Formation method of complex oxide pattern

Abstract

To provide a technique capable of finely processing a complex oxide to a submicron size or smaller.SOLUTION: A formation method of a complex oxide pattern includes: a first step of forming a resist film 20 containing hydrogen silsesquioxane (HSQ) on a substrate 10; a second step of exposing the resist film 20 with an energy line; a third step of developing the resist film 20 to form a mask 22; a fourth step of vapor-depositing a thin film 30 of a complex oxide on the mask 22; and a fifth step of removing the mask 22.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for forming a complex oxide pattern. [Background technology]

[0002] Composite oxides containing two or more metal elements are widely used as electronic materials such as ferroelectrics, magnetic materials, semiconductors, and superconductors due to their electronic and magnetic properties. In recent years, the development of composite oxides called multiferroic materials, which have multiple properties such as ferromagnetism, ferroelectricity, and ferroelasticity, has progressed. For example, Patent Document 1 describes BiFe 1-x A x A magnetic memory element using a thin film of a multiferroic material represented by O3 (wherein A is Co or Mn) has been proposed. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-009304 Summary of the Invention [Problem to be solved by the invention]

[0004] High integration of electronic devices such as magnetic memories is essential to improve their performance. High integration of complex oxide electronic devices, such as those described in Patent Document 1, requires precise microfabrication of the complex oxide. However, complex oxides are generally difficult to process because they are harder and more brittle than metal materials or silicon. Therefore, microfabrication of complex oxides to submicron sizes has not yet been achieved.

[0005] The present invention has been made in view of the above circumstances, and aims to provide a technique that enables microfabrication of complex oxides to submicron sizes or less. [Means for solving the problem]

[0006] A method for forming a complex oxide pattern according to one embodiment of the present invention includes the steps of: A first step of forming a resist film containing hydrogen silsesquioxane (HSQ) on a substrate; a second step of exposing the resist film to energy rays; a third step of developing the resist film to form a resist mask; a fourth step of depositing a thin film of a composite oxide on the resist mask; and a fifth step of removing the resist mask. [Effects of the Invention]

[0007] According to the present invention, a technique can be provided that enables microfabrication of composite oxides to submicron sizes or less. [Brief explanation of the drawings]

[0008] [Figure 1] 1(a) to 1(e) are diagrams illustrating a method for forming a complex oxide pattern according to one embodiment of the present invention. [Figure 2] 1 is a schematic diagram showing the thin film growth process using a pulsed laser deposition (PLD) device. [Figure 3] This is a scanning electron microscope (SEM) image of an HSQ mask. [Figure 4] This is an atomic force microscope (AFM) image of BiFe0.9Co0.1O3 (BFCO). [Figure 5] 5(a) to 5(i) are diagrams showing the observation results of polarization switching of BFCO nanodots by voltage application. [Figure 6] FIG. 1 shows the results of observing the ferroelectricity of a single BFCO nanodot by PFM. [Figure 7] 7(a) and 7(b) show the results of observing the ferroelectricity of a single BFCO nanodot by MFM. [Figure 8] This is an SEM image of La0.6Sr0.4MnO3 (LSMO) nanodots after lift-off. [Figure 9]AFM topography image of LSMO nanodots. [Figure 10] 10(a) and 10(b) show the observation results of ferromagnetic domains in LSMO nanodots. [Figure 11] This is an SEM image of BaTiO3 (BTO) nanodots after lift-off. [Figure 12] This is an AFM shape image of BTO nanodots. [Figure 13] FIG. 1 shows the results of PFM observation of the ferroelectricity of a single BTO nanodot. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same elements are given the same reference numerals, and duplicated explanations will be omitted as appropriate. Furthermore, the configurations described below are examples and do not limit the scope of the present invention in any way.

[0010] A method for forming a complex oxide pattern according to one embodiment of the present invention will be described with reference to FIGS. 1(a) to 1(e).

