofdeviceswhicharefabricatedby means of the

siliconplanartechnologywillbepresented. 3.1 SMT’s

_for

Radiant Signals

Radiant signals embrace the whole spectrum of electromagnetic waves. This spectrum includes, in order ofincreasing wavelength, gamma rays,

X-rays,

light waves andradio waves in a range roughly between 10-14 m and

104

m. Light only represents a small

portion ofthis spectrum fromultraviolet

(10

-7

m)

viavisible

(3

x 10-7

m-7x 10-7

_m)

to infrared

(10

-4

m).

For detecting visiblelight silicon deviceshavebeenusedon agreat scale during the last two decades. These devices are amply described and reviewed in

literature.S,6

Silicon with abandgap of1.1 eVshows photoconductivity between 400nmand 1150

nm. A photoconductor can easily be fabricatedby diffusingap-typestripintoann-type

substrate and by providing the strip with ohmiccontactsattheends. Another methodis

to use pn-junctions with either forward or reverse bias.Whena junction is exposedto

light photons create electron-hole pairs in andnearthe depletion layer. Becauseof the

electric field in thejunction, the electrons go towards the n-typeregion and the holes towards the p-typeregion. The result of thisseparationofchargecarriersisthata

poten-tial difference iscreatedacrossthejunction, which can beexternally measured and which

isameasureofthe incident lightintensity.

When a reverse voltage is applied to a pn-junction only averysmall, so-called dark

current can be observed. Free charge cardersarecreated when the junctionisexposedto

light, and this raises the reverse current roughly proportional to the light intensity. A

moreefficientphotodiodeisobtained whenalayerofintrinsicsilicon isused between the p-type and n-typeregions.Suchaphotodiodeisknownasa pinphotodiode.

Sensitivities towavelengthsbelow 300nm can be obtainedby Schottkybarrier

gold-silicon photodiodes. In an avalanche photodiode reversevoltage breakdown andcartier

multiplication is used to increase the sensitivity.

In

a phototransistor the basecollector junction is exposed tolight. When the baseis notconnected theincrease of the reverse

currentthroughthisjunction isamplified, resultingin alarge collector-currentchange. MOSFET’scanalso be used as light detectors.

Because theyareboth fabricatedinsilicon withplanarsilicontechnology,

optoelectro-nic devices and electronic signal-processingcircuits can easily be integrated onthe same

chip.

A

large number of such integratedstructures,sometimescalled

"smart

sensors",

are

described in the literature. Optoelectronic silicon devices are fabricated for line and

matrix scanningpurposes or are combined withcircuitssuchasSchmittTriggers,

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SILICON ANDHYBRIDMICRO-ELECTRONIC SENSORS 221 FIGURE4 -5V hv -IOV SiO n-type silicon

Schematic representation of athree-phase charge-coupleddevice(CCD).

Around 1970a veryinnovative optoelectronic device called a ChargeCoupledDevice

(CCD)

was proposed.This device consists of arrays of photosensitive MOScapacitors in

which electrical charge packets are stored (Figure

4).

Thechargecanbecreatedby light-induced electron-hole pair formation, the number ofcharge carders being ameasure of the incident light intensity.

By

applying voltagestothe electrodesin aproper sequence, thecharge packetscanbeshiftedthroughthe semi-conductor.WithCCD’sit ispossibleto

construct all solid-state linear and area optical imaging devices. CCDarrayswith more

than

106

pictureelements have been processed.Nevertheless,atpresentitisstill difficult to predict whether the existingTV camera tubeswill be replaced in thefuture by CCD devices.

3. 2 SMT’s

_for

Mechanical Signals

A great need exists (e.g. in the automotive industry) fortransducers that can measure

mechanical signals such as: pressure, torque, liquid flow, thickness, level, position, velocity, acceleration, tilt, etc. The scientific literature contains several interesting

des-criptionsofsilicondevices.

a) Pressure

sensor. Silicon is notpiezoelectric, but showsalargepiezoresistiveeffect.A pressure sensor making use of this effect was first described by Gieles.7 _{That device}

consists of a very thin

(15 tam)

n-type silicon pressure diaphragm with a diameter of

mm in whichaWheatstone bridge of fourp-dopedresistors isdiffused.Devices canbe made for the pressure range of 1-100 bar.

b)

Accelerometer. Asilicon accelerometer8

is described which consists ofaverythin

(<

15

tam)

silicon cantilever with a 200

tam-thick

siliconmass,which serves as an inertial

reference. A piezoresistive strain gaugeis diffusedinto the cantilever. Its resistanceis a measure for the accelerationof the structure. The wholestructureismadeinsilicon with a

V-groove

etching technique.

c)

Positiondetector. By using alaser,a mirrorattached to the object whoseposition

has tobemeasured, anda specially designed

_{photodetector,}

one can measureposition.9 The photodetector consists of an 8 x 6mm rectangular pn-diode (Figure

5).

