Structure, waveguide, optical modulator, and method for producing structure

Abstract

The purpose of the present invention is to form a BTO thin film or a BSTO thin film on a substrate that is composed of an amorphous material, the BTO thin film or BSTO thin film exhibiting an excellent electro-optic effect. The present invention relates to a structure which has a substrate that is composed of an amorphous material, a seed layer that is superposed on the substrate, an intermediate layer that is superposed on the seed layer, and a BTO thin film or a BSTO thin film that is superposed on the intermediate layer, wherein the intermediate layer contains, as a main component, a titanium oxide that contains at least one element selected from the group consisting of Pb, Zr, La, Ba, Sr, K, Li, Nb, B, Mg, As, P, and Ge.

Description

Structure, waveguide, optical modulator, and method for manufacturing structure

[0001] The present invention relates to a structure, a waveguide, an optical modulator, and a method for manufacturing a structure.

[0002] PZT (lead zirconate titanate), PLZT (lead lanthanum zirconate titanate), BTO (barium titanate), and BSTO (barium strontium titanate) are piezoelectric ceramics with a wide range of applications as ferroelectric crystals. PZT, PLZT, BTO, and BSTO are known to have a large electro-optic effect due to their high electric polarization. Because they can switch the phase of light by turning the applied voltage on and off, they can be used as optical control elements such as optical shutters and modulators. Such optical control elements are used, for example, in optical fiber communication systems.

[0003] Conventionally, bulk ferroelectric crystals such as lithium niobate have been used in optical control devices. However, in recent years, there has been a demand for further miniaturization, broader bandwidth, and lower power consumption of optical control devices. To meet these demands, the formation of a ferroelectric thin film such as PZT on a substrate and its application to optical control devices has been investigated (e.g., Patent Documents 1 and 2). These documents disclose structures including a substrate and a ferroelectric thin film such as PZT formed on the substrate. Patent Document 1 discloses a process of forming a seed layer on a silicon substrate and then spin-coating a metal alkoxide layer to form PLZT, and considers using ultrafine Ti powder as particles for forming the seed layer. Patent Document 2 also discloses an element in which a spinel layer is formed on a silicon substrate and a PLZT thin film is further formed thereon.

[0004] International Publication No. 2005 / 015292 Japanese Patent Application Laid-Open No. 63-221306

[0005] Conventionally, special substrates such as sapphire with a crystal orientation have been used to deposit ferroelectric thin films such as PZT on substrates and promote crystal growth. However, sapphire has a high dielectric constant, which slows the electro-optic modulation speed of optical control elements, limiting its applications. For this reason, studies have been conducted to deposit ferroelectric thin films such as PZT on substrates made of amorphous materials, such as silicon substrates with thermal oxide films. However, conventional techniques have not been able to deposit ferroelectric thin films that exhibit high electro-optical effects on substrates made of amorphous materials, and further technological development has been anticipated. In particular, BTO and BSTO are lead-free compositions and are therefore useful as electro-optical components. Therefore, a technology is needed to deposit ferroelectric thin films such as BTO on amorphous materials and promote crystal growth.

[0006] Therefore, in order to solve the problems of the conventional technology, the present inventors have carried out investigations with the aim of forming a BTO thin film or a BSTO thin film with excellent crystal orientation on a substrate made of an amorphous material.

[0007] Examples of specific embodiments of the present invention are given below.

[0008] [1] A structure comprising: a substrate made of an amorphous material; a seed layer laminated on the substrate; an intermediate layer laminated on the seed layer; and a BTO thin film or a BSTO thin film laminated on the intermediate layer, wherein the intermediate layer contains, as a main component, a titanium oxide containing at least one element selected from the group consisting of Pb, Zr, La, Ba, Sr, K, Li, Nb, B, Mg, As, P, and Ge. [2] The structure according to [1], wherein the intermediate layer contains, as a main component, a titanium oxide containing at least one element selected from the group consisting of Pb, Zr, La, Ba, and Sr. [3] The structure according to [1] or [2], wherein the intermediate layer is a BSTO film or a PZT film. [4] The structure according to any one of [1] to [3], wherein the seed layer contains lanthanum nitrate as a main component. [5] The structure according to any one of [1] to [4], wherein the BTO thin film or the BSTO thin film has a thickness of 50 nm or more. [6] The structure according to any one of [1] to [5], wherein at least one intermediate layer and at least one BTO or BSTO thin film are laminated in this order on a BTO or BSTO thin film. [7] The structure according to any one of [1] to [6], wherein the thickness of the intermediate layer is 50 nm or less. [8] The structure according to any one of [1] to [7], wherein the substrate is a silicon substrate. [9] The structure according to [8], wherein the silicon substrate has a thermal oxide film.

[10] A waveguide including the structure according to any one of [1] to [9].

[11] An optical modulator formed by forming an electrode on the structure according to any one of [1] to [9].

[12] A method for manufacturing the structure according to any one of [1] to [9], comprising the steps of: forming a seed layer on a substrate made of an amorphous material; forming an intermediate layer on the seed layer; and forming a BTO or BSTO thin film on the intermediate layer.

[13] A method for manufacturing the structure according to

[12] , wherein the seed layer contains lanthanum nitrate as a main component.

[14] The method for manufacturing the structure according to

[12] or

[13] , wherein after the step of forming a BTO thin film or a BSTO thin film on the intermediate layer, a step of forming an intermediate layer and a step of forming a BTO thin film or a BSTO thin film are further performed at least once each.

[15] The method for manufacturing a structure according to any one of

[12] to

[14] , wherein the BTO thin film or the BSTO thin film is formed by a sol-gel method.

[16] The method for manufacturing a structure according to any one of

[12] to

[15] , wherein the substrate is a silicon substrate.

[0009] According to the present invention, a BTO thin film or a BSTO thin film having excellent crystal orientation can be formed on a substrate made of an amorphous material. By forming a BTO thin film or a BSTO thin film having excellent crystal orientation, the structure of the present invention can be used, for example, in a waveguide or an optical modulator.

[0010] FIG. 1 is a cross-sectional view illustrating the structure of the structure. FIG. 2 is a cross-sectional view illustrating the structure of a waveguide. FIG. 3 is a schematic diagram illustrating a process for forming a waveguide. FIG. 4 is a plan view illustrating the structure of a waveguide. FIG. 5 is a plan view illustrating the structure of an optical modulator. FIG. 6 is an XRD diffraction pattern of the structure obtained in Comparative Example 1. FIG. 7 is an XRD diffraction pattern of the structures obtained in Examples 1 and 2 and the precursor of the structure of Example 1. FIG. 8 is an XRD diffraction pattern of the structures obtained in Examples 1 and 4. FIG. 9 is an XRD diffraction pattern of the structures obtained in Examples 3 and 4. FIG. 10 is an XRD diffraction pattern of the structures obtained in Examples 2, 5, and 6. FIG. 11 is an XRD diffraction pattern of the structures of Examples 3 and 5 at each temperature when heated from room temperature to 110° C. FIG. 12 is an image of the cross section of the BTO film of the structure of Example 4 observed with an electron microscope. FIG. 13 shows the results of an oscilloscope analysis of the optical modulation characteristics of the optical modulator.

[0011] The present invention will be described in detail below. The following description of the constituent elements may be based on representative embodiments or specific examples, but the present invention is not limited to such embodiments. In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0012] (Structure) This embodiment relates to a structure having a substrate made of an amorphous material, a seed layer laminated on the substrate, an intermediate layer laminated on the seed layer, and a BTO thin film or a BSTO thin film (hereinafter, these thin films are also collectively referred to as a ferroelectric thin film) laminated on the intermediate layer, wherein the intermediate layer contains, as a main component, a titanium oxide containing at least one element selected from the group consisting of Pb, Zr, La, Ba, Sr, K, Li, Nb, B, Mg, As, P, and Ge.

[0013] Fig. 1 is a cross-sectional view illustrating the structure of a structure according to this embodiment. As shown in Fig. 1, a structure 10 according to this embodiment includes a substrate 2, a seed layer 4, an intermediate layer 5, and a ferroelectric thin film 6, in this order. Although other layers may be provided between the layers, it is preferable that the substrate 2 and the seed layer 4 are directly stacked, and it is also preferable that the seed layer 4 and the intermediate layer 5 are directly stacked, and it is also preferable that the intermediate layer 5 and the ferroelectric thin film 6 are directly stacked.

