JPH06169070A - Application method of ferroelectric memory element - Google Patents

Abstract

PURPOSE:To prevent the reduction of a spontaneous polarization in a ferro- electric memory element while information is maintained. CONSTITUTION:The electrodes 3 and 3 of the respective memory cells 4 of a ferroelectric memory device 1 are connected to each other with a short- circuiting circuit 8 while it is neither in a writing period nor in a reading period but in an information holding period. The short-circuiting circuit 8 has an impedance lower than an insulation resistance value between the electrodes 3 and 3 of the respective memory cells 4 and the electrodes 3 and 3 are short-circuited by the short-circuiting circuit 8, so that a reverse polarization field which has a direction opposite to the direction of the spontaneous polarization in the memory cell 4 can be eliminated.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method of using a ferroelectric memory device. Specifically, the present invention relates to a method of using a ferroelectric memory device that stores information by reversing the direction of spontaneous polarization of a ferroelectric substance.

[0002]

BACKGROUND ART A ferroelectric memory device has been studied as a highly integrated nonvolatile memory device. A ferroelectric memory device usually includes a plurality of memory cells, and the memory cell is composed of a ferroelectric thin plate and a pair of electrodes formed by sandwiching the ferroelectric thin plate.

Therefore, a polarization process is performed by applying an electric field of a predetermined strength between the electrodes, and the polarization direction of the ferroelectric portion of each memory cell is oriented in the direction parallel to the direction of the electric field to spontaneous polarization. Information can be stored by storing the polarization direction of the.

[0004]

However, in the conventional ferroelectric memory device, the spontaneous polarization value (residual polarization value) of the ferroelectric thin plate decreases during the storage of written data (information). However, there is a problem that the ferroelectric memory device does not exhibit its function sufficiently. As a result of diligent studies by the inventors of the present invention, it has been clarified that this cause is due to pyroelectric charge generated on the surface of the ferroelectric thin plate due to temperature change during storage of the ferroelectric memory element.

That is, when the ferroelectric memory element is once brought into a high temperature state and then returned to room temperature, pyroelectric charge is generated on the surface of the ferroelectric thin film, and the pyroelectric charge causes the ferroelectric thin film to be formed. The electric field Ep in the direction opposite to the direction of spontaneous polarization P in 31
Cause 4 (a), (b) and (c) are explanatory views showing the mechanism of this pyroelectric charge generation.
Shows the state after the ferroelectric thin film 31 is polarized, and the dipoles 32 in the ferroelectric thin film 31 are aligned in a certain direction to generate spontaneous polarization P. At this time, in order to cancel the surface charge due to the polarized dipole 32, the unipolar charge 33 is formed in the surface layer inside (the crystal lattice) of the ferroelectric thin film 31.
Is occurring. Now, when the ferroelectric thin film 31 is brought into a high temperature state due to a change in ambient temperature, the size of the dipole 32 decreases, but the unipolar electric charge 33 is difficult to move and the response is poor. As shown, the spontaneous polarization P becomes small and the unipolar charge 33 becomes excessive. In this way, when the electric charge 33 of the surface layer becomes excessive, air charge, ions, etc.
Is attracted to the surface of the ferroelectric thin film 31,
It becomes the pyroelectric charge 34. After that, when the temperature of the ferroelectric thin film 31 is lowered again, the spontaneous polarization P is recovered again as shown in FIG. 4C, but the spontaneous polarization P is attached to the surface of the ferroelectric thin film 31. Since the pyroelectric charge 34 is not immediately discharged, the pyroelectric charge 34 causes an electric field Ep in the direction opposite to the direction of the spontaneous polarization P. As a result, the electric field Ep reduces the magnitude of the spontaneous polarization P of the ferroelectric thin plate 32.

The present invention has been made in view of the above-mentioned drawbacks of conventional examples, and an object of the present invention is to prevent reduction of the remanent polarization value during storage of written data. It is to provide a method of using a dielectric memory device.

[0007]

A method of using a ferroelectric memory according to the present invention comprises one or more memory cells each consisting of a ferroelectric and a pair of electrodes formed with the ferroelectric sandwiched therebetween. In a ferroelectric memory device that stores information according to the direction of spontaneous polarization of a ferroelectric substance of the memory cell, an impedance component lower than an insulation resistance value between electrodes of the memory cell is applied to the memory cell at least during a period of information storage. It is characterized in that it is connected between the electrodes.

