Buffer layer forming solution for PZT-based dielectric films and method for manufacturing a buffer layer.

Abstract

The lanthanum nitrate remains stably dissolved even after several days, maintaining the effect of dielectric alignment film formation, improving wettability to the substrate, and forming a uniform buffer layer without appearance defects even on metals such as platinum. [Solution] A buffer layer is formed by applying a buffer layer forming solution for PZT-based dielectric films onto a substrate and heating it. The buffer layer forming solution for PZT-based dielectric films is a solution in which lanthanum nitrate is dissolved in an organic solvent in a range of 0.8% to 5.5% by mass, and the mass concentration of water in the solution is 3% to 21% by mass.

Description

Technical Field

[0001] The present invention relates to a solution for forming a buffer layer between a dielectric film made of lead zirconate titanate (PZT) or lanthanum-doped lead zirconate titanate (PLZT) and a substrate, and a method for manufacturing a buffer layer using the solution.

Background Art

[0002] Non-Patent Document 1 and Patent Document 1 disclose a method for forming a buffer layer for highly orienting a thin film made of lead zirconate titanate (hereinafter referred to as PZT) on a substrate. In this method for forming the buffer layer, first, a solution of polyvinylpyrrolidone (hereinafter referred to as PVP) is applied on the substrate, then lanthanum nitrate hexahydrate is dissolved in a solvent, the solution is spin-coated on the PVP, and further heat-treated, whereby the buffer layer is formed on the substrate.

[0003] Patent Document 1 discloses a functional element using PZT, Non-Patent Document 2 and Patent Document 2 disclose using PZT in an optical module, and a buffer layer containing lanthanum is provided between a silicon substrate and PZT.

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

[0005] [Patent Document 1] International Publication No. 2004 / 079059 [Patent Document 2] International Publication No. 2014 / 083195 [Overview of the project] [Problems that the invention aims to solve]

[0006] Solutions prepared by dissolving lanthanum nitrate in an organic solvent have low stability, and the effectiveness of forming PZT and PLZT alignment films decreases over time. Furthermore, when it is necessary to form ferroelectric alignment films on metal electrodes such as platinum in the fabrication of optical devices, the wettability of the lanthanum nitrate solution on metals is poor, resulting in appearance defects after the deposition of the ferroelectric thin film.

[0007] In view of these challenges, the present invention aims to stably dissolve lanthanum nitrate even after a period of time has elapsed, sustain the effect of dielectric alignment film formation, improve wettability to the substrate, and form a uniform buffer layer without appearance defects on metals such as platinum. [Means for solving the problem]

[0008] The buffer layer forming solution for PZT-based dielectric films of the present invention is a solution in which lanthanum nitrate is dissolved in an organic solvent in an amount of 0.8% by mass or more and 5.5% by mass or less, wherein the mass concentration of water in the solution is 3% by mass or more and 21% by mass or less.

[0009] By dissolving lanthanum nitrate in a solution of an organic solvent with added water, the lanthanum nitrate remains stable even after several days, sustaining the effect of ferroelectric orientation film formation. Furthermore, with the buffer layer forming solution for PZT-based dielectric films of the present invention, the wettability to the substrate is improved by the addition of water, allowing for the uniform formation of a buffer layer even when the solution is dropped onto a metal such as platinum, thus reducing the likelihood of appearance defects.

[0010] In this case, if the water concentration in the lanthanum nitrate solution is less than 3% by mass, the orientation maintenance ability after several days is insufficient, and sufficient wettability on the metal film cannot be obtained. On the other hand, if the water concentration in the lanthanum nitrate solution exceeds 21% by mass, the viscosity of the solution becomes too low, making it difficult to form a uniform film when spin-coating. Furthermore, if the lanthanum nitrate concentration is less than 0.8% by mass or more than 5.5% by mass, it becomes difficult to form a uniform ferroelectric multilayer film. The concentration of water in the lanthanum nitrate solution is more preferably 3.5% by mass or more and 15% by mass or less, and even more preferably 5% by mass or more and 12% by mass or less. The concentration of lanthanum nitrate is more preferably 1% by mass or more and 5% by mass or less, and even more preferably 1.2% by mass or more and 3.5% by mass or less.