[0014] Because this embodiment has the above-described configuration, a ferroelectric thin film with excellent crystal orientation can be formed on a substrate made of an amorphous material. Furthermore, because this embodiment does not require the use of a lead-containing composition in each layer, a ferroelectric thin film made of a lead-free composition can be formed on a substrate made of an amorphous material. The structure of this embodiment has a BTO or BSTO thin film with excellent crystal orientation and can exhibit excellent electro-optical effects, so it can be used, for example, in waveguides and optical modulators. Furthermore, because the structure of this embodiment is a structure with a lead-free ferroelectric thin film, it has a wide range of applications and is particularly useful as an electro-optical component.

[0015] The electro-optic effect of the structure of this embodiment can be evaluated, for example, by measuring the half-wave voltage (Vπ) of the optical modulator. To measure the half-wave voltage (Vπ), an optical modulator is fabricated using the method described in the examples, and then a laser light source with a wavelength of 1550 nm or 1310 nm is used. The laser light is TE polarized and input to the optical modulator using a polarization-maintaining optical fiber. An electrical amplitude signal with a frequency of 14 kHz is input as a modulation voltage, and the intensity of the optical signal output from the modulator is measured. The half-wave voltage (Vπ) is then calculated from the output light intensity relative to the voltage of the input electrical signal. FIG. 5 shows a method for measuring the half-wave voltage (Vπ) using an optical modulator. Laser light is incident on the optical modulator 100 from the laser 20 using the optical fiber 25. The optical modulator 100 is composed of a Mach-Zehnder waveguide and electrodes 50a and 50b. An electrical signal is input to the electrode 50a using a function generator 52. The electrode 50b is grounded. The modulated optical signal output from the modulator 100 is extracted using an optical fiber 25 and analyzed using an optical detector 60 and an oscilloscope 70. When measured using a laser light source with a wavelength of 1550 nm, the half-wave voltage (Vπ) of the optical modulator is preferably 10 V or less, more preferably 5 V or less, and even more preferably 1 V or less. The lower limit of the half-wave voltage (Vπ) is not particularly limited, but is preferably 0.0001 V or more, for example. When measured using a laser light source with a wavelength of 1310 nm, the half-wave voltage (Vπ) of the optical modulator is preferably 10 V or less, more preferably 5 V or less, and even more preferably 1 V or less. The lower limit of the half-wave voltage (Vπ) is not particularly limited, but is preferably 0.0001 V or more, for example. The half-wave voltage (Vπ) is one index indicating the operating voltage of an optical modulator, and is defined as the voltage required to shift the electro-optical phase by 180°. Therefore, if the value of the half-wave voltage (Vπ) is equal to or less than the upper limit, the voltage required to shift the electro-optic phase by 180° can be suppressed, and it can be determined that the electro-optic effect is excellent.

[0016] Since the half-wave voltage is inversely proportional to the phase length, an index showing the half-wave voltage characteristics relative to the phase length is the VπL value, which is the product of the half-wave voltage and the phase length. When measured using a laser light source with a wavelength of 1550 nm, the VπL value of the optical modulator is preferably 5 Vcm or less, more preferably 2 Vcm or less, and even more preferably 1 Vcm or less. Furthermore, the VπL value is preferably 0.0001 Vcm or more. When measured using a laser light source with a wavelength of 1310 nm, the VπL value of the optical modulator is preferably 5 Vcm or less, more preferably 2 Vcm or less, and even more preferably 1 Vcm or less. Furthermore, the VπL value is preferably 0.0001 Vcm or more.

[0017] The electro-optic coefficient of the optical modulator including the structure of this embodiment is calculated using the following formula: r=λ×d / (n ３ ×Vπ×L×Γ) In the above formula, r is the electro-optic coefficient, λ is the wavelength of the laser light source, d is the electrode spacing, n is the effective refractive index of the waveguide, Vπ is the half-wave voltage, L is the electrode length, and Γ is the degree of overlap between the light and the electric field in the waveguide.

[0018] In this embodiment, the electro-optic coefficient calculated from the half-wave voltage and the electrode structure (length, electrode spacing) is preferably 30 pm / V or more, more preferably 60 pm / V or more, even more preferably 100 pm / V or more, and particularly preferably 200 pm / V or more. In this embodiment, a large electro-optic coefficient can be obtained in the optical modulator, making it possible to manufacture an optical modulator capable of operating at high speed.

[0019] <Substrate> The structure of this embodiment has a substrate made of an amorphous material. Examples of amorphous materials include glass, silicon, silicon oxide, silicon nitride, titanium oxide, and ITO. Among these, the amorphous material is preferably silicon with an oxide film.

[0020] Because substrates made of amorphous materials do not have a crystalline orientation, it has been difficult to form a ferroelectric thin film such as BTO on such substrates. However, the configuration of this embodiment makes it possible to form a ferroelectric thin film such as BTO on a substrate made of an amorphous material. Furthermore, using a substrate made of an amorphous material facilitates the manufacture of optical control elements and the integration of various elements on a silicon substrate. Therefore, in this embodiment, for example, when the structure is used in an optical modulator, the operating voltage can be reduced, and an optical modulator capable of high-speed operation can be manufactured at low cost.

[0021] The substrate made of an amorphous material used in this embodiment is preferably a silicon substrate, and the silicon substrate is preferably a substrate made of a thermal oxide film (e.g., SiO ２ In this case, it is preferable to use a silicon substrate with a thermal oxide film (SiO ２ The thickness of the Si substrate in the thermally-oxidized silicon substrate (SiO ２ / Si substrate) ２ The thickness of the film is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more. ２ The thickness of the film is preferably 100 μm or less.

[0022] <Seed Layer> The structure of this embodiment has a seed layer stacked on a substrate. In this embodiment, the structure having the seed layer allows crystals constituting the ferroelectric thin film to grow on a substrate made of an amorphous material. Furthermore, the structure having the seed layer allows a ferroelectric thin film with excellent crystal orientation to be formed.

[0023] In this embodiment, the seed layer preferably contains lanthanum nitrate as a main component. By forming a seed layer containing lanthanum nitrate as a main component, it becomes easier to form a ferroelectric thin film that is uniform and has good crystal orientation.

[0024] The content of lanthanum nitrate may be 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, still more preferably 90% by mass or more, and particularly preferably 95% by mass or more, based on the total mass of the seed layer. The content of lanthanum nitrate may be 100% by mass based on the total mass of the seed layer.

[0025] The thickness of the seed layer is preferably 1 nm or more, more preferably 2 nm or more, and even more preferably 3 nm or more. The thickness of the seed layer is preferably 30 nm or less, more preferably 25 nm or less, even more preferably 20 nm or less, and particularly preferably 15 nm or less. By setting the thickness of the seed layer within the above range, it becomes easier to form a ferroelectric thin film that is uniform and has good crystal orientation. When multiple seed layers are provided in the structure, the above film thickness is the total thickness of the multiple layers.

[0026] In this embodiment, a uniform seed layer can be formed over a relatively large area. For example, a seed layer having a substantially uniform thickness and no cracks or chips can be formed over a 0.5×0.5 cm ２ It can be formed into a size of 1.0 x 1.0 cm ２ It can be formed into a size of 2.0 x 2.0 cm ２ The above can be formed.

[0027] <Intermediate Layer> The structure of this embodiment has an intermediate layer stacked on the seed layer. In this embodiment, the structure has an intermediate layer, which allows crystals with excellent crystal orientation to grow on a substrate made of an amorphous material. The structure has a BTO thin film or BSTO thin film with excellent crystal orientation, which allows it to exhibit an excellent electro-optic effect. For this reason, the structure of this embodiment is preferably used in, for example, a waveguide or an optical modulator.

[0028] In this embodiment, it is preferable that the intermediate layer also has excellent crystal orientation. More specifically, it is preferable that the intermediate layer also has a c-axis orientation in which the crystal axes are aligned. By forming such an intermediate layer on the seed layer and constructing a ferroelectric thin film on the intermediate layer having the same crystal orientation as the ferroelectric thin film, a ferroelectric thin film with even better crystal orientation can be obtained, and as a result, a ferroelectric thin film that exhibits an excellent electro-optic effect can be formed.

[0029] The intermediate layer preferably has only a (100) diffraction peak and a (200) diffraction peak indicative of the crystal orientation in XRD diffraction, and a crystal state in which the half-width of each peak is small indicates the highest degree of orientation and is favorable. In this embodiment, such an intermediate layer is formed on the seed layer, and a ferroelectric thin film is further formed thereon, thereby making it possible to form a ferroelectric thin film having a crystal structure having only a (100) diffraction peak and a (200) diffraction peak.

