New materials for micro-scale sensors and actuators An engineering review

129 Pages·2007·9.12 MB·English

MaterialsScienceandEngineeringR56(2007)1–129
www.elsevier.com/locate/mser
New materials for micro-scale sensors and actuators
An engineering review
Stephen A. Wilsona,*, Renaud P.J. Jourdaina, Qi Zhanga, Robert A. Doreya,
Chris R. Bowenb,1, Magnus Willanderc,2, Qamar Ul Wahabd,3, Magnus Willandere,4,
Safaa M. Al-hillie,4, Omer Nure,4, Eckhard Quandtf,5, Christer Johanssong,6,
Emmanouel Pagounish,7, Manfred Kohli,8, Jovan Matovicj,9, Bjo¨rn Samelk,10,
Wouter van der Wijngaartk,10, Edwin W.H. Jagerl,11, Daniel Carlssonl,11,
Zoran Djinovicj,12, Michael Wegenerp,13, Carmen Moldovanm,14, Rodica Iosubm,
Estefania Abadn,15, Michael Wendlandto,16, Cristina Rusug,17, Katrin Perssong,17
aMicrosystemsandNanotechnologyGroup,MaterialsDepartment,CranfieldUniversity,Cranfield,BedfordshireMK430AL,UnitedKingdom
bMaterialsResearchCentre,DepartmentofMechanicalEngineering,UniversityofBath,BathBA27AY,UnitedKingdom
cPhysicalElectronics,DepartmentofScienceandTechnology,CampusNorrko¨ping,Linko¨pingUniversity,SE-60174Norrko¨ping,Sweden
dDepartmentofPhysics,MeasurementTechnology,BiologyandChemistry,Linko¨pingUniveristy,SE-58183Linko¨ping,Sweden
ePhysicalElectronicsandPhotonics,PhysicsDepartment,GothenburgUniversity,SE-41296Gothenburg,Sweden
fInstituteforMaterialsScience,FacultyofEngineering,UniversityKiel,Kaiserstr.2,24143Kiel,Germany
gImegoAB,ArvidHedvallsBacke4,SE-41133Go¨teborg,Sweden
hHelsinkiUniversityofTechnology,LaboratoryofMaterialsScience,Vuorimiehentie2A,02015TKK,Finland
iMicrosystems,ForschungszentrumKarlsruhe,IMT,Postfach3640,76021Karlsruhe,Germany
jInstituteofSensorandActuatorSystems,ViennaUniversityofTechnology,Floragasse7/2,A-1040Vienna,Austria
kMicrosystemTechnologyLab(MST),SchoolofElectricalEngineering(EE),RoyalInstituteofTechnology(KTH),
Osquldasvag10,S-10044Stockholm,Sweden
lMicromuscleAB,Teknikringen10,SE-58330Linko¨ping,Sweden
mMicrostructuresforBio-MedicalApplicationsResearchLaboratory,NationalInstituteforResearchandDevelopmentinMicrotehnologies,
IMT-Bucharest,31BErouIancuNicolaeStreet,077190Bucharest,Romania
nMicroandNanotechnologyDepartment,Fundacio´nTekniker,AvenidaOtaola20,20600EIBAR(Guipuzcoa),Spain
oMicroandNanosystems,DepartmentofMechanicalEngineering,ETHZurich,8092Zurich,Switzerland
pFunctionalPolymerSystems,FraunhoferInstituteforAppliedPolymerResearch,Geiselbergstrasse69,14476Potsdam-Golm,Germany
Received23February2007;receivedinrevisedform20March2007;accepted20March2007
Availableonline29June2007
* Correspondingauthor.Tel.:[phone]x2505;fax:[phone].
E-mailaddresses:s.a.wilson@cranfield.ac.uk(S.A.Wilson),[email protected](C.R.Bowen),[email protected](M.Willander),
[email protected](Q.U.Wahab),[email protected](M.Willander),[email protected](S.M.Al-hilli),
[email protected](O.Nur),[email protected](E.Quandt),[email protected](C.Johansson),[email protected].fi
(E.Pagounis),[email protected](M.Kohl),[email protected](J.Matovic),[email protected](B.Samel),
[email protected](W.vanderWijngaart),[email protected](E.W.H.Jager),[email protected]
(D.Carlsson),[email protected](Z.Djinovic),[email protected],[email protected](M.Wegener),
[email protected](C.Moldovan),[email protected](R.Iosub),[email protected](E.Abad),[email protected](M.Wendlandt),
[email protected](C.Rusu),[email protected](K.Persson).
0927-796X/$–seefrontmatter#2007ElsevierB.V.Allrightsreserved.
doi:10.1016/j.mser.2007.03.001
2 S.Wilsonetal./MaterialsScienceandEngineeringR56(2007)1–129
Abstract
Thispaperprovidesadetailedoverviewofdevelopmentsintransducermaterialstechnologyrelatingtotheircurrentandfuture
applications in micro-scale devices. Recent advances in piezoelectric, magnetostrictive and shape-memory alloy systems are
discussedandemergingtransducermaterialssuchasmagneticnanoparticles,expandablemicro-spheresandconductivepolymers
areintroduced.Materialsproperties,transducermechanismsandendapplicationsaredescribedandthepotentialforintegrationof
the materials with ancillary systems components is viewed as an essential consideration. The review concludes with a short
discussionofstructuralpolymersthatareextendingtherangeofmicro-fabricationtechniquesavailabletodesignersandproduction
engineersbeyondthelimitations of silicon fabrication technology.
