Board with dielectric thin film, optical waveguide element, and optical modulator

Abstract

To provide a board with dielectric thin film with which cracks hardly occur to a lithium niobate film, which is applicable to a wide range of light from visible light to infrared light, and with which it is possible to form an optical waveguide element and an optical modulator with which light losses are few.SOLUTION: Provided is a board 1 with dielectric thin film comprising a single-crystal board 2, a stress relaxation layer 31, and a dielectric thin film 3. The stress relaxation layer 31 includes a twin crystal structure of LiNbO3 composed of an epitaxial layer of a c axis orientation and containing a first crystal 3a and a second crystal 3b derived by rotating the first crystal 3a 180° around an axis c, and a LiNb3O8 phase. The dielectric thin film 3 is composed of a lithium niobate film which is an epitaxial film of the c axis orientation and includes a twin crystal structure of LiNbO3. The film thickness is 0.5 μm to 2 μm, the maximum domain width is larger than the maximum domain width of the stress relaxation layer 31, and the ratio of the film thickness of the stress relaxation film 31 to the film thickness of the dielectric thin film 3 is 5% to 25%.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a substrate with a dielectric thin film, and an optical waveguide element and an optical modulation element using the same. [Background technology]

[0002] Conventionally, optical waveguide elements and optical modulation elements include those that use a lithium niobate film epitaxially grown on a substrate. For example, Patent Document 1 describes a substrate with a dielectric thin film, which includes a dielectric thin film made of a c-axis oriented lithium niobate film epitaxially formed on the main surface of a single crystal substrate. Patent Document 1 describes that the dielectric thin film has a twin crystal structure including a first crystal and a second crystal obtained by rotating the first crystal by 180° around the c-axis, and that in a pole measurement by an X-ray diffraction method, the ratio of a first diffraction intensity corresponding to the first crystal to a second diffraction intensity corresponding to the second crystal is 0.5 or more and 2.0 or less.

[0003] Patent Document 2 describes an optical modulator having a lithium niobate film having a ridge-shaped portion, which is an epitaxial film formed on the main surface of a single crystal substrate. Patent Document 2 also describes that a lithium niobate film is epitaxially grown on a silicon single crystal substrate via an epi-buffer layer that promotes epitaxial growth. Patent Document 2 also describes that ZrO2 is preferable as the epi-buffer layer.

[0004] Furthermore, Patent Document 3 describes an optical element having a structure in which a ruthenium lower electrode, a lithium niobate thin film that is an electro-optical effect material heteroepitaxially formed thereon, and an upper electrode made of a metal material are sequentially laminated on a hexagonal sapphire substrate having a composition of Al2O3. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2018 / 016428 [Patent Document 2] JP 2014-142411 A [Patent Document 3] JP 2003-15098 A Summary of the Invention [Problem to be solved by the invention]

[0006] An optical waveguide element and an optical modulation element using a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on the substrate are more productive than an optical waveguide element and an optical modulation element using a lithium niobate single crystal substrate. This is because a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on the substrate can form a lithium niobate film of a desired thickness by adjusting the film formation conditions. In other words, when a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on the substrate is used, it is not necessary to adjust the thickness by cutting or polishing the lithium niobate single crystal substrate as in the case of using a lithium niobate single crystal substrate. Moreover, a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on the substrate is less expensive than a lithium niobate single crystal substrate.

[0007] However, in the optical waveguide element and the optical modulation element using the lithium niobate film epitaxially grown on the substrate, the thickness of the lithium niobate film may be a design limitation of the optical waveguide element and the optical modulation element. More specifically, the lithium niobate film epitaxially grown on the substrate preferably has a sufficient film thickness so that the optical waveguide element and the optical modulation element applicable to a wide range of light from visible light to infrared light can be obtained. However, the lithium niobate film epitaxially grown on the substrate is prone to cracking, and has the disadvantage that the thicker the film, the more likely the cracking occurs. For this reason, there is a demand for a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on a substrate, the lithium niobate film being less susceptible to cracking.