Contact

strips as shown are provided to both p-type and n-type layers. The pn-diodeis

reverse-biased at about 10

V.

When the laser light-spot reflectedviathemirrorisincidenttothe device, electron-hole pairs are generatedatthe light-spot position.Inthe structure shown the photocurrentwill be dividedbetween the contactsin proportion tothe conductivity between the light-spot position and the contacts. The

X-

and Y-signalsareexactlylinear to the coordinates ofthe light-spot.Linearityisbetter than0.5%full scale andresolution isabout 5 tam. The photodetectorcanbemade much larger.

However,

the device requires the diffusion depthandtheimpurity concentration to bethe sameinthe wholedetector, whichrequirescarefulprocessing.

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n-type Si

FIGURE5 Light-spot position-sensitive detector (PSD).

d)

Flowmeter. Averyimportantphysical quantity in the mechanical signaldomainis

gasorliquid flow.Manyprinciples formeasuringflow have been investigatedinpast years. A principle based on measuring temperature differences1 is suitablefor realization by

silicon planar technology. Three transistors areintegratedon asiliconchip of 2x 2mm2

(Figure

6).

The transistor in the centerisused as aheatingdevice, while the onestothe left and fight serve as temperature sensors.Whenan airflowpasses alongthe chip from left to right, the left-hand sensing transistor is cooledto a greater degreethanits right-hand counterpart. Thisdifference isdue tothe increasing thickness of the laminar boun-dary layerinthe direction of the flow. Thetemperaturedifferenceappears to be

propor-tional to the square root of the flowvelocityandchangessign whenthe flowdirectionis

reversed. Adding another two temperature-sensing transistors above and below the heatingtransistor makes itfeasible toconstructasilicon vectorflow sensor formeasuring

flow amplitudeandflow direction simultaneously. 3.3 SMT’s

_for

ThermalSignals

The temperature sensitivity of semiconductor devicesisusually consideredtobe an un-desirable property.Yetit canalsobe putto use to make very accurate temperature

sen-sensing transistor

-flo

L

Si chip heating transistor

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SILICON ANDHYBRID MICRO-ELECT.RONICSENSORS 223 sors in a wide varietyof applications. Simplediffused resistors canbeusedastemperature transducers,because the charge-carder density and the mobilityin silicon are a function

of the temperature.

Silicon diodes are also suitable as temperature sensors, because the voltage acrossa

forward-biased pn-junction is roughly proportional to the absolute temperature at constant current.

However,

the best silicon temperature transducers are madewiththe help ofmatched transistors.When the transistorsare operatedat different current

den-sities, the difference in emitter-base voltages islineartothe absolute temperature.

Dif-ferentcurrent densities canbeobtainedbyusing identicaltransistorsattwodifferent col-lector currents, orby using two transistorswithunequalemitterareas atequal collector

currents.

Devices based on thiseffect are called

PTAT

(proportionalto absolutetemperature)

devices. One oftheir disadvantagesis that they usuallyrequirethreecalibrationsteps for adjusting, e.g. the scale factor. Recently, a newIC design that only needs thecalibration

of oneresistorhas been described intheliterature.11_{The device}_operates

_between-50C

and

125C

with anabsoluteerrorof

+-

0.5C

andanexcellent long-term stability.By inte-grating processing circuits on the same chip, devices can be obtained with electronic

output signalsperfectlysuitedtointerfacingwith microprocessors.

3.4 SMT’s

_for

Magnetic Signals

Magnetic-fieldmeasurement equipment contains amagnetic-fieldsensor whose linearity, range,temperature sensitivity andsize are the most important properties. It seems, that

siliconlends itself wellto constructing sensors withsatisfactorycharacteristics.