[0030] The mismatch rate between the crystal lattice constant of the seed layer and the crystal lattice constant of the intermediate layer is preferably 3% or less, more preferably 2.5% or less, and even more preferably 2% or less. The mismatch rate between the crystal lattice constant of the seed layer and the crystal lattice constant of the intermediate layer may be 0%. In this specification, the mismatch rate is a value obtained by dividing the difference between the crystal lattice constant of the seed layer and the crystal lattice constant of the intermediate layer by the crystal lattice constant of the intermediate layer. Specifically, it is calculated using the following formula: Mismatch rate (%) = Difference between the crystal lattice constant of the seed layer and the crystal lattice constant of the intermediate layer / Crystal lattice constant of the intermediate layer × 100. For example, in a preferred embodiment, the crystal lattice constant of the seed layer containing lanthanum nitrate as a main component is C = 4.07 angstroms, and the crystal lattice constant of the intermediate layer made of a BTO thin film is C = 4.00 angstroms, resulting in a mismatch rate of 1.7%. By reducing the mismatch rate in this way, a crystalline phase serving as an intermediate layer can be favorably formed on the seed layer.

[0031] Similarly, the mismatch rate between the crystal lattice constant of the intermediate layer and the crystal lattice constant of the ferroelectric thin film is preferably 3% or less, more preferably 2.5% or less, and even more preferably 2% or less. The mismatch rate between the crystal lattice constant of the intermediate layer and the crystal lattice constant of the ferroelectric thin film may be 0%. In this specification, the mismatch rate is a value obtained by dividing the difference between the crystal lattice constant of the intermediate layer and the crystal lattice constant of the ferroelectric thin film by the crystal lattice of the ferroelectric thin film. Specifically, it is calculated using the following formula: Mismatch rate (%) = Difference between the crystal lattice constant of the intermediate layer and the crystal lattice constant of the ferroelectric thin film / Crystal lattice constant of the ferroelectric thin film × 100

[0032] The intermediate layer preferably contains, as a main component, titanium oxide containing at least one element selected from the group consisting of Pb, Zr, La, Ba, Sr, K, Li, Nb, B, Mg, As, P, and Ge, more preferably titanium oxide containing at least one element selected from the group consisting of Pb, Zr, La, Ba, and Sr, and particularly preferably titanium oxide containing at least one element selected from Ba and Sr. The atomic compositions of the intermediate layer and the ferroelectric thin film are preferably similar, and preferably they share at least one element. Among these, the intermediate layer is preferably a BTO thin film or a BSTO thin film, and particularly preferably a BSTO film.

[0033] It is preferable that the crystal structure of the main component constituting the intermediate layer and the crystal structure of the main component constituting the ferroelectric thin film are similar to each other. More specifically, it is preferable that the crystal structure of the main component constituting the intermediate layer and the crystal structure of the main component constituting the ferroelectric thin film are both perovskite structures, and ABX ３ A perovskite structure of the type is particularly preferred.

[0034] The content of the titanium oxide containing the above-mentioned predetermined element may be 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, still more preferably 90% by mass or more, and particularly preferably 95% by mass or more, based on the total mass of the intermediate layer. The content of the titanium oxide containing the above-mentioned predetermined element may be 100% by mass based on the total mass of the intermediate layer. By setting the content of titanium oxide within the above range, it becomes easier to form a ferroelectric thin film that is uniform and has good crystal orientation.

[0035] The thickness of the intermediate layer is preferably 1 nm or more, more preferably 2 nm or more, and even more preferably 3 nm or more. The thickness of the intermediate layer is preferably 300 nm or less, more preferably 200 nm or less, even more preferably 150 nm or less, even more preferably 100 nm or less, and particularly preferably 50 nm or less. By setting the thickness of the intermediate layer within the above range, it becomes easier to form a ferroelectric thin film that is uniform and has good crystal orientation. When multiple intermediate layers are provided in the structure, the above film thickness refers to the total thickness of the multiple layers.

[0036] <Ferroelectric Thin Film> The structure of this embodiment has a ferroelectric thin film on an intermediate layer. The intermediate layer and the ferroelectric thin film are preferably directly stacked, but another layer may be provided between them. Here, the ferroelectric thin film is preferably a film composed of an oxide with a perovskite structure, and is preferably a BTO (barium titanate) thin film or a BSTO (barium strontium titanate) thin film. When a BTO or BSTO thin film is used, a larger electro-optic coefficient (electro-optic effect) can be achieved compared to when a lithium niobate film is used. Furthermore, BTO and BSTO are ferroelectrics, and their polarization change is proportional to the magnitude of the electric field. This allows for a lower voltage required for electro-optic modulation and switching. Furthermore, the rapid polarization change allows for faster optical modulation and switching.

[0037] BTO and BSTO are each represented by the following structural formula: BTO: Ba ｘ Ti １－ｘ O ３ BSTO:Ba １－ｘ Sr ｘ TiO ３(O≦x<1) BSTO is BTO in which part of the barium in BTO is replaced with strontium. In this embodiment, BTO or BSTO to which Mn (manganese), Mg (magnesium), Nb (niobium), Co (cobalt), Fe (iron), Ni (nickel), Zn (zinc), Al (aluminum), Ta (tantalum), W (tungsten), or the like may be added may also be used.

[0038] The thickness of the BTO thin film or BSTO thin film is preferably 50 nm or more, more preferably 80 nm or more, even more preferably 100 nm or more, even more preferably 150 nm or more, even more preferably 170 nm or more, even more preferably 200 nm or more, and particularly preferably 300 nm or more. The thickness of the ferroelectric thin film is preferably 5 μm or less. By forming the ferroelectric thin film on a silicon substrate with an oxide film with a thickness within the above range, the effective dielectric constant can be reduced and the modulation speed can be increased. When multiple layers of BTO thin films or BSTO thin films are provided in the structure, the above thickness refers to the total thickness of the multiple layers.

[0039] The grain size of the crystals contained in the ferroelectric thin film is preferably equal to or smaller than the wavelength of light used in optical modulation or switching, for example, 100 nm or less. Furthermore, in the ferroelectric thin film, it is preferable that the grains are densely aggregated so as to prevent light from being scattered between them.

[0040] The crystal structure of the crystals contained in the ferroelectric thin film preferably has only a (100) diffraction peak and a (200) diffraction peak, which indicate the crystal orientation, and the crystal state in which the half-width of each peak is small is the most favorable and highly oriented. In this embodiment, a predetermined intermediate layer is formed on the seed layer, and a ferroelectric thin film is further formed thereon, thereby making it possible to form a ferroelectric thin film having a crystal structure having only a (100) diffraction peak and a (200) diffraction peak.

[0041] In the structure of this embodiment, at least one intermediate layer and at least one BTO thin film or BSTO thin film may be laminated in this order on the above-mentioned BTO thin film or BSTO thin film. That is, the structure of this embodiment may have a layer configuration of substrate / first seed layer / first intermediate layer / first ferroelectric thin film / second intermediate layer / second ferroelectric thin film, or a configuration of substrate / first seed layer / first intermediate layer / first ferroelectric thin film / second intermediate layer / second ferroelectric thin film / third intermediate layer / third ferroelectric thin film, or a configuration of substrate / first seed layer / first intermediate layer / first ferroelectric thin film / second intermediate layer / second ferroelectric thin film / third intermediate layer / third ferroelectric thin film / ... / nth intermediate layer / nth ferroelectric thin film (where n is an integer of 4 to 20). A second seed layer may be provided between the first ferroelectric thin film and the second intermediate layer, a third seed layer may be provided between the second ferroelectric thin film and the third intermediate layer, and an nth seed layer may be provided between the (n-1)th ferroelectric thin film and the nth intermediate layer. Furthermore, the ferroelectric thin films may have different crystal lattice sizes or may have the same crystal lattice size. Thus, in this embodiment, multiple layers of BTO or BSTO thin films can be formed, thereby obtaining a structure that exhibits an excellent electro-optic effect. Such a structure is preferably used for a waveguide, an optical modulator, or the like.