#2007Elsevier B.V.All rightsreserved.
Keywords: Piezoelectric;Magnetic;Shapememory;Polymer;Microstructure;Microtechnology
Contents
1. Introduction ... .... .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... ... 5
2. Ferroelectricceramics .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... ... 6
2.1. Piezoelectric properties and potentialapplications offerroelectric thin films .... .... .... ..... .... ... 7
2.1.1. Thinfilmdeposition.... .... .... .... .... ..... .... .... .... .... .... ..... .... ... 8
2.1.2. Piezoelectric properties offerroelectric thin films..... .... .... .... .... .... ..... .... ... 8
2.1.3. Polingandreliability issues... .... .... .... ..... .... .... .... .... .... ..... .... ... 9
2.1.4. Summary—ferroelectric thin fims... .... .... ..... .... .... .... .... .... ..... .... .. 10
2.2. Thickfilmfabrication formicro-scale sensors .... .... ..... .... .... .... .... .... ..... .... .. 10
2.2.1. Thickfilmdepositiontechniques ... .... .... ..... .... .... .... .... .... ..... .... .. 10
2.2.2. Inks .. .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 11
2.2.3. Transformation binders.. .... .... .... .... ..... .... .... .... .... .... ..... .... .. 12
2.2.4. Electricalproperties of PZTthick films... .... ..... .... .... .... .... .... ..... .... .. 12
2.2.5. Summary—ferroelectric thickfilms . .... .... ..... .... .... .... .... .... ..... .... .. 12
3. Piezoelectric semiconductors..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 13
3.1. GroupsIII–Vnitrides (GaN/AlN). .... .... .... .... ..... .... .... .... .... .... ..... .... .. 13
3.2. GroupsIII–Vmaterials.... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 15
3.3. ZnOmaterials . .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 15
3.4. Summary—piezoelectric semi-conductors ... .... .... ..... .... .... .... .... .... ..... .... .. 16
4. Zincoxidestructures forchemicalsensors ... .... .... .... ..... .... .... .... .... .... ..... .... .. 16
4.1. Synthesisand properties ofZnO nano-structures .. .... ..... .... .... .... .... .... ..... .... .. 17
4.2. Electrochemical potentialmethod. .... .... .... .... ..... .... .... .... .... .... ..... .... .. 18
4.3. Sitebindingmethod. ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 19
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S.Wilsonetal./MaterialsScienceandEngineeringR56(2007)1–129 3
5. Siliconcarbide forchemical sensingdevices.... .... .... ..... .... .... .... .... .... ..... .... .... 21
5.1. SiC singlecrystal growth .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 22
5.2. Gassensorprinciples ... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 23
5.3. SiC gassensor development .. .... .... .... .... ..... .... .... .... .... .... ..... .... .... 23
5.4. Otherinnovative SiC basedchemical gassensors.... ..... .... .... .... .... .... ..... .... .... 24
5.5. Conclusions. .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 25
6. Magnetostrictivethin films .... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 25
6.1. Giantmagnetostrictivethin films ... .... .... .... ..... .... .... .... .... .... ..... .... .... 25
6.2. Magnetostrictivethin filmactuators . .... .... .... ..... .... .... .... .... .... ..... .... .... 27
6.3. Magnetostrictivemagnetoresistive sensors. .... .... ..... .... .... .... .... .... ..... .... .... 27
6.4. Magnetostrictivemagnetoimpedance sensors ... .... ..... .... .... .... .... .... ..... .... .... 28
6.5. Magnetostrictiveinductivesensors .. .... .... .... ..... .... .... .... .... .... ..... .... .... 28
7. Magneticproperties of magneticnanoparticles .. .... .... ..... .... .... .... .... .... ..... .... .... 29
7.1. Single domains .. ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 29
7.2. Ne´elrelaxation... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 29
7.3. Brownian relaxation .... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 31
7.4. Biosensormethods usingmagnetic nanoparticles .... ..... .... .... .... .... .... ..... .... .... 31
7.5. Conclusions. .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 32
8. Magneticshape memoryalloys. .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 33
8.1. Production andchemical composition.... .... .... ..... .... .... .... .... .... ..... .... .... 34
8.2. Magneticand mechanical measurements.. .... .... ..... .... .... .... .... .... ..... .... .... 35
8.3. Magneticshape memoryactuators .. .... .... .... ..... .... .... .... .... .... ..... .... .... 40
8.4. Magneticshape memorysensors, thin filmsandcomposites . .... .... .... .... .... ..... .... .... 43
9. Shape memorythin filmsforsmartactuators ... .... .... ..... .... .... .... .... .... ..... .... .... 44
9.1. Microfluidic valves usingSMA thin films. .... .... ..... .... .... .... .... .... ..... .... .... 44
9.2. Roboticdevices usingSMA thinfilmcomposites.... ..... .... .... .... .... .... ..... .... .... 47
9.3. Microactuators offerromagnetic SMA thinfilmsfor informationtechnology.. .... .... ..... .... .... 49
9.4. Conclusions. .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 51
10. Shape memorymaterials. ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 51
10.1. Shape memoryalloys ... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 51
10.2. Micro-scale applications ofSMA... .... .... .... ..... .... .... .... .... .... ..... .... .... 53
10.3. Shape memorypolymers. .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 54
10.4. SMPapplications inMST.... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 55
10.5. Conclusion . .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 56
11. Expandable microsphere composites . .... .... .... .... ..... .... .... .... .... .... ..... .... .... 56
11.1. Direct mixingof themicrospheresin liquid.... .... ..... .... .... .... .... .... ..... .... .... 57
11.2. Surface immobilization ofthemicrospheres byincorporationinphotoresist .. .... .... ..... .... .... 58
11.3. Surface immobilization ofthemicrospheres throughself-assembly onachemically alteredsurface... .... 60
11.4. Incorporation ofthemicrospheres ina paste ... .... ..... .... .... .... .... .... ..... .... .... 61
11.5. Incorporation ofthemicrospheres as acomposite ina polymermatrix.. .... .... .... ..... .... .... 62
12. Electro-active polymermicroactuators .... .... .... .... ..... .... .... .... .... .... ..... .... .... 64
12.1. Conjugated polymer actuators . .... .... .... .... ..... .... .... .... .... .... ..... .... .... 65
12.2. Fabrication of PPy-microactuators .. .... .... .... ..... .... .... .... .... .... ..... .... .... 66
