Laminated film, substrate with laminated film, and electro-optical device using film or substrate

Abstract

A laminated film comprising a seed layer having lanthanum as a main component and a ferroelectric PLZT film laminated on the seed layer, wherein, when the concentration (at%) of each element of the PLZT film measured by XPS analysis is denoted as [Pb], [La], [Zr], and [Ti], respectively, a ratio defined by \{([Pb] + [La]) / ([Zr] + [Ti])\} in a surface layer portion of the PLZT film is 1.00 to 1.40 inclusive, and a difference between a position of a peak top of a diffraction peak of PLZT (200) measured by X-ray diffraction (XRD) using CuKα rays with a scan axis of 2θ / θ and a peak position in a fitting result by a split pseudo-Voigt function for the peak is less than 0.10°.

Description

Multilayer films, substrates with multilayer films, and electro-optical devices using these.

[0001] This invention relates to a multilayer film, a substrate with a multilayer film, and an electro-optical device using these.

[0002] This application claims priority based on Japanese Patent Application No. 2025-053006, filed in Japan on March 27, 2025, and the contents of that application are incorporated herein by reference.

[0003] Non-patent document 1 describes how PLZT((Pb, La)(Zr, Ti)O) is formed on a single crystal substrate or a seed layer formed on a Si (silicon) substrate using the sol-gel method. 3 A technique has been disclosed in which a thin film (lanthanum-doped lead zirconate titanate) is formed, the thin film is processed into a waveguide, and electrodes are added to it to use it as an optical modulator.

[0004] Furthermore, Patent Documents 1 and 2 disclose a technique for forming a PZT (lead zirconate titanate) thin film as an oriented film on a Si substrate by applying an oxygen-containing compound containing La as a buffer layer (seed layer), and using it as an optical modulator or ferroelectric memory. In addition, Patent Document 3 also discloses a technique for forming a PZT thin film as an oriented film on a Si substrate by applying an oxygen-containing compound containing La as a buffer layer.

[0005] In response to the miniaturization and integration of today's optical communication devices, optical modulators are required to exhibit sufficient optical modulation effects even over short distances. A performance indicator that represents this is the half-wavelength voltage (V). π ) and the product V of electrode length (L) π There is a value L (the product of the voltage required to change the optical phase by π and the electrode length; a lower value indicates that a smaller, low-voltage driven modulator can be constructed). This V π L is lithium niobate (LiNbO ３ Current thin-film LN modulators using bonded substrates (hereinafter abbreviated as LN) have been reported to have a voltage of as low as 2.0 V·cm (Non-Patent Document 2).

[0006] S. Abe et al., Photonic integration based on a ferroelectric thin-film platform, Sci. Reports 9 (2019) 16548. J. Mao et al., Heterogeneous silicon-on-lithium niobate electro-optic modulator for 100-Gbaud modulation, APL Photonics 7 (2022) 126103.

[0007] International Publication No. 2014 / 083195, International Publication No. 2004 / 079059, Korean Registered Patent No. 10-0610150

[0008] Further, for example, in a modulator using the PLZT thin film described above, low V π L is achieved, and there is a technique of using a PLZT thin film in an electro-optical device such as an optical modulator. In such a device, in order to orient the dielectric polarization of PLZT, an electric field is applied from the electrode portion to PLZT in advance, and an electric field poling treatment is performed.

[0009] However, in such an electro-optical device, as time elapses after the electric field poling treatment, V π L increases over time, leading to the problem that modulator characteristics are unstable. This problem can be avoided by applying a DC voltage (bias voltage) as a voltage for maintaining electric field orientation during modulator operation, but this brings about the problems that power consumption increases and the process of modulator operation becomes complicated.

[0010] In view of these problems, the present invention achieves low V π L by processing a PLZT thin film into an optical modulation element, enables small size and low-voltage driving, and achieves V π L with suppressed increase over time, eliminating the need for DC voltage application during modulator operation, and an object of the present invention is to configure an electro-optical device with stable characteristics.

[0011] 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 measured by XPS analysis are [Pb], [La], [Zr], and [Ti] respectively, the ratio defined as {([Pb] + [La]) / ([Zr] + [Ti])} in the surface layer of the PLZT film is 1.00 or more and 1.40 or less, and the difference between the peak top position of the diffraction peak of PLZT(200) measured by X-ray diffraction (XRD) using CuKα rays with a scan axis of 2θ / θ and the peak position in the fitting result of the divided pseudo-Voigt function for the said peak is less than 0.10°.

[0012] The ratio of {([Pb] + [La]) / ([Zr] + [Ti])} is between 1.00 and 1.40, so that (Pb + La) occupies the A site without deficiency in the perovskite crystal structure of PLZT, forming a crystal suitable for the expression of the electro-optic effect. As a result, when the multilayer film is processed into a waveguide and a Mach-Zehnder modulator is constructed, V π A multilayer film can be provided that allows L to be reduced to 1.8 V·cm or less, enabling the construction of an optical modulator capable of short-distance, low-voltage operation.

