FUNDAMENTALS OF THIN FILM PIEZOELECTRIC MATERIALS AND PROCESSING DESIGN FOR A BETTER ENERGY HARVESTING MEMS

Transcription

1 FUNDAMENTALS OF THIN FILM PIEZOELECTRIC MATERIALS AND PROCESSING DESIGN FOR A BETTER ENERGY HARVESTING MEMS Kiyotaka Wasa, Isaku Kanno, and Hidetoshi Kotera Microengineering, Kyoto University, Kyoto, Japan Abstract: Piezoelectric materials are essential for harvesting energy from mechanical vibrations. Thin films of piezoelectric materials including AlN and binary perovskite compounds PZT are widely studied for a fabrication of energy harvesting MEMS. The piezoelectric materials have held a key position in the energy harvesting devices. Varieties of bulk piezoelectric ceramic materials are studied and used in practice for the piezoelectric sensors and/or actuators. However, piezoelectric properties of the thin film materials are different from bulk properties due to their structural differences. The characters of the thin film piezoelectric materials are not fully understood yet. For making a better energy harvesting device, a fundamental discussion will be necessary for the thin film piezoelectric materials. In this paper, first the fundamentals of thin film piezoelectric materials and the process design are discussed, and then novel piezoelectric thin films of PZT-based ternary perovskite compounds are proposed as an example for making the improved energy harvesting MEMS. Keywords: Power MEMS, Piezoelectric energy harvesting, PZT-based ternary perovskite, PMnN-PZT thin films INTRODUCTION Thin films of AlN, ZnO, and Pb(Zr,Ti)O 3 (PZT) are widely studied for a fabrication of the piezoelectric actuators and sensors in MEMS [1]. Thin films of AlN and ZnO are used for the fabrication of the acoustic wave devices [2]. The thin films of the ferroelectric PZT perovskite ceramics, binary compounds of PbTiO 3 (PT) and PbZrO 3 (PZ), are used for the fabrication of gyro-sensors and ink-jet printer heads due to their excellent piezoelectric properties [3]. Recently much interests have been paid to the energy harvesting power MEMS using piezoelectric materials [4]. The piezoelectric materials have held a key position for the power MEMS. The power outputs are governed by the coupling factors k and the dielectric constantsε, and are proportional to the ratios k 2 /ε [5]. The PZT-based materials are considered to be a suitable material for the energy harvesting power MEMS due to their extremely high material coupling factors k. However, at present the ALN thin films with a small k value are also considered as a candidate for the piezoelectric materials in the power MEMS, since the values, k 2 /ε, obtained in the AlN piezoelectric thin films are not inferior to those of PZT thin films [5]. The quality factors for the piezoelectric PZT-based thin films are sensitive to the chemical composition and the microstructure. Most of the reported PZT-based thin films are intrinsic thin films. Further improvements of their chemical compositions are still expected for the PZT-based thin films. Recently the authors have improved their chemical compositions and developed thin films of the PZT-based ternary perovskite compounds [6]. In this paper, first the fundamentals of thin film piezoelectric materials and the process design are discussed, and then the PZT-based ternary compound piezoelectric thin films are shown in relation to their potential application for making the energy harvesting MEMS. PIEZOELECTRIC THIN FILMS Possible Materials Non-ferroelectric AlN is well known hexagonal piezoelectric materials with extremely high mechanical quality factor Q m. The AlN thin films are used as an acoustic device. However, the usage of the non-ferroelectric materials is