12.3. Operation and performance ... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 68
12.4. Applications and devices. .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 68
12.4.1. Bending actuators.... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 68
12.4.2. Valves... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 71
13. Electrochromic andelectroluminescent polymers. .... .... ..... .... .... .... .... .... ..... .... .... 72
13.1. Electrochromic materials. .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 73
13.2. Electrochromic devices.. .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 74
13.3. Electroluminescent materials .. .... .... .... .... ..... .... .... .... .... .... ..... .... .... 75
13.4. Electroluminescent devices ... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 76
13.5. Conclusions. .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .... 78
14. Ferroelectrets—cellular piezoelectric polymers .. .... .... ..... .... .... .... .... .... ..... .... .... 78
14.1. Foampreparation andoptimization.. .... .... .... ..... .... .... .... .... .... ..... .... .... 79
14.2. Voidchargingin cellularspace–charge electrets. .... ..... .... .... .... .... .... ..... .... .... 80
4 S.Wilsonetal./MaterialsScienceandEngineeringR56(2007)1–129
14.3. Piezoelectric properties.... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 81
14.4. Applications offerroelectrets.... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 82
14.5. Conclusionsandoutlook... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 82
15. Conductive polymers . .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 83
15.1. Mechanism ofpolymer conductivity—roleof doping ... ..... .... .... .... .... .... ..... .... .. 83
15.2. Conductive polymericmaterials—examples.. .... .... ..... .... .... .... .... .... ..... .... .. 85
15.2.1. Polypyrrole . ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 85
15.2.2. Polyaniline . ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 85
15.2.3. Polythiophene .... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 85
15.2.4. Polysiloxane ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 86
15.2.5. Polyphthalocyanine. .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 86
15.2.6. Fullerene... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 87
15.3. Applications ofconductive polymersin sensorsandactuators... .... .... .... .... .... ..... .... .. 87
15.3.1. Sensors.... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 87
15.3.2. Chemical microsensors.. .... .... .... .... ..... .... .... .... .... .... ..... .... .. 88
15.3.3. Electronicnoses... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 89
15.3.4. FETtypedevices.. .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 90
15.3.5. Biosensors.. ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 91
15.3.6. Actuators... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 91
15.4. Conclusions... .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 92
16. Polyimides .... .... .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 93
16.1. Properties ofpolyimides ... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 93
16.2. Processingof polyimides... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 93
16.2.1. Wetetch patterning. .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 93
16.2.2. Dryetch patterning. .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 94
16.2.3. Photodefinable polyimides.... .... .... .... ..... .... .... .... .... .... ..... .... .. 94
16.2.4. Laser ablation .... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 95
16.3. Polyimideapplications .... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 95
16.3.1. Highdensity interconnection flexiblesubstrates. ..... .... .... .... .... .... ..... .... .. 95
16.3.2. MEMSdevices.... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 95
17. Structuralpolymers .. .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 97
17.1. Selection ofstructuralpolymers formicro-scale devices. ..... .... .... .... .... .... ..... .... .. 98
17.1.1. Thermosets . ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... .. 98
17.1.2. Thermoplastics.... .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 100
17.1.3. Elastomers.. ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 101
17.2. Applications ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 101
17.2.1. Micro-scale sensors .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 101
17.2.2. Micro-scale actuators ... .... .... .... .... ..... .... .... .... .... .... ..... .... . 102
18. Integrationand interconnection ... .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 103
18.1. Wafer bonding. .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 104
18.1.1. Adhesivebonding.. .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 104
18.1.2. Metallic bonding .. .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 105
18.1.3. Glass-fritbonding.. .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 105
18.1.4. Silicondirect bonding... .... .... .... .... ..... .... .... .... .... .... ..... .... . 105
18.1.5. Plasma-enhancedbonding.... .... .... .... ..... .... .... .... .... .... ..... .... . 106
18.1.6. Anodicbonding... .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 106
18.2. Low temperature co-firedceramics andmicrosystems... ..... .... .... .... .... .... ..... .... . 107
18.2.1. MediumCTE LTCC.... .... .... .... .... ..... .... .... .... .... .... ..... .... . 108
18.2.2. Low CTELTCC... .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 109
18.3. Characterisation methods formicrosystem bonding .... ..... .... .... .... .... .... ..... .... . 110
18.4. Conclusion ... .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 112
Acknowledgements .. .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 112
References .... .... .... ..... .... .... .... .... .... ..... .... .... .... .... .... ..... .... . 112
S.Wilsonetal./MaterialsScienceandEngineeringR56(2007)1–129 5
1. Introduction
Amaterial can besaidtobe‘new’ or‘novel’ untilit findsitsway intomainstreamengineering technology.The
distinguishing criterion is not whether the end-use is in consumer products, sophisticated, specialised or niche
applications, butwhether materials performance is predictable and reliable. By implication, quality and processing
must be well understood and commercial supplies readily available. For these reasons, the time-scale in which a
materialremainsnewisrelateddirectlytothecommercialinterestthathasevolvedandconsequentlytothebusiness
opportunities that the material has inspired in its conceptual form.
