KR20170128101A - Method of producing ferroelectric thin film - Google Patents

Abstract

This ferroelectric thin film manufacturing method is a method of manufacturing a ferroelectric thin film by applying a CSD solution in which a main solvent containing an organometallic compound for forming a ferroelectric thin film is acetic acid isophthalate to a substrate to form a gel coating film, / Or a liquid containing ethanol as a main component is sprayed or dropped to remove the outer peripheral end of the gel coating film and the gel coating film that has been removed from the outer peripheral end of the gel coating film is heat treated to form a ferroelectric thin film. The ferroelectric thin film is preferably a BST-based film, a BT-based film or an ST-based film.

Description

METHOD OF PRODUCING FERROELECTRIC THIN FILM FIELD OF THE INVENTION [0001]

The present invention relates to a method for producing a ferroelectric thin film by coating and firing a raw material solution prepared by a sol-gel method or the like. More specifically, it is a method suitable for producing a ferroelectric thin film of a BST-based film (strontium barium-titanate-based film), a BT-based film (barium titanate film), or an ST-based film (strontium titanate-based film).

The sol-gel method is a method in which a sol composed of a metal alkoxide is converted into a gel having lost fluidity by a hydrolysis / polycondensation reaction and the gel is heated and fired to be an oxide. As a technique for forming a film on a substrate (wafer) For example, a dip coating method in which a substrate is immersed in a solution and then pulled up at a constant speed, a flow coating method in which a solution is spread on a substrate from the top of the substrate, a method in which a rotating plastic roll surface is wetted with a solution, Or a spin coating method in which a solution is dripped onto a rotating substrate and is diffused and spread on a substrate by a rotational force. In particular, in the case of the spin coating method, the film tends to become thick at the peripheral edge of the substrate, and the phenomenon of returning to the back surface of the substrate tends to occur.

In the case of the coating method using the sol-gel method or the like (hereinafter, collectively referred to as the CSD method, the solution being referred to as the CSD solution), if the film thickness to be applied one time is excessively large, cracks tend to occur in the film after the heat treatment . The film peeled off by this crack becomes a particle, which may cause the yield of the device to be lowered. Therefore, a method is adopted in which the film at the outer peripheral end of the substrate is removed before the heat treatment.

As a method of removing the film at the outer peripheral edge of the substrate, there is a method called edge bead rinse (EBR) in which the organic solvent is brought into contact with the outer peripheral edge of the substrate while rotating the substrate after applying the raw material solution to remove the film.

When removing a part of the coating film by the CSD solution by the EBR method, it is generally considered that the solution used for the CSD solution is used as the liquid for EBR. On the other hand, when a ferroelectric thin film such as a PZT film (lead zirconate titanate zirconate film) or an SBT film (bismuth strontium tantalum film) is formed by applying a raw material solution prepared by a sol-gel method or the like, A CSD solution containing a metal compound is applied to a substrate to form a gel coating film and water is sprayed or dropped as an EBR liquid to the outer peripheral end while rotating the substrate to remove the outer peripheral edge of the gel coating film, And then removing the gel-like coating film after heat treatment to form a ferroelectric thin film (see, for example, Patent Document 1).

In the invention of Patent Document 1, the reason why water is selected as the liquid for EBR is that when the ferroelectric thin film is a PZT film (lead zirconate titanate zirconate film) or SBT film (bismuth strontium tantalum film), the CSD solution When methanol, ethanol, butanol, or the like is used as the main solvent, when a solvent such as methanol, ethanol, butanol is sprayed or dripped onto the outer peripheral end after the application of the CSD solution, penetration into the radial direction of the solvent , The elongation toward the radially outward direction of the melted film affects each other and the film thickness in the vicinity of the spraying or dropping region becomes uneven due to the deviation of the acting force between them and cracks or local peeling This is because there is a problem.