limited due to their low coupling factors. Thin films of ferroelectric PZT perovskite ceramics are well known piezoelectric materials of high piezoelectricity. However, the high piezoelectricity could not be well utilized for the energy harvesting MEMS because of their high dielectric constants. Bulk PZT-based ternary perovskite ceramics show a variety of dielectric and piezoelectric properties. The coupling factor k, mechanical quality factor Q m,and the dielectric constants ε are controlled by the doping of relaxor ferroelectric materials such as Pb(Mg,Nb)O 3 (PMN) and Pb(Mn,Nb)O 3 (PMnN) into the binary compounds PZT [7-8]. Among the ternary perovskite compounds, the hard ferroelectric materials PMnN-PZT will be useful for making the piezoelectric energy harvesting MEMS. Typical phase diagram of the PMnN-PZT is shown in Fig.1 [9] /PMEMS2009/$ TRF 61 PowerMEMS 2009, Washington DC, USA, December 1-4, 2009

2 Fig. 1. Phase diagram of PZT-based ternary compounds: xpmnn-(1-x)pzt, 0<x<0.2 [9]. Basic Processing for PZT-Based Thin Films Thin films of perovskite materials are fabricated by sputtering, pulsed laser ablation (PLD), MOCVD, and sol-gel deposition [10]. The ferroelectric properties of these perovskite thin films are different from bulk ceramic properties probably due to the difference of microstructure and the presence of the growth stress in the thin films [11]. The PZT-based thin films of perovskite phase are fabricated by two different deposition conditions as shown in Tab.1. One is deposition at room temperature followed by post-annealing at crystallizing temperature Tcr of perovskite phase (low temperature process), the other is deposition at the crystallizing temperature Tcr of perovskite phase (high temperature process). The sintering process of the bulk ceramics is also shown in the Tab. 1. The piezoelectric properties of the thin films are governed by the material compositions and their structures. The low temperature process is similar to the sintering process. The high temperature process provides the Table 1. Basic fabrication processes for PZT- based thin films Process Structure Ceramics mixing sintering polycrystalline Thin (1) *1 deposition post anneal polycrystalline films at RT at Tcr*3 (2) *2 deposition at Tcr polycrystalline single crystal *4 * 1 low temperature process, * 2 high temperature process *3 Tcr: o C *4 deposition on single crystal substrates ferroelectric perovskite thin films without the postannealing. Polycrystalline thin films are deposited on the glass substrates. Single crystal thin films are deposited on the single crystal substrates using the heteroepitaxial growth. The polycrystalline films show the fiber structure and/or grained structure. The epitaxial films show strained and/or dislocated structure due to the thermal and lattice mismatch between the PZT-based materials and the substrates as shown in Fig.2. These structural properties reduce the coupling factor k and/or the mechanical quality factor Q m. One of the ideal structures of the PZT-based thin films is the relaxed single crystal structure as shown in Fig.2. The crystal quality including interface is improved by the buffer layers. The single crystal substrates and the buffer layers are shown in Tab.2. Among the several deposition processes described above, sputtering is one of the useful deposition processes, since the porosity of the sputtered thin films is extremely small. Recently we have found the sputtered epitaxial thin films show bulk single crystal-like relaxed structure, i.e. one of the ideal structures, when the epitaxial PZT-based thin films are quenched after the deposition [12]. substrate Single domain Strained single crystal PZT Tetra Absence of