JP6665673B2 - Manufacturing method of ferroelectric thin film - Google Patents

Description

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

  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 baked to form an oxide, and a film is formed on a substrate (wafer). Examples of such techniques include a dip coating method in which a substrate is immersed in a solution and then lifted at a constant speed, a flow coating method in which the solution is poured from above the substrate onto the substrate and spread, and a rotating plastic roll surface is wetted with the solution and the substrate is transported. Various methods are known, such as a roll coating method in which a solution is contacted with a roll while spinning, or a spin coating method in which a solution is dropped on a rotating substrate and the solution is flowed and spread on the substrate by rotational force. Among them, in the case of the spin coating method in particular, the film tends to be thick at the outer peripheral edge of the substrate, and the phenomenon of wrapping around the back surface of the substrate is likely to occur.

  In the case of a coating method using such a sol-gel method or the like (hereinafter, the solution is collectively referred to as a CSD solution), if the film thickness applied at one time is too thick, cracks occur in the film after the heat treatment. There is a problem that it is easy. Since the film peeled off by the cracks becomes particles, which may cause a reduction in the yield of the device, a method of removing the film at the outer peripheral end of the substrate before the heat treatment is adopted.

  As a method for removing the film at the outer peripheral edge of the substrate, a method called edge bead rinsing (EBR) for removing the film by applying an organic solvent to the outer peripheral edge of the substrate while rotating the substrate after applying the raw material solution is used. is there.

  When a part of the coating film formed by the CSD solution is removed by the EBR method, it is generally considered that the solution used for the CSD solution is used as the EBR liquid. On the other hand, when a raw material solution manufactured by a sol-gel method or the like is applied and fired to form ferroelectric thin films such as a PZT film (lead zirconate titanate film) and an SBT film (bismuth strontium tantalate film), A CSD solution containing an organometallic compound for forming a ferroelectric thin film is applied to a substrate to form a gel-like coating film, and water is sprayed or dropped as a liquid for EBR on the outer peripheral edge while rotating the substrate. Then, after removing the outer peripheral edge of the gel-like coating film, a method of forming a ferroelectric thin film by heat-treating the gel-like coating film after removing the outer peripheral edge portion is disclosed (for example, See Patent Document 1.).

  The reason why water is selected as the EBR liquid in the invention of Patent Document 1 is that when the ferroelectric thin film is a PZT film (lead zirconate titanate film) or an SBT film (bismuth strontium tantalate film), Due to the use of methanol, ethanol, butanol, etc. as the main solvent of the CSD solution, which is a solution, the solvent such as methanol, ethanol, butanol, etc. is sprayed or dropped onto the outer peripheral edge after the CSD solution is applied. Of the melted film and the radially outward expansion of the melted film interact with each other. This is because there is a problem that cracks and local peeling occur in the vicinity of the sprayed or dropped region after the heat treatment.

JP 2011-014649A (Claim 1, paragraph [0001], paragraph [0009])

  Even when the same organic solvent as the main solvent is used as the liquid for EBR without considering the type of the main solvent of the CSD solution, the same problems as described above may occur. In particular, the ferroelectric thin film is a strontium barium titanate-based film (hereinafter, also 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-based film. When using isoamyl acetate as a main solvent of a CSD solution as a raw material solution for the case where the film is a film (hereinafter, also referred to as an ST-based film), after applying the CSD solution, When water described in Patent Literature 1 is jetted or dropped as a liquid for EBR to the portion, the CSD solution containing isoamyl acetate as a main solvent does not mix with water. There was a problem that the film could not be removed. In addition, even when isoamyl acetate, which is the same as the main solvent, is used as the EBR liquid, the coating film swells and the film thickness becomes non-uniform, so that the removal rate of the film at the outer peripheral edge of the substrate cannot be increased. There was a point.

  An object of the present invention is to increase the removal rate of a film at the outer peripheral edge of a substrate when spin-coating a CSD solution whose main solvent is isoamyl acetate after spin coating to form a ferroelectric thin film. It is another object of the present invention to provide a method of manufacturing a ferroelectric thin film capable of preventing generation of particles by removing a film at an outer peripheral end portion of a substrate without causing cracks or local peeling.

  According to a first aspect of the present invention, there is provided a step of applying a CSD solution containing an organometallic compound for forming a ferroelectric thin film to a substrate to form a gel-like coating film; Spraying or dropping a liquid on the part, removing the outer peripheral edge of the gel-like coating, and heating the gel-like coating after the outer peripheral edge has been removed to form a ferroelectric thin film. Forming a ferroelectric thin film, wherein the main solvent of the CSD solution is isoamyl acetate, and the liquid is a liquid containing methanol and / or ethanol as a main component.