[0008] The present invention has been made in consideration of the above problems, and aims to provide a substrate with a dielectric thin film that has a lithium niobate film epitaxially grown on a substrate, in which cracks are unlikely to occur in the lithium niobate film, is applicable to a wide range of light from visible light to infrared light, and is capable of forming optical waveguide elements and optical modulation elements with little optical loss. Another object of the present invention is to provide an optical waveguide element and an optical modulation element which are provided with a substrate having a dielectric thin film made of a lithium niobate film, which is less likely to crack, is applicable to a wide range of light from visible light to infrared light, and has little optical loss. [Means for solving the problem]

[0009] A substrate with a dielectric thin film according to one embodiment of the present invention comprises a single crystal substrate, a stress relaxation layer formed on and in contact with a main surface of the single crystal substrate, and a dielectric thin film formed on and in contact with the stress relaxation layer, the stress relaxation layer being made of a c-axis oriented epitaxial film and including a twin crystal structure of LiNbO3 including a first crystal and a second crystal obtained by rotating the first crystal by 180° around the c-axis, and a LiNb3O8 phase, the dielectric thin film being made of a lithium niobate film which is a c-axis oriented epitaxial film and having the twin crystal structure of LiNbO3, a film thickness of 0.5 μm to 2 μm, a maximum domain width larger than the maximum domain width of the stress relaxation layer, and a ratio of the film thickness of the stress relaxation layer to the film thickness of the dielectric thin film being 5% to 25%. Effect of the Invention

[0010] The substrate with dielectric thin film of the present invention comprises a c-axis oriented epitaxial film formed on the main surface of a single crystal substrate, a twin crystal structure of LiNbO3 including a first crystal and a second crystal obtained by rotating the first crystal by 180° around the c-axis, a stress relaxation layer including a LiNb3O8 phase, and a dielectric thin film formed on the stress relaxation layer, the dielectric thin film being a lithium niobate film that is a c-axis oriented epitaxial film, has the twin crystal structure of LiNbO3, has a film thickness of 0.5 μm to 2 μm, has a maximum domain width larger than the maximum domain width of the stress relaxation layer, and the ratio of the film thickness of the stress relaxation layer to the film thickness of the dielectric thin film is 5% to 25%. Therefore, in the substrate with dielectric thin film of the present invention, the strain and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate and the lithium niobate film are absorbed and relaxed by the stress relaxation layer. Therefore, in the substrate with dielectric thin film of the present invention, cracks are unlikely to occur in the lithium niobate film.

[0011] In addition, since the dielectric thin film of the substrate of the present invention is made of a lithium niobate film, which is an epitaxial film oriented along the c-axis, and is unlikely to crack, the thickness of the dielectric thin film can be set to, for example, 0.5 μm or more, which is suitable for manufacturing optical waveguide elements and optical modulation elements using the substrate with the dielectric thin film. Therefore, the substrate with the dielectric thin film of the present invention can be preferably used for manufacturing optical waveguide elements and optical modulation elements that are applicable to a wide range of light from visible light to infrared light. Furthermore, in the substrate with a dielectric thin film of the present invention, the ratio of the thickness of the stress relaxation layer to the thickness of the dielectric thin film is 25% or less, so that the stress relaxation layer does not affect the characteristics of the optical waveguide element and the optical modulation element, and optical waveguide element and optical modulation element with low optical loss can be formed.