Japanese Laid-Open Patent Publication No. 2011-014649 (Claim 1, paragraph [0001], paragraph [0009])

Even when the same organic solvent as that of the main solvent is used as the liquid for EBR without considering the kind of the main solvent of the CSD solution, the same problem as described above may occur. In particular, the ferroelectric thin film may be a strontium titanate titanate-based film (hereinafter sometimes referred to as a BST-based film), a barium titanate-based film (hereinafter also referred to as a BT-based film) or a strontium titanate- ), The CSD solution is applied when acetic acid is used as the main solvent of the CSD solution as the raw material solution, and the water described in Patent Document 1 as the liquid for EBR is applied to the outer peripheral end of the CSD solution The CSD solution containing acetic acid as the main solvent in the case of spraying or dripping does not mix with water and thus there is a problem that the film on the outer peripheral end of the substrate can not be removed in the aqueous liquid EBR liquid. Further, even when the same acetic acid / soybean wheat as the main solvent is used as the liquid for EBR, the coating film is swollen and the film thickness becomes uneven, so that the removal rate of the film at the outer peripheral edge of the substrate can not be increased.

It is an object of the present invention to improve the removal rate of the film on the peripheral edge of the substrate when the CSD solution in which the main solvent is acetic acid isoam mill is subjected to edge bead rinsing after the spin coating to form the ferroelectric thin film, And a method for manufacturing a ferroelectric thin film which prevents the generation of particles by removing the film at the end without causing cracks or local peeling.

A first aspect of the present invention is a process for producing a ferroelectric thin film, comprising the steps of: applying a CSD solution containing an organometallic compound for forming a ferroelectric thin film to a substrate to form a gelated film; and injecting or dropping a liquid onto the outer peripheral end, And a step of forming a ferroelectric thin film by heat-treating the gel-like coating film after being removed from the outer peripheral end of the ferroelectric thin film, wherein the main solvent of the CSD solution is Acetic acid is a child wheat, and the liquid is a liquid containing methanol and / or ethanol as a main component.

A second aspect of the present invention is the invention based on the first aspect, wherein the ferroelectric thin film is a BST-based film, a BT-based film, or an ST-based film.

A third aspect of the present invention resides in an invention based on the first or second aspect, wherein the organometallic compound is a metal salt of a carboxylic acid represented by the general formula C _n H _{2n + 1} COOH (3 ≦ n ≦ 7) .

According to the method for producing a ferroelectric thin film of the first aspect of the present invention, when the main solvent of the CSD solution is acetic acid, the CSD solution is applied to the substrate and then a liquid containing methanol and / or ethanol as a main component The coating film at the outer circumferential end of the radially outward position from the point of spraying or dropping does not swell well because the acetic acid of the main solvent has a higher solubility than the other organic solvents in comparison with other organic solvents, It is possible to prevent cracks and local peeling in the film at the outer peripheral end portion.

According to the method for producing a ferroelectric thin film of the second aspect of the present invention, the main solvent in the production of the BST-based film, the BT-based film, or the ferroelectric thin film as the ST-based film includes the time- This good acetic acid is effective in producing the ferroelectric thin film because the child wheat is used more frequently.

According to the method for producing a ferroelectric thin film of the third aspect of the present invention, by using a metal salt of a carboxylic acid represented by the general formula C _n H _{2n + 1} COOH (3 ≦ n ≦ 7) Cracks or local peeling can be prevented because the coating film does not swell even more when the EBR-containing liquid for EBR and / or the ethanol-based liquid having a high solubility in the solvent is sprayed or dropped.

Fig. 1 is a cross-sectional view schematically showing a state in which a film on the outer peripheral end is removed while rotating a substrate in the embodiment of the present invention. Fig. (a) is a cross-sectional view showing a state before the outer peripheral end of the gel coating film is removed, and (b) is a sectional view showing a state after removing the outer peripheral end of the gel coating film.
Fig. 2 is a diagram schematically showing an end face portion at the outer peripheral end of the film. Fig.

BEST MODE FOR CARRYING OUT THE INVENTION Next, embodiments for carrying out the present invention will be described with reference to the drawings.