phase transition at MPB Multi domain Polycrystal PZT Tetra - Rhombo at MPB Fig. 2. Lattice images for heteroepitaxial thin films. Table 2. Cubic single crystal substrates. Single domain relaxed single crystal PZT Tetra Absence of Phase transition at MPB Substrates: Sapphire (a=0.4763nm), SrTiO 3 (ST)(a=0.3905nm), MgO(a=0.4203nm), LaAlO 3 (a=0.3792nm) Si (a=0.5431nm),gaas (a=0.5654nm) Buffer layers : YSZ ( a=0.516nm), SrRuO 3 ( a=0.393nm) Epitaxial relations: (111)PT//(0001)sapphire, (001)PT//(001)LaAlO 3, (001)PT//(001)ST, (101)SRO //PT //(001)ST, (001)PT//(001)MgO, (101)SRO// PT// (001)MgO, (111)PT//YSZ//(100)Si 62

3 PMnN-PZT THIN FILMS Deposition: Relaxed Single Crystal Thin Films A planar rf-magnetron sputtering was used for the heteroepitaxial growth of the PZT-based ternary compounds. Thin films of PMnN-PZT were directly sputtered from PMnN-PZT powder target on (001)MgO substrates. The powder target was composed of the mixture of PT, PZT, PbO, Nb 2 O 5 MnO 2, ZrO 2, and TiO 2. The high temperature process shown in Tab.1 was used for the deposition. The chemical composition of the sputtered thin films was easily controlled by the composition of the mixed powder. The epitaxial temperature was 500~650 o C. Typical sputtering conditions are shown in Tab. 1[13]. The sputtered films were quenched after the deposition to make the single crystal-like relaxed structure. Table 3. Sputtering conditions. RF-magnetron Sputter up( Balance mag.field) Target* 1 mixed powder: PT, PZT,PbO, Nb 2 O 5 MnO 2,ZrO 2, TiO 2 Substrates* 2 (001)MgO Sputtering gas 0.5 Pa (Ar/O 2 =20/1) Growth temp o C Surface (101)SRO/(001)Pt Growth rate 5-15 nm/min Film thickness nm Quenching rate 100 o C/min. in air * 1 Composition: xpmnn-(1-x)pzt + 10%PbO x-=0-0.2 * 2 Conductive base electrode: (001)Pt on (001)MgO Crystal Structure and Piezoelectric Properties XRD measurements suggested the PMnN-PZT thin films deposited on the SRO/Pt/MgO substrates show highly (001) orientation. Fig.3 shows the cross sectional TEM image with electron diffraction patterns. The TEM image suggested the sputtered films showed high density structure without grains and/or interfacial layers between thin films and substrates. The electron diffraction patterns describe the sputtered PMnN-PZT thin films were single c-domain/ single crystal thin films epitaxially grown on the SRO/Pt/MgO substrates. Tab. 4 shows the lattice parameters of (001)PMnN-PZT thin films for different compositions of the PZT. The in plane lattice parameters were almost the same to bulk values independent of the lattice parameters of the SRO. The sputtered films were stress free and/or relaxed. The PMnN-PZT ternary ceramics comprise PZT with donor additive Nb and acceptor additive Mn. However, it is known the ternary ceramics show hard ferroelectric response. Fig. 4 shows a typical P-E hysteresis curve for the single c-domain/ single crystal thin films of 0.06PMnN-0.94PZT(45/55). The hysteresis curve shows typical hard ferroelectric properties of square shaped loops with high Ec and large Pr (2Ec 230kV/cm and 2Pr 120 C/cm 2 ). The PMnN-PZT thin film showed dielectric anomaly at Tc=560 o C, Curie temperature, as shown in Fig.4. The Tc is higher than bulk ceramic values. The higher Tc will be caused by growth stress during the sputtering deposition. The dielectric constant decreases with the increase of Ti similar to the bulk ceramics. The observed relative dielectric constants were with tan = at 1kHz. The dielectric constant was much lower than bulk non-doped intrinsic PZT ( bulk values 700). The low dielectric constant reveals the sputtered thin films consist of highly (001) oriented crystal structure. PMnN-PZT Tab. 4. Lattice parameters of sputtered PMnN-PZT thin films epitaxially grown on (101)SRO/ (001)Pt/ (001)MgO for different compositions of PZT. Composition a-lattice c-lattice c/a SRO/Pt MgO Fig. 3. Cross sectional TEM images of sputtered PMnN- PZT thin films epitaxially grown on(110)sro/(001)pt (001)MgO: Composition, 0.06PMnN-0.94PZT(45/55), film thickness, 1.3 m. 0.06PMnN-0.94PZT(45/55) nm nm PMnN-0.94PZT(48/52) PMnN-0.94PZT(55/45) PZT(52/48) Bulk PZT: Tetra (50/50) nm c=0.4139nm SRO (101) a 0.393nm 63

