Fabrication and characterization of PZT string based MEMS devices 2016 Journal of Science Advanced Materials and Devices

Journal of Science: Advanced Materials and Devices (2016) 214e219 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original article Fabrication and characterization of PZT string based MEMS devices D.T Huong Giang a, b, *, N.H Duc a, G Agnus b, T Maroutian b, P Lecoeur b a Nano Magnetic Materials and Devices Department, Faculty of Engineering Physics and Nanotechnology, VNU University of Engineering and Technology, Vietnam National University, Hanoi, E3 Building, 144 Xuan Thuy Road, Cau Giay, Hanoi, Viet Nam b Institut d’Electronique Fondamentale, UMR CNRS and Universit e Paris-Sud, F-91405, Orsay, France a r t i c l e i n f o a b s t r a c t Article history: Received 22 May 2016 Received in revised form 26 May 2016 Accepted 26 May 2016 Available online June 2016 String based MEMS devices recently attract world technology development thanks to their advantages over cantilever ones Approaching to this direction, the paper reports on the micro-fabrication and characterization of free-standing doubly clamped piezoelectric beams based on heterostructures of Pd/ FeNi/Pd/PZT/LSMO/STO/Si The displacement of strings is investigated in both static and dynamic mode The static response exhibits a bending displacement as large as 1.2 mm, whereas the dynamic response shows a strong resonance with a high quality factor of around 35 depending on the resonant mode at atmospheric pressure These findings are comparable with those observed in large dimension membrane and cantilever based MEMS devices, which exhibit high potentials in variety of sensor and resonant actuator applications © 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Piezoelectric Clampedeclamped beam String based MEMS CeV characteristics Optical interferometer profiler Quality factor Introduction Micro- and Nanoelectromechanical system (MEMS and NEMS) devices find their use in sensing and actuating, drug delivery, DNA sequencing, homeland security, automotive industry [1] Practically, MEMS and NEMS can be realized in cantilever or string forms, which correspond to the single or double clamped beam like structures, respectively Cantilever based MEMS can be operated either in static or dynamic modes In the static mode of operation, the bending is measured In the dynamic mode, the change in resonant frequency of vibrating cantilever is determined String based MEMS are relatively new and still rare in literatures They are also potential to use as mass sensor [2], temperature sensor [3], as well as bio sensor [4] In comparison with cantilevers, the strings proceed a more simple bending mode, position and mass calculations In particular, the time consuming computation for strings is short So they can be served as real time devices Moreover, strings are mechanically more stable for which * Corresponding author Nano Magnetic Materials and Devices Department, Faculty of Engineering Physics and Nanotechnology, VNU University of Engineering and Technology, Vietnam National University, Hanoi, E3 Building, 144 Xuan Thuy Road, Cau Giay, Hanoi, Viet Nam E-mail address: giangdth@vnu.edu.vn (D.T.H Giang) Peer review under responsibility of Vietnam National University, Hanoi they always provide a high fabrication yield compared to cantilevers Micro strings can detect masses of femtograms in air and hundreds of attogram in high vacuum can be detected [2] On the other hand, the sensitive electronic components endure some intense vibrations, specially, in military and aerospace applications These vibrations have some disturbing effects on the stability and on the service life of the devices In this case, the string like structure can isolate such vibrations either at the rack, board level or at the component level [5] MEMS and NEMS have been developing rapidly for a wide variety of applications in the last decade A wide range of materials have been used in the design and fabrication of MEMS and NEMS devices and many advanced microfabrication techniques have been developed [1e7] However, as already mentioned above, most of the reported MEMS devices concerned to the cantilever structure and lead zirconate titanate piezoelectrics (PZT) thanks to their large electromechanical coupling coefficient Although most of devices are similar