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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 Universite Paris-Sud, F-91405, Orsay, France
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ARTICLE INFO
Article history: Received 22 May 2016 Received in revised form 26 May 2016 Accepted 26 May 2016 Available online 3 June 2016
Keywords:
Piezoelectric
Clamped—clamped beam
String based MEMS
C—V characteristics
Optical interferometer profiler
Quality factor
ABSTRACT
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/).
1. 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 [1—7]. 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/jjsamd.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/).
investigated in both static and dynamic mode. It exhibits high potentials in variety of sensor and resonant actuator applications.
2. Experimentation
The doubly clamped PZT beams were fabricated using micro-patterning procedure illustrated in Fig. 1. Heterostructures of PZT/ LSMO (with the respective piezoelectric Pb(ZrTi)O3 and bottom Lanthanum Strontium Manganite La067Sr033MnO3 electrode thickness of tPZT = 220 mm and tLSMO = 40 nm) were grown on the STO/Si, where the buffer Strontium Titanate SrTiO3 layer thickness tSTO = 10 nm (Fig. la). 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 2 Hz repetition and about 2.2 mJ cm-2 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 cooling-down 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).
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 x 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 -5 to 5 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.
3. Results and discussion
3.1. Microstructure
Fig. 3 shows the 6-26 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. 1. 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 ion-beam 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.
T ' I ' I ' I ' I 1 I 1 r~
^ • PZT
o O LSMO
v v Si
1=_I_I_I_I_I_I_I_I_I_I_I_I_I_I_I_=1
30 40 50 60 70 80 90 100 110 26 (deg.)
Fig. 3. XRD diffraction patterns of the PZT/LSMO/STO/Si heterostructure.
E (kV/cm)
■250 -200 -150 -100 -50 0 50 100 150 200 250
1 I 1 I 1 I 1 I 1 I 1 I 1
.......... ..........
■5 -4 -3 -2 -1 0 1 2 3 4 5 U(V)
Fig. 5. C—V characteristics of the PZT/LSMO/STO/Si based string.
A three-dimension AFM image (with scanning area 3.5 x 3.5 mm2) and surface roughness profile of PZT film deposited on LSMO bottom electrode layer before micro fabricating are illustrated in Fig. 4. 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 C—V 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 5 to -5 V and then reversely swept back to 5 V. Note that, the C—V characteristics exhibits the typical
Fig. 4. Three-dimension AFM morphologies (a) and the roughness profile (b) of 3.5 x 3.5 mm2 PZT thin films deposited on 40 nm-thick LSMO bottom electrode layer before micro fabricating.
Fig. 6. 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 [12—14]. A typical C—V 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 fields of the PZT film are of +83.5 and -12.5 kV cm-1, which yield an absolute coercive field of 48 kV cm-1. For a similar heterostructure of {Ta/IrMn/ Co/Ta}/PZT/LSMO/STO film, the coercive field of 34.05 kV cm-1 was reported [11].
3.3. Mechanical characterization
3.3.1. Static response
Shown in Fig. 6 is the deflection profile plotted in three-dimension for the investigated clamped—clamped beam. Here, the geometric plane of bridge is defined as coordinate plane with x-and 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 = 0 (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 5 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 0 to 5 V, the initial downward curvature along the width decreases, becomes flat at Vbias = 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 5 V.
Fig. 7. The deflection along the width (a) and the length (b) of the freestanding PZT bridge measured at different applying voltage from 0 to 5 V.
D.T.H. Giang et al. / Journal of Science: Advanced Materials and Devices 1 (2016) 214—219
Fig. 8. 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. 8 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 C—V 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 = -Z$tPZT/lVbias
It turns out that, the value d31 = -630 pm/V, which is rather higher than that (of about — 125 pm/V) reported for the clam-ped—clamped beam piezoelectric micro-scale resonator [19].
3.3.2. Dynamic behavior
Resonant behavior of the investigated PZT string is illustrated in Fig. 9. Here, the string was actuated by a sinusoidal potential with amplitude of 0.5 Vp-p and frequency ranging from 1 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 = fr/Df
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 10-12 g/Hz with a minimum detectable mass of 5 ng was reported [18].
Fig. 9. 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 0 to 5 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 = 2 V. The main resonant peak at 298.8 kHz remains in the bias voltages up to Vbias = 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 = 5 V. The dynamic behavior of this PZT string, thus, can only work at low bias voltages.
4. 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
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 — 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.
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