KR101145667B1 - Method for forming ferroelectric thin film including acid treatment - Google Patents

Abstract

The present invention relates to a method of forming a ferroelectric thin film including an acid treatment process, and more particularly, an acid treatment of a substrate surface to etch a surface having a basicity and to expose a surface having an acid; And forming a perovskite-type ferroelectric thin film on the substrate subjected to the acid treatment by hydrothermal synthesis.

Description

Method for forming ferroelectric thin film including acid treatment

The present invention relates to a method of forming a ferroelectric thin film, and more particularly, to a method of forming a thin film of a perovskite series ferroelectric.

"Ferroelectrics" means materials exhibiting spontaneous electrical polarization; That is, the ferroelectric maintains the polarization state even after the electric field is removed after the polarization is caused by the electric field. Representative examples of ferroelectrics having such properties include perovskite compounds such as SrBi ₂ Ta ₂ O ₉ (SBT) series, Pb (Zr, Ti) O ₃ (PZT) series, and the like. In addition, ferroelectrics generally have a very high dielectric constant.

Therefore, ferroelectrics can be applied to various devices requiring high dielectric constant and / or spontaneous polarization. For example, the ferroelectric may be usefully applied to a capacitor, a piezoelectric (piezoelectric), a pyroelectric (pyroelectric), an electro-optical device, a memory device, a sensor, an actuator, and the like.

In these devices, ferroelectrics are mainly used in the form of thin films. A specific example of a memory device using a ferroelectric is a ferroelectric random access memory (FeRAM). FeRAM is generally divided into 1T / 1C (one transistor / onecapacitor) -type and FET-type. In 1T / 1C-type FeRAM, ferroelectrics are used as the dielectric of the capacitor. In FET-type FeRAM, ferroelectrics are used as gate insulators. In FET-type FeRAM, ferroelectrics not only act as insulators, but also maintain charges on semiconductor surfaces by spontaneous polarization. FeRAM is non-volatile due to spontaneous polarization of ferroelectrics. Note that the capacitor insulation layer or gate insulation layer of the FeRAM is implemented in the form of a thin film.

Conventional dielectric thin film manufacturing methods are classified into dry methods such as vacuum deposition, sputtering, CVD, and wet methods such as chemical etching, thermal bunching, hydrothermal synthesis, sol-gel, and spray coating. have.

Among them, the dry method is hard to avoid the damage of the film because most of the heat treatment at high temperature for crystallization during film formation [L. D. Madsen and E. M. Griwold, "Domain structures in Pb (Zr, Ti) O 3 and PbTiO 3 thin films", J. Mater. R, 12 (10), 2612 (1997)], and the selection of the substrate is limited, there is a disadvantage that the peeling and cracking of the film occurs when heating [M. Yoshimura, S. E. Yoo, M. Hayashi and N. Ishizawa, "Preparation of BaTiO3 Thin Film by Hydrothermal Electrochemical Method", Jpn. J. Appl. Phys., 28 (11), L2007 (1989)].

In addition, the ferroelectric loses ferroelectricity at a temperature above Tc (phase transition temperature or Curie temperature) and exhibits phase dielectric properties. On the contrary, when the ferroelectric having a temperature above Tc is cooled, the ferroelectric loses phase dielectric properties and exhibits ferroelectricity again. It is called phase change.

In the method of forming a thin film made of ferroelectric material on a substrate at a temperature above the Curie temperature (Tc), by cooling the thin film to a temperature below Tc, the thin film exhibits ferroelectricity. The ferroelectric thin film formed through the phase transition process In the a-domain and the c-domain (c-domain) coexist. In the a-domain, ferroelectric crystal lattice is oriented in the a-axis direction. In the c-domain, ferroelectric crystal lattice is oriented in the c-axis direction. The a-region is advantageous for the realization of high dielectric constant, and the c-region is advantageous for the formation of spontaneous polarization. Therefore, in a device seeking only the high dielectric constant of the ferroelectric, or a device seeking only the spontaneous polarization phenomenon of the ferroelectric, coexistence of the a-domain and the c-domain may be undesirable (Ferroelectric Materials and Their Applications, Yuhuan Xu, 1991). .

In addition, in the ferroelectric thin film formed through the phase transition process, holes formed in the thin film due to poor layer covering may cause electrical short-circuit of the upper and lower electrodes, which may make it impossible to evaluate the electrical characteristics.

In contrast, the hydrothermal synthesis method of the wet method can easily control the reaction temperature, the reaction pressure, the concentration of the solute, the concentration of the solvent, the concentration of the other additives, the thermodynamic parameters, and the amount of addition by using a supercritical or subcritical aqueous solution. In addition, there is no need for a calcination or sintering process, and the synthesis at a relatively low temperature can suppress the formation of the a-sector, thereby obtaining a film having excellent crystallinity and improved crystallinity.

