Laminated film, substrate equipped with laminated film, electro-optical device using same, and method for manufacturing substrate equipped with laminated film

Abstract

The present invention comprises a buffer layer 210 composed of an oxide containing La as a main component, and a ferroelectric layer 220 formed on the buffer layer 210 and composed of PZT or PLZT. When 2θ / θ is defined as a scan axis, IA [counts • degree] is defined as the integrated intensity of a peak of the buffer layer appearing within a range where the diffraction angle (2θ) is 12.90° to 13.95° in an X-ray diffraction result using CuK α rays, and IB [counts • degree] is defined as the integrated intensity of a peak of the buffer layer appearing within a range where the diffraction angle (2θ) is 38.50° to 40.20°, the intensity ratio (= IB / IA) is 0.33 or greater.

Description

LAMINATED FILM, LAMINATED FILM-COATED SUBSTRATE, ELECTRO-OPTICAL DEVICE USING THEM, AND METHOD FOR PRODUCING LAMINATED FILM-COATED SUBSTRATE

[0001] The present invention relates to a laminated film, a substrate with the laminated film, an electro-optical device using the same, and a method for producing a substrate with the laminated film.

[0002] This application claims priority based on Japanese Patent Application No. 2024-103176 filed in Japan on June 26, 2024, and Japanese Patent Application No. 2025-106928 filed in Japan on June 25, 2025, the contents of which are incorporated herein by reference.

[0003] Non-Patent Document 1 discloses a method for forming a buffer layer on a substrate that highly orients a thin film made of lead zirconate titanate (hereinafter referred to as PZT). This buffer layer formation method involves first coating a solution of polyvinylpyrrolidone (hereinafter referred to as PVP) on the substrate, then dissolving lanthanum nitrate hexahydrate in a solvent, spin-coating the solution on the PVP, and then heat-treating the substrate to form a buffer layer on the substrate.

[0004] Patent Document 1 discloses a functional element using PZT, and Non-Patent Document 2 discloses the use of PZT in an optical module, in which a buffer layer containing lanthanum is provided between a silicon substrate and PZT.

[0005] Jong-Jin Choi and three others, "Growth of highly (100) oriented lead zirconate titanate films on silicon and glass substrates using lanthanum nitrate as a buffer layer," Applied Physics Letters, 2004, Vol. 85, pp. 4621-4623; J.P. George and seven others, "Lanthanide-Assisted Deposition of Strongly Electro-optic PZT Thin Films on Silicon: Toward Integrated Active Nanophotonic Devices," Applied Materials Interfaces, 2015, Vol. 7, pp. 13350-13359

[0006] WO 2004 / 079059

[0007] When a ferroelectric layer of PZT or lanthanum-doped lead zirconate titanate (hereinafter referred to as PLZT) is formed on a substrate, it is not sufficiently oriented, and if sufficient orientation is not obtained, it becomes difficult to use it in, for example, an optical modulator.

[0008] In view of the above problems, an object of the present invention is to obtain a laminated film having a highly oriented ferroelectric layer.

[0009] The laminated film of the present invention comprises a buffer layer made of an oxide containing La as a main component, and a ferroelectric layer made of PZT or PLZT formed on the buffer layer, and the integrated intensity of the peak of the buffer layer appearing within a diffraction angle (2θ) range of 12.90° to 13.95° in X-ray diffraction analysis using CuKα rays with a scan axis of 2θ / θ is expressed as IＡ [Counts·degree], and the integrated intensity of the peak of the buffer layer appearing within the range of the diffraction angle (2θ) of 38.50° to 40.20° is defined as I Ｂ In terms of [Counts·degree], the intensity ratio of the following formula (1) is 0.33 or more.

[0010]

[0011] X-ray diffraction (XRD) results show that Ａ / I Ｂ The intensity ratio is an index showing the phase state of the buffer layer. When the intensity ratio is 0.33 or more, the buffer layer is in a state where a phase transition from a crystalline phase with a peak appearing in a diffraction angle range of 12.90° to 13.95° to a crystalline phase with a peak appearing in a diffraction angle range of 38.50° to 40.20° has progressed sufficiently. The progress of this phase transition to a crystalline phase with a peak appearing in a diffraction angle range of 38.50° to 40.20° allows the ferroelectric layer on this buffer layer to be highly oriented.

[0012] In this case, the I that appears in the diffraction angle range of 12.90° to 13.95° in the X-ray diffraction results Ａ [Counts·degree] is the La(NO ３ ) ３ This is the integrated intensity of the (100) peak, which appears in the diffraction angle range of 38.50° to 40.20° in the X-ray diffraction results. Ｂ [Counts·degree] is the LaONO buffer layer ３ (004) or La ２ O ２ (CO ３ ) (060) peak.

[0013] The intensity ratio is an index showing the phase state of the buffer layer. When the intensity ratio is 0.33 or more, the buffer layer is La(NO ３ ) ３ From LaONO ３ Or La ２ O ２ (CO ３ The LaONO phase transition in this buffer layer has progressed sufficiently. ３Or La ２ O ２ (CO ３ ) has an effect on the orientation of the ferroelectric layer.

[0014] In the laminated film of the present invention, the scan axis is set to 2θ / θ, and the integrated intensity of the peak of PZT(100) or PLZT(100) appearing within the range of the diffraction angle (2θ) of 21.60° or more and 22.10° or less in the X-ray diffraction results measured from the ferroelectric layer side using CuKα radiation is defined as I ＸＹ [Counts·degree], the integrated intensity I in the plane of the following formula (2) is ＸＹ The ratio of the standard deviation to the maximum value is 0.13 or less.

[0015]

[0016] I ＸＹ is the integrated intensity of the PLZT(100) or PZT(100) peak appearing between 21.60° and 22.10° in the X-ray diffraction results. The standard deviation is the integrated intensity I obtained from the nine measurement points. ＸＹ The maximum value is the degree of variation of the nine integrated intensities I ＸＹ is the maximum integrated intensity [Counts·degree] among the

[0017] The ratio (=standard deviation / maximum value) of formula (2) is the integrated intensity I of the ferroelectric layer in the plane of the laminated film. ＸＹ When this ratio is 0.13 or less, the ferroelectric layer has a high degree of uniformity of orientation at multiple locations within the surface. This allows a uniform orientation film made of PLZT or PZT to be obtained even when applied to a large-area wafer, such as a 4-inch wafer.

