Static information storage and retrieval – Systems using particular element – Ferroelectric
Reexamination Certificate
1997-12-19
2001-06-12
Nelms, David (Department: 2818)
Static information storage and retrieval
Systems using particular element
Ferroelectric
C365S149000, C365S121000
Reexamination Certificate
active
06246602
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The entire disclosure of Japanese patent applications No. Hei 8-350046 filed on Dec. 27, 1996 and No. Hei 9-324143 field on Nov. 26, 1997 including specifications, claims, drawings and summaries are incorporated herein by reference in their entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a ferroelectric storage device, more particularly, elongation in the life of the ferroelectric storage device.
2. Description of the Earlier Technology
Ferroelectric memories having ferroelectric capacitors are known as nonvolatile semiconductor memories.
FIG. 12
shows a part of a conventional ferroelectric memory in a circuit diagram. The conventional ferroelectric memories comprise a ferroelectric capacitor
4
and a capacitor
6
for load.
FIG. 13
is a hysteresis curve showing a relationship between voltage and polarity state in the ferroelectric capacitor
4
, wherein the voltage means the potential of a bit line BL when voltage of a plate line PL
1
shown
FIG. 12
is used as a reference potential and the quantity of electric charge is used to show the level of the polarity state.
In
FIG. 13
, a first polarity state P
1
means a state wherein the capacitor
4
has remanence Z
1
and a second polarity state P
2
means a state wherein the capacitor
4
has remanence Z
2
, the remanence Z
1
corresponding to data “H” and the remanence Z
2
corresponding to data “L”. Data can be read out of the ferroelectric capacitor
4
by examining which of these polarity states the capacitor
4
has.
To examine which of these polarity states the ferroelectric capacitor
4
has, a voltage Vf is measured which occurs in both ends of the ferroelectric capacitor
4
when a read-out voltage Vp is applied to the plate line PL, after electricity is discharged from the capacitor
6
for a load shown at FIG.
12
and then the bit line BL is made into a floating state.
According to the graphical solution in
FIG. 13
, when electrostatic capacity of the capacitor
6
for load respects the slope of the line L
1
the voltage Vf occurring in both ends of the ferroelectric capacitor
4
is “V
1
” if the ferroelectric capacitor
4
has the first polarity state P
1
and the voltage Vf is “V
2
” if the second polarity state P
2
. Therefore, when a reference voltage is predetermined as shown in
FIG. 13
it can be examined which of these polarity states the ferroelectric capacitor
4
has by making a comparison between the voltage Vf occurring in read-out operation and the reference voltage Vref.
Data associated with the polarity state can be read out by means of examining the polarity state of the ferroelectric capacitor
4
in this way.
Such a conventional ferroelectric memory as described above, however, has the following problems. There is a bad electric property (which is referred to as “imprint effect”) in ferroelectric capacitors wherein the hysteresis curve distorts when the same polarity state is held for long time.
As a result of that, when a long time passes with the same data being stored in the ferroelectric capacitor
4
this ferroelectric capacitor gets the bad electric property. On getting the bad electric property, there changes the voltage Vf occurring in the both ends of the described-above ferroelectric capacitor
4
. In particular, when the opposite data has been written into the ferroelectric capacitor
4
having the bad electric property the opposite data can not be read out correctly. That is, as time passes the function of ferroelectric memory storage deteriorates.
SUMMARY OF THE INVENTION
An object of this invention is to provide a ferroelectric storage device which has long life in use as well as to solve such a problem.
A ferroelectric storage device according to the present invention comprises a ferroelectric storage cell which is capable of holding a polarity state to store data, the polarity state being associated with the data,
wherein the association between the data and the polarity state is changeable on a switch signal.
A ferroelectric storage device according to the present invention comprises a ferroelectric storage cell which is capable of holding a first polarity state or a second polarity state the direction of which is different from that of the first polarity state, the first and second polarity states being associated with two kinds of data to be stored,
wherein on a certain switch signal the polarity state of said ferroelectric storage cell is changed and a certain number of write operations or read-out operations are performed on said ferroelectric storage cell so as to further move the polarity state in the direction of the changed polarity and then the ferroelectric storage cell is changed back into the previous polarity state.
A controlling device for controlling a ferroelectric storage device according to the present invention is characterized in that the ferroelectric storage device comprises a ferroelectric storage cell which is capable of holding a polarity state to store data, the polarity state being associated with the data, wherein the association between the data and the polarity state is changeable on a certain switch signal.
A controlling device for controlling a ferroelectric storage device according to the present invention is characterized in that the ferroelectric storage device comprises a ferroelectric storage cell which is capable of holding a first polarity state or a second polarity state the direction of which is different from that of the first polarity state, the first and second polarity states being associated with two kinds of data to be stored,
wherein on a certain switch signal the polarity state of said ferroelectric storage cell is changed and a certain number of write operations or read-out operations are performed on said ferroelectric storage cell so as to further move the polarity state in the direction of the changed polarity and then the ferroelectric storage device is changed from a “non-reversal-of-operation” state to a “reversal-of-operation” state or from a “reversal-of-operation” state to a “non-reversal-of-operation” state, the “non-reversal-of-operation” state being a state wherein write operations input data is written into the ferroelectric storage cell as it is and in read-out operations read-out data is output as it is, and, the “reversal-of-operation” state is a state wherein write operations input data is written into the ferroelectric storage cell as it is reversed and in read-out operations read-out data is output as it is reversed.
A controlling device for controlling a ferroelectric storage device according to the present invention is characterized in that the ferroelectric storage device comprises a ferroelectric storage cell which is capable of holding a first polarity state or a second polarity state the direction of which is different from that of the first polarity state, the first and second polarity states being associated with two kinds of data to be stored,
wherein on a certain switch signal the polarity state of said ferroelectric storage cell is changed, and, a certain number of write operations or read-out operations are performed on said ferroelectric storage cell so as to further move the polarity state in the direction of the changed polarity and then the ferroelectric storage cell is changed back into the previous polarity state.
A storage mediums according to the present invention is a computer readable storage medium storing an executable computer program which is used to control a ferroelectric storage device comprising a ferroelectric storage cell which is capable of holding a polarity state to store data, the polarity state being associated with the data,
wherein the program makes a computer change the association between the data and the polarity state on a certain switch signal.
A storage medium according to the present invention is a computer readable storage mediums storing an executable computer program which is used to control a ferroelectric storage device comprising a ferroelectric storage cell which is capabl
Lam David
Merchant & Gould P.C.
Nelms David
Rohm & Co., Ltd.
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