Nondestructive evaluation of ferroelectric domain structure poled and driven by electric fields of surface electrode pairs by ultrasonic atomic force microscopy

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Scanning probe microscopy has revealed an origin of a polarization fatigue and a mechanism for improving ferroelectric thin films for memory devices and MEMS actuators. An observation method with a nanoscale resolution for observing domains during the device operation has not been established. In this study, ultrasonic atomic force microscopy (UAFM) that can measure elastic property on the nanoscale was combined with narrow gap of surface electrode pairs (SEP) fabricated on ferroelectric materials in order to establish the technique for the evaluation of ferroelectric materials in practical environment during the device operation. We used lead magnesium niobate - lead titanate single crystal (Pb(Mg_<2/3>Nb_<1/3>)O_3-PbTiO_3 : PMN-PT) as a sample that shows significantly high piezoelectric coefficient by poling in the direction along certain crystal orientation. As a result, an electric field of SEP was found to drive ferroelectric domain for the first time, although the topmost surface of the sample was depolarized by heating up during the vacuum deposition of Au/Cr thin film of SEP since low Curie temperature of 130℃. Then, the domain boundary of the depolarized structure was observed by UAFM, so that it was found to be tilted under the surface by small angle with respect to the observation plane. A subsurface imaging in PMN-PT with high dielectric constant is difficult in piezoresponse force microscopy that is generally used for imaging ferroelectric domain configuration, and it was demonstrated that UAFM is useful for analyzing 3D structure of subsurface defect. In conclusions, it was verified that the application of the electric field by SEP is useful for the nondestructive evaluation of ferroelectric domain structure by UAFM. From these results, the basis of UAFM evaluation of ferrolectrics using SEP was established, so that the purpose that was originally intended was achieved.