[0011] The present invention provides a method for manufacturing a buffer layer for a PZT-based dielectric film, which involves applying the buffer layer forming solution for the PZT-based dielectric film onto a substrate and heating it to form the buffer layer.

[0012] In this case, the substrate may be one whose surface is formed by a metal surface at least. Platinum (Pt) or gold (Au) can be used as the metal surface, and it is possible to have substrates formed from metal itself, or to have a metal layer formed on top of a silicon substrate. [Effects of the Invention]

[0013] According to the present invention, even when days have passed, lanthanum nitrate can be stably dissolved, the effect of forming a ferroelectric alignment film can be sustained, the wettability to a substrate can be improved, and a buffer layer that is uniform and has no appearance defect can be formed even on a metal such as platinum.

Brief Description of the Drawings

[0014] [Figure 1] It is a cross-sectional view of a substrate with a laminated film according to an embodiment of the present invention. [Figure 2] It is a graph showing the XRD intensity of the X-ray diffraction measurement of sample 10.

Modes for Carrying Out the Invention

[0015] Hereinafter, embodiments of the present invention will be described while referring to the drawings.

[0016] A substrate 1 with a laminated film shown in FIG. 1 (hereinafter referred to as a substrate with a thin film) includes a substrate 10 and a laminated film 20 formed on the substrate 10.

[0017] The substrate 10 has a metal layer 12 such as platinum (Pt), gold (Au), or copper (Cu) formed on the surface of a substrate body 11 such as a Si substrate without an oxide film, a Si substrate with an oxide film, a glass substrate, or a sapphire substrate as a metal surface. The size of the substrate 10 is not limited, but for example, it may be formed as a circular wafer having a planar shape of 10 mm or more and 350 mm or less and a diameter of 50 mmφ or more. The metal layer 12 constitutes a lower electrode, but a substrate composed only of the substrate body 11 without the metal layer 12 is also applicable. For example, it can also be applied to a Si substrate without a metal layer 12, a Si substrate with an oxide film, a glass substrate, or a sapphire substrate. In addition, for example, the substrate 10 may be a substrate having a metal surface (such as platinum, gold, or copper) formed of the substrate body 11 with metal. Although these thicknesses are not limited, the thickness of the substrate body 11 is 300 μm or more and 2000 μm or less, and the thickness of the metal layer 12 is 10 nm or more and 500 nm or less.

[0018] The laminated film 20 includes a buffer layer 210 formed on the substrate 10 and a PZT-based dielectric film 220 formed on the buffer layer 210.

[0019] The buffer layer 210 constitutes the underlayer of the PZT-based dielectric film 220 and controls the crystal orientation of the PZT-based dielectric film 220 formed thereon. It is made of lanthanum nitrate [La(NO3)3] and is sometimes referred to as a seed layer. The film thickness of this buffer layer is not limited, but for example, it is 4 nm or more and 50 nm or less, preferably 5 nm or more and 35 nm or less, and more preferably 8 nm or more and 25 nm or less. If the film thickness is less than 4 nm, it is difficult to form a uniform film, and if it exceeds 50 nm, when forming a ferroelectric layer on the buffer layer 210 to fabricate an optoelectronic device, there is a concern that light confinement in the ferroelectric layer becomes insufficient and efficient optoelectronic conversion cannot be performed.

[0020] The PZT-based dielectric film 220 is made of PZT (lead zirconate titanate: Pb(Zr,Ti)O3) or PLZT (lanthanum-doped lead zirconate titanate: [(Pb,La)(Zr,Ti)O3]).

[0021] (Manufacturing method of the substrate with a thin film 1) The manufacturing method of the substrate with a thin film 1 includes a first film-forming step of forming a buffer layer 210 on the substrate 10 and a second film-forming step of forming a PZT-based dielectric film 220 on the buffer layer 210. On the substrate 10, a metal layer 12 is formed on the surface of the substrate body 11 by a thin film technology such as sputtering.

[0022] [First film-forming step: Buffer layer 210] The first film formation process comprises a first coating step of coating the surface of the metal layer 12 on the substrate 10 with polyvinylpyrrolidone (hereinafter referred to as PVP), a second coating step of coating the PVP with lanthanum nitrate, a pre-calcination step of pre-calcining the lanthanum nitrate on the substrate 10, and a calcination step of calcining the lanthanum nitrate on the substrate 10.