Two effects can be employed in silicon: the Hall effect and themagneto-resistance

effect. Both effectsoriginate from the fact that an electronorholemovingina magnetic field experiencesthe Lorentz force, whichis mutually perpendiculartothedirectionsof the particle velocity and the magnetic field. Under influence of the Lorentz force the charge-carrierswill "pile-up"at oneend of the conductorandchargedepletionwilloccur

at theother end.This givesriseto an electrical fieldequal butopposite indirectiontothe Lorentz force. The effect in which a voltage is measured perpendicular to thecurrent directionis called the Hall effect. Whenthe Hallvoltageisshort-circuitedbyanexternal conductor orby the currentcontactsthe electric field isreducedto zero andtheLorentz force will deflect the charge-carders. This results in smaller mobilities and thus in

increased resistivity. This effectisreferredtothe geometrical magneto-resistanceeffect. Hall plates caneasily be fabricatedinsilicon by diffusingap-type Hall plate into an

n-type epilayer and by providing properly located contacts to the plate.12 _A _new _and

recently describedSMTformagnetic fields isbasedon atwo-collectortransistorstructure

inwhich the current distribution overthe collectorismodulated bya magnetic field.3

As

showninFigure 7 a split buffedlayeris symmetrically located beneath theemitterbase

Si02

_/-cl

p-type

Lorentz force

),,

buried layer

(8)

of the npn-structure,inorderto improvethesensitivity.Electrons leaving theemitterwill

be deflected under the Hall angle0r,whichisproportionaltothe magnetic-field

compo-nentparallel to thestrip emitterand the electron mobility.

The electron deflection causesanunbalance of thecollector currents whichis

propor-tional to the magneticfield.Linearity within0.5%ismeasured forfieldsup to 1T.Based

on this sensor afour-collector npn-transistor structure was alsodeveloped,which simul-taneously measures theX- and

Y-components

ofa magnetic field.Theemitterandbasein

this vectorsensorare circularandplacedinthe center between the four collectors.

3.5 SMT’s

for

Chemical Signals

The growing interest in better environmental controls has increased the demand for accurate sensorsfor chemical measurands. The wholefieldof analytical chemistry can be representedbyadiagramasshowninFigure 8.

Matter can manifest itselfin a solid, liquid or gas phase.

Moreover,

eachphasecan occur as aperturbanceinany otherphase,asindicatedby the examplesgiven.Inorderto

beabletomakefull use of theelectronicsignal-processing systemsavailabletoday,

chemi-cal sensors with similar performance/price ratios and electronic output signals are required.

Thischallenge has spurred manylaboratories onto work on usingsemiconductors and especiallysilicondevices for this purpose.

However,

aspastexperiencehasproved,silicon

devicesarevery sensitive to trace impuritiesand contamination.Therefore,the

semicon-ductor industry has taken greatpainstodeveloppassivationand encapsulationtechniques

to protect the silicon devices and circuits from the environment and to improve their

reliability. If silicon is going to be used for chemical sensors, new encapsulation

tech-niques will have to be invented and a very thorough understanding of the silicon measurand interface will have to be provided. These problems have not prevented research groups fromentering this difficult field,so that at present, next tothe

radiant-signal sensors, thechemical-signalfield is attractingthemostinterest,la

Inthe followingsectionthemost importantSMT’swillbe discussed.

a)

The ISFET. The first chemical SMT was invented in 1968 by Bergveldis _and

is

called anISFET (ion-sensitive

FET).

The structureof thisdeviceresemblesthat ofa

con-ventional

MOSFET,

except thatno gate electrode isused and thatanion-selectivelayer (Si3Na, A1203, polymers,

etc.)

is deposited on top of the thermally grown gate oxide

layer.Sodium concentrationand

pH

valueshave been measured.

b)

Hydrogen sensor. WhenPd isused inaMOSFETas gate material,it appearstobe possible tomake ahydrogensensor,t6 Itsoperationseemstobebasedonthe subsequent dissociation of the

_H2

at the Pdsurface,the diffusion ofH throughthe Pdlayerand the

0R

MINOR GAS

GAS IN AIR

FIGURE8

LIQUID FOG

SOLID DUST IN AIR

LIQUID IN WATER ALCOHOL IN WATER MILK SOLID IN SmCo MOISTURE IN PAPER ALLOYS

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SILICON ANDHYBRIDMICRO-ELECTRONIC SENSORS 225 capacitor FIGURE9 temperature sensor peltJer element

Sensor formeasuring dew pointandhumidity.

absorption of H at the

_Pd-SiO2

interface. The Pd work function is changed by this, leading to a changeinthe thresholdvoltageof the

MOSFET,

whichcanbe electronically detected.

c)

Humiditysensor. Humidityin aircanbe accurately measured byachemicalSMT.17

The siliconsensor chip consists ofa capacitive f’mger structure for detectingcapacitance

changes dueto dewformation on the chip and atransistor formeasuringthe chip

tem-perature (Figure

9).

