JP2002116151A - Method for spectroscopically evaluating bismuth layered structural ferroelectric material and manufacturing method for ferroelectric product - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an evaluating method capable of easily detecting, even in multiple phases, the microscopic area of a bismuth layered structural ferroelectric material which is difficult to detect by normal XRD(x-ray diffraction), and also capable of quantitatively determining its multiple phases. SOLUTION: The bismuth layered structural ferroelectric material including SBT(strontium bismuth tantalate), SBTN (niobium-doped SBT) or the like is analyzed by Raman spectroscopy, so that its phases can be quantitatively determined in addition to being identified.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for spectroscopic evaluation of a bismuth layered structure ferroelectric and a method for manufacturing a ferroelectric product using the same.

[0002]

2. Description of the Related Art Ferroelectric memory is a volatile DRA.
It has attracted attention as a non-volatile random access memory having features not found in M. PT (PbTi
O ₃ ), PZT ((Pb (Zr, Ti) O ₃ ) and PLZT ((Pb, La) (Zr, Ti) O ₃ ) are typical, but these ferroelectrics have large polarization fatigue deterioration. Therefore, SrBi ₂ Ta ₂ O ₉ (SBT), Sr
Bismuth layer-structured ferroelectrics such as Bi ₂ (Ta _x , Nb _1-x ) ₂ O ₉ (SBTN) are regarded as promising due to their high fatigue resistance, and development research on these materials is being actively conducted.

[0003]

In a bismuth layered structure ferroelectric material such as SBT or SBTN, which is currently receiving the most attention, unlike a PZT system in which the ferroelectric phase is a stable phase,
Since the non-ferroelectric phase is easily generated together with the ferroelectric phase as an impurity phase having a non-stoichiometric composition, it is important to improve the performance by increasing the generation rate of the ferroelectric phase. However, ordinary X-ray diffraction (XRD), which is a general method for evaluating inorganic solid materials such as conventional ferroelectrics, can only observe an average structure in a region of about 30 μm or more.
There is a problem that it is difficult to distinguish the mixture of a ferroelectric phase in TN and the like and a non-ferroelectric phase as an impurity layer. Recently, detailed analysis by X-ray reciprocal space mapping has shown that polyphase identification is possible, but this requires a great deal of skill and time. Therefore, conventionally and even now, SBT, SB
TN, etc. use ordinary X-ray diffraction (XRD) and physical properties of thin films,
The ferroelectric film produced by direct measurement of characteristics is evaluated. Therefore, an interpretation error has occurred.

Accordingly, the present invention provides a simple and effective method for analyzing and evaluating ferroelectric and non-ferroelectric phases contained in a bismuth layered ferroelectric which is difficult to distinguish by ordinary X-ray diffraction. To provide.

[0005]

In order to achieve the above object, the present invention provides a method for evaluating a bismuth layered ferroelectric material, comprising analyzing the bismuth layered ferroelectric material by Raman spectroscopy. I will provide a. Many bismuth layer structure ferroelectric materials have a layer crystal structure such as a perovskite structure, but especially in low-temperature film formation, a cubic fluorite-type (fluorite) structure which is a non-stoichiometric low-temperature stable phase or A crystalline phase having a pyrochlore structure is easily generated as an impurity phase. At this time, the main peak in the X-ray diffraction of the crystal phase having the fluorite-type structure or the pyrochlore structure overlaps with the strong peak of the crystal phase having the layered crystal structure such as the perovskite structure. It may be difficult to identify the mixture of phases. For example, while SBT or SBTN has a strong peak (characteristic peak) at (115), the main peaks at (111) and (222) in the fluorite-type structure and the pyrochlore-type crystal phase as its impurity phase. However, both appear at 2θ = about 29 ° and the peak areas overlap. In particular, in the case of a thin film, the width of the peak is widened, so that analysis is difficult.

On the other hand, in Raman spectroscopy, the spectral pattern of the crystal structure of each of these polyphases is unique and completely different from each other. This is because Raman scattering is sensitive to local structural changes such as coordination states and defects, and the number and position of modes are completely different if the crystal system or crystal structure is different. Utilizing this characteristic of the Raman spectrum, the ferroelectric phase and the non-ferroelectric phase of the bismuth layered structure ferroelectric material can be easily distinguished.

