JPH03113889A - Method for reproducing ferroelectric memory - Google Patents

Abstract

PURPOSE:To prevent the crosstalk at the time of reading out and to allow the non-destructive reproducing of data by stopping the impression of the voltage before the inversion polarization area generated by the impression of a reading out voltage grows and identifying the polarity of residual polarization from the value of the currents flowing in 1st and 2nd band-shaped electrodes. CONSTITUTION:The voltage is impressed to the 1st band-shaped electrodes 17, 18 positioned above and below the ferroelectric film formed with the residual polarization. The voltage impression is stopped before the inversion polarization formed in the ferroelectric film by the voltage impression grows to the size at which the polarization remains in the film even after the impressed voltage is removed. The currents flowing in the 1st and 2nd band-shaped electrodes 17, 18 are subjected to current-voltage conversion by an amplifier 24, by which the currents are amplified. The amplified currents are inputted to a comparator circuit 26 which discriminates the polarity of the recorded residual polarization, by which the binary data is reproduced. The loss of the data by the crosstalk at the time of reading out is prevented in this way and the non-destructive reading out of the recorded data is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for reproducing a ferroelectric memory using a ferroelectric material as a recording material.

[Prior Art] Ferroelectric materials undergo polarization when a voltage is applied, and their polarization characteristics have hysteresis with respect to the applied voltage. Therefore, after a voltage is applied, even if the applied voltage is removed, residual polarization with a value of Po or -Po remains in the ferroelectric material. A ferroelectric memory has been proposed that uses a ferroelectric material as a recording material and records binary data by associating, for example, data "0" with residual polarization Po and data "1" with Po. Initially, ferroelectric films, which could be driven at low voltages equivalent to those of semiconductor memories, had to be made thin in order to increase the effective voltage applied, which caused problems in terms of durability. However, the sol-gel method, MOD method, and MOCVD method have made it possible to obtain ferroelectric thin films with good hysteresis and durability.

Generally, a ferroelectric memory has a large number of strip-shaped electrodes on the top and bottom of a ferroelectric film, which are perpendicular to each other with the film in between.
A memory cell is formed in a region where the upper strip electrode and the lower strip electrode overlap. - A particular memory cell is selected by specifying one upper strip electrode and one lower strip electrode, and V between the designated upper strip electrode and the lower strip electrode.

By applying a voltage of -vo or -vo, '0' or '1' is recorded in the selected memory cell. The recorded data, for example,
/2Vo, applying 11/2Vo to the lower electrode for a predetermined time,
At this time, from the amount of current flowing through the electrode, it is "0", but "
1” is determined.

[Issue 8 to be solved by the invention] In such a ferroelectric memory, for example, during writing, 1 1/2 vo is applied to the upper electrode and 1/2 vo is applied to the lower electrode.
To the memory cell to which Vo is applied and “1” is recorded,
When reading, 1/2Vo is applied to the upper electrode, and 1/2Vo is applied to the lower electrode.
When /2VO is applied, polarization in the opposite direction to the residual polarization corresponding to "1" is formed in the memory cell, and the recorded state after reading becomes "0". In other words, the ferroelectric memory is destroyed. It is a memory, and the data is destroyed by one readout. However, it is desirable that the memory be a nondestructive memory that can be repeatedly played back, and it is desired to provide a ferroelectric memory that can be read out nondestructively.

Furthermore, ferroelectric memory has the problem of crosstalk during reading. In a ferroelectric memory, when reading data, for example, 1/2Vo and -1/2Vo are applied to a predetermined upper band-shaped electrode and a lower band-shaped electrode, respectively. At this time, other memory cells having an upper or lower strip electrode as one of the electrodes also have 1/2Vo or 11/2V.
o is applied.

The residual polarization of a memory cell to which 1/2Vo or 11/2Vo is applied does not return to its original value after the applied voltage is removed. Polarization characteristics explaining this situation are shown in FIG. For example, when a voltage of Vl is applied to a ferroelectric material having residual polarization -Po, the polarization shifts to point A according to the major loop 42. When the applied voltage is removed, the residual polarization becomes -P1. When a voltage v1 is further applied to the ferroelectric material with residual polarization - Pl, the polarization changes according to a minor loop 44 similar to this formed inside the major loop 42 and moves to point B, and the applied voltage When is removed, the residual polarization becomes -P2. Therefore, the residual polarization of the memory cell decreases in magnitude each time a voltage is applied. For example, in the electrode pattern shown in FIG. 4, one of the upper electrodes 46 is selected and 1/2V is applied, 11/2Vo is sequentially applied to the lower electrode 48, and all the lower electrodes 48 are applied with 1/2V. After applying the voltage, the upper electrode 46
When reading data from all memory cells by performing the same operation by changing A voltage of 11/2 Vo is applied N-1 times to the memory cell having an upper electrode of N next month. For this reason, there is a problem in that the residual polarization of the memory cell becomes extremely small, making it impossible to read it as data.

