Multilayer films, substrates with multilayer films, electro-optical devices using the same, and methods for manufacturing substrates with multilayer films.

Abstract

This system provides a modulator capable of achieving a VπL of 2.0V·cm or less, enabling compact size, low voltage operation, and response to high-frequency signals. [Solution] A laminated film comprising a seed layer mainly composed of lanthanum and a ferroelectric PLZT film laminated on the seed layer, wherein when the concentrations (at%) of each element of the PLZT film measured by XPS analysis are [Pb], [La], [Zr], and [Ti] respectively, in the surface layer of the PLZT film, The amount of Pb+La defined as {([Pb]+[La]) / ([Pb]+[La]+[Zr]+[Ti])}(%) is 35% or more and 40% or less. The amount of La expressed as {[La] / ([Pb]+[La]+[Zr]+[Ti])}(%) is between 2% and 5%.

Description

Technical Field

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

Background Art

[0002] Non-Patent Document 1 discloses a technique of forming a PLZT ((Pb, La)(Zr, Ti)O3: lanthanum-doped lead zirconate titanate) thin film by the sol-gel method on a single crystal substrate or a seed layer formed on a Si (silicon) substrate, processing it as a waveguide, and applying electrodes to use it as an optical modulator. Also, Patent Documents 1 and 2 disclose a technique of forming a PLZT thin film as an alignment film on a Si substrate by applying an oxide composed of La nitrate as a buffer layer (seed layer) and using it as an optical modulator.

[0003] In an optical modulator, it is required to correspond to the miniaturization and integration of current optical communication devices and exhibit a sufficient optical modulation effect even at short distances. As a performance index representing this, there is the product VπL of the half-wave voltage and the electrode length (which is the product of the voltage required to change the optical phase by π and the electrode length, and indicates that a smaller and lower-voltage-driven modulator can be configured with a lower value). In the current thin-film LN modulator using a lithium niobate (LiNbO3 (hereinafter abbreviated as LN) bonded substrate, a low value of about 2.0 V·cm has been reported (Non-Patent Document 2).

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Non-Patent Document 2

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

[0006] Here, PLZT and LN are applied as optical modulators based on their electro-optic effect (the property that the refractive index changes with voltage), and the magnitude of this effect is characterized by a value called the Pockels coefficient (a proportionality constant that represents the change in refractive index with respect to the applied electric field). PLZT is known as a material that exhibits a higher Pockels coefficient than LN, with a value of about 500 pm / V for bulk single crystals compared to 31 pm / V for LN. However, when applied to devices, the VπL is almost always reported to be lower than that of thin-film LN, at 2.0 V·cm or higher, for reasons such as the inferior crystallinity of thin films compared to single crystals, the influence of crystal orientation on the substrate on the electro-optic response, and the influence of the materials of the lower and upper layers of the PLZT constituting the waveguide on the electro-optic response.

[0007] In view of these challenges, the present invention aims to provide a modulator that can achieve a VπL of 2.0 V·cm or less using a PLZT thin film, is compact and low-voltage driven, and can respond to high-frequency signals. [Means for solving the problem]

[0008] The present invention provides a laminated film comprising a seed layer mainly composed of lanthanum and a ferroelectric PLZT film laminated on the seed layer, wherein when the concentrations (at%) of each element in the PLZT film, as measured by XPS analysis, are [Pb], [La], [Zr], and [Ti] respectively, the surface layer of the PLZT film contains, The amount of Pb+La defined as {([Pb]+[La]) / ([Pb]+[La]+[Zr]+[Ti])}(%) is 35% or more and 40% or less. The amount of La expressed as {[La] / ([Pb]+[La]+[Zr]+[Ti])}(%) is between 2% and 5%.

[0009] By keeping the Pb+La content between 35% and 40%, the inclusion of non-PLZT phases is prevented, and the dielectric loss tangent tanδ can be reduced to a small value, for example, 0.05 or less. This allows for the construction of an optical modulator that can be used even when applied to high-frequency applications. Furthermore, by having a La content of 2% to 5%, the VπL, which represents the optical modulation capability, can be reduced to, for example, 2.0 V·cm or less, providing a laminated film that can suitably be used to construct a compact and highly efficient optical modulator. Furthermore, the concentration of each element in the surface layer of the PLZT film can be measured by X-ray photoelectron spectroscopy (XPS).

