JPH05218445A - Ferroelectric memory - Google Patents

Abstract

PURPOSE:To obtain a ferroelectric memory, which has a long life as a memory, from which data can be read out in a non-destructive readout and is suitable for an increase in integration. CONSTITUTION:When a voltage higher than a voltage to generate a resistive electric field is applied to electrodes 2 and 3 holding a ferroelectric film 1 between them by a write means 21, a residual dielectric polarization is generated in the film 1 and memory of data in the film 1 is conducted with this polarization. On the other hand, when a voltage lower than the voltage to generate the electric field is applied to the electrodes by a readout means 2, a current of a strength corresponding to the direction of the residual polarization in the film 1 flows. The strength of this current is detected by a readout means 23 and readout of the data from the film 1 is conducted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferroelectric memory utilizing the hysteresis characteristic of remanent polarization of a ferroelectric substance.

[0002]

2. Description of the Related Art Ferroelectric materials have a hysteresis characteristic of remanent polarization with respect to an applied electric field and are used as a non-volatile memory by utilizing this hysteresis characteristic.
That is, when an electric field is applied to the ferroelectric material, polarization occurs in the ferroelectric material. This polarization remains as a remnant polarization in the positive direction even if the electric field is removed. Next, if the electric field applied to the ferroelectric material is in the negative direction and then the negative electric field is eliminated,
Remanent polarization in the negative direction remains in the ferroelectric material. Therefore, the remanent polarizations in the positive and negative directions can be written in the ferroelectric material as data of "0" and "1", and the data "0" and "1" can be read in the positive and negative directions of these remanent polarizations.

The structure of such a ferroelectric memory is roughly classified into two types. One of them is a simple matrix structure in which each stripe electrode is wired in a matrix and the electrodes in the matrix are arranged to face each other with a ferroelectric substance interposed therebetween. Then, the ferroelectric portion corresponding to the intersection of each stripe electrode is formed as one memory cell. With such a simple matrix configuration,
It has a simple structure and enables high-density data storage.

Further, another ferroelectric memory has an active matrix structure, and one switching element is provided for one ferroelectric memory cell. With such a structure, the structure becomes complicated, and there is a limit to high density. By the way, as a method of reading data from a ferroelectric memory, (1) a destructive read method that uses a polarization reversal current that requires rewriting of the selected memory cell.

(2) In a ferroelectric memory having a simple matrix structure as described in JP-A-2-154389, a self-reversal phenomenon of the ferroelectric thin film itself, that is, an initial polarization when an external pulse is applied. A method of performing write / read while suppressing the effect on non-selected memory cells by writing / reading the phenomenon of returning to the state with low impedance. and so on.

[0006]

However, since the polarization inversion is repeated in the above destructive readout method, the property of the ferroelectric substance is deteriorated and the residual polarization becomes small. For this reason,
A rewriting circuit having a short memory life and a complicated structure is required. Further, although the method utilizing the self-reversal phenomenon has high feasibility, the present situation is that a specific mechanism and device structure for realizing this have not been found yet.

In order to solve the above problems, it is an object of the present invention to provide a ferroelectric memory which has a long life as a memory and which can read data by nondestructive reading and which is suitable for high integration. ..

[0008]

According to the present invention, a ferroelectric film formed by combining at least two ferroelectric materials having different remanent polarizations and coercive electric fields, and a ferroelectric film sandwiched between the ferroelectric films are arranged to face each other. It is a ferroelectric memory which is provided with each electrode and which is intended to achieve the above object.

In this case, the ferroelectric film is a combination of the first ferroelectric material having a large remanent polarization at a low coercive electric field and the second ferroelectric material having a small remanent polarization at a high coercive electric field. There is.

[0010]

By providing such means, when a voltage higher than the voltage for generating the coercive electric field by the writing means is applied to each electrode sandwiching the ferroelectric film, the ferroelectric film is remnantly polarized. Data is stored in the ferroelectric film with this residual polarization.

On the other hand, when a voltage lower than the voltage for generating a coercive electric field is applied to each electrode by the reading means, a current having a magnitude corresponding to the direction of remanent polarization in the ferroelectric film is detected, and the ferroelectric film is detected. The data is read from.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a ferroelectric memory. The ferroelectric film 1 is provided with a matrix-shaped electrode in which a plurality of stripe electrodes 2, 2, ... And 3, 3 ,. These matrix-shaped electrodes are arranged so as to face each other with the ferroelectric film 1 interposed therebetween, and the intersections of the stripe electrodes on the upper surface and the lower surface of the ferroelectric film 1 correspond to each other. However, memory cells 1a, 1b, ... Are formed in the ferroelectric film 1 at the intersections of these stripe electrodes.