4 &"!!! &!!!! %!!! $!!! #!!! "!!! -.*+,& '## "# $# %# &# #!&##!'##!&# # '## &##!%#!$#!"# 5*6./78*197*6*:/-7:1:;<=/.</1>cooling!! &!! "!! '!! #!! (!! $!! )*+,*-./0-*1234 heating Fig. 4. P-E hysteresis curve and temperature variations of dielectric constants: (001)PMnN-PZT thin films epitaxially grown on (101)SRO/ Pt/MgO substrates. Composition, 0.06PMnN-0.94PZT (45/55), Piezoelectric film thickness,1.9 m. PMN-PZT FBAR ()*+, *e 31 =d 31 / *s 11 = h 2 /3s 11 L 2 /V, h and s 11 are thickness and elastic compliance of MgO substrates, *s 11 is the compliance of PMnN-PZT thin films, L is length of the cantilever, shows the deflection of the cantilever at the applied voltage V [14]. Taking typical experimental values, =840nm at 10V, h=0.3mm, L=10mm, 1/s x10 9 N/m 2 (MgO), 1/*s 11 92x10 9 N/m 2 (PMnN-PZT), the e 31f and d 31 for the PMnN-PZT thin films were e 31f = 10.8C/m 2, d 31 = 83x10-12 m/v, respectively. The observed transverse piezoelectric constants are almost the same to bulk PZT ceramic values. Bulk PMnN-PZT ceramics show the doping of PMnN into PZT remarkably enhances the Q m. In order to confirm the effect of the PMnN doping on the Q m, PMnN-PZT thin film FBAR structure was fabricated and evaluated their Q m values. The (001)MgO substrates of 0.3 mm in film thickness were used for the fabrication of the FBAR structure. The FBAR structure comprised film thickness of the PMnN-PZT thin films of 280 to 320nm thick, SRO/Pt base conductive electrodes, and Al top electrodes. Size of the Al top electrodes were 500nmx500nm. The effective coupling factor k eff was evaluated by the relation k eff =[(fp 2 -fs 2 )/fp 2 ] 1/2, where fp and fs denote the parallel and series resonant frequency. The k t was evaluated by the relation k t 2 = (π/8) 2 (k eff 2 )/(1- k eff 2 ). The Q m was obtained by the phase change of the impedance at the antiresonant frequency fp using the relation Q m =1/2 ω (dφ/dω). These impedance properties were evaluated by the network analyzer [6]. Typical impedance properties are shown in Fig.6 for the PMnN-PZT FBAR. From these impedance measurements we have investigated the effect of the doping of PMnN into PZT on k t and Q m at GHz FBAR structure. Typical thin films of PMnN-PZT, 0.1PMnN -0.9PZT(55/45), 300nm in film thickness showed fs=3.373 GHz and fp=3.870ghz. Taking the observed fp and fs values, the k eff =0.487 and the kt = From the phase properties Q m =185. Thin films Fig. 5. Cantilever beam and FBAR structures. Fig. 5 shows the construction of the cantilevers and the FBARs for the measurements of piezoelectric properties. The effective piezoelectric coefficients for thin films e 31f are estimated by using following relation [1]: e 31f =*e 31 x [s 11 / (s 11 +s 12 )], where Impedance Z (db! Φ Frequency (GHz) Fig. 6. Typical impedance properties of PMnN-PZT thin film FBAR. Φ Z Phase _ (rad) 64