[0023] The first coating step comprises the steps of applying the PVP solution to the substrate 10 and volatilizing the solvent.

[0024] The PVP solution consists of PVP and a solvent, with a PVP concentration of 0.05 wt% to 0.5 wt%. Examples of solvents include 2-methoxyethanol, 1-propanol, methanol, and ethanol. The PVP solution was dropped onto the substrate and spin-coated. Alternatively, dip-coating or other methods may be used instead of spin-coating. After applying the PVP solution, the substrate was placed on a heated surface, for example, at 150°C, to evaporate the solvent. This formed a PVP film, which acted as a surfactant, effectively uniformly depositing lanthanum nitrate.

[0025] The second coating step comprises the steps of applying a lanthanum nitrate solution onto the PVP and volatilizing the solvent.

[0026] The lanthanum nitrate solution consists of lanthanum nitrate hexahydrate, an organic solvent, and water. The organic solvent is, for example, 2-methoxyethanol, 1-propanol, methanol, or ethanol. The water is pure water or the like. The lanthanum nitrate solution of this embodiment may contain unavoidable impurities. Furthermore, the lanthanum nitrate solution of this embodiment is a buffer layer forming solution for PZT-based dielectric films. The concentration of this lanthanum nitrate (excluding hydrate) is 0.8% by mass or more and 5.5% by mass or less. If the lanthanum nitrate concentration is less than 0.8% by mass or more than 5.5% by mass, it is difficult to form a uniform ferroelectric multilayer film. The concentration of this lanthanum nitrate solution is preferably 1% by mass or more and 5% by mass or less, and more preferably 1.2% by mass or more and 3.5% by mass or less. Alternatively, the concentration of this lanthanum nitrate solution may be 1% by mass or more and 4% by mass or less, or 1.2% by mass or more and 5.5% by mass or less.

[0027] Furthermore, the mass concentration of water in the lanthanum nitrate solution is between 3% by mass and 21% by mass. If the water concentration in the lanthanum nitrate solution is less than 3% by mass, the orientation maintenance ability after several days is insufficient, and sufficient wettability on the metal film cannot be obtained. On the other hand, if the water concentration in the lanthanum nitrate solution is too high, exceeding 21% by mass, the viscosity of the solution becomes too low, making it difficult to form a uniform film when spin coating is performed. The water concentration in this lanthanum nitrate solution is preferably between 3.5% by mass and 15% by mass, and more preferably between 5% by mass and 12% by mass. Alternatively, the water concentration in this lanthanum nitrate solution may be between 3.4% by mass and 21% by mass, or between 5% by mass and 15% by mass. Note that the mass concentration of water in the lanthanum nitrate solution includes the water of hydration derived from the lanthanum nitrate raw material.

[0028] In the second coating step, the lanthanum nitrate solution is dropped onto the PVP and applied as a thin film by spin coating. Alternatively, dip coating or other methods may be used instead of spin coating. After applying the lanthanum nitrate solution, the substrate is heated to, for example, 150°C to 250°C to evaporate the organic solvent and water.

[0029] The calcination process involves heating the substrate to a temperature between 300°C and 400°C to calcine the lanthanum nitrate. After heating to the calcination temperature, the substrate is placed on a heating section with a temperature lower than the calcination temperature to gradually lower the temperature, and then removed from the heating section and allowed to cool in the air.

[0030] The firing process involves firing the substrate to crystallize lanthanum nitrate. The firing temperature is 450°C to 650°C, preferably 480°C to 650°C, and more preferably 500°C to 600°C. The holding time for maintaining the firing temperature is 1 second to 1000 seconds, preferably 30 seconds to 180 seconds. Furthermore, in the firing process, the heating time t1 from room temperature to the firing temperature is 200 seconds to 1000 seconds, preferably 240 seconds to 1000 seconds, and more preferably 240 seconds to 750 seconds.