[0013] When the difference between the peak top position of the diffraction peak of PLZT(200) and the peak position in the fitting result using the divided pseudo-Voigt function for that peak is less than 0.10°, the symmetry of the diffraction peak by X-ray diffraction (XRD) is high, and a PLZT film is formed that exhibits good dielectric properties with a uniform composition distribution within the PLZT film. In particular, by maintaining high residual polarization, even after time has elapsed since the completion of the polishing process, V π Large increases in L become less likely.

[0014] Furthermore, the concentration of each element in the surface layer of the PLZT film can be measured by X-ray photoelectron spectroscopy (XPS).

[0015] 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.

[0016] 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. Al can be used as the intermediate layer. ２ O ３ , ZrO ２ , TiO ２ A metal oxide film layer consisting of the above can be used.

[0017] The electro-optical device of the present invention comprises the aforementioned multilayer film-coated substrate.

[0018] Then, when the laminated film is processed into a waveguide to form a Mach-Zehnder modulator, the product V of the half-wavelength voltage and the electrode length is π L is 1.8 V·cm or less.

[0019] Electro-optical devices include, for example, optical modulators, optical switches, and phase shifters. π Since L is 1.8 V·cm or less, a compact and highly efficient electro-optical device can be provided.

[0020] The aforementioned V π It is even preferable that L remains below 1.8 V·cm even after 24 hours have elapsed since electric field orientation (polling). Thus, the electro-optic device of the present invention has a low V even after 24 hours have elapsed since electric field orientation. π It is possible to maintain L.

[0021] In the electro-optical device of the present invention, the contact portion of the electrode with the PLZT film is preferably made of one of the elements Cr, Pt, Au, or Cu, or an alloy containing these elements.

[0022] According to the present invention, by processing a PLZT thin film into an optical modulation element, a low V π This allows us to achieve L and construct a small, low-voltage driven electro-optic device. Moreover, even after time has elapsed, V π Because a large increase in L is unlikely to occur, it is not necessary to apply a DC voltage to maintain the orientation of dielectric polarization during modulator operation, and an electro-optic device with stable characteristics can be constructed.

[0023] This is a cross-sectional view of a laminated substrate according to an embodiment of the present invention. This is a cross-sectional view of an optical modulator using the laminated substrate of Figure 1. This is a diagram illustrating the manufacturing method of the optical modulator of the embodiment. This is a diagram showing an evaluation system for evaluating the characteristics of the optical modulator of the embodiment.

[0024] Embodiments of the present invention will be described below with reference to the drawings.

[0025] The laminated substrate (hereinafter referred to as the thin-film substrate) 1 shown in Figure 1 comprises 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 can be, for example, a Si substrate without an oxide film, SiO ２ Ya Al ２ O ３ Such as oxide-coated Si substrates, glass substrates, sapphire substrates, etc., can be used. The size of the substrate body 10 is not limited, 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 one side of 10 mm or more and 300 mm or less, or as a circular wafer with a diameter of 50 mmφ or more and 350 mmφ or less.

[0027] The laminated film 20 comprises a seed layer 210 formed on the substrate body 10 and a ferroelectric PLZT film 220 formed on the seed layer 210.

[0028] The seed layer 210 consists of an oxygen-containing compound mainly composed of La. The oxygen-containing compound that makes up the seed layer 210 is, for example, La(NO). ３ ) ３ LaONO ３ La ２ O ２ CO ３ La ２ O ３ These include oxynitrates, oxycarbonates, oxynitrides, and oxides. Thus, the seed layer 210 is preferably composed of an oxygen-containing compound containing La, and the main component means the metal element component with the highest content ratio among the components constituting the seed layer 210.

[0029] 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, when a ferroelectric layer is formed on the seed layer 210 to fabricate a photoelectric device, there is a concern that light confinement in the ferroelectric layer will be insufficient, and efficient photoelectric conversion will not be possible.

[0030] PLZT film 220 is PLZT (Lanthanum-doped lead zirconate titanate: [(Pb,La)(Zr,Ti)O ３ It consists of ) and has a perovskite crystal structure. The thickness of this PLZT film 220 is not limited, but for example it is 100 nm to 900 nm, and preferably 250 nm to 700 nm.

[0031] In this PLZT film 220, Pb and La occupy the A sites of the perovskite crystal structure, and Zr and Ti occupy the B sites.

[0032] Here, when the concentration (at%) of each element is [Pb], [La], [Zr], and [Ti] respectively, the ratio defined as {([Pb] + [La]) / ([Zr] + [Ti])} in the surface layer of the PLZT film is between 1.00 and 1.40, and the difference between the peak top position of the PLZT(200) diffraction peak measured by X-ray diffraction (XRD) using CuKα rays with a scan axis of 2θ / θ and the peak position in the fitting result of the Split pseudo-Voigt function for that peak is less than 0.10°. PLZT(200) indicates diffraction at the (200) plane of PLZT.