The method for producing a ferroelectric thin film of the present invention is a method for producing a perovskite-type oxide thin film, preferably a BST-based film, a BT-based film or an ST-based film, wherein the main solvent containing an organometallic compound is (CSD solution application step) in which a CSD solution is applied to a substrate to form a gel coating film, and a step of spraying or dropping a liquid containing methanol and / or ethanol as a main component onto the outer peripheral end while rotating the substrate, A step of removing the coating film (EBR step), and a step of forming a ferroelectric thin film by heat-treating the gel-like coating film that has been removed on the outer peripheral end thereof (heat treatment step). The BST-based film, the BT-based film or the ST-based film may contain calcium (Ca) and / or zirconium (Zr) as dopants, respectively. The amount of calcium as a dopant is preferably the number of moles of 0 to 20% of the total number of moles of barium and / or strontium, and the amount of zirconium as a dopant is preferably the number of moles of 0 to 20% of the number of moles of titanium.

<CSD solution>

The CSD solution used in this ferroelectric thin film production method is obtained by dissolving a metal compound with a solvent and adding a stabilizer and the like. In the BST-based film (strontium barium titanate-based film), a Ba raw material, a Sr raw material, A Ba raw material and a Ti raw material are used for the BT-based film (barium titanate based film), and a Sr raw material and a Ti raw material are used for the ST based film (strontium titanate based film). This CSD solution is a perovskite-type oxide-forming solution containing barium and / or strontium. In the case of a solution for forming a BST-based film, the organic barium compound of Ba raw material, the organic strontium compound of Sr raw material and the titanium alkoxide of Ti raw material are dissolved in an organic solvent. In the case of the solution for forming the BT-based film, the organic barium compound as the Ba raw material and the titanium alkoxide as the Ti raw material are dissolved in the organic solvent. In the case of the solution for forming the ST-based film, the organic strontium compound of the Sr raw material and the titanium alkoxide of the Ti raw material are dissolved in an organic solvent. When a BST-based film, a BT-based film or an ST-based film contains calcium (Ca) and / or zirconium (Zr) as a dopant, an organic calcium compound as a Ca raw material and an organic zirconium compound as a Zr raw material Is selected. When zirconium (Zr) is contained as a dopant in the ferroelectric thin film, the organic zirconium compound is preferably zirconium alkoxide.

The organic barium compound and the organic strontium compound are preferably a metal salt of a carboxylic acid represented by the general formula C _n H _{2n + 1} COOH (3 ≦ n ≦ 7). When calcium (Ca) is contained as a dopant in the ferroelectric thin film, the organic calcium compound is also preferably a metal salt represented by the above general formula. As such a carboxylic acid, the following compounds may be specifically exemplified depending on the number of n in the above general formula.

When n = 3, n-butyric acid can be mentioned. When n = 4,? -methylbutyric acid or isovaleric acid (? -methylbutyric acid) can be mentioned. When n = 5, 2-ethylbutyric acid, 2,2-dimethylbutyric acid, 3,3-dimethylbutyric acid, 2,3-dimethylbutyric acid, 3-methylpentanoic acid or 4-methylpentanoic acid may be mentioned. When n = 6, 2-ethylpentanoic acid, 3-ethylpentanoic acid, 2,2-dimethylpentanoic acid, 3,3-dimethylpentanoic acid, 2,3-dimethylpentanoic acid or 4-methylhexanoic acid have. When n = 7, 2-ethylhexanoic acid, 3-ethylhexanoic acid, 2,2-dimethylhexanoic acid or 3,3-dimethylhexanoic acid can be exemplified.

Examples of the titanium alkoxide include tetraethoxy titanium, tetraisopropoxy titanium, tetra n-butoxy titanium, tetra i-butoxy titanium, tetra t-butoxy titanium and dimethoxy diisopropoxy titanium. Examples of the zirconium alkoxide include zirconium n-butoxide, zirconium n-propoxide, zirconium isopropoxide, and zirconium ethoxide.

In the method for producing a ferroelectric thin film of the present invention, the organic barium compound and / or the organic strontium compound, if necessary, the organic calcium compound and / or the organic zirconium compound, the titanium alkoxide and the organic solvent are mixed at a suitable ratio To obtain a CSD solution. In order to homogenize the solution, heating and refluxing is carried out.