[0008]

In the present invention, since the impedance component lower than the insulation resistance value between the electrodes of the memory cell is connected and stored between the electrodes of the memory cell, the polarization charge ( In particular, the charge moves through the impedance component so as to cancel the anti-polarization field (electric field Ep opposite to the spontaneous polarization) generated by the pyroelectric charge appearing on the surface of the ferroelectric due to the temperature change. Thus
Since the anti-polarization field is canceled by the electric charge attracted and moved by the polarization charge or the pyroelectric charge, the depolarization of the spontaneous polarization due to the anti-polarization field is prevented.

[0009]

1 is a block diagram showing a method of using a ferroelectric memory device according to an embodiment of the present invention. In this method of using a ferroelectric memory device, a ferroelectric body having a plurality of memory cells 4 each composed of a ferroelectric thin plate 2 and a pair of electrodes 3 formed by sandwiching the ferroelectric thin plate 2 is used. The memory device 1 is used.

The electrodes 3 and 3 of each memory cell 4 are provided with a switch 11 when the memory is stored except when writing or reading.
Via a short circuit (or low impedance circuit)
8 and is connected to the switch 11a,
Writing circuit 7 by switching the contact of 11b
Is connected to the reading circuit 9 by switching the contacts of the switches 11a and 11b at the time of reading.

The short circuit 8 includes both electrodes 3 of the memory cell 4.
Any one that short-circuits between the electrodes 3 and has an impedance lower than the insulation resistance value between the electrodes 3 of the memory cells 4 may be used.
A resistor having a resistance value lower than the insulation resistance value between them may be used, or only a conductive wire (for example, a silver wire) may be used.

When the write command is input from the input device 5, the control circuit 6 causes the switches 11a and 1
1b is controlled, and the electrodes 3 and 3 of each memory cell 4 are connected to the write circuit 7. Next, the forward and reverse voltages according to the write data are applied to the electrodes 3 and 3 of each memory cell 4, and the ferroelectric thin plate 2 between the electrodes 3 and 3 is spontaneously polarized in the direction according to the forward and reverse of the voltage. Data is stored according to the polarization direction.

When a read command is input from the input device 5, the electrodes 3, 3 of each memory cell 4 are connected to the read circuit 9, and the read circuit 9 causes each memory cell 4 to be read.
The normal and reverse directions of the spontaneous polarization of the
The data stored in is read. The read data is output from the output device 10, for example.

In the method of using the ferroelectric memory device of this embodiment, the short circuit 8 having an impedance lower than the insulation resistance value between the electrodes 3 and 3 of the memory cell 4 is connected between the electrodes 3 and 3 of the memory cell 4. Since the data is saved while connected to
Pyroelectric charges appear on the surface of the ferroelectric thin plate 2 due to temperature change, and even if an anti-polarization field in the direction opposite to the spontaneous polarization occurs inside the ferroelectric thin plate 12, it is attracted to the pyroelectric charge and short-circuited. The electric charge moves through 8 and the electric charge having the opposite polarity to the pyroelectric charge is induced across the electrodes 3 and 3. As a result, the antipolarization field generated by the pyroelectric charge is canceled by the electric field generated by the induced charge, and it is possible to prevent the spontaneous polarization from being depolarized by the antipolarization field of the pyroelectric charge.

The method of using the ferroelectric memory device according to this embodiment is applicable to a bulk memory device using a ferroelectric thin plate or a thin film memory device using a ferroelectric thin film. However, the same result can be obtained. Further, the number, shape and arrangement of the memory cells of the ferroelectric memory element may be arbitrary. Further, the impedance component connecting the electrodes may be provided by any means capable of bypassing the charge between the electrodes of the memory cell.

Concrete Example In order to confirm the effect of the present invention, three memory cells 14,
A ferroelectric memory element 12 having 15 and 16 is manufactured,
Two of the memory cells 14 and 15 were used in the method of use of the present invention, and the other memory cell 16 was used in the conventional method of use, and the changes in the remanent polarization value Pr due to the temperature shock test were compared. The results are shown below.

Preparation of Sample FIG. 2A is a perspective view of the ferroelectric memory 12 prepared and used in this embodiment, and FIG. 2B is its sectional view.
First, a ceramic represented by (Pb _0.98 La _0.02 ) (Ti _0.49 Zr _0.51 ) _1-X O ₃ was fired and polished to form a ceramic thin plate (ferroelectric body) 13 having a thickness of 0.1 mm. Next, both sides of the ceramic thin plate 13 are covered with a mask and silver is vapor-deposited, and the electrodes 14a, 14b; 15a, 15b;
Pairs were formed to form three memory cells 14, 15 and 16. Electrodes 14a, 14b; 15a, 15b; 16a, 1
6b is a square having a length of 5 mm and a width of 5 mm.