[0010] In the multilayer film of the present invention, the dielectric loss tangent tanδ of the PLZT film is preferably 0.05 or less. A dielectric loss tangent tanδ of 0.05 or less results in low power consumption and low dielectric loss, making it possible to construct an optical modulator more suitable for high-frequency applications.

[0011] The laminated substrate of the present invention comprises a substrate body and the laminated film provided directly on the substrate body or via an intermediate layer. The substrate itself can be, for example, a Si substrate without an oxide film, a Si substrate with an oxide film, a glass substrate, a sapphire substrate, etc. A metal layer such as platinum (Pt) or gold (Au) or a metal oxide film layer can be used as an intermediate layer.

[0012] The electro-optical device of the present invention includes the substrate with the laminated film.

[0013] When the laminated film is processed into a waveguide to form a modulator, VπL is 2.0 V·cm or less. The electro-optical device is, for example, an optical modulator, an optical switch, or a phase shifter. Since VπL is 2.0 V·cm or less, a small-sized and highly efficient electro-optical device can be provided.

[0014] The method for manufacturing the substrate with the laminated film of the present invention includes a seed layer forming step of forming a seed layer by applying and heating a seed layer forming solution mainly composed of lanthanum on the substrate, and a PLZT film forming step of forming a ferroelectric PLZT film by applying a PLZT sol-gel solution on the seed layer and performing heat treatment. When the composition ratio of Pb:La:Zr:Ti in the PLZT sol-gel solution is A:B:C:D, C + D = 100, and A:B is (106 to 109):(3 to 6).

[0015] Since Pb volatilizes partially during the subsequent firing and thus increases in amount, if it becomes too much, the concentration of ([Pb]+[La]) when forming the film becomes high, and the high-frequency responsiveness when used as an electro-optical device deteriorates. By preparing a solution for thin film formation with A:B being (106 to 109):(3 to 6), the concentration of ([Pb]+[La]) in the film can be appropriately controlled.

Advantages of the Invention

[0016] According to the present invention, a VπL of 2.0 V·cm or less can be achieved, a modulator that can be driven at a small size and low voltage and can also respond to high-frequency signals can be configured.

Brief Description of the Drawings

[0017] [Figure 1] It is a cross-sectional view of the substrate with the laminated film according to an embodiment of the present invention. [Figure 2]It is a cross-sectional view of an optical modulator using a substrate with a laminated film shown in FIG. 1. [Figure 3] It is a diagram for explaining a method of manufacturing an optical modulator of an embodiment. [Figure 4] It is a diagram showing an evaluation system for evaluating the characteristics of an optical modulator of an embodiment.

Mode for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0019] A substrate 1 with a laminated film (hereinafter referred to as a substrate with a thin film) shown in FIG. 1 includes a substrate body 10 and a laminated film 20 formed on the substrate body 10.

[0020] The substrate body 10 is not particularly limited, and for example, a Si substrate without an oxide film, a Si substrate with an oxide film such as SiO2 or Al2O3, a glass substrate, a sapphire substrate, etc. can be used. The size of the substrate body 10 is not limited either, but for example, it may be formed as a planar rectangular shape with a thickness of 300 μm or more and 2000 μm or less and a side length of 10 mm or more and 300 mm or less, or a circular wafer with a diameter of 50 mmφ or more and 350 mmφ or less.

[0021] The laminated film 20 includes a seed layer 210 formed on the substrate body 10 and a PLZT film 220 of a ferroelectric material formed on the seed layer 210.

[0022] The seed layer 210 is made of an oxide mainly composed of La. The oxides constituting the seed layer 210 are, for example, La(NO3)3, LaONO3, La2O2CO3, etc.

[0023] The thickness of the seed 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, there is a concern that when a ferroelectric layer is formed on the seed layer 210 to fabricate a photoelectric device, the light confinement in the ferroelectric layer will be insufficient, and efficient photoelectric conversion will not be possible.

[0024] The PLZT film 220 is made of PLZT (lanthanum-doped lead zirconate titanate: [(Pb,La)(Zr,Ti)O3]) and has a perovskite crystal structure. The thickness of this PLZT film 220 is not limited, but is for example 100 nm to 900 nm, and preferably 250 nm to 700 nm.