As the organic solvent, acetic acid and soybean wheat are used as main solvents. As used herein, the term &quot; main solvent &quot; refers to a solvent that contains 40% by mass or more of the solvent. The acetic acid isoam mill is excellent in the solubility of the carboxylic acid salt. In order to obtain the concentration and wettability of the CSD solution suitable for application, acetic acid may be mixed with a solvent such as a carboxylic acid as an auxiliary solvent in addition to the child wheat. However, the organic solvent of the present invention does not contain an alcohol such as methanol, ethanol, butanol or the like in view of low time-course stability when a carboxylic acid salt is dissolved. The total concentration of the organometallic compound in the CSD solution is preferably about 0.1 to 20% by mass in terms of the metal oxide.

In the CSD solution, β-diketones (for example, acetyl acetone, heptafluorobutanoyl pivaloyl methane, dipivaloyl methane, trifluoroacetylacetone, benzoyl acetone, etc.) Ketone acids (e.g., acetoacetic acid, propionylacetic acid, benzoyl acetic acid, etc.) at a molar ratio of 0.2 to 3 times with respect to the metal.

<CSD Solution Coating Step>

By applying the CSD solution to the substrate, a gel coating film is formed on the entire surface of the substrate. Examples of the substrate material include silicon wafers (single crystal), metals such as platinum and nickel, ruthenium oxide, iridium oxide, strontium ruthenate (SrRuO ₃ ) or cobalt-dispersed tantalum oxide ((La _x Sr _1-x ) CoO ₃ ) Silicon, glass, alumina, quartz, etc. having a coating film of a perovskite-type conductive oxide or the like. The method of applying the CSD solution on the substrate is a spin coat method.

<EBR Process>

As shown in Fig. 1 (a), while rotating the substrate 2 on which the gel-like coating film 1 is formed after applying the CSD solution, methanol and / or ethanol as the main component The outer peripheral end portion of the gel coating film 1 is removed as shown in Fig. Methanol in the range of 1 to 2 carbon atoms as the liquid for EBR, alcohol limited to ethanol, and alcohols having 3 or more carbon atoms such as 1-propanol, 2-propanol and 1-butanol, , 82.6 ° C and 117.7 ° C, respectively, because methanol and ethanol have boiling points of 64.7 ° C and 78.4 ° C, respectively, which rapidly volatilize after the injection or dropping onto the substrate and swell the coating film. When the same acetic acid and mother liquor as the main solvent of the CSD solution is used as the EBR liquid, the acetic acid-soybean wheat has high affinity with the coating film, so that the coating film can be effectively removed. However, Is a factor for promoting the swelling of the coating film, and it is not possible to reliably remove the film at the outer peripheral edge of the substrate. For the above reasons, the liquid for EBR of the present invention is a liquid containing methanol and / or ethanol as a main component. Here, the main component means a ratio of not less than 75 mass%, preferably not less than 80 mass%, based on 100 mass% of the liquid for EBR.

The rotation speed of the substrate at the time of EBR is, for example, 1000 to 3000 rpm. The position of the injection or dropping is appropriately set corresponding to the position of the object to be removed. For example, when removing the radially outer side from the outer peripheral edge 5 mm of the substrate, the liquid 4 is injected or dropped at a position of 5 mm radially inward from the outer peripheral edge of the substrate, The film in the range of 5 mm outside of the film is removed. The amount of spraying or dropping may be an amount sufficient to wash out the gel coating film appropriately set from the thickness of the coating film and present in the range to be removed. It is sufficient if the liquid 4 is jetted or continuously dropped for 2 to 5 seconds in the above rotational speed. In this EBR process, the nozzle 3 may be moved radially outward as necessary, while rotating the substrate 2 so that the gel-like coating film 1 at the outer peripheral end easily flows.

<Heat Treatment Step>

The heat treatment process includes a drying process, a plasticizing process, and a crystallization annealing process.

(Drying step)

After removing the peripheral edge portion, the gel-like coating film is dried to remove the solvent. The drying temperature varies depending on the type of the solvent, but is usually about 80 to 200 ° C, preferably 100 to 180 ° C. However, since the solvent is removed during the heating at the subsequent heating step for converting the metal compound in the raw material solution into the metal oxide, the step of drying the coating film is not necessarily required.