In the present invention, microscopic Raman spectroscopy is mainly used. Raman spectroscopy has features of short time (about 1 minute), non-destructive, and high resolution (about 1 μm). This feature is 2
This is contrasted with the fact that θ scan XRD takes about one hour and the resolution is not high, or that electron microscope observation requires a long time and is a destructive inspection. Raman spectroscopy not only allows easy identification of bismuth layered ferroelectric materials but also enables semi-quantitative evaluation of impurity phases present in bismuth layered ferroelectric materials. . In particular, in a dielectric material that is desired to be formed at a lower temperature, an impurity phase (eg, a fluorite phase or a pyrochlore phase) is more likely to be generated. It is extremely important in the production, and the significance of providing a method for quantitatively evaluating the impurity phase present in the present invention is extremely high in this field.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION A bismuth layered structure ferroelectric material to which the present invention is directed exhibits ferroelectric properties by including a layered structure in a crystal structure, as typically seen in a perovskite structure. Bismuth-containing compound, usually a bismuth-containing composite metal oxide, has the chemical formula Bi ₂ A _m-1 B _m O
_{3m + 3} = (Bi ₂ O ₂ ) ²⁺ (A _m-1 B _m O _{3m + 1} ) ^2- (where m = 1 to 4; A = Na ⁺ ,
K ⁺ , Ba ²⁺ , Ca ²⁺ , Sr ²⁺ , Pb ²⁺ , Bi ³⁺ ; B = Fe ³⁺ , Ga ³⁺ , T
i ³⁺ , Ta ³⁺ , Nb ⁵⁺ , V ⁵⁺ , Mo ⁶⁺ , W ⁶⁺ ), for example, Bi ₄ V ₂ O ₁₁ , Bi ₄ Ti ₃ O ₁₂ , Bi ₂ SrTa ₂ O ₉ , Bi ₂
SrNb ₂ O ₉ , SrBi ₂ (Ta _x , Nb _1-x ) ₂ O ₉ , Bi ₂ WO _{4 and the} like are included. FIG. 1 shows a perovskite crystal structure of Bi ₂ SrTa ₂ O ₉ , which includes a layer structure of TaO ₆ and Bi ₂ O ₂ , and exhibits ferroelectricity in the direction of the spontaneous polarization axis (Ps) in the figure. . Such a compound is attracting attention as a material for a nonvolatile random access memory as a ferroelectric material having excellent characteristics such as good fatigue characteristics.

Methods for producing such compounds are known, including film forming methods. However, as described above, research and development for lowering the temperature, miniaturization, integration, and the like of the process are currently in progress for practical use. Are rapidly evolving. The bismuth layered structure ferroelectric material to which the present invention is directed may be a known material, a material to be developed in the future, or any of them. This is because the method of the present invention is characterized by a method for evaluating the obtained bismuth layered structure ferroelectric material, irrespective of the type and manufacturing method of the bismuth layered structure ferroelectric material. Typical film forming methods include metal organic chemical vapor deposition and sol-gel.
The substrate for forming the film is not limited.

The Raman analysis method used in the present invention is known. Since the spectrum of light scattered from the sample by irradiating the sample with visible light indicates the vibration of molecules, the change of the state of rotation, the state of the lattice vibration of the crystal, etc., the substance is analyzed from this. As a light source, versatile argon laser light (514.5 nm) can be used. By using a microscope, nondestructive evaluation can be performed from a minute region of a sample or a microscopic / integrated device (submicron region / about 30 μm in conventional XRD). So-called microscopic observation is possible. Since the scattered light can be detected and separated by the CCD, it has a characteristic that it can be completed in a short time (about 1 minute).

Raman analyzers are known and commercially available, and various improvements have been proposed in patent literature and the like, and any of them may be used. Also,
Obtaining pure crystals of the bismuth layered ferroelectric material, and obtaining the non-stoichiometrically generated impurity phases of the bismuth layered ferroelectric material as pure crystals, these pure crystals Although many Raman spectra are known, even if they are not known, it is necessary to produce a bulk pure crystal including at least an impurity phase, and to identify the crystal structure thereof by using conventional XRD and X-ray reciprocal lattice spatial mapping. Since it is possible to make full use of this, it is also possible to obtain a Raman spectrum of the pure crystal.
Therefore, once the Raman spectrum of each pure phase of the polyphase including the impurity phase constituting a certain bismuth layered structure ferroelectric material is measured and obtained, the Raman analysis of the specific material to be measured is performed, and the obtained Raman spectrum is obtained. Comparing the spectrum with the known Raman spectrum of the pure crystal to identify the phases contained in the bismuth layered ferroelectric material and knowing its abundance is easy by waveform analysis. That is, it is possible to know the generation and the purity of the bismuth layer structure ferroelectric phase in the bismuth layer structure ferroelectric material.