[0033] The ratio of {([Pb] + [La]) / ([Zr] + [Ti])} being between 1.00 and 1.40 allows (Pb + La) to occupy the A site without deficiency in the perovskite crystal structure of PLZT, thus forming a crystal suitable for the expression of the electro-optic effect. As a result, when the multilayer film is processed into a waveguide to construct a Mach-Zehnder modulator, the product of the half-wavelength voltage and the electrode length V πA multilayer film can be provided that allows L to be 1.8 V·cm or less, enabling the construction of an optical modulator capable of short-distance, low-voltage operation. π The lower limit of L is not particularly limited, but it is 0.1 V·cm.

[0034] As will be described later, since Pb volatilizes during calcination when forming the PLZT film, the composition ratio (at%) 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]. However, in the PLZT film, the sum of [Pb] and [La] is set to be equal to the sum of [Zr] and [Ti], or 1.40 times or less the sum of [Zr] and [Ti].

[0035] If the ratio of {([Pb] + [La]) / ([Zr] + [Ti])} is less than 1.00, the amount of [Pb] + [La] constituting the A site of PLZT is small, and the amount of [Pb] + [La] (especially the amount of Pb) necessary to form the perovskite phase with electro-optic effect is insufficient, resulting in the formation of a bilochlore phase, which deteriorates the dielectric properties over time. π L increases significantly, and V over time π The stability of L decreases.

[0036] When the ratio of {([Pb] + [La]) / ([Zr] + [Ti])} exceeds 1.40, a large amount of [Pb] + [La] is generated, occupying sites other than the A site of the perovskite structure PLZT. This negatively affects the dielectric properties, and immediately after the completion of the polishing process of the optical modulation element made of PLZT film, V π Even if L can be reduced to 1.8 V·cm or less, over time V π The increase in L is large, resulting in a lack of stability over time. π It is preferable that L be 1.2 V·cm or less immediately after the completion of the Poling process, and more preferably 1.0 V·cm or less.

[0037] Therefore, the ratio of {([Pb] + [La]) / ([Zr] + [Ti])} is set to 1.00 or more and 1.40 or less, and it is preferable that this ratio be between 1.05 and 1.35.

[0038] The average composition (at%) of the entire PLZT film is defined as the ratio {([Pb] + [La]) / ([Zr] + [Ti])}, which is between 1.00 and 1.40, and more preferably between 1.05 and 1.35. However, when measuring the composition by X-ray photoelectron spectroscopy, the measurement is performed in a range of approximately 5 nm depth from the surface of the PLZT film; therefore, the ratio was defined based on the composition of the surface layer of the PLZT film. In other words, the surface layer is the layer at a depth of 5 nm from the surface.

[0039] Furthermore, because the difference between the peak top position of the diffraction peak of PLZT(200) and the peak position in the fitting result using the divided pseudo-Voigt function for that peak is less than 0.10°, the symmetry of the XRD diffraction peak is high, and a PLZT film is formed that exhibits good dielectric properties with a uniform composition distribution within the PLZT film. In particular, by maintaining high residual polarization, even after time has elapsed since the completion of the polishing process, V π Large increases in L become less likely.

[0040] The peak top position refers to the 2theta point where the intensity is maximum within the range of 43° to 46° where the diffraction peak of PLZT(200) appears, in the actual measurement data (intensity (counts) vs. 2theta or intensity (cps) vs. 2theta) without fitting. On the other hand, the peak position in the fitting result refers to the 2theta point displayed as the diffraction peak fitting result after fitting has been performed using the analysis software (PDXL).

[0041] If the difference between the peak top position of the diffraction peak of PLZT(200) and the peak position in the fitting result is 0.10° or more, even if the amount of [Pb] + [La] is appropriate, the asymmetry of the XRD peaks is strong and they are not uniformly distributed within the film, or other phases such as pyrochlore phases are formed, which worsens the dielectric properties, and V increases with time elapsed since the completion of the polishing process. π L increases significantly, and V over time πThe stability of L decreases. It is preferable that this difference be 0.08° or less. Furthermore, the lower limit of this difference is not particularly limited, but it is 0.01° or more.

[0042] This difference is the absolute value of the result calculated as (peak top position of diffraction peak) - (peak position in the fitting result) (which can be either positive or negative), with a sampling step of 0.01°, and the difference is rounded to two decimal places.

[0043] 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 220.

[0044] 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.

[0045] In this embodiment, the [Pb] content of the PLZT film 220 is preferably 48 at% to 55 at%, and the [La] content is preferably 1.8 at% to 4.5 at%. Furthermore, the [Zr] + [Ti] content of the PLZT film 220 is preferably 42 at% to 50 at%.

