Static information storage and retrieval – Systems using particular element – Ferroelectric
Reexamination Certificate
2002-03-21
2003-01-21
Tran, Andrew Q. (Department: 2824)
Static information storage and retrieval
Systems using particular element
Ferroelectric
C365S173000, C365S171000, C257S295000
Reexamination Certificate
active
06510074
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ferroelectric memory element having a high polarization moment and a high Curie temperature, in particular to a ferroelectric memory element having a high polarization moment and a high Curie temperature when a ferroelectric recording layer is made into a thin film of less than 50 nm thickness, and relates to an electronic apparatus provided with this ferroelectric memory element.
2. Description of the Related Art
Currently, the use of ferroelectric materials comprising transition metal oxides, typically represented by PbZr
1−x
Ti
x
O
3
(PZT) and SrBi
2
Ta
2
O
3
(SBT), in the recording layer of a non-volatile memory element has allowed the realization of high memory capacity and low power consumption of such memory devices. A typical elemental structure is produced by laminating a ferroelectric material such as PZT or SBT on top of the lower electrode made of such materials as Pt, Ir, and the like and further laminating such materials as Pt or Ir as the upper electrode to form a capacitor device. It is desirable for such memory elements to have characteristics of high polarization moment and Curie temperature. As requirements of the sensor amplifier, the polarization moment should be at least 10 &mgr;C/cm
2
. Also, from the viewpoint of reliability of recorded data, it is desirable that the Curie temperature be higher than 200° C. Further, it is necessary that a structural phase transition does not occur within the temperature range of usage (for example, −10 to 100° C.).
In recent years, on the other hand, further refinement in the size of the elemental structure is being made for the purpose of producing memories of gigabit capacity. To realize such an objective, ferroelectric materials must also be made available in thin films. For example, a film thickness of 150 nm has been sufficient when the design rule was 0.35 &mgr;m, but when the design rule becomes finer than 0.10 &mgr;m, a film thickness of 50 nm or even 20 nm is needed. It has, therefore, become necessary to provide a ferroelectric material that can provide high performance even in such thin film regions.
In thin film regions of less than 50 nm thickness, it is possible to epitaxially deposit a ferroelectric material on the lower electrode. According to a publication in the Japanese Journal of Applied Physics, Vol. 38, (1999), pp. 5305, it is reported that a higher polarization moment and Curie temperature than those in bulk crystals are obtained in the recording layer by growing a tetragonal crystal of BaTiO
3
oriented in a <001> direction. The film thickness in this case is 12 nm. The lower electrode comprises SrRuO
3
deposited on the (001) surface of a SrTiO
3
substrate. In this case, the length of the a-axis of tetragonal BaTiO
3
is compressed by the epitaxial growth so as to match the a-axial length 391 pm of SrTiO
3
. In this case, to maintain the Poisson's ratio, the c-axial length of BaTiO
3
is elongated even up to 437 pm. The polarization moment of BaTiO
3
having the c-elongated crystal structure is about 35 &mgr;C/cm
2
, and the Curie temperature is estimated to be higher than 350° C. On the other hand, in a bulk BaTiO
3
material, the polarization moment is 29 &mgr;C/cm
2
and the Curie temperature is 120° C. Therefore, the technique of crystal lattice elongation caused by epitaxial growth in a thin film region is an extremely useful technique for raising the polarization moment and Curie temperature. In particular, it can be said that BaTiO
3
has become a strong candidate for a ferroelectric memory material because of its ability to significantly increase its Curie temperature.
However, the trend towards high density, high capacity ferroelectric memory elements indicates that an even higher polarization moment than that achievable by the conventional technology is required. The reason is that, as the memory element becomes finer and the area of the capacitor decreases, a ferroelectric material comprising the memory layer is required to generate an even higher polarization moment in order to secure a charge level for activating the sensor amplifier.
SUMMARY OF THE INVENTION
A first object of the present invention is, therefore, to provide a ferroelectric memory element having a ferroelectric recording layer that can generate a high polarization moment and a high Curie temperature.
The next consideration relates to the epitaxial growth technique for improving the properties of BaTiO
3
. As described in the description of the prior technologies, it is first necessary to form an oriented lower electrode comprising a perovskite-type oxide. However, because most ferroelectric memory elements are formed on an amorphous oxide, especially SiO
2
, it has been difficult to form an oriented lower electrode having a perovskite-type oxide. For this reason, it has been a topic of investigation to find a method for producing an oriented film comprising a perovskite-type oxide even on an oxide material such as amorphous SiO
2
or SiOF. Further, another topic of investigation has been to use, in the recording layer, BaTiO
3
that has a trigonal crystal structure and that generates a higher polarization moment and a higher Curie temperature than those in a bulk material of BaTiO
3
. Therefore, it is a second object of the present invention to provide a ferroelectric memory element having a ferroelectric recording layer comprising an electrode of an oriented film of a perovskite-type oxide that can be formed on an amorphous oxide, and a BaTiO
3
recording layer of a trigonal crystal structure formed by an epitaxial growth technique on top of the oxide layer, thereby resulting in a ferroelectric memory element having a ferroelectric recording layer that can generate a high polarization moment and a high Curie temperature originating from the extraordinarily high stresses caused by epitaxial growth.
Further, it is desirable to guarantee reliability of the memory element, especially a repeatability of over 10
12
cycles. Therefore, a third object of the present invention is to provide a ferroelectric memory element having an electrode comprising an oriented film of a perovskite-type oxide that can be formed on an amorphous oxide, and a BaTiO
3
recording layer of a trigonal crystal structure formed by an epitaxial growth technique on top of the oxide layer, thereby resulting in a ferroelectric memory element having a ferroelectric recording layer that not only can generate a high polarization moment and a high Curie temperature originating from the extraordinarily high stresses caused by epitaxial growth, but can also provide a high reliability concerning the reading and writing repeatability.
A fourth object of the present invention is to provide an electronic apparatus which has a ferroelectric memory element such as described above.
Therefore, a first aspect of the present invention relates to a ferroelectric memory element having a recording layer comprising BaTiO
3
having a trigonal crystal structure and oriented in a <111> direction in a pseudo-cubic system so as to utilize a polarization moment in the <111> direction in a pseudo-cubic system for reading recorded signals.
Accordingly, a high polarization moment and a high Curie temperature are obtained in the recording layer.
In the ferroelectric memory element, the recording layer is formed preferably at a thickness of 50 nm or less.
Accordingly, the memory element in the first aspect can respond to the demand of the design rule of microelements of less than 0.10 &mgr;m in the thin film layer of the ferroelectric recording layer.
In the ferroelectric memory element in the first aspect, it is preferable that a unit cell of BaTiO
3
crystal structure is elongated in the <111> direction in a pseudo-cubic system, compared with a unit cell of a trigonal crystal structure of a bulk material of BaTiO
3
in a most stable state.
Accordingly, a high polarization moment and a high Curie temperature can be obtained in the recording layer.
In
Higuchi Takamitsu
Iwashita Setsuya
Miyazawa Hiromu
Harness & Dickey & Pierce P.L.C.
Seiko Epson Corporation
Tran Andrew Q.
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