5 of intrinsic PZT(48/52) showed fs= 3724MHz and fp=4451mhz. Their k eff =0.547 and k t = The Q m was 114. The kt of the intrinsic PZT thin films was slightly higher than the PMnN doped PZT thin films. The doping increases the Q m almost two times in magnitude. Typical variations of Q m and k t with the doping of PMnN to PZT are shown in Fig.7. Qm kt Qm xpmnn-(1-x)pzt(45/55) x (%) xpmnn-(1-x)pzt kt 0.8 Fig. 7. Typical variations of Q m and k t with doping of PMnN into PZT thin films for xpmnn-(1- x)pzt(45/55) thin films: PZT near MPB composition. DISCUSSION Single c-domain/single crystal thin films of the PZT-based ternary compounds were successfully synthesized by the sputtering. The resultant films show the hard ferroelectric properties with high kt and high Q m. Their piezoelectric properties are superior to the conventional binary PZT-based thin films due to the doping effects and the structural perfection. Porous thin films of PZT show small Q m values. The high Q m is achieved at the bulk-like pore free high density structure. The high Q m values of the piezoelectric thin films are essential for the long term stability of the piezoelectric devices. The energy harvesting system consists of a mechanical vibration portion and the piezoelectric power generating portion. If the piezoelectric power generator is actuated by the transverse piezoelectric mode, the maximum power generated by the mechanical vibration operated at resonance is given by P max =k 31 2 mq 2 A 2 /4ω, where k 31 is the electromechanical coupling coefficient, m is the mass of the cantilever, Q is the mechanical quality factor of the vibration portion, A is the acceleration magnitude of the input vibrations and ω is the resonance frequency [5]. Under a given mechanical vibration condition, the output powers are proportional to the coupling factor k 31 2 and/or the ratio, e 31f 2 /ε, where ε is the dielectric constants of the piezoelectric thin films. The ratio e 31f 2 /ε is tentatively defined as a power generation factor. Table 4 shows the piezoelectric properties for the PZT-based thin films and the power generation factor, e 31f 2 /ε, in comparison with the AlN thin films. Table 4. Typical dielectric and piezoelectric properties of PZT-based thin films and AlN thin films. PZT PMnN-PZT AlN Substrate (001)MgO (001) MgO Sapphire Structure Epi Epi Epi ε/ε Ec(kV/cm) Pr (µc/cm 2 ) *e 31 (C/m 2 ) e 31f (C/m 2 ) d 31 (pc/n) e 2 31f /ε (GPa) Ref [14-15] *1 [13] *2 [2] *1 Conventional sputtering. PZT(53/47). e 31f *e 31 x 1.3. *2 Sputtering and quenching. High Q m 185,high kt (=0.7). 0.06PMnN-0.94PZT(45/55) It is seen the power generation factor, e 31f 2 /ε, for the AlN thin films are comparable to those of conventional PZT thin films [14-15]. This is mainly caused by the small dielectric constants of AlN. Thin films of single c-domain PMnN-PZT have a high potential for the better energy harvesting devices. The output powers of the present ternary PZT-based thin films will be one order in magnitude higher than those of the conventional PZT thin films and/or AlN thin films. For the better energy harvesting MEMS, the selection of the substrate materials are essential for the optimum design of the mechanical vibration portion. The piezoelectric single crystal thin films are deposited on a single crystal substrate. The substrate materials are selected for epitaxy. It is noted the resultant single crystal thin films could be transferred onto another substrate having an optimum elastic constant which achieves high mechanical Q in the mechanical vibration port. The transfer process does not affect on their structural and piezoelectric properties of the epitaxial single crystal thin films [16]. In this paper the sputtered single crystal piezoelectric thin films are described as an example. 65

