d31 piezoelectric coefficient of as-grown films was measured using piezoelectrically actuated cantilevers. The results indicate that films annealed after each ...
Integrated Ferroelectrics, 63: 49–54, 2004 C Taylor & Francis Inc. Copyright ISSN: 1058-4587 print/ 1607-8489 online DOI: 10.1080/10584580490458603
Integrated PNZT Structures for MEMS Gyroscope CORINA NISTORICA, JIAN ZHANG, P. PADMINI, SUSHMA KOTRU, and R. K. PANDEY Electrical & Computer Engineering Department, The University of Alabama, Tuscaloosa, AL 35487-0286, U.S.A (Received August 8, 2003; in final form January 5, 2004) The effect of processing on the ferroelectric and piezoelectric properties of solgel grown Pb1.1 Nb0.04 Zr0.2 Ti0.8 O3 (PNZT) thin films was investigated. The effective d31 piezoelectric coefficient of as-grown films was measured using piezoelectrically actuated cantilevers. The results indicate that films annealed after each layer possess a significant internal field, leading to high piezoelectric coefficients in the as-grown PNZT films. But the films annealed in the final step possess a very small internal field, and, consequently, small piezoelectric response. Keywords: PNZT films; asymmetry; poling; piezoelectric; ferroelectric
INTRODUCTION Based on their high piezoelectric response, lead zirconate titanate (PZT) thin films are natural candidates for microsensors and microactuators. Usually, PZT composition is chosen near the morphotropic phase boundary. However, the piezoelectric properties of bulk ceramics can be optimized by addition of dopants [1]. Only a few papers have been published regarding Nb doped PZT in thin film form [2, 3].
EXPERIMENTAL Sol-gel thin PNZT films were prepared by spin coating the Pb1.1 Nb0.04 Zr0.2 Ti0.8 O3 solution onto platinized silicon substrates at 3000 rpm for 30 seconds. The substrates used were (100) Si wafers with a 150 nm thick (111)-Pt bottom electrode. Multiple coatings were used to [561]/49
50/[562]
C. NISTORICA et al.
prepare films in the thickness range of 400–1200 nm. The coated substrates were dried at 400◦ C for 10 minutes and annealed at higher temperature in flowing oxygen. Two different schemes for annealing were used: annealing after each coating at 700◦ C for one minute (called layer-by-layer annealing) and a final annealing at 650◦ C for 15 minutes after all layers were deposited and dried (called final annealing). The films were annealed in a tube furnace through direct insertion. The area of the top electrodes used for electrical characterization was 0.091 mm2 while an electrode area of 6 mm2 was used for the piezoelectric measurements. The density of the films crystallized in one final step was lower (120 nm per layer) than in the case of the layer-by-layer annealed films (100 nm per layer). This can amount to differences in the mechanical, dielectric and ferroelectric properties of the films grown using the two methods. The polarization versus electric field (P − E) measurements were performed at a frequency of 1 kHz. The evolution of the ferroelectric polarization and coercive fields was determined as a function of applied electric field for the two types of processing undergone by the PNZT films. Films that were crystallized in a final annealing step presented symmetrical P − E loops, with similar values of the coercive fields for positive and negative bias as shown in Figs. 1 and 2. This indicates that internal bias fields were not present in these films. However, when the PNZT films were annealed after each coating step, severe asymmetric switching behavior was observed due to a strong internal bias field, as shown in Figs. 3 and 4. The films were selfpolarized by an internal bias field upon cooling through the phase transition
FIGURE 1 P − E hysteresis loop for sol-gel PNZT films annealed in a final step.
INTEGRATED PNZT STRUCTURES FOR MEMS GYROSCOPE
[563]/51
FIGURE 2 Coercive voltages for sol-gel PNZT films annealed in a final annealing step.
temperature. The self-polarization of the films was very stable and subsequent room temperature poling did not increase the internal bias field or the piezoelectric response significantly. This property can be very useful for pyroelectric and piezoelectric applications since the poling can be avoided. The piezoelectric coefficient d31 was calculated using the measured deflection of cantilever beams actuated by the converse piezoelectric effect. Cantilevers with a thickness of 700 µm, width of 4 mm and a length of 28 mm
FIGURE 3 P-E hysteresis loop for sol-gel PNZT films annealed layer-by-layer.