and exploit d31 mode, the range of application is quite wide Among them, the string like structures are designed and fabricated acting as resonator for filtering electrical signal [8], responsibility to acoustic and temperature changes [9], capacitive shunt electrostatic MEMS switch [10] This paper reports the micro-fabrication and characterization of free-standing piezoelectric strings based on the heterostructure of Pd/FeNi/Pd/PZT/LSMO/STO/Si The displacement of this string is http://dx.doi.org/10.1016/j.jsamd.2016.05.004 2468-2179/© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) D.T.H Giang et al / Journal of Science: Advanced Materials and Devices (2016) 214e219 investigated in both static and dynamic mode It exhibits high potentials in variety of sensor and resonant actuator applications Experimentation The doubly clamped PZT beams were fabricated using micropatterning procedure illustrated in Fig Heterostructures of PZT/ LSMO (with the respective piezoelectric Pb(ZrTi)O3 and bottom Lanthanum Strontium Manganite La0.67Sr0.33MnO3 electrode thickness of tPZT ¼ 220 mm and tLSMO ¼ 40 nm) were grown on the STO/Si, where the buffer Strontium Titanate SrTiO3 layer thickness tSTO ¼ 10 nm (Fig 1a) In this step, before fabricating PZT films, a LSMO layer was firstly epitaxially grown on 500 mm-thick STO/ Si(001) substrate by pulsed laser deposition (PLD) The PZT films were then grown further at 600  C on the LSMO/STO substrate In the process, a KrF excimer laser of 248 nm wavelength was used with Hz repetition and about 2.2 mJ cmÀ2 energy density in an O2 gas pressure of 120 mTorr for LSMO deposition and in a N2O ambient of 260 mTorr for PZT deposition, followed by a coolingdown procedure under 300 Torr of pure oxygen atmosphere [11] In order to prepare the bottom contact, firstly, a hole was opened through the PZT layer by UV lithography and Ar ion-beam etching processes (Fig 1b) Then, the Pd bottom contact pad was fabricated using UV lithography, RF-sputtering and liftoff techniques (Fig 1c) The sandwich Pd/NiFe/Pd was sputtered on the top of the PZT layer (Fig 1d) It serves as the top electrode as well as the protective layer (Fig 1d) As can be seen below, this metallic layer can prevent the beam from the breaking during etching of Si layer The doubly clamped PZT microcantilever was formed by releasing the PZT film from Si substrate This process was performed by sacrificial etching of underlying silicon structure using XeF2 gas (Fig 1e) Finally, chip was mounted on a plastic printed board The bottom and top contacts were electrically connected using wire bonding (Fig 1f) 215 A top-view scanning electron microscopy (SEM) image of fabricated PZT micro string is shown in Fig 2a,b It is clearly seen that the string is of square-shaped configuration The higher magnification SEM image (Fig 2b), however, shows several small cracks at the edges of the bridge, where the metallic layer Pd/NiFe/ Pd was not deposited on the top This verifies the role of metallic capping layers in preventing the cantilever from breaking during etching So through appropriate control of deposition conditions, relatively flat double clamped beam were achieved From the SEM image, the real PZT area covered by cap layers of free-standing bridge is determined to be of 45 Â 75 mm2 The X-ray diffraction (XRD) system (Rigaku 3272) with Cu-Ka radiation was used to examine the crystal orientation of the PZT films The surface morphology of the PZT films was characterized by atomic force microscopy (AFM) measurements A ferroelectric test system (Precision LC Radiant Technology) was used to measure their electrical properties The deflection in an applied bias dc voltage bias (from À5 to V) was measured using optical interferometer profiler The resonant frequencies, modal shapes, and quality factors of the epitaxial PZT membranes are characterized using a Polytec IVS-400 laser doppler vibrometer All experimental measurements are performed at room temperature Results and discussion 3.1 Microstructure Fig shows the q-2q X-ray diffraction patterns of the successfully fabricated PZT based cantilever In the log-scale, not only the typical patterns spectrum of the PZT film and Si substrate, but also that of the minor portion of LSMO phase are exhibited The results reflect well the fact that, the PZT and LSMO films displayed