[0046] Figure 2 is a cross-sectional view showing an example of an optical modulator 100 as an electro-optical device 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.

[0047] This optical modulator 100 exhibits favorable electro-optical effects because its core layer is formed of a highly oriented PLZT film 220. However, the optical modulator using the thin-film substrate 1 of the present invention is not limited to a Mach-Zehnder modulator.

[0048] In this embodiment, the electrode can be a single layer or a multilayer structure, but it is desirable that the contact portion with the PLZT film 220 be made of one of the metals Cr, Pt, Au, or Cu, or an alloy containing at least one of these metals. The contact portion with the PLZT may be made of these metals, and another metal film may be deposited on top of it. When the electrode has a multilayer structure, the surface (the side opposite to the PLZT film) can be made of Al or the like.

[0049] (Method for manufacturing the thin-film substrate 1) The method for manufacturing the thin-film 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.

[0050] [First film formation process: seed layer 210] The first film formation process comprises a first coating process in which the surface of the substrate body 10 is coated with polyvinylpyrrolidone (hereinafter referred to as PVP), a second coating process in which the PVP is coated with lanthanum nitrate, a pre-calcination process in which the lanthanum nitrate on the substrate is pre-calcined, and a calcination process in which the lanthanum nitrate on the substrate is calcined.

[0051] 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).

[0052] 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. The solvent is, for example, 2-methoxyethanol, 1-propanol, methanol, or ethanol. The PVP solution is dropped onto the substrate and spin-coated (PVP solution coating step). Alternatively, dip coating or the like may be performed 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 and has the effect of uniformly forming a lanthanum nitrate film.

[0053] 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).

[0054] 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.

[0055] The concentration of this lanthanum nitrate (excluding hydrate) is 0.8% by mass or more and 5.5% by mass or less. If the lanthanum nitrate concentration is less than 0.8% by mass or more than 5.5% by mass, it is difficult to form a uniform ferroelectric multilayer film. The concentration of this lanthanum nitrate solution is preferably 1% by mass or more and 5% by mass or less, and more preferably 1.2% by mass or more and 3.5% by mass or less.

[0056] 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. Additionally, the addition of water improves wettability to the substrate, allowing for the uniform formation of the seed layer and reducing the likelihood of appearance defects.

[0057] 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.

[0058] 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, for example, 150°C to 250°C to evaporate the solvent (solvent evaporation step).

[0059] The calcination process involves heating the substrate to a temperature between 250°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 to cool in the air.

[0060] 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, the heating time from room temperature to firing temperature t １ The firing time 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. Furthermore, it is preferable that the atmosphere during firing be air.

[0061] By adjusting this heating time t1, the lanthanum nitrate after firing is brought into a phase state suitable for the formation of the PLZT film 220 (La ２ O ２ CO ３ ) is controlled. If the heating time t1 is less than 200 seconds, the La of lanthanum nitrate２ O ２ CO ３ The phase transition to becomes insufficient, making it difficult to form the PLZT film 220 on the seed layer 210 with high orientation. When the heating time t1 exceeds 1000 seconds, La ２ O ２ CO ３ There is a concern that other unrelated phases may be mixed in. Also, the heating rate v1 from room temperature to firing temperature should be 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, La ２ O ２ CO ３ There is a concern that other unrelated phases may be mixed in, and if the heating rate v1 exceeds 10°C / second, La ２ O ２ CO ３ There is a risk that the phase transition to [the other state] may be insufficient.

[0062] 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.

[0063] [Second film formation process: PLZT film 220] The second film formation process comprises 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 pre-calcination step of pre-calcining the thin film forming material, and a calcination step of calcining the thin film forming material.

[0064] The thin-film formation solution coating process involves dropping the thin-film formation solution onto a substrate that already has a seed layer 210 formed on it, and then performing spin coating.

[0065] This thin-film forming solution is a compound containing Pb, La, Zr, and Ti. The composition ratio (at%) of Pb:La:Zr:Ti in the solution is, for example, C+D=100 and A:B is (106-113):(3-6), where A:B:C:D.

[0066] Of these, Pb is increased because it partially volatilizes during subsequent calcination. By controlling this increase in Pb and preparing the thin-film forming solution, the film composition, the ratio of {([Pb] + [La]) / ([Zr] + [Ti])}, and the difference between the peak top position of the diffraction peak of PLZT(200) and the peak position in the fitting result can be appropriately controlled.

[0067] 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.

[0068] The pre-sintering process involves heating the substrate to 300°C to 450°C to pre-sinter the thin-film forming material. After heating to the pre-sintering temperature, the substrate is placed on a heating section with a temperature lower than the pre-sintering temperature to gradually lower the temperature, and then removed from the heating section to cool in the air.