[0025] In this PLZT film 220, Pb and La occupy the A sites of the perovskite crystal structure, while Zr and Ti occupy the B sites. Here, when the concentration (at%) of each element is [Pb], [La], [Zr], and [Ti] respectively, in the surface layer of the PLZT film, The amount of Pb+La defined as {([Pb]+[La]) / ([Pb]+[La]+[Zr]+[Ti])}(%) is 35% or more and 40% or less. The amount of La expressed as {[La] / ([Pb]+[La]+[Zr]+[Ti])}(%) is between 2% and 5%.

[0026] As will be described later, since Pb volatilizes during the calcination process when forming the PLZT film, the composition ratio of the PLZT sol-gel solution is set so that the sum of Pb and La is greater than the sum of Zr and Ti, and in the PLZT film, the sum of Pb and La may be excessive. If the amount of Pb+La is too high, lead oxide and other substances are formed, affecting the dielectric loss tangent tanδ.

[0027] In this embodiment, the laminated film 20 or thin film-attached substrate 1 allows for the construction of a compact, low-voltage optical modulator by setting the Pb+La content to 35% to 40% and the La content to 2% to 5%. In other words, a PLZT film exhibiting an electro-optic effect can be formed by having a Pb+La content of 35% or more, while a Pb+La content of 40% or less prevents the inclusion of non-PLZT phases and allows the dielectric loss tangent tanδ to be reduced to a small value, for example, 0.05 or less. This makes it possible to construct an optical modulator that can be used even when the laminated film 20 or thin-film substrate 1 of this embodiment is applied to high-frequency applications. Furthermore, by having a La content of 2% to 5%, the VπL, which represents the optical modulation capability, can be reduced to, for example, 2.0 V·cm or less, providing a laminated film that can suitably be used to construct a compact and highly efficient optical modulator. The concentrations of each element in the surface layer of the PLZT film 220 can be measured by X-ray photoelectron spectroscopy (XPS). In this embodiment, the surface layer of the PLZT film 220 corresponds to the detection depth of photoelectrons when using monochromatic Al Kα rays (25 W) as the X-ray source in XPS analysis, with a pass energy of 112 eV and a photoelectron extraction angle of 45° relative to the sample surface, and is approximately 5 nm from the surface of the PLZT film. Furthermore, the concentrations of each element in the PLZT film 220 can also be measured by Auger electron spectroscopy (AES) or energy-dispersive spectroscopy (TEM-EDS) under transmission electron microscopy. In this case, it is preferable that the concentration of each element in the PLZT film 220 be 35% to 40% for Pb + La and 2% to 5% for La in the surface layer, and 48% to 53% for Pb + La and 1.5% to 3.5% for La in the center and lower layers of the film.

[0028] Figure 2 is a cross-sectional view showing an example of an optical modulator 100 fabricated using the thin-film substrate 1 shown in Figure 1. This optical modulator 100 has a general-purpose Mach-Zehnder interference waveguide structure. In the illustrated example, the PLZT film 220 is dry-etched to form two waveguides 221 separated by a branch not shown. 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 a highly oriented PLZT film 220, which favorably exhibits the electro-optic effect. However, the optical modulator using the thin-film substrate 1 of the present invention is not limited to a Mach-Zehnder modulator.

[0029] (Manufacturing method for the thin film substrate 1) The manufacturing method for the thin-film coated substrate 1 comprises a first film formation step of forming a seed layer 210 on the substrate body 10, and a second film formation step of forming a PLZT film 220 on the seed layer 210.

[0030] [First film formation process: Seed layer 210] The first film formation process comprises a first coating step of coating the surface of the substrate body 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, and a calcination step of calcining the lanthanum nitrate on the substrate.

[0031] The first coating step comprises a step of applying a PVP solution to the substrate body 10 (PVP solution application step) and a step of volatilizing the solvent (solvent volatilization step).

[0032] The PVP solution consists of PVP and a solvent, with a PVP concentration of 0.05% by mass or more and 0.5% by mass or less. Examples of solvents include 2-methoxyethanol, 1-propanol, methanol, and ethanol. The PVP solution is dropped onto the substrate and spin-coated (PVP solution coating step). Alternatively, dip-coating or other methods may be used instead of spin-coating. After coating the PVP solution, the substrate is placed on a heated area, for example, heated to 150°C, to evaporate the solvent (solvent evaporation step). This forms a PVP film, which acts as a surfactant to uniformly deposit lanthanum nitrate.