[0031] The heating time t1 may be between 240 seconds and 720 seconds. By adjusting the heating time t1, the lanthanum nitrate after firing is controlled to a phase state suitable for the formation of the ferroelectric thin film 220. If the heating time t1 is less than 200 seconds, the phase transition of lanthanum nitrate to LaONO3, which is effective in oriented the PZT-based dielectric film, will be insufficient, making it difficult to form the ferroelectric thin film 220 with high orientation on the buffer layer 210. If the heating time t1 exceeds 1000 seconds, there is a concern that other phases besides LaONO3 may be mixed in. The heating rate v1 from room temperature to firing temperature is between 0.5°C / second and 10°C / second, preferably between 0.70°C / second and 2.5°C / second. If the heating rate v1 is less than 0.5°C / second, there is a concern that other phases besides LaONO3 may be mixed in, and if the heating rate v1 exceeds 10°C / second, the phase transition to LaONO3 may be insufficient. The thickness of the buffer layer 210 can be increased by repeating the process from the first coating step of applying PVP through the second coating step to the firing step.

[0032] [Second film deposition process: Dielectric film 220] The second film formation step comprises the steps of applying a thin film forming solution onto the buffer layer 210, volatilizing the solvent, pre-calcining the thin film forming material, and calcining the thin film forming material.

[0033] The coating process involves dropping a thin-film forming solution onto a substrate with a buffer layer 210 already formed, and then performing spin coating. Alternatively, dip coating or other methods may be used instead of spin coating. After applying the thin-film forming solution, the solvent is evaporated. For example, the substrate coated with the thin-film forming solution is placed on a heating section heated to 150°C.

[0034] The calcination process involves heating the substrate to 300°C to 450°C to calcinate the thin-film forming material. After heating to the calcination temperature, the substrate is placed on a heating section with a temperature lower than the calcination temperature to gradually lower the temperature, and then removed from the heating section to cool in the air.

[0035] The firing process involves firing the substrate to crystallize the PZT-based thin film forming material. The firing temperature is 500°C to 750°C, preferably 550°C to 650°C. The holding time for maintaining the firing temperature is 1 second to 500 seconds, preferably 30 seconds to 90 seconds. Furthermore, the heating time t2 from room temperature to the firing temperature is 30 seconds to 1000 seconds, preferably 240 seconds to 720 seconds. The heating rate from room temperature to the firing temperature is 0.65°C / second to 10°C / second, preferably 0.90°C / second to 2.5°C / second. The thickness of the ferroelectric thin film 220 can be increased, for example, by repeating the process from applying the thin film forming solution to the firing process of firing the thin film forming material. After the firing process, the thin film-coated substrate 1 is completed.

[0036] The lanthanum nitrate solution used for forming the buffer layer 210 in the manufacturing method of the thin-film substrate 1 of this embodiment contains water at a predetermined concentration, which provides stable orientation ability for the dielectric film 220. Even after being stored in a bottle after preparation and used after several days in a predetermined environment, its orientation ability can be well maintained. For example, the lanthanum nitrate solution of this embodiment can maintain its orientation ability well even after 10, 30, or 90 days of storage after preparation. For example, the lanthanum nitrate solution of this embodiment can be stored for 10, 30, or 90 days in an environment with a temperature of 25°C and a humidity of 50%. Furthermore, because it contains water, this lanthanum nitrate solution has excellent wettability when applied to a substrate to form a film. Therefore, even if an electrode metal layer 12 such as platinum (Pt) is formed on the surface of the substrate 10, the buffer layer 210 can be uniformly formed on the metal layer 12. Therefore, by forming a buffer layer 210 with such a lanthanum nitrate solution and then forming a PZT-based dielectric film, it is possible to form a PZT-based dielectric film that is stably oriented and uniformly oriented in-plane, and the optical modulator formed using this thin-film substrate 1 can also exhibit uniform characteristics.

[0037] Furthermore, the present invention can be implemented in any way not limited to the above description and illustrated examples. The thin-film substrate 1 of the present invention can be used not only in optical modulators but also in optical switches and phase shifters. Furthermore, the method for forming the dielectric film on the buffer layer is not limited to the above description, and may also be formed by other methods such as chemical solution deposition, chemical vapor deposition, sputtering, and vapor deposition. [Examples]

[0038] Two types of substrates were prepared: one consisting of a single Si wafer, and another with a metal layer formed on the surface of the Si wafer. These substrates were then used as samples, and laminated substrates were created from them, each consisting of a buffer layer and a PLZT film deposited on this buffer layer. The films in each sample were then analyzed.