The spectroscopic evaluation method for a bismuth layered structure ferroelectric material according to the present invention can be applied not only to the research and development of a bismuth layered structure ferroelectric material but also to a ferroelectric memory or the like containing the bismuth layered structure ferroelectric material. It can also be used for processes in the production line and product inspection. The latter is also possible online.

[0013]

EXAMPLE In this example, SrBi ₂ (Ta _0.7 , Nb _0.3 ) ₂ O ₉ (SB
TN) A thin film was formed by the MOCVD method. Pt as substrate
_{/ Ti / SiO 2 / Si (} 001) was used. Using Sr, Ta, Nb triple ethoxide and trimethyl bismuth having a desired stoichiometric composition as raw materials, SrBi ₂ (Ta _0.7 , Nb _0.3 ) ₂ O ₉ is deposited at 670 ° C. in an oxygen atmosphere to a thickness of 200.
nm. It is known that SBTN having this composition has larger remanent polarization than SBT. The structure of the obtained thin film was examined by conventional XRD and X-ray reciprocal lattice space mapping, and it was confirmed that SBTN having a perovskite structure was generated.

Next, in the same manner, except that the material Bi / (T
a + Nb) from 1.00 above to 1.36 and 0.8
2, and the substrate temperature was set to 600 ° C., and thin films of the fluorite phase and the pyrochlore phase were formed. The conventional XRD and X
The structure was examined by line reciprocal space mapping, and it was confirmed that a fluorite phase and a pyrochlore phase were formed, respectively.

The Raman spectra of these thin films are 1 m
A W Ar laser beam (wavelength: 514.5 nm) is focused on a sample to a spot having a diameter of about 1 to 2 μm, and the backscattered light is
It was detected and measured by a spectroscope equipped with a CD detector (R6400 manufactured by Atago Bussan). The wave number resolution was about 2 cm ^-1 . The measurement time was about 1 minute. The SBTN thus obtained is shown in FIG.
1 shows a typical X-ray diffraction pattern of (SrBi ₂ (Ta _0.7 , Nb _0.3 ) ₂ O ₉ ), a fluorite phase, and a pyrochlore phase by 2θ scan (deg / CuKα). As is well known, SBTN or SBT grown on a Pt-coated Si (001) substrate is strongly (115) oriented, and a strong peak of (115) is generated around 2θ = 29 °, and SBTN or It is used as a characteristic peak of SBT. However, although the fluorite type phase (111) and the pyrochlore phase (222) are the main peaks, they are near 29 °, and the SBTN (11)
It overlaps with the peak area of 5). Since the fluorite-type phase and the pyrochlore phase do not have any other strong peaks, it is difficult to identify these crystal phases and determine the mixture of the phases only by XRD of a normal 2θ scan. Although not shown, the same applies to the SBT.

FIG. 3 shows Raman spectra of the same SBTN, fluorite phase, and pyrochlore phase. SBTN is 28,5
It has a strong mode at 8,210,593,824 cm ^-1 . Since this feature is in good agreement with the properties reported for bulk SBTN, it can be seen that this thin film consists almost entirely of single-phase SBTN, with negligible distortion. On the other hand, the Raman spectra of the fluorite phase and the pyrochlore phase show rather broad peaks, and the mode of the pyrochlore phase matches well with that of the bulk, but the mode of the fluorite phase does not match. These imperfect matches are due to defects in these structures, and there are many reports of this. But,
Importantly, the spectra (Raman shift, Raman selection rule) of these three crystalline phases are characteristic of each phase, so that these Raman spectra can be used as fingerprints of these phases. .

In addition, the discrimination between each phase is of course.
However, even if these phases are mixed, the identification and existence of those phases
Semi-quantitative analysis of abundance is possible. FIG. 4 shows the same as above.
Although the raw material ratio is slightly excessive in Bi [Bi / (Ta +
Nb) = 1.3], and the SBTN thin film formed at 600 ° C.
3 shows a Raman spectrum of the film. This thin film is pure SBT
Presence of fluorite-type phase and pyrochlore phase in addition to N-phase
Is the deviation from the Raman spectrum of the pure SBTN phase,
Especially low wave number (100cm^{- 1}Easily)
Understand. However, the ramance of pure SBTN phase
Pectol (dot-dash line) and fluorite phase or pike rock
Weighted average spectral pattern with Raman spectrum of phase
Ratio (dashed line) to the Raman spectrum (solid line) of the thin film.
In comparison, as shown in the figure, the above thin film was 70% SBT
It is clear that the mixed phase is N / 30% pyrochlore.
You.