[0011] First, a resist film 20 containing hydrogen silsesquioxane (HSQ) is formed on a substrate 10 (FIG. 1(a)). The resist film 20 is formed, for example, by uniformly applying a solution containing HSQ onto the substrate 10 by spin coating and heating it at a temperature of 100°C to 200°C.

[0012] In photolithography, the size of the smallest resolvable pattern is proportional to the wavelength of the exposure light source, so nano-level fine processing is performed by using energy rays with shorter wavelengths for exposure. HSQ has the general formula (HSiO 1.5 ) nIt is a compound represented by the formula: and is primarily used as a resist material in semiconductor manufacturing, forming a latent image by energy beam irradiation. Because nano-level patterns can be drawn on an HSQ film by energy beam irradiation, it is possible to form fine patterns of complex oxides. HSQ also has excellent resistance to high temperatures, and does not decompose even in the high-temperature environment of the complex oxide film formation process described below, nor does HSQ contaminate the complex oxide. Therefore, nano-level patterns can be formed on a substrate 10 without degrading the complex oxide.

[0013] Next, the resist film 20 is exposed to energy beams E (FIG. 1(b)). Specifically, a desired pattern is drawn on the resist film 20 by irradiating the resist film 20 with energy beams E while controlling the irradiated area. The HSQ in the resist film 20 exposed to the energy beams E forms a silicon oxide with a low dielectric constant that is insoluble in a developer. Examples of energy beams that can expose HSQ include excimer lasers such as KrF, ArF, Xe, F2, and Ar, electron beams (EB), extreme ultraviolet rays (EUV), and X-rays.

[0014] Subsequently, the exposed resist film 20 is developed to form a mask 22 of a desired pattern (FIG. 1(c)). Specifically, by immersing the exposed resist film 20 in a developer, the portions 21 not irradiated with the energy rays E shown in FIG. 1(b) are removed, and a mask 22 is formed on the substrate 10 as shown in FIG. 1(c). As the developer, for example, a basic aqueous solution such as a tetramethylammonium hydroxide (TMAH) aqueous solution is used.

[0015] Next, a thin film 30 of a complex oxide is vapor-deposited on the mask 22 (FIG. 1(d)). Generally, methods for forming a thin film of a complex oxide include physical vapor deposition (PVD) and chemical vapor deposition (CVD). Specific examples of PVD include pulsed laser deposition (PLD) and sputtering. Specific examples of CVD include metal organic (MO) CVD and mist CVD. To deposit the thin film on the substrate 10 in the presence of the HSQ mask 22 without impairing the performance of the complex oxide itself, the thin film 30 is preferably deposited by PVD or CVD while the substrate 10 is heated. The process of depositing the thin film 30 by PLD will be described below as an example. A conventionally known PLD apparatus is used for the PLD method. FIG. 2 is a schematic diagram showing the thin film growth process using a PLD apparatus.

[0016] The PLD apparatus 100 has a vacuum chamber 102. A substrate heating mechanism 104 for heating the substrate 10 is installed in the upper part of the vacuum chamber 102. The vacuum chamber 102 also has a gas inlet 106 for introducing atmospheric gas into the vacuum chamber 102, and a vacuum pump 108 for evacuating gas from the vacuum chamber 102 and creating a vacuum. A laser oscillator 200 is installed outside the vacuum chamber 102. A pulsed laser L emitted from the laser oscillator 200 is incident on the vacuum chamber 102. A raw material target 8 of a polycrystalline sintered body is installed in the lower part of the vacuum chamber 102.

[0017] In the process of growing the thin film 30, the target 8 is intermittently irradiated with a pulsed laser L. This generates plasma of the raw material 9 from the target 8. The plasma of the raw material 9 moves toward the substrate 10. Atoms of the raw material 9 that reach the substrate 10 diffuse on the surfaces of the substrate 10 and mask 22 (not shown), resulting in a thin film. During this thinning process, the substrate 10 is heated by a substrate heating mechanism 104. The heating temperature of the substrate is, for example, room temperature to 1000°C. By growing the thin film 30 while heating the substrate 10 in this manner, a high-quality thin film can be deposited in the presence of an HSQ mask without impairing the performance of the complex oxide itself.

