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Sensors and Materials, Vol. 36, No. 8 (2024) 3419–3431
MYU Tokyo
S & M 3741

Fracture Mechanisms of Lead Zirconate Titanate PiezoelectricThin Films Determined by Mechanicaland Electrical Cyclic Loading Tests
Yuga Kumakiri,1 Tomohiro Date,2 Noriyuki Shimoji,2Koji Terumoto,2 and Takahiro Namazu1,*
1Kyoto University of Advanced Science, 18 Yamanouchigotanda-cho, Ukyo-ku, Kyoto 615-8577, Japan
2Rohm Co., Ltd., 21 Saiinmizosaki-cho, Ukyo-ku, Kyoto 615-8585, Japan(Received May 23, 2024; accepted July 2, 2024)

Keywords: PZT film, dielectric withstand voltage, cyclic loading test, cracking, short circuit, fracture
mechanism

In this paper, we describe the fracture mechanisms of lead zirconate titanate (PZT)
piezoelectric thin films synthesized by sol-gel processing. One of the technical concerns inpiezoelectric thin films is how their surface fractures because, in most cases, the films failelectrically, and the fracture surface is remade after its first fracture. To estimate the failure
mechanisms, cantilever-type and clamped capacitor-type actuators made of PZT piezoelectricthin films deposited on a Si wafer were prepared and subjected to electrical and mechanicalstresses. The cantilever-type actuators showed a decreasing trend in dielectric withstand voltage
with increasing number of mechanical loading cycles. The clamped capacitor-type actuators
showed a decreasing trend in withstand voltage with increasing cyclic voltage amplitude.
Through mechanical and electrical experiments, we found that the origin of cracking differedfrom that of short circuit. This finding indicates that the PZT films fractured mechanically, thenfractured electrically. The focused ion beam fabrication of a surface defect and scanning electron
microscopy observation around the defect suggest a reasonable fracture mechanism.

1.

Introduction

A lead zirconate titanate (PZT) film is well known as a representative piezoelectric film,
which is utilized as an actuator in MEMS. The applications of the PZT MEMS include
microactuators,(1–4) mechanical sensors,(5,6) energy harvesters,(7–11) ultrasound transducers,(12,13)
and others.(14–17) The PZT’s piezoelectric effect is used for passive applications, such asmechanical sensors and energy harvesters, whereas its inverse piezoelectric effect is used for
active applications, such as microactuators and ultrasound transducers. One of the advantages of
the use of PZT in MEMS is that it enables us to move micromechanical elements rapidly at morethan several tens of kHz in the case of using thin PZT films. Another advantage is that thick PZTfilms can provide high power density, which can be utilized for energy-harvesting devices and*Corresponding author: e-mail: namazu.takahiro@kuas.ac.jp

https://doi.org/10.18494/SAM5153

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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high-power actuators, owing to an increase in the active volume of the films. In addition, thicker
films tend to be less clamped to the substrate, which allows for higher domain wall mobility andmay enhance their piezoelectric response. Considering the practical use of PZT films asfunctional materials, such rapid response and high-power output are really attractive, which isconsidered to be the main reason why the PZT films are being widely used as actuators in
MEMS. Moreover, the PZT films can be produced relatively easier than other functional
materials, such as shape memory alloys(18,19) and ferromagnetic and exothermic reactive
films,(20,21) by the sol-gel method and sputtering.
In considering the mechanical reliability of PZT MEMS devices, one of the significant
concerns in the PZT MEMS is that it is very difficult to understand how it fractures during
operation.(22–33) If the PZT film is used as an actuator, a voltage is applied to the film for
actuation. During the operation at a high voltage, the PZT MEMS suddenly collapses. After the
rupture, the PZT MEMS changes shape within a very short period of time, and an irregularpattern can be observed as if the PZT film is partially melted. This means that the PZT MEMS
fractured owing to both electrical and mechanical issues. Commonly, the fracture mechanism of
hard materials such as metals and ceramics is discussed through the observation of the fracturesurface where significant information, such as crack initiation, crack propagation, and fatigue
striation, is included. In the case of MEMS devices with a PZT actuation mechanism, however,no marks remain on the fracture surface because second-order fracture occurs, which meansthat the evidence explaining the fracture disappeared completely. Therefore, it is challenging tocorrectly understand how the PZT MEMS failed in experiments. At the same time, revealing thefracture mechanism is of considerable interest to material researchers investigating themechanical reliability of PZT MEMS devices.
The objective of this study is to clarify the fracture mechanism of PZT MEMS devices basedon experimental findings. Two types of MEMS device with an actuation mechanism using the
PZT film produced by the sol-gel method were prepared. A cantilever-type PZT MEMS device
was subjected to cyclic mechanical loading, whereas a clamped capacitor-type PZT MEMS
device was subjected to cyclic electrical loading. On the basis of experimental results derived
from the two loading tests, a reasonable fracture mechanism is discussed from the viewpoint of
the difference between the origins of electrical and mechanical fractures.

