Keisaku Nakao

Retention Characteristics of a Ferroelectric Memory Based on SrBi2(Ta, Nb)2O9

The polarization decay process in SrBi 2 (Ta, Nb) 2 O 9 capacitors and retention characteristics of a 288-bit ferroelectric memory device fabricated from SrBi 2 (Ta, Nb) 2 O 9 were studied. The remanent polarization decay at room temperature showed good linearity when plotted against logarithmic retention time over a wide range of 10 -3 -10 5 s. The distribution of times to failure of a 288-bit memory was fit to a model having a linear relationship between log(log t f ) and 1/T for the period of infant failures and to the Arrhenius model having the form log t f vs 1/T for the period of random failures, where t f is the time to failure and T is the temperature. The activation energy was found to be 0.35eV for infant failures and 1.15eV for random failures. Possible causes for the difference in activation energies are discussed.

Voltage Shift Effect on Retention Failure in Ferroelectric Memories

We investigated the origin of retention failure in ferroelectric memories (FeRAMs) with SrBi2(Ta, Nb)2O9 (SBTN) memory cell capacitors by considering the time-dependent behavior of polarization vs. voltage (P-V ) curves of the capacitors during high-temperature storage. Since the SBTN capacitors exhibited no marked decrease in the nonvolatile component of polarization even after high-temperature storage, we focused on the effect of voltage shift observed in P-V curves. We calculated bitline voltage along the storage from the P-V curves and the bitline capacitance, and successfully estimated a decrease in the bitline voltage, which is in agreement with the retention failure in FeRAMs. In addition, the calculation indicated that the lifetime limited by the retention failure in FeRAMs with SBTN capacitors at 125°C exceeds 10 years.

Thermal Aging Effect in Poled Ferroelectric SrBi2(Ta,Nb)2O9 Capacitors

Changes in the electrical properties of poled ferroelectric SrBi2(Ta,Nb)2O9 (SBTN) thin-film capacitors caused by high-temperature storage were studied. Current–voltage (J–V) characteristics of SBTN capacitors before and after high-temperature storage indicated that the current in SBTN is predominantly carried by electrons and limited by electrode interfaces. The voltage shift in the polarization–voltage (P–V) curve caused at high temperatures was ascribed to a bulk effect because there were no definite changes in the interface-limited J–V characteristics before and after high-temperature storage. Assuming the pinning of domains by capturing electrons emitted from traps distributed in the energy gap, we describe the decay in switchable polarization with the power of time. The activation energy for the decay in switchable polarization associated with electron capture was determined to be 0.23 eV based on the temperature dependence of the decay in switchable polarization.

Observation of Light Scattering in Lithium Niobate

Results of experiments on the dynamics of scattering in a quasi-c-cut plate of LiNbO3:Fe crystal are presented. With the incident laser beam along the +c direction, the back and forward-scattered light forms many spots and rings after a few seconds of illumination. The pattern is caused by the inter-reflection and photorefractive effect, and are explained with reference to theories of degenerated four-wave mixing and anisotropic light scattering in photorefractive materials. No scattering pattern is obtained when the incident beam is in the opposite direction.

Ferroelectric Random Access Memory (feram) Technology

There has been considerable focus dedicated in recent years to ferroelectric materials for nonvolatile memory applications. With ferroelectric memories (FeRAMs) rather than conventional memories, high-speed writing, low dissipation, power, and high endurance can be achieved for the first time. Several attempts are made to make ferroelectric nonvolatile memories in the 1950s and 1960s. Unfortunately, these efforts were not successful because the ferroelectric material used in these memories experienced serious problems, including fatigue, imprint, and degradation because of the integration process. Due to these material issues, the first commercial FeRAMs were not available until almost 30-year later. In this chapter, some of the major breakthroughs in FeRAM technology are discussed highlighting, material issues, the integration process, different circuit designs, and some very recent achievements in the industry. Thus, with some of the improved processing techniques described in the foregoing, it is believed that high-density FeRAMs will soon be available as they are clearly within the scope of current technology.