[0033] The second coating step comprises a step of applying a lanthanum nitrate solution onto the PVP (lanthanum nitrate solution application step) and a step of volatilizing the solvent (solvent volatilization step).

[0034] 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. Note that the lanthanum nitrate solution in this embodiment may contain unavoidable impurities. 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 laminated 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.

[0035] Furthermore, the mass concentration of water in the lanthanum nitrate solution is between 3% and 21% by mass. The presence of water in the lanthanum nitrate solution ensures that the lanthanum nitrate remains stable even after several days of storage, thus maintaining the effect of ferroelectric alignment film formation. In addition, the addition of water improves the wettability to the substrate, allowing for the uniform formation of the seed layer and reducing the likelihood of appearance defects. 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 a sufficient improvement in wettability cannot be obtained. On the other hand, if the water concentration in the lanthanum nitrate solution is 21% by mass or higher, the viscosity of the solution becomes too low, making it difficult to form a uniform film when spin-coating. The water concentration in the lanthanum nitrate solution is preferably 3.5% by mass or more and 15% by mass or less, and more preferably 5% by mass or more and 12% by mass or less. Note that the mass concentration of water in the lanthanum nitrate solution includes the water of hydration derived from the lanthanum nitrate raw material.

[0036] This lanthanum nitrate solution is dropped onto the PVP and coated in a thin film by spin coating (lanthanum nitrate solution coating step). Alternatively, dip coating or other methods may be used instead of spin coating. After coating the lanthanum nitrate solution, the substrate is heated to a temperature of 150°C to 250°C to evaporate the solvent (solvent evaporation step).

[0037] 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.

[0038] 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.

[0039] The heating time t1 is adjusted to control the lanthanum nitrate after firing to a phase state (LaONO3) suitable for the formation of the PLZT film 220. If the heating time t1 is less than 200 seconds, the phase transition of lanthanum nitrate to LaONO3 will be insufficient, making it difficult to form the PLZT film 220 on the seed layer 210 with high orientation. 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 0.5°C / second or more and 10°C / second or less, preferably 0.70°C / second or more and 2.5°C / second or less. 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, there is a risk that the phase transition to LaONO3 will be insufficient. The thickness of the seed 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.

[0040] [Second film formation process: PLZT film 220] The second film formation process includes a step of applying a thin film forming solution onto the seed layer 210 (thin film forming solution application step), a step of volatilizing the solvent (solvent volatilization step), a calcination step of calcining the thin film forming material, and a firing step of firing the thin film forming material.

[0041] In the thin-film formation solution coating process, the thin-film formation solution is dropped onto a substrate that already has a seed layer 210 formed on it, and then spin-coated. This thin-film forming solution is a compound containing Pb, La, Zr, and Ti. When the composition ratio (at%) of Pb:La:Zr:Ti in the solution is A:B:C:D, then C+D=100 and A:B is (10⁶~10⁹):(3~6). The amount of Pb is increased because some of it volatilizes during subsequent firing, but if it becomes too high, the concentration of ([Pb]+[La]) in the formed film becomes high, resulting in poor high-frequency response when used in electro-optical devices. By preparing a thin-film formation solution with A:B in a ratio of (10⁶~10⁹):(3~6), the concentration of ([Pb]+[La]) in the film can be appropriately controlled.

[0042] Note that this coating process may be performed by dip coating or other methods instead of spin coating. After applying the thin film forming solution, the solvent is evaporated. For example, a substrate coated with the thin film forming solution is placed on a heating section heated to 150°C.

[0043] 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.

[0044] 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 PLZT 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.

[0045] In the thin-film substrate 1 of this embodiment, the phase state of the seed layer 210 is controlled by heat treatment during manufacturing, so that the PLZT film 220 is formed in an oriented manner. Furthermore, the thin-film substrate 1 has uniform orientation of the PLZT film 220 at various points on the surface. Therefore, the uniformity of the characteristics of the optical modulator formed using the thin-film substrate 1 can be sufficiently improved. In this case, the lanthanum nitrate solution for forming the seed layer 210 contains water at a predetermined concentration, which stabilizes its orientation ability for the dielectric film 220. Furthermore, because it contains water, this lanthanum nitrate solution has excellent wettability when applied to the substrate body 10 for film formation. As a result, the seed layer 210 can be uniformly formed on the surface of the substrate body 10. Therefore, by forming a seed layer 210 with such a lanthanum nitrate solution and then forming a PLZT film, it is possible to form a PLZT film that is stably oriented and has high in-plane uniformity of orientation, and the optical modulator formed using this thin-film-attached substrate 1 can also exhibit uniform characteristics.