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

  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 isoamyl acetate, the CSD solution is applied to the substrate, and then the methanol and / or ethanol is applied to the rotating substrate. When a liquid containing as a main component is jetted or dropped, this liquid has a higher solubility in isoamyl acetate as a main solvent than other organic solvents, so that the liquid is located at a position radially outward from the jetted or dropped portion. The coating film on the outer peripheral end does not easily swell and does not become partially thick, and as a result, cracks and local peeling of the film on the outer peripheral end can be prevented.

  According to the method for producing a ferroelectric thin film of the second aspect of the present invention, the main solvent used for producing a ferroelectric thin film that is a BST-based film, a BT-based film, or an ST-based film contains the organometallic compound described above. Since isoamyl acetate having good stability over time in the case of dissolving is frequently used, it is effective in producing the ferroelectric thin film.

According to the method for producing a ferroelectric thin film of the third aspect of the present invention, the metal salt of a carboxylic acid represented by the general formula C _n H _{2n + 1} COOH (where 3 ≦ n ≦ 7) is used as the organometallic compound. By using this, even when a liquid mainly containing methanol and / or ethanol for EBR having high solubility in the main solvent is jetted or dropped, the coating film is less likely to swell, so that cracks and local peeling are prevented. Can be prevented.

FIG. 4 is a cross-sectional view schematically showing a state in which a film at an outer peripheral end is removed while rotating the substrate in the embodiment of the present invention. (A) is a sectional view showing a state before removing the outer peripheral end of the gel-like coating film, and (b) is a sectional view showing a state after removing the outer peripheral end of the gel-like coating film. It is a figure which shows the cross section in the outer peripheral end part of a film typically.

  Next, an embodiment 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 oxide thin film, preferably a BST-based film, a BT-based film or an ST-based film, wherein the main solvent containing the organometallic compound is isoamyl acetate. Applying a CSD solution to a substrate to form a gel-like coating film (CSD solution applying step), and spraying or spraying a liquid containing methanol and / or ethanol as a main component on the outer peripheral edge while rotating the substrate. A step of dropping and removing the gel-like coating film at the outer peripheral end (EBR step), and a step of heat-treating the gel-like coating film removed at the outer peripheral end to form a ferroelectric thin film (Heat treatment step). The BST-based film, the BT-based film, or the ST-based film may include calcium (Ca) and / or zirconium (Zr) as dopants, respectively. The amount of calcium as a dopant is preferably 0 to 20% of the total number of moles of barium and / or strontium, and the amount of zirconium as a dopant is preferably 0 to 20% of the number of moles of titanium.

<CSD solution>
The CSD solution used in the method for producing a ferroelectric thin film is obtained by dissolving a metal compound in a solvent and adding a stabilizer and the like. In a BST-based film (a strontium barium titanate-based film), a Ba material is used. , Sr raw material, and Ti raw material are used. For a BT-based film (barium titanate-based film), a Ba raw material and a Ti raw material are used. For an ST-based film (a strontium titanate-based film), an Sr raw material and a Ti raw material are used. . 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 film, an organic barium compound as a Ba raw material, an organic strontium compound as a Sr raw material, and a titanium alkoxide as a Ti raw material are dissolved in an organic solvent. In the case of a solution for forming a BT-based film, an organic barium compound as a Ba raw material and a titanium alkoxide as a Ti raw material are dissolved in an organic solvent. In the case of a solution for forming an ST-based film, an Sr raw material organic strontium compound and a Ti raw material titanium alkoxide are dissolved in an organic solvent. When the BST-based film, the BT-based film or the ST-based film contains calcium (Ca) and / or zirconium (Zr) as a dopant, the organic metal compound is 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 included as a dopant for the ferroelectric thin film, the organic zirconium compound is preferably a zirconium alkoxide.

The organic barium compound and the organic strontium compound are preferably metal salts of a carboxylic acid represented by the general formula C _n H _{2n + 1} COOH (provided that 3 ≦ n ≦ 7). When calcium (Ca) is contained as a dopant for the ferroelectric thin film, the organic calcium compound is also preferably a metal salt represented by the above general formula. Specific examples of such a carboxylic acid include the following compounds according to the number of n in the above general formula.