[0039] The Si wafer is a Si wafer with a thermal oxide film (thickness: 3 μm), has a diameter of 4 inches, and a thickness of 0.525 mm. After cleaning the Si wafer under the following conditions, a metal layer was formed on the surface of the substrate with a metal layer to produce two types of substrates.

[0040] [Washing conditions] The cleaning process was carried out in the following order: first cleaning, then third cleaning. In the first cleaning step, the Si substrate was immersed in acetone and ultrasonically cleaned for 2 minutes. In the second cleaning step, the Si substrate was immersed in pure water and ultrasonically cleaned for 2 minutes. In the third wash, the Si substrate was immersed for 20 minutes after the solution was heated to 75 degrees Celsius (RCA SC1 wash). The solution consisted of pure water, hydrogen peroxide (35 wt%), and aqueous ammonia (29 wt%), with a volume ratio of pure water:hydrogen peroxide (35 wt%):ammonia (29 wt%) = 3:1:1. After the second wash and before the third wash, the Si substrate may be heat-treated at a temperature between 500°C and 800°C.

[0041] [Metal layer formation] A two-layer metal layer consisting of Pt (30 nm) and Ti (10 nm) was deposited on a Si wafer using the DC sputtering method. The sputtering apparatus used was a magnetron sputtering apparatus (ULVAC QAM-4-ST), and the film was deposited under the following conditions. • Magnetic field strength: 1000 Gauss (directly above the target, vertical component) ·Achieved vacuum level: 5.0×10 -4 Pa or less • Sputtering gas: High-purity argon • Sputtering gas pressure inside the chamber: 0.18 Pa ·DC power: 150W • Board rotation speed: 10 rpm The film thicknesses of Ti and Pt thin films, which had been pre-deposited by sputtering for a predetermined time, were measured using a film thickness gauge (DEKTAK, ULVAC, Inc.), and the deposition rate under each deposition condition was calculated. Based on this, the deposition time was adjusted so that the film thicknesses of Pt and Ti were 30 nm and 10 nm, respectively.

[0042] Then, a first deposition process was carried out to deposit a buffer layer on each substrate, and a second deposition process was carried out to deposit a PLZT film on the buffer layer.

[0043] [First film formation process: buffer layer] The first film formation process consisted of a first coating step in which the surface of the substrate was coated with polyvinylpyrrolidone (hereinafter referred to as PVP), a second coating step in which the PVP was coated with lanthanum nitrate, a pre-calcination step in which the lanthanum nitrate on the substrate was pre-calcined, and a calcination step in which the lanthanum nitrate on the substrate was calcined, as follows.

[0044] In the first coating step, the PVP solution was applied to the substrate, and then the solvent was heated to evaporate it. The PVP solution was prepared by weighing 0.075 g of polyvinylpyrrolidone (k=15, average molecular weight 10,000) into a glass container, adding 19.7 g of 2-methoxyethanol (purity >99.0%), and stirring for 30 minutes. While k=30 (average molecular weight 40,000) or k=90 (average molecular weight 360,000) PVP may also be used, using lower molecular weight PVP allows for easier dissolution in the solvent. Furthermore, 5% to 20% by mass of pure water may be added to improve the solubility of the PVP.

[0045] Before adding the solution dropwise, the PVP solution was stirred to completely dissolve the PVP. Then, 1 mL of the PVP solution was added dropwise onto the substrate, and spin coating was performed using a spin coater. The spin coating was performed at 500 rpm for 5 seconds, followed by 4000 rpm for 30 seconds. After spin coating, the substrate coated with the PVP solution was placed on a hot plate heated to 150°C for 1 minute to evaporate the solvent.

[0046] In the second coating step, a lanthanum nitrate solution was applied to the PVP, and then the solvent was evaporated by heating. The lanthanum nitrate solution was prepared by weighing 0.15 g to 1.56 g of lanthanum nitrate hexahydrate (purity >99.0%) into a glass container, adding 2-methoxyethanol and pure water to make a total volume of 20 g, and stirring for 30 minutes. The amount of pure water was varied in the range of 0.1% to 30% by mass. The concentration of lanthanum nitrate (La(NO3)3) excluding the hydrate corresponds to 0.59% by mass to 5.9% by mass. The amount of water added means, for example, adding 0.2g of water for every 20g of total solution volume if the concentration is 1% by mass.

