Ferroelectric thin film coated substrate, producing method...

Details Ferroelectric thin film coated substrate, producing method... Ferroelectric thin film coated substrate, producing method...

1996-05-08

2002-08-27

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of inorganic material

C428S446000, C428S697000, C428S702000, C361S312000, C361S322000, C427S079000

Reexamination Certificate

active

06440591

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a ferroelectric thin film coated substrate to be used in a ferroelectric memory device, a pyroelectric sensor device, a piezoelectric device, etc., a producing method thereof, and a capacitor structure element using the ferroelectric thin film coated substrate.
BACKGROUND OF THE INVENTION
Since ferroelectrics has a lot of advantages, such as spontaneous polarization, high dielectric constant, an electro-optic effect, a piezoelectric effect and a pyroelectric effect, it is applied to the development of various devices, such as a capacitor, an oscillator, a light modulator and an infrared sensor. Conventionally, in these applications, monocrystal, made of triglycine sulfate (TGS), LiNbO
3
and LiTaO
3
which are materials of the ferroelectrics, or a ceramic, which is made of BaTiO
3
, PbTiO
3
, Pb (Zr
1−
Ti
x
)O
3
(PZT), PLZT, etc., was cut and was ground so as to have a thickness of approximately 50 &mgr;m. However, it is difficult and costly to produce large-sized monocrystal, and its processing is difficult due to cleavage. Moreover, since ceramics are generally fragile and it is difficult to process the ceramics so that it has a thickness of less than 50 &mgr;m due to a crack, etc. at a processing step, a lot of effort is required and its production cost becomes higher.
Meanwhile, as a technique for forming a thin film is developed, an application field for such a ferroelectric thin film is spreading at present. As an example of the applications, when high dielectric constant properties are applied to a capacitor for various semiconductor devices, such as DRAM, high integration of a element is realized by decreasing the capacitor size and reliability is improved. In particular, a high density ferroelectric nonvolatile memory (FRAM) which is operated at high speed has been developed by combining a ferroelectric thin film and a semiconductor memory element, such as DRAM, recently. The ferroelectric nonvolatile memory does not require backup battery because of utilization of the ferroelectric properties (hysteresis effect) of the ferroelectrics. The development of these devices require materials which have properties, such as large remanent spontaneous polarization (Pr), a small coercive electric field (Ec), a low leakage current and excellent endurance to repetition of polarization reversal. Moreover, in order to lower an operating voltage and to suitably perform semiconductor fine processing, it is desirable that the above properties are realized by a thin film with a thickness of less than 200 nm.
In order to achieve the application of the ferroelectric thin film to the FRAM, etc., an oxide ferroelectric thin film having a perovskite structure, such as PbTiO
3
, PZT, PLZT, etc. is tried to be formed by thin film forming methods, such as sputtering method, vacuum evaporation method, sol-gel method and MOCVD (Metal-Organic Chemical Vapor Deposition) method.
In the above ferroelectric materials, Pb (Zr
1−x
Ti
x
)O
3
(PZT) is now being studied most intensively, and a thin film with excellent ferroelectric properties is obtained by the sputtering method and the sol-gel method. For example, a thin film, whose remanent spontaneous polarization Pr takes a large value in the range of 10 &mgr;C/cm
2
to 26 &mgr;C/cm
2
, is obtained. However, although the ferroelectric properties of PZT greatly depend upon composition x, PZT contain Pb whose vapor pressure is high, so decrease in a film thickness arises a problem that the leakage current and fatigue in the endurance to polarization reversal are caused because film component is liable to change at the time of film formation and of heat treatment, a pinhole is produced, a low dielectric constant layer is obtained due to reacting between a ground electrode Pt and Pb, etc. For this reason, it is desired that another materials whose ferroelectric properties and the endurance to polarization reversal are excellent are developed. Moreover, in the case of the application to an integrated device, a fine grain of a thin film, which corresponds to fine processing, is required.
Bi layered oxide materials such as SrBi
2
Ta
2
O
9
have attracted interest as the materials with fatigue free property. A thin film of SrBi
2
Ta
2
O
9
is produced by a MOD method. The MOD method is a method for forming a film including the following processes. Namely, like the sol-gel method, metalorganic raw materials are mixed so that fixed film composition is obtained, and a raw material solution for application whose concentration and viscosity are adjusted is produced. A substrate is spin-coated with the produced raw material solution and the substrate is dried. Then, in order to remove the organic element and solvent, the substrate is heated to form the amorphous film. These processes are repeated until the fixed film thickness is obtained, and finally, the substrate is crystallized by sintering. Therefore, the film thickness is controlled by adjusting the thickness of a once-applied film (see Extended Abstracts (The 55th Autumn Meeting, 1194): The Japan Society of Applied Physics, 20 p-M-19).
The most serious problem of SrBi
2
Ta
2
O
9
thin film formation is that since the sintering temperature is very high, namely, 750° C.-800° C., a long sintering time, namely, longer than 1 hour, is required. In such a manufacturing process, when the processes, such as film formation and heat treatment are performed for a long time at the temperature of not lower than 650° C., a mutual diffusion reaction between a platinum metal electrode as a substrate and the ferroelectrics and reactions between silicon or oxidized silicon under the ground electrode and the electrode or the ferroelectrics are taken place. Moreover, the film composition is changed due to volatilization of a composition element from the ferroelectric thin film, and thus the application to the actual device producing process becomes difficult. Moreover, since only a film, having surface morphology of a large grain size of approximately 0.3 am, is obtained at present, the film cannot be applied to the submicron fine processing which is required for the development of highly integrated devices. Moreover, in the case of the coated film, since a coating method is disadvantageous of a step coverage, there arises a problem of disconnection of a wiring, etc. Therefore, as to SrBi
2
Ta
2
O
9
, its ferroelectric properties and its fatigue free property are excellent, but it still has a serious problem for application to devices.
In addition, in order to realize high integration of the ferroelectric nonvolatile memory at present, it is proposed to use a polycrystal silicon plug for wiring between a MOS transistor and a ferroelectric capacitor, but in the case where a ferroelectric thin film is produced by a long-time and high temperature process used for SrBi
2
Ta
2
O
9
, there arises a problem that its properties deteriorate. due to mutual diffusion between the polycrystal silicon used for wiring and the ferroelectric thin film. In order to solve such a problem, a structure where various diffusion barrier layers are inserted is examined, but even in such a structure, the permitted limit of a forming temperature of the ferroelectric thin film is up to 650° C., and on another short-time heat treatment process, the permitted limit is up to approximately 700° C. However, at present in the ferroelectric thin film made of SrBi
2
Ta
2
O
9
or the like, in general, as the film forming temperature is higher, the ferroelectric properties as well as crystallinity are more improved. Therefore, when the film forming temperature is lowered, the crystallinity and the ferroelectric properties are deteriorated, so it is difficult to achieve both the improvement in the ferroelectric properties and the low film forming temperature in the ferroelectric thin film.
On the other hand, an example of oxide ferroelectrics excluding Pb, which exerts a bad influence upon the leakage current and the resistance to polarization reversal, is Bi
4
Ti
3
O
12
having a layered perovski

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