The chipismountedon aminiaturePeltierelement,whichisusedas

the cooling device. In operation,thetemperatureofthe chipislowereduntilwatervapor from theair starts to condensate on the chip, and asharp increaseinthecapacitance is

measured.The temperature at whichcondensation startsismeasured next,giving a

tem-perature from which the relative air humidity can be calculated. The chip size is 2 x 2 mm, and thewidthandinterspacingof theelectrodesare 10#m.The non-linearity

of thedeviceislessthan0.1 Koveratemperaturerange of50K.

4. TRANSDUCERS INTHICK- ORTHIN-FILM TECHNOLOGY

Thick- and thin-ftim technologies have long been used for makingelectronic circuits, but

they cannow also beapplied tomake transducers. Examples of transducersproducedby these methods will be given, although we will first turn our attention to some general features.

a)

Substrates. The essence ofthese techniquesistoapply patternsto an isolating

sub-strate. Asubstrate material must notonly have thedesiredelectricalproperties,butmust

also be able to resist the process conditions. Examples ofmaterialsused are: alumina, beryllia, sapphire and glass. The choice inmaterialsprovides an importantfreedom, so

that the mechanical and thermalproperties (elasticity,heat conductivity, thermal coef-ficient of expansion,

etc.)

canbetakenintoaccount.

b)

Patterns. Some ofthe examplesgivenherearebasedon aspecific patterngivingthe

desired properties. Thin-f’flmtechnologyoften makes use ofaphotolithographicprocess,

while thick-f’flm patterns aremade by screenprinting.Multilayerscaneasily be made and addtotheversatilityofthese techniques.

c)

Materials. Still other examples get their desired properties mainly by making a choice of the manymaterials that canbe used.A variety of metals, metaloxides,alloys and semiconductors canbe evaporatedorsputteredand appliedasathin-f’flm. Thick-f’tim pastes are commerciallyavailableforconductors,resistors(includingntcandptc)and

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die-A B

FIGURE 10 Sensor for measuring sunlight power density (Solarimeter) a) frontside withblack centerand reflectingringb) backsidewiththermocouplesbetween centerand edge.

lectrics. The formulation of special thick-film pastes has a great potential for adding

materials with specifiedcharacteristics to asubstrate.Passive componentswillbemostly used,resultingintransducersof the modulating type.

d)

Other features. Adding active components to a substrate makes ahybrid circuit.

The same processmakes iteasyto handle electronic signals close to theplacewhere the

conversion is accomplished. Adequatepotting materials andpassivationlayers provide a

good resistance against environmental circumstances.

A

hybridtransducer can exhibit a

large operatingtemperature range, mainlyasthe result of carefulmaterial selectionand highprocessingtemperatures.

4.1 HybridTransducers

for

Radiant Signals

a)

Sensor for measuring sunlight power density. A device for measuring the power density ofincidentsunlight hasbeen realized(Figure

10).

The center of one side of an

_A12

_O3

substrateismade black.Onthe other side, the heat of the absorbed sunlightis removed at theedge by aheat sink.The resulting

tem-perature difference between the center and the edge ismeasured by a series of thermo-couples formed bytwo differentthick-filmmaterials. The initial experimentsweredone

withAu and

Pt-Au.

The temperature difference isproportional to thepower density of theincidentsunlight.

b)

Sensor for measuring radial electron-beam density. A pattern ofconcentric elec-trodesisusedtomeasurethe radial electron density of an electron beam. Thechargeof

the incidentelectronsiscoupledto anamplifierbyanRCnetwork(Figure

11).

FIGURE 11 Sensor for measuringradialelectronbeam density. Fifty concentricelectrodes con-nectedtoresistorsat theedgevia a dielectriclayer.

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SILICON ANDHYBRIDMICRO-ELECTRONIC SENSORS 227

c)

Light meter.Sputteredorevaporated films ofmaterialssuchasCdSareused as

light-sensitiveresistors.

4.2 Hybrid Transducers

_for

Mechanical Signals

a)

Pressure sensor.The piezoresistiveeffectin athick-filmresistor isused byCattaneo,

et al. for measuringpressure.18 _{The pressure deforms}_{a ceramic}_diaphragm_on_which_four

thick-f’drn resistors have been deposited in a Wheatstone bridge configuration. The

signalismodulated by the displacementofthe diaphragm.

b)

Strain gauge. Berthe et al. described thin-f’tlm straingauges,inwhichanisolation layeris first sputteredon ametal substrate.19_On

top ofthis aWheatstone bridge of thin-f’ilm resistors is formed with the aid of a photolithographic process. Thin-f’dm strain

gaugescanbe small andareused formeasuringlowforces.

c)

Displacement sensor. Athin-film microdisplacement transducer hasbeendeveloped by Klaassenet al.2 _{The transducer}

consistsof two glass plates: the upperismovable and has an electrode capacitivety coupled to the electrodes evaporated, on thelower glass plate.The capacitive couplingisposition-dependent. Theminimumwidthandspacingof

the electrodesis 100/am. Inthe sensor 90 phase-shiftedsinusoidalsine-wavevoltagesare usedonadjacent fingersofthelowerpattern.Distancesdownto25nmcan be detected.