[0018] The laminated film-coated substrate of the present invention is characterized by comprising a substrate body and the laminated film provided on the substrate body directly or via an intermediate layer. The substrate body can be, for example, a Si substrate without an oxide film, a Si substrate with an oxide film, a glass substrate, a sapphire substrate, or the like.

[0019] The electro-optical device of the present invention is characterized by comprising the above-described laminated film-coated substrate, and examples of the electro-optical device include an optical modulator, an optical switch, and a phase shifter.

[0020] The method for manufacturing a laminated film-coated substrate of the present invention comprises a first film formation step of forming a buffer layer made of an oxide mainly composed of La on a substrate body directly or via an intermediate layer, and a second film formation step of forming a ferroelectric layer on the buffer layer, wherein the first film formation step sets the firing temperature for firing lanthanum nitrate at 450°C or higher and 650°C or lower, and further sets the heating time t1 from room temperature to the firing temperature at 200 seconds or longer.

[0021] By lengthening the temperature rise time t1 in the first film formation step, La(NO ３ ) ３ From LaONO ３ Or La ２ O ２ (CO ３ ) can promote the phase transition to LaONO. ３ Or La ２ O ２ (CO ３ ) has an effect on the orientation of the ferroelectric layer. ３ Or La ２ O ２ (CO ３ ) cannot be sufficiently transitioned to the ferroelectric layer, resulting in a low orientation of the ferroelectric layer.

[0022] According to the present invention, a ferroelectric layer can be highly oriented, and can be suitably used in electro-optical devices such as optical modulators.

[0023] FIG. 1 is a cross-sectional view of a laminated film-coated substrate according to an embodiment of the present invention. FIG. 2 is a graph showing the XRD intensity of an X-ray diffraction measurement of the laminated film-coated substrate of FIG. 1. FIG. 3 is a graph showing the XRD intensity of FIG. 2 on an enlarged scale. FIG. 4 is a graph showing the XRD intensity of an X-ray diffraction measurement of a laminated film-coated substrate of a comparative example on an enlarged scale. FIG. 5 is a cross-sectional view of an optical modulator using the laminated film-coated substrate of FIG. 1. FIG. 7A to FIG. 7F are diagrams for explaining a manufacturing method of an optical modulator of an example. FIG. 6 is a diagram showing an evaluation system for evaluating the characteristics of the optical modulator of an example.

[0024] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0025] The laminated film-coated substrate 1 shown in FIG. 1 includes a substrate body 10 and a laminated film 20 formed on the substrate body 10 .

[0026] The substrate body 10 is not particularly limited, and may be, for example, a Si (silicon) substrate without an oxide film, a Si substrate with an oxide film, a glass substrate, a sapphire substrate, etc. The size of the substrate body 10 is also not limited, and may be formed as, for example, a circular wafer with a diameter of 10 mm to 350 mm, or a rectangular wafer with one side of 10 mm to 350 mm.

[0027] The laminated film 20 includes a buffer layer 210 and a ferroelectric layer 220 formed on the buffer layer 210. In the illustrated example, the buffer layer 210 is directly laminated on the substrate body 10, but there is a SiO ２ , Al ２ O ３ , ZrO ２ , HfO ２ For example, the buffer layer 210 may be formed on the first surface 11 of the substrate body 10, or the buffer layer 210 may be formed on the intermediate layer formed on the first surface 11.

[0028] The buffer layer 210 is made of an oxide containing La as a main component. The oxide constituting the buffer layer 210 is, for example, La(NO ３ ) ３ , LaONO ３ , La ２ O ２ CO ３ 2 shows the results of X-ray diffraction (XRD) of this buffer layer 210, measured using CuKα radiation with a scan axis of 2θ / θ. For the measurement, the buffer layer 210 and the ferroelectric layer 220 thereon are formed on a substrate, and the measurement is performed from the surface side of the ferroelectric layer 220. For the measurement in this embodiment, the buffer layer 210 and the ferroelectric layer 220 thereon are formed as thermal oxide films (SiO２ The integrated intensity of the peak of the buffer layer appearing within the range of diffraction angles (2θ) of 12.90° to 13.95° shown in FIG. 2 is defined as I Ａ [Counts·degree], and the integrated intensity of the peak of the buffer layer appearing within the range of the diffraction angle (2θ) of 38.50° to 40.20° is defined as I Ｂ In terms of [Counts·degree], the intensity ratio of the following formula (1) is 0.33 or more. Note that the diffraction angle is the angle between the incident X-ray and the scattered X-ray in the XRD measurement, and is twice the scattering angle θ.

[0029]

[0030] The intensity ratio shown in formula (1) is an index showing the phase state of the buffer layer 210. A higher intensity ratio indicates that a phase having an effect on the orientation of PLZT is formed as the buffer layer. In this embodiment, La(NO ３ ) ３ The (100) peak appears between 12.90° and 13.95°, and LaONO ３ (004) or La ２ O ２ (CO ３ ) (060) peak appears at an angle of 38.50° or more and 40.20° or less.

[0031] In the X-ray diffraction results of the buffer layer 210, La(NO ３ ) ３ The integrated intensity of the (100) peak is I Ａ [Counts·degree], and LaONO appears between 38.50° and 40.20° ３ (004) or La ２ O ２ (CO ３ ) (060) peak is I Ｂ [Counts·degree].

[0032] In this X-ray diffraction result, phase identification can be performed using the pattern disclosed in the PDF (Powder Diffraction File) card number (PDF-2 2025) in the database of the International Centre for Diffraction Data (ICDD (registered trademark)). The PDF card numbers of each crystalline phase and the peak assignments of interest are as follows:

[0033] ・La (NO ３ ) ３ : No. 00-052-1103 The peak at 12.90° or more and 13.95° or less is triclinic La(NO ３ ) ３ Belongs to (100)

[0034] ・LaONO ３ : No. 00-047-0890 The peak around 38.50° to 40.20° is tetragonal LaONO ３ Belongs to (004)

[0035] ・La ２ O ２ (CO ３ ): No. 00-048-1113 The peak around 38.50° to 40.20° is monoclinic La ２ O ２ (CO ３ ) (060)

[0036] In the laminated film 20 or the laminated film-coated substrate 1 of this embodiment, La(NO ３ ) ３ (100) to LaONO ３ (004) or La ２ O ２ (CO ３ In order to control the phase transition from the (060) plane to the (060) plane sufficiently, the intensity ratio is 0.33 or more. The intensity ratio of the buffer layer 210 of this embodiment may be 0.35 or more, more preferably 0.39 or more, and even more preferably 0.49 or more.

