Sample Geometry Effects on Electric-Field-Induced Displacements in Piezoelectric Thin Films Measured by Atomic Force Mic
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C11.29.1
Sample Geometry Effects on Electric-Field-Induced Displacements in Piezoelectric Thin Films Measured by Atomic Force Microscopy Hirotake Okino, Hirofumi Matsuda 1 , Takashi Iijima1, Shintaro Yokoyama2, Hiroshi Funakubo2 and Takashi Yamamoto Department of Communications Engineering, National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan 1 Smart Structure Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba 305-8568, Japan 2 Department of Innovative and Engineered Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
ABSTRACT Electric-Þeld-induced displacements of PZT Þlm capacitor Pt/PZT(5µm)/Pt/SiO 2 /Si(100) were calculated by Þnite element method with various parameters of sample geometry: the diameter of top electrode φTE ranging from 0.2 µm to 1000 µm and whether PZT Þlm was continuous or side-etched. If φTE was larger than 40µm, surface longitudinal displacement (corresponding to AFM-measured strain) was not equal to net longitudinal displacement of PZT Þlm, including a contribution of the bending motion of substrate. In contrast, if φ TE was smaller than 4µm and PZT Þlm was continuous, effective d33 evaluated from net longitudinal displacement was smaller than intrinsic d33 , because the side PZT Þlm clamped the edge of the capacitor disk and prevented the whole disk from elongating longitudinally. It was also revealed that d 33 value calculated from net longitudinal displacement of PZT Þlm depended on the Poisson’s ratio of PZT and was not equal to intrinsic d33 , excluding the case that φTE was smaller than 4µm and PZT Þlm was side-etched. In conclusion, it is suggested that smaller φ TE (< 4 µm, in our case) and side-etch treatment permit a precision measurement of d33 ; however this condition is difÞcult to be satisÞed experimentally.
INTRODUCTION In 1990s, the motivation to develop ferroelectric random access memories propelled advances in thin Þlm processing for ferroelectric materials. Recently, using this progressed ferroelectric thin Þlm fabrication technology, many researchers have been trying to develop another devices including micro electromechanical systems (MEMS) devices such as micro-actuators, micro-transducers, and micro-pumps. In case of materials research for ferroelectric MEMS, it has been important to measure electric-Þeld-induced displacements and to evaluate piezoelectric constants (particularly longitudinal piezoelectric constant d 33 ) of ferroelectric Þlms. Currently, several techniques such as atomic force microscopy (AFM)[1–5] , interferometry[6–8] and direct methods[9] have been employed to evaluate the electric-Þeld-induced displacements. The double-beam interferometer[6] has been the most precision technique; however it has needed a well-skilled operator and some sample processing. Thus, among these techniques, AFM has been the most effective technique for materials research, because AFM is easy to operate and has sufÞcient
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