  When n = 3, n-butyric acid is exemplified. When n = 4, α-methylbutyric acid or isoxouric acid (β-methylbutyric acid) is exemplified. When n = 5, examples thereof include 2-ethylbutyric acid, 2,2-dimethylbutyric acid, 3,3-dimethylbutyric acid, 2,3-dimethylbutyric acid, 3-methylpentanoic acid and 4-methylpentanoic acid. When n = 6, mention may be made of 2-ethylpentanoic acid, 3-ethylpentanoic acid, 2,2-dimethylpentanoic acid, 3,3-dimethylpentanoic acid, 2,3-dimethylpentanoic acid or 4-methylhexanoic acid. Can be When n = 7, examples thereof include 2-ethylhexanoic acid, 3-ethylhexanoic acid, 2,2-dimethylhexanoic acid, and 3,3-dimethylhexanoic acid.

  Examples of the titanium alkoxide include tetraethoxytitanium, tetraisopropoxytitanium, tetra-n-butoxytitanium, tetra-i-butoxytitanium, tetra-t-butoxytitanium, dimethoxydiisopropoxytitanium and the like. Examples of the zirconium alkoxide include zirconium n-butoxide, zirconium n-propoxide, zirconium isopropoxide, zirconium ethoxide, and the like.

  In the method for producing a ferroelectric thin film of the present invention, an organic barium compound and / or an organic strontium compound, and if necessary, an organic calcium compound and / or an organic zirconium compound, a titanium alkoxide, and an organic solvent have a desired metal component concentration. As described above. In addition, heating and reflux are performed to homogenize the solution.

  As the organic solvent, isoamyl acetate is used as a main solvent. As used herein, the term “main solvent” means that the solvent contains 51% by mass or more. Isoamyl acetate is excellent in solubility of carboxylate. In order to make the above-mentioned CSD solution have a concentration and wettability suitable for coating, a solvent such as carboxylic acid may be mixed as an auxiliary solvent in addition to isoamyl acetate. However, the organic solvent of the present invention does not include alcohols such as methanol, ethanol, and butanol because the stability with time when the carboxylate is dissolved is low. The total concentration of the organometallic compound in the solution is preferably about 0.1 to 20% by weight in terms of a metal oxide.

  In the CSD solution, if necessary, β-diketones (eg, acetylacetone, heptafluorobutanoylpivaloylmethane, dipivaloylmethane, trifluoroacetylacetone, benzoylacetone, etc.) as stabilizers, ketones Acids (for example, acetoacetic acid, propionylacetic acid, benzoylacetic acid, etc.) may be blended in a molar ratio to the metal of about 0.2 to 3 times.

<CSD solution application process>
A gel-like coating film is formed on the entire surface of the substrate by applying the CSD solution to the substrate. As the substrate material, a silicon wafer (single crystal), and platinum, metals such as nickel, ruthenium oxide, iridium oxide, strontium ruthenate (SrRuO ₃₎ or lanthanum cobaltate strontium _{((La x Sr 1-x} ) CoO 3 ), A substrate of silicon, glass, alumina, quartz or the like having a coating of a perovskite-type conductive oxide or the like. The method of applying the CSD solution on the substrate is a spin coating method.

<EBR process>
As shown in FIG. 1 (a), while rotating the substrate 2 on which the gel-like coating film 1 after the application of the CSD solution is formed, methanol and / or ethanol are mainly supplied from the upper nozzle 3 to the outer peripheral end thereof. By spraying or dropping the liquid 4 as a component, the outer peripheral end of the gel-like coating film 1 is removed as shown in FIG. As the liquid for EBR, the alcohol having a carbon number in the range of 1 to 2 is limited to methanol and ethanol, and the alcohol having 3 or more carbon atoms, for example, 1-propanol, 2-propanol and 1-butanol has a boiling point of The boiling points of methanol and ethanol are 64.7 ° C and 78.4 ° C, whereas they are 97.2 ° C, 82.6 ° C and 117.7 ° C, respectively, and are quickly volatilized after being sprayed or dropped onto the substrate. However, this is because swelling of the coating film is small. When isoamyl acetate, which is the same as the main solvent of the CSD solution, is used as the liquid for EBR, isoamyl acetate has a high affinity with the coating film and can effectively remove the coating film. Difficulty is a factor that promotes 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 EBR liquid of the present invention is a liquid containing methanol and / or ethanol as a main component. Here, the main component refers to a ratio containing at least 75% by mass, preferably at least 80% by mass with respect to 100% by mass of the EBR liquid.