2.

Specimen

Figures 1(a) and 1(b) show schematics of the cantilever-type and clamped capacitor-type PZTMEMS devices used for fracture mechanism investigation, respectively. The cantilever-type
PZT MEMS device chip has multiple cantilever arrays with a width of 560 μm and lengths of
500–2000 μm, which consists of the PZT film actuator, upper and lower electrode layers, and Si
substrate covered with SiO2 film. The clamped capacitor-type PZT MEMS device has almost thesame configuration other than its in-plane shape and non-self-standing property. The clamped
capacitor of the PZT film has a circular shape with a diameter of 2800 μm. The two devices areproduced through almost the same fabrication process flow, which is explained as follows: An
n-type silicon-on-insulator (SOI) wafer with a 20 μm active layer, a 1.5 μm buried oxide (BOX)Sensors and Materials, Vol. 36, No. 8 (2024)

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Fig. 1. (Color online) Schematics of the cross sections of PZT MEMS devices: (a) cantilever type and (b) clamped
capacitor type.

layer, and a 200 μm handle layer is employed as a starting material. As the first step, wetoxidation is carried out to grow a SiO2 layer of 1.5 μm thickness on the entire surface of thewafer. After that, a 0.15-μm-thick Pt film used as the lower electrode is deposited on the top
surface of the wafer by sputtering. On the Pt film layer, a 2.0-μm-thick PZT film is deposited by
the sol-gel method, followed by sputtering a 0.1-μm-thick Ir/IrO film onto the PZT film layer.
The Ir/IrO film is utilized as the upper electrode. After these deposition processes, a device
pattern for each layer from the upper electrode to the lower electrode is formed. The three layers
are patterned one by one by photolithography using a positive photoresist and then etched byplasma etching using a photoresist mask. Then, an Al film is deposited on the top surface andpatterned for making a wiring to apply an electrical voltage to the two electrode layers. In thecase of the clamped capacitor-type PZT MEMS device, the fabrication is complete because no
self-standing processes are necessary. In the case of the cantilever-type PZT MEMS device, thecantilever shape is processed from the top surface using photolithography and Si dry etching
techniques. After the surface micromachining processes, Si dry etching from the back side of
the wafer is carried out to produce a through hole to make the cantilever actuator structure selfstanding. Finally, by dry etching the BOX layer to be removed, the self-standing cantilever-type
PZT MEMS device is complete and ready for the fracture test.

3.

Cantilever Fracture Testing

To experimentally investigate how mechanical loading affects the withstand voltage of the
PZT film layer, the cantilever-type PZT MEMS devices were subjected to mechanical and3422