[0012] The optical waveguide element and optical modulation element of the present invention include a substrate with a dielectric thin film having a lithium niobate film that is resistant to cracks. Therefore, even if annealing is performed during the manufacturing process of the optical waveguide element and / or optical modulation element, the lithium niobate film of the substrate with the dielectric thin film is resistant to cracks, resulting in excellent productivity. Furthermore, since the lithium niobate film of the substrate with the dielectric thin film is resistant to cracks, the optical waveguide element and optical modulation element have excellent durability. In addition, the substrate with a dielectric thin film of the present invention has a dielectric thin film made of a lithium niobate film having a sufficiently thick thickness, and the ratio of the thickness of the stress relaxation layer to the thickness of the dielectric thin film is appropriate, so that it can be applied to a wide range of light from visible light to infrared light, and becomes an optical waveguide element and an optical modulation element with little optical loss. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view showing a substrate 1 with a dielectric thin film according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a plan view showing an example of an optical waveguide element 100 using the substrate 1 with the dielectric thin film shown in FIG. [Diagram 3] FIG. 3 is a cross-sectional view of the optical waveguide element 100 shown in FIG. 2 taken along line AA'. [Figure 4] FIG. 4 is a plan view showing an example of a Mach-Zehnder type optical modulation element 200A using the dielectric thin film-formed substrate 1 shown in FIG. [Diagram 5] FIG. 5 is a cross-sectional view of the light modulation element 200A shown in FIG. 4 taken along the line BB'. [Figure 6] Fig. 6 is a photograph showing an image obtained by mapping analysis of the interface between the single crystal substrate 2 and the stress relaxation layer 31 of Example 5. Fig. 6(a) and Fig. 6(b) are photographs of the first crystal 3a and the second crystal 3b forming the twin structure of LiNbO3 contained in the stress relaxation layer 31. Fig. 6(c) is a photograph of the LiNb3O8 phase 3c contained in the stress relaxation layer 31. [Figure 7]Fig. 7 is a photograph showing an image obtained by mapping analysis of the interface between the single crystal substrate 2 and the dielectric thin film 3 of Comparative Example 1. Fig. 7(a) and Fig. 7(b) are photographs of the first crystal 3a and the second crystal 3b forming the twin structure of LiNbO3 contained in the dielectric thin film 3. Fig. 7(c) is a photograph of the LiNb3O8 phase 3c contained in the dielectric thin film 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The present inventors have conducted extensive research as described below in order to solve the above problems and to suppress the occurrence of cracks in a lithium niobate film in a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on a single crystal substrate. That is, in a substrate with a dielectric thin film in which an epitaxially grown lithium niobate film is formed on and in contact with a single crystal substrate, large distortions and stresses are generated at the interface between the single crystal substrate and the lithium niobate film due to differences in lattice constants and linear expansion coefficients between the single crystal substrate and the lithium niobate film.

[0015] For this reason, in the conventional technology, cracks were likely to occur in the lithium niobate film when the lithium niobate film was epitaxially grown in the manufacturing process of the substrate with the dielectric thin film. Furthermore, the above-mentioned strain and stress become larger as the thickness of the lithium niobate film is increased, so that the thicker the lithium niobate film is, the more likely the substrate with the dielectric thin film is to have cracks in the lithium niobate film during and after the manufacturing process. Furthermore, when a process such as annealing is performed when manufacturing an optical modulation element or the like using a substrate with the dielectric thin film, the strain and stress caused by the difference in linear expansion coefficient between the single crystal substrate and the lithium niobate film are likely to become larger. For this reason, when manufacturing an optical modulation element or the like using a substrate with the dielectric thin film, cracks were likely to occur in the lithium niobate film during and after the manufacturing process.

[0016] The present inventors have proposed a method for suppressing the occurrence of cracks in the lithium niobate film by using a c-axis oriented epitaxial film having a twin crystal structure of LiNbO3 including a first crystal and a second crystal obtained by rotating the first crystal by 180° around the c-axis as the lithium niobate film. In this case, it is presumed that the twin crystal structure reduces the distortion and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate and the lithium niobate film, making the lithium niobate film less susceptible to cracks.

[0017] However, even when the above-mentioned epitaxial film having a twin crystal structure and a c-axis orientation is used as the lithium niobate film, it is not possible to sufficiently suppress the occurrence of cracks in the lithium niobate film epitaxially grown on the single crystal substrate. Therefore, there is a demand for a substrate with a dielectric thin film in which the occurrence of cracks in the lithium niobate film is further suppressed.

[0018] Therefore, the inventors considered that it would be sufficient to epitaxially grow a c-axis oriented lithium niobate film having the above-mentioned twin crystal structure on a single crystal substrate via a stress relaxation layer, and conducted extensive research focusing on the film formation conditions of the lithium niobate film. As a result, it was found that, before epitaxially growing a c-axis oriented lithium niobate film having the above-mentioned twin structure on a single crystal substrate, a stress relaxation layer consisting of a c-axis oriented epitaxial film containing the above-mentioned twin structure and LiNb3O8 phase and having a maximum domain width smaller than that of the lithium niobate film having the above-mentioned twin structure can be formed using a material having a higher Nb content and a lower Li content than the material used to form the lithium niobate film having the above-mentioned twin structure.

[0019] When a c-axis oriented lithium niobate film having the above-mentioned twin crystal structure and a thickness of 2 μm or less is epitaxially grown on and in contact with the stress relaxation layer formed in this manner, so that the ratio of the thickness of the stress relaxation layer to the thickness of the epitaxially grown lithium niobate film is 5% or more, it is estimated that the distortion and stress generated at the interface between the single crystal substrate and the lithium niobate film are relaxed by the stress relaxation layer, which has a sufficient thickness relative to the thickness of the epitaxially grown lithium niobate film, and the occurrence of cracks in the lithium niobate film is suppressed.

