MEMS-双压电制动器Characterization of TiAl alloy

Materials Science in Semiconductor Processing5(2002)35–38 Characterization of TiAl alloyfilms for potential applicationin MEMS bimorph actuatorsX.X.Qu a,*,Q.X.Zhang a,Q.B.Zou a,N.Balasubramanian a,P.Yang b,K.Y.Zeng ca Institute of Microelectronics,Science Park Road11,Science Park II,117685Singaporeb Singapore Synchrotron Light Source(SSLS),National University of Singapore,5Research Link,117603Singaporec Institute of Materials Research&Engineering,3Research Link,117602SingaporeAbstractIn this letter,we demonstrate the feasibilityof apply ing TiAl alloyfilm for the fabrication of bimorph actuators.The TiAl alloyfilms were prepared bythermal annealing at4001C of Ti/Al multilayers,which were deposited by DC magnetron sputtering from Ti and Al targets.The microstructure and surface morphologyof TiAl alloyfilms were analyzed by X-ray diffraction and scanning electron microscopy,which showed that TiAl alloyfilm is formed in the mixed phases of TiAl3and Ti36Al64,depending on the deposition conditions.The resistivityof TiAlfilm is about 9mO cm,and the stress is about200MPa.Our nano-indentation measurements showed that the Young’s modulus and hardness of TiAl alloyfilms are175and6.5GPa,respectively,which are larger than that of Al and comparable to Si. We have successfullyfabricated the bimorph actuators based on the TiAl alloyfilms and our test cantilevers up to 500m m long showed verystraight with tip bending as small as75m m,indicating negligible stress gradient in TiAlfilm. Our preliminarytesting results indicated that TiAl alloyfilm has potential application for bimorph actuators.r2002 Published byElsevier Science Ltd.Bimorph structures have manyapplications in variousMEMS[1–10],which include m-switch,m-valve,m-pump,and m-mirrors[2–5].Particularly,m-mirrors can be usedfor optical switches implemented in optical routers forthe Internet.The bimorph structures can be actuated indifferent mechanisms,either thermal,electrostatic,piezoelectric,or in other ways[1–7].In all the cases,the bimorph structure maydeflect to a large degreeduring actuation,and high stress will be induced in thelayers of the bimorph structures.Therefore to a largeextent the properties of the bimorph structure materialsdetermine the lifetime and reliabilityof the bimorphactuators.Up to now,most of the bimorph structureswere fabricated bybilay ers of oxide/conducting materi-al,with the latter mainlypolySi or Al.The performanceof the bimorph-based MEMS is still far from optimiza-tion.The basic idea of this work is that we select adifferent material for the fabrication of the bimorphstructures and improve the performance of the MEMS.In this work,TiAl alloyfilms were used to replacepure Alfilm for the fabrication of bimorph actuators.Basically,there are many issues with Al-based bimorphstructures.Even though the conductivityof Al is lowenough for the thermallyactuated Al-based bimorphMEMS,it suffered from the instabilityand high stressgradient.It is also well known that during thermalprocess Al will form hillocks.The static position of thebimorph structures is also drifting with the lifetime ofactuation.However,TiAl-based bimorph structureshave manyadvantages compared to Al;firstly,TiAlalloy,particularly TiAl3,is more stable than Al itself,which can effectivelyprevent the formation of hillocksup to5001C;secondly,the surface of the TiAl alloyfilmis smoother than Al after anythermal process;thirdly,the stress of TiAl3can be controlled and the stressgradient is negligible.It is also well known that bulkTiAl is proven to be a perfect material in a number ofapplications,due to its large elastic modulus,hardness,temperature stabilityand small density.The purpose of this letter is to report our results ofdeposition,characterization,and testing for TiAl alloy *Corresponding author.1369-8001/02/$-see front matter r2002Published byElsevier Science Ltd.PII:S1369-8001(02)00055-0films,and to demonstrate TiAl-based cantilevers and bimorph structures.Details of the bimorph actuators,fabrication and simulation,testing and reliabilitystudy will be presented in later papers.TiAl films were formed byannealing Ti/Al multilay ers in N 2environment at 4001C for 30min.Ti/Al multi-layers were deposited by DC magnetron sputtering Ti and Al targets,respectively.The vacuum of PVD chamber is better than 10À8Torr and Ar pressure during Ti and Al deposition is 2.5and 3mTorr,respectively.The thickness of Ti and Al sublayers is 10and 30nm,respectively,and in total 12layers of Ti/Al were deposited.Finallya cover lay er of Ti 10nm was deposited.The annealing of Ti/Al multilayers to form TiAl alloywas studied and it was found that 30min at 4001C is enough to form a uniform TiAl alloyfilm.The TiAl films were characterized byX-raydiffrac-tion (XRD)and scanning electron microscopy(SEM)to analyze the phases and surface morphology.The stress was measured bythe FSM sy stem byusing the standard laser scanning method.Sheet resistance was measured bythe 4-point probe method,and the resistivityof the film material was calculated.MTS nano-indentation system was employed to measure the Young’s modulus and hardness of the TiAl alloyfilm.The TiAl-based bimorph actuators were fabricated according to stan-dard process steps.The actuators and test structures were released in the STS system by Si dry etching with gases of SF 6or XeF 2.The phases of TiAl alloyfilms were analy zed byXRD,as shown in Fig.1.There are two phases:TiAl 3and Ti 36Al 64,both are tetragonal.It is interesting to note that the two phases are highly(111)oriented.These phases were formed during the 4001C furnace annealing in N2environment.The (111)orientation maybe related to the Si(001)substrate surface.It has beenfurther found that up to 8001C annealing