4. 3 HybridTransducers

for

ThermalSignals

a)

Thermocouple. Using thin-f’timlayers of differentmaterials toformathermocouple

iswell known. Suchadevice is very usefulinsurface temperature measurement,

particu-larly when a fast response is required. Special thick-film pastes consistingof Pd-Pt-Au and Au-Pd have beendevelopedforthermocoupleuse.2

b)

Thermistor. The formulation and use of a stable thick-film thermistor paste has been reported by Ikegamiet al.z The use of thiselementisillustrated by a number of examples,including thecompensationofthermal effects.

c) Temperature

sensor.Leppgvuoridescribesa capacitivethick-filmtemperaturesensor

makinguse ofthe temperature dependence ofthe

_er

of ferroelectric materialsabove the

Curietemperature,za Amixture ofBa Ti

_Oa

and SrTi

_Oa

isincorporatedinathick-Film pastetoserve as anactiveelement.

for

Magnetic Signals

Thin-film layers of magnetic material have already been under investigation for along

time. Recently Druyvesteijn reportedthe application ofNi-Fethin-films inaread head

for a magnetic recorder.4

By

chemicaletchingapatternisformedin a0.05/amthick

Ni-Fe f’tlm. Conductorsare formedontop ofit inasandwichofMo Au Mo

(0.02

0.3

0.05/am).

Variationinthemagnetization ofthe tapecausesthemagnetization inthe

Ni-Fef’timtorotate, whichisdetectedas a variation intheresistance.

for

Chemical Signals

a)

Moisture sensor. The absorption ofwater inaporous material causes achangein

impedance. Leppivuori reportssome experiments concerning the effect of humidityon

the conductivity and permittivity of a thick-film capacitor.2

Two

types of thick-film moisture sensors for permanentinstallationinhighway structures havebeendescribedby

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228 MIDDELHOEK,

TABLE

Comparisonoftechnologies for sensors

Property Si-Techn. ThinFilm Thick Film

Cost mass production + 0

Cost small production + +

Reliabilityencapsulated + + +

Reliabilitynon-encapsulated 0

Versatility 0 0 +

Electronic compatibility + 0 0 Possibility of smartsensors +

Sensorsize + 0 0 Largearrays 0 + + Elementdensity + 0 Reproducibility 0 0 0 Accuracy 0 + + Temperaturerange + +

Longterm stability + + 0

Energyconsumption + 0 0

Handling 0 +

Suitability + Good 0 Moderate

Bad

Lucaset al.2s" _{one using an}

interdigitated thick-film capacitor as a sensingelement,and another consisting of athick-film heater and thermistor screened and fired on opposite sides ofan aluminasubstrate.With a fixedpowerinputthe temperature can be relatedto

thesoft-moisture content.

b) pH

sensor. Afromowitz fabricated a pH-sensitive implantable electrode,in whicha

conductor of Pt-Pd-Au is covered by a pH-sensitive glass layer.26

Corning 0150 glass, normally used for

pH

measurements, isappliedvia a thick-filmpaste, thus forming the

sensitive film.

An

impedanceconversion isrealizedonthesamesubstrate.

5.

CONCLUSIONS

AND

FUTURE

OUTLOOK

Furtherpenetration ofmicro-electronicsintomanynewmarketsishamperedby the lack

of transducerswithperformance/price ratios comparable to those of integrated circuits. Silicon planar, thin-f’tim and thick-film technologies all show promise with respect to

f’fllingthisgap. In Table Ithe authors have madeanattempt to evaluate the suitabilityof

the three technologies for thefabricationof transducers.

This reviewpaperdescribes alarge number of alreadyinventeddevices, basedonthese technologies. Becauseresearch anddevelopmentlaboratoriesofindustriesanduniversities are focusingtheir efforts more andmore on thisburgeoning field,wemay becertainthat

inthenextdecade manymorelow-cost andinnovativesensors, displays and actuatorswill

follow.

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