[0037] This intensity ratio is theoretically ３ ) ３ If the (100) peak becomes so small that it cannot be detected, the value can be considerably large, but in reality, the value rarely exceeds 1.0, and 1.0 is the upper limit.

[0038] Since the buffer layer 210 has an intensity ratio of 0.33 or more, when the ferroelectric layer 220 is formed on the buffer layer 210, the orientation of the ferroelectric layer 220 becomes high. When the substrate 1 with the laminated film is subjected to X-ray diffraction measurement, for example, when the ferroelectric layer is made of PLZT, a high peak of oriented PLZT (100) appears as shown in FIG. 3 .

[0039] On the other hand, the calcination of lanthanum nitrate produces La(NO ３ ) ３ From LaONO ３ Or La ２ O ２ (CO ３ ) is insufficient, LaONO ３ (004) or La ２ O ２ (CO ３ ) (060) peak does not appear significantly between 38.50° and 40.20°. ３ Or La ２ O ２ (CO ３ When a ferroelectric layer is formed on the buffer layer 210 in which the phase transition to PLZT (100) is insufficient and the ferroelectric layer is subjected to X-ray diffraction measurement, a low peak appears due to insufficient orientation of PLZT (100), as shown in Figure 5. As described above, when the intensity ratio is less than 0.33, the ferroelectric layer 220 cannot be formed on the buffer layer 210 with high orientation.

[0040] The thickness of the buffer layer 210 is not limited, but is, for example, 4 nm to 50 nm, and preferably 8 nm to 25 nm. If the thickness is less than 4 nm, it is difficult to form a uniform film, and if it exceeds 50 nm, when a ferroelectric layer is formed on the buffer layer 210 to fabricate a photoelectric element, there is a concern that light confinement in the ferroelectric layer will be insufficient, preventing efficient photoelectric conversion.

[0041] The ferroelectric layer 220 is made of PZT (lead zirconate titanate: Pb(Zr,Ti)O ３ ) or PLZT (lanthanum-doped lead zirconate titanate: [(Pb,La)(Zr,Ti)O ３ ]).

[0042] Furthermore, when the XRD intensity of the ferroelectric layer was confirmed at any nine points within the surface of the ferroelectric layer side of the laminated film-coated substrate 1 by X-ray diffraction measurement using CuKα rays with a scan axis of 2θ / θ, the in-plane integrated intensity I ＸＹ The ratio of the standard deviation to the maximum value of the integrated intensity I of the ferroelectric layer 220 in the plane of the laminated film-coated substrate 1 is ＸＹ The integrated intensity I ＸＹ The buffer layer 210 and the ferroelectric layer 220 thereon are formed as a thermal oxide film (SiO ２ The film was formed on a Si substrate with a ferroelectric layer 220, and measurements were taken at nine points from the surface of the ferroelectric layer 220 on the side where the ferroelectric layer 220 was formed. From the X-ray diffraction results obtained at each measurement point, the integrated intensity of the peak of PLZT(100) or PZT(100) appearing in the range of diffraction angles (2θ) of 21.60° to 22.10° is shown. The standard deviation is the integrated intensity I of the nine points. ＸＹ The maximum value is the integrated intensity I ＸＹ Hereinafter, the ratio of formula (2) may be referred to as standard deviation / maximum value.

[0043]

[0044] In the laminated film 20 or laminated film-coated substrate 1 of this embodiment, the standard deviation / maximum value is 0.13 or less to increase the uniformity of the orientation of the ferroelectric layer 220 at multiple locations within the plane. This allows the uniformity of the characteristics of, for example, multiple optical modulators formed using the laminated film-coated substrate 1 to be sufficiently increased. On the other hand, if the standard deviation / maximum value exceeds 0.13, the yield of multiple optical modulators formed using the laminated film-coated substrate decreases. Note that the standard deviation / maximum value in this embodiment may be 0.10 or less. The lower limit of this ratio is 0, which is the theoretical case where there is no variation at all (standard deviation is 0).

[0045] 6 is a cross-sectional view showing an example of an optical modulator 100 fabricated using the laminated film-coated substrate 1 shown in FIG. 1, and this optical modulator 100 has a general-purpose Mach-Zehnder interference waveguide structure. In the illustrated example, the ferroelectric layer 220 is dry-etched to have two waveguides 221 separated by a branch (not shown). Also, reference numeral 40 denotes an electrode, and reference numeral 50 denotes a cladding layer. In this optical modulator 100, the core layer is formed of the highly oriented ferroelectric layer 220, thereby favorably achieving an electro-optic effect.

[0046] (Method for manufacturing substrate 1 with laminated film) The method for manufacturing substrate 1 with laminated film includes a first film formation process for forming a buffer layer 210 on the substrate body 10 (hereinafter sometimes referred to as the substrate), and a second film formation process for forming a ferroelectric layer 220 on the buffer layer 210.

[0047] [First Film Forming Process: Buffer Layer 210] The first film forming process includes a first coating process of coating the surface of the substrate with polyvinylpyrrolidone (hereinafter referred to as PVP), a second coating process of coating the PVP with lanthanum nitrate, a pre-baking process of pre-baking the lanthanum nitrate on the substrate, and a baking process of baking the lanthanum nitrate on the substrate.

[0048] The first coating step includes a step of applying a PVP solution to a substrate and a step of volatilizing the solvent.

[0049] The PVP solution consists of PVP and a solvent, with the PVP concentration being 0.05 wt% or more and 0.5 wt% or less. Examples of the solvent include 2-methoxyethanol, 1-propanol, methanol, and ethanol. The PVP solution was dropped onto the substrate and spin-coated. Note that dip coating may be used instead of spin coating. After applying the PVP solution, the substrate is placed on a heating unit heated to, for example, 150°C to volatilize the solvent. This forms a PVP film, which acts as a surfactant and has the effect of forming a uniform film of lanthanum nitrate.

[0050] The second coating step includes applying a lanthanum nitrate solution onto the PVP and evaporating the solvent.