purely 00l-type peaks of the orientated perovskite structure, which confirm the preferentially c-axis oriented epitaxial growth of the films on the STO/Si substrate Fig Process flow used for fabrication of MEMS based PZT structures: (a) heterostructure of PZT/LSMO grown on the STO/Si; (b) opening the hole through the PZT layer by Ar ionbeam etching; (c) deposition of Pd bottom contact via the hole; (d) deposition of Pd/NiFe/Pd top contact layer; (e) releasing the PZT film from Si substrate; (f) wire bonding electrical contacts 216 D.T.H Giang et al / Journal of Science: Advanced Materials and Devices (2016) 214e219 Fig Top-view SEM image of investigated PZT string based MEMS (a) and bridge focus (b) Fig XRD diffraction patterns of the PZT/LSMO/STO/Si heterostructure Fig CeV characteristics of the PZT/LSMO/STO/Si based string A three-dimension AFM image (with scanning area 3.5 Â 3.5 mm2) and surface roughness profile of PZT film deposited on LSMO bottom electrode layer before micro fabricating are illustrated in Fig The roughness analysis using horizontal straight line method turns out that the mean film roughness is of about 6.8 nm 3.2 Electric characterization Shown in Fig 5a is the CeV characteristics performed at the frequency of 10 kHz for the investigated PZT string The drive is connected to the bottom electrode (i.e in the positive branch) and the dc voltage was swept from to À5 V and then reversely swept back to V Note that, the CeV characteristics exhibits the typical Fig Three-dimension AFM morphologies (a) and the roughness profile (b) of 3.5 Â 3.5 mm2 PZT thin films deposited on 40 nm-thick LSMO bottom electrode layer before micro fabricating D.T.H Giang et al / Journal of Science: Advanced Materials and Devices (2016) 214e219 217 Fig 3D plots of PZT-bridge surface observed from top side in zero- (a) and 5V-applied voltages (b) butterfly shape with a large asymmetry As usual, this asymmetric phenomenon can be attributed to the dissimilar electrodes, mobile charge and interface charge traps [12e14] A typical CeV symmetry, however, is recently reported for the SRO/PZT/Cu structure [15] and Pt/ZnO/PZT/Pt//Ti/SiO/Si heterojunction [16] The coercivity is shifted to the positive applied voltage and an enhancement of the capacitance is accompanied Indeed, the coercive elds of the PZT lm are of ỵ83.5 and 12.5 kV cmÀ1, which yield an absolute coercive field of 48 kV cmÀ1 For a similar heterostructure of {Ta/IrMn/ Co/Ta}/PZT/LSMO/STO film, the coercive field of 34.05 kV cmÀ1 was reported [11] 3.3 Mechanical characterization 3.3.1 Static response Shown in Fig is the deflection profile plotted in threedimension for the investigated clampedeclamped beam Here, the geometric plane of bridge is defined as coordinate plane with xand y-axis aligned along to the length and the width, respectively The displacement is measured along the vertical direction of the film (i.e in z-axis) It is clearly seen that, due to the presence of residual stress, the deflection of the PZT bridge already exist in zero-applied voltage, Vbias ¼ (Fig 6a) The bending upward curve is observed along the length (x-axis) and the downward one is found along the width (y-axis) of the bridge The maximum bending is observed at the central point (0,0) of the plane In a bias dc voltage of V, the resident bending tends to be compensated thanks to an induced contract deflection across the bridge, which makes the deflection curvature changing in to the positive sign along the width and decreasing along the length (Fig 6b) These behaviors are described in more detail analysis and illustrated in Fig 7a,b Varying the bias voltage from to V, the initial downward curvature along the width decreases, becomes flat at Vbias ¼ 2.5 V The upward curvature is established and enhanced with further increasing bias voltage (Fig 7a) For the deflection along the length, the initial upward curvature always remains The single maximum as high as 327 nm is observed in zero-bias voltage It develops into a more complex deformation with double maximum height of 135 nm companying with a minimum one of 121 nm at the bias voltage of V Fig The deflection along the width (a) and the length (b) of the freestanding PZT bridge measured at different applying voltage from to V 218 D.T.H Giang et al / Journal of Science: Advanced Materials and Devices (2016) 214e219 