Compositional dependence of the Young's modulus and piezoelectric coefficient of (110)-oriented pulsed laser deposited PZT thin films

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Compositional Dependence of the Young’s Modulus

and Piezoelectric Coefficient of (110)-Oriented

Pulsed Laser Deposited PZT Thin Films

Hammad Nazeer, Minh D. Nguyen, Özlem Sardan Sukas, Guus Rijnders,

Leon Abelmann, Member, IEEE, and Miko C. Elwenspoek

Abstract— In this contribution, we report on the compositional

dependence of the mechanical and piezoelectric properties of Pb(ZrxTi1−x)O3 (PZT) thin films fabricated by pulsed laser

deposition (PLD). These films grow epitaxially on silicon with a (110) preferred orientation and have excellent piezoelectric properties, which make them outstanding candidates for applica-tion in microelectromechanical system devices. Vibrometric mea-surements on capacitors showed that the effective longitudinal piezoelectric coefficient (d33,f) of 100-nm thick PZT films has

a maximum value of 72 pm/V for a composition of x = 0.52. The Young’s modulus was determined by measuring the differ-ence in the flexural resonance frequencies of cantilevers before and after the deposition of the PZT thin films. The compositional dependence of the Young’s modulus shows an increase in value for the Zr-rich compositions, which is in agreement with the trend observed in their bulk ceramic counterparts. From the obtained dielectric constant and d33,f, we show that the

cal-culated coupling coefficients of the PLD-PZT thin films have higher values for most of the compositions than their ceramic

counterparts. [2013-0039]

Index Terms— Pb(ZrxTi1−x)O3 (PZT), piezoelectric

coeffi-cient, Young’s modulus, coupling coefficient. I. INTRODUCTION

I

N THE micro- and nano industry, the ever-growing demand for powerful actuators and sensitive sensors is addressed by the use of piezo-based transducers [1], [2]. Pb(ZrxTi1−x)O3 Manuscript received June 28, 2013; revised April 23, 2014; accepted May 2, 2014. This work was supported in part by the SmartMix Program, the Netherlands Ministry of Economic Affairs; and in part by the Netherlands Ministry of Education, Culture, and Science. Subject Editor D. DeVoe.

H. Nazeer, Ö. Sardan Sukas, and L. Abelmann are with the Transducers Science and Technology Group, MESA+ Research Institute for Nanotech-nology, University of Twente, Enschede 7522 NB, The Netherlands (e-mail: h.nazeer@utwente.nl; o.sardansukas@utwente.nl; l.abelmann@utwente.nl).

M. D. Nguyen is with the Inorganic Materials Science Group, MESA+ Research Institute for Nanotechnology, University of Twente, Enschede 7522 NB, The Netherlands; with SolMates B.V., Enschede 7522 NB, The Netherlands; and also with the International Training Institute for Materials Science, Hanoi University of Science and Technology, Hanoi 10000, Vietnam (e-mail: d.m.nguyen@utwente.nl).

G. Rijnders is with the Inorganic Materials Science Group, MESA+ Research Institute for Nanotechnology, University of Twente, Enschede 7522 NB, The Netherlands (e-mail: a.j.h.m.rijnders@utwente.nl).

M. C. Elwenspoek is with the Transducers Science and Technology Group, MESA+ Research Institute for Nanotechnology, University of Twente, Enschede 7522 NB, The Netherlands; and also with the Freiburg Institute for Advanced Studies, Albert-Ludwigs University, Freiburg 79085, Germany (e-mail: m.c.elwenspoek@utwente.nl).

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JMEMS.2014.2323476

(PZT) thin films are often used as piezo-materials because they have excellent ferroelectric and piezoelectric properties. These properties can be tuned by controlling the composition of the material by changing the Zr/Ti ratio [3], [4]. For instance, the composition Pb(Zr0.52Ti0.48)O3 is used in different types

of applications due to its higher piezoelectric properties [5]. Recently [4] discussed the use of a Ti-rich composition (x = 0.2) in energy-harvesting devices. They combined the high in-plane transverse piezoelectric coefficient and low dielectric constant of Pb(Zr0.2Ti0.8)O3 to obtain a high figure

of merit for power and voltage generation. A similar trade-off can be achieved for the longitudinal piezoelectric coefficient and the Young’s modulus of the material, which are analyzed in this paper.

If one looks at micro-electromechanical systems (MEMS), it is apparent that with the development of various types and applications, such as sensors and actuators, the requirement for materials with specific properties is getting very strict. It is realized that because of their tunable properties, PZT thin films are very suitable for such micro- and nano systems [6]. However, in order to efficiently use PZT thin films in these systems a better understanding of the piezoelectric and ferro-electric properties, as well as the mechanical behaviour of PZT thin films of various compositions, is necessary. For instance, the compositional dependence of these properties is dissimilar from their ceramic counterparts due to reasons like clamping of the films to the substrates and the different orientation of the films [7], [8].