[0020] More specifically, even when the single crystal substrate and the lithium niobate film are in contact with each other through a stress relaxation layer, strain and stress occur due to the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate and the lithium niobate film. However, when the single crystal substrate and the lithium niobate film are in contact with each other through a stress relaxation layer, before the above strain and stress cause cracks to occur in the lithium niobate film, fine nanocracks, which are the initial stage of cracks, occur in the stress relaxation layer. The extension of the nanocracks that occur in the stress relaxation layer is limited within the range of the stress relaxation layer. It is presumed that this causes the above strain and stress to be absorbed and relaxed by the stress relaxation layer, suppressing the occurrence of cracks in the lithium niobate film.

[0021] The inventors have also discovered that by setting the thickness of the lithium niobate film grown on and in contact with the stress relaxation layer to 0.5 μm or more, a substrate with a dielectric thin film can be obtained that is suitable for use in manufacturing optical waveguide elements and optical modulation elements that can be applied to a wide range of light from visible light to infrared light, and further, by setting the ratio of the thickness of the stress relaxation layer to the thickness of the lithium niobate film to 25% or less, a substrate with a dielectric thin film can be obtained that is capable of forming optical waveguide elements and optical modulation elements with sufficiently low optical loss.

[0022] In addition, the stress relaxation layer is made of a c-axis oriented epitaxial film whose maximum domain width is smaller than that of the lithium niobate film having the above-mentioned twin crystal structure, and contains a LiNbO3 twin crystal structure and a LiNb3O8 phase, so that it has good bonding with the c-axis oriented lithium niobate film having the above-mentioned twin crystal structure epitaxially grown thereon, and has little effect on the characteristics of the optical modulation element having a substrate with a dielectric thin film.

[0023] In contrast, for example, if an epi-buffer layer made of ZrO2, which is a material completely different from lithium niobate, is provided instead of the above-mentioned stress relaxation layer, poor bonding with the lithium niobate film is likely to occur, and this will have a significant impact on characteristics such as optical loss in an optical modulation element having a substrate with a dielectric thin film.

[0024] Furthermore, the inventors have confirmed that a substrate with a dielectric thin film in which a single crystal substrate and a lithium niobate film are in contact with each other via the above-mentioned stress relaxation layer is less susceptible to cracks in the lithium niobate film even when annealed, and optical waveguide elements and optical modulation elements with low optical loss can be manufactured with good yield, and have arrived at the present invention.

[0025] The present invention includes the following aspects. [1] A single crystal substrate; a stress relief layer formed on and in contact with a main surface of the single crystal substrate; a dielectric thin film formed on and in contact with the stress relaxation layer, the stress relaxation layer is made of a c-axis oriented epitaxial film, and includes a twin crystal structure of LiNbO3 including a first crystal and a second crystal obtained by rotating the first crystal by 180° about the c-axis, and a LiNb3O8 phase; the dielectric thin film is made of a lithium niobate film which is a c-axis oriented epitaxial film, has a twin crystal structure of the LiNbO3, has a film thickness of 0.5 μm to 2 μm, and has a maximum domain width larger than the maximum domain width of the stress relaxation layer; A substrate with a dielectric thin film, wherein the ratio of the thickness of the stress relaxation layer to the thickness of the dielectric thin film is 5% to 25%.

[0026] [2] The substrate with a dielectric thin film according to [1], wherein the stress relaxation layer has a maximum domain width of 20 nm to 70 nm.

[0027] [3] The substrate with a dielectric thin film according to [1], wherein the single crystal substrate is a sapphire single crystal substrate, the main surface of which is a c-plane. [4] The substrate with a dielectric thin film described in [1], wherein the twin crystal structure of the dielectric thin film has a ratio of a first diffraction intensity corresponding to the first crystal to a second diffraction intensity corresponding to the second crystal of 0.5 or more and 2.0 or less in a pole measurement by an X-ray diffraction method.

[0028] [5] An optical waveguide element comprising a substrate with a dielectric thin film according to any one of [1] to [4]. [6] An optical modulation element comprising a substrate with a dielectric thin film according to any one of [1] to [4].