52/[564]
C. NISTORICA et al.
FIGURE 4 Coercive voltages for sol-gel PNZT films annealed layer-by-layer.
were used. The piezoelectric characterization of the sol-gel films grown using the layer-by-layer annealing method, indicate that the as-deposited films had large spontaneous polarization which produced d31 piezoelectric coefficients between 17 pm/V and 52 pm/V. The large d31 coefficients for the as-grown films and the strong asymmetry in the poling behavior are related to the internal bias field in the film [4, 5]. Samples were also poled with the top electrode made either negative or positive. The applied electric field was two times the coercive field of the sample. For The PNZT films having strong internal bias fields, there exists a significant difference in the magnitude of the d31 coefficient obtained after poling with the top electrode positive compared to poling with the top electrode negative. Similar directional anisotropy in the d31 piezoelectric response has been reported previously for sputtered PZT thin films [6]. The PNZT films display a small increase in the magnitude of d31 when poled by a field having the direction parallel to the direction of the internal bias field, as shown in Fig. 5. This is due to the strong internal field, which poles the sample almost completely. When poled with the top electrode negative, the poling field was antiparallel to the internal field and the piezoelectric coefficient showed large changes. The piezoelectric coefficient decreased at first, since the poling field was switching the orientation of the domains, decreasing the polarization. The piezoelectric coefficient began increasing when the applied field was strong enough to increase the polarization in the opposite direction relative to the initial one.
INTEGRATED PNZT STRUCTURES FOR MEMS GYROSCOPE
[565]/53
FIGURE 5 Piezoelectric coefficient d 31 for sol-gel PNZT films annealed layer-bylayer.
In the case of films that were prepared using one final annealing step, the positive and negative poling produced symmetrical results as shown in Fig. 6. The piezoelectric coefficients obtained for the same amount of poling time were relatively lower compared to the piezoelectric coefficients of the films annealed after each layer.
FIGURE 6 Piezoelectric coefficient d 31 for sol-gel PNZT films annealed in a final step.
54/[566]
C. NISTORICA et al.
ACKNOWLEDGMENTS The authors thankfully acknowledge the generous support of this research by the U.S. Federal Aviation Administration (FAA) and the valuable help we received from our colleagues in the NSF-MRSEC Center for Materials for Information Technology (MINT), the Central Analytical Facilities (CAF), and the Laboratory for Electronic Materials and Device Technology (EMD Tech), all at the University of Alabama, Tuscaloosa, AL.
REFERENCES [1] B. Jaffe, W. R. Cook, and H. Jaffe, Piezoelectric Ceramics, London, Academic (1971). [2] W. S. Kim, S.-M. Ha, H. H. Park, and C. E. Kim, “The effects of cation-substitution on the ferroelectric properties of sol-gel derived PZT thin film for FRAM applications,” Thin Solid Films 355–356, 531–535 (1999). [3] T. Haccart, E. Cattan, and D. Remiens, “Evaluation of Niobium effects on the longitudinal coefficients of Pb(Zr, Ti)O3 thin films,” Appl. Phys. Lett. 76, 3292–3294 (2000). [4] A. L. Kholkin, K. G. Brooks, D. V. Taylor, S. Hiboux, and N. Setter, “Self-polarization effect in Pb (Zr,Ti)O3 thin films,” Integrated Ferroelectrics 525–533 (1998). [5] W. L. Warren, D. Dimos, G. E. Pike, B. A. Tuttle, M. V. Raymond, R. Ramesh, and J. T. Evans, Jr., “Voltage shifts and imprint in ferroelectric capacitors,” Appl. Phys. Lett. 67, 866–868 (1995). [6] J. F. Shepard, F. Chu, I. Kanno, and S. Trolier-Mc-Kinstry, “Characterization and aging response of the d 31 piezoelectric coefficient of lead zirconate titanate thin films,” J. Appl. Phys. 85, 6711–6716 (1999).