PZT thin films can be obtained through different processes like sol-gel [9], sputter- [10] and pulsed laser deposition (PLD) [11] techniques. Previously we reported the excellent ferroelectric properties of PLD-PZT with a (110) preferred orientation [12]. In this work, we investigate the compositional dependence of the effective longitudinal piezoelectric coeffi-cient (d33,f), the Young’s modulus E, dielectric constant ε33

and coupling coefficient k of these PLD-PZT thin films in order to efficiently use these films as active device layers in MEMS devices. We used micrometer-sized measurement devices to characterize these dependencies.

The d33,f was determined by measuring the out-of-plane

displacement of PZT thin film capacitors, as described in section II-A. The Young’s modulus of the PZT thin films was determined by measuring the change in the resonance frequency of cantilevers before and after deposition of the

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This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

2 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS

Fig. 1. Schematic view of a PZT thin film capacitor.

PZT thin films. The method used for determining the Young’s modulus is briefly introduced in section II-B. In sections III-A and III-B, the fabrication of capacitor structures and silicon cantilevers and the deposition of PZT thin films by PLD are explained. The Young’s modulus and the d33,f depend on the

orientation of the PZT thin films. Therefore, X-ray diffraction (XRD) measurements were performed. These measurements and the techniques used to measure the Young’s modulus and d33,f are described in sections III-C, III-D and III-E.

Finally, the compositional dependence of the d33,f, the Young’s

modulus, the dielectric constant and the coupling coefficient of the PLD-PZT thin films are discussed in section IV.

II. THEORY

A. Longitudinal Piezoelectric Coefficient d33,f

The d33,fcan be determined by either measuring the charge

generated due to an applied external mechanical stress (direct piezoelectric effect) or measuring the displacement in the PZT caused by the application of an electric field (converse piezo-electric effect). Homogeneous uniaxial stress is required in the direct piezoelectric effect measurements, which is difficult to apply. Bending in the film due to application of the non-homogeneous stress results in a large amounts of charge due to the transverse piezoelectric effect [13]. For this reason, we measured the d33,f using the converse piezoelectric effect.

Applying an ac voltage on the top and bottom electrodes of the PZT capacitors, as shown in Fig. 1, results in a piezoelectric displacement. The d33,f is then determined by measuring the

out-of-plane displacement of these capacitors by using this relation [14]:

d33,f= S3

V /tf. (1)

Here d33,f is the effective longitudinal piezoelectric

coeffi-cient (“effective” means that the thin film is clamped to the substrate which reduces d33 with respect to a system that is

not clamped), S is strain, V is the voltage over the capacitor and tf is the thickness of the film.

B. Analytical Model for the Young’s Modulus of PZT Thin Films

The in-plane Young’s modulus of PZT thin films can be determined by using micromachined silicon cantilevers as test structures (see Fig. 2). The flexural rigidity and mass of the

Fig. 2. Schematical representation of a cantilever fabricated from a silicon-on-insulator wafer. The difference in resonance frequency of the cantilevers before and after the PZT deposition is used for Young’s modulus determination.

cantilevers will change due to the addition of the PZT thin films. These changes result in a difference in the resonance frequency of the cantilevers before and after the deposition of the PZT thin films. This difference is measured, after which the Young’s modulus of the PZT thin films is calculated using the equations given in [15].

III. EXPERIMENTALDETAILS A. Fabrication of PZT Capacitors

To measure the d33,f and dielectric constant of PLD-PZT,

capacitors were fabricated on (001) silicon wafers. To obtain epitaxial growth of the PZT thin films, a 50 nm thick buffer layer of yttria-stabilized zirconia (YSZ) was first deposited on silicon by PLD. This layer prevents the diffusion of lead into the silicon during PZT deposition and also acts as a crystalliza-tion template for epitaxial growth of the PZT thin films. Next, 50 nm of strontium ruthenate (SRO) was deposited as a bottom electrode. The PLD process then continued with the PZT thin film until the desired thickness of 100 nm was achieved. The parameters for the process used are given in [12]. Deposition of the stack was completed with the 50 nm thick top electrode of SRO. The 200× 200 µm2 _{capacitors were patterned by}

a standard photolithographic process, followed by argon-ion beam milling of the top SRO electrodes with an etching rate of 10 nm/minute and a wet etch to remove the PZT layer in the diluted HF:HNO3:H2O solution.

