Kye-Nam Lee

Performance and Reliability of Low-Temperature Processed SrBi2Ta2 O 9 Capacitors for FeRAM Applications

Low-temperature processed SrBi 2 Ta 2 O 9 (SBT) capacitors were tested as the ferroelectric memory cells of fully functional ferroelectric random access memory (FeRAM) devices. The 100 nm thick SBT films were deposited by the spin-on coating technique using a metallorganic decomposition source and crystallized by rapid thermal annealing at 700°C for 1 min followed by a furnace annealing at 650°C for I h under oxygen atmosphere, considered a low thermal budget process. The fabricated Pt/SBT/Pt capacitors showed reasonable ferroelectric performances with a AP (switching polarization-nonswitching polarization) of approximately 10 μC/cm 2 after the full process integration. The FeRAM chip-level reliability analysis showed that the major reason for the function failure was from the opposite state retention characteristics due mainly to the small AP values. A 10-year guaranteed lifetime can be achieved when the operation voltage is higher than approximately 4 V at the test condition of 85°C operation and 125°C storage.

Noble FeRAM technologies with MTP cell structure and BLT ferroelectric capacitors

A 16 Mb 1TIC FeRAM with a novel cell structure has been successfully developed with 0.25 /spl mu/m process technology using (Bi,La)/sub 4/Ti/sub 3/O/sub 12/ (BLT) capacitors for the first time. The developed FeRAM is highly scalable and reliable as a result of applying an MTP (merged top electrode and plate line) structure and BLT stacked capacitor, respectively.

A Current-Gain Scheme for High Density and Low Voltage FeRAM

The proposed current-gain scheme provides a key technical solution for a high density, low cost and high performance ferroelectric random access memory. The proposed sensing scheme shows maximum sensing-signal window because of divided sub-bitline (SBL) structure. The unit cell array section is composed of the cell array of 64 rows and 128 columns with SBL, SBL switch (SBSW) devices and current-gain transistor (CGT) device. The global main bitline (MBL) is biased by MBL sensing load (MSL) device and connected to common MBL bus (CMB) through block selection switch (BSS) device. The device sizes of CGT and MSL devices are key factors for determining the transfer characteristics of SBL and MBL. The 128 sense amplifiers in peripheral circuit region are shared to all cell array blocks through CMB with 128 MBL columns of each cell array block. The address access time of the 16 Mb chip is evaluated to less than 70 ns at 3 V.

Retention Characteristics of (Bi, La)4Ti3O12-Based FeRAMs

We have investigated the retention characteristics of ferroelectric random access memories (FeRAMs) using a bismuth layered perovskite, (Bi, La)4Ti3O12 (BLT). Due to the charge retention loss in ferroelectric storage capacitors, the sensing signal margin in FeRAM devices decreases from 920 mV at the retention time of 0.01 s to 770 mV at 100 s. From the logarithmic time dependence of the loss of obtained cell signals, the average margin of 520 mV is expected to remain after 10 y at 90°C. In addition, the slower retention loss rate in ferroelectric capacitors in long time retention region, suggests the high device reliability against retention degradation.

Study on the Variations of Microstructures and Domain Structures of Bi3.35La0.85Ti3O12 Ferroelectric Thin Films Formed by Two-Step Rapid Thermal Annealing (RTA) Process Utilizing Piezoresponse Force Microscope (PFM)

Recently, La-substituted Bi4Ti3O12 (BLT) has been widely studied as a candidate material for FeRAM due to its superior properties like high fatigue endurance with relatively lower crystallization temperature. In this study, we attempted to examine ferroelectric properties of various BLT thin films that were made using two-step rapid thermal annealing (RTA) process. The microstructure of BLT thin film is appeared as a critical factor to maximize ferroelectric properties. We found that the 2nd annealing temperature in two-step RTA process played an important role in determining the crystalline orientation of BLT thin films. Grain orientations of the BLT thin films were interpreted based on x-ray diffraction (XRD) in conjunction with piezoresponse images that were obtained utilizing piezoresponse force microscope (PFM). In addition, the results were correlated with the hysteresis loops.

Ferroelectric Properties of Lead-Free (Bi,La)4Ti3O12 Thin Film Deposited on MTP Cell Structure for High Density FeRAM Device

Ferroelectric properties of Pb-free (Bi,La)4Ti3O12 (BLT) films were optimized on a newly developed MTP cell structure. BLT films were coated on Pt/IrOx/Ir bottom electrode using sol-gel solutions. The composition of the optimized BLT film was about Bi3.25La0.75Ti3.0O12, which was analyzed by ICP-MS method. The switchable polarization obtained in a 100nm-thick BLT film was about 20 uC/cm2 at the 3 V applied voltage, and the optimized BLT film showed little fatigue loss about 10% up to 1×1011 cycles. The imprint properties of the BLT film were also characterized at 25 °C and 90 °C operating temperature after 125 °C data storage. Regardless of operating temperature, switchable polarization of BLT had a sufficiently large margin for device operation up to 10 years.