In the downsizing the Metal-SiO2-SiN-SiO2-Semiconductor (MONOS) type Flash memory, charges are stored in further localized area of the cell transistor. Then, a high-resolution evaluation measurement is necessary to detect such localized charges. The charges stored in the gate dielectric thin film induce a permittivity change in the film. We have succeeded in clarifying the position where electrons/holes are located in the gate thin SiO2-SiN-SiO2 film of the MONOS memory. Following our previous work (ICN+T2006,605), we succeeded in visualizing the charge distribution to measure a higher-order nonlinear dielectric constant by using Scanning Nonlinear Dielectric Microscopy (SNDM). SNDM is one of the microwave microscopy measurement techniques using an AFM where a ring electrode is used in conjugation with a cantilever. Alternating electric field Ecosωpis biased between this electrode and the sample, the capacitance variation of the surface region of the sample is detected. The capacitance variation ΔCs(t) is, ΔCs(t)/Cs0 ~ ε333/ε33 Ecosωp+ε3333/4ε33 E2cos2ωp+higher order.In this formula, ε33 is a linear dielectric constant, ε333 is the lowest-order and ε3333 is 2nd order nonlinear dielectric constant. The ε333/ε33 and ε3333/ε33 can be obtained by detecting cosωp (ωp), and cos2ωp (2ωp) components of SNDM signals, respectively. From the higher order signal, we can obtain higher-resolution SNDM image, because the electric field of higher order dump faster than that of lower order, then it penetrates into limited area.
The SNDM images of electrons/holes in a MONOS memory can be interpreted as that of the capacitance variation (dC/dV) of the polarization of the electron-hole pair, which dC/dV is determined from the ωp component. On the other hand, in the higher-order SNDM image, it is deferent from ωp case. The 2ωp component contains d2C/dV2 and higher order non-linear dielectric constant of the ONO film. The higher order SNDM image shows the polarization of the electron-hole pair, since d2C/dV2 is not 0 due to the flat band voltage shift of the electron-hole pair system. Therefore the stored charge in ONO film can be visible in high resolution. As mentioned above, SNDM is a powerful method for visualizing charge spatial location in a dielectric thin film. |