B. Fabrication of Cantilevers for Young’s Modulus Measurement

Silicon cantilevers of varying lengths from 250 µm to 350 µm in steps of 10 µm were fabricated using a dedicated SOI/MEMS fabrication process, similar to what we reported in [15]. A schematic illustration showing the main steps of the fabrication procedure can be seen in Fig. 3. Scanning electron micrographs (SEM) and optical images were used to characterize these cantilevers; see Fig. 4. After characterization of the fabricated silicon cantilevers, 10 nm thick buffer layers of YSZ and SRO and then 100 nm thick PZT thin films were deposited. In contrast to the capacitor structures, we deposited thin buffer layers of YSZ and SRO to prevent the influence of the additional layer on the resonance frequency.

C. XRD Measurements

The orientation of the deposited PZT thin films was ana-lyzed by θ–2θ X-ray diffraction (XRD) scans (XRD, Bruker

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Fig. 3. Illustration of the fabrication process of the cantilevers. (a) Devices etched on the silicon on insulator (SOI) wafer with deep reactive ion etching (DRIE). (b) Application of polyimide pyralin as protective layer on the front side and wafer through etching from the backside of the wafers. (c) Polyimide pyralin and photoresist removal from the front and backsides and subsequent etching of the buried oxide (BOX) layer using vapor hydrofluoric acid (VHF).

Fig. 4. Scanning electron micrograph of fabricated cantilevers. Cantilevers were fabricated from a 3 µm thick silicon device layer. The length of the cantilevers varies from 250 µm to 350 µm in steps of 10 µm. The cantilevers have a constant width of 30 µm. See-through to substrate shows rough walls due to DRIE from the backside of the wafers.

D8 Discover) with a Cu Kα cathode in the Bragg–Brentano geometry. The θ–2θ scans were performed for all composi-tions of the PZT thin films (x = 0.2 − 0.8). Fig. 5 shows the XRD patterns of the PZT thin films deposited on SRO/YSZ buffered Si cantilevers and Si substrates, respectively. The PZT films grown on Si substrates have a pure (110)-orientation (Fig. 5(b)). Due to the various compositions (or Zr/Ti ratios), change in position of (110)-oriented peaks is observed. This peak-position shifts towards a higher angle when the Zr/Ti

Fig. 5. XRD patterns of PZT thin films with different compositions grown on: (a) Si cantilevers and (b) Si substrates.

Fig. 6. X-ray phi-scan profiles of PZT(002) and Si(202) reflections of the (110)-oriented PZT(x _{= 0.52) thin films grown on Si substrate and} Si cantilever. The insets show the glue step of Si substrate and Si cantilever on the heaters.

ratio is varied ranging from 80/20 to 20/80 (or x is decreased). The in-plane epitaxial relationships between the films and substrates were estimated by the X-ray phi-scan (φ-scans) measurement, as illustrated in Fig. 6. In the structure of PZT

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film/Si substrate, four identical sets of peaks of PZT(002) deflections are positioned around the reflections corresponding to Si(202), showing the epitaxial growth of film on substrate. However, instead of a single peak positioned at the Si(202) position, the intensity is divided over two peaks situated at +10° and −10° with respect to Si. Since two PZT(002) peaks are expected in a perfect crystal, this means that twin domains exist in the (110)-oriented PZT thin film. The following relationship in the structure is deduced: PZT(110)∥ SRO(110) ∥ YSZ(001) ∥ Si(001), since the rotation angle φ of the (002) peak of SRO and the (202) peak of YSZ coincides with those of PZT and Si, respectively [16].

The PZT films grown on Si cantilevers consist of mainly (110)-orientation and minor orientations of (001), (111), (210) and (211), as shown in Fig. 5(a). The mixed orientations in PZT/Si cantilevers, in comparison with pure (110)-orientation in PZT/Si substrates, are due to the less homogeneous tem-perature of Si cantilevers during deposition (see the insets in Fig. 6). The phi-scan measurement in Fig. 6 shows that the PZT film grown on Si cantilever is not an epitaxial structure due to worse in-plane orientation. Moreover, the (222) diffraction peaks of the pyrochlore phase are observed in PZT/Si cantilevers. The formation of a metastable stoichio-metric pyrochlore becomes more significantly in the tetragonal PZT (Ti-rich) films grown on Si cantilevers at low deposited-temperature in relation to the growth of a Ti-rich pyrochlore (Pb2Ti2O6) phase.

D. Measurements of the Longitudinal Piezoelectric Coefficient d33,f

The piezoelectric displacement of the PZT thin film capac-itors was measured to determine the d33,f. A MSA-400 micro

system analyzer scanning laser Doppler vibrometer was used for measuring the displacement of the capacitors. An ac-voltage of magnitude 6 Vp-p(peak to peak) was applied

to the top and bottom electrodes at a frequency of 8 kHz. This voltage actuates the PZT and the resulting displacement of the top electrode was measured.