[0069] 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 from room temperature to the firing temperature is t. ２ The heating time 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 atmosphere during firing is preferably an oxygen atmosphere. The thickness of the PLZT film 220 can be increased by repeating the process from applying the thin film forming solution to firing the thin film forming material. For example, a PLZT film with a thickness of 100 nm to 150 nm can be formed in one coating and firing, and by repeating this multiple times, a thickness of about 500 nm can be achieved. In this case, it is thought that the crystal structure becomes more stable when the film obtained in the initial coating and firing is fired multiple times.

[0070] After the above firing process, the thin-film coated substrate 1 is completed.

[0071] In the thin-film substrate 1 of this embodiment, the PLZT film 220 is formed in an oriented manner because the phase state of the seed layer 210 is controlled by heat treatment during manufacturing. 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.

[0072] 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.

[0073] 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.

[0074] 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-coated substrate 1 can also exhibit uniform characteristics.

[0075] As mentioned above, the laminated film 20 formed on this thin-film substrate 1 has a ratio of {([Pb] + [La]) / ([Zr] + [Ti])} of 1.00 to 1.40, so that (Pb + La) occupies the A site without deficiency in the perovskite crystal structure PLZT, forming a crystal suitable for the expression of the electro-optic effect. As a result, when the laminated film is processed into a waveguide and a Mach-Zehnder modulator is constructed, V π L can be reduced to 1.8V·cm or less, enabling short-distance, low-voltage operation and allowing for the construction of a compact, highly efficient optical modulator.

[0076] When the difference between the peak top position of the diffraction peak of PLZT(200) and the peak position in the fitting result using the divided pseudo-Voigt function for that peak is less than 0.10°, the symmetry of the XRD diffraction peak is high, and a PLZT film is formed that exhibits good dielectric properties with a uniform composition distribution within the PLZT film. In particular, by maintaining high residual polarization, even after time has passed since the polishing treatment, Vπ A large increase in L becomes less likely. According to this embodiment, after 24 hours have elapsed since the PLZT film was polished, V π L can maintain a low value. V after 24 hours following this polling process. π L is preferably 1.8 V·cm or less, and more preferably 1.5 V·cm or less.

[0077] Also, this V π It is preferable that L remains stable at a low value even after 24 hours or more have elapsed since the Polling process, and V after 72 hours since the Polling process. π L is preferably 1.8 V·cm or less, and more preferably 1.5 V·cm or less. Furthermore, V after 500 hours from the polishing treatment. π L is preferably 2.0 V·cm or less, and more preferably 1.6 V·cm or less.

[0078] The present invention can be implemented in any way not limited to the above description and illustrated examples.

[0079] For example, on the surface of the substrate body 10, Al ２ O ３ , ZrO ２ , TiO ２ An intermediate layer composed of a metal oxide layer made of the following materials may be formed. In this case, the thickness of these layers is not limited, but for example, the substrate body is 300 μm or more and 2000 μm or less, and the intermediate layer is 10 nm or more and 500 nm or less.

[0080] 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.

[0081] 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 of the optical modulator fabricated using the sample were confirmed.

[0082] 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.

[0083] [Cleaning Process] The cleaning process consisted of the following three steps in order: first cleaning to third cleaning. In the first cleaning, the Si substrate was immersed in acetone and ultrasonic cleaning was performed for 2 minutes. In the second cleaning, the Si substrate was immersed in pure water and ultrasonic cleaning was performed for 2 minutes. In the third cleaning, the solution was heated to 75 degrees Celsius, and then the Si substrate was immersed for 20 minutes (RCA SC1 cleaning). The solution consisted of pure water, hydrogen peroxide (35% by mass), and ammonia water (29% by mass), with a volume ratio of pure water:hydrogen peroxide (35% by mass):ammonia water (29% by mass) = 3:1:1. After the second cleaning and before the third cleaning, the Si substrate may be heat-treated at a temperature of 500°C to 800°C.

[0084] [First film formation process: seed layer] The first film formation process consisted of the following steps: a first coating process in which the surface of the substrate was coated with polyvinylpyrrolidone (hereinafter referred to as PVP), a second coating process in which the PVP was coated with lanthanum nitrate, a pre-calcination process in which the lanthanum nitrate on the substrate was pre-calcined, and a calcination process in which the lanthanum nitrate on the substrate was calcined.

[0085] In the first coating step, the PVP solution was applied to the substrate, and then the solvent was heated to evaporate it.

[0086] 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% by mass), and 2.0 g of pure water, 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 low molecular weight PVP allows for easy dissolution in the solvent, enabling uniform coating without appearance defects during film formation.

[0087] The PVP solution was stirred before dropping 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.

[0088] After spin coating, as a step to evaporate the solvent, the Si substrate coated with PVP solution was placed on a hot plate heated to 150°C for 1 minute.