[0051] The lanthanum nitrate solution consists of lanthanum nitrate hexahydrate and a solvent. The concentration of lanthanum nitrate hexahydrate is 0.5 wt% or more and 3 wt% or less. The solvent is, for example, 2-methoxyethanol, 1-propanol, methanol, ethanol, etc. The lanthanum nitrate solution is dropped onto PVP and spin-coated. Note that dip coating may be used instead of spin coating. After applying the lanthanum nitrate solution, the substrate is heated to, for example, 150°C or more and 250°C or less to volatilize the solvent.

[0052] In the pre-baking step, the substrate is heated to 300° C. or higher and 400° C. or lower to pre-baked the lanthanum nitrate. After heating to the pre-baking temperature, the substrate is placed on a heating unit whose temperature is lower than the pre-baking temperature and gradually cooled, and then removed from the heating unit and allowed to cool in the air.

[0053] In the baking process, the substrate is baked to crystallize lanthanum nitrate. The baking temperature is 450°C or higher and 650°C or lower, preferably 480°C or higher and 650°C or lower, and more preferably 500°C or higher and 600°C or lower. The baking temperature is maintained for 1 second or higher and 1000 seconds or lower, preferably 30 seconds or higher and 180 seconds or lower. Furthermore, in the baking process, the temperature rise time t from room temperature to the baking temperature is １ is 200 seconds or more and 1000 seconds or less, preferably 240 seconds or more and 1000 seconds or less, and more preferably 240 seconds or more and 750 seconds or less. １ may be 240 seconds or more and 720 seconds or less.

[0054] The temperature rise time t1 is adjusted to control the lanthanum nitrate after firing to a phase state suitable for forming the ferroelectric layer 220. If the temperature rise time t1 is less than 200 seconds, the lanthanum nitrate LaONO ３ Or La ２ O ２ (CO ３ ) is insufficient, and the ferroelectric layer 220 cannot be formed with high orientation on the buffer layer 210. ３ Or La ２ O ２ (CO ３ There is a concern that other phases may be mixed in.

[0055] The temperature rise rate v1 from room temperature to the firing temperature is 0.5° C. / sec or more and 10° C. / sec or less, and preferably 0.70° C. / sec or more and 2.5° C. / sec or less. When the temperature rise rate v1 is less than 0.5° C. / sec, LaONO ３ Or La ２ O ２ (CO ３ ) may be mixed with other phases than LaONO. ３ Or La ２ O ２ (CO ３ ) phase transition becomes insufficient.

[0056] The thickness of the buffer layer 210 can be increased by repeating the first coating step of applying PVP, the second coating step, and the baking step.

[0057] [Second Film Forming Process: Ferroelectric Layer 220] The second film forming process includes a process of applying a film forming solution for PLZT or PZT onto the buffer layer 210, a process of volatilizing the solvent, a pre-baking process of pre-baking the film forming material, and a baking process of baking the film forming material.

[0058] In the coating process, the film-forming solution is dropped onto the substrate on which the buffer layer 210 has already been formed, and spin coating is performed. Note that dip coating or the like may be performed instead of spin coating. After the film-forming solution is applied, the solvent is evaporated. For example, the substrate on which the film-forming solution has been applied is placed on a heating unit heated to 150°C.

[0059] In the pre-baking step, the substrate is heated to 300° C. or higher and 450° C. or lower to pre-baked the film-forming material. After heating to the pre-baking temperature, the substrate is placed on a heating unit whose temperature is lower than the pre-baking temperature and gradually cooled, and then removed from the heating unit and allowed to cool in the air.

[0060] In the baking process, the substrate is baked to crystallize the film-forming material. The baking temperature is 500°C or higher and 750°C or lower, preferably 550°C or higher and 650°C or lower. The baking temperature is maintained for 1 second or higher and 500 seconds or lower, preferably 30 seconds or higher and 90 seconds or lower. Furthermore, the temperature rise time t from room temperature to the baking temperature is ２The heating time is 30 seconds or more and 1000 seconds or less, preferably 240 seconds or more and 720 seconds or less. The heating rate from room temperature to the baking temperature is 0.65°C / second or more and 10°C / second or less, preferably 0.90°C / second or more and 2.5°C / second or less. The film thickness of the ferroelectric layer 220 can be increased by repeating, for example, the process from applying the film-forming solution to the baking process of baking the film-forming material. After the baking process, the substrate 1 with the laminated film is completed.

[0061] In the laminated film-coated substrate 1 of this embodiment, the phase state of the buffer layer 210 is controlled by heat treatment during manufacturing, so that the ferroelectric layer 220 is formed with an orientation. Furthermore, in the laminated film-coated substrate 1, the orientation of the ferroelectric layer 220 is uniform throughout the surface. Therefore, the uniformity of the characteristics of the optical modulator formed using the laminated film-coated substrate 1 can be sufficiently improved.

[0062] The present invention can be practiced without being limited to the above-described and illustrated examples.

[0063] The laminated film-coated substrate 1 of the present invention can be used not only for optical modulators but also for optical switches, phase shifters, etc. The method for forming the ferroelectric layer on the buffer layer is not limited to the above description, and other methods such as chemical solution deposition, chemical vapor deposition, sputtering, and evaporation may also be used.

[0064] The samples were substrates with laminated films, each consisting of a buffer layer formed on a Si substrate with different film thicknesses and firing conditions, and a PLZT or PZT ferroelectric layer formed on this buffer layer.The films of each sample were analyzed, and the characteristics of the optical modulators fabricated using the samples were confirmed.

[0065] The Si substrate used was a Si (silicon) wafer with a thermal oxide film (film thickness: 3 μm). The Si substrate was 4 inches in size and 0.525 mm in thickness.

[0066] The method for manufacturing a substrate with a laminated film includes a cleaning step for cleaning a Si substrate, a first film formation step for forming a buffer layer on the cleaned Si substrate, and a second film formation step for forming a ferroelectric layer made of PLZT or PZT on the buffer layer.

[0067] [Cleaning Process] The cleaning process was performed in the following order: first cleaning, second cleaning, and third cleaning. In the first cleaning, the Si substrate was immersed in acetone and ultrasonically cleaned for 2 minutes. In the second cleaning, the Si substrate was immersed in pure water and ultrasonically cleaned for 2 minutes. In the third cleaning, the solution was heated to 75°C and the Si substrate was immersed in the solution for 20 minutes (RCA SC1 cleaning). The solution consisted of pure water, hydrogen peroxide (35 wt%), and ammonia water (29 wt%), with a volume ratio of pure water:hydrogen peroxide (35 wt%):ammonia water (29 wt%) = 3:1:1. After the second cleaning and before the third cleaning, the Si substrate may be heat-treated at 500°C to 800°C.