[0029] The substrate with dielectric thin film, the optical waveguide element, and the optical modulation element of the present embodiment will be described in detail below with reference to the drawings as appropriate. The drawings used in the following description may show characteristic parts in an enlarged scale for the sake of convenience in order to make the features of the present invention easier to understand. Therefore, the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited to them, and may be modified as appropriate within the scope of the present invention.

[0030] [Substrate with thin dielectric film] Fig. 1 is a schematic cross-sectional view showing a dielectric thin film-attached substrate 1 according to one embodiment of the present invention. As shown in Fig. 1, the dielectric thin film-attached substrate 1 of this embodiment has a single crystal substrate 2, a stress relief layer 31 formed in contact with a main surface 2a of the single crystal substrate 2, and a dielectric thin film 3 formed in contact with the stress relief layer 31.

[0031] The stress relaxation layer 31 of the substrate 1 with the dielectric thin film of this embodiment is made of an epitaxial film oriented along the c-axis, and contains a twin crystal structure of LiNbO3 and a LiNb3O8 phase 3c, which will be described later. The dielectric thin film 3 of the dielectric thin film-coated substrate 1 of this embodiment is made of a lithium niobate film which is an epitaxial film oriented along the c-axis, has a twin crystal structure of LiNbO3 similar to the twin crystal structure of LiNbO3 of the stress relaxation layer 31, has a film thickness of 0.5 μm to 2 μm, and has a maximum domain width larger than that of the stress relaxation layer 31. In the substrate 1 with the dielectric thin film of this embodiment, the ratio of the thickness of the stress relaxation layer 31 to the thickness of the dielectric thin film 3 is 5% to 25%.

[0032] (Single crystal substrate 2) The single crystal substrate 2 may be any known single crystal substrate as long as it is capable of forming the stress relaxation layer 31 including the twin crystal structure of LiNbO3 and the LiNb3O8 phase 3c as an epitaxial film. The single crystal substrate 2 preferably has a refractive index lower than that of lithium niobate (LiNbO3), and may be, for example, a sapphire single crystal substrate or a silicon single crystal substrate.

[0033] In the substrate 1 with the dielectric thin film of this embodiment, it is particularly preferable to use a sapphire single crystal substrate as the single crystal substrate 2. Since the sapphire single crystal substrate has a lower refractive index than LiNbO3, for example, when the dielectric thin film 3 of the substrate 1 with the dielectric thin film is used as an optical waveguide layer of an optical modulation element, it can play the role of a cladding layer. Therefore, when the single crystal substrate 2 is a sapphire single crystal substrate, the dielectric thin film 3 can be suitably used as an optical waveguide layer of an optical modulation element without providing a separate cladding layer between the single crystal substrate 2 and the dielectric thin film 3.

[0034] The stress relaxation layer 31 of the dielectric thin film-attached substrate 1 of this embodiment and the dielectric thin film 3 formed on the single crystal substrate 2 via the stress relaxation layer 31 are likely to be formed as c-axis oriented epitaxial films for single crystal substrates 2 of various crystal orientations. For this reason, the crystal orientation of the single crystal substrate 2 is not particularly limited.

[0035] In the substrate 1 with the dielectric thin film of this embodiment, the stress relaxation layer 31 is made of a c-axis oriented epitaxial film and contains a twin crystal structure of LiNbO3 and a LiNb3O8 phase 3c, and the dielectric thin film 3 formed on the single crystal substrate 2 via the stress relaxation layer 31 is made of a lithium niobate film which is a c-axis oriented epitaxial film and has a twin crystal structure of LiNbO3, and has three-fold symmetry. Therefore, it is desirable that the crystal orientation of the main surface 2a of the single crystal substrate 2 has the same symmetry as that of the dielectric thin film 3. Therefore, when a sapphire single crystal substrate is used as the single crystal substrate 2, for example, it is preferable that the main surface 2a is a c-plane. Also, when a silicon single crystal substrate is used as the single crystal substrate 2, for example, it is preferable that the main surface 2a is a (111) plane.

[0036] (Stress relief layer 31) The stress relaxation layer 31 is made of a c-axis oriented epitaxial film. The epitaxial film forming the stress relaxation layer 31 is mainly composed of lithium niobate (LiNbO3). Lithium niobate has a large electro-optic constant and is therefore suitable as a material for the optical waveguide layer of an optical modulation element.