A new material that promises to provide tangible improvements over the established norm will soon attract
commercialinterestandits’potentialusewillcomeunderscrutiny.Thefirstquestionstoberaisedrelatetopossible
integrationintoexistingsystemsorpossiblecreationofanewproductline.Iftechnologicalbarrierstointegration
exist,betheseeitherrealorperceived,thencommercialinterestwillimmediatelycool.Fortheparticularcaseof
micro-systemstechnology(MST),wherethecreationoffinescaleintegratedsystemsisakeymotivationalfactor,
thepotentialcostsofproductdevelopmentcanoftenovershadowanyimprovementsinperformancethatmightbe
gained.Thisispartlyaconsequenceoflocalintegrationwithmicroelectronicsandpackaginganditispartlydueto
the capital equipment costs involved. In the main, however, it is due to the time and uncertainty involved in
establishing a new fabrication route that meets predefined standards of quality and reliability. Hence, to gain
acceptance in micro-technology the newmaterial must offer distinctperformance advantages and it must also be
compatible with various ancillary systems components and packaging. In all cases, it is highly probable that
production will entail a lengthy sequence of process steps and consequently the material will need to tolerate
repeated thermal cycling as system fabrication proceeds. It is not uncommon for the materials covered in this
review, namely transducer materials, to rely on some aspect of their micro-structural composition that is highly
sensitive to processing conditions. As an example, effects of grain size or morphology are often critical and
optimumperformancecanbeimpairedbyexcursionsoutsidealimitedtemperaturerange.Therefore,theprocesses
involved in creating the materialmay only be one partof the equation and compatibility with secondary systems
fabrication processes is equally essential.
Full-integration of micro-electronic and micro-mechanical components on a single wafer has been achieved
commercially using silicon processing technology. Some examples of products made in this way include micro-
gyroscopesandmicro-mirrorarrays.Whilstthisintegrateddesignapproachappearstobecommerciallyattractive
it has, however, proven to be relatively rare owing to the complexity of the design process and, consequently,
high development costs. Furthermore, due to processing restrictions the mechanical components of these
fully-integrated devices are often constructed simply from silicon and silicon oxide with selective metallization.
An alternative approach, adopted much more commonly, is via a hybrid design where component parts are
created separately for subsequent assembly into a complete system. For small or medium-scale batch production
this is an attractive option, as it removes many of the restrictions imposed by the need for process compatibility.
Furthermore, test procedures can be performed at the wafer-scale before final assembly to enhance quality and
overall yield. It is in this context that new transducer materials have the best chance of success. Key considera-
tions are the availability of material-specific replication technologies, device-specific geometric requirements
(feature types, planar or 3D, aspect ratios), the required dimensional tolerances and accuracy, surface quality or
integrity, volumetric production rate and material cost, which can often be of secondary importance in this
context.
Overall it can be said that the most significant barriers to progress are firstly the availability of production
technologiesandsecondlytheavailabilityofknowledge.Thisarticlethereforeseekstoreviewrecentdevelopmentsin
transducermaterialstechnologyandtoplacetheminthecontextoftheircurrentandfutureapplicationsinmicro-scale
systemsfabrication.Inadditiontoexaminingrecentadvancesinpiezoelectric,magnetostrictiveandshapememory
alloys systems, emerging transducer materials such as magnetic nanoparticles, expandable micro spheres and
conductivepolymersarealsodiscussed.Theirunderlyingproperties,transducermechanismandendapplicationsare
described, along with the processing technologies to form them in particulate, bulk or film geometry. Aspects of
processing that may influence integration of the materials with their related components areviewed as an essential
consideration. From a global perspective, there are of necessity some important omissions. It seems certain that
materials incorporating carbon nanotube technology and nanocomposites will reach industrial maturity in the very
nearfutureandthattheirimpactwillbesignificant.Thissubjectmatterhasbeenextensivelyreviewedelsewhereand
6 S.Wilsonetal./MaterialsScienceandEngineeringR56(2007)1–129
thematerialsarenotcoveredinthisreview.Rathertheintentionistohighlightarangeofmaterialsthatcouldbeused
inconjunctionwithstandardmicro-fabricationtechniquestoextendtherangeofdevicesthatcanbemadebeyondthe
limitations of silicon fabrication technology.