[0042] Furthermore, in the structure of this embodiment, a ferroelectric thin film may be directly stacked on the ferroelectric thin film to increase the thickness. That is, the structure of this embodiment may have a layer structure of substrate / first seed layer / first intermediate layer / first ferroelectric thin film / second ferroelectric thin film, or a structure of substrate / first seed layer / first intermediate layer / first ferroelectric thin film / second ferroelectric thin film / third ferroelectric thin film, or a structure of substrate / first seed layer / first intermediate layer / first ferroelectric thin film / second ferroelectric thin film / third ferroelectric thin film / ... / nth ferroelectric thin film (where n is an integer of 4 to 20). In this way, by depositing the ferroelectric thin film with or without an intermediate layer, it is possible to increase the thickness of the ferroelectric thin film while maintaining the crystal orientation. When the structure of this embodiment is used as a component of, for example, a waveguide, an optical modulator, an optical integrated circuit, an optical switching circuit, or the like, it is necessary for the ferroelectric thin film to have a certain degree of thickness or more, and therefore the structure of this embodiment is preferably used for the above-mentioned applications.

[0043] In this embodiment, even when multiple ferroelectric thin films are formed, the thickness of each ferroelectric thin film can be set to 50 nm or more, preferably 60 nm or more, more preferably 70 nm or more, and even more preferably 80 nm or more. In this way, a ferroelectric thin film having a predetermined thickness or more can be formed by a single spin coating. Furthermore, by forming multiple ferroelectric thin films as described above, a structure having a total thickness of 300 nm or more can be finally fabricated.

[0044] <Use of Structure> The use of the structure of this embodiment is not particularly limited. The structure of this embodiment can be used, for example, as an optical element, an optical intensity modulator, an optical phase modulator, a directional coupler, a ring resonator, an optical switching element, a beam steering element, a multimode interference element, a variable wavelength filter, a variable optical intensity filter, etc. Since the structure of this embodiment has an excellent electro-optical effect, it is preferably used as a component of an optical element or an optical control device, and is particularly preferably used as a component of, for example, a waveguide, an optical modulator, an optical integrated circuit, an optical switching circuit, etc.

[0045] When the structure is used as a component of a waveguide, an optical modulator, an optical integrated circuit, an optical switching circuit, etc., the ferroelectric thin film preferably has peaks of (100) and (200) and a total thickness of 200 nm or more, preferably 300 nm or more. A structure having such a ferroelectric thin film can exhibit an excellent electro-optical effect.

[0046] (Method for Manufacturing a Structure) This embodiment relates to a method for manufacturing the above-mentioned structure, which includes the steps of forming a seed layer on a substrate made of an amorphous material, forming an intermediate layer on the seed layer, and forming a BTO thin film or a BSTO thin film on the intermediate layer. The substrate used in the manufacturing method of this embodiment is preferably a silicon substrate.

[0047] In this embodiment, the seed layer preferably contains lanthanum nitrate as a main component. The content of lanthanum nitrate may be 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, and particularly preferably 95% by mass or more, based on the total mass of the seed layer. The content of lanthanum nitrate may be 100% by mass based on the total mass of the seed layer.

[0048] In this embodiment, a step of applying a hydrophilic polymer-containing substance onto a substrate made of an amorphous material is preferably provided. Here, the hydrophilic polymer is preferably a nonionic hydrophilic polymer. Examples of such hydrophilic polymers include polyvinylpyrrolidone, polyvinyl alcohol, polyvinylamide, polyacrylamide, polyacrylic acid, polyethylene oxide, and polyamine. Among these, polyvinylpyrrolidone is particularly preferred as the hydrophilic polymer.

[0049] The weight-average molecular weight of the hydrophilic polymer is not particularly limited, but is preferably about 1,000 to 100,000.

[0050] In the step of applying a hydrophilic polymer-containing material to a substrate, the hydrophilic polymer-containing material (hydrophilic polymer-containing solution) is applied to the substrate. The application method is not particularly limited, and for example, spin coating, dip coating, LSMCD (Liquid Source Missed Chemical Deposition), electrostatic spraying, and the like can be employed. Among these, spin coating is preferred as the application method. In this case, the concentration of the hydrophilic polymer contained in the hydrophilic polymer-containing material (hydrophilic polymer-containing solution) is preferably 0.001% by mass or more, more preferably 0.05% by mass or more, and even more preferably 0.1% by mass or more. Furthermore, the concentration of the hydrophilic polymer is preferably 50% by mass or less, more preferably 10% by mass or less, even more preferably 2% by mass or less, and particularly preferably 1% by mass or less.

[0051] The hydrophilic polymer-containing material (hydrophilic polymer-containing solution) preferably contains a solvent in addition to the hydrophilic polymer. Examples of the solvent include 2-methoxyethanol, butanol, ethanol, isopropanol, methanol, and water.

[0052] By applying the hydrophilic polymer-containing material onto the substrate, a hydrophilic polymer coating film (film) is formed on the substrate. The thickness of the coating film (film) in this case is preferably, for example, 10 nm or less.

[0053] A drying step may be carried out after the hydrophilic polymer-containing material is applied to the substrate. In this case, the drying temperature is preferably, for example, 50 to 300°C, and the drying time is preferably 1 second to 10 minutes. Furthermore, a step of cleaning the substrate may be carried out before the hydrophilic polymer-containing material is applied to the substrate. In this case, a step of heating and drying the substrate after cleaning may be carried out.

[0054] In the step of forming a seed layer, a seed layer solution is applied onto a substrate or a hydrophilic polymer coating film. The application method is not particularly limited, and for example, spin coating, dip coating, LSMCD (Liquid Source Misted Chemical Deposition), electrostatic spraying, and the like can be used. Among these, spin coating is preferred as the application method. In this case, the concentration of seed particles contained in the seed layer solution is preferably 0.001% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.5% by mass or more. Furthermore, the concentration of seed particles is preferably 50% by mass or less, more preferably 10% by mass or less, and even more preferably 3% by mass or less.

[0055] The seed layer solution preferably contains a solvent for dispersing the seed particles, such as 2-methoxyethanol, butanol, ethanol, isopropanol, methanol, or water.

[0056] In the present embodiment, in the step of forming a seed layer on a substrate made of an amorphous material, the above-described hydrophilic polymer may be mixed into the seed layer solution, in which case the seed layer solution containing the hydrophilic polymer is applied onto the substrate made of an amorphous material.

[0057] In the step of forming the seed layer, a baking step (drying step) is preferably provided after the seed layer solution is applied. The baking (drying) temperature is, for example, 150 to 500°C, and the drying time is preferably 1 to 10 minutes. By providing such a drying step, the solvent can be removed.

[0058] In this embodiment, the step of applying a hydrophilic polymer-containing material to a substrate and the step of forming a seed layer may be performed once, or the step of applying a hydrophilic polymer-containing material and / or the step of forming a seed layer may each be performed multiple times. For example, the method for producing a structure in this embodiment may include the steps of applying a hydrophilic polymer-containing material to a substrate made of an amorphous material, forming a seed layer on the surface of the substrate where the hydrophilic polymer-containing material has been applied, applying the hydrophilic polymer-containing material on the seed layer, forming a seed layer on the surface of the seed layer where the hydrophilic polymer-containing material has been applied, forming an intermediate layer on the seed layer, and forming a BTO thin film or a BSTO thin film on the intermediate layer.

[0059] After the step of forming the seed layer, a firing step for crystallizing the seed layer may be performed. The firing temperature in the firing step is preferably 400°C or higher, more preferably 450°C or higher, and even more preferably 500°C or higher. The firing temperature in the firing step is preferably 700°C or lower, more preferably 650°C or lower, even more preferably 600°C or lower, and particularly preferably 550°C or lower. The firing time in the firing step is preferably 1 to 10 minutes. By setting the firing temperature in the firing step within the above range, the crystal growth properties of the ferroelectric thin film can be improved. More specifically, the crystal structure of the ferroelectric thin film can be made to have a c-axis in the substrate normal direction.

[0060] It is believed that the coating (film) containing the hydrophilic polymer is burned away during the baking process described above. Therefore, it is difficult to analyze the presence of the coating (film) containing the hydrophilic polymer in the structure of this embodiment. However, by forming the coating (film) containing the hydrophilic polymer in the manufacturing process of the structure, a seed layer containing lanthanum nitrate can be formed on a substrate made of an amorphous material, and a ferroelectric thin film with excellent crystal orientation can be formed on such a seed layer.

[0061] In the step of forming the intermediate layer, an intermediate layer solution is applied onto the seed layer. The application method is not particularly limited, and examples thereof include spin coating, dip coating, LSMCD (Liquid Source Mixed Chemical Deposition), and electrostatic spraying. Among these, spin coating is preferred. In this case, the concentration of titanium oxide containing at least one element selected from the group consisting of Pb, Zr, La, Ba, Sr, K, Li, Nb, B, Mg, As, P, and Ge contained in the intermediate layer solution is preferably 0.001% by mass or more, more preferably 0.1% by mass or more, and even more preferably 0.5% by mass or more. Furthermore, the concentration of the titanium oxide is preferably 50% by mass or less, more preferably 10% by mass or less, and even more preferably 3% by mass or less.