[0046] Furthermore, as mentioned above, by setting the laminated film 20 formed on this thin-film substrate 1 to have a Pb+La content of 35% to 40% and an La content of 2% to 5%, a compact, low-voltage driven optical modulator can be constructed. By having a Pb+La content of 35% to 40%, a PLZT film composition that exhibits electro-optical properties can be achieved, while preventing the inclusion of non-PLZT phases and allowing the dielectric loss tangent tanδ to be reduced to a small value, for example, 0.05 or less. This enables the construction of an optical modulator that can be used even when applied to high-frequency applications. Furthermore, by having a La content of 2% to 5% in the laminated film 20, the VπL representing the optical modulation capability can be set to, for example, 2.0 V·cm or less, providing a laminated film that can suitably construct a compact and highly efficient optical modulator. In addition, according to the laminated film 20 in this embodiment, the VπL representing the optical modulation capability can be set to less than 2.0 V·cm or 1.9 V·cm or less, allowing for the suitability of constructing a compact and highly efficient optical modulator.

[0047] The present invention can be implemented in any way not limited to the above description and illustrated examples. For example, an intermediate layer may be formed on the surface of the substrate body 10, consisting of a metal layer made of platinum (Pt), gold (Au), or copper (Cu), or a metal oxide layer made of Al2O3, ZrO2, TiO2, etc. In this case, the thickness of these layers is not limited, but for example, the substrate body may be 300 μm or more and 2000 μm or less, and the intermediate layer may be 10 nm or more and 500 nm or less.

[0048] 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 PLZT film on the seed layer is not limited to the above description, and may also be formed by other chemical solution deposition methods, chemical vapor deposition methods, sputtering methods, or vapor deposition methods. [Examples]

[0049] A substrate with a multilayer film consisting of a seed layer and a PLZT film deposited on the seed layer was used as a sample. The film of each sample was analyzed, and the characteristics and dielectric loss tangent of the optical modulator fabricated using the sample were confirmed.

[0050] A Si wafer with a thermal oxide film (thickness: 3 μm) was used as the Si substrate. The Si substrate had a diameter of 4 inches and a thickness of 0.525 mm. In addition to the Si substrate with a thermal oxide film, Si substrates without an oxide film, glass substrates, sapphire substrates, etc., can also be used. This Si wafer was cleaned under the following conditions.

[0051] [Washing process] 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% by mass), and aqueous ammonia (29% by mass), with a volume ratio of pure water:hydrogen peroxide (35% by mass):ammonia (29% by mass) = 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.

[0052] [First film formation process: Seed 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.

[0053] 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 17.5 g of 2-methoxyethanol (purity >99.0%) and 2.0 g of pure water, 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 a lower molecular weight PVP allows for easier dissolution in the solvent.

[0054] The PVP solution was stirred before dropwise addition to completely dissolve the PVP. Then, 1 mL of the PVP solution was dropped onto the Si 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, as a step to evaporate the solvent, the Si substrate coated with the PVP solution was placed on a hot plate heated to 150°C for 1 minute.

[0055] 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 0.467 g of lanthanum nitrate hexahydrate (purity >99.0%) into a glass container, adding 17.5 g of 2-methoxyethanol and 2.0 g of pure water to make a total volume of 20 g, and stirring the mixture for 30 minutes.

[0056] 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.

[0057] The calcination process involved placing the substrate on a hot plate heated to 300°C for 5 minutes, then gradually lowering the Si substrate temperature 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.

[0058] 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 seed 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 served as the base for the PLZT film was set to two layers (thickness 11 nm).

[0059] [Second film formation process: PLZT film] The second film formation process comprises a step of coating the PLZT sol-gel solution onto the seed 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 15% PLZT E1 solution manufactured by Mitsubishi Materials Corporation, with compositions differing in the amount of Pb and La as shown in Table 1.

[0060] The coating process involved dropping 1 ml of PLZT sol-gel solution onto a substrate with a pre-deposited seed layer, followed by spin coating 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 PLZT sol-gel solution was placed on a hot plate heated to 150°C for 1 minute to evaporate the solvent.

[0061] 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.

