JP2004152922A - Manufacturing method of ferroelectric film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the inclination of composition in the thickness direction of a film without dropping a film manufacturing speed extremely in the manufacturing method of a ferroelectric film. <P>SOLUTION: In the manufacturing method of the ferroelectric film, at least two kinds or more of sol-gel solutions 1, 2 having different compositions are superposed before calcination and, thereafter, the calcination is effected. A drying film of multilayer structure having different compositions is formed to compensate segregation before calcination, whereby the ferroelectric film reduced in the inclination of composition in the thickness direction of the same can be manufactured without increasing the manufacturing process remarkably. On the other hand, this drying film contributes to the improvement of the performance and the reliability of an optical device such as an optical deflection element or the like or an electronic device such as an FeRAM, a piezo-electric element or the like. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a ferroelectric film, and more particularly to a ferroelectric film having a configuration for reducing a composition gradient in a film thickness direction when a ferroelectric film is formed by a sol-gel method. The present invention relates to a method for manufacturing a body film.
[0002]
[Prior art]
Conventionally, a ferroelectric film such as PZT [Pb (Zr, Ti) O ₃ ] has been used in an optical element such as an optical deflection element, and a capacitor of an FeRAM (ferroelectric random access memory). A ferroelectric film such as PZT is also used as the dielectric film.
[0003]
One of the methods for forming such a ferroelectric film is the sol-gel method, but the sol-gel method can be manufactured by a simple method of coating and heating and baking, without requiring an expensive apparatus. This is a film forming method suitable for mass production.
[0004]
Further, a film thickness exceeding 100 nm per layer can be produced by the sol-gel method. Here, a manufacturing process of the PZT film by the conventional sol-gel method will be described with reference to FIGS. FIG. 5 is a flow chart of film formation by a conventional sol-gel method, and FIG. 6 is an explanatory diagram of a film forming process of a PZT film by a sol-gel method.
[0005]
First, a sol-gel raw material solution as a precursor (precursor) is prepared through hydrolysis and condensation polymerization of a metal alkoxide.
In this case, the sol-gel raw material solution for PZT includes Pb (CH ₃ COO) ₂ .3H ₂ O [lead acetate], Ti (i-OC ₃ H ₇ ) ₄ [titanium] Isopropoxide], Zr (i-OC ₃ H ₇ ) ₄ [zirconium isopropoxide], and CH ₃ COCH ₂ COCH ₃ (2,4-pentanedione) as a stabilizer as a solvent CH ₃ OC ₂ A solution diluted with H ₄ OH [methoxyethanol] is used.
[0006]
5 and FIG. 6A, the sol-gel coating film 32 is formed by applying the sol-gel raw material solution to a thickness of 100 to 200 nm on the SrTiO ₃ substrate 31 by spin coating or dipping. I do.
[0007]
5 and 6 (b). Next, the sol-gel coating film 32 is heated to 300 to 400 [deg.] C. to remove the organic components, and is dried to form a polymer gel state, thereby obtaining a dry gel film 33.
[0008]
Next, as shown in FIGS. 5 and 6C, the PZT film 34 having a perovskite type crystal structure is formed by firing in a high-temperature oxygen atmosphere of 600 ° C. or higher to crystallize the dried gel film 33.
[0009]
In order to produce a thicker PZT film, the above-described series of steps of coating, drying, and baking may be repeated as many times as necessary to obtain a required film thickness.
[0010]
As described above, it is known that a titanate-based composite oxide having a perovskite-type crystal structure exhibits ferroelectricity, and a thin film of this oxide can be used as a dielectric film, a piezoelectric film, a pyroelectric film, or the like.
[0011]
[Patent Document 1]
JP-A-6-305714
[Problems to be solved by the invention]
The above-mentioned PZT is a ceramic in which lead titanate (PbTiO ₃ : PTO) and lead zirconate (PbZrO ₃ : PZO) are mixed. However, since the crystallization temperatures of PTO and PZO are different, a film is formed on a substrate. Sometimes, when raising the temperature during firing, there is a problem that PTO having a low crystallization temperature is crystallized first and the composition may be inclined in the film thickness direction. This is described with reference to FIG. .
[0013]
FIG. 7 is an explanatory diagram of the composition distribution of the PZT layer 35 formed as described above. The distribution of the PZT film 34 is such that the Ti composition gradually decreases in the growth direction for each PZT film 34. When a film having a uniform composition distribution cannot be obtained and the composition is used for an optical deflection element or the like, the composition distribution becomes a refractive index distribution, which affects the element characteristics.
[0014]
In order to reduce such a composition gradient, the film thickness per layer may be reduced. In such a case, the number of times of repeating a series of coating, drying and baking to obtain a required film thickness Therefore, there is a problem that the film forming speed is extremely reduced because of the necessity of increasing the film formation speed.
[0015]
Therefore, an object of the present invention is to reduce the composition gradient in the film thickness direction without extremely lowering the film forming speed.