[0037] The composition of the epitaxial film forming the stress relaxation layer 31 is such that the composition of the twin crystal structure of LiNbO3 and the composition of the LiNb3O8 phase 3c coexist. Therefore, the composition of the epitaxial film forming the stress relaxation layer 31 has a smaller amount of Li2O than the composition of the twin crystal structure of LiNbO3 at a ratio corresponding to the content of the LiNb3O8 phase 3c. Specifically, the composition of the epitaxial film forming the stress relaxation layer 31 preferably has a smaller amount of Li2O than the composition of the twin crystal structure of LiNbO3 at a ratio of 35 to 48 mol%.

[0038] The stress relaxation layer 31 includes a twin crystal structure of LiNbO3 and a LiNb3O8 phase 3c. The twin crystal structure of LiNbO3 forming the stress relaxation layer 31 includes a first crystal 3a and a second crystal 3b obtained by rotating the first crystal 3a by 180° around the c-axis. As shown in FIG. 1, the first crystal 3a, the second crystal 3b, and the LiNb3O8 phase 3c forming the stress relaxation layer 31 are all grown substantially perpendicular to the main surface 2a of the single crystal substrate 2.

[0039] In this embodiment, the first crystal 3a included in the twin crystal structure of LiNbO3 forming the stress relaxation layer 31 refers to a LiNbO3 crystal whose in-plane orientation is the same as that of the single crystal substrate 2. The second crystal 3b refers to a LiNbO3 crystal whose in-plane orientation is rotated 180° with respect to that of the single crystal substrate 2.

[0040] The first crystal 3a and the second crystal 3b forming the stress relaxation layer 31 are both close to single crystals. However, if the first crystal 3a and the second crystal 3b are too close to a complete single crystal, the effect of relaxing the distortion and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film forming the dielectric thin film 3 due to the above-mentioned twin crystal structure is reduced, and cracks may easily occur in the lithium niobate film. For this reason, it is preferable that the half-width of the rocking curve of the (006) reflection of the first crystal 3a and the second crystal 3b measured by the X-ray diffraction method is in the range of 0.3° or more and 0.6° or less. When the half-width of the rocking curve of the (006) reflection of the first crystal 3a and the second crystal 3b is within this range, the first crystal 3a and the second crystal 3b can be regarded as optically single crystals. Moreover, the first crystals 3a and the second crystals 3b are not too close to being completely single crystals, and therefore the occurrence of cracks in the lithium niobate film can be effectively suppressed.

[0041] In a cross-sectional view, the ratio of the area of ​​the LiNbO3 twin structure to the total area of ​​the LiNbO3 twin structure and the LiNb3O8 phase 3c contained in the epitaxial film forming the stress relaxation layer 31 can be, for example, in the range of 0.6 to 0.95, and preferably in the range of 0.7 to 0.9. The ratio of the area of ​​the LiNb3O8 phase 3c to the above total area can be, for example, in the range of 0.05 to 0.4, and preferably in the range of 0.1 to 0.3.

[0042] When the ratio of the area of ​​the LiNbO3 twin structure to the total area of ​​the LiNbO3 twin structure and the LiNb3O8 phase 3c contained in the epitaxial film forming the stress relaxation layer 31 is 0.6 or more (in other words, the ratio of the area of ​​the LiNb3O8 phase 3c to the above total area is 0.4 or less), the crystal orientation of the lithium niobate film of the dielectric thin film 3 formed on the stress relaxation layer 31 is likely to be aligned with the crystal orientation of the stress relaxation layer 31. Therefore, when the dielectric thin film 3 of the substrate 1 with the dielectric thin film is used as an optical waveguide layer of an optical modulation element, the stress relaxation layer 31 is unlikely to affect the characteristics such as optical loss of the optical modulation element.

[0043] In addition, when the ratio of the area of ​​the LiNbO3 twin structure to the total area of ​​the LiNbO3 twin structure and the LiNb3O8 phase 3c contained in the epitaxial film forming the stress relaxation layer 31 in plan view is 0.95 or less (in other words, the ratio of the area of ​​the LiNb3O8 phase 3c to the total area is 0.05 or more), the stress relaxation layer 31 contains a sufficient amount of LiNb3O8 phase 3c, so that the maximum domain width of the stress relaxation layer 31 tends to be small. As a result, the stress relaxation layer 31 has a remarkable effect of relaxing the distortion and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film forming the dielectric thin film 3. Therefore, the occurrence of cracks in the lithium niobate film can be more effectively suppressed.