As another wet method, the sol-gel method is a chemical method of coating a multicomponent solution having a selective composition on a substrate, drying, and finally producing a thin film having a desired composition and properties through a final reaction. This method can simply adjust the composition, improve the uniformity of the thin film, can be synthesized at low temperatures, inexpensive, and easy to manufacture in many other fields.

Among the wet methods, hydrothermal synthesis and sol-gel have many advantages, while hydrothermal synthesis has excellent crystal nucleation and growth, but has a disadvantage of roughness on the surface of the membrane. There is a problem that is accompanied by cracking and peeling phenomenon.

The present invention is to solve the problems of the prior art as described above, before forming the perovskite-type ferroelectric thin film by the hydrothermal synthesis method is subjected to acid treatment on the substrate to form a thin film uniformly by etching the surface having a basic It provides a ferroelectric thin film manufacturing method that can be.

The present invention to achieve the above object,

Acid treating the substrate surface to etch a surface having basicity and to expose a surface having acidity; And forming a perovskite type ferroelectric thin film by hydrothermal synthesis on the substrate subjected to the acid treatment.

In the hydrothermal synthesis method, a substrate is added to a strong base reaction solution containing a perovskite-type ferroelectric generation precursor and water, and reacted at a temperature lower than the phase transition temperature of the perovskite-type ferroelectric, and the temperature is 195 to 205. It is preferable that it is ° C.

The reaction solution may further include a surfactant or a mineralizer, and the mineralizer uses one or more selected from the group consisting of KOH, NaOH, LiOH, RbOH, and NH ₄ OH.

In addition, the present invention is a substrate containing a perovskite-type ferroelectric thin film, characterized in that the a-segment formation is suppressed by the hydrothermal synthesis method, and the layer covering of the thin film is improved by performing an acid treatment process on the substrate before the hydrothermal synthesis process To provide.

According to the present invention, after the acid treatment process on the substrate, by producing a lead-free piezoelectric thin film through hydrothermal synthesis, it is possible to form a uniform thin film, the thermal stress between the substrate and the thin film and the material movement between the substrate (thermal) In addition to minimizing diffusion, the formation of oxygen vacancies is minimized, thereby obtaining a high quality piezoelectric thin film.

1 is a three-dimensional view showing the perovskite crystal structure at a temperature of Tc or higher.
2 is a three-dimensional view showing a perovskite crystal structure in a polarized state.
Figure 3 is a low magnification (500X) SEM photograph showing the surface of the shape of the BiFeO ₃ thin film prepared on the SrTiO ₃ (001) is not an acid treatment to the existing surface of the substrate.
Figure 4 is a low magnification (500X) SEM photograph showing the surface of the shape of the BiFeO ₃ thin film production an acid treatment on the SrTiO ₃ (001) substrate surface in accordance with one embodiment of the present invention.
Figures 5a and 5b is a diagram showing the XRD analysis results of the BiFeO ₃ thin-film fabricated on a SrTiO ₃ (001) substrate surface to the acid treatment according to an embodiment of the present invention.
Figures 6a and 6b is a cross-sectional SEM photograph of the BiFeO ₃ thin film production an acid treatment on the SrTiO ₃ (001) substrate surface in accordance with one embodiment of the present invention.
Figure 7 is a diagram showing a hysteresis curve of the piezoelectric SrRuO ₃ (001) to acid treatment to the surface / SrTiO ₃ thin film prepared on the BiFeO ₃ (001) substrate in accordance with one embodiment of the present invention.

The terms or words used in this specification and claims are not to be construed as being limited to their ordinary or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best describe their invention. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

Therefore, the embodiments described in the specification and the drawings shown in the drawings are only the most preferred embodiment of the present invention, and do not represent all of the technical idea of the present invention, which can be replaced at the time of the present application It should be understood that there may be various equivalents and variations.

The present invention comprises the steps of acid treating a substrate surface to etch a surface having a basic and to expose a surface having an acid; And forming a perovskite-type ferroelectric thin film on the substrate subjected to the acid treatment by hydrothermal synthesis.

In the present invention, the perovskite ferroelectric means a material having a perovskite crystal structure. The perovskite crystal structure is also called "ABO ₃ ".

1 is a three-dimensional diagram showing a perovskite crystal structure at a temperature of Tc or more, one anion (A) at eight corners, one other anion (B) at the core, and six locations; Three cations (O3) are arranged in the body core of. Here, the metal ions in the B position and the oxygen ions surrounding them form an octahedron, and these octahedrons are arranged in a cubic structure.