[0062] After the coating step of the intermediate layer solution, a heat treatment step may be performed between each coating step. The heat treatment temperature is, for example, 150 to 800° C., and the heat treatment time is preferably 1 to 30 minutes.

[0063] In the step of forming the intermediate layer, ２ It is preferable to further perform a heat treatment in an atmosphere to crystallize the intermediate layer. ２ It is particularly preferable to perform the heat treatment in an atmosphere. The heat treatment temperature is preferably 400°C or higher, more preferably 450°C or higher, even more preferably 500°C or higher, and particularly preferably 550°C or higher. The heat treatment temperature is preferably 1000°C or lower, more preferably 850°C or lower, even more preferably 800°C or lower, and particularly preferably 750°C or lower. The heat treatment time is preferably 1 to 60 minutes, more preferably 1 to 40 minutes, and even more preferably 1 to 30 minutes. By setting the heat treatment temperature within the above range, the crystal growth properties of the intermediate layer can be improved. More specifically, the crystal structure of the intermediate layer can be made to have its c-axis in the direction normal to the substrate.

[0064] In the step of forming a BTO thin film or a BSTO thin film on the intermediate layer, a ferroelectric thin film is preferably formed on the intermediate layer by a sol-gel method. Specifically, a sol-gel liquid for forming a ferroelectric thin film is applied to the intermediate layer. The application method is not particularly limited, and methods such as spin coating, dip coating, LSMCD (Liquid Source Mixed Chemical Deposition), and electrostatic spraying can be used. Among these, spin coating is preferred. In the step of forming a ferroelectric thin film, the application step may be repeated until a predetermined film thickness is achieved. In this embodiment, for example, the application step may be repeated approximately 2 to 30 times. A heat treatment step may be performed between each application step. The heat treatment temperature is preferably 150 to 800°C, and the heat treatment time is preferably 1 to 20 minutes.

[0065] After the coating process, the ２ It is preferable to further perform a heat treatment in an atmosphere to crystallize the ferroelectric material. ２ It is particularly preferable to perform the heat treatment in an atmosphere. The heat treatment temperature in this case is preferably 400°C or higher, more preferably 450°C or higher, even more preferably 500°C or higher, and particularly preferably 550°C or higher. The heat treatment temperature is preferably 1000°C or lower, more preferably 900°C or lower, even more preferably 800°C or lower, and particularly preferably 750°C or lower. The heat treatment time is preferably 1 to 60 minutes, more preferably 1 to 40 minutes, and even more preferably 1 to 20 minutes. By setting the heat treatment temperature within the above range, the crystal growth properties of the ferroelectric thin film can be improved. More specifically, the crystal structure of the ferroelectric thin film can be made to have its c-axis in the direction normal to the substrate.

[0066] In this embodiment, after the step of forming the BTO thin film or BSTO thin film on the intermediate layer, the step of forming the intermediate layer and the step of forming the BTO thin film or BSTO thin film may be further performed at least once. For example, the method for manufacturing a structure in this embodiment may include the steps of applying a hydrophilic polymer-containing substance to a substrate made of an amorphous material, forming a first seed layer on the surface of the substrate on which the hydrophilic polymer-containing substance has been applied, forming a first intermediate layer, forming a first BTO thin film or BSTO thin film, forming a second intermediate layer, and forming a second BTO thin film or BSTO thin film, or the steps of applying a hydrophilic polymer-containing substance to a substrate made of an amorphous material, forming a first seed layer on the surface of the substrate on which the hydrophilic polymer-containing substance has been applied, forming a first intermediate layer, forming a first BTO thin film or BSTO thin film, and forming a second intermediate layer. The method may include a step of forming a first seed layer on the surface of the substrate coated with the hydrophilic polymer-containing material, a step of forming a first intermediate layer, a step of forming a first BTO thin film or BSTO thin film, a step of forming a second BTO thin film or BSTO thin film, a step of forming a second intermediate layer, a step of forming a second BTO thin film or BSTO thin film, a step of forming a third intermediate layer, a step of forming a third BTO thin film or BSTO thin film, a step of forming an nth intermediate layer, and a step of forming an nth BTO thin film or BSTO thin film (where n is an integer of 4 to 20). Furthermore, a step of forming a second seed layer may be provided before the step of forming the second intermediate layer, a step of forming a third seed layer may be provided before the step of forming the third intermediate layer, or a step of forming an nth seed layer may be provided before the step of forming the nth intermediate layer. In this embodiment, the intermediate layer and the ferroelectric thin film may be repeatedly stacked in this order on the first BTO thin film or the BSTO thin film. In this case, it is preferable to provide a step of forming the second intermediate layer after forming the first ferroelectric thin film, performing a heat treatment, and crystallizing the ferroelectric.

[0067] In this embodiment, after the step of forming a BTO thin film or a BSTO thin film on the intermediate layer, a step of forming a BTO thin film or a BSTO thin film may be performed at least once. For example, the method for manufacturing a structure in this embodiment may include the steps of applying a hydrophilic polymer-containing substance to a substrate made of an amorphous material, forming a first seed layer on the surface of the substrate on which the hydrophilic polymer-containing substance has been applied, forming a first intermediate layer, forming a first BTO thin film or a BSTO thin film, and forming a second BTO thin film or a BSTO thin film. Alternatively, the method may include a step of forming a first BTO or BSTO thin film, a step of forming a second BTO or BSTO thin film, and a step of forming a third BTO or BSTO thin film, or a step of forming a first seed layer on the surface of the substrate coated with the hydrophilic polymer-containing material, a step of forming a first intermediate layer, a step of forming a first BTO or BSTO thin film, a step of forming a second BTO or BSTO thin film, a step of forming a third BTO or BSTO thin film, and a step of forming an n-th BTO or BSTO thin film (where n is an integer of 4 to 20). In this embodiment, ferroelectric thin films may be repeatedly stacked on the first BTO or BSTO thin film. In this case, it is preferable to form a first ferroelectric thin film, perform a heat treatment, crystallize the ferroelectric, and then form a second ferroelectric thin film.

[0068] The structure of this embodiment is preferably manufactured through the manufacturing process described above. Specifically, the structure of this embodiment is preferably manufactured by forming a hydrophilic polymer coating (film) on a substrate made of an amorphous material, followed by a step of forming a seed layer, a step of forming an intermediate layer on the seed layer, and a step of forming a ferroelectric thin film on the intermediate layer. Although the hydrophilic polymer coating (film) is considered to be burned away in the firing step, the structure of this embodiment may contain components derived from this hydrophilic polymer coating (film). For example, components or elements derived from the hydrophilic polymer may be present between the substrate and the seed layer.

[0069] For example, lanthanum nitrate, which is used for the seed layer, tends to crystallize during heating, making it difficult to form a dense, uniform seed layer. In other words, it is difficult to uniformly crystallize the ferroelectric thin film on the seed layer. On the other hand, a uniform film can be formed on a substrate made of an amorphous material on a hydrophilic polymer. Therefore, the lanthanum nitrate can connect the microcrystals to prevent unevenness during the heating process, thereby enabling the seed layer to be densified. Although the hydrophilic polymer is burned off during the final high-temperature sintering process, the temperature is maintained at approximately 300–400°C during the crystallization process of lanthanum nitrate. Therefore, depending on the sintering conditions, components derived from the hydrophilic polymer may be detected.

[0070] Furthermore, the present embodiment may also be a structure manufactured by applying a seed layer solution containing a hydrophilic polymer and lanthanum nitrate onto a substrate made of an amorphous material to form a seed layer, further forming an intermediate layer on the seed layer, and then forming a ferroelectric thin film on the intermediate layer. In this case, the seed layer may contain components or elements derived from the hydrophilic polymer.

[0071] (Waveguide) This embodiment relates to a waveguide (optical waveguide) including the above-described structure. In this embodiment, the waveguide may be a phase modulation type waveguide or a Mach-Zehnder interference waveguide.