The present invention aims to solve the problem of crosstalk during reading and to provide a method for reproducing ferroelectric memory in which data can be reproduced non-destructively.

[Means for Solving the Problems] In the ferroelectric memory reproducing method of the present invention, a large number of first strip-shaped electrodes are provided on the upper surface of a ferroelectric thin film, and the first strip electrodes are A large number of second strip-shaped electrodes are provided on the lower surface of the ferroelectric film to intersect with the strip-shaped electrodes, and the second strip-shaped electrodes are arranged in a manner that depends on the direction of the voltage applied between the first strip-shaped electrode and the second strip-shaped electrode. In a ferroelectric memory that records binary data by associating two residual polarizations formed by a ferroelectric film with 0 or 1, first and second ferroelectric films located above and below the ferroelectric film on which residual polarizations are formed are provided. The process of applying a voltage to the strip electrode and the application of the voltage before the inverted polarization formed in the ferroelectric film due to the voltage application grows to a size that remains in the film even after the applied voltage is removed. and a step of reproducing binary data by detecting the current flowing through the first and second strip electrodes and determining the polarity of residual polarization recorded in the ferroelectric film.

[Function] In the ferroelectric memory, in the ferroelectric film 30 on which data is recorded, the polarization of the recording area 33 is aligned in the same direction, as schematically shown in FIG. 2A. During readout, when a voltage of opposite polarity is applied to the electrodes 31 and 32 provided above and below this ferroelectric film 30, as shown in FIG. 2B, the polarization is opposite to the already formed polarization. An inverted polarization region 34 having a polarization of is generated near the electrodes 31 and 32. It is known that when the applied voltage is removed in this state, the opposite polarizations touch at the tip of the reversed polarization region 34 and the reversed polarization region 34 disappears because it is extremely unstable in terms of electrostatic energy. There is. When the voltage application to the ferroelectric film 30 in the state shown in FIG. 2B continues, the reversed polarization region 34 grows and reaches the opposite surface as shown in FIG. 2C. This state is energetically stable, and even if the applied voltage is removed, the reversed polarization region 34 does not disappear and remains in the film in the state shown in the figure. This indicates the depolarization of the hysteresis property of the ferroelectric. As the voltage continues to be applied, the inverted polarization region 34 expands and eventually reaches the second D.
As shown in the figure, the recording area 33 of the memory cell becomes occupied by reversed polarization.

In the ferroelectric memory reproducing method of the present invention, the voltage application is stopped in the state shown in FIG. 2B before the inverted polarization region 34 generated by the application of the read voltage shifts to the state shown in FIG. 2C. The data recorded on the ferroelectric film, that is, the polarity of residual polarization, can be easily determined from the current values flowing through the first and second strip electrodes 31 and 32 provided on the upper and lower surfaces of the ferroelectric film. Ru. As a result, the residual polarization after the read voltage is removed returns to the value before the voltage was applied. Therefore, data recorded in the ferroelectric memory can be read out non-destructively. Furthermore, since the residual polarization of memory cells other than those to be accessed does not decrease, data loss due to crosstalk during reading can be prevented.

[Embodiment 2] A basic circuit diagram of a ferroelectric memory according to the present invention is shown in FIG. The column decoder 11 and the row decoder 12 each include 512 bit lines 17 and word lines 18, and these bit lines 17 and word lines 18 are provided so as to be orthogonal to each other via a ferroelectric thin film. The bit line 17 and the word line 18 have a width of, for example, 2 μm, and both signal lines 17 and 18 have a width of 2 μm that intersects with each other via a ferroelectric film.
One memory cell is formed in a region having an area of x2 μm. That is, 512×512-262144 memory cells are provided in a matrix. During writing and reading, a specific memory cell is selected from among the 262,144 memory cells, and data is written to or read from the selected memory cell.