[0089] In the second coating step, a lanthanum nitrate solution was applied to the PVP, and then the solvent was evaporated by heating.

[0090] The lanthanum nitrate solution was prepared by weighing 0.467 g of lanthanum nitrate hexahydrate (purity > 99.0% by mass) 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.

[0091] 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.

[0092] The calcination process involved placing the substrate on a hot plate heated to 250°C for 3 minutes, then on a hot plate heated to 320°C for 5 minutes, and then, in order to gradually cool the Si substrate, placing it on a hot plate heated to 150°C for 30 seconds, before removing it from the hot plate and allowing it to cool in the air.

[0093] In the firing process, the substrate was fired in a firing apparatus at a firing temperature of 590°C for a holding time of 60 seconds, using air as the firing atmosphere. For the firing process of each sample, the heating time from room temperature to the firing temperature of 590°C was t １The firing time 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 step of applying PVP to the second coating step and then to the firing step, and for each sample, the number of layers that would serve as the base for the PLZT film was set to two layers (thickness 11 nm).

[0094] [Second Film Formation Process: PLZT Film] The second film formation process comprises the steps of applying the PLZT sol-gel solution onto the seed layer, volatilizing the solvent, calcining the PLZT, and firing the PLZT. The PLZT sol-gel solution used was 15% by mass PLZT E1 solution manufactured by Mitsubishi Materials Corporation, with compositions differing in Pb and La content as shown in Table 1.

[0095] 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.

[0096] The pre-firing process involved placing the substrate on a hot plate heated to 320°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.

[0097] In the firing process, the substrate is fired in the aforementioned RTA apparatus at a firing temperature of 640°C, a temperature holding time of 60 seconds, and a firing atmosphere of O ２ The samples were subjected to a firing process. For each sample, the heating time from room temperature to the firing temperature of 640°C was t. ２ The time was set to 640 seconds. PLZT was crystallized by the firing process.

[0098] The PLZT film was formed by repeating the process from coating the PLZT sol-gel solution to firing three or six times, adjusting the film thickness to approximately 300 nm or approximately 600 nm.

[0099] In this case, ellipsometry was performed to calculate the film thickness of the PLZT film. A J. A. Woolam M-2000 was used for the measurement, and measurements were taken in reflection mode at three incident / reflection angles: 60°, 70°, and 80°. The obtained results were fitted with Cauchy's optical model, and the film thickness was measured.

[0100] [Evaluation of the film] The composition (at%) of the PLZT film deposited on the substrate was analyzed, and an optical modulator was formed, and the V at that time was measured. π L was evaluated. (Compositional analysis) The compositional analysis of the PLZT film was performed 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.

[0101] (X-ray structural analysis) The orientation of the deposited PLZT film and the crystalline phase of the buffer layer were analyzed using a fully automated multi-purpose X-ray diffractometer (Rigaku, SmartLab). The scan axis was set to 2θ / θ, and measurements were performed in the angular range of 10° to 60° with a sampling step of 0.01°, a scan speed of 1° / min, IS = 0.300 mm, RS1 = 5 mm, and RS2 = 5 mm. For the measurements, a slit was set up with an incident solar slit of 5°, an IS longitudinal of 10 mm, a PSA of 0.5°, and a measurement solar slit of 5°. A scintillation counter (Rigaku, SC-70) was used as the detector. CuKα rays were used as the X-ray source. The obtained XRD data was analyzed using Rigaku Corporation's integrated powder X-ray analysis software PDXL2 Version 2.8.4.0, and peak search and fitting were performed. In the XRD intensity evaluation, the shape and position of the PLZT(200) peak appearing between 43.00° and 46.00° were assessed.

[0102] Peak fitting was performed automatically with "Refine background" enabled. The peak shape was set to a segmented pseudo-Voigt function. Kβ lines and filter edges were not removed, and the alpha cut value for peak search was set to 3.00, with an alpha cut range of 0.50 to 20.00. Amorphous peaks were excluded from detection. Peak optimization was performed only once after opening the measurement file.

[0103] In the XRD analysis results, the peak top position refers to the 2theta value where the intensity is maximum within the range of 2theta = 43° to 46° where the PLZT(200) peak appears, in the actual measurement data (relationship between intensity (counts) vs. 2theta or intensity (cps) vs. 2theta) without fitting. On the other hand, the peak position in the fitting results refers to the 2theta value displayed as the fitting result of the aforementioned peak after fitting has been performed using the analysis software (PDXL).

[0104] (Optical Modulation Characteristics) Waveguide patterns and electrodes were fabricated using electron beam lithography and etching processes.

