Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1997-03-11
2001-03-06
Wojciechowicz, Edward (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S298000, C257S300000, C257S308000, C257S310000
Reexamination Certificate
active
06198119
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an FRAM utilizing non-volatility, as an infrared ray sensor utilizing the pyroelectric effect, a memory element and a ferroelectric element widely used as a communications part, a memory cell thereof, and a method of producing the same.
A non-volatile semiconductor memory, such as a ROM (read-only memory), which is able to hold stored data even when the power source is turned off, has an inherent problem in that a limitation is imposed on the number of times data and the speed is low. On the other hand, a RAM (random access memory) features a high data rewriting speed. In particular, an FRAM which uses a ferroelectric material exhibits very excellent non-volatility as well as the ability to rewrite data a great number of times, such as 1010 to 1012 times, owing to the use of two residual polarizations of different polarities. Besides, the rewriting speed is as fast as a value in the order of &mgr;s or shorter, and so this memory is drawing attention as an ideal memory of the next generation.
Efforts have been made to develop a FRAM having a large capacity, featuring non-volatility and operating at high speeds. However, the affinity to wear of the film is a serious problem in that the spontaneous polarization (Pr) of the ferroelectric material decreases with an increase in the number of times of writing. It has been known that the capacity can be increased and the durability can be improved (1) by employing a ferroelectric material having a large spontaneous polarization (Pr), and (2) by employing a ferroelectric material which is immune to wear. Oxides of the perovskite structure have been widely used as such materials. Among them, it has been known that PZT (Pb(Zr
1−x
Ti
x
)O
3
) which is a single lattice having a crystal structure which is the perovskite structure, exhibits a large Pr without crystalline anisotropy. A diode made of this material which is used as a non-volatile memory has been disclosed in International Electron Device Meeting Technical Digest 1989, p. 255 (IEEE IEDM Tech. Dig.: 255-256, 1989). There has further been known SrBi2Ta2O9 of a Bi laminar ferroelectric material having a crystalline structure in which a plurality of single lattices of the perovskite structure are overlapped one upon the other. In this material, Pr exhibits a crystalline anisotropy only in a direction perpendicular to the c-axis. Though the Pr value is not so large, the material exhibits an excellent resistance to wear. Examples of using this material have been disclosed in Patents WO93/12542 and PCT/US92/10627.
SUMMARY OF THE INVENTION
According to the above-mentioned prior art, however, it has been impossible to provide a ferroelectric material exhibiting a high spontaneous polarization (Pr) which is indispensable for further increasing the degree of integration, a low coersive electric field (Ec) and an excellent resistance to wear. In a memory using PZT (Pb(Zr
1−x
Ti
x
)O
3
) of a single lattice having a crystalline structure which is a perovskite structure an attempt, has been made to lower the operating voltage entailed by an increase in the degree of integration. The operating voltage of the memory varies in proportion to the product of the thickness of the ferroelectric thin film and the coersive electric field. In order to lower the operating voltage, therefore, a process control has been studied in order to decrease the thickness of the ferroelectric thin film to be not larger than 100 nm. According to the above-mentioned prior art for decreasing the thickness of the film, however, a transition layer is formed due to the diffusion reaction of elements on the interface between the ferroelectric thin film and the metal electrode, bringing about problems such as a decrease in the spontaneous polarization and a wear of the film permitting the coersive electric field to increase. This stems from the fact that the ferroelectric material (PZT) of the oxide is so close to the metal electrode (Pt) that oxygen of PZT diffuses into Pt through the interface. Therefore, the number of times of writing the data by inverting the electric field is greatly limited.
In the Bi laminar ferroelectric material according to a prior art having the crystalline structure in which a plurality of single lattices of the perovskite structure are superposed one upon the other, no oxygen diffuses through the interface with the metal electrode, formation of the transition layer is suppressed, and excellent resistance is exhibited against wear of film. However, the spontaneous polarization is small and, when the electric field is applied in parallel with the c-axis, the polarization exhibits a strong crystalline anisotropy only in a direction perpendicular to the c-axis. This is because in the crystalline structure of the Bi laminar ferroelectric material, Ta atoms which cause spontaneous polarization due to a blocking layer of a Bi—O layer composed of two layers are polarized in directions opposite to each other along the c-axis direction. In the Bi laminar ferroelectric material, furthermore, the crystals grow preferentially in a direction perpendicular to the c-axis. In the structure consisting of the upper electrode, ferroelectric material and lower electrode according to the prior art, therefore, the amorphous Bi laminar ferroelectric material is formed on the amorphous lower electrode and is then subjected to a quick-heating processing to grow crystals having random crystalline directions (the orientation degree is not larger than 60%). Among them, only those crystals of the Bi laminar ferroelectric material having the c-axis perpendicular to the direction of the applied electric field work to produce polarization. Therefore, the practically effective crystals are 50%, which is not efficient. According to the prior art, furthermore, the electrode is composed of a metal such as Pt, Al, Au, or Ni inhibiting the transmission of light, and is not suitable as an element for optical response systems.
The present invention has been accomplished in order to solve the above-mentioned problems, and provides a ferroelectric thin film of perovskite crystalline structure without two blocking layers that suppress spontaneous polarization, provides a ferroelectric element having a ferroelectric thin film sandwiched between the upper and lower electrodes or between the right and left electrodes, provides a ferroelectric memory cell, and provides a method of producing the ferroelectric element.
Another object of the present invention is to provide a ferroelectric thin film of perovskite structure including two blocking layers and exhibiting a large spontaneous polarization and a weak coersive electric field by arbitrarily imparting lattice distortion by replacing elements having dissimilar ionic radii, to provide a ferroelectric element having a ferroelectric thin film sandwiched between the upper and lower electrodes or between the right and left electrodes, and to provide a method of producing the ferroelectric element.
The present invention further provides a method of producing a ferroelectric element having an excellent resistance to wear by suppressing the formation of a transition layer by using an electrically conductive oxide as the electrode that is in contact with the ferroelectric thin film, as well as a method of producing the ferroelectric element.
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patent: 0514149 (1992-11-01), None
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patent: 94/03908 (1994-02-01), None
Nakamura, T. et al., “Preparation of C-Axis-Oriented Bi4Ti3012 Thin Films by Metalorganic Chemical Vapor Deposition”, inJpn. J. Appl. Phys., vol. 32, No. 9B, Sep. 1, 1993, pp. 4086-4088.
Masaji Yoshida, “Chemical Vapor Deposition of (BA,SR)TI03”, Extended Abstracts, vol. 93/2, Jan. 1, 1993, p. 264.
Mendiola, J. et al.
Maeda Kunihiro
Nabatame Toshihide
Oishi Tomoji
Suzuki Takaaki
Takahashi Ken
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Wojciechowicz Edward
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