  The rotation speed of the substrate during EBR is, for example, 1000 to 3000 rpm. The position of the jet or the drop is appropriately set according to the position of the object to be removed. For example, when removing the outer periphery in the radial direction from the outer periphery 5 mm of the substrate, the liquid 4 is ejected or dropped at a position 5 mm radially inward from the outer periphery of the substrate, and 5 mm outside the ejection or dripping position is ejected. Remove the area of the film. The amount of spraying or dropping is appropriately set based on the thickness of the coating film and the like, and may be an amount sufficient to wash away the gel coating film existing in the range to be removed. At the above rotational speed, it is sufficient if the liquid 4 is continuously jetted or dropped for 2 to 5 seconds. In this EBR step, the nozzle 3 may be moved outward in the radial direction as needed while rotating the substrate 2 so that the gel-like coating film 1 at the outer peripheral end can easily flow. .

<Heat treatment step>
The heat treatment step includes a drying step, a calcining step, and a crystallization annealing step.

(Drying process)
After removing the outer edge, 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 200C, and preferably in the range of 100 to 180C. However, the solvent is removed during the heating in the next step for converting the metal compound in the raw material solution into the metal oxide, so that the step of drying the coating film is not necessarily required.

(Calcination process)
Thereafter, as a calcining step, the applied substrate is heated, and the organometallic compound is completely hydrolyzed or thermally decomposed to be converted into a metal oxide, thereby forming a film made of the metal oxide. This heating is generally performed in an atmosphere containing steam in the sol-gel method requiring hydrolysis, for example, in an air or steam-containing atmosphere (for example, a nitrogen atmosphere containing steam), and in the MOD method for thermal decomposition, oxygen-containing is used. It takes place in an atmosphere. The heating temperature varies depending on the type of the metal oxide, but is usually in the range of 150 to 550 ° C, preferably 300 to 450 ° C. The heating time is selected so that the hydrolysis and thermal decomposition proceed completely, but is usually about 1 minute to 1 hour.

  In the case of a sol-gel method or the like, it is often difficult to obtain a film thickness required for a perovskite-type oxide thin film in a single application. Therefore, if necessary, the above steps from the application of the CSD solution to calcination may be performed. By repeating the application, the CSD solution is repeatedly applied, and each time the application is performed, a metal oxide film having a desired film thickness is obtained while removing the gel-like coating film from the outer peripheral end.

(Crystallization annealing step)
The thus-obtained coating film is amorphous or crystalline and has insufficient crystallinity, so that it has low polarizability and cannot be used as a ferroelectric thin film. Therefore, finally, as a crystallization annealing step, firing 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 crystal structure. The firing for crystallization may not be performed once at the end, but may be performed after the above-described calcination for each applied coating film, but it is necessary to repeat firing at a high temperature many times. Therefore, it is economically advantageous to perform the operation at the end.

The firing temperature for this crystallization may be a relatively low temperature of usually 500 to 800C, for example, 550 to 700C. 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 is usually air or oxygen. The perovskite-type oxide thin film thus formed is uniformly formed on the substrate, and there is no crack or local peeling even at the outer peripheral end thereof, and therefore, a ferroelectric thin film free of particles is obtained. Can be.
from here

<Preparation of CSD solution for perovskite oxide thin film containing Ba, Sr, and Ca>
Seven types of CSD solutions 1 to 7 were prepared by mixing the carboxylate salts of Ba, Sr, and Ca and the alkoxides of Ti, Zr so as to have the metal atom ratios shown in Table 1. Specifically, a carboxylic acid represented by the general formula M (C _n H _{2n + 1} COO) ₂ (where 3 ≦ n ≦ 7, M means Ba, Sr, Ca) as a raw material for Ba, Sr, and Ca. Salt was used. Titanium tetraisopropoxide was used as a Ti raw material. Further, zirconium n-butoxide was used as a Zr raw material. These were weighed so as to have the metal atom ratios shown in Table 1, and acetylacetone was mixed as a stabilizer in an amount of 1 times the total mole number of Zr and Ti. Further, isoamyl acetate was used as a main solvent in 100% by mass of the CSD solution so that the total concentration of the composition in terms of oxide was the concentration of mass% shown in Table 1 (in Table 1, indicated by “oxide concentration”). The carboxylic acid having the same n number as that of the carboxylate used in the CSD solution is added as a co-solvent in an amount of 11 to 24% by mass, and the mixture is added at 150 ° C for 1 hour in a nitrogen atmosphere. The mixture was refluxed to prepare seven types of CSD solutions 1 to 7 for perovskite-type oxide thin films. Table 1 shows the content of isoamyl acetate as the main solvent and the content of carboxylic acid as the auxiliary solvent used in the seven types of CSD solutions 1 to 7, respectively.