[0072] The waveguide in this embodiment may have a structure as shown in FIG. 2, for example. FIG. 2 is a cross-sectional view of the waveguide in the width direction (a cross-sectional view perpendicular to the direction in which the waveguide pattern extends). As shown in FIG. 2, the waveguide 15 has a substrate 2, a seed layer 4, an intermediate layer 5, and a ferroelectric thin film 6, in this order, and a ridge portion (convex portion) 14 is formed in the ferroelectric thin film 6. The surface including the ridge portion 14 is covered with a top clad layer 8. Examples of materials that can be used to form the top clad layer 8 include acrylic resins such as polymethyl methacrylate (PMMA), epoxy resins, and silicon oxide.

[0073] FIG. 3 is a schematic diagram illustrating the process of forming a waveguide 15. As shown in FIG. 3, a resist pattern 12 for the waveguide is formed on the ferroelectric thin film 6 of the structure 10 (FIG. 3(b)). Next, a ridge portion 14 is formed by an etching process, and the resist 12 for the waveguide is removed (FIG. 3(c)). Thereafter, a top clad layer 8 is formed to obtain the waveguide 15 (FIG. 3(d)). The maximum thickness of the top clad layer 8 is preferably 1 μm or more, more preferably 2 μm or more, and even more preferably 3 μm or more. The maximum thickness of the top clad layer 8 is preferably 100 μm or less. The maximum thickness of the top clad layer 8 is the thickness represented by t in FIG. 2.

[0074] The ridge length (waveguide length) of the waveguide 15 is preferably 10 mm or less, and more preferably 5 mm or less. The ridge length is preferably 0.001 mm or more. The ridge height (waveguide height) is preferably 1 μm or less. The ridge height is preferably 100 nm or more, and more preferably 200 nm or more. The ridge width (waveguide width) is preferably 3 μm or less, and more preferably 2 μm or less. The ridge width is preferably 0.5 μm or more, and more preferably 1 μm or more. In FIG. 2, the ridge height (waveguide height) is represented by h, and the ridge width (waveguide width) is represented by w.

[0075] The thickness of the waveguide 15 (excluding the thickness of the substrate) is preferably 15 μm or less, more preferably 10 μm or less, and even more preferably 7 μm or less. There is no particular lower limit to the thickness of the waveguide 15, but it is preferably 5 μm or more, for example. In this way, in this embodiment, it is also possible to make the waveguide thinner.

[0076] Fig. 4 is a plan view of the waveguide shown in Fig. 2, and is a schematic diagram visualizing the waveguide pattern. As shown in Fig. 4, in the waveguide, the value of input / output loss with respect to the waveguide length can be measured by inputting laser light to the end face of the waveguide and measuring the light intensity of the laser light emitted from the opposite side of the waveguide.

[0077] In this embodiment, when the optical propagation loss of a waveguide including the structure is calculated, the optical propagation loss is preferably 10 db / mm or less, more preferably 5 db / mm or less, even more preferably 3 db / mm or less, and particularly preferably 1 db / mm or less. The lower limit of the optical propagation loss is not particularly limited, and may be 0.01 db / mm.

[0078] (Optical Modulator) This embodiment relates to an optical modulator formed by forming electrodes on the above-described structure. That is, the optical modulator of this embodiment is a waveguide-type optical modulator. Such an optical modulator is used in optical waveguide devices, optical communication devices, etc.

[0079] The optical modulator of this embodiment may have, for example, a configuration as shown in Fig. 5. As shown in Fig. 5, the optical modulator 100 further includes electrodes 50a and 50b in addition to the above-described waveguide. The electrodes 50a and 50b can be made of, for example, Au, Al, Cu, or the like. A plurality of the electrodes 50a and 50b are arranged on either side of the waveguide, and the refractive index of the optical modulator is changed by an electric field applied between the pair of electrodes.

[0080] The spacing between the electrodes is preferably, for example, 3 μm or more, more preferably 5 μm or more, and even more preferably 6 μm or more. The spacing between the electrodes is preferably 10 μm or less, more preferably 8 μm or less, and even more preferably 7 μm or less. By keeping the spacing between the electrodes within the above range, the modulation voltage input to the electrodes can be efficiently applied to the waveguide portion.

[0081] Electrode 50b in FIG. 5 is a ground electrode, and an electrical signal at a predetermined voltage is input to electrode 50a. Laser light is input to the end face of the waveguide, and the light intensity of the laser light emitted from the opposite side of the waveguide is measured to measure the half-wave voltage (Vπ), which serves as an indicator of the operating voltage of the optical modulator. When measured using a laser light source with a wavelength of 1550 nm, the half-wave voltage (Vπ) of the optical modulator is preferably 10 V or less, more preferably 5 V or less, and even more preferably 1 V or less. The lower limit of the half-wave voltage (Vπ) is not particularly limited, but is preferably 0.0001 V or more, for example. When measured using a laser light source with a wavelength of 1310 nm, the half-wave voltage (Vπ) of the optical modulator is preferably 10 V or less, more preferably 5 V or less, and even more preferably 1 V or less. The lower limit of the half-wave voltage (Vp) is not particularly limited, but is preferably 0.0001 V or more, for example. In this embodiment, the electro-optic coefficient calculated from the half-wave voltage and the electrode structure (length, electrode spacing) is preferably 30 pm / V or more, more preferably 50 pm / V or more, even more preferably 60 pm / V or more, and particularly preferably 100 pm / V or more.

[0082] In this embodiment, the phase length (electrode length) of the optical modulator can be shortened, allowing for miniaturization. For example, the phase length (electrode length) of an optical modulator including the structure of this embodiment is preferably 20 mm or less, more preferably 10 mm or less, even more preferably 5 mm or less, even more preferably 4 mm or less, and particularly preferably 3 mm or less. By setting the phase length (electrode length) of the optical modulator within the above range, it is possible to effectively suppress attenuation of high-frequency electrical signals input to the electrodes.

[0083] In this embodiment, a large electro-optic coefficient can be obtained in the optical modulator, making it possible to manufacture an optical modulator capable of operating at high speeds. For example, when an optical modulator is manufactured using the structure of this embodiment, it is possible to obtain optical modulation of 40 Gbit / s or more, 50 Gbit / s or more, 60 Gbit / s or more, 70 Gbit / s or more, 80 Gbit / s or more, 90 Gbit / s or more, and even 100 Gbit / s or more. The accuracy of the optically modulated signal can be evaluated by the shape of the eye pattern of the optically modulated signal and the Q factor, which is defined by the signal amplitude and signal noise. For example, the Q factor, which is defined by the signal amplitude and signal noise, is preferably 3.5 or more, more preferably 4.0 or more, even more preferably 4.5 or more, and particularly preferably 5.0 or more.

[0084] In this embodiment, the refractive index of the optical modulator can be controlled with high precision even when the voltage between the electrodes is small. As such, the optical modulator of this embodiment is an optical modulator that can operate at low voltage and high speed. Furthermore, the optical modulator of this embodiment achieves high efficiency and ultra-high speed modulation because it obtains a high electro-optic effect.

[0085] The features of the present invention will be explained in more detail below with reference to examples and comparative examples. The materials, amounts used, ratios, treatment contents, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.

[0086] Example 1 Preparation of Seed Layer Solution 0.311 g of lanthanum nitrate hexahydrate and 20 ml of 2-methoxyethanol were mixed and stirred to obtain a seed layer solution. The seed layer solution was diluted immediately before film formation to a seed layer solution:2-methoxyethanol ratio of 2:1 (weight ratio). 0.05 g of polyvinylpyrrolidone (PVP) and 20 ml of 2-methoxyethanol were mixed and stirred to obtain a PVP solution.

[0087] <Formation of seed layer> A silicon substrate with a thermal oxide film (SiO ) cut into a 20 mm x 20 mm square was used as a substrate. ２ A silicon substrate (Si substrate) was prepared. The thermal oxide film had a thickness of 3 μm, and the silicon substrate had a thickness of 525 μm. A PVP (polyvinylpyrrolidone) solution was spin-coated onto the thermal oxide film of the substrate at 4000 rpm / 1 minute, and then a seed layer solution diluted with 2-methoxyethanol was spin-coated at 4000 rpm / 1 minute, and the substrate was heated in air at 300°C for 5 minutes. A PVP solution was further spin-coated onto the substrate at 4000 rpm / 1 minute, and then a seed layer solution diluted with 2-methoxyethanol was spin-coated at 4000 rpm / 1 minute, and the substrate was heated in air at 300°C for 5 minutes. The substrate was then heated in air at 500°C for 15 minutes, forming a seed layer with a thickness of approximately 10 nm.