[0062] 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 the firing process of each sample, the heating time t2 from room temperature to the firing temperature of 640°C was set to 640 seconds. PLZT was crystallized by the firing process.

[0063] 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 and fitted to determine the film thickness.

[0064] [Membrane evaluation] The composition of the PLZT film deposited on the substrate was analyzed, and an optical modulator was formed, and the V at that time was analyzed. π Capacitor structures were fabricated using L and PLZT films, and the dielectric loss tangent was evaluated. (composition analysis) The composition of the PLZT film was analyzed by X-ray photoelectron spectroscopy (ULVAC-PHI, PHI Quantera). A monochromatic Al Kα-ray (25 W) was used as the X-ray source, and spectra were acquired with a pass energy of 112 eV and a measurement interval of 0.1 eV / step. The photoelectron extraction angle relative to the sample surface was 45°, and the analysis area was 100 μmφ. From the spectra obtained from the PLZT film surface, the concentrations (at%) of Pb, La, Zr, and Ti were determined at three locations, and the average composition was determined by averaging the three measured values.

[0065] (Optical modulation characteristics) Waveguide patterns and electrodes were fabricated using electron beam lithography and etching processes. A 20mm x 20mm plate was cut from a sample in which a PLZT film had been deposited on a substrate (Figure 3(a)), and this plate was washed with acetone. In Figure 3(a), the reference numeral 10A is the substrate body, the reference numeral 210A is the seed layer formed on the first surface 11A of the substrate body 10A, and 220A is the PLZT layer. Then, a resist 60A was spin-coated onto the surface of the PLZT layer 220A (Figure 3(b)). 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, development was performed to transfer the waveguide pattern 70A onto the resist 60A (Figure 3(c)). Then, the PLZT layer 220A was reactively ion-etched using a reactive ion etching system to fabricate a Mach-Zehnder interference waveguide 221A (Figure 3(d)). This etching was performed using a mixed gas of CHF3 and Ar, with a process pressure of 1.8 Pa and RF power of 150 W. The etching depth of the PLZT layer 220A was approximately 100 nm, and this was controlled by adjusting the etching time.

[0066] Next, an electrode 40A was fabricated to apply voltage to the waveguide 221A during phase modulation. To fabricate electrode 40A, a resist was first applied to the PLZT layer 220A of a plate, and then irradiated with an electron beam using an electron beam lithography system. After development, the electrode structure was transferred to the resist. Next, Cr (approximately 10 nm) and Al (approximately 400 nm) were deposited using a vacuum deposition system to complete electrode 40A (Figure 3(e)).

[0067] Furthermore, a cladding layer 50A was formed on the waveguide 221A using polymethyl methacrylate resin (PMMA) to complete the optical modulator 100A (Figure 3(f)). The electrode spacing was 5 μm and the electrode length was 4 mm.

[0068] For the fabrication of this optical modulator, Allresist GmbH's electron beam resist (RP6200) was used as the resist, Elionix Corporation's electron beam lithography system (G100) was used for electron beam lithography, and Samco Corporation's reactive ion etching system (RIE-10NR) was used for etching.

[0069] To induce the electro-optic effect of PLZT, an electric field orientation treatment was performed. A DC voltage was applied using an electrode 40A fabricated on a PLZT layer 220A on a hot plate at 90°C. The polarity of the electric field was ensured to be the same in both phase modulation sections. The power supply was adjusted to achieve a DC electric field strength of 56V / μm. After maintaining the voltage for 30 minutes, the temperature was lowered to room temperature, completing the electric field orientation treatment.

[0070] VπL was evaluated as a light modulation characteristic. (The product of half-wavelength voltage and electrode length VπL) Figure 4 shows an evaluation system 300 for evaluating the optical modulation characteristics of optical modulator 100A. In optical modulator 100A, the width of the waveguide 221A for phase modulation of the PLZT layer 220A is set to 1.4 μm. The Mach-Zehnder interference waveguide is equipped with multi-mode interference (MMI) circuits 80A and 80B on the input and output sides. At the input side MMI 80A, the light intensity is divided into 50% each and split into two phase modulation waveguides 221A, and the light is combined again at the output side MMI 80B.

[0071] A laser beam with a wavelength of 1550 nm was incident on the optical modulator 100A from a laser light source 310 via an optical fiber 320, and a modulation voltage (triangular wave: sine wave) was input to the electrodes by a function generator 330. One electrode was used as the 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 by an oscilloscope 360. The VπL of the optical modulator 100A was then measured using each sample. The polarization of the laser beam incident on the optical modulator 100A was set to TE mode.

