ELECTRICAL AND RELIABILITY PROPERTIES OF PZT THIN-FILMS FOR ULSI DRAM APPLICATIONS

The electrical and reliability characteristics of ferroelectric capacitors fabricated using sol-gel derived 50/50 lead-zirconate-titanate (PZT) thin films have been examined for ULSI DRAM (dynamic random access memory) applications. Various electrode materials, film thicknesses (200 nm to 600 nm) and capacitor areas were used in this study. A large stored-charge density (Q(c)') of 15-mu-C/cm2 (at 125 kV/cm) was measured using different methods. It was found that Q(c)'. did not alter with either area or thickness for a given electric field, indicating encouraging prospects for use in ULSI DRAMs. Unipolar switching characteristics, studied using a simple RC circuit, were found to be pseudolinear with an electric field-dependent value of effective dielectric constant. Reasonable read-write times were projected for a 64 Mbit cell. The discharge times, measured with an electrometer, are large. Leakage current densities of PZT films were found to be much lower than that of other alternative dielectrics (e.g., a Ta2O5/SiO2 sandwich layer) when compared at a given charge-storage density (i.e., a fixed effective electric field). The breakdown field and time-to-breakdown (under constant voltage stress) degrade with decreasing thickness. Dopants such as Lanthanum are expected to reduce leakage. The films exhibit no aging effects, while unipolar (DRAM operation) fatigue in the 350 nm devices causes no degradation in Q(c)'. even after 10(13) cycles. The results indicate that PZT thin films exhibit material properties that might satisfy the requirements of ULSI DRAMs.