E. Measurements of the Young’s Modulus

To determine the in-plane Young’s modulus of PZT thin films, the resonance frequencies of cantilevers were mea-sured by using a MSA-400 micro system analyzer scanning laser–Doppler vibrometer. Thermally excited vibrations of the cantilevers were measured in ambient conditions. Curve fitting with a theoretical expression for a second-order mass–spring system with damping was used to calculate the free reso-nance frequencies. The resoreso-nance frequency measurements were conducted both before and after the deposition of the PZT thin films for cantilevers of varying length and for different compositions. As an example, the measurements for a cantilever of length ∼250 µm, width ∼30 µm, and thickness ∼3 µm before and after deposition of the Pb(Zr0.2Ti0.8)O3

thin film are shown in Fig. 7. The Young’s modulus can be calculated from the shift in resonance frequency as described in [15]. To reduce the uncertainty in the calculated value of the Young’s modulus, we measured the thickness of the

Fig. 7. The measured resonance frequencies of a cantilever before and after PZT deposition. Normalised amplitude shows a decrease in the resonance frequency of the cantilever measured after deposition of the Pb(Zr0.2Ti0.8)O3.

This expected decrease is attributed to the addition of the PZT thin film on the cantilever.

Fig. 8. (a) Polarization hysteresis (P–E) loops, and (b) Remnant polarization (Pr) and coercive field (Ec), of PZT thin films as a function of composition.

cantilevers by using high-resolution SEM. As a result we could measure the Young’s modulus with a standard error of less than ±1.8 GPa.

F. Measurements of the Ferroelectric Properties

The polarization hysteresis (P–E) loop measurements were performed using the ferroelectric mode of the aixACCT TF-2000 Analyzer. The P–E loops were measured with an applied ac-electric field of ±300kV/cm at 1 kHz frequency at room temperature. Fig. 8(a) indicates the difference in ferroelectric properties among PZT thin films with different compositions. The tetragonal film with x = 0.2 exhibits a well saturated rectangular loop with a large remnant

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polar-Fig. 9. (a) Dielectric constant-electric field and (b) Dielectric loss-electric field curves of PZT thin film capacitors, as a function of composition.

ization (Pr) and a large coercive field (Ec). Slim ferroelectric P–E hysteresis loop was observed in rhombohedral film with

x_{= 0.8. The plots of the average values of the P}r and Ec are

shown as a function of composition (Zr/Ti ratio) of PZT thin films in Fig. 8(b). With increasing Ti-rich composition, larger

Pr and Ec values are observed.

A Süss MicroTech PM300 manual probe-station equipped with a Keithley 4200 Semiconductor characterization system was used for the capacitance measurement. The capacitance-electric field (C–E) curves were measured using a slowly sweeping dc-electric field of ±300 kV/cm and a 1 kHz

ac-electric field of 10 kV/cm. The dielectric constant (ε33–E)

and dielectric loss (tanδ–E) versus electric field, as shown in Fig. 9, can be calculated from these capacitance–electric field (C–E) and conductance–electric field (G–E), which were obtained from capacitance measurement, as follows [16]:

ε33 = Ct

ε0A. (2)

tanδ = _2πfCG . (3)

where t is PZT film thickness, A is the area of capacitor, ε0(= 8.854 × 10−12 F/m) is the vacuum permittivity and f is

the measured frequency.

IV. RESULTS ANDDISCUSSION A. Crystal Structure

It is known that the piezoelectric and mechanical properties of PZT thin films depend on the crystal orientation [17], [18].

Fig. 10. The d33,fvalues as a function of Zr content (x) for different PZT

compositions. Based on our measurements we find a maximum value of d33,f

at x _{= 0.52. The trend of the d}33,f values for PZT thin films is compared

with the bulk PZT ceramics [19] in clamped condition.

X-ray diffraction (XRD) measurements reveal that all PZT thin films investigated in this study grow with a (110) preferred orientation; see Fig. 5. Therefore, if there are any variations in the d33,f values and the Young’s modulus, then these can

not be caused by the crystal orientation but must be due to a difference in composition.