<Example 1>
1 mL of CSD solution 1 was dropped on the Pt film surface of a substrate having a SiO ₂ film, a TiO ₂ film, and a Pt film formed in this order on a silicon wafer having a diameter of 4 inches, and the substrate was first coated with a spin coater at a rotation speed of 500 rpm. The CSD solution 1 was spin-coated on the Pt film of the substrate (hereinafter, simply referred to as the substrate) by rotating the substrate at a rotation speed of 3000 rpm for 15 seconds. 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 peripheral end of the substrate to dissolve the gel-like coating film, and EBR treatment was performed. This substrate was heated on a hot plate heated to 300 ° C. for 5 minutes to thermally decompose organic substances. This operation was repeated, and after coating was performed twice in total, baking was performed at 700 ° C. for 5 minutes by rapid heating (RTA) to obtain a BST-based film as a perovskite oxide thin film on the substrate.

<Examples 2 to 11, Comparative Examples 1 to 8>
Using the seven types of CSD solutions 1 to 7 shown in Table 2, the CSD solutions 1 to 7 were prepared on the same substrate as in Example 1 in the same manner as in Example 1 by using Examples 2 to 11 and Comparative Examples 1 to 7. Each 8 was spin-coated. Next, EBR treatment was performed in the same manner as in Example 1 using the EBR liquids shown in Table 2. Thereafter, the same procedure as in Example 1 was performed to obtain a BST-based film, an ST-based film, and a BT-based film as perovskite-type oxide thin films on the substrate.

<Comparison test and evaluation>
The outer peripheral edges of the 19 types of films obtained in Examples 1 to 11 and Comparative Examples 1 to 8 were observed with a stylus type surface shape measuring device (Veeco Instruments, Inc., device name: Dectak), and the outer peripheral edges of the films were observed. The hump ratio as the removal ratio of the coating film in the part was calculated. The outer peripheral edge of the film was visually observed to check for cracks and film peeling. Here, the “hump ratio” is a ratio ((hump height h) of a value obtained by subtracting the average film thickness from the film thickness of the thickest portion measured from the substrate surface to the average film thickness a as shown in FIG. 2 (( h / a) × 100 (%)). Table 2 shows the results.

  As is clear from Table 2, in each of Comparative Example 1 using 1-propanol and Comparative Example 2 using 2-propanol as the liquid for EBR, there was no peeling or cracking of the film, but methanol and ethanol were used. The hump ratio was as high as "6%" due to the higher boiling point and swelling of the coating film. In Comparative Example 3 using 1-butanol as the liquid for EBR, although there was no peeling or cracking of the film, the hump ratio was “8%” because the boiling point was higher than that of 1-propanol or 2-propanol. it was high.

  In Comparative Examples 4 and 6, in which the same solvent as the main solvent, isoamyl acetate was used as the liquid for EBR, neither peeling nor cracking of the film was observed, but the boiling point was higher than that of methanol or ethanol, and the film was swollen. The rate was as high as 5%.

  Further, in Comparative Example 7 using water as the liquid for EBR and Comparative Example 8 using water and propylene glycol mixed at a mass ratio of 80:20 as the liquid for EBR, in both cases, the outer peripheral edge of the film was used. There was a part where the coating film could not be removed, and the hump ratio could not be measured normally. Moreover, peeling of the film was locally confirmed.

  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 outer peripheral end of the film was “4 to 4.5%”. And the removal rate of the coating film on the outer peripheral edge of the film was high. Also, no peeling or cracking of the film was observed over the entire outer peripheral edge of the film.

  The ferroelectric thin film manufactured by the method of the present invention can be used for devices such as a capacitor, a ferroelectric memory (FeRAM), and a piezoelectric element.

DESCRIPTION OF SYMBOLS 1 Gel-like 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-like coating film, and spraying or dropping a liquid on an outer peripheral end while rotating the substrate. Removing the outer peripheral edge of the gel coating film, and heating the gel coating film removed from the outer peripheral edge to form a ferroelectric thin film In the method for producing a thin film,
A method for producing a ferroelectric thin film, wherein a main solvent of the CSD solution is isoamyl acetate, and the liquid is a liquid containing methanol and / or ethanol as a main component.   2. The ferroelectric thin film according to claim 1, wherein the ferroelectric thin film is a BST-based film (a strontium titanate-based film), a BT-based film (a barium titanate film), or an ST-based film (a strontium titanate film). Method. The method for producing a ferroelectric thin film according to claim 1, wherein the organometallic compound is a metal salt of a carboxylic acid represented by the general formula C _n H _{2n + 1} COOH (where 3 ≦ n ≦ 7).

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