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electrical loading cycles. First, quasistatic cantilever bending tests were performed to measure
the force value where the PZT MEMS device fractures mechanically. Cantilevers with lengths
longer than 1200 μm were chosen, and a normal force was applied at the point where the distancefrom the fixed end was only 1000 μm in all the samples using a tungsten probe from theoverhead on the PZT film side until fracture, which indicates that tensile stress was generated in
the film during the bending test.(34–36) A load cell with a capacity of 2 N was placed beneath thedevice chip to measure the applied force. Figure 2(a) shows a representative bending force–timerelationship obtained in the cantilever bending test. The bending force was applied manually
under the conditions of 2.5 mN loading step and 5 s holding time. It was found that the bending
force–time relationship is almost linear at an almost constant loading speed, although it wasapplied by hand. At 63.8 mN, the applied force dropped considerably, which indicates that thedevice fractured mechanically in a brittle manner. In this study, five samples in total were
subjected to the quasistatic bending test, and they fractured at the bending force ranging from60.1 to 64.0 mN (mean: 63.7 mN), although those data are omitted here.
As a second step, cyclic mechanical loading was applied to the PZT MEMS devices. The
cyclic mechanical load with a triangle waveform was applied manually using a tungsten probe.
The maximum bending force was set to be 47.9 mN, which is 75% of the mean force where the
PZT MEMS devices failed mechanically under quasistatic tension. As shown in Fig. 2(b), whichis the partial data of the test, a triangle waveform of the applied bending force can be confirmed
regardless of manual force application. We attempted to maintain the frequency and maximum
force to be around 0.04 Hz and 47.9 mN, respectively, as much as possible. The cyclic bending
loading was applied until 46 and 75 cycles. After 0, 46, and 75 loading cycles, three samples foreach were subjected to the withstand voltage evaluation experiment. Figure 3 shows therelationship between the withstand voltage and the number of loading cycles. Before applying
bending loading to the PZT MEMS device, the device failed when a voltage of 132 V on average
was supplied between the upper and lower electrode layers. The voltage when the device failed
electrically was judged as the withstand voltage of the device without any mechanical damage.
With increasing number of loading cycles, the withstand voltage was found to decrease

Fig. 2. Representative results of mechanical loading tests: (a) bending force–time relationship in quasistatic
bending test and (b) bending force–time waveform in cyclic bending test.

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Fig. 3.

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Relationship between withstand voltage and number of mechanical preloading cycles.

monotonically. After 75 mechanical loading cycles, it was 80 V on average, which is around 60%of the initial value. In this case, the withstand voltage (Vw) decreased with increasing number ofloading cycles (N) at Vw = 132e−0.007N, which strongly indicates that cyclic mechanical loading
affected the electrical reliability of the PZT film used in the MEMS device.
Figure 4 shows a magnified SEM image taken around the cracking part along with the energy
dispersive X-ray (EDX) analysis results. In this experiment, the cantilever device was broken
mechanically when the withstand voltage was applied. In the vicinity of the fractured part, acrack was observed as seen in the CCD image shown in the upper-right side of the figure. In the
SEM image, the crack was found to be introduced in the PZT film layer. It propagated from thecrack initiation part, which is probably the fractured portion of the cantilever device, to the
cantilever’s sidewall. Note that the portion around the crack only appears melted and the melted
portion continues along the crack. That is, a crack that we observed in the CCD image wasrepresented as the combination of the crack and melted materials along the crack. In the crosssection of the cantilever device, both the columnar layer and the flat portion can be seen. The
former is the non-melted PZT film layer and the latter is the melted portion. According to the
EDX analysis results, if oxygen is excluded, the melted material, part B, contained large amountsof Pb, Zr, and Ti, which are the major components of the PZT film indicated as part A. Ir and Al,
which are respectively used as the upper electrode and wiring materials in the device, were
included in the melted region. Those EDX results imply that, by supplying a voltage around its
withstand voltage, the PZT film layer was melted and the cantilever device was broken
mechanically. However, the correlation between PZT melting and cracking is still unknown inthis experiment.

4.

Clamped Capacitor Fracture Testing

To understand the correlation between electrical degradation and mechanical cracking, theeffect of cyclic electrical loading on the fracture of the PZT film layer was examined using the3424

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Fig. 4. (Color online) SEM image of failed PZT MEMS device along with ED-analyzed material element
percentage at columnar PZT film and melted portion.