B. Piezoelectric Coefficient

The composition of PZT has a strong effect on the d33,f

value, as seen in Fig. 10. For a film thickness of 100 nm, a maximum d33,f value of 72 pm/V was observed at a

composition of Pb(Zr0.52Ti0.48)O3. The optimum composition

is in agreement with bulk PZT ceramics in unclamped condi-tion [19], but the value is 68% lower. There are two reasons, which can be used to explain the reduction of d33,fin PZT thin

films: substrate clamping and film/electrode interface effects. Clamping of the thin film with the substrate causes a reduction in the d33,f value as compared to the corresponding bulk

material (d33) [20], due to the limitation of domain wall motion

in films [21]. The relation between the d33,f and d33 using

the compliance coefficients data of the bulk Pb(Zr0.52Ti0.48)O3

ceramic is given in [22] as:

d_33,f_{= d}₃₃_{+ 1.38d}₃₁ (4)

Since relevant compliance coefficient data are not available for all PZT ceramic compositions, we used Eq. 4 and d33

and d31 of the corresponding composition [19] to calculate

the dceramic

33,c values of PZT ceramics in clamped condition

(Fig. 10). It is evident from the comparison of the composi-tional dependence of the PZT ceramics in clamped condition

dceramic

33,c and d33,f that the d33,f value of PLD-PZT thin film is

36% lower for x = 0.52, whereas it shows higher values for other compositions.

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Fig. 11. Compositional dependence of Young’s modulus of the PZT thin films plotted as a function of Zr content (x) in Pb(ZrxTi1−x)O3 thin films.

The trend is compared with the data published by Jaffe et al. [19] for bulk PZT ceramics.

In addition to substrate clamping effects, the presence of a thin layer with a low dielectric constant between the film and the electrode, a so-called interfacial “dead" layer; can also have a large effect on the piezoelectric properties. The interfacial layer may also alter the distribution of the electric field in the PZT film, leading to a decrease in piezoelectric coefficient [23]. Nguyen indicated that an interfacial layer thickness of about 6 nm was calculated for the 100-nm-thick epitaxial PZT film on SRO/YSZ/Si substrate [16].

The maximum of d33,f at x = 0.52 composition is in

agreement with the piezoelectric response reported in literature for PZT thin films obtained by a sol-gel method [24], [25]. However, the effect is more pronounced in our PZT thin films, with a shallow maximum at x = 0.52.

C. Young’s Modulus

The Young’s modulus strongly depends on the film com-position, as is shown in Fig. 11. The dependence of Young’s modulus on the PZT composition shows an increase in value for the Zr-rich compositions, which is in agreement with the published data for bulk PZT ceramics [19], also shown in Fig. 11. The value of the Young’s modulus for the composition with the maximum d33,f was found to be 113.5 GPa with a

standard error of ±1.5 GPa at x = 0.52. This value is 57% higher than for bulk PZT ceramic. It is also much higher than values reported in literature for sol-gel films (Young’s modulus: 84 GPa [26]), but have the same order as values reported for sputter deposited PZT (109 GPa [10]). The dip in the Young’s modulus lies at a lower Zr content than found for bulk PZT ceramics (x = 0.52, see Fig. 11). This behaviour is under investigation.

D. Dielectric Constant and Coupling Factor

The compositional dependence of the dielectric constant (ε33) and dielectric loss (tanδ) is shown in Fig. 12(a).

Fig. 12. (a) Dielectric constant and dielectric loss of the PZT film capacitors as a function of composition. (b) Compositional dependence of the relative dielectric constant of the PZT film capacitors in compared with the bulk PZT ceramics data obtained from Jaffe et al. [19].

The values of ε33 and tanδ were defined from ε33-E and

tanδ-E curves, respectively, and at 0 V point (see more in Fig. 9). It is shown that the film with a Zr/Ti ratio of 52/48 (x = 0.52) exhibits highest dielectric constant. Such depen-dence was also reported previously [27]. Rhombohedral films show higher dielectric constants than tetragonal films, due to more possible polarization directions in the rhombohedral phase (eight [111] directions) in comparison to the tetragonal phase (six [100] directions). The dielectric loss is slightly increased with increasing Zr/Ti ratio (or increasing x), as 5.3% and 8.1% for x = 0.2 and x = 0.8, respectively. The electromechanical coupling factor (k33,f), which is an

extremely useful figure of merit, is defined as [28]:

k33,f= d33,f

!

E₃₃

ε (5)

where ε = ε0εr, E33 is the out-of-plane Young’s modulus.

The E33 can be calculated as: E33 = E/3(1-ν) = E/2.1,

where E (in-plane Young’s modulus) is the measured Young’s modulus in this study, ν(= 0.3) is Poisson’s ratio of PZT film. For this calculation, the “intrinsic” or “relative” dielectric constant (εr) was estimated by constructing a linear fit to

the highest electric field (from 200 to 300 kV/cm range) and extrapolating back to zero field, as shown in Fig. 9. This is required to remove the contribution of 180° domain-wall to the dielectric constant, because such domain-wall does not contribute to the electromechanical properties. Similar to the dielectric constant, the film at x = 0.52 also exhibits large relative dielectric constant (Fig. 12(b)). Compared to bulk PZT ceramics, the PZT films show a much broader with lower val-ues of εr for Ti-rich compositions and higher values for Zr-rich

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Fig. 13. Compositional dependence of the coupling factor of the PZT film capacitors in compared with the bulk PZT ceramics in clamped condition.

