| Summary: | 碩士 === 國立交通大學 === 電子物理系 === 90 === Non-volatile ferroelectric random access memories (FeRAMs) have the potential to replace current state-of-the-art non-volatile memories such as flash and EEPROMs because of their lower power consumption, lower writing voltage, faster writing speed, and better endurance. Such attributes make this technology very attractive for applications such as pagers, cellular phones, and smart cards. However, the ferroelectric properties and their reliability are still of major interest.
The main theme of this work will focus on the reliability concerns in ferroelectric capacitors. Besides reviewing the most investigated subjects such as fatigue, imprint and aging, we intensively investigate the mechanisms for the polarization loss including polarization relaxation and polarized pulse effect. To evaluate the relaxation effect on bitline sense voltage, a SPICE compatible polarization relaxation model has been developed. A relaxation voltage source, responsible for polarization relaxation effects, is incorporated into the conventional ferro-capacitor model. The sense voltage in a 2T/2C cell is significantly reduced from the relaxation model. The relaxation effect on minimum sense voltage attributed to capacitance mismatch in a 1T/1C cell is also evaluated. As opposed to the result in the cell structure of 2T/2C, the worst case occurs when polarization is not relaxed at all.
Polarization charge loss in PZT ferroelectric memory under low-voltage and high-speed operation is observed. This effect significantly reduces the sensing margin and causes severe reliability issue in advanced ferroelectric memory, particularly for low-voltage applications. This polarization loss is attributed to slowing-down of polarization caused by band bending from Schottky potential at the electrode/ferroelectric interface. A solution to eliminate the polarization loss is proposed and verified.
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