[0068] [First Film Forming Process: Buffer Layer] The first film forming process includes a first coating process of coating the surface of the cleaned Si substrate with polyvinylpyrrolidone (hereinafter referred to as PVP), a second coating process of coating the PVP with lanthanum nitrate, a pre-baking process of pre-baking the lanthanum nitrate on the substrate, and a baking process of baking the lanthanum nitrate on the substrate.

[0069] The first coating step includes a step of applying a PVP solution to a Si substrate that has been subjected to substrate cleaning, and a step of volatilizing the solvent.

[0070] The PVP solution was prepared by weighing 0.05 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 PVP with k = 30 (average molecular weight 40,000) or k = 90 (average molecular weight 360,000) may also be used, using a low molecular weight PVP allows for easier dissolution in the solvent. The PVP solution was stirred before dripping, and after the PVP was completely dissolved, 1 mL of the PVP solution was dripped onto a Si substrate and spin-coated using a spin coater. Spin-coating was performed at 500 rpm for 5 seconds, followed by 4000 rpm for 30 seconds.

[0071] After the spin coating, the Si substrate coated with the PVP solution was placed on a hot plate heated to 150° C. for 1 minute to volatilize the solvent.

[0072] The second coating step includes applying a lanthanum nitrate solution onto the PVP and evaporating the solvent.

[0073] The lanthanum nitrate solution was prepared by weighing 0.311 g of lanthanum nitrate hexahydrate (purity >99.0%) into a glass container, adding 2-methoxyethanol, and stirring for 30 minutes. Before adding the 2-methoxyethanol to the lanthanum nitrate hexahydrate, the lanthanum nitrate hexahydrate may be heated up to 200°C to remove the water of hydration.

[0074] The lanthanum nitrate solution was stirred before dripping to completely dissolve the lanthanum nitrate. Then, 1 mL of the lanthanum nitrate solution was dripped onto the PVP and spin-coated using a spin coater. The spin-coating was performed by holding at 500 rpm for 5 seconds, followed by 4000 rpm for 30 seconds. After spin-coating, the Si substrate coated with the lanthanum nitrate solution was placed on a hot plate heated to 150°C for 1 minute to volatilize the solvent.

[0075] In the pre-baking step, the Si substrate was placed on a hot plate heated to 300°C for 5 minutes, and then placed on a hot plate heated to 150°C for 30 seconds to gradually lower the temperature of the Si substrate. The Si substrate was then removed from the hot plate and allowed to cool in the air.

[0076] In the firing process, the Si substrate was fired in a firing apparatus at a firing temperature of 590°C, a temperature holding time of 60 seconds, and in an air atmosphere. １The time t was varied in the range of 59 seconds to 720 seconds. This is expressed as a heating rate of 0.82°C / second to 10.00°C / second. A Rapid Thermal Annealing device (RTA-8000) manufactured by Advance Riko Co., Ltd. was used as the baking device (hereinafter referred to as the RTA device). Lanthanum nitrate was crystallized by performing the baking process. The thickness of the buffer layer can be increased by repeating the first coating step of applying PVP, the second coating step, and the baking step. In each sample, the number of layers serving as the base for the ferroelectric layer was set to two (thickness 11 nm) and four (thickness 22 nm). The heating time t １ The thickness and thickness of the film are shown in Table 1.

[0077] [Second Film Forming Process: Ferroelectric Layer] The second film forming process includes the steps of applying a sol-gel solution for PLZT or PZT onto the buffer layer, volatilizing the solvent, pre-firing the PLZT or PZT, and firing the PLZT or PZT. For samples 1 to 8, Mitsubishi Materials Corporation's film-forming E1 solution (concentration 15%, composition ratio: Pb:La:Zr:Ti = 115:8:65:35) was used as the PLZT sol-gel solution (samples 1 to 8). Samples were also fabricated with varying Pb:La:Zr:Ti composition ratios, as shown in Table 1. Specifically, samples were prepared using a solution in which the Zr:Ti ratio was fixed at 65:35 and the Pb:La ratio was 106:6 (sample 9), 108:4 (sample 10), 110:2 (sample 11), and 110:4 (samples 12 and 13), and samples were also prepared using a solution in which the Zr:Ti ratio was varied and the Pb:La:Zr:Ti ratio was 111:4:52:48 (sample 15).

[0078] The same process was performed when forming a PZT layer, and the PZT sol-gel liquid was E1 liquid for film formation manufactured by Mitsubishi Materials Corporation (concentration 15%, composition ratio: Pb:Zr:Ti = 112:65:35) (Sample 14).

[0079] In the coating process, 1 mL of the sol-gel solution was dropped onto a Si substrate on which a buffer layer had already been formed, and spin-coated using a spin coater. The spin coating was performed by holding at 500 rpm for 5 seconds, followed by holding at 4000 rpm for 30 seconds. After spin coating, the Si substrate on which the sol-gel solution had been applied was placed on a hot plate heated to 150°C for 1 minute to volatilize the solvent.

[0080] In the pre-baking step, the Si substrate was placed on a hot plate heated to 300°C for 5 minutes, and then placed on a hot plate heated to 150°C for 30 seconds to gradually lower the temperature of the Si substrate. The Si substrate was then removed from the hot plate and allowed to cool in the air.

[0081] In the baking process, the Si substrate was baked in the RTA apparatus at a baking temperature of 640°C, a temperature holding time of 60 seconds, and a baking atmosphere of O ２ In the firing treatment of each sample, the temperature was raised from room temperature to the firing temperature of 640°C for t ２ The time is varied within a range of 64 seconds to 720 seconds. The PLZT or PZT is crystallized by performing a baking treatment.

[0082] The ferroelectric layer was formed by repeating the steps from the application of the sol-gel solution to the baking process. Ellipsometry measurements were performed to calculate the film thickness of the ferroelectric layer of the sample. A spectroscopic ellipsometer (M-2000 manufactured by J.A. Woollam) was used for the measurements, and measurements were performed in reflection mode at three angles of incidence / reflection: 60°, 70°, and 80°. The Cauchy optical model was applied to the obtained results and fitting was performed to evaluate the film thickness.