these two phases are still stable and (111)oriented.We have compared the XRD spectra for annealing at 4001C,5001C,up to 8001C,and we saw that both the phases exist in all the annealed samples,indicating that 4001C annealing temperature can form TiAl alloy.The surface morphologyof TiAl alloyfilms was analyzed by SEM and typical pictures are shown in Fig.2.We found that after 4001C annealing induced alloying and crystallization,the surface was still very smooth and uniform.No hillocks were observed.However,longer annealing time and higher annealing temperature can cause the formation of hillocks.We have noted that TiAl films are quite stable without hillocks if further annealed at a temperature below 4001C.For all the actuator operation,even in the case of thermal actuation,the thermal stabilityensures the stabilityof the actuators fabricated bythis alloy material.Sheet resistance of TiAl alloyfilms and Ti/Al multi-layers was measured,and was about 59mO /&and 0.18O /&,respectively.The thickness of both TiAl alloy film and Ti/Al multilayers was about 500nm.We did not see measurable change in thickness after thermal annealing of Ti/Al multilayers,as can be confirmed from cross section SEM inspection.The sheet resistance increased to three times that of the as-deposited,indicating TiAl alloyformation.Further annealing did not show much change in the sheet resistance,indicating complete alloying.From the sheet resistance,we get the resistivityof the TiAl alloyto be 9mO cm.This is in between A12.65mO cm and Ti 42mO cm.We know that the stress in as-deposited Ti and Al film is compressive and tensile,respectively.The annealing at 4001C of Ti/Al multilayers to form TiAl alloy film has induced tensile stress of about 200MPa,which is quite stable during further annealing process.Even though the stress looks quite high,we did not see measurable stress gradient in the TiAl alloyfilm,which can be confirmed bytest cantilever structures released later.This maybe correlated to the highlyoriented TiAl alloyfilm.This characteristic is much better than that of Al,which can be greatlyaffected byinterface,or hillocks formationtoFig.1.The XRD spectra of TiAlalloyfilms.Fig.2.SEM morphologyof (a)as-deposited Ti/Al multilay ers;(b)TiAl alloyfilms.X.X.Qu et al./Materials Science in Semiconductor Processing 5(2002)35–3836induce quite non-uniform stress and high stress gradient.The stress in TiAl is believed to be mainlyfrom the thermal stress,and it can be relaxed once the structure is released without affecting the form of the fabricated cantilevers or bimorph actuators.Young’s modulus and hardness of TiAl alloyfilms were measured using nano-indentation technique.The measurement gives the average Young’s modulus and hardness of the TiAl alloyfilm as 175and 6.5GPa,respectively,which is much higher than that of Al and comparable to Si or polySi.The data is consistent for measurement on different sites of the samples.Such data indicate that TiAl alloyis more reliable and have higher lifetime if it is made for bimorph actuators.Bimorph actuators and test cantilevers were fabri-cated from TiAl film.The cantilever test structures were shown in Fig.3.These TiAl cantilevers were 500m m long,10m m wide and 500nm thick,which were released bySi dryetch with SF 6or XeF 2.One end of those cantilevers was anchored on Si substrate.Optical inspection indicated that these floating cantilevers were straight,with tip bending up or down within the range of 75m m,indicating reliable cantilevers structures,without anystress gradient in them.The high tensile stress in the cantilevers mayhave been relaxed after release.Furthermore,we did not see much change of the surface morphologyof the cantilever structures after dry etch release.The cantilever fabrication process was found to be stable and reliable across wafers.The bimorph structure is fabricated with TiAl/SiO 2or TiAl/SiO 2/TiAl depending on the design.Fig.4illus-trates the schematic process flow for one type of thermal balanced bimorph structure.The silicon wafer starts with thermal oxide as a dielectric and etch-stop layer for final releasing.Poly-Si is deposited and patterned as the sacrificial layer,followed by the deposition of the first TiAl layer of 0.5m m.Then a silicon dioxide of 1m m thickness is deposited as the base structural layer byPECVD.The second layer of 0.5m m thick TiAl is deposited.The patterning of TiAl and oxide layers can be done at respective steps or together bythe dryetch process.The final step is to release the sacrificial poly-Si layer by using SF 6or XeF 2dryetch.The results on these structures will be reported elsewhere.In summary,this letter presented our results on the characterization of the TiAl alloyfilm and fabrication of bimorph and test cantilever structures.Our demonstra-tion indicated that TiAl mayhave potential application for reliable and stable bimorph actuators.References[1]Dhuler,Vijayakumar R,Wood,Robert L.Microelectro-mechanical positioning apparatus,US5962949:October 5,1999.[2]Motamedi,Edward M,Andrews,Angus P,Park,Sangtae.Microelectro-mechanical optical resonator and method,US5903380:May11,1999.[3]Apte,Raj B,Paoli,Thomas L,Maeda,Patrick Y,Fisli,Tibor.Micro-positioned laser source for raster output scanners,US5854705:December 29,1998.[4]Carr,William,Sun,Xi-qing.Multi-dimensional physicalactuation of m -structures,US5870007:February1999.[5]Carr,William N,Sun,Xi-qing.Optical m shutter array,US5781331:July14,1998.[6]Quanbo Zou,Uppili Sridhar,Rongming Lin.A studyonmicro-machined bimetallic actuation.Sensors Actuators 1999;A78:212.[7]Quanbo Zou,et SaT:A novel single-chipfabrication technique for three-dimensional MEMS.Sen-sors Actuators1999;A72:115–24.Fig.3.Test cantilevers 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