Fig. 13 shows the coupling factor of the PZT films and bulk PZT ceramics as a function of the film composition. The k33,f

of the PZT films are higher than those of the corresponding bulk ceramics. It is then expected that the PLD PZT films in this study are more suitable for applications in both sensors and actuators.

V. CONCLUSION

We report on the compositional dependence of the effective longitudinal piezoelectric coefficient, the Young’s modulus, dielectric constant and coupling coefficient of Pb(ZrxTi1−x)O3

thin films with a (110)-preferred orientation, grown on micro-machined silicon cantilevers by pulsed laser deposition.

The d33,f shows a maximum value of 72 pm/V at a Zr

content of x = 0.52 for a film thickness of 100 nm. The dependence of the piezoelectric coefficient of PLD-PZT on composition is much weaker than in bulk PZT ceramics material, which has a sharp peak at x = 0.52. When correcting for the clamping effect, the d33,f in bulk ceramics exceeds the

PLD-PZT thin film value by 36% at this composition. At other compositions the PLD-PZT has a substantially higher value.

The Young’s modulus of films at x = 0.52 composition is 113.5 GPa with a standard error of ±1.5 GPa. When further increasing the Zr content, the Young’s modulus increases, which is in agreement with data published for bulk PZT ceramics [19]. For lower Zr content, the Young’s modulus does not appear to follow the curves measured for ceramics, showing a minimum of 105 GPa at x= 0.2. The data suggest a second minimum for the Young’s modulus in the region of

x_{= 0.4 to x = 0.52.}

The compositional dependence of the dielectric constant of the PLD-PZT thin films follows the same trend as predicted by bulk PZT ceramics. Instead of a distinct peak as observed in ceramics, we found a broader maximum at the x = 0.52 composition. A comparison of the electromechanical coupling factor k33,fof the PLD-PZT thin films and bulk PZT ceramics

in clamped condition reveals that our PLD-PZT thin films have a much higher coupling factor because of higher Young’s modulus values.

ACKNOWLEDGMENT

The authors also thank the assistance of M.J. de Boer for etching, R.G.P. Sanders for laser Doppler vibrometer measurements, J.G.M. Sanderink and H.A.G.M. van Wolferen for SEM, and L.A. Woldering and N.R. Tas for helpful discussions.

REFERENCES

[1] J. R. Bronson, J. S. Pulskamp, R. G. Polcawich, C. M. Kroninger, and E. D. Wetzel, “PZT MEMS actuated flapping wings for insect-inspired robotics,” in Proc. 22nd IEEE Int. Conf. MEMS, Sorrento, Italy, Jan. 2009, pp. 1047–1050.

[2] H.-J. Nam et al., “Silicon nitride cantilever array integrated with silicon heaters and piezoelectric detectors for probe-based data storage,” Sens.

Actuators A, Phys., vol. 134, no. 2, pp. 329–333, 2007.

[3] Z. Q. Zhuang, M. J. Haun, S.-J. Jang, and L. E. Cross, “Composition and temperature dependence of the dielectric, piezoelectric and elastic properties of pure PZT ceramics,” IEEE Trans. Ultrason., Ferroelectr.,

Freq. Control, vol. 36, no. 4, pp. 413–416, Jul. 1989.

[4] D. Isarakorn et al., “The realization and performance of vibration energy harvesting mems devices based on an epitaxial piezoelectric thin film,”

Smart Mater. Struct., vol. 20, no. 2, p. 025015, Jan. 2011.

[5] B. Xu, L. E. Cross, and J. J. Bernstein, “Ferroelectric and antiferroelectric films for microelectromechanical systems applications,”

Thin Solid Films, vols. 377–378, pp. 712–718, Dec. 2000.

[6] B. Piekarski, M. Dubey, E. Zakar, R. Polcawich, D. DeVoe, and D. Wickenden, “Sol-gel PZT for MEMS applications,” Integr.

Ferroelectr., Int. J., vol. 42, no. 1, pp. 25–37, 2002.

[7] S. Hiboux, P. Muralt, and T. Maeder, “Domain and lattice contributions to dielectric and piezoelectric properties of Pb(Zrx,Ti1−x)O3 thin

films as a function of composition,” J. Mater. Res., vol. 14, no. 11, pp. 4307–4318, 1999.

[8] D. V. Taylor and D. Damjanovic, “Piezoelectric properties of rhombohedral Pb(Zr, Ti)O3 thin films with (100), (111), and

‘random’ crystallographic orientation,” Appl. Phys. Lett., vol. 76, no. 12, pp. 1615–1617, Jan. 2000.

