Static information storage and retrieval – Systems using particular element – Ferroelectric
Patent
1999-01-29
2000-12-05
Nelms, David
Static information storage and retrieval
Systems using particular element
Ferroelectric
365149, G11C 1122
Patent
active
061575631
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a memory system included in electronic equipment, in particular, a ferroelectric memory system including a memory cell provided with a ferroelectric capacitor.
BACKGROUND ART
Recently, a ferroelectric memory system is proposed in which a capacitance film of a ferroelectric material is disposed in a capacitor of a memory cell so as to make stored data nonvolatile. A ferroelectric material is a material having a characteristic of polarization changed in accordance with a change in the polarity of a voltage (an electric field) in a hysteresis loop as is shown in FIG. 20. Specifically, in a ferroelectric material, as an applied voltage (electric field) is increased, the polarization is increased along a change curve and reaches a saturation polarization value at a point A, and as the voltage (electric field) is decreased, the polarization is gradually decreased not through the above-described change process, and even when the electric field becomes 0, the polarization does not become 0 but residual polarization at a point B remains. When a negative electric field is applied to a ferroelectric material and the electric field is increased in the negative direction, a saturation polarization value is attained at a point C, and when the electric field is decreased to 0, residual polarization at a point D remains. In this manner, a ferroelectric material has a characteristic that residual polarization remains in accordance with the strength and the polarity of an electric field applied theretofore, namely, the so-called hysteresis characteristic. At this point, a saturation polarization value corresponds to a point where the two change curves in the hysteresis curve of FIG. 20 become substantially the same curve, namely, a point where these curves are substantially in contact with each other. Furthermore, the behavior of polarization inversion depends not upon a voltage but upon an electric field given as a value obtained by dividing a voltage by a film thickness, but in the following description, the thickness of a ferroelectric film is fixed, and hence, an operation characteristic in accordance with a voltage will be described.
Therefore, when a ferroelectric capacitor including a ferroelectric film sandwiched between two conductive films is provided in a memory cell and residual polarization of the ferroelectric film in accordance with the polarity and the amplitude of a signal voltage is used as stored data, the stored data can be nonvolatile, and thus, the so-called nonvolatile memory system can be realized.
For example, U.S. Pat. No. 4,873,664 discloses the following two types of ferroelectric memory systems:
In the first type of the nonvolatile memory system, a memory cell includes one transistor and one ferroelectric capacitor (1T1C) per bit. In this case, one dummy memory cell (reference cell) is provided to, for example, every 256 main memory cells (normal cells).
In the second type of the nonvolatile memory system, no dummy memory cell is provided and a memory cell includes two transistors and two ferroelectric capacitors (2T2C) per bit. In this case, a pair of complementary data are stored in a pair of ferroelectric capacitors.
Furthermore, as is disclosed in, for example, U.S. Pat. No. 4,888,733, a memory cell can include two transistors and one ferroelectric capacitor (2T1C) per bit.
Moreover, as a ferroelectric material used in a ferroelectric capacitor, KNO.sub.3, PbLa.sub.2 O.sub.3 --ZrO.sub.2 --TiO.sub.2, PbTiO.sub.3 --PbZrO.sub.3 and the like are known. Furthermore, PCT International Publication No. WO93/12542 discloses a ferroelectric material suitable to a ferroelectric capacitor used in a ferroelectric memory system that is much less fatigued than PbTiO.sub.3 --PbZrO.sub.3.
On the other hand, ferroelectric materials generally used at present cannot be free from fatigue, and when they are used for a long period of time, their ability to store data can be spoiled.
Therefore, as is disclosed in U.S. Pat. No. 5,532,953, in order not to spoil the
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Hirano Hiroshige
Takeo Masato
Auduong Gene N.
Matsushita Electronics Corporation
Nelms David
Robinson Eric J.
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