JPH06103753A - Ferroelectric memory - Google Patents

Abstract

PURPOSE:To provide the ferroelectric memory which can be actually produced as a device by specifying the current and voltage characteristics of a nonlinear resistor thin film forming a unit memory cell. CONSTITUTION:A thin film 16 having such current and voltage characteristics that the voltage at the time of passing 1 microampere current attains about 1 volt to 6 volts is used in the unit memory cell formed by laminating the ferroelectric thin film 14 and the nonlinear resistor thin film 16 and the ferroelectric memory constituted of striped electrodes 13, 17. As a result, the device which has no crosstalk between the memory cells and obviates the destruction of the memory state of the non-selected memory cells is attained. The ferroelectric memory which can be actually produced as the device, is thus obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a large-capacity solid-state non-volatile memory applicable to image files, audio files, etc., and more particularly to a ferroelectric memory capable of writing and reading without destroying the memory state of non-selected cells. .

[0002]

2. Description of the Related Art Conventionally, there is known a ferroelectric memory utilizing the polarization characteristic of a ferroelectric material having a hysteresis characteristic as shown in FIG.

According to this hysteresis characteristic, when a voltage V is applied to the ferroelectric material to polarize it once, even if the voltage is returned to "0", the remanent polarization value Pr shown at the point A or the point C is shown. Hold the state of.

Therefore, by associating each of the remanent polarization values shown at the points A or C with the digital signals "1" and "0", it is possible to function as a memory.

When information is recorded using such characteristics, a voltage V of a sufficient magnitude exceeding the coercive voltage Vc is obtained.
By applying s (saturation voltage), “0” is recorded, and a sufficiently large voltage −Vs (saturation voltage) exceeding the coercive voltage −Vc is applied to record the state of “1”. .

When the positive read pulse Vs is applied when the state of "1" is recorded, the polarization state changes from the point A to the point C. At this time, charges corresponding to both polarization differences 2Pr are discharged.

On the other hand, in the state of "0", C → B →
Since the polarization state changes with C, both polarization differences are zero.

Therefore, the storage state is "1" or "0" by detecting the amount of charge generated by applying a positive voltage.
Can be read.

FIG. 2 is a diagram showing a specific structure of a memory device having a conventional structure using the above-mentioned polarization.

FIG. 2A shows a simple matrix structure, and FIG. 2B shows an active matrix structure.

In this memory, a pair of stripe-shaped lower electrodes 2 and upper electrodes 3 which intersect each other are arranged on the surface of a substrate 1 serving as a support, and a ferroelectric thin film 4 is provided between the electrodes 2 and 3. A memory cell is provided at the intersection of the upper and lower stripe electrodes 2 and 3.

Such a memory has stripe electrodes 2, 3
To write or read voltage to,
It is called a simple matrix system after the liquid crystal display device.

FIG. 2C shows an equivalent circuit of a memory device having an active matrix structure.

In FIG. 3, the simple matrix structure is 3 × 3.
The case of the matrix is shown.

FIG. 3A is a cell layout diagram, and FIG. 3B is an equivalent circuit diagram.

Now, V / 2 is applied to the A2 terminal shown in FIG.
When a voltage of -V / 2 is applied to the B3 terminal, the cell C3
The voltage V is applied to 2.

When the storage state of the cell C32 is to be read out, 2/5 of C22, C21, C12 and C33 are stored.
A voltage of 1 / 5V is applied to each of V, C31, C11, C13, and C23.

When the simple matrix structure is n × n, (n-1) / (2n-) is applied to the non-selected cells related to the stripe lines to which the voltages of +1/2 V and -1/2 V are applied.
1) .V is 1 / (2n−) for the other non-selected cells.
1) The voltage of V is applied.

By applying such a voltage, even if the applied voltage is set so that the polarization of the ferroelectric memory cell is not reversed, that is, the applied voltage is equal to or lower than the coercive voltage, the voltage is as shown by the dotted line in FIG. After the removal, the polarization state is depolarized as A → B ′ → A ′.

This phenomenon is extremely fundamental to the ferroelectric substance. Therefore, in the conventional simple matrix structure ferroelectric memory as shown in FIG. 2A, the polarization state of the non-selected cell is depolarized, That is, it was impossible to avoid destruction of the memory state.

[0021]

Therefore, the present applicants propose that unit memory cells in which a ferroelectric thin film and a resistor thin film having a non-linear resistor are formed in a layered structure are arranged above and below each other. A ferroelectric memory having a simple matrix structure arranged and arranged at each intersection of X and Y stripe electrodes formed so as to be orthogonal to each other was proposed (Application No. Japanese Patent Application No. 3-239696).