(Plasticizing process)

Thereafter, as a tentative firing step, the applied substrate is heated to completely hydrolyze or thermally decompose the organic metal compound to form a film made of a metal oxide. This heating is usually carried out in an atmosphere containing water vapor in a sol-gel method requiring hydrolysis, for example, in an air or a steam atmosphere (for example, a nitrogen atmosphere containing water vapor), and in the MOD method for pyrolysis Oxygen atmosphere. The heating temperature varies depending on the kind of the metal oxide, but is usually in the range of 150 to 550 占 폚, preferably 300 to 450 占 폚. The heating time is selected so that hydrolysis and pyrolysis proceed completely, but is usually about 1 minute to 1 hour.

In the case of the sol-gel method or the like, it is often difficult to obtain a film thickness necessary for the perovskite-type oxide thin film by one application. Therefore, if necessary, repeating the steps from the application of the CSD solution to the plasticity, And a film of a metal oxide having a desired film thickness is obtained while removing the gel coat film to the outer peripheral edge each time the coating is carried out.

(Crystallization annealing process)

The coating film obtained in this way is amorphous or crystallized insufficiently even in crystalline form, and thus has a low polarizability and can not be used as a ferroelectric thin film. Therefore, finally, as the crystallization annealing step, calcination is performed at a temperature equal to or higher than the crystallization temperature of the metal oxide to obtain a crystalline metal oxide thin film having a perovskite type crystal structure. The calcination for crystallization may be carried out continuously after the above-mentioned plasticity for each coated film, but it is necessary to repeat the calcination at a high temperature several times. This is economically advantageous.

The firing temperature for the crystallization is usually a relatively low temperature of 500 to 800 ° C, for example, 550 to 700 ° C. Therefore, a substrate having heat resistance enough to withstand this firing temperature is used. The firing (annealing) time for crystallization is usually about 1 minute to 1 hour, and the firing atmosphere is not particularly limited, but usually air or oxygen. The perovskite-type oxide thin film formed in this way is uniformly formed on the substrate, and there is no crack or locally peeled off at the outer circumferential end thereof, and therefore, a ferroelectric thin film free from adhesion of particles can be obtained.

Example

<Preparation of CSD solution for perovskite type oxide thin film containing Ba, Sr, Ca>

Seven kinds of CSD solutions 1 to 7 were prepared by mixing the carboxylate of Ba, Sr and Ca and the alkoxide of Ti and Zr so as to have the metal atom ratios shown in Table 1. Concretely, a carboxylate represented by the general formula M (C _n H _{2n + 1} COO) ₂ (3 ≦ n ≦ 7, M means Ba, Sr and Ca) as a raw material of Ba, Sr and Ca Were used. Titanium tetraisopropoxide was used as a Ti raw material. In addition, zirconium n-butoxide was used as a Zr raw material. These were weighed so as to have the metal atomic ratios shown in Table 1, and acetylacetone in a molar amount equivalent to the total number of moles of Zr and Ti was mixed as a stabilizer. In addition, in order to make the total concentration of the composition in terms of the metal oxide equivalent as shown in Table 1 (expressed as &quot; oxide concentration &quot; in Table 1), acetic acid / % Of carboxylic acid having the same n number as that of the carboxylate used in the CSD solution as a co-solvent was added at a ratio of 11 to 24 mass%, and the mixture was refluxed in a nitrogen atmosphere at 150 캜 for 1 hour, Of CSD solutions 1 to 7 for perovskite-type oxide thin films were prepared. Table 1 shows the content ratio of the main solvent acetic acid and the carboxylic acid in the auxiliary solvent used in the seven kinds of CSD solutions 1 to 7, respectively.

&Lt; Example 1 >

1 ml of the CSD solution 1 was dropped onto the surface of the Pt film of the substrate on which the SiO ₂ film, the TiO ₂ film and the Pt film were formed in this order on a silicon wafer having a diameter of 4 inches. The substrate was first spin- And then rotated at a rotation speed of 3000 rpm for 15 seconds to spin-coat the CSD solution 1 on the Pt film (hereinafter simply referred to as the substrate) of the substrate. Thereafter, while rotating the substrate at a rotation speed of 2800 rpm with a spin coater, methanol was sprayed at a position 5 mm radially inward from the outer circumferential end of the substrate to dissolve the gel coating film, and EBR treatment was performed. The substrate was heated on a hot plate heated to 300 캜 for 5 minutes to pyrolyze the organic material. This operation was repeated, and the coating was applied twice in total, followed by firing at 700 占 폚 for 5 minutes by rapid heating (RTA) to obtain a BST-based film as a perovskite-type oxide thin film on the substrate.