[0018] Next, the mask 22 is removed, and the thin film 30 on the mask 22 is peeled off (FIG. 1(e)). The mask 22 can be removed, for example, by immersing the substrate 10 on which the mask 22 and thin film 30 have been formed in a solution capable of dissolving the mask 22 and then washing the substrate 10. Examples of solutions capable of dissolving the mask 22 include alkaline solutions such as aqueous sodium hydroxide and tetramethylammonium hydroxide solutions, acidic solutions such as hydrofluoric acid, and solutions in which ammonium fluoride has been added to hydrofluoric acid (buffered hydrofluoric acid). The solution used to dissolve the mask 22 can be selected depending on whether the target composite oxide is resistant to acids or bases. If necessary, the solution in which the substrate 10 is immersed may be heated to promote dissolution of the mask 22. In this manner, a fine pattern of the thin film 30 of the composite oxide is formed on the substrate 10.

[0019] Generally, there are two types of methods for forming fine patterns of complex oxides aimed at increasing the integration density of electronic devices: a top-down process and a bottom-up process. A top-down process is a method for forming the desired shape or structure by removing insoluble portions from a large block of material. An example of this is an etching method in which a sample prepared on a substrate is etched into the desired shape. A bottom-up process is a method for assembling small structures at the atomic or molecular level while controlling the assembly so that the desired shape or structure is formed. The method for forming a complex oxide pattern according to this embodiment described above is a bottom-up process.

[0020] In etching methods, which are top-down processes, complex oxides generally have high etching resistance, making it extremely difficult to determine nanometer-level fine etching conditions for each complex oxide. Furthermore, a metal film (hard mask) deposited on top of the complex oxide as an etching mask is likely to contaminate the complex oxide and deteriorate its interface characteristics. On the other hand, the method for forming a complex oxide pattern according to this embodiment avoids the complex oxide etching process, and therefore can form a nanometer-level fine pattern without causing any deterioration of the complex oxide.

[0021] The method for forming a complex oxide pattern according to this embodiment is applicable to many complex oxides such as ferroelectrics, piezoelectrics, magnetic materials, semiconductors, superconductors, and catalysts. [Example]

[0022] Examples of the present invention will be described below, but these examples are for the purpose of conveniently explaining the present invention. It is merely an example and is not intended to limit the invention in any way.

[0023] In all of the following examples, crystal structure evaluation was performed using X-ray diffraction (XRD) (2θ-ω measurement perpendicular to the surface, SmartLab, Rigaku Corporation). Surface morphology observation was performed using a scanning electron microscope (SEM) (S-4800, Hitachi High-Technologies Corporation, accelerating voltage 15 kV), an atomic force microscope (AFM), a piezoelectric microscope (PFM), and a magnetic force microscope (MFM) (MFP-3D, Oxford Instruments, or Cypher S, Oxford Instruments; cantilever: ASYELEC.01-R2, Oxford Instruments, or MFMR, NanoWorld).

[0024] Example 1 In Example 1, BiFe was deposited on a substrate. 0.9 Co 0.1Nanodot patterns of O3 (BFCO) were fabricated. In this example, Nb:SrTiO3 (100 orientation) (Nb:STO, Nb content: 0.5 wt%) was selected as the substrate. Hydrogen silsesquioxane (HSQ) (Applied Quantum Materials) was dissolved in methyl isobutyl ketone to prepare a 6 wt% solution. The HSQ solution was dropped onto the Nb:STO substrate and spin-coated at 4000 rpm for 1 minute. A heat treatment was then performed at 150°C for 2 minutes to form a 110 nm-thick HSQ film on the substrate.