Further, each stripe electrode 2, 2, ... And 3, 3
Of the striped electrodes 2, 2, ...
, And the column switching control section 5 is connected to the stripe electrodes 3, 3, ....

The ferroelectric film 1 is formed by combining at least two respective ferroelectric materials having different remanent polarization and coercive electric field, and has a hysteresis characteristic as shown in FIG.

Specifically, the ferroelectric film 1 has a large remanent polarization P at a low coercive electric field e _{pc as} shown in the hysteresis characteristic of FIG.
The first ferroelectric material 10 having _pr and a small remanent polarization P _qr at a high coercive electric field e _{qc as} shown in the hysteresis characteristic of FIG.
It is a mixed film in which the second ferroelectric material 11 having

Here, the relationship between the magnitudes of the coercive electric fields e _pc and e _qc and the remnant polarizations P _pr and P _qr is e _pc <e _qc P _pr > P _qr .

The first ferroelectric material 10 is an element that generates a donor for lead zirconate titanate, such as Nb or L.
a, Bi is added, and the second ferroelectric material 11
Is an element that generates an acceptor, such as Cr, Fe, Mm, Co, or Cu, to lead zircon titanate. 5 and 6 show the cross-sectional structure of each memory cell in the ferroelectric film 1.

By the way, when the ferroelectric film 1 is theoretically analyzed, if the free energy is f, then f = (α / 2) P ² + (Β / 4) P ⁴ + (Α '/ 2) Q ² + (Β '/ 4) Q ⁴ + KPQ- (P + Q) e (1)

Here, α and α ′ are temperature functions, and α <0 and α ′ <0 in the ferroelectric state. P and Q are first ferroelectric materials 1 having different hysteresis characteristics
0, the polarization of the second ferroelectric material 11, e is the strength of the external electric field, and k is the interaction coefficient between the first and second ferroelectric materials 10 and 11. On the other hand, the time dependence of each polarization P, Q is given by the following equation.

[0021]

[Equation 1] Here, γ ₁ and γ ₂ are viscosity coefficients of the first and second ferroelectric materials 10 and 11 having the polarizations P and Q, respectively.

The coefficients are α = −1, α ′ = − 1 β = 1, β ′ = 3 γ ₁ <γ ₂ k = 1.

However, each memory cell 1a, 1b ... In the ferroelectric film 1 in which the first and second ferroelectric materials 10 and 11 are combined has a hysteresis characteristic as shown in FIG.

A writing circuit 21 and a reading circuit 22 are connected to the row switching control unit 4 and the column switching control unit 5 via a switching circuit 20, and a detection circuit 23 is connected to the row switching control unit 4 and the column switching control unit 5.

The writing circuit 21 includes matrix switching control units 4,
5 through, for each stripe electrodes 2 and 3 corresponding to the memory cells selected by these switching control unit 4, 5, it higher than the voltage that generates the coercive field e _c shown in FIG. 2, or coercive field - applying a voltage lower than the voltage for generating e _c,
It has a function of generating positive and negative remanent polarization, and writing the remanent polarization in the positive direction as data “0” and the remanent polarization in the negative direction as data “1”.

The read-out circuit 22 includes the stripe electrodes 2, 3
On the other hand, it has a function of applying a voltage lower than the voltage generating the coercive electric field e _c and reading the data “1” and “0” from the ferroelectric film 1 from the current flowing at this time. Specifically, the read circuit 22 applies the drive waveform signal shown in FIG. 7 to each of the stripe electrodes 2 and 3. The drive waveform signal has a positive amplitude e ₀ lower than the voltage generating the coercive electric field e _c and is negative. amplitude e ₁ is in the amplitude satisfy a relation of amplitude e ₀ and lower than the voltage that generates the electric field e'change point of the hysteresis, that is _{e 1 = (e 0/2} ) <e 0 <e c of .. The drive waveform signal is not limited to the sine wave, and may be a rectangular wave, a triangular wave, or a waveform obtained by combining these waveforms.

The detection circuit 23 detects the magnitude of the current flowing when the drive waveform signal is applied to each stripe electrode 2 and 3, and the data stored in the ferroelectric film 1 is "1" or "0". It has a function of determining whether or not. Next, the operation of data writing and reading with respect to the ferroelectric memory configured as described above will be described.

The row and column switching control sections 4 and 5 select and control the stripe electrodes 2 and 3 for selecting the memory cell 1a, for example, among the stripe electrodes 2 and 3. Also, the switching circuit 2
0 is switch-connected to the write circuit 21, and the write circuit 21 is connected to each of the row and column switch control units 4 and 5.

