Retention behavior of ferroelectric memory devices depending on the capacitor processes

Retention characteristics of a ferroelectric random access memory (FRAM) devices are strongly influenced by the integration processes and the ferroelectric material itself. In this study, we investigated the retention behavior of the PZT-based 4Mbit FRAM devices. Various heat treatment conditions were introduced to crystallize PZT films and the effect of La-doping was also observed. The nonvolatile charge of the PZT capacitor generally decreased as the increase of the baking time and finally the device failed to maintain the data. The decay rate of a sample, which was obtained through fitting the measured polarization values to the stretched exponential equation, was closely related to the crystallization process of PZT. It was also confirmed that the amount of the polarization decay varied due to the write/read voltages and baking temperature. Additionally, 1/T dependence of the logarithmic decay rate gave the activation energies for the polarization decay. The obtained activation energies were in the range between 0.95 and 0.97 eV. This suggests that the polarization decay of the PZT capacitor seems to be related to the migration of the oxygen vacancy in the film.