[0047] The total water volume listed in Table 1 includes the amount of hydrated water derived from lanthanum nitrate hexahydrate, and the mass concentration (mass%) is calculated as the weight ratio of water contained in the total solution weight. Similarly, the nitrate (La) concentration is calculated as the mass concentration (mass%) of La(NO3)3, excluding the mass of hydrated water, as the weight ratio of La(NO3)3 to the total solution weight. Furthermore, infrared spectroscopy, gas chromatography, microwave spectroscopy, and the Karl Fischer method can be used to quantify the water content in lanthanum nitrate solution.

[0048] Before adding the solution dropwise, the lanthanum nitrate solution was stirred to completely dissolve the lanthanum nitrate. Then, 1 mL of the lanthanum nitrate solution was added dropwise onto the PVP-coated substrate, and spin coating was performed using a spin coater. The spin coating was performed at 500 rpm for 5 seconds, followed by 4000 rpm for 30 seconds. After spin coating, the substrate coated with the lanthanum nitrate solution was placed on a hot plate heated to 150°C for 1 minute to allow the solvent to evaporate.

[0049] In the calcination process, the substrate was placed on a hot plate heated to 300°C for 5 minutes, then placed on a hot plate heated to 150°C for 30 seconds to gradually lower the temperature of the substrate, and finally removed from the hot plate and allowed to cool in the air.

[0050] In the firing process, the substrates were fired in a firing apparatus at a firing temperature of 590°C, a temperature holding time of 60 seconds, and in an atmospheric firing environment. For each sample, the heating time t1 from room temperature to the firing temperature of 590°C was set to 720 seconds. A Rapid Thermal Annealing apparatus (RTA-8000) manufactured by Advance Engineering Co., Ltd. was used as the firing apparatus (hereinafter referred to as the RTA apparatus). Lanthanum nitrate was crystallized by the firing process. The thickness of the buffer layer could be increased by repeating the process from the first coating process of applying PVP to the second coating process and then to the firing process. For each sample, the number of layers that formed the base for the PLZT film was set to 4 layers (thickness 22 nm).

[0051] [Second film formation process: PLZT film] The second film formation step comprises a step of coating the PLZT sol-gel solution onto a buffer layer, a step of volatilizing the solvent, a calcination step of calcining the PLZT, and a calcination step of calcining the PLZT. The PLZT sol-gel solution used was E1 solution for thin film formation manufactured by Mitsubishi Materials Corporation (concentration 15%, composition ratio: Pb / La / Zr / Ti = 115 / 8 / 65 / 35).

[0052] The coating process involved dropping 1 ml of PLZT sol-gel solution onto a substrate with a buffer layer already formed, followed by spin coating using a spin coater. Spin coating was performed at 500 rpm for 5 seconds, followed by 4000 rpm for 30 seconds. After spin coating, the substrate coated with PLZT sol-gel solution was placed on a hot plate heated to 150°C for 1 minute to evaporate the solvent.

[0053] The pre-firing process involved placing the substrate on a hot plate heated to 300°C for 5 minutes, then gradually lowering the temperature of the substrate by placing it on a hot plate heated to 150°C for 30 seconds, and finally removing it from the hot plate and allowing it to cool in the air.

[0054] In the firing process, the substrate was fired using the aforementioned RTA apparatus at a firing temperature of 640°C, a temperature holding time of 60 seconds, and in an O2 atmosphere. For each sample, the heating time t2 from room temperature to the firing temperature of 640°C was set to 64 seconds. PLZT was crystallized by the firing process.

[0055] The PLZT film was formed by repeatedly applying the PLZT sol-gel solution and then firing it, adjusting the film thickness to 300 nm. In this case, ellipsometry was performed to calculate the film thickness of the PLZT film. A JAWoolam M-2000 was used for the measurement, and measurements were taken in reflection mode at three incident / reflection angles: 60°, 70°, and 80°. Cauchy's optical model was applied to the obtained results for fitting, and the film thickness was measured.

[0056] [evaluation] The effects of storage time on lanthanum nitrate solution were investigated. Solutions prepared with various water content and lanthanum nitrate concentrations were evaluated immediately after preparation, and again after filling glass vials with the prepared solutions and storing them for 10 days at 25°C and 50% humidity. A buffer layer was then formed with each solution, and the XRD (X-ray Diffraction) intensity ratio, uniformity of coating, and uniformity of coating on a Pt-coated substrate were evaluated for the PLZT film deposited on top of the buffer layer.

