Method for forming composite oxide pattern

Abstract

This method for forming a composite oxide pattern includes: a first step of forming, on a substrate 10, a resist film 20 containing hydrogen silsesquioxane (HSQ); a second step of exposing the resist film 20 by using energy rays; a third step of developing the resist film 20 to form a mask 22; a fourth step of depositing a composite-oxide thin film 30 on the mask 22; and a fifth step of removing the mask 22.

Description

Method for forming complex oxide pattern

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

[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 a BiFe １－ｘ A ｘ O ３ A magnetic memory element using a thin film of a multiferroic material represented by the formula (wherein A is Co or Mn) has been proposed.

[0003] Japanese Patent Application Laid-Open No. 2019-009304

[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.

[0006] A method for forming a complex oxide pattern according to one embodiment of the present invention comprises: 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 complex oxide on the resist mask; and a fifth step of removing the resist mask.

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

[0008] 1(a) to 1(e) are diagrams for explaining a method for forming a complex oxide pattern according to one embodiment of the present invention. A schematic diagram showing the thin film growth process using a pulsed laser deposition (PLD) device. A scanning electron microscope (SEM) image of an HSQ mask. BiFe ０．９ Co ０．１ O ３ 5(a) to 5(i) are atomic force microscope (AFM) images of BFCO nanodots. 5(a) to 5(i) are diagrams showing the observation results of polarization switching by voltage application to BFCO nanodots. 5(a) and 5(b) are diagrams showing the observation results of ferroelectricity of a single BFCO nanodot by PFM. 5(a) and 5(b) are diagrams showing the observation results of ferroelectricity of a single BFCO nanodot by MFM. 5(b) shows the observation results of La after lift-off. ０．６ Sr ０．４ MnO ３ 10(a) and 10(b) are SEM images of (LSMO) nanodots, AFM shape images of LSMO nanodots, and observation results of ferromagnetic domains of LSMO nanodots. BaTiO after lift-off. ３ 1A and 1B are SEM and AFM images of (BTO) nanodots, respectively, and are diagrams showing the results of PFM observation of the ferroelectricity of a single BTO nanodot.

[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 then heating the solution at a temperature of 100°C to 200°C.

[0012] In so-called photolithography, the size of the smallest resolvable pattern is proportional to the wavelength of the exposure light source, and therefore nano-level fine processing is carried out by using energy rays with shorter wavelengths for exposure. HSQ is a compound represented by the general formula (HSiO １．５ ) ｎ It is a compound represented by the formula (I), which is mainly used as a resist material for semiconductor manufacturing, and forms 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 the substrate 10 without degrading the complex oxide.

[0013] Next, the resist film 20 is exposed to energy rays E (FIG. 1B). Specifically, a desired pattern is drawn on the resist film 20 by irradiating the resist film 20 with energy rays E while controlling the irradiated area. The HSQ in the resist film 20 exposed to the energy rays E forms silicon oxide with a low dielectric constant that is insoluble in a developer. Examples of energy rays that can expose HSQ include KrF, ArF, Xe, and F. ２ Examples of such light include excimer lasers such as 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 having 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 following describes the process of depositing the thin film 30 by PLD, 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. The atoms of the raw material 9 that reach the substrate 10 diffuse on the surfaces of the substrate 10 and the 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.

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

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

[0024] Example 1 In Example 1, a BiFe layer was formed on a substrate. ０．９ Co ０．１ O ３ In this example, a nanodot pattern of Nb:SrTiO (BFCO) was fabricated as a substrate. ３ (100 orientation) (Nb:STO, Nb content: 0.5 wt%) was selected. Hydrogen silsesquioxane (HSQ) (manufactured by Applied Quantum Materials) was dissolved in methyl isobutyl ketone to prepare a 6 wt% solution. The HSQ solution was dropped onto an 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. ２The HSQ / Nb:STO(100) 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 to 30 nm in the nanodot area. Substrate temperature: 515°C, oxygen partial pressure: 1.4 Pa, laser fluence: 1.0 J / cm. ２ Repetition frequency: 2Hz Number of shots: 18,000

[0027] Next, the HSQ mask was lifted off using the following procedure to obtain BFCO nanodots. 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, and no significant degradation due to liftoff was observed.

[0028] Figure 3 shows 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 results of observing polarization switching of BFCO nanodots upon voltage application. Figure 5(a) is an AFM height image of the BFCO nanodots, Figure 5(b) is its in-plane PFM image, and Figure 5(c) is its perpendicular PFM image. Figure 5(d) is an AFM height image of the BFCO nanodots when a voltage of -18 V is applied, Figure 5(e) is its in-plane PFM image, and Figure 5(f) is its perpendicular PFM image. Figure 5(g) is an AFM height image of the BFCO nanodots when a voltage of +15 V is applied, Figure 5(h) is its in-plane PFM image, and Figure 5(i) is 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 applying an electric field, suggesting that polarization switching 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 ferromagnetism observation of a single BFCO nanodot by MFM. 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 formed on a substrate. ０．６ Sr ０．４ MnO ３ Nanodots of (LSMO) were prepared. ０．６ Sr ０．４ MnO ３ is a room temperature ferromagnetic oxide.

[0034] In electron beam lithography, the dose was 680 μC / cm ２An 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°C, Oxygen partial pressure: 10 Pa, Laser fluence: 1.4 J / cm ２ Repetition frequency: 2Hz Number of shots: 7200

[0035] Next, the HSQ mask was lifted off using the following procedure to obtain LSMO nanodots. 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, and no significant degradation due to liftoff was observed.

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

[0037] 9 shows an AFM image of the LSMO nanodots, which had a width of 200 nm (reflecting the shape of the mask) and a height of 60 nm.

[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 the 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, BaTiO ３ (BTO) nanodots were prepared. ３ is a typical ferroelectric compound oxide.

[0040] In the same manner as in Example 1, a dose of 640 μC / cm was applied to a Nb:STO (100) substrate. ２An HSQ mask was formed. BTO was deposited on 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°C, Oxygen partial pressure: 10 Pa, Laser fluence: 1.3 J / cm ２ Repetition frequency: 2Hz Number of shots: 2400

[0041] Next, the HSQ mask was lifted off using the following procedure to obtain BTO nanodots. 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, and no significant degradation due to liftoff was observed.

[0042] Figure 11 shows an SEM image of the BTO nanodots after lift-off. As shown in Figure 11, a good nanodot shape reflecting the HSQ mask shape was confirmed.

[0043] 12 shows an AFM image of the BTO nanodots, which had a width of 200 nm (reflecting the shape of the mask) 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 in the amplitude image. This suggests that the 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.

[0046] The present invention can be used for microfabrication of complex oxides.

[0047] 10 substrate, 20 resist film, 22 mask, 30 thin film.

Claims

1. A method for forming a complex oxide pattern, comprising: 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 complex oxide on the mask; and a fifth step of removing the mask.

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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