2. Ferroelectric ceramics18
Polycrystallineleadzirconatetitanate(PZT)ceramicsareofmajorimportanceinmicrotechnology,particularlyin
the field of sensors and actuators, because of their superior piezoelectric and pyroelectric properties and their high
dielectricconstants[1].Devicesthatincorporatethesematerialsastheiractivecomponentincludemicro-pumpsand
valves, ultrasonic motors, thermal sensors, probes for medical imaging and non-destructive testing, accelerometers
and quite recently a new range of electronic components that includes filters, memory devices and switches. New
applications continue to emerge and a major research effort has been underway to address the manufacturing
technologyrequiredtoincorporatethesematerialswithassociatedstructuralcomponentsandelectroniccircuitryat
the wafer scale. Two distinct approaches are available which have very different process requirements and which
consequentlyrequiredifferentfabricationtechniques.Thebottom-upapproachisbythinfilmdeposition,performed
viaspincoatingofasol–gelprecursororsputtering.Thinfilmcompositionshavebeendevelopedthathavegreatly
reducedprocessingtemperatures(600–7008C)incomparisontostandardbulkceramicsintering(1100–14008C)and
this has led to commercialization by the major electronics corporations in the form of ferroelectric memories and
electroniccomponents.Asinglelayeristypicallyaround0.1mmandfilmsarebuiltuptotherequiredthicknessby
depositing several layers in succession.
The processing issues that surround production of electromechanical devices on the micro-scale are arguably
evenmorecomplex,however,duetotherangeofancillarysystemcomponentsthatareneeded.Theavailableforce
thatcanbegeneratedbytheceramicisdirectlyrelatedtotheamountofelectro-activematerialthatisavailableand
manypiezoelectricdeviceswithpotentialcommercialapplicationssuchasmicro-pumpsrequiremuchthickerfilms
to be effective, typically in the size range 10–80mm. Thesevalues have been achieved by multi-layer deposition
using composite thick film techniques and significant progress has been made, which makes these materials
suitableforanumberofapplications.Thistechniqueisdetailedbelow.Inpracticeresidualtensilestressisacritical
issue, inherent to the process, which becomes progressively more significant as film thickness increases. Tensile
stressesresultfromsubstrateclampingasthematerialcrystallizesatelevatedtemperaturesoftenleadingtoreduced
fracture toughness or cracking and somewhat lower electro-active coefficients.
The alternative, top-down approach for micro-scale device fabrication is by assembly of net shape components,
usuallybyadhesivebonding.Thisisroutinelyadoptedforone-offdevicefabricationintheresearchenvironment.On
thewafer scale there are important questions of positional accuracy both laterally and in terms of parallelism with
underlyingmaterials.Thisbecomesmoresignificantaslayerthicknessesarereducedbelow(cid:2)80mm.Thenatureof
thebondisofcriticalimportancetodeviceperformanceandhencethesurfaceroughnessandparticularlytheflatness
oftheceramiccomponentareverysignificant.Recently,ithasbeenshownthatbulkPZTceramicscanbethinnedin
situ to thicknesses well below 50mm, using ultra-precision grinding, after bonding towafer-scale components [2].
Thistechniquehasseveraladvantages:(a)theelectro-activepropertiesoftheceramiccanbefullyexploited;(b)films
canbemadeinthe20–50mmthicknessrange,whichisdifficulttoachievebyothermethods;(c)ceramicfilmscanbe
engineeredintoresidualcompressiontooptimizedeviceperformance;(d)themachiningtechniquescanbeusedin
sequencewithstandardmicro-fabricationprocesses,suchasphotolithography,withouttheneedforahightemperature
excursion,thereby extendingdesign flexibility andthe range of devices that can be produced; (e) PZT filmsin this
thicknessrangecanbecanbeactivatedwellbelow100V,thisishighlysignificantincommercialtermsastheyare
thencompatiblewithcurrentCMOSdrivecircuitry.Recentresearchworkisthisareahasleadtomajorimprovements
intechniqueandthemethodcanbeconsideredviableforflexible,batch-scaleassemblyandsystemsintegration.The
key issues that are involved in producing exceptionally smooth, flat surfaces in PZT by means of ultra-precision
grinding have been discussed by Arai et al. [3–5].
As noted,ferroelectric ceramics are ofwidespread technological importanceand forthisreason theyremainthe
subjectofintenseresearchactivity.Materialsdevelopmenthasfocussedonthreeparticularareas.Oneofthesecanbe
18StephenA.Wilson,RenaudP.J.Jourdain,QiZhang,RobertA.Dorey
S.Wilsonetal./MaterialsScienceandEngineeringR56(2007)1–129 7
saidtobemarket-driventhroughstrongcommercialinterestinnewfuelinjectionsystemsformotorvehicles.Thisisa
highpower,hightemperature,lowvoltageapplicationwhichissatisfiedbymulti-layerceramicstacks.Theceramic
layersaretypicallylessthan50mminthicknessandtheyareco-firedwithmetallicinterlayerstoproduceaninter-
digitatedstructure.Asthelayersarethinalowappliedvoltagecanbeusedtogenerateastrongelectricfieldinthe
ceramic[4].Afurtherareaofbothcommercialandtechnologicalinterestisinhighfrequencymedicalultrasonicsfor
imagingandultrasound-guidedtherapy.Thisalsotendstobeahighpowerapplicationwherethegoalistoreducethe
energylosses thatresultfrom internal power dissipation. These can generatesignificantamounts ofheatleading to
thermal instability and loss of performance [6–8].
