JP2003298014A - Ferroelectric memory - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferroelectric memory, in the memory cell structure to have the limited operation voltage region, which has been manufactured to show small difference in the processes and the characteristics which do not change as much as possible, in voltages other than the operating voltage. <P>SOLUTION: The ferroelectric memory is manufactured using a mixture of bismuth strontiumate-tantalate and bismuth strontiumate-niobate as a ferroelectric. The composition ratio of materials is determined from the given operating voltage and film thickness condition, considering coercive electric field as the material depending on difference of composition ratio. Accordingly, it is possible to follow the specifications of the operation voltage and film thickness within a certain range. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a ferroelectric memory,
In particular, the present invention relates to a ferroelectric memory in which a single ferroelectric capacitor located at an intersection between word lines and bit lines intersecting each other is used as one memory cell.

[0002]

2. Description of the Related Art A ferroelectric memory having a ferroelectric capacitor as a memory element has an operating speed comparable to that of a DRAM,
Further, since it has a feature that it is non-volatile like a flash memory, it is expected as a memory element that may replace a conventional memory.

As a conventional memory device structure, one memory cell is composed of a transistor and a capacitor like a DRAM, and a ferroelectric material is used for the capacitor so that the memory can be retained. There are one (1T1C type structure) and one (2T2C type structure) in which another combination of a transistor and a capacitor is added for reference to each memory cell. However, considering the high integration in the future, the 1T1C type and 2T2C type structures have limitations, and a smaller memory element structure is required. In a ferroelectric memory, the ferroelectric material itself has a memory holding function, and at least a ferroelectric capacitor can operate the memory at a minimum. From this point of view, what is currently being studied is Japanese Patent Laid-Open No. 9-116
107 or a special table 2001-515256, a ferroelectric memory (1C type structure) in which one memory cell is composed of only one ferroelectric capacitor,
High integration is expected.

[0004]

However, in the 1C type ferroelectric memory, since the operating voltage region is limited by the material and film thickness of the ferroelectric capacitor, the ferroelectric memory having the conventional structure or more is required. In addition, it is necessary to design the ferroelectric material and the film thickness. Also, when making differently according to the specifications of multiple operating voltages,
It is important to keep the process differences as small as possible and to make the characteristics so that they are the same except for the operating voltage.

[0005]

In order to solve the above-mentioned problems, in a ferroelectric memory according to claim 1 of the present invention, bismuth tantalate strontate is used as a ferroelectric material for forming a ferroelectric capacitor. It is characterized in that a mixture of bismuth strontium niobate is used.

According to the above characteristics, the ferroelectric memory can be manufactured by freely adjusting the operating voltage to some extent only by the difference in the mixing ratio of the materials and the crystallization and processing conditions of the ferroelectric material itself. Have an effect.

Further, in the ferroelectric memory according to claim 2 of the present invention, the coercive voltage determined from the mixture ratio of bismuth strontium niobate and the film thickness is not less than ⅔ of the operating voltage and not more than the operating voltage. It is characterized by adjusting so that

By designing according to the above characteristics,
It is possible to operate the ferroelectric memory at an appropriate operating voltage.

A ferroelectric memory according to a third aspect of the present invention is characterized in that the material contains bismuth strontium niobate as a component in an amount of 50% or more.

According to the above characteristics, (1) the surface roughness of the ferroelectric material can be reduced, and the reliability of the material is increased, and (2) the material is compared with the case where the amount of bismuth tantalate strontate is large. Since the coercive electric field is high, the film thickness of the material for obtaining the ferroelectric memory of the same operating voltage can be thinned. (3) Lowering the process temperature compared to the case where there is much bismuth tantalate strontate. As a result, the peripheral circuits are less damaged.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1) FIG. 1 is a block diagram of a ferroelectric memory targeted by the present invention, and FIG. 2 is a perspective view schematically showing the memory array. As shown in FIG.
The memory array includes a ferroelectric thin film, a plurality of word lines arranged on one surface of the ferroelectric thin film, and a plurality of word lines arranged on the other surface of the ferroelectric thin film in a manner intersecting the word lines. And bit lines.

The operating principle of the ferroelectric memory shown in FIG. 1 will be described.

When performing a write or rewrite operation on the selected memory cell, a write voltage is applied to the word line 14 or bit line 16 corresponding to the selected memory cell 18. When performing a read operation, a read voltage is applied to the word line 14 corresponding to the selected memory cell 18 to detect a signal corresponding to the data held by the selected memory cell. Generally, the write voltage and the read voltage are called operating voltage.

In the write and read operations, if no measures are taken against the non-selected memory cells, the same voltage as the operating voltage is applied to the non-selected memory cells via the shared word line or bit line, and the memory is lost. There is a case.
To prevent this, during write and read operations,
A method of applying a voltage to a word line and a bit line not corresponding to the selected memory cell so that a certain potential difference lower than that of the selected memory cell is applied to the non-selected memory cell is used. The potential difference applied to the non-selected memory is generally 1/2 to 1/3 of the operating voltage.