As shown in FIG. 2, at temperatures below Tc, B ions deviate from the center of the crystal up or down, resulting in polarization. In this polarized state, the perovskite crystal structure has a tetragonal or rhombohedral form. Here, the polarization vector is formed in the c-axis direction. A may be, for example, Bi, Ba, Pb, K, Na, or the like. B may be, for example, Fe, Ti, Zr, Nb, Ta and the like.

More specific examples of the perovskite ferroelectric include BiFeO ₃ , BaTiO ₃ , PbTiO ₃ , Pb (Zr, Ti) O ₃ , (Pb, La) (Zr, Ti) O ₃ , KNbO ₃ Etc.

In the case of obtaining a BiFeO ₃ thin film, a Bi-containing compound or a Fe-containing compound can be used as the "probe compound for producing a pebskoid-type piezoelectric thin film". Examples of Bi-containing compounds include Bi (NO ₃ ) ₃ , Bi (NO ₃ ) _{3 to} 5H ₂ O, Bi ₂ O ₃ , Bi (OH) _3, and the like. Examples of the Fe-containing compound include Fe (NO ₃ ) ₃ , Fe (NO ₃ ) _{3 to} 9H ₂ O, Fe ₂ O ₃ , Fe (OH) _3, and the like.

Hydrothermal synthesis method is easy to control synthesis temperature, synthesis pressure, solute concentration, solvent concentration and concentration of other additives, thermodynamic parameters, and addition amount by using supercritical aqueous solution. Since the process does not require, and a heteroepitaxial thin film can be synthesized at a temperature of about 200 ° C. at a relatively low temperature, the process cost is low and a high quality thin film can be obtained.

In the method of forming a thin film made of ferroelectric material on a substrate at a temperature above the Curie temperature (Tc), the thin film is cooled to a temperature below Tc, thereby exhibiting ferroelectricity in the thin film. The a-domain and the c-domain coexist, where the a-domain is advantageous for the realization of high dielectric constant and the c-domain is advantageous for the formation of spontaneous polarization. Therefore, as in the present invention, the thin film formation by the hydrothermal synthesis method can suppress the formation of the a-region by synthesis at a relatively low temperature, and thus has an advantage of obtaining a film having excellent crystallinity and improved crystallinity.

However, the hydrothermal synthesis method has a disadvantage of poor coverage uniformity in spite of being inexpensive in manufacturing cost and producing a thin film having excellent crystallinity. Go through

The acid treatment is described below.

On the surface of the single crystal substrate, a basic surface and an acidic surface coexist. Therefore, when the thin film is formed using the hydrothermal synthesis method, since the solvent reacts well with an acidic surface because a strong base solution is mainly used, an acid treatment process is used to obtain uniformity in forming the thin film.

First, an acid treatment is performed on the substrate to etch a relatively basic surface and to reveal an acidic surface. For example, immersion of the SrTiO ₃ substrate surface in buffered HF (BHF) solution etches the SrO layer and reveals the TiO ₂ layer. This is because the solvent is mainly used for the strong base in the hydrothermal synthesis process, it reacts well with the acidic surface to bring about the effect of improving the layer covering. The strong base solution is preferably used in KOH, NaOH or NH ₄ OH.

In the case of the lead-free piezoelectric thin film manufactured by hydrothermal synthesis on an acid-treated substrate as described above, the thermal stress between the substrate and the thin film and the thermal diffusion between the substrate and the thin film are minimized as well. The formation of oxygen vacancy is minimized to obtain a high quality piezoelectric thin film.

Hydrothermal synthesis of the reaction solution proceeds at a temperature lower than the phase transition temperature of the perovskite ferroelectric to be obtained. In addition, the hydrothermal synthesis temperature of the reaction solution should be considered for high pressure formation and ionization of the precursor compound. If the hydrothermal synthesis temperature of the reaction solution is too low it is not possible to ionize the precursor compounds, there is not enough high pressure to form an epitaxial thin film, if too high may increase the pressure in the autoclave to stress the thin film. In consideration of this point, the hydrothermal synthesis temperature of the reaction solution may generally be about 150 to about 250 ° C, preferably about 195 to about 205 ° C.

The hydrothermal synthesis pressure of the reaction solution does not need to be particularly limited. However, if the hydrothermal synthesis pressure of the reaction solution is too low, a good crystalline thin film may not be obtained and ferroelectric phase may not be formed, and if too high, stress may be applied to the thin film. In consideration of this point, the hydrothermal synthesis pressure of the reaction solution may generally be about 1 to about 5 MPa, preferably about 1.5 to 3 MPa.

The hydrothermal synthesis time of the reaction solution is not particularly limited and may be appropriately selected to obtain a thickness of the ferroelectric layer to be obtained.

Meanwhile, in the hydrothermal synthesis process, in order to promote the synthesis of the ferroelectric, the reaction solution may further include a surfactant or a mineralizer.