Thesecondmajorareaofresearchispushedbynewtechnologythathasemergedintheformofferroelectricsingle
crystalmaterials.Thistypeofmaterialhasrecentlybecomeavailableincommercialquantitiesandtheelectro-active
propertiesexhibitedareamarkedextensionbeyondthoseofconventionalpolycrystallineceramics.Thecrystalsare
relaxorferroelectric materialsandtheyaretypicallybasedontheleadmagnesiumniobate–leadtitanate(PMN-PT)
solid solution, although many other compositions are also in research. Relaxors are characterised by a diffuse
dielectric phase transition, that is to say their dielectric permittivity is both frequency and temperature dependent.
Theirphysicalbehaviourisasyetnotfullyunderstoodbut,importantly,theyarefoundtoexhibitverylargedielectric
permittivities and very high piezoelectric coefficients. In operation, their electro-mechanical behaviour is
predominantly electrostrictive in nature resulting in exceptionally low hysteretic losses even at high frequencies.
Whilstthesematerialshaveshownclearsuperiorityforsomeelectro-acousticapplications,theiradoptionforusein
actuatorsisstillat averyearlystage.Theuppertemperaturelimit ofoperation can berelativelylowataround50–
808Candthis,togetherwithamarkedenvironmentalvariabilityofproperties,clearlyimposessomerestrictionson
design.Nevertheless,thesematerialsdoshowveryinterestingnewcapabilitiesandtheyareanexcitingtechnological
innovation [9–15].
The third main focus of research is driven by environmental concerns over the industrial use of compounds
containinglead.Whilstitcanbearguedthatthetoxicityoflead-containingceramicsorglassesisverysignificantly
reducedincomparisontothatofthebasemetal,thereispressuretoreduceitsconsumption.Thishasledtoaconcerted
effortworld-widetoidentifyequivalentelectro-activematerialsthatarelead-free.To-date,despitesomesignificant
investmentoftimeandresources,littleprogress hasbeen madeindevelopingmaterialsthatareabletooutperform
standardPZTceramics.Severalinterestingcompositionshavebeenidentified,however,thathaveusefultransducer
properties and work seems sure to continue [16–20].
2.1. Piezoelectric properties and potential applications of ferroelectric thin films
Thin films are generally considered to have thicknesses less than 1 micron. Interest in ferroelectric thin films
has been considerable over the last 20 years, driven by the possibility of using them for non-volatile memory
applications and new microelectromechanical systems (MEMS). Thin film piezoelectric materials also offer a
numberofadvantagesinMEMSapplications,duetotherelativelylargedisplacementsthatcanbegenerated,the
high energy densities, as well as high sensitivity sensors with wide dynamic ranges and low power requirements
[21].
Piezoelectric MEMS devices contain at least two elements: a bulk silicon frame and a piezoelectric deflection
element built onto it, which also has electrodes to apply or detect voltage potentials. The silicon substrate often
providesonlythestructuralelement,definingthemechanicalproperties,whiletheaddedfunctionalmaterialsuchas
piezoelectricthinfilmsprovideadirecttransformationbetweenadrivingsignaloraread-outsignalandasensororan
actuator parameter.
Asamplingofrecentdevelopmentsinpiezoelectrictransductiondevicesusingthinfilmsincludesleadzirconate
titanate(PZT)basedultrasonicmicromotors[22–24],cantileveractuators,probesforatomicforcemicroscopy[25],
micropumps [26], ultrasonic transducers for medical applications [27,28] and uncooled thermal imaging as
pyroelectric arrays [29,30]. The aims of this section are as follows:
(cid:3) To introduce the current fabrication techniques for piezoelectric thin films.
(cid:3) To discuss the important piezoelectric coefficients and the key issues or factors influencing the piezoelectric
properties of ferroelectric thin films.
(cid:3) To discuss piezoelectric thin film poling and reliability issues.
8 S.Wilsonetal./MaterialsScienceandEngineeringR56(2007)1–129
2.1.1. Thin film deposition
MostoftheexistingphysicalandchemicalcoatingtechniqueshavebeeninvestigatedforthedepositionofPZT.
Thephysicalmethodsincludeionbeamsputtering[31],rfmagnetronsputtering[32,33],dcmagnetronsputtering[34]
and pulsed laser deposition (PLD) [35–37]. Chemical methods include metal-organic chemical vapour deposition
(MOCVD)[38–42]andchemicalsolutiondeposition(CSD)[43,44].TodaythereisacleartrendtoapplyMOCVDor
CSDsincea particular advantagewith MOCVD is thatconformal coating of three-dimensionalobjects is possible.
CSDisalowcosttechniqueforsmall-scaleproduction,asrequiredinthesensorindustry.SinceforCSDthefilmis
initiallyamorphous,post-annealingtreatmentsarenecessarytocrystallizethefilm.Alltheothermethodsdescribed
aboveallowinsitugrowth.AlthoughtheCSDtechniqueseemsverydifferentfromthevacuumdepositiontechniques
like sputtering or PLD, there are nevertheless some common features:
(cid:3) Thecrystallinityandtextureofthefilmarestronglydependentonthecrystalstructureofthesubstrate,forexample:
lattice parameters and thermal expansion coefficients matching, surface defects, etc.