[0016]
[Means for Solving the Problems]
FIG. 1 is an explanatory diagram of the basic configuration of the present invention. Here, means for solving the problem in the present invention will be described with reference to FIG.
1 (a) and 1 (b) In order to achieve the above-mentioned object, the present invention relates to a method for forming a ferroelectric film 3 using a sol-gel method. It is characterized in that firing is performed after the gel solutions 1 and 2 are stacked before firing.
[0017]
As described above, at least two or more types of sol-gel solutions 1 and 2 having different compositions are stacked on the substrate 4 before firing, then fired together and crystallized, whereby the constituent metal in the upper layer during firing is fired. The composition gradient due to segregation can be reduced by the diffusion of the components to the lower layer side.
[0018]
In addition, as the ferroelectric film 3 in which the above method is effective, PbZr _x Ti _1-x O ₃ [where 0 <x <1] or Pb _1-y A _y (Zr _x Ti _1-x ) _{1 −y / 4} O ₃ [where 0 <x, y <1] is a typical one.
[0019]
In addition, of two or more sol-gel solutions 1 and 2 having different compositions, a sol-gel applied to the upper layer has a Ti composition ratio constituting PTO which is more likely to be segregated than PZO in the sol-gel solution 1 applied to the lower layer. By making the composition ratio smaller than the Ti composition ratio in the gel solution 2, the composition gradient can be reduced.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Here, the film forming process of the PZT film according to the embodiment of the present invention will be described with reference to FIGS. 2 to 4. FIG. 2 is a flow chart of the film forming by the sol-gel method according to the embodiment of the present invention. FIG. 3 and FIG. 4 are explanatory views of a PZT film forming process by the sol-gel method according to the embodiment of the present invention.
Referring to FIG. 2, a first sol-gel raw material solution having a PZT (60/40) composition and a second sol-gel solution having a PZT (40/60) composition are used as precursors through hydrolysis and polycondensation of a metal alkoxide. Prepare a gel raw material solution.
[0021]
In this case, as a sol-gel raw material solution for PZT, Pb (CH ₃ COO) ₂ .3H ₂ O [lead acetate] and Ti (i-OC ₃ H) which are organic compounds of constituent metal elements are used as in the conventional case. _{7) 4} [titanium _{isopropoxide], Zr (i-OC 3 H} 7) 4 [zirconium isopropoxide], and, _{CH 3} COCH ₂ COCH 3 as stabilizer (2,4-pentanedione) in a solvent A solution diluted with a certain CH ₃ OC ₂ H ₄ OH [methoxyethanol] is used. In the case of the first sol-gel raw material solution having a PZT (60/40) composition, Zr (i-OC ₃ The molar ratio of H ₇ ) ₄ / Ti (i-OC ₃ H ₇ ) ₄ is set to 60/40.
On the other hand, PZT (40/60) second sol composition - when the gel raw material solution, _Zr in the solution _{(i-OC 3 H 7)} 4 / Ti molar ratio of _{(i-OC 3 H 7)} 4 Is set to 40/60.
[0022]
2 and 3A, the first sol-gel raw material solution is applied on the SrTiO ₃ substrate 11 to a thickness of 10 to 100 nm, for example, 50 nm by spin coating to obtain a Zr-rich sol-gel. The coating film 12 is formed.
[0023]
2 and 3 (b), on a hot plate, baking is performed by heating to 300 to 400 ° C., for example, 350 ° C. to remove organic components in the Zr-rich sol-gel coating film 12 and then drying. The amorphous Zr-rich PZT film 13 is made into a polymer gel state.
[0024]
2 and 3 (c), a second sol-gel raw material solution is applied on the amorphous Zr-rich PZT film 13 to a thickness of 10 to 100 nm, for example, 50 nm by spin coating to obtain a Ti-rich sol. Forming a gel coating film 14;
[0025]
2 and FIG. 4 (d), on a hot plate, baking is performed by heating to 300 to 400 ° C., for example, 350 ° C., and the organic components in the Ti-rich sol-gel coating film 14 are removed and dried. An amorphous Ti-rich PZT film 15 is formed in a polymer gel state.
[0026]
Referring to FIGS. 2 and 4 (e), the amorphous Ti-rich PZT film 15 and the amorphous Zr-rich PZT film 13 are heated and fired in an electric furnace in an oxygen atmosphere at a temperature of 600 ° C. or more, for example, 700 ° C. to form a perovskite crystal structure. Of the PZT crystal film 16 is formed.
[0027]
In this firing step, the amorphous Zr-rich PZT film 13 is crystallized from the SrTiO ₃ substrate 11 side. At this time, the PTO which tends to segregate first crystallizes, but the Ti composition ratio in the amorphous Zr-rich PZT film 13 Is 0.4, so crystallization starts near the composition exceeding 0.5.