Fig z-deflection at the surface central point (0,0) of the PZT bridge as a function of applied bias voltage The positive or negative deflections correspond to the upward and downward of surface of the string Fig presents the variation of the vertical displacement (z) as a function of applied bias voltage for the central point of the bridge surface There, the positive or negative sign corresponds to the upward and downward of surface This figure resembles not only the butterfly shape but also the electrical coercive field of the CeV loop shown earlier in Fig 5a Note that, in this investigation, the total (absolute) deflection of the string is of about 1.2 mm A smaller piezoelectric response is usually expected for string like structure due to the double clamping mechanism Presently, however, the displacement magnitude is found to be comparable with those in large dimension membranes and cantilevers [17,18] The piezoelectric constant of d31 can be calculated from the slope of butterfly loop as it passes the zero applied field region Indeed, the transverse piezoelectric strain coefficient d31 of the unimorph cantilever is expressed as d31 ¼ Àz$tPZT =l:Vbias (1) It turns out that, the value d31 ¼ À630 pm/V, which is rather higher than that (of about e 125 pm/V) reported for the clampedeclamped beam piezoelectric micro-scale resonator [19] 3.3.2 Dynamic behavior Resonant behavior of the investigated PZT string is illustrated in Fig Here, the string was actuated by a sinusoidal potential with amplitude of 0.5 Vp-p and frequency ranging from to 500 kHz In zero-applied dc voltage, the resonant structure exhibits three main resonant peaks at 104.7, 298.8 and 319.5 kHz corresponding three different modes of vibration, where the second resonance is prominent Quality factor (Q-factor) is a measure of total energy dissipation compared to stored energy in a sensor structure It is defined as the ratio between the resonant frequency and the width of the resonant peak (Df) at its haft height, i.e.: Q ¼ fr =Df (2) It turns out from experimental results that Q-factor of about 34, 31 and 40 for the first, second and third resonant modes, respectively, at ambient pressure These values are comparable with those of about 50 reported for 1500 mm-diameter membranes, where a mass sensitivity in the order of 10À12 g/Hz with a minimum detectable mass of ng was reported [18] Fig Frequency response of the beam exited by a sinusoidal signal with the same amplitude of 0.5 V at different dc bias voltage offset from to V With the increasing dc bias voltage, the position of all resonant peaks tents to shift to lower frequencies In particular, the amplitude of the lowest and highest resonant peaks are strongly suppressed and almost disappears at Vbias ¼ V The main resonant peak at 298.8 kHz remains in the bias voltages up to Vbias ¼ 2.5 V, at which two new peaks appear at 250 kHz range of the resonant structure These two new peaks are broadened at higher bias voltage and the resonant structure is totally destroyed at Vbias ¼ V The dynamic behavior of this PZT string, thus, can only work at low bias voltages Conclusions We have presented the micro-fabrication and characterization of free-standing strings based on the heterostructures of Pd/FeNi/ Pd/PZT/LSMO/STO/Si In this fabrication technology, the PZT film D.T.H Giang et al / Journal of Science: Advanced Materials and Devices (2016) 214e219 was epitaxially grown preferentially c-axis oriented It was prevent from the breaking during etching procedure thanks to the metallic {Pd/FeNi/Pd} top electrode The static response shows a bending displacement as large as 1.2 mm, whereas the dynamic response exhibits a strong harmonic oscillation resonance with a high quality factor of about 35 depending on the resonant mode at atmospheric pressure These performances are comparable with those observed in large dimension cantilever based MEMS devices Moreover, they profit advantages of the string like structure such as simple bending mode, mechanically stable, small intrinsic energy loss and real-time use, which can be developed for mass sensing applications Acknowledgments The paper is dedicated to the memory of Dr Peter Brommer e a former physicist of the University of Amsterdam This work was partly supported by the National Program for Space Technology of Vietnam under the granted