In this case, the unit memory cell includes an upper stripe electrode / non-linear resistor / second electrode / PZT /
A ferroelectric memory having a laminated structure of a lower stripe electrode / silicon oxide film / semiconductor substrate has been proposed.

However, although the above-mentioned proposal describes the basic structure, the current-voltage characteristic of the non-linear resistor is not concrete.

That is, a unit memory cell having a ferroelectric thin film and a resistor thin film having a non-linear current-voltage characteristic formed in a layered structure is formed above and below the memory cell so as to be orthogonal to each other. In a ferroelectric memory having a simple matrix structure arranged at each intersection of X and Y stripe electrodes, the unit memory cell includes an upper stripe electrode / non-linear resistor / second electrode / PZT / lower stripe electrode / silicon oxide. In a ferroelectric memory characterized by having a film / semiconductor laminated substrate structure, the characteristics of the non-linear resistor were unclear, and a device could not be actually manufactured.

Therefore, the present invention provides a ferroelectric memory which can be actually manufactured as a device by making the effective non-linear resistance current-voltage characteristics concrete for the above ferroelectric memory. The purpose is to do.

[0026]

That is, in the present invention, crosstalk between memory cells is prevented by forming a ferroelectric memory having a simple matrix structure in which nonlinear resistors having clear current-voltage characteristics are laminated. In addition, a device in which the storage state of the non-selected memory cell is not destroyed by the write / read operation is realized.

First, the outline of the ferroelectric memory device of the present invention will be described.

FIG. 4 shows the current-voltage characteristics of the nonlinear resistor thin film, and the relationship between the basic characteristics of the nonlinear resistor laminated on the ferroelectric thin film and the memory cell characteristics will be described.

The current-voltage characteristics of this non-linear resistor thin film are configured so as to have positive / negative symmetry.

This non-linear resistor has, for example, an odd-numbered ZnO-Bi ₂ O ₃ repeating structure whose layered structure is centrally symmetric,
Alternatively, it can be composed of a polycrystalline material or the like.

Then, FIG. 5 shows a series-parallel circuit as an example of an equivalent circuit of the nonlinear resistor.

Here, Rg is the internal resistance of the polycrystalline grain, which is usually several ohms. Rb and Cb are the direct current resistance component and the electrostatic capacitance component of the grain boundary layer, which are several hundred ohms and about 500 pF, respectively.

Further, Ri and Ci are the direct current resistance component and the capacitance component at the interface between the grain and the grain boundary, and each is 10 ⁷ to
It has a value of about 10 ⁸ ohms and about 100 pF.

In this structure, the equivalent DC resistance becomes extremely large when the voltage becomes lower than the threshold voltage Vth.

Therefore, the combined impedance is determined by the equivalent capacitance Cnl of the nonlinear resistor.

Here, the threshold voltage means that the nonlinear resistor is in the off state (high resistance state) to the on state (low resistance state).
It is the voltage when switching to.

Next, the memory operation when the nonlinear resistor and the ferroelectric thin film are laminated will be described.

In this case, the applied voltage Vex is divided by the non-linear resistor and the ferroelectric thin film having the electric capacitance Cfe,
Cnl / (Cnl + Cfe) · Vex is applied to the ferroelectric side, and Cfe / (Cnl + Cf) is applied to the non-linear resistor side.
e) -Vex is applied.

Therefore, the smaller the value of Cnl + Cfe, the smaller the voltage applied to the ferroelectric side, and the more effective non-destructive reading can be performed.

On the other hand, when the external voltage increases and the voltage Cfe / (Cnl + Cfe) · Vex applied to the side of the non-linear resistor exceeds the threshold voltage Vth, the resistance of the non-linear resistor decreases rapidly, and at the same time, the resistance of the non-linear resistor increases. Applied voltage Zfe
When / (Zfe + Znl) · Vex becomes large and exceeds the coercive voltage Vc of the ferroelectric substance, the polarization is inverted.

Therefore, in order to prevent the voltage from being applied to the ferroelectric substance of the non-selected cell, it is better that the electric capacitance is smaller in the OFF state of the nonlinear resistor.

[0042]

First, the structure of a ferroelectric memory device of the present invention will be described.

FIG. 9 shows a unit memory cell of the ferroelectric memory device of the present invention.