[0083] The thickness of the ferroelectric layer of each sample and the temperature rise time t during the firing process of PLZT or PZT ２ The results are shown in Table 1.

[0084] [Film Evaluation] The crystalline phase of the buffer layer formed on the Si substrate and the orientation of the ferroelectric layer were analyzed by X-ray diffraction (XRD). The X-ray diffraction apparatus used was a fully automated multipurpose X-ray diffraction apparatus (SmartLab) manufactured by Rigaku Corporation. A Cu tube was used to generate X-rays, and Kα rays were used for diffraction.

[0085] A. Crystalline Phase of the Buffer Layer The scan axis was 2θ / θ, and the angle range was 10° to 60°. The scan was performed at a sampling step of 0.01°, with a scan speed of 10° / min, IS = 0.300 mm, RS1 = 5 mm, and RS2 = 5 mm. Measurements were performed on the center of a 4-inch wafer placed on a measurement stage. The measurement was performed with an incident-side Soller slit of 5°, an incident-length limiting slit (IS length) of 10 mm, a parallel slit analyzer (PSA) of 0.5°, and a measurement-side Soller slit of 5°. A scintillation counter (Rigaku, SC-70) was used as the detector.

[0086] The obtained XRD intensity data was subjected to a peak search using integrated powder X-ray analysis software (PDXL2 Version 2.8.4.0, Rigaku), and La(NO) appearing between 12.90° and 13.95° was identified. ３ ) ３ Integrated intensity of the (100) peak I Ａ (Counts·degree) was calculated, and LaO(NO ３ ) (004) or La ２ O ２ (CO ３ ) (060) peak integrated intensity I Ｂ [Counts·degree] was calculated. At this time, the peak search was performed automatically with "Refine background" turned on. The peak shape was a divided pseudo-Voigt function. At this time, Kβ rays and filter edges were not removed, and the α cut value for the peak search was set to 3.00, with the α cut range being a minimum of 0.50 and a maximum of 20.00. Amorphous peaks were excluded from detection. Peak optimization was performed only once after the measurement file was opened. Then, the intensity ratio (formula (1)) was calculated.

[0087] The evaluation of the crystalline phase was carried out by determining that the intensity ratio was 0.33 or more as La(NO ３ ) ３ From LaO (NO ３ ) or La ２ O ２ (CO ３The phase transition from 0.1 to 0.2 is considered to have progressed sufficiently and the buffer layer is in a good phase state, and an intensity ratio of less than 0.33 is considered to have a poor phase state. Ａ and the integrated intensity I Ｂ The intensity ratios are shown in Table 1.

[0088] B. Orientation of Ferroelectrics Mapping measurements were performed using CuKα radiation at nine points on the ferroelectric layer, with the scan axis set to 2θ / θ, to confirm the in-plane distribution of the XRD intensity of the ferroelectric layer. When measuring the integrated intensity, the orientation flat of the Si substrate (wafer) was placed in front of the stage, and the central coordinates were set to (0 mm, 0 mm), with measurements taken at (-40 mm, 0 mm), (-20 mm, 0 mm), (0 mm, 0 mm), (20 mm, 0 mm), (40 mm, 0 mm), (0 mm, 40 mm), (0 mm, 20 mm), (0 mm, -20 mm), and (0 mm, -40 mm). The entrance longitudinal limiting slit (IS longitudinal) was set to 2 mm.

[0089] The XRD intensity data obtained at each measurement point was subjected to a peak search under the above conditions using integrated powder X-ray analysis software (PDXL2, Rigaku), and the integrated intensity I of the PLZT (100) or PZT (100) peak appearing between 21.60° and 22.10° was calculated. ＸＹ The integrated intensity I ＸＹ The standard deviation [Counts degree] of the 9 points is calculated. ＸＹ The maximum value [Counts·degree] was identified, and the ratio of the standard deviation and the maximum value in the above formula (2) was calculated.

[0090] The orientation of the ferroelectric layer was evaluated as follows: a standard deviation / maximum value of 0.13 or less was considered to be good, indicating that the orientation of the PLZT or PZT was highly uniform across the surface of the Si substrate (wafer); a standard deviation / maximum value of more than 0.13 was considered to be poor, indicating that the degree of orientation of the PLZT or PZT varied significantly across the surface. The standard deviation / maximum values ​​for each sample are shown in Table 1.

[0091] [Optical Modulation Characteristics] A. Fabrication of Optical Modulator A waveguide pattern and electrodes were fabricated using electron beam lithography and etching processes. First, a 20 mm x 20 mm plate was cut from the sample ( FIG. 7A ) and washed with acetone. In FIG. 7A , reference numeral 10A denotes the substrate body, reference numeral 210A denotes a buffer layer formed on the first surface 11A of the substrate body 10A, and reference numeral 220A denotes a ferroelectric layer. Then, a resist 60A was spin-coated onto the surface of the ferroelectric layer 220A ( FIG. 7B ). Spin-coating was performed at 2000 rpm for 30 seconds and dried on a hot plate at 120°C for 2 minutes. After irradiation with an electron beam using an electron beam lithography system, the resist was developed and transferred to the waveguide pattern 70A onto the resist 60A ( FIG. 7C ).

[0092] Then, the ferroelectric layer 220A was subjected to reactive ion etching in a reactive ion etching apparatus to fabricate a Mach-Zehnder interference type waveguide 221A (FIG. 7D). ３ A mixed gas of Ar and N was used, and the process was performed at a pressure of 1.8 Pa and an RF power of 150 W. The etching depth of the ferroelectric layer 220A was about 100 nm, and was controlled by adjusting the etching time.

[0093] Next, we fabricated an electrode 40A that applies a voltage to the waveguide 221A during phase modulation. To fabricate the electrode 40A, we first coated a resist on the ferroelectric layer 220A of the plate, irradiated it with an electron beam using an electron beam lithography system, and then developed the resist to transfer the electrode structure. Next, we evaporated Cr (approximately 10 nm) and Al (approximately 400 nm) using a vacuum evaporation system to complete the electrode 40A (Figure 7E).

[0094] Furthermore, a cladding layer 50A was formed on the waveguide 221A using polymethyl methacrylate resin (PMMA), completing the optical modulator 100A (FIG. 7F). The electrode spacing was 6 μm, and the electrode length was 4 mm.

