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

扫描探针显微镜已经揭示了极化疲劳的起源和改善用于存储器件和MEMS致动器的铁电薄膜的机制。尚未建立用于在器件操作期间观察畴的具有纳米级分辨率的观察方法。本研究将超声波原子力显微镜（UAFM）在纳米尺度上测量弹性性质的能力与铁电材料表面制备的窄间隙表面电极对（SEP）相结合，建立了在器件运行过程中对铁电材料在实际环境中的性能进行评估的技术。我们采用镁酸铅-钛酸铅单晶（Pb（Mg_<2/3>Nb_<1/3>）O_3-PbTiO_3：PMN-PT）作为样品，通过沿沿着一定晶向极化，显示出显著的高压电系数。结果表明，尽管由于居里温度较低（130℃），在真空沉积SEP的Au/Cr薄膜过程中，样品的最顶表面因加热而去极化，但首次发现SEP的电场可以驱动铁电畴。然后，通过UAFM观察去极化结构的畴边界，发现其相对于观察平面在表面下倾斜小角度。压电响应力显微镜通常用于铁电畴结构的成像，但在高介电常数的PMN-PT中，亚表面的成像是困难的，而UAFM可用于分析亚表面缺陷的三维结构。结果表明，SEP电场的应用对于利用UAFM无损评价铁电畴结构是有效的。从这些结果中，建立了使用SEP进行铁电材料UAFM评估的基础，从而达到了最初预期的目的。

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.