[0105] A 20 mm x 20 mm plate was cut from a sample in which a PLZT film had been deposited on a substrate (Figure 3A), and this plate was washed with acetone. In Figure 3A, reference numeral 10A indicates the substrate body, reference numeral 210A indicates the seed layer formed on the first surface 11A of the substrate body 10A, and reference numeral 220A indicates the PLZT film. Then, a resist 60A was spin-coated onto the surface of the PLZT film 220A (Figure 3B). 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 to the resist 60A (Figure 3C). Then, the PLZT film 220A was reactively ion-etched using a reactive ion etching system to fabricate a Mach-Zehnder interference type waveguide 221A (Figure 3D). This etching was performed using CHF ３ A mixed gas of ions and Ar was used, the process pressure was 1.8 Pa, and the process was carried out with 150 W of RF power. The etching depth of the PLZT film 220A was approximately 100 nm, and this was controlled by adjusting the etching time.

[0106] Next, electrodes 40A were fabricated to apply voltage to the waveguide 221A during phase modulation. To fabricate electrodes 40A, a resist was first applied to a plate-shaped PLZT film 220A, and then irradiated with an electron beam using an electron beam lithography system. After development, the electrode structure was transferred to the resist. Next, metal electrodes were deposited using a vacuum deposition system or a sputtering system to complete electrodes 40A (Figure 3E). Four types of electrodes were deposited: a laminated structure of Cr (10 nm) and Al (400 nm) as an adhesion layer using the deposition method, a single-layer structure of Au (200 nm) without an adhesion layer, a single-layer structure of Pt (200 nm) without an adhesion layer using the sputtering method, and a single-layer structure of Cu (200 nm) without an adhesion layer. In the Cr and Al laminated electrode structure, the Cr layer was formed in contact with the PLZT film as an adhesion layer. Since Au, Pt, and Cu do not have an adhesion layer, these single layers are formed in contact with the PLZT film.

[0107] Furthermore, a cladding layer 50A was formed on the waveguide 221A using polymethyl methacrylate resin (PMMA) to complete the optical modulator 100A (Figure 3F). The electrode spacing was 5 μm (when the PLZT film thickness was approximately 300 nm) or 7 μm (when the PLZT film thickness was approximately 600 nm), and the electrode length was 3 mm.

[0108] For the fabrication of this optical modulator, Allresist GmbH's electron beam resist (ARP6200) 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.

[0109] To induce the electro-optic effect of PLZT, an electric field orientation (poling) treatment was performed. First, to partially open the cladding layer 50A above electrode 40A, the electrode was exposed by locally irradiating it with an electron beam using an electron beam lithography system and performing a development treatment. Then, a DC voltage was applied using electrode 40A, which was fabricated on the PLZT film 220A, on a 90°C hot plate. At this time, the polarity of the electric field was made the same in the two phase modulation sections. The power supply was adjusted so that the DC electric field strength was 56 V / μm. After holding the voltage for 90 minutes, the temperature was lowered to room temperature, and the electric field orientation treatment was completed.

[0110] As for optical modulation characteristics, V π L was evaluated. (V is the product of half-wavelength voltage and electrode length) π L) Figure 4 shows an evaluation system 300 for evaluating the optical modulation characteristics of optical modulator 100A. The optical modulator 100A has a waveguide 221A for phase modulation of the PLZT film 220A with a width of 1.4 μm. The Mach-Zehnder interference waveguide is equipped with multimode circuits (Multi-Mode Interference: hereinafter referred to as MMI) 80A and 80B on the input and output sides. The light is divided into two phase modulation waveguides 221A at the input side MMI 80A, with the light intensity divided into 50% each, and the light is combined again at the output side MMI 80B.

[0111] Laser light with a wavelength of 1550 nm was incident on the optical modulator 100A from the laser light source 310 via the optical fiber 320, and the modulation voltage (triangular wave: sine wave) was input to electrode 40A by the function generator 330. One electrode 40A (the electrode located outside the waveguide 221A in Figure 4) was used as the ground electrode. The modulated output light was sent to the photodetector 350 via the optical fiber 340, and the signal from the photodetector 350 was further analyzed by the oscilloscope 360. The V of the optical modulator 100A using each sample was then analyzed. π L (voltage when the phase of the light is shifted by 90° (π / 2)) was measured. The polarization of the laser light incident on the optical modulator 100A was set to TE mode.

[0112] V over time π In measuring the change in L, the fabricated element is left undisturbed in an atmospheric environment at 25°C and 45% humidity without applying an electrical signal. After a certain period of time, the probe is brought back into contact with the element and an electrical signal is applied to measure the change in V. π L was measured. Considering that it is possible to perform the operation of reapplying voltage (poling) to realign the orientation of the dielectric polarization, V was measured 24 hours (denoted as h) after the completion of the polishing process so that this operation is practically feasible. π The change in L was evaluated. V after 24 hours π For samples with small L changes, V was measured 72 hours and 500 hours after the completion of the Poling treatment. π We also calculated the change in L.

