Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-02-12
2001-03-06
Wojciechowicz, Edward (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S310000, C257S532000, C438S003000, C438S240000, C438S253000
Reexamination Certificate
active
06198120
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ferroelectric thin film device having a Si substrate, and more specifically, relates to a ferroelectric thin film device comprising a highly-oriented ferroelectric thin film provided on a Si substrate, and suitable for use as capacitors for DRAM and ferroelectric RAM (Fe RAM), as well as for application to a pyroelectric element, a micro-actuator, a thin film capacitor, a small piezoelectric element, and the like. The present invention also relates to a production method for the ferroelectric thin film device.
2. Description of the Related Art
In recent years, studies have actively been conducted on formation of thin films of Pb type and non-Pb type perovskite compounds such as BaTiO
3
(abbreviated to “BTO” hereinafter), SrTiO
3
(abbreviated to “STO” hereinafter), (Ba, Sr)TiO
3
(abbreviated to “BSTO” hereinafter), PbTiO
3
, (Pb, La)TiO
3
(abbreviated to “PLT” hereinafter), PZT, PLZT, Pb(Mg, Nb)O
3
(abbreviated to “PMN” hereinafter), and the like.
Particularly, when a Pb type perovskite compound having high residual polarization, such as PZT, PLZT, or the like can be epitaxially grown, spontaneous polarization can be arranged in one direction, thereby obtaining higher polarization values and switching characteristics. Therefore, under present conditions, application to a high-density recording medium is significantly increased, and there is thus a strong demand for developing a technique for epitaxially growing a Pb type perovskite compound.
However, in application in which spontaneous polarization is arranged in one direction, e.g., in the direction of the film thickness, a structure in which a ferroelectric thin film is held between conductive layers (electrode layers) on a Si substrate, i.e., a metal-ferroelectric material-metal (MFM) structure, is required. A triaxially-oriented ferroelectric oxide thin film having good crystallinity is difficult to obtain from the following reasons:
(1) When a metal thin film of Ag, Au or the like is formed as a conductor on a Si substrate, mutual diffusion occurs between the metal thin film and the Si substrate as a base during growth of the ferroelectric oxide thin film.
(2) A method using a Pt thin film as a metal thin film can be considered. Although Pt can be epitaxially grown on an oxide single crystal substrate such as MgO, SrTiO
3
, or the like, direct epitaxial growth of Pt on a Si substrate cannot be realized at present.
(3) A method using an oxide such as (La, Sr)CoO
3
(abbreviated to “LSCO” hereinafter) for a conductive thin film can be considered. In this case, it is necessary to insert other layers between the Si substrate and the LSCO layer, for example, as in PLZT/LSCO/BiTO/YSZ/Si, thereby causing difficulties in improving epitaxy of the ferroelectric layer as the uppermost layer. Here, BiTO represents Bi
4
Ti
3
O
12
, and YSZ represents ZrO
2
to which Y (yttrium) is added.
(4) There is also a method comprising forming a TiN thin film by ion beam deposition, forming SrRuO
3
(abbreviated to “SRO” hereinafter) as a buffer layer on the Pt thin film formed on the TiN thin film, and the epitaxially growing a (Ba, Sr)TiO
3
(BSTO) thin film thereon. This method has problems in that a multilayer structure of BSTO/SRO/Pt/TAN/Si causes a significant increase in cost, and deterioration in crystallinity with lamination of the thin films. In addition, SRO is considered to impart ferroelectricity to BSTO by applying a stress stain to BSTO, but there is now no success of growth of a Pb type perovskite compound.
For the forgoing reasons, there is a need for a ferroelectric thin film device comprising a ferroelectric thin film of a perovskite oxide, which is highly oriented and formed in a simple film structure on a Si substrate without requiring a complicated multilayer structure, and a method of producing a ferroelectric thin film device, which is capable of efficiently producing the ferroelectric thin film device.
SUMMARY OF THE INVENTION
The present invention is directed to a ferroelectric thin film device and a method of producing a ferroelectric thin film device that satisfied this need. The ferroelectric thin film device comprises: a Si substrate; a TiN thin film epitaxially grown on the Si substrate in which the Ti is partially substituted by Al; a metal thin film epitaxially grown on the TiN thin film; and a ferroelectric thin film grown and oriented on the metal thin film and composed of an oxide having a perovskite structure. The amount of Al substituted at Ti sites in the TiN thin film is about 1 to 30% in terms of Al atoms, and the oxygen content of the TiN thin film is about 5% or less in terms of oxygen atoms.
The method of producing a ferroelectric thin film device comprises the steps of: epitaxially growing, on a Si substrate, a TiN thin film in which the Ti is partially substituted by Al; epitaxially growing a metal thin film on the TiN thin film; and orienting and growing a ferroelectric thin film of an oxide having a perovskite structure on the metal thin film. The amount of Al substituted at the Ti sites in the TiN thin film is in the range of about 1 to 30% in terms of Al atoms, and the oxygen content of the TiN thin film is about 5% or less in terms of oxygen atoms.
According to the present invention, it is possible to obtain the element having a structure in which the ferroelectric thin film is oriented and grown on the Si substrate without requiring a complicated multilayer structure.
Namely, although a complicated multilayer structure is conventionally required for orienting and growing a perovskite type oxide ferroelectric thin film on the Si substrate, the construction of the present invention enables epitaxial growth of the perovskite type oxide ferroelectric thin film on the Si substrate by using only two buffer layers including the TiN thin film and the metal thin film.
Particularly, the present invention enables growth of a Pb perovskite type oxide ferroelectric thin film having a high degree of orientation (at least monoaxial orientation).
In addition, when a Pt thin film is formed as the metal thin film, the Pt thin film also functions as an oxidation barrier layer for the TiN thin film, and exhibits a catalytic ability in the process for forming the ferroelectric thin film, thereby obtaining the ferroelectric thin film having excellent crystallinity.
Furthermore, when the ferroelectric thin film is epitaxially grown, all thin films are epitaxial films, and thus mutual diffusion does not occur at the interfaces between the respective thin films, thereby obtaining the ferroelectric thin film device which is thermally stable.
Furthermore, the present invention permits epitaxial grown of the perovskite type oxide ferroelectric thin film on the Si substrate by using only two buffer layers including the TiN thin film and the metal thin film, thereby decreasing the process time and production cost.
As a result, it is possible to easily realize epitaxial growth of a perovskite type oxide ferroelectric material, particularly a Pb type perovskite compound, on the single crystal Si substrate, which is conventionally very difficult, and thus provide a ferroelectric thin film device which can be applied to capacitors for DRAM and FeRAM, a pyroelectric element, a micro-actuator, a thin film capacitor, a small piezoelectric element, and the like.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
REFERENCES:
patent: 6046469 (2000-04-01), Yamazaki et al.
patent: 09252094 (1997-09-01), None
“Synthesis of Epitaxial Pt on (100)Si Using TiN Buffer Layer by Pulsed Laser Deposition”; P. Tiwari, et al.; American Institue of Physics,Appl. Phys. Lett.; 65 (21); Nov. 21, 1994; pp. 2693-2695.
Li Xiao-min
Sakurai Atsushi
Shiratsuyu Kosuke
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Wojciechowicz Edward
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