&Lt; Examples 2 to 11 and Comparative Examples 1 to 8 >

CSD solutions 1 to 7 were prepared in the same manner as in Example 1 on the basis of the same substrate as that used in Example 1 except that the seven types of CSD solutions 1 to 7 shown in Table 2 were used in Examples 2 to 11 and Comparative Examples 1 to 8 Respectively. Subsequently, the EBR treatment using the liquid for EBR shown in Table 2 was carried out in the same manner as in Example 1. A BST-based film, an ST-based film, and a BT-based film, which are perovskite-type oxide films, were obtained on the substrate in the same manner as in Example 1.

<Comparative Test and Evaluation>

The outer peripheral end portions of the 19 types of films obtained in Examples 1 to 11 and Comparative Examples 1 to 8 were observed with a contact type surface shape measuring instrument (manufactured by Veeco Instruments, Dectak), and a hump hump rate was calculated. The outer peripheral edge of the film was observed with naked eyes, and the presence or absence of cracking or film peeling was examined. Here, the term "hump ratio" refers to a ratio (h / a (h / a)) of a value obtained by subtracting the average thickness from the thickness of the thickest portion measured from the substrate surface (hump height h) ) X 100 (%)). The results are shown in Table 2.

The average film thickness a is a value obtained by subtracting the film thickness from the thickest part of the peripheral edge of the film (the hump measuring position) to the center of the substrate to 0.3 mm from the thickest part of the peripheral edge of the film to the center of the substrate to 0.8 mm And calculated by averaging.

As evident from Table 2, in Comparative Example 1 using 1-propanol and Comparative Example 2 using 2-propanol as liquids for EBR, no peeling or cracking of the membrane was observed, but the boiling point was higher than that of methanol or ethanol, As the coating swelled, all of the hump rates were as high as "6%". In Comparative Example 3 in which 1-butanol was used as a liquid for EBR, the film had no peeling or cracking but had a higher boiling point than 1-propanol or 2-propanol, so the hump rate was as high as 8%.

In Comparative Examples 4 and 6 in which the same acetic acid / soybean wheat as the main solvent was used as the liquid for EBR, the film had no peeling or cracking, but the boiling point was higher than that of methanol or ethanol and the film swelled, % ", Respectively.

In Comparative Example 7 using water as the liquid for EBR and Comparative Example 8 using water and propylene glycol as the liquid for EBR in a ratio of 80:20 in mass ratio, , So that the measurement of the hump rate could not be normally performed. The peeling of the film was confirmed locally.

On the other hand, in Examples 1 to 11 in which a liquid containing methanol and / or ethanol as a main component was used as the liquid for EBR, the hump ratio at the peripheral edge of the membrane was low as "4 to 4.5%", . No separation or cracking of the film was observed over the entire periphery of the peripheral edge of the film.

The ferroelectric thin film produced by the method of the present invention can be used for a device such as a capacitor, a ferroelectric memory (FeRAM), or a piezoelectric device.

1 gel coating film
2 substrate
3 nozzle
4 liquid
h hump height
a Average film thickness

Claims (3)

A step of applying a CSD solution containing an organometallic compound for forming a ferroelectric thin film to a substrate to form a gel coating film; and a step of spraying or dropping a liquid onto the outer peripheral end portion while rotating the substrate to remove the outer peripheral end of the gel coating film And a step of forming a ferroelectric thin film by heat-treating the gel-like coating film after being removed from the outer peripheral end, the method comprising the steps of:
Wherein the main solvent of the CSD solution is acetic acid isoam, and the liquid is a liquid containing methanol and / or ethanol as a main component. The method according to claim 1,
Wherein the ferroelectric thin film is a BST-based film (strontium barium titanate-based film), a BT-based film (barium titanate film), or an ST-based film (strontium titanate film). 3. The method according to claim 1 or 2,
Wherein the organometallic compound is a metal salt of a carboxylic acid represented by the general formula C _n H _{2n + 1} COOH (3 ≦ n ≦ 7).

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