Next, an electric field of 30 kV / cm is applied in the same direction between the electrodes 14a, 14b; 15a, 15b; 16a, 16b of the respective memory cells 14, 15, 16 so that the respective memory cells 14, 15, 16 are connected. It was polarized in the direction of the arrow in FIG. Immediately after polarization, the value of the spontaneous polarization P of each memory cell 14, 15, 16 was measured, and as shown in FIG.
Both have about 35 μcoul / cm ² and three memory cells 1
There was no difference between 4, 15 and 16.

Next, three memory cells 14, 15, 1
One of the six memory cells 14 has electrodes 14a and 14b.
Short the gap with a silver wire (impedance component) 17
One memory cell 15 has 10 MΩ between electrodes 15a and 15b.
The resistor (impedance component) 18 is connected. Also,
The remaining one memory cell 16 was opened without connecting the electrodes 16a and 16b. The memory cells 14, 15,
16 electrodes 14a, 14b; 15a, 15b; 16a,
The insulation resistance between 16b differs depending on the direction of the applied electric field,
1 when evaluated at a voltage of 10 V, which is sufficiently smaller than the polarization reversal electric field
It was × 10 ¹² Ω (however, the electric field direction and the spontaneous polarization direction were antiparallel).

As a result of the temperature shock test, the ferroelectric memory element 12 manufactured in this manner
After putting it in a temperature bath of 0 ° C. for 10 minutes, it was taken out and a temperature shock test was conducted by a method of allowing it to cool naturally to room temperature. Next, the value of the spontaneous polarization P of each memory cell 14, 15, 16 was measured.

The measurement results are shown in FIG. Electrodes 16a, 16
Spontaneous polarization P of the memory cell 16 (conventional example) in which the space b is released
The value of is about 10 μcoul / cm ² and the initial value is about 35 μcoul / cm ^2.
Much smaller than. On the other hand, the electrodes 14a,
The values of the spontaneous polarization P of the memory cell 14 [Example] in which 14b are short-circuited by the silver wire 17 and the memory cell 15 [Example] in which the electrodes 15a and 15b are connected by the resistor 18 are both about 30.
μcoul / cm ² was slightly smaller than the initial value of about 35 μcoul / cm ² . This shows that the present invention significantly improves the change in the value of the spontaneous polarization P due to temperature shock.

[0022]

According to the present invention, the anti-polarization field (spontaneous polarization) generated by the polarization charge appearing on the surface of the ferroelectric substance (in particular, the pyroelectric charge appearing on the surface of the ferroelectric substance due to temperature change). Since the information can be retained in the state where the electric field Ep) in the opposite direction is canceled, it is possible to prevent the depolarization of the spontaneous polarization during the information storage, and to improve the stability and reliability of the ferroelectric memory device over time. Can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a method of using a ferroelectric memory device according to an embodiment of the present invention.

2A is a perspective view for explaining an experiment for confirming the same effect, and FIG. 2B is a sectional view of FIG.

FIG. 3 is a diagram showing the experimental results of the above, and is a diagram comparing changes in spontaneous polarization values due to a temperature shock test with a conventional example.

4 (a), (b) and (c) are explanatory views showing a mechanism of a decrease in spontaneous polarization value of a ferroelectric memory element.

[Explanation of symbols]

1, 12 Ferroelectric Memory Element 2 Ferroelectric Thin Plate 4, 14, 15, 16 Memory Cell 3, 14a, 14b, 15a, 15b, 16a, 16b
Electrode 8 Short circuit (impedance component) 17 Silver wire (impedance component) 18 Resistance (impedance component)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasunobu Yoneda 2 26-10 Tenjin Tenjin, Nagaokakyo City, Kyoto Prefecture Murata Manufacturing Co., Ltd.

Claims (1)

[Claims] 1. A memory device comprising one or more memory cells each composed of a ferroelectric substance and a pair of electrodes formed with the ferroelectric substance sandwiched therebetween, wherein information is stored according to a spontaneous polarization direction of the ferroelectric substance of the memory cell. In the ferroelectric memory element, the ferroelectric memory element is characterized in that an impedance component lower than an insulation resistance value between the electrodes of the memory cell is connected between the electrodes of the memory cell at least during a period of information storage. How to use.