[0028]
Next, as the crystallization proceeds, the Ti composition ratio decreases, but since Ti diffuses from the amorphous Ti-rich PZT film 15 and is supplied, the reduction of the Ti composition ratio is compensated, and PbZr ₀ with small composition fluctuation is compensated. _.5 the Ti _0.5 O ₃ composition PZT crystal film 16 in the vicinity.
[0029]
In order to produce a PZT film thicker than that shown in FIG. 4F, a series of the above-described steps of coating, drying, coating, drying, and firing are repeated as many times as necessary to obtain a required film thickness.
[0030]
Further, as shown in the right side of FIG. 4F, the composition distribution of the multilayered PZT crystal layer 17 has a slight composition gradient in the growth direction for each PZT crystal layer 16, The composition gradient can be reduced as compared with the related art.
[0031]
As described above, in the embodiment of the present invention, two-layer amorphous PZT films having different compositions are formed using sol-gel raw materials having different compositions, the compositions are compensated in advance, and then firing for crystallization is performed. As a result, the composition gradient in the film thickness direction can be reduced.
[0032]
Also, by reducing the composition gradient, it is possible to obtain desired element characteristics when the PZT film is applied to various devices, and particularly when the PZT film is used for an optical element such as an optical deflection element. In addition, an optical element having reduced refractive index distribution and excellent characteristics can be realized.
[0033]
The embodiments of the present invention have been described above. However, the present invention is not limited to the configurations described in the embodiments, and various modifications are possible.
For example, in the above embodiment, a spin coating method is used as a method for applying the sol-gel raw material solution, but the method is not limited to the spin coating method, and a dipping method or a spray method may be used. is there.
[0034]
In the above embodiment, a PZT (60/40) solution and a PZT (40/60) solution are used to form a PbZr _0.5 Ti _0.5 O ₃ film. A combination of a (55/45) solution and a solution of another composition such as a PZT (45/55) solution may be used.
[0035]
In the above-described embodiment, the two layers of amorphous PZT films having different compositions are formed and then baked together. However, the three or more layers of amorphous PZT films having different compositions are formed and then baked. It may be fired.
[0036]
Further, in the above-described embodiment, the description is made as a PZT film forming process, but the present invention is not limited to the PZT film, and the present invention is not limited to the PZT film forming process of a lead-containing perovskite crystal containing at least Zr and Ti. is intended to be applied, for _example, like _{Pb 1-y La y (Zr} x Ti 1-x) 1-y / 4 O 3 also is intended to target.
[0037]
Further, in the above embodiments, the use of the ferroelectric film is not particularly mentioned, but it is used for an optical device such as an optical deflection element, a capacitor dielectric film of a FeRAM, a piezoelectric element, a pyroelectric element, and the like. Needless to say.
[0038]
【The invention's effect】
According to the present invention, since a dried film having a multilayer structure having a different composition is formed so as to compensate for segregation before firing, a ferroelectric material having a small composition gradient in a film thickness direction without greatly increasing a manufacturing process. It becomes possible to form a body film, which greatly contributes to the improvement of the performance and reliability of an optical device such as an optical deflection element or an electronic device such as an FeRAM or a piezoelectric element.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of a basic configuration of the present invention.
FIG. 2 is a flow chart of a film formation by a sol-gel method according to an embodiment of the present invention.
FIG. 3 is an explanatory diagram of a film forming process of a PZT film halfway by a sol-gel method of the present invention.
FIG. 4 is an explanatory view of a step of forming a PZT film by the sol-gel method of the present invention after FIG.
FIG. 5 is a flow chart of film formation by a conventional sol-gel method.
FIG. 6 is an explanatory diagram of a film forming process of a PZT film by a conventional sol-gel method.
FIG. 7 is an explanatory diagram of a composition distribution of a conventional multilayer PZT film.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 sol-gel solution 2 sol-gel solution 3 ferroelectric film 4 substrate 11 SrTiO ₃ substrate 12 Zr-rich sol-gel coating film 13 amorphous Zr-rich PZT film 14 Ti-rich sol-gel coating film 15 amorphous Ti-rich PZT film 16 PZT crystal Film 17 PZT crystal layer 31 SrTiO ₃ substrate 32 Sol-gel coating film 33 Dry gel film 34 PZT film 35 PZT layer

Claims (3)

In a method of forming a ferroelectric film using a sol-gel method, at least two types of sol-gel solutions having different compositions are stacked before firing, and then firing is performed. Film forming method. The ferroelectric _{layer, PbZr x Ti 1-x O} 3 [where 0 <x <1] or _{Pb 1-y La y (Zr} x Ti 1-x) 1-y / 4 O 3 [where 0 2. The method for forming a ferroelectric film according to claim 1, wherein the method comprises any one of <x, y <1>. The Ti composition ratio of the sol-gel solution applied to the lower layer of the two or more sol-gel solutions having different compositions is made smaller than the Ti composition ratio of the sol-gel solution applied to the upper layer. 3. The method for forming a ferroelectric film according to item 2.

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