(cid:3) Thequalityoftheinterfaceisdependentonthesubstratechemistry,forexample:reactivityofthesubstratesurface
with the deposited phase constituents, diffusion coefficients, etc.
(cid:3) Thelatticeenergyhastobebroughttothesystem,eitherthermallyorbyaphysicalway,sincetheinitialstateisa
disordered one (gas or liquid phase, plasma, particle beam, etc.).
(cid:3) Nucleationandgrowthoftheperovskiterequireaprecisestoichiometry,otherwisecompetingphaseswithfluorite
(Pb Ti O ) and pyrochlore (PbTi O ) structures will nucleate [45].
2+x 2(cid:4)x 7(cid:4)y 3 7
(cid:3) The growth is nucleation controlled [46,47].
2.1.2. Piezoelectric properties of ferroelectric thin films
Thepiezoelectricpropertiesofferroelectricmaterials,suchasPbZr Ti O ,arehighlydependentoncomposition
1(cid:4)x x 3
[21].Aschematicdiagramoftheleadzirconate(PZ)–leadtitanate(PT)phasediagramisshowninFig.1.PZThastwo
main ferroelectric phases; rhombohedral for x<0.48 and tetragonal for x>0.48 under standard conditions. The
rhombohedralphaseisdividedinto‘hightemperature’and‘lowtemperature’phaseswithcrystalsymmetriesR2mand
R3c, respectively. The boundary between the tetragonal and rhombohedral phases is sharply defined and virtually
independentoftemperatureandtheboundaryisknownasthemorphotropicphaseboundary(MPB).Theboundary
wasdefinedbyJaffeetal.[48]tobeatacompositionof53%Zrand47%TiinPZTceramics,andisdefinedasthe
point of equal coexistence for tetraganol/rhombohedral phases. In bulk ceramics, maxima in the piezoelectric
coefficients are generally observed at the MPB. The same behaviour is often [49–55], but not universally [54–56],
reported in thin films.
InMEMStechnology,mostofthepiezoelectricthinfilmsarepolycrystallinematerials.Thepiezoelectriceffectis
averagedoverallthegrains.Theoptimumpiezoelectricpropertiesofferroelectricmaterialscanonlybeobtainedfor
Fig.1. PhasediagramofthePbZrO3–PbTiO system[48].
3
S.Wilsonetal./MaterialsScienceandEngineeringR56(2007)1–129 9
polycrystalline materials after an appropriate ‘poling’ treatment. Poling is the term used to describe a preliminary
procedurethatmustbecarriedout,wherebyastrongelectricfieldisusedtoswitchtheinitial,quasi-randominternal
polarisationofthepoly-domainstructureintoameta-stablealignmentinthedirectionoftheappliedfield.Asaresult,
there is a net polarisation and a net piezoelectric effect. This can simplify processing, since single crystals are not
required for good electromechanical properties.
Thepiezoelectricpropertiesoffilmsarealmostalwayssmallerthanthoseofcorrespondingbulkceramics.Thisis
duetosubstrateclamping,whichreducestheamountofstrainwhichthefilmcanexhibitforagivenappliedelectric
fieldorstress[56,57].Thefilmispartofacompositestructureconsistingofthepiezoelectricfilmandsiliconsubstrate.
The film is effectively clamped in the film plane, but free to move in the out-of-plane direction. Therefore, the
clamping effect is thickness dependent, and the piezoelectric coefficients, such as d , increase with increasing
33,f
thicknessoverarangeoffilmthickness[21,58–62].Inthinfilmceramics,itisconventionaltoassigntheindex3tothe
polingdirection,usuallyperpendiculartothefilmplane.Thedirectionsof1,2arethereforeintheplaneofthefilm.In
apolycrystallinefilm,directions1and2areequivalentwhichimpliesthatthein-planestrains(d andd )duetoan
31 32
applied electric field though the film thickness (E ) are isotropic and d =d .
3 31 32
Therelativecoefficientsofpiezoelectricthinfilmsaretheeffectivevaluesofd ande ,whichareobtainedas
33,f 31,f
follows from the bulk tensor properties [63,56]:
d (cid:4)2sE d31
d ¼ 33 13 (1)
33;f ðsE þsE Þ
11 12
d
e ¼ 31 (2)
31;f ðsE þsE Þ
11 12
The d coefficient can be directly measured as the strain per unit electric field through the film thickness
33,f
(x /E ) provided that x =x =s =0, where x and x are in-plane strains, s off-plane stress, x is off-plane
3 3 1 2 3 1 2 3 3
strain and sE is a compliance of the thin film. This measurement has been achieved with a double-beam Mach-
ij
Zehnder interferometer [64] that measures the thickness change of a film clamped on a much thicker substrate
(assuring x =x =0) at s =0. The measurement of the transverse piezoelectric coefficient e , has been
1 2 3 31f
undertakenwithacantileverbendingmethod,collectingthechargesasafunctionofx andx atzeros andelectric
1 2 3
field [65].