[0025] Next, the obtained HSQ / Nb:STO(100) HSQ film was subjected to electron beam lithography using an electron beam lithography system (ELS-550 manufactured by Elionix) at an acceleration voltage of 50 kV, a beam current of 400 pA, and a dose of 600 to 640 μC / cm. 2 The HSQ / Nb:STO(100) substrate was then immersed in a 2.38% aqueous solution of tetramethylammonium hydroxide (TMAH) for 15 minutes to develop the HSQ film. After development, the substrate was washed with ultrapure water to obtain an HSQ mask.

[0026] BFCO was deposited on the HSQ mask by pulsed laser deposition (PLD) under the following conditions: The thickness of the obtained BFCO film was 60 nm outside the mask and 20-30 nm in the nanodot area. Substrate temperature: 515℃ Oxygen partial pressure: 1.4Pa Laser fluence: 1.0J / cm 2 Repeat frequency: 2Hz Number of shots: 18,000

[0027] Next, the HSQ mask was lifted off to obtain BFCO nanodots using the following procedure. A 5 mol / L NaOH aqueous solution was prepared and heated to 60–80°C on a hot plate. The BFCO / HSQ / Nb:STO(100) was immersed in this heated solution for approximately 30 minutes and then washed with distilled water. The X-ray diffraction (XRD) patterns of the BFCO / HSQ / Nb:STO(100) before and after liftoff were measured and compared. The XRD patterns before and after liftoff were nearly identical, indicating no significant degradation due to liftoff.

[0028] Figure 3 is an SEM image of the HSQ mask. In Figure 3, except for the dark areas, the HSQ was removed by development, exposing the Nb:STO. The mask height was approximately 110 nm.

[0029] Figure 4 shows an AFM image of the BFCO nanodots. The nanodots were found to have the same shape and size as the mask holes, and their height was approximately 30 nm, which was sufficient for evaluating ferroelectricity.

[0030] Figures 5(a) to 5(i) show the polarization switching observed in BFCO nanodots upon application of a voltage. Figure 5(a) shows an AFM height image of the BFCO nanodot, Figure 5(b) shows its in-plane PFM image, and Figure 5(c) shows its perpendicular PFM image. Figure 5(d) shows an AFM height image of the BFCO nanodot when a voltage of -18 V is applied, Figure 5(e) shows its in-plane PFM image, and Figure 5(f) shows its perpendicular PFM image. Figure 5(g) shows an AFM height image of the BFCO nanodot when a voltage of +15 V is applied, Figure 5(h) shows its in-plane PFM image, and Figure 5(i) shows its perpendicular PFM image. As shown in Figures 5(b), 5(c), 5(e), 5(f), 5(h), and 5(i), the left-right contrast was reversed by the application of an electric field. This suggests that polarization switching has occurred.

[0031] Figure 6 shows the results of PFM observation of the ferroelectricity of a single BFCO nanodot. As shown in Figure 6, a hysteresis loop was observed in the phase image and a butterfly loop was observed in the amplitude image. This suggests that the single BFCO nanodot has ferroelectricity.

[0032] Figures 7(a) and 7(b) show the results of MFM observation of the ferromagnetism of a single BFCO nanodot. Figure 7(a) is an AFM height image of a single BFCO nanodot, and Figure 7(b) is its MFM image. As shown in Figure 7(b), a domain structure resulting from the ferromagnetism of the single BFCO nanodot was observed.

[0033] Example 2 In Example 2, La was deposited on the substrate. 0.6 Sr 0.4 MnO3 (LSMO) nanodots were prepared. 0.6 Sr 0.4 MnO3 is a room temperature ferromagnetic oxide.