[0095] In the fabrication of the optical modulator, an electron beam resist (RP6200) manufactured by Allresist GmbH was used as the resist, an electron beam lithography system (G100) manufactured by Elionix Corporation was used for electron beam lithography, and a reactive ion etching system (RIE-10NR) manufactured by Samco Corporation was used for etching.

[0096] An electric field orientation process was performed to induce the electro-optical effect of PLZT or PZT. DC voltage was applied using an electrode 40A fabricated on the ferroelectric layer 220A on a hot plate at 90°C. The polarity of the electric field was the same for the two phase modulation sections. The DC electric field strength was preferably 25 V / μm to 60 V / μm, and the holding time was preferably 30 minutes to 200 minutes, and was set arbitrarily within this range. Specifically, the power supply was adjusted so that the DC electric field strength was 30 V / μm. After holding the voltage for 30 minutes, the temperature was lowered to room temperature, completing the electric field orientation process.

[0097] B. Evaluation of Optical Modulation Characteristics Figure 5 shows an evaluation system 300 for evaluating the optical modulation characteristics of the optical modulator 100A. In the optical modulator 100A, the width of the phase modulation waveguide 221A in the ferroelectric layer 220A was set to 1.4 μm. The Mach-Zehnder interference waveguide is equipped with multi-mode circuits (hereinafter referred to as MMI) 80A and 80B on the input and output sides. The light intensity is divided into 50% each by the input side MMI 80A and branched into two phase modulation waveguides 221A, and the light is combined again by the output side MMI 80B.

[0098] Laser light with a wavelength of 1550 nm was incident on this optical modulator 100A from a laser light source 310 via an optical fiber 320, and a modulation voltage (triangular wave) was input to the electrodes by a function generator 330. One of the electrodes was used as a ground electrode. The modulated output light was sent to a photodetector 350 via an optical fiber 340, and the signal from the photodetector 350 was analyzed using an oscilloscope 360. The half-wave voltage (the voltage when the phase of the light is shifted by 90°) of the optical modulator 100A using each sample was then determined. The polarization of the laser light incident on the optical modulator 100A was TE mode.

[0099] The optical modulation characteristics were evaluated such that a half-wave voltage of less than 7.0 V was considered good, and a half-wave voltage of 7.0 V or more was considered poor. The half-wave voltage of the optical modulator using each sample and the evaluation of its optical modulation characteristics are shown in Table 1. The evaluation was graded A and B as pass, C as fail, with A being better than B.

[0100]

[0101] A. Regarding Samples 1 to 4 Samples 1 to 4 are common in that they were manufactured with a buffer layer thickness of 22 nm, a ferroelectric layer (PLZT layer) thickness of 300 nm, and the temperature rise time t2 during the firing treatment of the PLZT fixed at 64 seconds, but Samples 1 to 4 differ in that the temperature rise time t1 during the firing treatment of the lanthanum nitrate was varied.

[0102] Among samples 1 to 4, samples 3 and 4 in which the temperature rise time t1 was set to 240 seconds or more had a half-wave voltage of less than 7 V. In addition, the buffer layers of samples 3 and 4 had an intensity ratio I Ｂ / I Ａ becomes 0.49, 0.40, and La (NO ３ ) ３ From LaONO ３ Or La ２ O ２ (CO ３ In this way, by heating lanthanum nitrate at a low rate (heating rate: 0.70°C / sec or more and 2.5°C / sec or less), a phase transition to LaONO ３ Or La ２ O ２ (CO ３ This has the effect of promoting the phase transition from ZnO to ZnO, making it possible to form highly oriented PLZT. This reduces the half-wave voltage and improves the optical modulation characteristics.

[0103] Samples 1 and 2 are comparative examples, and the temperature rise time t1 was set to 59 seconds or 120 seconds, and the temperature of lanthanum nitrate was raised at a high speed. Ｂ / I Ａ becomes 0.26, 0.25, and La(NO ３ ) ３ From LaONO ３ Or La ２ O ２ (CO ３ In Samples 1 and 2, the PLZT could not be formed with a high orientation, and the half-wave voltage was high and the light modulation characteristics were poor.

[0104] In addition, in Samples 1 to 4, the longer the temperature rise time t1 of lanthanum nitrate, the smaller the standard deviation / maximum value of the PLZT layer, and the smaller the integrated intensity I ＸＹ It was confirmed that the uniformity of

[0105] B. Samples 4 and 5 Samples 4 and 5 have a buffer layer thickness of 22 nm, a PLZT layer thickness of 300 nm, and a heating time t １ They have in common that they are manufactured with the heating time t2 fixed at 720 seconds, which is a time at which the phase transition is easily promoted, but they differ in that the temperature rise time t2 during the firing process of the PLZT is changed to either 64 seconds or 640 seconds.

[0106] In both samples 4 and 5, the intensity ratio I obtained from the X-ray diffraction measurement of the buffer layer Ｂ / I Ａ is 0.39 or more, and La(NO ３ ) ３ From LaONO ３ Or La ２ O ２ (CO ３ ) phase transition was progressing, and the half-wave voltage was 5.3 V or less, and the optical modulation characteristics were good.

[0107] In Samples 4 and 5, it can be seen that the standard deviation / maximum value increases when the temperature rise time t2 during the firing process of PLZT is long (Sample 5). ３ ) ３ From LaONO ３ Or La ２ O ２ (CO ３ ) for the buffer layer in which the phase transition to ) has sufficiently progressed, shortening the temperature rise time t2 during the firing treatment of the PLZT can reduce the integrated intensity I ＸＹ The variation in the integrated intensity I ＸＹ It was confirmed that the uniformity of

[0108] C. Regarding Samples 6 to 8 Samples 6 to 8 have in common the fact that they were manufactured with a buffer layer thickness of 11 nm and a PLZT layer thickness of 300 nm. However, Samples 6 to 8 differ in the temperature rise time t １The difference is that the time t2 during the firing treatment of the PLZT was changed to 59 seconds or 720 seconds, and the temperature rise time t2 during the firing treatment of the PLZT was changed to 64 seconds or 640 seconds. Sample 6 is a comparative example.