clamped capacitor-type PZT MEMS devices. The main reason why the clamped capacitor-type
PZT MEMS devices were chosen is that, in the latter, the area of the PZT film was larger thanthat in the cantilever-type devices, which allows an easier observation of the degradation of the
PZT layer. First, cyclic electrical loading was applied to the PZT MEMS devices to observe thesurface degradation of the PZT film. The voltage amplitude was set to 0, 50, 75, and 100 V, andthe loading frequency was maintained at 50 Hz. Each loading was applied until the number ofloading cycles reached 1,000,000. After the cyclic loading tests, a quasistatic electrical loading
test was carried out at a constant rate of 0.5 V/s to evaluate the withstand voltage of the PZTfilm. Figure 5 shows the relationship between the voltage amplitude in the cyclic loading test andthe withstand voltage. Three samples were subjected to the experiment under each voltage
amplitude condition. Before cyclic electrical loading, the PZT MEMS device failed electricallywhen 216.5 V was applied on average. This voltage is the withstand voltage of the PZT film.
With the increase in amplitude to 50 V, the mean withstand voltage decreased to 149.5 V, which
is roughly 70% of the initial value. The withstand voltage (Vw) was found to decrease
monotonically with increasing voltage amplitude (VA), which can be expressed as Vw =210e−0.005VA. This relationship implies that the PZT MEMS device’s performance, showing thewithstand voltage, is determined by cyclic electrical loading as well as cyclic mechanical loading
because the PZT film was subjected to cyclic mechanical stress originating from cyclic electricalvoltage application.
After the PZT MEMS failure during the quasistatic electric loading test, the PZT film surface
was observed with a CCD camera as shown in Fig. 6 [voltage amplitudes of (a) 0, (b) 50, and (c)75 V during cyclic electrical loading test]. With the voltage amplitude of 0 V indicating no cyclic

Sensors and Materials, Vol. 36, No. 8 (2024)

Fig. 5.

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Relationship between withstand voltage and voltage amplitude during cyclic electrical preloading test.

Fig. 6. (Color online) Representative membrane surface observation results after withstand voltage evaluation
tests. After a cyclic electrical preloading test at voltage amplitudes of (a) 0, (b) 50, and (c) 75 V, the dielectricwithstand voltage was measured by quasistatic electrical loading.

electric loading, even though the device fractured, the PZT film showed almost a flat surface
except for one portion indicated as A. In the magnified image (second row), some gray dots on
the flat surface were found, which are dust on the lens of the CCD system. In the detailed imageof A (third row), several dark color regions can be seen. It is considered that the PZT MEMS
device failed at portion A where the PZT film was melted when 216.5 V was applied. Aroundthose portions, it can be observed that very small cracks were introduced. On the other hand, the3426

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PZT MEMS devices tested with the voltage amplitudes of 50 and 75 V showed similar trends. In
the images of the entire surface (first row), a dark color fluctuating pattern like a snake can beclearly seen. According to the snapshot taken during the quasistatic electrical loading test, thepattern made was found to be due to the electric short circuit. Figure 4(b) shows that the shortcircuit started from the point in the upper left portion and then propagated to the bottom side
where the electrode existed. As shown in Fig. 4(c), the same phenomenon started from the upper
right point and then propagated to the electrode as well. The interesting thing about this is thatthe propagation direction was split into two in the middle and one of the two stopped. Thisimplies that once one short circuit reached an electrode, the other one stopped immediately, andthe short circuit phenomenon observed in this study finished in a very short time. From theexperimental findings, it can be confirmed that those devices finally fractured electrically. The
portion of the surface other than the trail of electrical short circuit apparently appears flat in theentire surface image. However, the magnified images (second row) depict that there are so many
cracks introduced into the entire surface of the clamped capacitor actuator. One island made bycracking measures almost 10 μm in planar diameter, which seems to consist of multiple crystal
grains. On the PZT capacitor surface, small black dots were detected. Those are considered as
the particles of melted material flown in all directions when the electric short circuit occurredvery rapidly. The number of particles appeared to increase when the voltage amplitude wasincreased to 75 V, indicating that the short circuit probably occurred with a large impact. In thedetailed images of B and C (third row), note that multiple cracks on the two devices started fromonly one point, indicated as the center of the solid circle in each image. The multiple cracks
would have started at the center of the circle, which was the origin of the cracks, and then would
have propagated radially. Furthermore, one more important finding is that the origin of thecracks was definitely different from that of the electrical short circuit. That would be strong
evidence that the PZT MEMS devices degraded both mechanically and electrically with theapplication of cyclic electrical loading to the PZT film. Also, it can be considered that the two
experimental findings, namely, radial crack propagation and the difference between the originsof the mechanical and electrical degradations, suggest that mechanical cracking might haveoccurred first, followed by the electrical short circuit.
As shown in the cross-sectional image in Fig. 7(a), the crack is found to have penetrated the
top Ir/IrO electrode and PZT actuation layers, and not to have penetrated the bottom Pt electrodelayer and the SiO2 layer beneath the bottom electrode. With careful observation, the width of thecrack in the PZT layer was found to be larger than that in the top electrode layer. The PZT film
and the two electrode metals are well known as brittle and ductile materials, respectively. Thus,
the difference in crack width implies that the cracks observed on the PZT capacitor surface wereintroduced into the PZT layer first, then immediately propagated into the top electrode layer.
Since the bottom Pt electrode layer was tightly restrained to the SiO2 film, the cracks would nothave propagated to the bottom electrode. According to Fig. 7(b) showing the cross-sectional
image taken where the electric short circuit occurred, it was found that the PZT film was melted
and the melted material was put on the PZT capacitor as with the cantilever-type PZT MEMS
device already shown in Fig. 4. No visible cracks could be detected in the cross section where theshort circuit occurred. Even from this experimental finding, it can also be considered that theSensors and Materials, Vol. 36, No. 8 (2024)