[0034] The dose in electron beam writing is 680μC / cm 2 An HSQ mask was formed on a Nb:STO (100) substrate in the same manner as in Example 1, except that LSMO was deposited on the HSQ mask by PLD under the following conditions: The thickness of the obtained LSMO film outside the mask was 66 nm. Substrate temperature: 830℃ Oxygen partial pressure: 10Pa Laser fluence: 1.4J / cm 2 Repeat frequency: 2Hz Number of shots: 7200

[0035] Next, the HSQ mask was lifted off to obtain LSMO nanodots using the following procedure. A 5 mol / L NaOH aqueous solution was prepared and heated to 60 °C on a hot plate. The LSMO / HSQ / Nb:STO(100) was immersed in this heated solution for 35 minutes and then washed with distilled water. The X-ray diffraction (XRD) patterns of the LSMO / HSQ / Nb:STO(100) before and after liftoff were measured and compared. The XRD patterns before and after liftoff were nearly identical, indicating no significant degradation due to liftoff.

[0036] Figure 8 shows an SEM image of the LSMO nanodots after lift-off. As shown in Figure 8, we confirmed that the nanodots had a good shape that reflected the HSQ mask shape.

[0037] Figure 9 shows an AFM image of the LSMO nanodots, which were 200 nm wide (reflecting the mask shape) and 60 nm high.

[0038] Figures 10(a) and 10(b) show the observation results of ferromagnetic domains in LSMO nanodots. Figure 10(a) is an AFM height image of an LSMO nanodot, and Figure 10(b) is its MFM image. As shown in Figure 10(b), ferromagnetic domains corresponding to the nanodots were observed.

[0039] Example 3 In Example 3, nanodots of BaTiO3 (BTO) were fabricated on a substrate. BaTiO3 is a typical ferroelectric composite oxide.

[0040] In the same manner as in Example 1, a dose of 640 μC / cm was applied to a Nb:STO (100) substrate. 2 An HSQ mask was formed as follows. BTO was evaporated onto the HSQ mask by PLD under the following conditions. The thickness of the obtained BTO film outside the mask was 60 nm. Substrate temperature: 765℃ Oxygen partial pressure: 10Pa Laser fluence: 1.3J / cm 2 Repeat frequency: 2Hz Number of shots: 2400

[0041] Next, the HSQ mask was lifted off to obtain BTO nanodots using the following procedure. A 5 mol / L NaOH aqueous solution was prepared and heated to 60 °C on a hot plate. The BTO / HSQ / Nb:STO(100) was immersed in this heated solution for 15 minutes and then washed with distilled water. The X-ray diffraction (XRD) patterns of the BTO / HSQ / Nb:STO(100) before and after liftoff were measured and compared. The XRD patterns before and after liftoff were nearly identical, indicating no significant degradation due to liftoff.

[0042] An SEM image of the BTO nanodots after lift-off is shown in Figure 11. As shown in Figure 11, we confirmed that the nanodots had a good shape that reflected the HSQ mask shape.

[0043] Figure 12 shows an AFM image of the BTO nanodots. The nanodots had a width of 200 nm (reflecting the mask shape) and a height of 40 nm.

[0044] Figure 13 shows the results of PFM observation of the ferroelectricity of a single BTO nanodot. As shown in Figure 13, a hysteresis loop was observed in the phase image and a butterfly loop was observed in the amplitude image. This suggests that a single BTO nanodot has ferroelectricity.

[0045] The present invention has been described above based on the embodiments. These embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of the components and treatment processes, and that such modifications are also within the scope of the present invention. [Explanation of symbols]

[0046] 10 substrate, 20 resist film, 22 mask, 30 thin film.

Claims

1. a first step of forming a resist film containing hydrogen silsesquioxane (HSQ) on a substrate; a second step of exposing the resist film to energy rays; a third step of developing the resist film to form a mask; a fourth step of depositing a thin film of a composite oxide on the mask; and a fifth step of removing the mask.

2. 2. The method for forming a complex oxide pattern according to claim 1, wherein in the fourth step, the thin film of the complex oxide is deposited by physical vapor deposition (PVD) or chemical vapor deposition (CVD) while heating the substrate.

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