Research Project VT/CN-03/13-15 and Vietnam National University, Hanoi under the granted Research Project QG 15.28 References [1] S Tadigadapa, K Mateti, Piezoelectric MEMS sensors: state-of-the-art and perspectives, Meas Sci Technol 20 (2009) 092001 [2] S.S Schmid, S Dohn, A Boisen, Real-time particle mass spectrometry based on resonant micro strings, Sensors 10 (2010) 8092e8100, http://dx.doi.org/ 10.3390/s100908092 [3] A.K Pandey, O Gottlieb, O Shtempluck, E Buks, Performance of an aupd micromechanical resonator as a temperature sensor, Appl Phys Lett 96 (2010) 203105, http://dx.doi.org/10.1063/1.3431614 [4] A.K Naik, M.S Hanay, W.K Hiebert, X.L Feng, M.L Roukes, Towards singlemolecule nanomechanical mass spectrometry, Nat Nanotechnol (2009) 445e450 [5] Y Meyer, M Collet, Active vibration isolation of electronic components by piezocomposite clampedeclamped beam, Mech Syst Signal Process 25 (2011) 1687e1701, http://dx.doi.org/10.1016/j.ymssp.2010.12.015 219 [6] K Prashanthi, M Mandal, S.P Duttagupta, R Pinto, V.R Palkar, Fabrication and characterization of a novel magnetoelectric multiferroic MEMS cantilevers on Si, Sensors Actuators A 166 (2011) 83e87, http://dx.doi.org/10.1016/ j.sna.2010.12.013 [7] G Yugandhar, G Venkateswara Rao, K Srinivasa Rao, Modeling and simulation of piezoelectric MEMS sensor, Mater Today Proc (2015) 1595 [8] B Piekarski, D de Voe, M Dubey, R Kaul, J Conrad, R Zeto, Surface micromachined piezoelectric resonant beam filters, Sensors Actuators A 90 (2001) 313 [9] T Tamagawa, D.L Polla, Lead zirconate titanate thin films in surface micromachined sensor structures, in: IEEE Int Electron Devices Meeting, San Francisco, 1990 [10] G.M Rebeiz, R.F MEMS, Theory, Design, and Technology, John Wiley & Sons, 2003 [11] D.T Huong Giang, N.H Duc, G Agnus, T Maroutian, P Lecoeur, Electric fieldcontrolled magnetization in exchange biased IrMn/Co/PZT multilayers, Adv Nat Sci Nanosci Nanotechnol (2013) 025017, http://dx.doi.org/10.1088/ 2043-6262/4/2/025017 [12] Y Lin, B.R Zhao, H.B Peng, Z Hao, B Xu, Z.X Zhao, J.S Chen, Asymmetry in the hysteresis loop of Pb (Zr0.53Ti0.47)O3/SiO2/Si structures, J Appl Phys 86 (1999) 4467 [13] B Xiao, X Gu, N Izyumskaya, V Avrutin, J.-Q Xie, H Morkoỗ, Structural and electrical properties of Pb(Zr,Ti)O3 lms grown by molecular beam epitaxy, Appl Phys Lett 91 (2007) 182906 [14] Z.X Zhu, J.-F Li, F.-P Lai, Y Zhen, Y.-H Lin, C.-W Nan, L Li, Phase structure of epitaxial Pb(Zr,Ti)O3 thin films on Nb-doped SrTiO3 substrates, Appl Phys Lett 91 (2007) 222910 [15] L.D Filip, L Pintilie, V Stancu, I Pintilie, Simulation of the capacitance-voltage characteristic in the case of epitaxial ferroelectric films with Schottky contacts, Thin Solid Films 592 (2015) 200e206, http://dx.doi.org/10.1016/ j.tsf.2015.08.046 [16] X Meng, C Yang, W Fu, J Wan, Preparation and electrical properties of ZnO/ PZT films by radio frequency reactive magnetron sputtering, Mater Lett 83 (2012) 179e182, http://dx.doi.org/10.1016/j.matlet.2012.06.015 [17] D Isarakorn, A Sambri, P Janphuang, D Briand, S Gariglio, J.-M Triscone, et al., Epitaxial piezoelectric MEMS on silicon, J Micromechanics Microengineering 20 (2010) 055008, http://dx.doi.org/10.1088/0960-1317/20/5/ 055008 [18] D Isarakorn, D Briand, A Sambri, S Gariglio, J.-M Triscone, F Guy, et al., Finite element analysis and experiments on a silicon membrane actuated by an epitaxial PZT thin film for localized-mass sensing applications, Sensors Actuators B Chem 153 (2011) 54e63, http://dx.doi.org/10.1016/ j.snb.2010.10.009 [19] Strojniski Vestnik, Characterizing effective d31 values for PZT from the nonlinear oscillations of clamped-clamped micro-resonators, J Mech Eng 59 (2013) 50e55, http://dx.doi.org/10.5545/sv-jme.2012.673  ... micro -fabrication and characterization of free-standing strings based on the heterostructures of Pd/FeNi/ Pd /PZT/ LSMO/STO/Si In this fabrication technology, the PZT film D.T.H Giang et al / Journal. .. 218 D.T.H Giang et al / Journal of Science: Advanced Materials and Devices (2016) 214e219 Fig z-deflection at the surface central point (0,0) of the PZT bridge as a function of applied bias voltage...D.T.H Giang et al / Journal of Science: Advanced Materials and Devices (2016) 214e219 investigated in both static and dynamic mode It exhibits high potentials in variety of sensor and resonant actuator