[9] N. Ledermann, P. Muralt, J. Baborowski, M. Forster, and J.-P. Pellaux, “Piezoelectric Pb(Zrx, Ti1−x)O3thin film cantilever and bridge acoustic

sensors for miniaturized photoacoustic gas detectors,” J. Micromech.

Microeng., vol. 14, no. 12, pp. 1650–1658, Aug. 2004.

[10] T.-H. Fang, S.-R. Jian, and D.-S. Chuu, “Nanomechanical properties of lead zirconate titanate thin films by nanoindentation,” J. Phys., Condens.

Matter, vol. 15, no. 30, pp. 5253–5259, Jul. 2003.

[11] M. Dekkers, M. D. Nguyen, R. Steenwelle, P. M. te Riele, D. H. A. Blank, and G. Rijnders, “Ferroelectric properties of epitaxial Pb(Zr,Ti)O3thin films on silicon by control of crystal orientation,” Appl.

Phys. Lett., vol. 95, no. 1, p. 012902-1–012902-3, Jul. 2009. [12] M. D. Nguyen et al., “Characterization of epitaxial Pb(Zr,Ti)O3 thin

films deposited by pulsed laser deposition on silicon cantilevers,”

J. Micromech. Microeng., vol. 20, no. 8, p. 085022, Jul. 2010. [13] K. Yao and F. E. H. Tay, “Measurement of longitudinal piezoelectric

coefficient of thin films by a laser-scanning vibrometer,” IEEE Trans.

Ultrason., Ferroelectr., Freq. Control, vol. 50, no. 2, pp. 113–116, Feb. 2003.

[14] K. Lefki and G. J. M. Dormans, “Measurement of piezoelectric coefficients of ferroelectric thin films,” J. Appl. Phys., vol. 76, no. 3, pp. 1764–1767, Aug. 1994.

[15] H. Nazeer, M. D. Nguyen, L. A. Woldering, L. Abelmann, G. Rijnders, and M. C. Elwenspoek, “Determination of the Young’s modulus of pulsed laser deposited epitaxial PZT thin films,” J. Micromech.

Microeng., vol. 21, no. 7, p. 074008, Jun. 2011.

[16] M. D. Nguyen, “Ferroelectric and piezoelectric properties of epitaxial PZT films and devices on silicon,” Ph.D. dissertation, Inorganic Mater. Sci., Univ. Twente, Enschede, The Netherlands, 2010.

[17] Q. M. Wang, Y. Ding, Q. Chen, M. Zhao, and J. Cheng, “Crystalline orientation dependence of nanomechanical properties of Pb(Zr0.52Ti0.48)O3 thin films,” Appl. Phys. Lett., vol. 86, no. 16,

(8)

[18] S. Corkovic, R. W. Whatmore, and Q. Zhang, “Development of residual stress in sol-gel derived Pb(Zr,Ti)O3 films: An experimental study,”

J. Appl. Phys., vol. 103, no. 8, pp. 084101-1–084101-12, 2008. [19] B. Jaffe, W. R. Cook, and H. Jaffe, Piezoelectric Ceramics. London,

U.K.: Academic, 1971.

[20] P. Muralt, “PZT thin films for microsensors and actuators: Where do we stand?,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 47, no. 4, pp. 903–915, Jul. 2000.

[21] F. Xu, S. Trolier-McKinstry, W. Ren, B. Xu, Z.-L. Xie, and K. Hemker, “Domain wall motion and its contribution to the dielectric and piezoelectric properties of lead zirconate titanate films,” J. Appl. Phys., vol. 89, no. 2, pp. 1336–1348, 2001.

[22] F. Xu, F. Chu, and S. Trolier-McKinstry, “Longitudinal piezoelectric coefficient measurement for bulk ceramics and thin films using pneumatic pressure rig,” J. Appl. Phys., vol. 86, no. 1, pp. 588–594, Jul. 1999.

[23] A. K. Tagantsev, M. Landivar, E. Colla, and N. Setter, “Identification of passive layer in ferroelectric thin films from their switching parameters,”

J. Appl. Phys., vol. 78, no. 4, pp. 2623–2630, Aug. 1995.

[24] D.-J. Kim, J.-P. Maria, A. I. Kingon, and S. K. Streiffer, “Evaluation of intrinsic and extrinsic contributions to the piezoelectric properties of Pb(Zr1−x,Tx)O3thin films as a function of composition,” J. Appl. Phys.,

vol. 93, no. 9, pp. 5568–5575, Apr. 2003.

