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

Journal of Science: Advanced Materials and Devices 1 (2016) 214e219

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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 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 3 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 ( />
Keywords:
Piezoelectric
Clampedeclamped beam
String based MEMS
CeV characteristics
Optical interferometer profiler
Quality factor

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: (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

/>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
( />

D.T.H. Giang et al. / Journal of Science: Advanced Materials and Devices 1 (2016) 214e219

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 micropatterning procedure illustrated in Fig. 1. 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 2 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 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 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. 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 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.


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Fig. 2. Top-view SEM image of investigated PZT string based MEMS (a) and bridge focus (b).

Fig. 3. XRD diffraction patterns of the PZT/LSMO/STO/Si heterostructure.
Fig. 5. 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. 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 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 5 to À5 V and then reversely swept
back to 5 V. Note that, the CeV characteristics exhibits the typical

Fig. 4. 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 1 (2016) 214e219

217

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 [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 fields of the PZT
film are of þ83.5 and À12.5 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. 6 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 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 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 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.


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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 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. 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 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 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 ¼ 2 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 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


D.T.H. Giang et al. / Journal of Science: Advanced Materials and Devices 1 (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.
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