[0113] These results are shown separately in Table 1 and Table 2. Table 1 shows the relationship between the charged composition ratio (at%) of the PLTZ sol-gel solution, the film thickness and composition ratio (at%) of the PLZT film, and the ratio of {([Pb]+[La]) / ([Zr]+[Ti])}, and Table 2 shows the peak position of XRD (200), V π L and evaluation results. The XRD (200) peak position is the diffraction peak position of the (200) plane of PLZT.

[0114]

[0115]

[0116] From the results in Table 1 and Table 2, it can be seen that for Sample No. in which the ratio of {([Pb]+[La]) / ([Zr]+[Ti])} is 1.00 or more and 1.40 or less, and the difference between the measurement result and the fitting result for the peak top position of the diffraction peak of PLZT (200) measured by X-ray diffraction (XRD) is less than 0.10°. 2, 3, 5 to 9, not only the V π L immediately after poling is 1.8 V·cm or less, but also the V π L is still 1.8 V·cm or less even 24 hours after the completion of poling, and the change in V π L is small even with the passage of time. In addition, for these Sample Nos. 2, 3, 5 to 9, the V π L is still 1.8 V·cm or less even 72 hours after the completion of poling, and the V π L is 2.0 V·cm or less even after 500 hours have passed, and remains stable at a low numerical value.

[0117] Specifically, in Sample Nos. 2 and 3, the ratio of {([Pb]+[La]) / ([Zr]+[Ti])} is appropriate, and the difference from the fitting result of the XRD peak is small (the asymmetry of the peak is small), so that composition gradient in the film and formation of heterogeneous phases are suppressed. Thereby, the film has good dielectric properties, and the change in V π L is small even with the passage of time.

[0118] Samples Nos. 5 and 6 are prepared by increasing the number of spin coating times of the PLZT sol-gel solution and the number of heat treatments so that the film thickness under the conditions of Samples Nos. 3 and 4 is 550 nm or more. There is no significant change in the ratio of {([Pb]+[La]) / ([Zr]+[Ti])}, but multiple heat treatments cause a change in the crystal state, which reduces the difference from the fitting result of the XRD peak. Accordingly, at low film thicknesses, V π It was found that even for the composition of No. 4, which had low L stability, the stability was improved by increasing the film thickness (No. 6).

[0119] In contrast, Sample No. 1 had an excessively high Pb content in the sol-gel solution, so the ratio of {([Pb]+[La]) / ([Zr]+[Ti])} was higher than 1.40, resulting in Pb+La occupying sites other than the A-site of the perovskite-structured PLZT film. This has an adverse effect on dielectric properties, and over time V π L increased.

[0120] For Sample No. 4, although the amount of (Pb]+[La]) is appropriate, the asymmetry of the XRD peak is strong, indicating that these components are not uniformly distributed in the film, or there is formation of a heterogeneous phase such as a pyrochlore phase. This degrades the dielectric properties, leading to temporal V π L with low stability.

[0121] Sample No. 2 has an electrode with a laminated structure of Al / Cr, which is laminated in a state where Cr is in contact with the PLZT film. Samples Nos. 7 to 9 are obtained by changing the electrode type to a single layer of Pt, Au, or Cu under the same film formation conditions as Sample No. 2. Even under the same PLZT film formation conditions, the initial V π L values fluctuate, but under all conditions, the V π L 24 hours after the completion of poling was 1.8 V·cm or less, showing high stability similar to Sample No. 2.

[0122] The present invention achieves low V π The PLZT film realizing low V L can be suitably used for electro-optic devices such as optical modulators, for example.

[0123] 1. Thin-film coated substrate (multilayer coated substrate) 10, 10A Substrate body 20 Multilayer film 210, 210A Seed layer 220, 220A PLZT film 231, 231A Waveguide 40, 40A Electrode 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 ratio defined as {([Pb] + [La]) / ([Zr] + [Ti])} in the surface layer of the PLZT film is 1.00 or more and 1.40 or less, and the difference between the position of the peak top of the diffraction peak of PLZT(200) measured by X-ray diffraction (XRD) using CuKα rays with a scan axis of 2θ / θ and the peak position in the fitting result of the divided pseudo-Voigt function for said peak is less than 0.10°.

2. 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.

3. An electro-optical device comprising a substrate with a multilayer film as described in claim 2.

4. When the laminated film is processed into a waveguide to constitute a Mach-Zehnder modulator, the product V of the half-wavelength voltage and the electrode length π The electro-optical device according to claim 3, characterized in that L is 1.8 V·cm or less.

5. The product V between the half-wavelength voltage and the electrode length. π The electro-optic device according to claim 4, characterized in that L remains 1.8 V·cm or less even after 24 hours have elapsed since electric field orientation.

6. An electro-optical device according to claim 3, characterized in that the contact portion of the electrode with the PLZT film is made of one of the metals Cr, Pt, Au, or Cu, or an alloy containing at least one of the aforementioned metal elements.

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