PZTthin®lmsforpiezoelectricmicroactuatorapplications H.Kueppers a,* ,T.Leuerer a ,U.Schnakenberg a ,W.Mokwa a ,M.Hoffmann b,1 , T.Schneller b ,U.Boettger b ,R.Waser b a IWE 1, RWTH Aachen, Sommerfeldstr. 24, D-52074 Aachen, Germany b EMRL, RWTH Aachen, Sommerfeldstr. 24, D-52074 Aachen, Germany Received15June2001;receivedinrevisedform15November2001;accepted18November2001 Abstract TheobjectiveoftheseinvestigationswasthedevelopmentofatechnologyfortheintegrationofpiezoelectricPZTthin®lmsdepositedby chemical solution deposition (CSD) on silicon-based cantilevers for micromirror and microrelay applications. The fabrication process is presented in detail with attention to the PZT thin ®lm. The electrical properties of these thin ®lms have been studied with hysteresis, capacitance±voltage (C±V) and current density±electric ®eld (S±E) measurements. Furthermore, laser interferometry investigations on completedmicroactuatorsaredepicted.For390 mmlongcantileverstructurestipde¯ectionsupto20 mmatanappliedvoltageof10Vwere measured.Thepiezoelectriccoef®cient d 31 wasmeasuredtobe 12pC/N.Resonancefrequencymeasurementsona190 mmlongcantilever resultedinavalueof23kHz. # 2002ElsevierScienceB.V.Allrightsreserved. Keywords: PZT;Microactuator;CSD;Piezoelectriccoef®cient d 31 1. Introduction PZTthin®lmshavebeenusedinthefabricationofsilicon- basedsensorsandactuators[1±5].Inscanningforcemicro- scopypiezoelectriccantileverstructuresarewellestablished. Hereshortcantileversareusedassensorelementswithfast responsetimeandhighresonancefrequency.Thetipde¯ec- tion has to be small to obtain ultrahigh resolution in the nanometerrange.Inactuatorapplications,likemicromirrors, thetipde¯ectionshouldbehightoachievelargeanglesat lowappliedvoltages.Inthispaperwepresenttheintegration of sol±gel PZT thin ®lms on silicon cantilever structures. ThedepositionandstructuringofthesePZT®lms,basedona newchemicalsolutiondeposition(CSD)butoxyethanole[6] approach,willbepresented.Inaddition,weinvestigatedthe piezoelectriccoef®cient d 31 bycantileverde¯ectionmeasure- mentsusinglaserinterferometry. 2. Fabrication process Fig.1showsacross-sectionalviewofthemicroactuator structure. A thin piezoelectric PZT layer was deposited between two platinum electrodes on a silicon cantilever astheactivematerial.Thefabricationprocessisshownin detailinFig.2.Onanoxidised(100)-orientedsiliconwafer aLPCVD-Si 3 N 4 layerwasdepositedandstructuredusinga RIEprocess.Instep2a680nmlowstresspoly-siliconlayer andathinphosphorsilicateglass(PSG)layerwasdeposited. On these layers the bottom electrode was sputtered which consistsofa10nmtitaniumlayeranda100nmPtlayer.The depositiontechniqueofthebottomelectrodewasoptimised to achieve h111i-oriented and hillock-free layers [7]. A PZTthin®lmwithaZr/Tiratioof45/55wasdepositedbya modi®ed butoxyethanole-based CSD route [8,9] to yield a highpiezoelectriccoef®cient[6]. For this CSD route titanium tetraisopropoxide, zirco- nium tetrapropoxide, and dehydrated lead acetate, with a Pbexcessof16%,weredissolvedin2-butoxyethanolein dry nitrogen atmosphere using the Schlenck technique. Afterre¯uxingthereactionmixtureforseveralhoursthe solvent and the by-products were removed by means of vacuum distillation. Due to the redilution a 1.12 molar stock solution was obtained diluted with butoxyethanole. Prior to spin-coating, this solution was ®ltered with a 0.2 mmPTFE®lter.Thesolutionwasspin-coatedinseveral layers on the wafer. Each layer was pyrolysed using hot plates.Toreachatotalthicknessof350nmsevencoating cycles were carried out. For crystallisation of the amor- phous ®lms a rapid thermal annealing (RTA) step was performedat700 8C. SensorsandActuatorsA97±98(2002)680±684 * Correspondingauthor.Tel.: 49-241-807810; fax: 49-241-8888392. E-mail addresses: kueppers@iwe.rwth-aachen.de(H.Kueppers), hoffmann@iwe.rwth-aachen.de(M.Hoffmann). 1 Co-correspondingauthor.Tel.: 49-241-807817; fax: 49-241-8888300. 0924-4247/02/$±seefrontmatter # 2002ElsevierScienceB.V.Allrightsreserved. PII:S0924-4247(01)00850-0