[25] S. Trolier-McKinstry and P. Muralt, “Thin film piezoelectrics for MEMS,” J. Electroceram., vol. 12, nos. 1–2, pp. 7–17, 2004. [26] S. Yagnamurthy, I. Chasiotis, J. Lambros, R. G. Polcawich,

J. S. Pulskamp, and M. Dubey, “Mechanical and ferroelectric behavior of PZT-based thin films,” J. Microelectromech. Syst., vol. 20, no. 6, pp. 1250–1258, Dec. 2011.

[27] X.-H. Du, J. Zheng, U. Belegundu, and K. Uchino, “Crystal orientation dependence of piezoelectric properties of lead zirconate titanate near the morphotropic phase boundary,” Appl. Phys. Lett., vol. 72, no. 19, pp. 2421–2423, 1998.

[28] H. Zhu, D. P. Chu, N. A. Fleck, S. E. Rowley, and S. S. Saxena, “Polarization change in ferroelectric thin film capacitors under external stress,” J. Appl. Phys., vol. 105, no. 6, p. 061609, 2009.

Hammad Nazeer obtained his Bachelor in 1997

and his Masters in 2000 in electrical engineering from the N.E.D. University of Engineering and Technology, Pakistan. In 2007, he joined the Trans-ducers Science and Technology Group, University of Twente, The Netherlands, and he received his PhD in 2012. His research interests focus on the analytical relation to determine the Young’s modulus and residual stress of Pb(Zrx,Ti1-x)O3 (PZT) thin films with various Zr/Ti ratios and orientations.

Minh D. Nguyen started his studies in chemistry

(1995-1999). After his master in Materials Science (2001), he received his PhD in 2010 in Physics from the University of Twente, The Netherlands. His research interests focus on various piezoelectric MEMS devices, concentrates on piezoelectric micro-diaphragms and micro-cantilevers for micro-fluidics and micro-biosensors applications. These devices are based on the textured and epitaxial Pb(Zr,Ti)O3 (PZT) and Pb(Mg1/3Nb2/3)O3/PbTiO3 (PMN-PT) thin films, fabricated on Si wafers using pulse laser deposition (PLD) and sol-gel techniques.

Özlem Sardan Sukas received her BSc and MSc

degrees in mechanical engineering from Middle East Technical University, Turkey, in 2004, and Koç University, Turkey, in 2006, respectively. Dur-ing her PhD, she worked on "Topology Optimized Electrothermal Microgrippers for Nanomanipulation and Assembly" within the EU project NanoHand. Receiving her PhD degree from Technical University of Denmark in 2010, she started as a Postdoctoral Fellow at University of Twente, The Netherlands. She worked as a MEMS researhcer at SmartTip BV, The Netherlands, for a year in 2013. Her research has included development of MEMS actuators for various applications. Her recent research focuses on realization of cantilever-based piezoMEMS bio-sensors.

Guus Rijnders finished his PhD work on "Initial

Growth of Complex Oxides: Study and Manipula-tion," in 2001, after which he became an assistant professor at the Low Temperature Division, Uni-versity of Twente. In 2003 he joined the Inorganic Materials Science Group, University of Twente, where he became an associate professor in 2006. Since April 1st 2010, he has been appointed full pro-fessor in Inorganic Materials Science. His research focuses on the structure-property relation of atom-ically engineered complex (nano)materials, espe-cially thin film ceramic oxides.

Leon Abelmann (M’08) (1965) is leader of the

NanoEngineering group at KIST Europe, and holds a professorship at the MESA+ Research Institute at the University of Twente, as well as the Physics and Mechantronics Department of Saarland University. He started his career on a grant from the Royal Dutch Academy of Sciences and a subsequent Inno-vation Grant from the Netherlands Organisation for Scientific Research (NWO). He obtained a tenure track at the University of Twente in 2001, and was appointed full professor in 2013. In 2014, he accepted a position as group leader at KIST Europe in Saarbrucken, where he strives to strengthen the link between KIST and the EU research community.

Miko C. Elwenspoek (born December 9,1948 in

Eutin, Germany) worked on liquids, nuclear physics, light scattering, biophysics (at the Free University of Berlin) and in Nijmegen, The Netherlands, on crystal growth, before he joined the University of Twente in 1987. In Twente, he took charge of the micromechanics group that was part of the Sensors and Actuators Lab, now MESA+ Research Institute. Since then his research focused on Microelectro-mechanical Systems. In 1996 he became full pro-fessor in transducers science and technology at the Faculty of Electrical engineering, University of Twente. One year later, he became Simon Stevin Master of the Dutch Technology Foundation. He is fellow of the Institute of Physics and of the Freiburg Institute of Advanced Studies. In 2001, 2008 and in 2011 students elected him as the best lecturer in their program. He was a co-founder Advanced Technology at the UT and of Master Nanotechnology. Since January 2007, he has been director of the honours programme at the University of Twente, and from 2011 to end 2013, director of the Electrical Engineering program.