The structure of this unit memory cell is such that, from the top, the upper stripe electrode 17 / nonlinear resistor layer 16 / second electrode 15 / ferroelectric layer 14 / lower stripe electrode 13 / silicon substrate surface oxide layer 12 / silicon substrate 11 are arranged. Has become.

Next, what kind of current-voltage characteristics the non-linear resistor has is optimum will be described.

FIG. 6 is an example of a hysteresis curve when several types of non-linear resistors are connected in series to a ferroelectric single cell. The inner and outer curves in the figure are 2.5 V and 5 respectively.
It is a measurement result in the unit of applied voltage of V. That is, the measurement was performed with applied voltages of 2.5 V and 5 V, because the maximum voltage applied to the non-selected cells is ½ of the voltage applied to the selected cells.

FIG. 6A is a hysteresis curve in the case of not having a non-linear resistor.

As shown in the figure, the polarization state changes greatly even at a voltage of 2.5V.

That is, this indicates that the memory state is destroyed by the write / read operation.

FIG. 6B shows a threshold voltage (V of 0.9V).
is a hysteresis curve in the case of connecting a non-linear resistor having th).

This is the case where the Vth of the non-linear resistor is too small, and the polarization state of the non-linear resistor changes greatly even with an applied voltage of 2.5V.

FIG. 6C shows a case where a non-linear resistor having a Vth of 2.76 V is connected.

This is a case where the Vth of the non-linear resistor has an appropriate value and shows sufficient polarization when the applied voltage is 5 V, but when the applied voltage is 2.5 V, the polarization state is almost changed. Absent.

FIG. 6D is a hysteresis curve when a non-linear resistor having a Vth of 6.2 V is connected.

In this case, Vth is too large, and the polarization state is hardly changed even when the applied voltage is 5V.

The above measurement was performed using the measurement circuit shown in FIG.

A graphical representation of the above measurement results is shown in FIG.
Shown in.

The horizontal axis represents the threshold voltage Vth at 1 μA of the diode as the non-linear resistor used for the measurement, and the vertical axis represents the ratio of the measured values of the polarization 2Pr at the applied voltages of 2.5V and 5V. , The unit is%.

In this figure, the smaller the value on the vertical axis is, the more the polarization is not changed by the applied voltage of 2.5 V as compared with the applied voltage of 5 V. Therefore, it is more non-destructive read-out. It can be said that there is. From this figure, it can be seen that the nonlinear resistor having the threshold voltage Vth of about 1 to 6 V required to conduct the current of 1 μA is most effective.

[0060]

As described above in detail, according to the present invention, by using the non-linear resistor thin film in which the voltage-current characteristic is embodied, the write / read operation can be performed without destroying the contents of the non-selected cells. It is possible to provide a ferroelectric memory that can be actually realized as a dielectric memory device.

[Brief description of drawings]

FIG. 1 is a diagram showing polarization (hysteresis) characteristics of a ferroelectric material.

FIG. 2 is a diagram showing a conventional ferroelectric memory structure and an equivalent circuit.

FIG. 3 is a diagram showing a cell layout diagram of a 3 × 3 simple matrix structure and an equivalent circuit.

FIG. 4 is a diagram showing current-voltage characteristics of a nonlinear resistor thin film used in the present invention.

5 is a diagram showing a serial-parallel circuit as an equivalent circuit of the non-linear resistor of FIG.

FIG. 6 is a diagram showing hysteresis measurement results when the type of a nonlinear resistor connected in series to a ferroelectric single cell is changed.

7 is a diagram showing a measurement circuit used in FIG.

8 is a diagram showing a graph showing the measurement results of FIG.

FIG. 9 is a diagram showing a unit memory cell of a ferroelectric memory according to the present invention.

[Explanation of symbols]

 11 Silicon Substrate 12 Silicon Substrate Surface Oxide Film 13 Lower Stripe Electrode 14 Ferroelectric Layer 15 Second Electrode 16 Nonlinear Resistor Layer 17 Upper Stripe Electrode

Claims (1)

[Claims] 1. An X, Y stripe in which unit memory cells in which a ferroelectric thin film and a resistor thin film having a non-linear current-voltage characteristic are formed in a layered structure are formed so as to be orthogonal to each other above and below the memory cell. In the ferroelectric memory having the simple matrix structure configured by being arranged at each intersection of the electrodes, the resistance thin film having the non-linear current-voltage characteristic has a voltage of about 1 when a current of 1 microamperes flows. A ferroelectric memory characterized by having a current-voltage characteristic such that the voltage changes from 6 V to 6 V.