[0109] For both samples 7 and 8, the temperature rise time t1 during the calcination treatment of lanthanum nitrate was set to 720 seconds, and the temperature was raised at a slow rate (temperature rise rate: 0.82°C / second), resulting in an intensity ratio I Ｂ / I Ａ is high at 0.57 or 0.95, and LaONO ３ Or La ２ O ２ (CO ３ ) phase transition was promoted, resulting in a half-wave voltage of less than 7 V, and the light modulation characteristics were excellent. Among them, sample 8 was particularly excellent, with a half-wave voltage of, for example, 6 V or less (5.8 V). Furthermore, sample 8 had a small standard deviation / maximum value, and the integrated intensity I ＸＹ On the other hand, in Sample 7, the standard deviation / maximum value increased, and the integrated intensity I ＸＹ There was variation and uniformity could not be achieved.

[0110] In this way, in Samples 7 and 8, the longer the temperature rise time t2 during the firing process of PLZT, the smaller the standard deviation / maximum value, and the larger the integrated intensity I ＸＹ The integrated intensity I of sample 7 was high. ＸＹ The reason for the low uniformity of the thin buffer layer is that the crystal uniformity is poor to begin with.

[0111] Looking at samples 1, 2, 3, and 7, for PLZT layers formed under conditions in which the buffer layer had a thickness of 11 nm or 22 nm, the temperature rise time t2 during the PLZT baking treatment was 64 seconds, and the PLZT layer had a thickness of 300 nm, the standard deviation / maximum value when the buffer layer had a thickness of 11 nm was larger than the standard deviation / maximum value when the buffer layer had a thickness of 22 nm. On the other hand, in sample 8, in which the temperature rise time t2 during the PLZT baking treatment was longer, the PLZT was exposed to a high temperature for a longer period, which is thought to have made the crystalline state of the PLZT more uniform.

[0112] From samples 7 and 8, the intensity ratio of the buffer layer I Ｂ / IＡ When the thickness of the buffer layer is small and the temperature rise time t2 of the PLZT is large, it is found that the orientation at various points within the plane of the PLZT layer is important, and the standard deviation / maximum value can be reduced.

[0113] In sample 6, the temperature rise time t1 during the lanthanum nitrate firing treatment was set to a short time of 59 seconds, and in the X-ray diffraction measurement of the buffer layer of sample 6, no peak was confirmed between 38.50° and 40.20°, the orientation of the PLZT layer was low, and the half-wave voltage exceeded 7 V, resulting in poor light modulation characteristics.

[0114] D. Regarding Sample 14, the ferroelectric layer of Sample 14 was a PZT layer, the buffer layer was 11 nm thick, and the PZT layer was formed to a thickness of 300 nm. １ The time t2 during the firing treatment of PZT was set to 640 seconds. Ｂ / I Ａ A highly oriented PZT was obtained with a standard deviation / maximum value of less than 0.10, and high uniformity was obtained.

[0115] E. Samples 9 to 13 and 15 In Samples 9 to 13 and 15, the composition ratio of the sol-gel solution: Pb:La:Zr:Ti was varied, the thickness of the buffer layer was set to 11 nm, and the thickness of the ferroelectric layer was set to 300 nm or 600 nm (Sample 13). The temperature rise time t １ The heating time t2 during the firing treatment of the PLZT was set to 720 seconds, and the heating time t2 during the firing treatment of the PLZT was set to 640 seconds. Ｂ / I Ａ is 0.33 or more, the orientation of PLZT is high, and I ＸＹ The standard deviation / maximum value was 0.06 or less, and high uniformity was obtained. The half-wave voltage was low at 4.3 V or less, and the light modulation characteristics were good.

[0116] From the results of D and E (samples 9 to 15), the intensity ratio I Ｂ / I Ａ is 0.33 or more, I ＸＹIt can be seen that by making the standard deviation / maximum value of 0.06 or less, a uniform ferroelectric layer with high orientation can be obtained regardless of the composition of the ferroelectric layer, PLZT or PZT.

[0117] The present invention can be suitably used in electro-optical devices such as optical modulators, by providing a laminated film having a highly oriented ferroelectric layer.

[0118] REFERENCE SIGNS LIST 1 Substrate with laminated film 10, 10A Substrate body 20 Laminated film 210, 210A Buffer layer 220 Ferroelectric layer 220A PLZT layer 231, 231A Waveguide 40, 40A Electrode 50, 50A Cladding layer 100, 100A Optical modulator 300 Evaluation system

Claims

1. A ferroelectric layer is provided which is made of a buffer layer made of an oxide mainly composed of La, and which is made of PZT or PLZT and formed on the buffer layer. The scan axis is set to 2θ / θ, and the integrated intensity of the peak of the buffer layer which appears within the range of the diffraction angle (2θ) of 12.90° to 13.95° in the X-ray diffraction results using CuKα rays is defined as I Ａ [Counts·degree], and the integrated intensity of the peak of the buffer layer appearing within the range of the diffraction angle (2θ) of 38.50° to 40.20° is defined as I Ｂ A laminated film characterized in that the intensity ratio of the following formula (1), expressed in [Counts·degree], is 0.33 or more.

2. I above Ａ [Counts·degree] is the La(NO ３ ) ３ is the integrated intensity of the (100) peak, Ｂ [Counts·degree] is the LaONO buffer layer ３ (004) or La ２ O ２ (CO ３ 2. The laminated film according to claim 1, wherein the integrated intensity of the peak of (060) is 3. The scan axis is set to 2θ / θ, and the integrated intensity of the peaks appearing within the range of diffraction angles (2θ) of 21.60° to 22.10° in the X-ray diffraction results measured at each of nine points in the plane from the ferroelectric layer side using CuKα radiation is defined as I. ＸＹ [Counts·degree], the integrated intensity I in the plane of the following formula (2) is ＸＹ 2. The laminated film according to claim 1, wherein the ratio of the standard deviation to the maximum value is 0.13 or less.

4. A substrate with a laminated film, comprising a substrate body and the laminated film according to any one of claims 1 to 3 provided on the substrate body directly or via an intermediate layer.

5. An electro-optical device comprising a substrate with a laminated film, characterized in that the substrate with a laminated film comprises a substrate body and a laminated film according to any one of claims 1 to 3 provided on the substrate body directly or via an intermediate layer.

6. A method for manufacturing a substrate with a laminated film, comprising: a first film formation step of forming a buffer layer made of an oxide whose main component is La directly on the substrate body or via an intermediate layer; and a second film formation step of forming a ferroelectric layer on the buffer layer, wherein the first film formation step is characterized in that the firing temperature for firing lanthanum nitrate is 450°C or higher and 650°C or lower, and the temperature rise time t1 from room temperature to the firing temperature is 240 seconds or longer.

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