3 to 5 show a ferroelectric memory including three types of ferroelectric capacitors having different coercive voltages when a certain operating voltage is applied, at an operating voltage and a voltage of 1/3 of the operating voltage. FIG. 4 is a diagram showing what kind of hysteresis curve is drawn respectively. Here, the coercive voltage of the ferroelectric capacitor is represented by the product of the coercive electric field of the ferroelectric material itself and the film thickness of the ferroelectric material. When the coercive voltage is appropriate as shown in FIG. 3, the write operation is performed firmly, and the memory is not lost when not selected. On the other hand, when the coercive voltage is too large as shown in FIG. 4, a firm write operation is not performed, and when the coercive voltage is too small as shown in FIG. 5, the memory is lost during non-selection. Therefore, in order to operate the ferroelectric memory of FIG. 1, it is necessary to control the coercive voltage of the ferroelectric capacitor well.

FIG. 6 shows how the coercive electric field changes when the composition ratio of bismuth tantalate strontate (hereinafter referred to as “SBT”) and bismuth strontium niobate (hereinafter referred to as “SBN”) is changed. It is the figure which showed how it changed into. When the ratio of SBN is increased, the coercive electric field is increased. From this, it is understood that the coercive voltage is increased as the ratio of SBN is increased, when the ferroelectric capacitors have the same film thickness.

FIG. 7 shows SB with the film thickness fixed at 120 nm.
6 shows changes in the operating voltage of a ferroelectric memory cell when the composition ratio of T and SBN is changed. SBT and S
It can be seen that the operating voltage can be changed by changing the composition ratio with BN.

(Example 2) By setting the SBN ratio to 50% or more, the effect of lowering the process temperature also appears as described above. This effect is shown in FIG.

FIG. 8 shows the characteristics when a ferroelectric capacitor having a ferroelectric material film thickness of 120 nm was prepared at a process temperature of 650 ° C. using materials having SBN ratios of 20% and 80%. It is a thing. It can be seen that when the SBN ratio is 20%, the ferroelectric capacitor cannot be formed and is a paraelectric substance, but when the SBN ratio is 80%, the ferroelectric capacitor is formed. Further, as shown in FIG. 7, if the film thickness is the same, the higher the SBN ratio, the higher the coercive voltage. Therefore, when the operating voltage is set, if the SBN ratio is high, the film thickness of the ferroelectric material increases. Can be kept thin and the process temperature moves further down.

[0021]

According to the ferroelectric memory of the present invention, it is possible to freely adjust the operating voltage to some extent only by changing the mixing ratio of the materials and the crystallization of the ferroelectric material itself and the processing conditions. The effect that a body memory can be produced is obtained. Further, by using the matters described in claim 3, (1) it is possible to further reduce the surface roughness of the ferroelectric material and increase the reliability of the material, and (2) when there is a large amount of bismuth tantalate tantalate. Since the coercive electric field of the material is high in comparison, it is possible to reduce the film thickness of the material for obtaining the ferroelectric memory of the same operating voltage. (3) The process compared to the case where the amount of bismuth tantalate strontate is large. Since the temperature can be lowered, the effect that the damage to the peripheral circuits is small can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram of a ferroelectric memory targeted by the present invention.

FIG. 2 is a perspective view schematically showing the memory array of FIG.

FIG. 3 is a diagram showing an operating voltage and a hysteresis curve at a voltage ⅓ of the operating voltage when the coercive voltage of the ferroelectric capacitor is appropriate.

FIG. 4 is a diagram showing an operating voltage and a hysteresis curve at a voltage ⅓ of the operating voltage when the coercive voltage of the ferroelectric capacitor is too large.

FIG. 5 is a diagram showing an operating voltage and a hysteresis curve at a voltage ⅓ of the operating voltage when the coercive voltage of the ferroelectric capacitor is too small.

FIG. 6 is a diagram showing how the coercive electric field changes when the composition ratio of bismuth tantalate strontate and bismuth niobate strontate is changed.

FIG. 7 is a diagram showing a change in operating voltage of a ferroelectric memory cell due to a difference in composition ratio between bismuth tantalate strontate and bismuth niobate strontate.

FIG. 8 is a diagram showing the difference in the effect of lowering the process temperature depending on the ratio of bismuth niobate strontate.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Eiji Natori             Seiko, 3-3-3 Yamato, Suwa City, Nagano Prefecture             -In Epson Corporation F-term (reference) 5F083 FR01 GA27 JA17 LA12 LA16

Claims (3)

[Claims] 1. A ferroelectric memory in which a memory cell composed of a single ferroelectric capacitor is arranged at each intersection of a plurality of word lines and a plurality of bit lines. 2. A ferroelectric memory, wherein the ferroelectric material forming the is a mixture of bismuth tantalate strontate and bismuth strontium niobate. 2. A strontium acid in which the coercive voltage determined by the film thickness and the mixture ratio of bismuth strontium niobate in the ferroelectric material is not less than ⅔ of the operating voltage and not more than the operating voltage. 2. The ferroelectric memory according to claim 1, wherein a ferroelectric material having a mixture ratio of bismuth niobate and a film thickness is used. 3. The ferroelectric memory according to claim 1, wherein the ferroelectric material contains bismuth strontium niobate as a component in an amount of 50% or more.