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Fig. 7. SEM images of cross section of failed PZT MEMS device after quasistatic electrical loading test: (a)cracked and (b) short circuit portions.

electrical short circuit might have occurred after the mechanical cracking has been introduced
into the PZT film, although no direct evidence to explain this could be obtained in theexperiment. To experimentally explain which occurred first, an electrical short circuit ormechanical cracking, an artificial mechanical defect was processed by focused ion beam (FIB)
fabrication on the PZT MEMS device. Figure 8 shows a schematic of the FIB-processedmechanical defect along with the photograph. A focused Ga ion beam at the acceleration voltageof 30 kV was irradiated from overhead of the PZT capacitor surface until the fabrication depth
reached the Si handle wafer. By shifting the fabrication portion while gradually decreasing theacceleration voltage, a slope was produced to easily observe the cross section of the fabricatedvertical wall. The artificial defect was 200 μm in width and 5 μm in depth at the deepest portion.
On the FIB-processed surface, a river-like pattern can be observed as if water flows from the top
surface to the inside. It might be cracks introduced into the PZT film layer owing to externalenergy application originating from FIB irradiation.
After fabricating the artificial mechanical defect, static electrical voltage was applied
between the upper and lower electrodes until failure. The representative observation results
obtained after the application of electrical voltages of 0, 15, and 20 V are shown in Figs. 9(a)–9(c), respectively. In Fig. 9(a), the PZT capacitor surface just after the defect fabrication appears
flat without any other defects. In the magnified image of D, however, cracks were found to be
introduced entirely onto the top surface of the capacitor. The crack density of the clamped
capacitor after the FIB fabrication seems to be lower than that after the cyclic and quasistatic
electrical loading tests shown in Figs. 6(b) and 6(c). After applying 15 V, as shown in the firstrow image in Fig. 9(b), it appears like nothing occurred. However, as seen in the detailed imageof D (second row), one portion of the sidewall deformed slightly as if it started to melt. The
sidewall of the fabricated mechanical defect other than the melted portion had many particle-likeconvexes, indicating that the PZT film layer started degrading arising from the application of 15
V. In addition, new small debris were found to be attached to the PZT capacitor surface
especially around the melted portion. After the voltage application of 20 V, in the entire surface
image of the clamped capacitor shown in Fig. 9(c), it was clearly found that the electrical shortcircuit formed, which started from the edge of the artificial mechanical defect to the electrode.

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Fig. 8. (Color online) Schematic of artificial mechanical defect produced by FIB process along with the photographof the produced defect on the membrane-type PZT MEMS device.