The write circuit 21 in this state, "1" or "0" voltage higher than the voltage generated the coercive field e _c in accordance with the data, or outputs a voltage lower than the voltage for generating a coercive field -e _c To do.

Here, a voltage higher than the voltage for generating the coercive electric field e _c is passed through the stripe electrodes 2 and 3 to the memory cell 1.
When added to a, polarization occurs in this memory cell 1a.
This polarization remains as the residual polarization P _k even if the voltage is not applied. As a result, the remanent polarization P _k is written in the memory cell 1 a as data “0”.

In addition, a voltage lower than the voltage for generating the coercive electric field -e _c passes through the stripe electrodes 2 and 3 and the memory cell 1b.
When applied, this is the memory cell 1b polarization in the opposite direction is generated in the residual polarization P _k, which remains as residual polarization -P _k. As a result, the remanent polarization −P _k is written in the memory cell 1a as data “1”.

On the other hand, when the read circuit 22 is connected to the row and column switch control sections 4 and 5 by the switch connection of the switch circuit 20, the read circuit 22 outputs the drive waveform signal shown in FIG. Add to 3. At this time, the coercive electric field e
_{When a} positive amplitude e _{0, which} is lower than the voltage generating _c , is applied to the memory cell 1a, a current having a magnitude corresponding to the remanent polarization P _k of the data “0” flows. At this time, the detection circuit 23 detects the magnitude of this current and determines data “0”.

By the way, since the ferroelectric film 1 has the hysteresis characteristic shown in FIG. 2, each remanent polarization P _k , -P is _obtained .
Each electric field at _k , namely positive amplitude e ₀ and negative amplitude e
_Since there is a large difference in the differential permittivity (slope of hysteresis) for each electric field generated in ₁ , there is a large difference in the magnitude of the output current. Therefore, the detection circuit 23 detects the magnitude of this current and determines the data "0".

When a negative amplitude e ₁ is applied to each of the sprite electrodes 2 and 3 of the memory cell 2a, a current having a magnitude corresponding to the remanent polarization -P _k of data "1" flows. At this time, the detection circuit 23 detects the magnitude of this current and detects the data "1".
To judge.

In this case, since the positive amplitude e ₀ and the negative amplitude e ₁ of the drive waveform signal are set in the above relationship, a voltage is applied also in the direction of depolarizing the remanent polarizations P _k and -P _k. However, since it is a sine wave voltage, it is not actually depolarized. Therefore, if an electric field lower than the coercive electric field e _c is applied, the polarization state will not be inverted or destroyed.

As described above, according to the above-described embodiment, the coercive electric field e _{c is} applied to each of the stripe electrodes 2 and 3 which sandwich the ferroelectric film 1.
Of the ferroelectric film 1 by applying a voltage higher than the voltage that generates
Storing data in, whereas, since the read out data of the ferroelectric film 1 by applying a voltage lower than the voltage for each stripe electrodes 2 and 3 to generate the coercive field e _c, the ferroelectric film The data written in 1 can be read without being destroyed, and the residual polarization of the ferroelectric film 1 does not become small, and the memory can have a long life without degrading the performance. Further, since the voltage for reading the data is lower than the voltage for generating the coercive electric field e _c , the stroke of each memory cell can be improved, and there is no performance deterioration due to the fatigue associated with use. The present invention is not limited to the above-mentioned embodiment, and may be modified within the scope of the invention.

[0037]

As described above in detail, according to the present invention, it is possible to provide a ferroelectric memory having a long life as a memory and capable of reading data by nondestructive reading and suitable for high integration.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a ferroelectric memory according to the present invention.

FIG. 2 is a diagram showing a hysteresis characteristic of the ferroelectric memory.

FIG. 3 is a diagram showing a hysteresis characteristic of a first ferroelectric material forming the same ferroelectric memory.

FIG. 4 is a diagram showing a hysteresis characteristic of a second ferroelectric material forming the same ferroelectric memory.

FIG. 5 is a sectional view of the same ferroelectric memory.

FIG. 6 is a sectional view of the ferroelectric memory.

FIG. 7 is a diagram showing drive waveforms output from a read circuit of the same ferroelectric memory.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Ferroelectric film, 2, 3 ... Matrix-like electrode, 4 ... Row switching control part, 5 ... Column switching control part, 10 ... 1st ferroelectric material, 11 ... 2nd ferroelectric material, 20 ... Switching Circuit, 21 ...
Write circuit, 22 ... Read circuit, 23 ... Detection circuit.

Claims (1)

[Claims] 1. A ferroelectric film formed by combining at least two ferroelectric materials having different remanent polarization and coercive electric field, and electrodes arranged opposite to each other with the ferroelectric film sandwiched therebetween. A ferroelectric memory characterized by.