[0088] <Formation of intermediate layer> BSTO solution ((Ba, Sr)TiO ３ A sol-gel solution for BST (manufactured by Mitsubishi Materials Corporation) was diluted 20 times with a solvent (n-pentyl acetate). The diluted BST solution was spin-coated onto the seed layer at 4000 rpm / minute and heated in air at 300°C for 5 minutes. This was then heated in an oxygen atmosphere at 750°C for 20 minutes. In this way, a BSTO thin film (intermediate layer) with a thickness of 10 nm or less was formed on the seed layer.

[0089] <Deposition of BTO Thin Film> A BTO solution (manufactured by Mitsubishi Materials Corporation) was spin-coated at 2000 rpm / minute onto the BSTO intermediate layer formed on the seed layer, and the resulting mixture was heated in air at 300°C for 5 minutes. This was followed by heating in an oxygen atmosphere at 700°C for 15 minutes. In this way, a BTO film with a thickness of approximately 80 nm was formed on the intermediate layer, forming a structure.

[0090] Example 2 <Formation of seed layer> A silicon substrate (SiO 2 ) with a thermal oxide film cut into a 20 mm x 20 mm square was used as a substrate in the same manner as in Example 1. ２ A seed layer was formed on a silicon substrate.

[0091] <Formation of intermediate layer> PZT solution (Pb / Zr / TiO ３ A sol-gel solution for PZT (manufactured by Mitsubishi Materials Corporation) was diluted 20 times with a solvent (n-pentyl acetate). The diluted PZT solution was spin-coated onto the seed layer at 4000 rpm / minute and heated in air at 300°C for 5 minutes. This was then heated in an oxygen atmosphere at 550°C for 10 minutes. In this way, a PZT thin film (intermediate layer) with a thickness of 10 nm or less was formed on the seed layer.

[0092] <Deposition of BTO Thin Film> A BTO solution (manufactured by Mitsubishi Materials Corporation) was spin-coated at 2000 rpm / minute onto the PZT intermediate layer formed on the seed layer, and the resulting structure was heated in air at 300°C for 5 minutes. The structure was then heated in an oxygen atmosphere at 700°C for 15 minutes. In this way, a BTO film with a thickness of approximately 80 nm was formed on the intermediate layer, forming a structure.

[0093] Example 3 An intermediate layer was formed on the BTO film (ferroelectric thin film) of the structure obtained in Example 1 in the same manner as in Example 1, and a BTO thin film (ferroelectric thin film) was formed on this intermediate layer in the same manner as in Example 1. Furthermore, an intermediate layer was formed on this BTO thin film in the same manner as in Example 1, and a BTO thin film was formed on this intermediate layer in the same manner as in Example 1. A structure having the structure of substrate / seed layer / BSTO intermediate layer (1) / BTO film (1) / BSTO intermediate layer (2) / BTO film (2) / BSTO intermediate layer (3) / BTO film (3) was obtained.

[0094] Example 4 A BTO thin film (ferroelectric thin film) was formed on the BTO film (ferroelectric thin film) of the structure obtained in Example 1 by the same method as in Example 1. Furthermore, a BTO thin film (ferroelectric thin film) was formed on this BTO thin film by the same method as in Example 1. A structure having the structure of substrate / seed layer / BSTO intermediate layer / BTO film (1) / BTO film (2) / BTO film (3) was obtained.

[0095] (Example 5) An intermediate layer was formed on the BTO film (ferroelectric thin film) of the structure obtained in Example 2 in the same manner as in Example 2, and a BTO thin film (ferroelectric thin film) was formed on this intermediate layer in the same manner as in Example 2. Furthermore, an intermediate layer was formed on this BTO thin film in the same manner as in Example 2, and a BTO thin film was formed on this intermediate layer in the same manner as in Example 2. A structure having the structure of substrate / seed layer / PZT intermediate layer (1) / BTO film (1) / PZT intermediate layer (2) / BTO film (2) / PZT intermediate layer (3) / BTO film (3) was obtained.

[0096] (Example 6) A BTO thin film (ferroelectric thin film) was formed on the BTO film (ferroelectric thin film) of the structure obtained in Example 2 by the same method as in Example 2. Furthermore, a BTO thin film (ferroelectric thin film) was formed on this BTO thin film by the same method as in Example 2. A structure having the structure of substrate / seed layer / PZT intermediate layer (1) / BTO film (1) / BTO film (2) / BTO film (3) was obtained.

[0097] Comparative Example 1 A structure was fabricated in the same manner as in Example 1, except that no intermediate layer was formed and a BTO film was formed on the seed layer.

[0098] Comparative Example 2: A silicon substrate with a thermal oxide film (SiO ２ A structure was fabricated in the same manner as in Example 1, except that a BSTO thin film (intermediate layer) and a BTO film were formed in this order on a silicon substrate (Si substrate).

[0099] (Results) Figure 6 shows the XRD diffraction pattern of the structure in Comparative Example 1. When a BTO film was formed on the seed layer, many diffraction peaks were observed, indicating that a crystalline film with poor uniaxial orientation was formed. Figure 7 shows the XRD diffraction patterns of the structures in Example 1 and Comparative Example 2. As can be seen from the result of Comparative Example 2 ((1) in Figure 7), the BSTO thin film (intermediate layer) was formed on a silicon substrate with a thermal oxide film (SiO ２ On the other hand, it was confirmed that no crystalline film was formed on the silicon substrate with thermal oxide film (SiO ２ When a seed layer and a BSTO thin film (intermediate layer) were formed in this order on a silicon substrate (Si substrate) ((2) in Figure 7), an intermediate layer with a c-axis orientation in which the crystal axes were aligned was formed on the seed layer. When a BTO thin film was further formed on top of that ((3) in Figure 7), a crystalline film with a c-axis orientation in which the crystal axes were aligned was formed. In Figure 7, the diffraction peaks of 22.2° and 45.3° correspond to the (100) and (200) directions, respectively, confirming that the BTO was an oriented crystalline film.

[0100] FIG. 8 shows XRD patterns of the structures in Example 1 ((4) in FIG. 8) and Example 4 ((5) in FIG. 8). Even when two BTO thin films were continuously laminated on the BTO thin film formed in Example 1, a c-axis-oriented crystalline film with aligned crystal axes was formed ((5) in FIG. 8). In FIG. 8, the diffraction peaks of 22.2° and 45.3° correspond to the (100) and (200) orientations, respectively, confirming that the BTO was an oriented crystalline film. Furthermore, the XRD diffraction peak intensity of the structure in Example 4 ((5) in FIG. 8) was approximately three times that of the structure in Example 1 ((4) in FIG. 8).

[0101] 9 shows XRD patterns of the structures in Example 4 ((5) in FIG. 9) and Example 3 ((6) in FIG. 9). Even when two sets of an intermediate layer and a BTO thin film were continuously laminated on the BTO thin film formed in Example 1, a c-axis oriented crystalline film with aligned crystal axes was formed ((6) in FIG. 9). In FIG. 9, the diffraction peaks of 22.2° and 45.3° correspond to the (100) and (200) directions, respectively, confirming that BTO was an oriented crystalline film.

[0102] FIG. 10 shows XRD patterns of Example 2 ((1) in FIG. 10), Example 5 ((2) in FIG. 10), and Example 6 ((3) in FIG. 10). Even when two BTO thin films were successively laminated on the BTO thin film formed in Example 2, a c-axis oriented crystalline film with aligned crystal axes was formed ((2) in FIG. 10). Furthermore, even when two sets of an intermediate layer and a BTO thin film were successively laminated on the BTO thin film formed in Example 2, a c-axis oriented crystalline film with aligned crystal axes was formed ((3) in FIG. 10). In FIG. 10, the diffraction peaks of 22.2° and 45.3° correspond to the (100) and (200) orientations, respectively, confirming that BTO was an oriented crystalline film. The XRD diffraction peak intensity of the structure in Example 5 ((2) in FIG. 10 ) was about three times or more the peak intensity of the XRD diffraction peak intensity of the structure in Example 2 ((1) in FIG. 10 ), and the XRD diffraction peak intensity of the structure in Example 6 ((3) in FIG. 10 ) was three times or more the peak intensity of the XRD diffraction peak intensity of the structure in Example 2 ((1) in FIG. 10 ).

[0103] 7 to 10, the numerical values ​​in the graphs indicate the respective peak intensities.