[0057] (Orientation of PLZT film) The orientation of the deposited PLZT film was analyzed using a fully automated multi-purpose X-ray diffractometer (Rigaku SmartLab). The scan axis was set to 2θ / θ, and measurements were performed in the angular range of 10° to 60° with a sampling step of 0.01°, a scan speed of 10° / min, IS=0.300mm, RS1=5mm, and RS2=5mm. For the measurements, a slit was set up with an incident solar slit of 5°, an IS longitudinal of 10mm, a PSA of 0.5°, and a measurement solar slit of 5°. A scintillation counter (Rigaku, SC-70) was used as the detector. The obtained XRD data was analyzed using integrated powder X-ray analysis software (PDXL2, Rigaku), and peak searches were performed. For the evaluation of XRD intensity, the integral intensity of the PLZT(100) peak appearing between 21.00° and 23.00° was evaluated. The ratio Ib / Ia of the integral intensity Ib of a sample deposited 10 days after solution preparation to the integral intensity Ia of a sample deposited on the day the solution was prepared was evaluated, and the effect of lanthanum nitrate solution on PLZT film orientation before and after storage was compared.

[0058] (Uniform application) The uniformity of PLZT film deposition on SiO2 / Si substrates and Pt-deposited Si substrates using each lanthanum nitrate solution was visually assessed and evaluated in three stages: A: Very good (no color unevenness, etc., observed on the wafer surface), B (no noticeable color unevenness, etc., on the wafer surface), and C (significant color unevenness or patterns observed). These results are shown in Table 1.

[0059] [Table 1]

[0060] As shown in Table 1, samples 3-6, 9-11 (lanthanum nitrate concentration: 1.2% to 4.7% by mass, total water content: 3.4% to 20.4% by mass) all showed improved XRD intensity ratio (Ib / Ia) after 10 days and were uniformly coated on both silicon substrates and substrates with metal layers. Figure 2 shows the XRD intensity of the solution of sample 10 immediately after preparation and 10 days later, indicating that the XRD intensity increased with the addition of water.

[0061] In contrast, samples 1, 2, 7, 8, and 12 are comparative examples. Sample 1, which contained no water added to the lanthanum nitrate solution, and Sample 2, which had only a small amount of water added (total water content of 1.4% by mass), showed a decrease in XRD intensity ratio after 10 days, and particularly poor uniform coating on substrates with metal layers. Although Samples 1 and 2 appeared uniform in appearance when coated on Si substrates, the decrease in XRD intensity ratio after 10 days raises concerns about reduced stability during longer-term storage. Furthermore, sample 7, which had a large amount of added water and a total water content of 30.4% by mass, and sample 8, which had a low lanthanum nitrate concentration of 0.59% by mass, could not be uniformly coated on either a silicon substrate or a substrate with a metal layer. Conversely, sample 12, which had a high lanthanum nitrate concentration of 5.9% by mass, also failed to achieve uniform coating on either the silicon substrate or the substrate with a metal layer. Note that the XRD intensity ratios for samples 7, 8, and 12 were not measured after 10 days.

[0062] From the above results, it can be seen that a solution in which lanthanum nitrate is dissolved in an amount of 0.8% to 5.5% by mass, and in which the mass concentration of water in the solution is 3% to 21% by mass, can be obtained to achieve good coating properties. [Explanation of Symbols]

[0063] 1. Thin-film coated substrate (multilayer coated substrate) 10 circuit boards 11 Main board 12 metal layer 20 Multilayer film 210 buffer layers 220 PZT dielectric film

Claims

1. A buffer layer forming solution for PZT-based dielectric films, characterized in that lanthanum nitrate is dissolved in an organic solvent in an amount of 0.8% by mass or more and 5.5% by mass or less, and the mass concentration of water in the solution is 3% by mass or more and 21% by mass or less.

2. A method for producing a buffer layer for a PZT-based dielectric film, characterized by forming a buffer layer by applying the buffer layer forming solution for PZT-based dielectric films described in claim 1 onto a substrate and heating it.

3. The method for manufacturing a buffer layer for a PZT-based dielectric film according to claim 2, characterized in that the substrate has at least a surface formed of metal.

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