Epitaxial Pb(Zr,Ti)O3 thin films for a MEMS application

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2011 Adv. Nat. Sci: Nanosci. Nanotechnol. 2 015005

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 015005 (5pp) doi:10.1088/2043-6262/2/1/015005

Epitaxial Pb(Zr,Ti)O3 thin films for aMEMS application

Minh D Nguyen1,2,3, Hung N Vu3, Dave H A Blank2 and Guus Rijnders2

1 SolMateS B.V., Drienerlolaan 5, Building Hogekamp, 7522 NB Enschede, The Netherlands2 Inorganic Materials Science, MESA Institute for Nanotechnology, University of Twente, PO Box 217,7500 AE Enschede, The Netherlands3 International Training Institute for Materials Science, Hanoi University of Technology, 1 Dai Co VietRoad, Hanoi, Vietnam

E-mail: minh.nguyen@solmates.nl and minh.nguyen@itims.edu.vn

Received 28 September 2010Accepted for publication 9 February 2011Published 7 March 2011Online at stacks.iop.org/ANSN/2/015005

AbstractThis research presents the deposition and device fabrication of epitaxial Pb(Zr, Ti)O3 (PZT)thin films for applications in microelectromechanical systems (MEMS). A piezoelectricmicro-membrane is described as an example. Using the pulsed laser deposition (PLD)technique and the MEMS microfabrication process, the piezo-membranes with diametersranging from 200 to 500 µm were obtained. The displacement of piezo-membranes increasedfrom 5.1 to 17.5 nm V−1 with a piezoelectric-membrane diameter in the range of 200–500 µm.Furthermore, the effect of PZT film-thickness on the mechanical properties has beeninvestigated. By using the conductive-oxide SrRuO3 (SRO) layers as the electrodes, thedegradation of both ferroelectric and piezoelectric properties is prevented up to 1010 switchingcycles.

Keywords: epitaxial thin films, Pb(Zr, Ti)O3, piezo MEMS devices, conductive-oxideelectrodes

Classification numbers: 4.10, 4.11, 6.12

1. Introduction

Thin film processing is quite important for development ofthe miniaturization of electronic devices with low operatingvoltage. Epitaxial growth of PZT thin films on a siliconsubstrate is considered to be a key technology for fabricatingthe thinner and smaller electronic devices, because theirleakage currents are expected to be lower than those ofpolycrystalline films [1, 2]. Furthermore, epitaxial PZT filmsalso exhibited better ferroelectric and piezoelectric propertiesthan polycrystalline PZT films [3].

Moreover, thin film processing techniques havebeen receiving much attention for applications inpiezo-MEMS devices, such as biosensors, micro-pumpsand micro-machined ultrasonic transducers (MUTs). Incomparison with other common deposition techniques, likesputter deposition [4, 5], evaporation [6, 7], metalorganicchemical vapor deposition (MOCVD) [8, 9] and sol–gelprocessing [10, 11], the PLD technique offers some

advantages. Most important is its ability to transfer materialstoichiometrically from a multicomponent target to a growingfilm. This is very important for complex oxides such as PZT,because their physical properties strongly depend on theprecise control of the chemical composition. Second, PLDis a promising technique for thick film fabrication because itoffers the advantage of a high deposition rate. Because of thehigh deposition rate in the pulse, moreover, the mass transferfrom the ablation plume to solid thin films is improved. Thisallows deposition at high growth temperatures, especially forthe epitaxial growth of volatile materials. In the case of PZTthin film deposition, it results in a less significant loss of themore volatile elements, like lead (Pb).

The aim of this work is to investigate the piezoelectricand ferroelectric properties of the PZT grown on siliconmembranes using SrRuO3 (SRO) layers as the top- andbottom-electrodes. Furthermore, the effects of membranegeometry and oxide-electrodes are discussed.

2043-6262/11/015005+05$33.00 1 © 2011 Vietnam Academy of Science & Technology

Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 015005 M D Nguyen et al

Figure 1. Fabrication process of the piezoelectric PZT film membrane on pre-patterned SOI substrates.

