Fabrication of Piezoelectric Diaphragm Using Lead Zirconate Titanate (PZT) Films
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Fabrication of Piezoelectric Diaphragm using Lead Zirconate Titanate (PZT) Films E. Hong1, S.V. Krishnaswamy1,2, C.B. Freidhoff2 and S. Trolier-McKinstry1 Materials Research Institute, The Pennsylvania State University, University Park, PA 16802 2 Northrop Grumman Electronics Systems, Baltimore, MD 21203 1
ABSTRACT Piezoelectric diaphragms were fabricated using bulk micromachining. The diaphragms had a unimorph structure, where Pb(Zr0.52Ti0.48)O3 (PZT) and thermally grown silicon oxide (SiO2) films were used as the active and passive layers, respectively. To actuate the diaphragms, two modes were designed: d31 and d33-mode. For d31-mode diaphragms, a Si wafer with Pt/Ti/SiO2 (0.5 µm) was coated with ~1.2 µm PZT. A Cr /Au top electrode was then evaporated. Each layer including the bottom electrode was patterned into a circular shape. To fabricate d33-mode diaphragms, a Si wafer with thermal SiO2 (0.5 µm) was coated with ~0.3 µm ZrO2 and ~1.6 µm of PZT. On top of these layers, a Cr/Au top electrode was deposited and patterned into a ringshaped interdigitated transducer. Finally, both d31 and d33-mode diaphragms were released using deep reactive ion etching. Diameters of the fabricated diaphragms were in the range of 600 µm and 1000 µm. For d31-mode diaphragms, the dielectric constant and loss of the released piezoelectric layer at 1 kHz were > 800 and < 2%, respectively. The remanent polarization was ~20 µC/cm2 and the coercive field was ~61 kV/cm. Ferroelectric measurements showed welldeveloped hysteresis loops for the d33-mode diaphragms. Both d31 and d33-mode diaphragms behave as membranes rather than plates. Their measured resonance frequencies were consistent with calculations from an analytic model for circular membranes and ANSYS finite element analysis. INTRODUCTION Piezoelectric materials have been incorporated into many microelectromechanical systems (MEMS) to provide sensing and actuation functions [1]. To actuate cantilever, bridge and diaphragm structures, unimorphs, consisting of a piezoelectric layer and a passive layer, are usually adopted. In many cases, a d31-mode has been used, where a piezoelectric layer is sandwiched between two metal electrodes. The piezoelectric film is then poled and driven through its thickness. The strain generated through the d31 coefficient results in flexure of the structure. In contrast, d33-mode can be also considered. Here, a piezoelectric layer is poled parallel to the passive layer using an interdigitated transducer (IDT). When an electric field is applied to the IDT, the strain generated by d33 coefficients results in the bending of the structure. In many perovskite ferroelectric materials, the d33 constant is two times larger than d31, and hence the interest in the d33-mode in our study. Originally, this idea was proposed by Kugel et al. for bulk PZT actuators [2]. In addition, Xu et al. reported the sensing properties of PZT films deposited on silicon wafer using this mode [3]. Until now, there are few reports on d33-mode actuated micromachined devices. In this st
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