Observation of nanometer-scale crystal grain orientation in ferroelectric thin films using polarization near-field scanning optical microscopy (NSOM)

Abstract

We developed an observation technique for crystal orientation in the nanometer-scale grain using polarization near-field scanning optical microscopy (NSOM), and applied it to Pb(Zr,Ti)O₃ (PZT) ferroelectric thin films. PZT is a ferroelectric RAM material. Because ferroelectric RAM cell sizes have become smaller and are now, being measured in the submicron scale range, the grain sizes in PZT that constitute the cells are about 100 nm. The observation for crystal grain orientation of such ferroelectric RAM cells has been difficult with current methods such as X-ray diffraction method or micro-Raman spectroscopy. PZT is a uniaxial crystal because of its tetragonal structure and we found that the birefringence retardation of PZT depends on its crystal grain orientation. The nanometer-scale grain was observed by NSOM, which is not limited by the diffraction limit of conventional optical microscopy. To achieve the observation of birefringence retardation, NSOM and polarization optical elements were integrated. For this integration, the optical compensation of polarized light was indispensable because a near-field probe in NSOM might show birefringence. Then, a polarization compensation method at the tip of the near-field probe was developed. Using this polarization NSOM, a new technique for observing the crystal grain orientation by birefringence retardation was developed.