Chapter/Index: Introduction | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Appendix
One of the main disadvantages of Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM) is their volatility, that is, the stored data is lost upon power failure. Commercial non-volatile memories have been Flash memory, Complementary Metal Oxide Semiconductors (CMOS) using battery as backup, and Electrically Erasable Programmable Read Only Memory (EEPROM). Based on the spontaneous polarization and its reversibility with the application of an external electric field in ferroelectric films, the ferroelectric random access memories (FRAMs), which are a type of non-volatile emerging memories, have been developed. In the FRAMs, the data is stored in a ferroelectric thin-film capacitor by localized polarization switching in the microscopic regions of ferroelectric thin films. The FRAMs are non-volatile because the polarization remains in the same state when the power is off. The upward and downward polarizations in FRAM's unit cells are referred to “0” and “1” state or vice versa. FRAMs also have other interesting features, e.g. a high-speed rewriting capacity. In practice, the macroscopic dipole moment, which induces the spontaneous polarization, in actual ferroelectric crystals is compensated by free surface charges, and thus there is no electric field outside the crystal. However, ferroelectrics insulators can physically be “switched” by applying an external electric field, resulting in an electrical current pulse between the two surfaces of the crystal. FRAM technology has practically been developed mainly for producing low-density products in which well-known large-feature-size CMOS technology is applied. Most FRAM technologies are based on 1T1C (1 transistor and 1 capacitor) [1-3] and 2T2C (2 transistors and 2 capacitors) architectures, in combination with COB (capacitor over bitline) and CUB (capacitor under bit-line) cell structures. Figure 3534 shows the FRAM 1T1C or 2T2C architectures.
There are still some challenges in applications of FRAM in industry: In practice, it is necessary that ferroelectric thin films for FRAMs have the characteristics below:
[1] D. J. Jung, N. S. Kang, S. Y. Lee, B. J. Koo, J. W. Park,
Y. S. Chun, M. H. Lee, B. G. Lee, S. I. Lee, T. E. Shim
and C. G. Hwang, VLSI Tech. Symp., p. 139 (1997).
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