Nanoscale electromechanical phenomena in ferroelectric thin films
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Nanoscale electromechanical phenomena in ferroelectric thin films
C. S. Ganpule, A. L. Roytburd, V. Nagarajan, A. Stanishevsky, J. Melngailis, E. D. Williams, and R. Ramesh Materials Research Science and Engineering Center, University of Maryland, College Park, MD 20742
ABSTRACT
Focused ion beam milling was used to fabricate ferroelectric islands in Pb-Zr-Ti-O thin films. The islands ranged in size from 200µm×200µm to 0.3µm×0.3µm. The inverse piezoelectric effect was studied in these islands as a function of their size by tracking the surface displacement of the top electrode of the island (under an applied electric field) using an atomic force microscope (AFM). It was found that there was a substantial increase in the piezoresponse as the size of the island decreased below 100µm×100µm. This increase was attributed to the elastic deformation of the substrate.
INTRODUCTION
Ferroelectric (FE) films are increasingly being used as micro-actuator elements in micro-electromechanical systems (MEMS). In such applications it is desirable to get the maximum surface displacement in the ferroelectric film-substrate couple for a given applied electric field. Constraints induced by the substrate generally decrease the piezoresponse of films [1,2]. This is true if one considers the electrically induced strains in the film. However, it is possible to get surface displacement of the film-substrate couple, which is comparable or larger than that of a free-standing film. This is due to the elastic deformation of the substrate and its contribution to the film-substrate surface displacement.
EXPERIMENTAL DETAILS
Experimental evidence of the important role of a substrate follows from atomic force microscopy (AFM) investigation of the piezoresponse of delineated islands of a ferroelectric film as a function of its in-plane size. Pb1.0(Nb0.04Zr0.28 Ti0.68 )O3 (PNZT) thin films with a thickness of 1600Å and grain size of ~1000 Å were deposited by solgel processing onto platinized Si wafers with a bottom electrode of the conducting oxide, La-Sr-Co-O. The films had a symmetric electrode structure with top electrodes of Pt on La-Sr-Co-O. A strong structural orientation with the preferred c-axis along the normal to the films was observed. Test capacitor structures in the size range of 200µm -0.3µm were CC1.5.1
fabricated by focused Ga ion beam milling [3,4]. This structure in which the ferroelectric layer surrounding it was milled out to produce an island like geometry is shown in figure 1. As a comparison, structures with similar size but with only the top electrode patterned by focused ion beam milling were produced, as illustrated in figure 2.
Figure 1. Scanning ion beam image of ferroelectric PNZT island milled by focused ion beam milling (size of islands: (a) 0.8µm x 0.8µm, (b) 0.4µm x 0.4µm, (c) 0.25µm x 0.25µm and (d) 0.07µm x 0.07µm).
An accelerating voltage of 50kV, dose of 8x1017 ions/cm2 and a beam size of 6-20nm (MICRION 2500) was used to delineate the structures. As milled structures showed ionbeam induced damage but a 6
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