FIG. 5 is also the same as above, except that the raw material ratio is slightly Bi-deficient [Bi / (Ta + Nb) = 0.8].
The Raman spectrum of the SBTN thin film formed at 600 ° C. is shown, which is 40% as in the case of FIG.
It is clear that this is a SBTN / 60% fluorite type mixed phase. FIG. 6 shows the measured Raman spectra of various bismuth layered structure ferroelectric materials. However, Bi ₂ Sr ₂ CaCu
₂ O ₈ is a reference superconductor material. It is clear that the method for spectroscopic evaluation of a bismuth layered structure ferroelectric material by Raman spectroscopy of the present invention can be effectively used for thin films of these bismuth layered structure ferroelectric materials. Further, the usefulness of the method for evaluating a bismuth layered structure ferroelectric material of the present invention is not necessarily related to whether or not a particular bismuth layered structure ferroelectric material has a peak overlap between phases in a normal XRD. Since there is a basis for simplicity, reliability, quantification, etc., it is clear that it is useful for evaluation of all bismuth layered structure ferroelectric materials.

[0019]

As described above, according to the method for evaluating a bismuth layer-structured ferroelectric material using Raman spectroscopy of the present invention, it is possible to easily detect a minute region even in a multiphase in a short time. Simple and extremely effective analysis and inspection of bismuth layered structure ferroelectric material, which is attracting attention as a ferroelectric material for next-generation non-volatile random access memory, because it is possible and quantification of the multiphase is also possible. Means are provided.

[Brief description of the drawings]

FIG. 1 shows a crystal structure of Bi ₂ SrTa ₂ O ₉ .

FIG. 2: X of the SBTN phase, the fluorite phase and the pyrochlore phase
3 shows a line diffraction pattern.

FIG. 3 shows Raman spectra of an SBTN phase, a fluorite phase, and a pyrochlore phase.

FIG. 4 is a diagram showing a SBTN thin film (7
2 shows a Raman spectrum of a 0% SBTN / 30% pyrochlore mixed phase).

FIG. 5 shows a Raman spectrum of an SBTN thin film (40% SBTN / 60% fluorite mixed phase) formed under another condition.

FIG. 6 shows measured Raman spectra of various bismuth layered structure ferroelectric materials.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hiroshi Funakubo 4259 Nagatsutacho, Midori-ku, Yokohama-shi, Kanagawa F-term in Tokyo Institute of Technology (reference) 2G043 AA03 CA05 EA03 FA02 JA01 KA02 KA09 LA03 NA06 4G048 AA05 AC02 AD06 4G077 AA03 BC33 BC36 DB08 GA06 HA06 4M106 AA13 AB08 BA05 CB21 DH12 DH32 DH60

Claims (6)

[Claims] 1. A spectroscopic evaluation method for a bismuth layered structure ferroelectric material, comprising analyzing the bismuth layered structure ferroelectric material by Raman spectroscopy. 2. A ferroelectric material having a layered structure of bismuth having a structure
Formula Bi_TwoA_m-1B_mO_{3m + 3}= (Bi_TwoO_Two)²⁺(A_m-1B_mO_{3m + 1})^2-(Where
m = 1-4; A = Na⁺, K⁺, Ba²⁺, Ca²⁺, Sr²⁺, Pb^{Two +}, Bi³⁺; B
= Fe³⁺, Ga³⁺, Ti³⁺, Ta³⁺, Nb⁵⁺, V⁵⁺, Mo⁶⁺, W⁶⁺)
2. The method according to claim 1, which comprises a ferroelectric phase.
A spectroscopic evaluation method for a ferroelectric material having a layered structure having a layered structure. 3. The spectroscopic evaluation method for a bismuth layered ferroelectric material according to claim 1, wherein the bismuth layered structure ferroelectric material is a thin film. 4. The ferroelectric bismuth layered structure according to claim 1, wherein the types and abundances of the ferroelectric phase and the non-ferroelectric phase constituting the bismuth layered structure ferroelectric material are analyzed. Spectroscopic evaluation method of body material. 5. The ferroelectric material having a layered structure of bismuth having a structure of Sr
Bi_TwoTa_TwoO₉Or SrBi2 (Ta_x, Nb_1-x)_TwoO₉And the screw
SrBi present in mass layered ferroelectric materials_TwoTa _TwoO₉Or
SrBi_Two(Ta_x, Nb_1-x)_TwoO₉Phase and fluorite phase and / or
5. The method according to claim 2, wherein the amount of the pyrochlore phase is analyzed.
Spectroscopic Evaluation of Bismuth Layered Structure Ferroelectric Materials
Value method. 6. A method for manufacturing a ferroelectric product, comprising the method for spectroscopically evaluating a bismuth layered structure ferroelectric material according to claim 1 in a manufacturing process.