Fig. 9. (Color online) Representative membrane surface observation results: (a) just after FIB fabrication and  
quasistatic electrical voltage application at (b) 15 and (c) 20 V. At 20 V application, short circuit occurred in the PZTMEMS device, indicating that 20 V was the dielectric withstand voltage of the device.

In the detailed image of D (second row), the defect structure completely collapsed and the PZTfilm melted from the partially melted portion, as found from the magnified image in Fig. 9(b), tothe outside radially. After the melting phenomenon was observed, it was confirmed that no new
cracks were introduced into the clamped capacitor. The PZT MEMS device with the artificial
mechanical defect collapsed at 20 V, which means that the device’s withstand voltage was 20 V.
Initially, the device exhibited the withstand voltage of 216.5 V, which gradually decreased to124.5 V with increasing voltage amplitude to 100 V in the cyclic electrical loading test. The
withstand voltage obtained in this experiment was 20 V, which is very low, roughly one-sixth toone-eleventh of the values obtained under cycling electrical loading conditions. The number andmagnitude of mechanical damage before the short circuit occurs directly determine thedurability of the PZT MEMS device to the applied voltage for actuating the device. That is,Sensors and Materials, Vol. 36, No. 8 (2024)

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Fig. 10. Schematic of fracture mechanism of PZT MEMS device. By applying external force, the PZT film issubjected to stress, and then cracks are introduced into the film. Current flows on the crack surface; consequently,
the PZT film melts, leading to short circuit in a very short period of time.

through the experiment with the introduction of an artificial mechanical defect using FIB, weconfirmed that the electrical short circuit leading to catastrophic failure would have occurred
after mechanical cracking had been introduced into the PZT film layer.
The fracture mechanism of the PZT MEMS device is summarized in Fig. 10 on the basis of
the experimental findings obtained in this study. The PZT film has a columnar polycrystalline
structure sandwiched by the upper and lower electrode layers made of Ir/IrO and Pt, respectively.
Once stress is applied to the PZT film by cyclic volume shrinkage originating from cyclic
electrical voltage applications or by direct cyclic mechanical forcing, cracks are introduced into
the PZT film because of its brittleness. Then, the cracks easily propagate into the upper andlower electrode layers because these layers are directly attached to the PZT film. At the highelectrical voltage applied between the two electrode layers, current accidentally starts flowing
on the surface of the crack because the surface is fresh, and no native oxide layer prohibits thecurrent flow. Once surface current flows, the PZT film starts melting locally because of thelarge current flow. The phenomenon can be seen as short circuit between the upper and lower
electrodes, which becomes a trigger that gives rise to short circuit between the PZT film’s melted
point and the electrode of the device. That phenomenon completes within a very short period oftime, so it is difficult to directly observe how the PZT film fractures in the experiment. In thisstudy, although all were indirect evidence, with the experimental findings, the reasonablefracture mechanism of the PZT MEMS devices could be illustrated.

5.

Conclusions

In this paper, we conducted two different types of cyclic loading test to experimentally
understand the fracture mechanism of MEMS devices equipped with a PZT film actuator madeby the sol-gel method. Two types of PZT MEMS device were prepared, namely, cantilever type

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and clamped capacitor type, for cyclic mechanical and electrical loading tests, respectively. In
the mechanical loading tests of the cantilever devices, it was found that the withstand voltage of
the PZT MEMS devices decreased monotonically with increasing number of mechanical loading
cycles. The cross-sectional observation result of the failed device showed that the PZT film
cracked and melted. In the electrical loading tests of the clamped capacitor-type devices, thewithstand voltage also decreased with increasing voltage amplitude during cyclic electricalloading. We found that the origin of the short circuit causing electrical failure was definitely
different from that of cracking. Through the withstand voltage evaluation experiments of theclamped capacitor-type device with an FIB-processed artificial defect, it was clarified thatcracking occurred first, followed by short circuit. This finding indicated that cracks generated in
the PZT film became a trigger for introducing electrical short circuit. To extend the lifetime of
PZT MEMS actuators, defects leading to cracking should be reduced or the PZT film should betoughened.

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