Types of surfactants used in hydrothermal synthesis include CTAB (Cetyl trimethylammonium bromide), EDTA (ethylenediaminetetraacetic acid), PVB (Polyvinyl butyral), PVP (Polyvinyl pyrrolidone), and the like. Increase the properties to get a good quality thin film.

In addition, by using a mineralizer, the required hydrothermal synthesis time can be shortened. Examples of the mineralizer may include KOH, NaOH, LiOH, RbOH, NH ₄ OH, or a mixture thereof. If the amount of the mineralizer added is too small, the effect of addition may be insignificant, and the ionization of the precursor compound may not be sufficient to form a thin film. If too large, the ionization may proceed rapidly to form a polycrystalline thin film. In consideration of this point, the concentration of the mineralizer in the reaction solution may be typically 4 M to 10 M, preferably 7 to 8 M.

Hereinafter, the present invention will be described in detail with reference to the following Examples, but the following examples are merely to illustrate the present invention, and the present invention is not limited thereto. The analysis of the product prepared in the Examples of the present invention may be performed by the following method. Carried out.

The materials of the substrate used in this example were SrTiO ₃ (001) substrates and SrRuO ₃ (001) / SrTiO ₃ (001) substrates. SrRuO ₃ (001) / SrTiO ₃ (001) substrates were used in the manufacture of thin films to evaluate electrical properties. 0.005 m of Bi (NO ₃ ) _3? 5H ₂ O, Fe (NO ₃ ) _3? A reaction solution for hydrothermal synthesis was prepared by mixing 9H ₂ O and 40 ml of 12M KOH aqueous solution. SrTiO ₃ (001) to SrRuO ₃ (001) / SrTiO ₃ (001) substrates treated with the reaction solution and BHF solution were introduced into the reactor. At this time, the substrate was immersed in the reaction solution in the reactor. Then, hydrothermal synthesis of the reaction solution was carried out at a temperature of 200 ° C. and a pressure of 1.5 MPa for 10 hours. SrTiO ₃ through this process (001) substrate and a _{SrRuO 3 (001) / SrTiO 3} (001) to form a BiFeO ₃ film on the substrate surface.

3 and 4, it can be seen that the layer covering was not good on the oxide substrate not subjected to the acid treatment on the surface, and the layer covering was improved on the substrate subjected to the acid treatment on the surface.

Referring to Figure 5a, a BiFeO ₃ (001) peak and BiFeO ₃ (002) peak, SrTiO ₃ (001) peak and a SrTiO ₃ (002) peak is shown strongly. Since the SrTiO ₃ peak originates from the substrate, it was confirmed that the component of the thin film prepared in the example was BiFeO ₃ .

Referring to Figure 5b, BiFeO ₃ (220) in the BiFeO ₃ thin film prepared in this embodiment plane and SrTiO ₃ (220), so if the have a 4-fold symmetry, while matching BiFeO ₃ thin film SrTiO ₃ hetero-epitaxially on the substrate You can see that it grew.

6A and 6B, it was confirmed that the BiFeO ₃ thin film prepared in this example had a thickness of about 2.5 μm and the surface had a mosaic shape.

Referring to FIG. 7, it was confirmed that the piezoelectric constant ( d ₃₃ ) of the BiFeO ₃ thin film manufactured in the present example was about 20 pm / V and had hysteresis characteristics.

Although the present invention has been described with reference to one embodiment shown in the accompanying drawings, this is merely exemplary, and it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible. Could be. Accordingly, the true scope of protection of the invention should be defined only by the appended claims.

Claims (9)

Acid treating the substrate surface such that the substrate surface is acidic;
Injecting a substrate subjected to the acid treatment into a reaction solution containing a perovskite ferroelectric precursor and water; And
Reacting at a temperature lower than the phase transition temperature of the perovskite ferroelectric to form a perovskite ferroelectric thin film by hydrothermal synthesis;
The reaction solution is at least one mineralizer selected from the group consisting of KOH, NaOH, LiOH, RbOH and NH ₄ OH; And CTAB (Cetyl trimethylammonium bromide), EDTA (ethylenediaminetetraacetic acid), PVB (Polyvinyl butyral) or PVP (Polyvinyl pyrrolidone) Method for producing a ferroelectric thin film, characterized in that it comprises a surfactant.
delete The method according to claim 1,
The reaction temperature of the hydrothermal synthesis method is a ferroelectric thin film production method, characterized in that 195 ~ 205 ℃.
The method according to claim 1,
Pressure during hydrothermal synthesis of the reaction solution is a ferroelectric thin film manufacturing method, characterized in that 1.5 to 3 MPa. delete delete delete delete delete

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