2. Experimental procedure

2.1. Membrane fabrication

Figure 1 shows the flowchart of the piezoelectric PZTmembrane fabrication process. Devices were fabricated ona (100) Silicon-on-Insulator (SOI) wafer with a 5-µm-thicksilicon layer and a 2-µm-thick SiO2 buried oxide (BOX)layer, as shown in figure 1(F1). The silicon-membrane wasfabricated using deep reactive-ion etching (DRIE) from thebackside of the wafer. The BOX layer that acts as an etch stopin the DRIE process is then removed by buffered-hydrofluoricacid (BHF) (figure 1(F2)). The resulting thickness of theSi-membrane was 5 µm. Then, the pre-patterned SOI waferwas cut into substrates with dimensions of 20 × 20 mm2.These are called pre-patterned SOI substrates. The device isover an area of 10 × 10 mm2 at the center of the pre-patternedSOI substrate.

The deposition of a multilayer SRO/PZT/SRO/YSZbegan with the deposition of a 100-nm-thick Yttria-stabilizedzirconia (YSZ) buffer-layer on top of the SOI. The YSZbuffer-layer acts as a barrier layer against lead diffusionduring PZT film deposition and prevents the formation ofan excessive SiO2 amorphous layer on the surface of the Sisubstrate. Moreover, it also acts as a crystallization templatefor the piezoelectric stack, allowing epitaxial layer growth.

Next, a 100-nm-thick SRO electrode and a 1-µm-thickPZT layer were deposited. Deposition of the piezoelectricstack was completed with a 100-nm-thick SRO top electrodelayer deposition (figure 1(F3)). All layers in the piezoelectricstack were deposited by pulsed laser deposition. Details ofthe optimal deposition conditions of the multilayer stack werepublished previously [12, 13].

Figures 1(F4)–(F9) show the fabrication of piezoelectricmembranes by the MEMS microfabrication process, includingphotolithography and etching. First, the top SRO layer wasetched by Ar-ion beam etching (steps F4 and F5). Next, thePZT film was removed by sequential wet-etching with HF andHCl solutions (steps F7 and F8). The top-view structure of themembranes is shown in figure 2 after release of the photoresistlayer (figure 1(F9)).

In fabricating a PZT thin film membrane actuator, siliconresidues may result from an uneven etch rate while releasing

Figure 2. Microscope images (top-view) of the fabricatedmembrane arrays with the top-electrodes of 100–160 µm in a200-µm-diameter silicon membrane.

the membrane from the SOI substrate. The silicon residuescan reduce the effective area of the membrane, and thendecrease the properties of the actuator. Figure 3(a) shows anSEM image of the backside of a PZT thin film membraneactuator. The membrane is uniform in thickness and withoutany residue appears circular in shape. This indicates that thediameter of the fabricated membrane actuator is correctedwith that of the membrane design. The surface and deflectionprofiles of a 200-µm-diameter circular-membrane, measuredusing a white-light interferometer, are shown in figure 3(b).Without applying bias voltage, the upward bending of thepiezoelectric membrane results from built-in gradient tensilestress between the PZT thin film and the silicon membrane.

2.2. Characterization methods

The polarization hysteresis (P–E) loop was measured at±200 kV cm−1 amplitude and 1 kHz frequency, using aferroelectric film test system (TF 2000 Analyzer). A SüssMicroTech PM300 manual probe-station equipped with aKeithley 4200 Semiconductor characterization system is usedfor the capacitance measurement. The capacitance–electricfield (C–E) was performed using an ac signal of 1 kV cm–1

and a 10 kHz frequency with the dc bias sweeping from−200 to + 200 kV cm−1 and then back to −200 kV cm−1.The corresponding dielectric constant was calculated fromthis C–E curve. The orientation of the film was observed by

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 015005 M D Nguyen et al

Figure 3. (a) SEM images of the backside view and (b) measured membrane deflection (or initial bending) of a fabricated PZT thin-filmmembrane actuator.

Figure 4. (a) Polarization hysteresis (P–E) loop and (b) dielectric constant (ε–E) curve for the PZT film membrane after device fabrication.The P–E loop was determined at ± 200 kV cm−1 amplitude and 1 kHz frequency while the dielectric constant curve was measured using anac signal at 1 kV cm−1 and 10 kHz frequency.

high-resolution x-ray diffraction (XRD, Bruker D8 Discover)with a Cu-Kα cathode in the Bragg–Brentano geometry.