[0104] As can be seen from the above results, it was found that a BTO film with excellent uniaxial orientation can be fabricated by forming a BSTO film as an intermediate layer. As shown in FIG. 6, when a BTO film was fabricated directly on a seed layer, a crystal phase other than the c-axis phase was formed (Comparative Example 1). To solve this problem, it was effective to provide an intermediate layer that alleviates the lattice mismatch between the seed layer and the ferroelectric layer. The atomic composition of the seed layer in Example 1 was La. ２ O ２ CO ３Although the atomic composition of the seed layer is lanthanum nitrate (BSTO), its similarity to that of BTO is low, and it is believed that the chemical fusion between layers at the crystalline phase interface is low. Therefore, the lattice mismatch mitigation effect is also small, making it difficult to achieve c-axis oriented growth of the BTO layer. On the other hand, as can be seen from the results of Examples 1 and 2, the intermediate layer (BSTO or PZT) forms a c-axis oriented film on the seed layer, thereby reducing the lattice mismatch between layers. For example, the mismatch between the seed layer (lanthanum nitrate) and PZT is 0.7%, and the atomic composition of BSTO and PZT is highly similar to that of BTO, which is expected to mitigate the lattice mismatch. Therefore, by forming an intermediate layer on the seed layer, a ferroelectric thin film with excellent uniaxial orientation was formed.

[0105] <Thermal Stability Test> The structures of Examples 3 and 5 were heated from room temperature to 110°C, and the XRD diffraction patterns at each temperature were analyzed. The diffraction intensity did not change at each temperature, confirming the thermal stability of the crystal structure (Figure 11). Furthermore, the XRD diffraction pattern when returned to room temperature after heating was the same as that before heating.

[0106] <Electron Microscope Analysis> The cross section of the BTO film of the structure of Example 4 was observed with an electron microscope, and it was confirmed that the grain size was 100 nm or less (FIG. 12).

[0107] <Fabrication of Waveguide> A linear waveguide pattern was formed on the BTO film of the structure obtained in Example 1 using an electron beam resist and an electron beam lithography system. Specifically, a resist pattern for a linear waveguide was fabricated on the BTO film of the structure using an electron beam resist (RP6200, manufactured by ALLRESIST) and an electron beam lithography system (G100, manufactured by Elionix) ( FIG. 3( b) ). In the waveguide pattern, the ridge lengths were 2 mm, 4 mm, 6 mm, and 8 mm, the ridge height was 100 nm, and the ridge width was 3 μm. An RIE dry etching system (RIE-10NR, manufactured by Samco, CHF ３The BTO film was etched using a SiO2 / Ar mixed gas (1.8 Pa) to form a linear waveguide pattern as shown in FIGS. 2 to 4. FIG. 2 is a cross-sectional view of the waveguide in the width direction (a cross-sectional view perpendicular to the direction in which the waveguide pattern extends). In FIG. 2, the ridge height is represented by h and the ridge width by w. FIG. 3 is a plan view of the waveguide, a schematic diagram visualizing the waveguide pattern. In FIG. 3, the ridge length is represented by h. The electron beam resist was then removed by oxygen etching (FIG. 3(c)). Next, a top clad layer with a maximum thickness of approximately 1 μm was formed using polymethyl methacrylate resin (PMMA), resulting in a ridge-shaped waveguide 10 (FIG. 3(d)). In FIG. 2, the maximum thickness of the PMMA layer is represented by t.

[0108] <Fabrication of Optical Modulator> The optical modulator 100 was fabricated using a general-purpose Mach-Zehnder interference waveguide structure as shown in FIG. 5. The Mach-Zehnder interference waveguide structure shown in FIG. 5 is a Y-shaped branched waveguide structure in which the amount of light is branched at a 1:1 ratio. Such a Mach-Zehnder interference waveguide structure was formed by the following method. First, a resist pattern for a Y-branched waveguide was fabricated on the BTO film of the structure obtained in the example and comparative example using an electron beam resist and an electron beam lithography system (G100, manufactured by Elionix). Electrodes 50a and 50b were arranged to sandwich the Y-branched waveguide, with an electrode length of 4 mm, an electrode width of 20 mm, and an electrode spacing of 5 μm. An RIE dry etching system (RIE-10NR, CHF, manufactured by Samco) was used. ３ The BTO film was etched using a SiO2 / Ar mixed gas (1.8 Pa) to form a Y-branched waveguide pattern. A 50 nm layer of titanium was deposited on the BTO film with the waveguide pattern formed thereon using a thermal vacuum deposition method, followed by a 0.4 μm layer of aluminum. The electron beam resist was then removed by oxygen etching. Next, a top clad layer with a maximum thickness of approximately 1 μm was formed using polymethyl methacrylate resin (PMMA), completing the optical modulator 100.

[0109] <Optical Modulation Evaluation> The half-wave voltage of the fabricated optical modulator was evaluated using laser light sources with wavelengths of 1550 nm and 1310 nm. The laser light was TE polarized and input to the optical modulator using a polarization-maintaining optical fiber. The modulation voltage was a triangular wave voltage (20 Vpp, 14 kHz) input to electrode 50a using function generator 52. Electrode 50b was used as a ground electrode. The output light from the optical modulator was incident on an optical fiber and connected to an optical detector 60. The output light intensity detected by the optical detector 60 was evaluated using an oscilloscope 70.

[0110] Figure 13 shows the results of an oscilloscope analysis of the optical modulation characteristics. The optical signal intensity output from the modulator was evaluated versus the input voltage when an electrical amplitude signal at a frequency of 14 kHz was input to electrode 50a using function generator 52. A DC electric field of approximately 50 to 60 V / μm was applied between electrodes 50b-50a-50b to polarize the BTO in one direction, and the electric field was oriented at 90°C. The polarization was maintained even after cooling to room temperature and the application of the electric field was stopped. By using electrodes 50b (both sides) as ground electrodes, the modulator operates as a push-pull type modulator. The wavelength of the laser light incident on the modulator was 1.55 mm. Figure 13 shows the output light intensity versus the voltage of the input electrical signal. As shown in Figure 13, a sinusoidal optical signal intensity change based on the electro-optic effect was obtained. The half-wave voltage was 5.1 V.

[0111] 2 substrate 4 seed layer 5 intermediate layer 6 ferroelectric thin film 8 top clad layer 10 structure 12 resist 14 ridge portion 15 waveguide 20 laser 25 optical fiber 30 objective lens 40 optical power meter 50a, 50b electrodes 52 function generator 60 optical detector 70 oscilloscope 100 optical modulator

Claims

1. A structure comprising: a substrate made of an amorphous material; a seed layer laminated on the substrate; an intermediate layer laminated on the seed layer; and a BTO thin film or BSTO thin film laminated on the intermediate layer, wherein the intermediate layer contains, as a main component, a titanium oxide containing at least one element selected from the group consisting of Pb, Zr, La, Ba, Sr, K, Li, Nb, B, Mg, As, P, and Ge.

2. The structure according to claim 1, wherein said intermediate layer contains, as a main component, titanium oxide containing at least one element selected from the group consisting of Pb, Zr, La, Ba and Sr.

3. The structure of claim 1, wherein the intermediate layer is a BSTO film or a PZT film.

4. The structure of claim 1, wherein said seed layer comprises lanthanum nitrate as a major component.

5. The structure according to claim 1, wherein the thickness of the BTO thin film or the BSTO thin film is 50 nm or more.

6. The structure according to claim 1, wherein at least one intermediate layer and at least one BTO or BSTO thin film are laminated in this order on the BTO or BSTO thin film.

7. The structure of claim 1, wherein the thickness of the intermediate layer is 50 nm or less.

8. The structure of claim 1, wherein the substrate is a silicon substrate.

9. The structure of claim 8, wherein said silicon substrate has a thermal oxide layer.

10. A waveguide comprising a structure according to any one of claims 1 to 9.

11. An optical modulator comprising the structure according to any one of claims 1 to 9 and an electrode formed thereon.

12. A method for manufacturing a structure according to any one of claims 1 to 9, comprising the steps of: forming a seed layer on a substrate made of an amorphous material; forming an intermediate layer on the seed layer; and forming a BTO thin film or a BSTO thin film on the intermediate layer.

13. The method for producing the structure of claim 12, wherein the seed layer comprises lanthanum nitrate as a major component.

14. The method for manufacturing the structure according to claim 12, wherein after the step of forming a BTO thin film or a BSTO thin film on the intermediate layer, a step of forming an intermediate layer and a step of forming a BTO thin film or a BSTO thin film are further performed at least once each.

15. The method for producing the structure according to claim 12, wherein the BTO thin film or the BSTO thin film is formed by a sol-gel method.

16. The method for manufacturing a structure according to claim 12, wherein the substrate is a silicon substrate.

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