To explain writing, first, a column address and a row address corresponding to a memory cell to be written are input to a column decoder 11 and a row decoder 12, respectively, and a predetermined memory cell is selected. When a write instruction pulse is input to the switch 16 via the terminal f, the pulse generator 13 and the terminal 16a are electrically connected, and the voltage pulse generated from the pulse generator 13 is selected via the column decoder. is applied to the bit line 17. At the same time, the write instruction pulse is input to the pulse generator 14 via the terminal g, the pulse generator 14 generates a voltage pulse, and the generated voltage pulse is transmitted to the selected word line 18 via the row decoder 12. is applied to The two pulse generators 13 and 14 are driven by a clock signal generated from a clock generation circuit 15 and generate synchronized voltage pulses. When writing "1" to the selected memory cell, the pulse generator 13 generates a voltage pulse of 1/2 Vo, and the pulse generator 14 generates a voltage pulse of +1/2 Vo. When writing "0#", the pulse generator 14 generates a voltage pulse of +1/2 Vo. Pulse generator 13
Does it generate a voltage pulse of +1/2Vo? S14 generates a voltage pulse of 11/2Vo.

Next, the read operation will be explained with reference to the drawings. A column address and a row address corresponding to a memory cell to be read are input to a column decoder 11 and a row decoder 12, respectively, and the memory cell to be read is selected. When a read instruction pulse is input to the switch 16 via the terminal f, the pulse generator 13 and the terminal 1
6b is electrically connected, and the pulse generator 13
5 voltage pulses with a pulse width of 100 ns are generated. This pulse is connected to NAND gate 21 and AND
The signal is input to gate 22. Initially, there is no output from comparator 26, so NAND gate 21 outputs 5V and A
ND gate 22 outputs 5V. The output voltage from the AND gate 22 is amplified by the amplifier 23 and sent to the column decoder 11.
is manually applied to the selected bit line 17.

Further, the read instruction pulse is input to the pulse generator 14 via the terminal g, and a voltage pulse of -5V with a pulse width of 100 ns, which is synchronized with the pulse generated from the pulse generator 13, is output from the pulse generator 14. Ru. This pulse is applied to the selected word line 18 via the row decoder 12.

When a voltage is applied to the bit line 17 and word line 18 of the memory cell, the polarization of the memory cell in which "1" is recorded changes, and current flows through the bit line 17 and word line 18. This current is subjected to current-to-voltage conversion by the amplifier 24, and the resulting voltage is amplified and output, and is also input to the comparison circuit 26. The comparison circuit 26 compares the manually input voltage signal with the voltage set by the threshold setting power supply, and outputs 5V when the input voltage signal exceeds the set voltage. When 5V is output from the comparison circuit 26, NA
The output of the ND gate 21 becomes 0■, and the output of the AND gate 22
The output of is also OV. Therefore, even before the pulse outputted from the pulse generation circuit 13 reaches Ov, when 5V is outputted from the comparison circuit 26, the application of voltage to the memory cell is stopped.

The voltage set by the threshold setting power supply is set appropriately depending on the ferroelectric memory used, but if the set voltage is too high, it will lead to destructive reading, and if it is too low, the output current will be too small, making reading unreliable. Sexuality decreases. Therefore, recording and reproduction are performed while changing the threshold value in a ferroelectric memory applied in advance, and a threshold value that allows repeated reading is experimentally selected.

[Effects of the Invention] According to the ferroelectric memory reproducing method of the present invention, data loss due to crosstalk during reading can be prevented, and data recorded in the ferroelectric can be read out non-destructively. Become.

[Brief explanation of drawings]

Fig. 1 is a basic circuit diagram of the ferroelectric memory of the present invention, Figs. 2A to 2D are diagrams showing the state of polarization of the ferroelectric film, and Fig. 3 shows the polarization characteristics of the ferroelectric material. FIG. 4 is a diagram showing an electrode pattern of a dielectric memory. 11... Column decoder, 12... Row decoder, 13°4... Pulse generator, 15... Clock generation circuit,
7...Bit line, 18...Word line.

Claims (1)

[Scope of Claims] A large number of first band-shaped electrodes are provided on the upper surface of the ferroelectric thin film, and a large number of second band-shaped electrodes that intersect with the first band-shaped electrodes via the ferroelectric film are provided on the upper surface of the ferroelectric thin film. Two residual polarizations provided on the lower surface of the ferroelectric film and formed depending on the direction of a voltage applied between the first strip electrode and the second strip electrode correspond to 0 or 1. In a ferroelectric memory that records binary data using a remanent polarization, the process includes: applying a voltage to first and second strip-shaped electrodes located above and below the ferroelectric film in which remanent polarization is formed; stopping the application of voltage before the inverted polarization formed in the dielectric film grows to a size that remains in the film even after the applied voltage is removed; A method for reproducing a ferroelectric memory comprising the steps of detecting a flowing current and reproducing binary data by determining the polarity of residual polarization recorded in the ferroelectric film.