Apart from mechanical clamping due to the inert substrate, there are several other factors which influence the
piezoelectricresponseofferroelectricthinfilms,includingorientationofthefilm[50,66–68],grainsize[69],thelevel
ofpolarizationandbreakdownfieldstrength[70,71].Theinfluenceofdefectsonthedomain-wallcontributionstothe
piezoelectriceffectinthinfilmshasnotyetbeenstudiedindetail.Thus,itispresentlynotclearwhether,forexample,
the effect of acceptor and donor dopants on the properties of PZT films would lead to the same effects as in bulk
materials.
Filmorientationcanhaveasubstantialeffectonpiezoelectriccoefficients.Piezoelectriccoefficientsareoptimized
when the polarization axis, namely c-axis or (001), is perpendicular to the film surface. It has been recently
demonstrated[58]thatthesol–gelderivedPZTthinfilmswithhigherc-axisorientationexhibitedlargerpiezoelectric
coefficients. For random polycrystalline films, poling is often necessary to reorient the domains along the poling
direction.
InmanyofthestructuresappliedtoMEMStechnology,thepiezoelectricfilmispartofacompositestructure,i.e.
thepiezoelectricfilmisclampedtoanotherelasticbody.Thecouplingcoefficientnotonlydependsonthematerial
parameters, but film stresses also play a role and such film stresses introduced during processing at elevated
temperature are unavoidable. The residual stress can be as high as 10–100MPa [72], which induces a pre-strain,
orapre-curvaturetomicromechanicalstructures.Thisstresshastobetakenintoaccountinthedesignphaseofthe
devices.
2.1.3. Poling and reliability issues
Theeffectsofpolinginthinfilmsdifferfromthatinceramics,sincetheclampingeffectofthesubstratepinsthe
motion of a-domains [56,73]. In bulk ceramics, the clamping is effectively zero, and domains are relatively free to
moveinalignmentwiththepolingfield.Therearefewstudiestodatethatarespecificallyrelatedtothinfilmpolingfor
10 S.Wilsonetal./MaterialsScienceandEngineeringR56(2007)1–129
Fig.2. PZTactuatedcoupledcantileverbandpassfilters/parallelplatevariablecapacitoractuatedbyfourthinfilmPZTcantileverunimorphs.
(ImagescourtesyofPaulKirby,CranfieldUniversity,UK).
piezoelectricmeasurement,butitiswellknownthatthestraininducedbypolingcanbecloseenoughtothetensile
strengthofthefilmwhichcaninducecrackingordelamination.Polingusuallytakesplaceatelevatedtemperatures
(<1508C) and at high field (200–300kV/cm) as this increases domain wall mobility and enables better alignment
along the field direction. Some examples of PZT thin film devices are shown in Fig. 2.
A further important point of performance is stability during operation and with time. The effective measured
piezoelectriccoefficientsdecaywithtimeafterpolinginaprocessknownaspiezoelectricageing,duringwhichthe
domainsinthepoledsamplereverttoamorethermodynamicallystableconfiguration.Depolarization(fatigue)may
occur and,ifintegrationof the film into the MEMS structure is notoptimised, delamination ofthe PZT film orthe
electrodes may occur [74]. From an industrial point of view, the evaluation of ageing and fatigue is certainly an
important task, however, only limited studies have been reported so far [75–77].
2.1.4. Summary—ferroelectric thin fims
FerroelectricthinfilmscontinuetorepresentanareaofdynamismandtechnicaladvanceinMEMS.Overthelast20
years, considerable progress has been made in optimizing the deposition conditions for thin films to improve the
available piezoelectric activity although the growth of good quality PZT thin films still requires some effort. In
processing such films, wet chemical methods continue to appear attractive for many applications. Recently, the
attention has shifted from preparing novel ferroelectric films to the integration of such films in complex devices.
The overall estimation of performance is best seen in device applications since the performance of the devices
dependsnotonlyonthepropertiesofthematerials,such asfilmorientation, grainsize, thickness,etc.,butalsothe
composite structure of the devices in many cases.
Inthefuture,thematerialscommunityrequiresgreaterknowledgeof,andabilitytocontrol,themicrostructureof
films,andmuchmoreeffectiveinteractionwithdevicetechnologiststobringcommercialsystemsintowidespreaduse.
2.2. Thick film fabrication for micro-scale sensors
Thickfilmsaregenerallyconsideredtobethosewiththicknessesgreaterthan1mm,however,suchadefinitionis
impreciseasmanythinfilmtechnologiescannowachievefilmthicknessesinexcessof1mm.Thickfilmsarerequired
to increase the amount of functional material present in order to achieve higher displacements or increased power
comparedtothinfilms,e.g.foracoustictransducersormicropumps(Fig.3).Forthepurposesofthisdiscussion,PZT
thickfilmswillbeconsideredtobethosethatareformedusingapowdersuspensionbasedprocessingroute.These
suspensions are typically made up of the desired ceramic powder (to impart the required functional properties), a
carrier fluid and additives designed to improve the stability of the ink and processing of the ceramic material. For
furtherinformationonissuesassociatedwiththickfilmprocessingandpatterningofthickfilmstructuresthereaderis
directed towards an earlier review [78].
2.2.1. Thick film deposition techniques
Many different forming techniques can be used to deposit thick films due to the ability to tailor the fluidic
characteristics(e.g.surfacetension,viscosity,shearbehaviour)ofthepowdersuspensions.Despitethedifferencein