The Polytech MSA-400 micro system analyzer is usedfor the analysis and visualization of structural vibration andsurface topography in microstructures such as membranes andMEMS devices in general. Full integration of a microscopewith a scanning laser Doppler vibrometer (LDV) was carriedout for measurements of the actuation of membranes.

3. Results and discussion

3.1. Structure and ferroelectric properties of membranes

From x-ray diffraction (XRD) (not shown here), the PZTfilms that were grown on SRO/YSZ/Si substrates had a (110)orientation. As shown in the phi-scan plots [13], the rotationangle phi-scan (φ) of the (202) peak of YSZ coincideswith that of Si. Moreover, four identical sets of SRO(002)peaks are positioned around the reflections correspondingto Si(202). Since the SRO(002) peak is divided over twopeaks situated at + 10◦ and −10◦ with respect to Si, thismeans that twin domains exist in the thin film. Theseresults indicate that the cube-on-cube epitaxial orientation

relationship with the substrate is obtained for all YSZ, SROand PZT layers, described by (110) PZT/(110)c SRO/(001)YSZ/(001) Si [14].

The ferroelectric properties for the PZT film membraneswere characterized using polarization and dielectric constantmeasurements, as shown in figure 4. The remnant polarizationPr was measured as 18.6 µC cm−2 and the dielectric constantcalculated from the capacitance measurement was 1360.

3.2. Electromechanical behavior

The LDV technique was used to measure theelectromechanical behavior (displacement) of the membranesactuated with a sinusoidal voltage. Membrane displacementwas measured at 6 Vp–p (peak-to-peak) amplitude and 8 kHzfrequency (well below resonance). Figure 5(a) shows atypical piezoelectric vibration response of a membrane of200 µm diameter. The laser scanning area covered both thetop electrode and the surrounding PZT film. To investigatethe effect of membrane geometry on membrane displacement,the measured central displacements for PZT membraneswith various diameters are illustrated in figure 5(b). Thedisplacement increases from 5.1 to 17.5 nm V−1 when the

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 015005 M D Nguyen et al

Figure 5. (a) Upward-displacement of the 200-µm-diameter membrane, which was carried out at 6 Vp–p amplitude and 8 kHz frequency(off-resonant conditions). (b) Plot of the measured central displacement as a function of the diameter of the membrane.

Figure 6. Membrane central displacement as a function of bias applied voltage (a) and PZT film-thickness (b) for the membrane of 200 µmin diameter.

diameter is in the range of 200–500 µm. The top-electrodesize is roughly 60% of the diameter of the membrane.

The effect of the actuation voltage amplitude on the200-µm-diameter membrane was evaluated by applyinga sine wave signal of an ac-voltage between 1 and6 Vp–p(peak-to-peak) at a frequency of 8 kHz. The results,shown in figure 6(a), indicate that the displacement wasproportional to the applied voltage.

Figure 6(b) illustrates that the displacement ofthe 200-µm-diameter membrane increased from 2.9 to5.1 nm V−1 with film thickness in the range of 0.25–1 µmand slightly decreased for the thicker films (1–2 µm).Like the polarization property, in the range of 0.25–1 µmfilm thickness, this variation in the piezoelectric propertywas attributed to the interfacial layer at the film/electrodeinterface. With increasing film thickness, the effect of theinterfacial layer is decreased, thus leading to a large d33

coefficient of PZT thin films. A slight decrease in thedisplacement with the thicker films (1–2 µm) is observed,because the PZT films get softened at these films [15].

Polarization fatigue is responsible for the decrease inthe displacement of the PZT/Pt piezoelectric stacks with

Figure 7. Remnant polarization and displacement as a function ofthe number of working cycles of the 200-µm-diameter membrane.The polarization was carried out using a bipolar switching pulse of200 kV cm−1 pulse height and 5 µs pulse width, and was measuredat ± 200 kV cm−1 and 1 kHz. The piezoelectric displacement wascarried out at 6 Vp–p amplitude and 8 kHz frequency.