[0072] (Dielectric loss tangent tanδ) Furthermore, to evaluate the dielectric loss tangent tanδ of PLZT films of each composition, PLZT films were deposited on a Si substrate with a thermal oxide film that had been sputtered with Pt (30 nm). Then, Pt sputtering was performed through a metal mask with a 1.1 mm diameter hole to create an upper Pt electrode on the PLZT film. The etching solution Pure Etch PT203 (manufactured by Hayashi Pure Chemical Industries, Ltd.) was applied to the areas covered by the metal mask where the upper Pt film was not deposited, exposing the lower Pt electrode. This formed a capacitor structure in which PLZT was sandwiched between the lower and upper Pt. For the PLZT film with this structure, the dielectric loss tangent tanδ was measured by applying a voltage signal with a frequency of 1 kHz and a maximum amplitude of 15 V using a TFanalyzer2000 ferroelectric test system (manufactured by Agxact Systems, Inc.).

[0073] For the evaluation of optical modulation characteristics, a VπL of 2.0V or less and a dielectric loss tangent tanδ of 0.05 or less were considered to have good optical modulation characteristics. The results are shown in Table 1.

[0074] [Table 1]

[0075] From the results in Table 1, it can be seen that for PLZT films where the concentration of each element (at%) is 40% or less for Pb + La and 2% to 5%, films No. 5 to 7 have a VπL of 2.0 V·cm or less and a dielectric loss tangent tanδ of 0.05 or less, indicating excellent optical modulation characteristics. In this case, for samples No. 5 to 7, when the composition ratio (at%) of Pb:La:Zr:Ti in the PLZT sol-gel solution is A:B:C:D, C+D=100, and A:B was (10⁶ to 10⁹):(3 to 6).

[0076] In contrast, samples No. 1 and No. 2 had large amounts of Pb+La, and the dielectric loss tangent tanδ was too large to measure. Samples No. 3 and No. 4 had VπL exceeding 2.0 V·cm. This was due to large amounts of Pb+La in No. 3 and large amounts of La in No. 4. Furthermore, VπL could not be measured for No. 8 because the PLZT sol-gel solution did not contain La. [Explanation of symbols]

[0077] 1. Thin-film coated substrate (multilayer coated substrate) 10,10A Main board 20 Multilayer film 210, 210A seed layer 220 PLZT membrane 220A PLZT membrane 231,231A waveguide 40,40A electrodes 50,50A cladding layer 100, 100A Optical Modulator 300 Evaluation System

Claims

1. A laminated film comprising a seed layer mainly composed of lanthanum and a ferroelectric PLZT film laminated on the seed layer, wherein when the concentrations (at%) of each element of the PLZT film measured by XPS analysis are [Pb], [La], [Zr], and [Ti] respectively, the surface layer of the PLZT film, The amount of Pb + La, defined as {([Pb] + [La]) / ([Pb] + [La] + [Zr] + [Ti])} (%), is between 35% and 40%. A laminated film characterized in that the amount of La, expressed as {[La] / ([Pb]+[La]+[Zr]+[Ti])} (%), is 2% or more and 5% or less.

2. The laminated film according to claim 1, characterized in that the dielectric loss tangent tanδ of the PLZT film is 0.05 or less.

3. A substrate with a laminated film, characterized by comprising a substrate body and a laminated film according to claim 1 provided directly on the substrate body or via an intermediate layer.

4. An electro-optical device comprising a substrate with a multilayer film as described in claim 3.

5. The electro-optical device according to claim 4, characterized in that when the laminated film is processed into a waveguide to form a modulator, the product VπL of the half-wavelength voltage and the electrode length is 2.0 V·cm or less.

6. The process comprises a seed layer formation step of forming a seed layer by applying a seed layer forming solution mainly composed of lanthanum onto a substrate and heating it, and a PLZT film formation step of forming a ferroelectric PLZT film by applying a PLZT sol-gel solution onto the seed layer and heat-treating it. A method for manufacturing a laminated substrate, characterized in that when the composition ratio of Pb:La:Zr:Ti in the PLZT sol-gel solution is A:B:C:D, C+D = 100 and A:B is (10⁶-10⁹):(3-6).

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