6 Numbers of the piezoelectric PZT thin films studied for making the energy harvesting MEMS are polycrystalline thin films deposited by the low temperature process, for instance, the sol-gel method. These polycrystalline PZT-based ternary perovskite thin films also show high coupling [17]. However, the polycrystalline thin films exhibit high dielectric constants due to the presence of (111) oriented grains. The (111) oriented grains should be reduced for the better energy harvesting MEMS. CONCLUSION If the conventional binary PZT thin films are replaced by the ternary PZT thin films of single c- domain/single crystal structure, the output powers of the energy harvesting system will surely increase. The high Q m values and the high Curie temperature of the ternary perovskite will improve the temperature stability and/or the long term stable operation. The transfer technology of epitaxial piezoelectric thin films onto the vibrating beam is useful for the total material design of the power MEMS. ACKNOWLEDGEMENTS This study is a part of Advancing Technology Excellence Nano-Medicine project, which is under Kyoto City Collaboration of Regional Entities assigned by JST. REFERENCES [1] Trolier-McKinstry S, Muralut P 2004 Thin Film Piezoelectric for MEMS J. Electroceramics [2] Muralt P, Antifakos J, Cantoni M, Lane R, Martin F 2005 Is there a better material for thin film BAW applications than AlN Proc. of 2005 IEEE Ultrasonics Symp. vol [3] Fujii E, Takayama R, Nomura K, Murata A, Hirasawa T, Tomozawa A, Fujii S, Kamada T, Torii H 2007 Preparation of (001)-Oriented Pb(Zr,Ti)O 3 Thin Films and Their Piezoelectric Applications IEEE Trans. on UFFC [4] Renaud M, Karakaya K, Sterken T, Fiorini P, van Hoof C, Puers R 2008 Fabrication, modeling and characterization of MEMS piezoelectric vibration harvesters Sensors and Actuators A [5] van Schaijk R, Elfrink R, Kamel T M, Goedbloed M 2008 Piezoelectric AlN energy harvesers for wireless autonomous transducer solutions Proc. of 2008 IEEE Sensors Conference [6] Yamauchi N, Shirai T, Matsushima T, Matsunaga T, Wasa K, Kanno I, Kotera H 2009 High coupling piezoelectric thin films of PZT-based ternary perovskite compounds for GHz FBAR Appl. Phys. Lett [7] Cross L.E 1993 Ferroelectric Ceramics Setter N and Colla E.L ed. (Birkhäuser-Verlag, Basel) [8]Takahashi M, Tsubouchi N, Ohno T 1971 Piezoelectric properties of the ternary and quaternary systems containing PbTiO 3 -PbZrO 3 IEC Report Japan 1971CPM ,. [9] Zhang T, Wasa K, Kanno I, Zhang S-Y 2008 Ferroelectric properties of Pb(Mn 1/3 Nb 2/3 )O 3 - Pb(Zr,Ti)O 3 thin films epitaxially grown on (001) MgO substrates J. Vac. Sci. Technol. A26(4) [10] K. Wasa, H. Adachi, M. Kitabatake 2004 Thin Film Materials Technology (Springer, William Andrew Pub., New York ) p.408. [11] V. Nagarajan,C.S. Ganpule, B. K. Nagaraj, S. Aggarwai, S. P. Alpay, A.L. Roytburd, E.D. Williams, R. Ramesh 1999 Effect of mechanical constraint on the dielectric and piezoelectric behaviour of epitaxial Pb(Mg 1/3 Nb 2/3 ) O 3 (90%)- PbTiO 3 (10%) Appl. Phys. Lett. vol [12] Wasa K, Nakamura K, Matsunaga T, Kanno I, Suzuki T, Okino H, Yamamoto T, Seo S.H, Noh D.Y 2006 Electromechanical coupling factors of single-domain 0.67Pb (Mg 1/3 Nb 2/3 ) PbTiO 3 single-crystal thin films Appl. Phys. Lett [13] Wasa K, Kanno I, Kotera H, Yamauchi N, Matsushima T 2008 Thin films of PZT-based ternary perovskite compounds for MEMS Proc. of 2008 IEEE International Ultrasonics Symp. p [14] Kanno I, Kotera H, Wasa K 2003 Measurement of transverse piezoelectric properties of PZT thin films Sensors and Actuators A [15] Kanno I, Fujii S, Kamada T, Takayama R 1997 Piezoelectric characteristics of c-axis oriented Pb(Zr,Ti)O 3 thin films Appl. Phys. Lett [16] Terada K, Suzuki T, Kanno I, Kotera H 2007 Fabrication of single crystal PZT thin films on glass substrates Vacuum [17] Zhang T, Wasa K, Zhang S-Y, Chen Z.J, Zhou F, Zhang Z, Yang Y 2009 High piezoelectricity of Pb(Zr,Ti)O 3 -based ternary compound thin films on silicon substrates Appl. Phys. Lett