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 2 (2011) 015005 M D Nguyen et al

increasing switching cycles [16]. This problem can be solvedby using conductive-oxide SRO electrodes in the piezoelectricstack [15]. The SRO electrodes can reduce or preventthe accumulation of oxygen vacancies near the PZT/SROinterface by changing its oxygen non-stoichiometry. Nodecrease in the remnant polarization was hence observed inthe PZT/SRO capacitors with increasing switching cycles.Figure 7 reveals the phenomenon of displacement of the200-µm-diameter membrane as a function of working cycle.It is noted that the displacement fatigue did not occur up to1010 cycles.

4. Conclusion

We have demonstrated the behavior of PZT thin film asan integrable actuator in MEMS technology. Piezoelectricmembranes based on epitaxial PZT thin films grown onsilicon membranes were designed and analyzed successfully.Using SRO layers as the top- and bottom-electrodesallowed the use of typical wet and dry etching processes,thus permitting a design based completely on standardmicrofabrication processes. The good quality, crack-freemembrane is mechanically strong enough to support theferroelectric micro-sensors, which may have each elementsize in the range of 200–500 µm. Membranes actuated bythe PZT thin films show deflection of several nanometersand these make possible the use of this kind of actuators forresonant actuator applications, as in accelerometers.

Next, the effects of membrane geometry on thepiezoelectric displacement were investigated. Thedisplacement increased with an increase in the membranediameter. Furthermore, the piezoelectric membranes withconductive-oxide SRO electrodes showed neither polarizationnor actuation fatigue up to 1010 cycles, which clearly indicatesthat the piezoelectric PZT film membranes can work stablyfor a long period of time. As a result, the SRO/PZT/SROthin-film stacks have great potential to serve as sensors andactuators for many MEMS applications.

Acknowledgments

The authors gratefully acknowledge the support of theSmart-Mix Program (Smart-Pie) of The Netherlands Ministryof Economic Affairs and The Netherlands Ministry ofEducation, Culture and Science, as well as the VietnameseOverseas Scholarship Program (MOET-VOSP Project 322).

References

[1] Kondo M, Maruyama K and Kurihara K 2002 Fujitsu Sci.Tech. J. 38 46

[2] Kim C J, Yoon D S, Lee J S, Choi C G, Lee W J and No K1994 J. Appl. Phys. 76 7478

[3] Akai D, Yokawa M, Hirabayashi K, Matsushita K, Sawada Kand Ishida M 2005 Appl. Phys. Lett. 86 202906

[4] Hata T, Kawagoe S, Zhang W, Sasaki K and Yoshioka Y 1998Vacuum 51 665

[5] Bouregba R, Sama N, Soyer C and Remiens D 2009 J. Appl.Phys. 106 044101

[6] Tokumitsu E, Ueno S, Nakamura R I and Ishiwara H 1995Integr. Ferroelectr. 7 215

[7] Park B E, Shouriki S, Tokumitsu E and Ishiwara H 1998Japan. J. Appl. Phys. 37 5145

[8] Pan C Y, Chen Y L and Tsai D S 2002 J. Mater. Res. 17 1536[9] Otani Y, Okamura S and Shiosaki T 2004 J. Electroceram.

13 15[10] Han H, Zhong J, Kotru S, Padmini P, Song X Y and Pandey

R K 2006 Appl. Phys. Lett. 88 092902[11] Vu H N, Le M V, Bui H T and Nguyen M D 2009 J. Phys.:

Conf. Ser. 187 012063[12] Nguyen M D, Steenwelle R J A, te Riele P M, Dekkers J M,

Blank D H A and Rijnders G 2008 Proc. EUROSENSORSXXII (Dresden, Germany) p 810

[13] Nguyen M D, Nazeer H, Karakaya K, Pham S V, SteenwelleR, Dekkers M, Abelmann L, Blank D H A and Rijnders G2010 J. Micromech. Microeng. 20 085022

[14] Dekkers M, Nguyen M D, Steenwelle R, te Riele, BlankD H A and Rijnders G 2009 Appl. Phys. Lett. 95 012902

[15] Nguyen M D 2010 Ferroelectric and piezoelectric properties ofepitaxial PZT films and devices on silicon PhD ThesisUniversity of Twente, The Netherlands

[16] Gerber P, Kügeler C, Ellerkmann U, Schorn P, Böttger U andWaser R 2005 Appl. Phys. Lett. 86 112908

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