[0044] In a cross-sectional view, the ratio of the area of ​​the LiNbO3 twin structure and the LiNb3O8 phase 3c to the total area of ​​the LiNbO3 twin structure and the LiNb3O8 phase 3c contained in the epitaxial film forming the stress relaxation layer 31 can be calculated, for example, by the method shown below. The cross section of the substrate 1 with the dielectric thin film is observed at a magnification of 100,000 to 400,000 times by a scanning transmission electron microscope (manufactured by FEI). By using the scanning transmission electron microscope, an image in which the twin structure of LiNbO3 and the LiNb3O8 phase 3c can be clearly distinguished can be obtained. In the obtained image, the area of ​​the twin structure of LiNbO3 and the area of ​​the LiNb3O8 phase 3c existing in a range of 200 nm to 800 nm in length on the interface between the single crystal substrate 2 and the stress relaxation layer 31 are measured. Using the results, the ratio of the area of ​​the twin structure of LiNbO3 and the ratio of the area of ​​the LiNb3O8 phase 3c to the total area of ​​the area of ​​the twin structure of LiNbO3 and the area of ​​the LiNb3O8 phase 3c are calculated.

[0045] In the twin crystal structure of LiNbO3 in the stress relaxation layer 31 of this embodiment, in a pole measurement by X-ray diffraction, the ratio of the first diffraction intensity corresponding to the first crystal 3a to the second diffraction intensity corresponding to the second crystal 3b is preferably 0.5 to 2.0, more preferably 0.8 to 1.25, and more preferably closer to 1.0. In the pole measurement by X-ray diffraction, the ratio of the first diffraction intensity corresponding to the first crystal 3a to the second diffraction intensity corresponding to the second crystal 3b corresponds to the ratio of the first crystal 3a to the second crystal 3b.

[0046] The more uniform the ratio of the first crystals 3a and the second crystals 3b in the stress relaxation layer 31, the more uniform the ratio of the first crystals 3a and the second crystals 3b in the twin crystal structure of LiNbO3 in the stress relaxation layer 31, and the more uniform the ratio of the first crystals 3a and the second crystals 3b in the twin crystal structure of LiNbO3 in the dielectric thin film 3. The more uniform the ratio of the first crystals 3a and the second crystals 3b in the twin crystal structure of LiNbO3 in the dielectric thin film 3, the more effectively the twin crystal structure can relieve distortion and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film. Therefore, cracks are less likely to occur in the lithium niobate film.

[0047] The epitaxial film forming the stress relaxation layer 31 may consist only of a twin structure of LiNbO3 and a LiNb3O8 phase 3c, or may contain phases other than the twin structure of LiNbO3 and the LiNb3O8 phase 3c, and is preferably composed only of a twin structure of LiNbO3 and a LiNb3O8 phase 3c.

[0048] The maximum domain width of the stress relaxation layer 31 is smaller than that of the dielectric thin film 3 (in other words, the maximum domain width of the dielectric thin film 3 is larger than that of the stress relaxation layer 31). As a result, in the substrate 1 with the dielectric thin film of this embodiment, the strain and stress generated at the interface between the single crystal substrate 2 and the dielectric thin film 3 are relaxed by the stress relaxation layer 31.

[0049] In this embodiment, the maximum domain width of the stress relaxation layer 31 means the following dimension: That is, the maximum domain width in the direction perpendicular to the thickness direction is measured for each of the first crystals 3a and the second crystals 3b and the LiNb3O8 phases 3c included in the twin crystal structure of LiNbO3 that exists in a length range of 200 nm to 800 nm on the interface between the single crystal substrate 2 and the stress relaxation layer 31, and the maximum dimension of the maximum domain width of the multiple first crystals 3a, multiple second crystals 3b, and multiple LiNb3O8 phases 3c that exists in a length range of 200 nm to 800 nm is extracted from the results.

[0050] In this embodiment, the maximum domain width is used as an index of the domain width of the stress relaxation layer 31. The maximum domain width of the stress relaxation layer 31 can be measured easily and accurately, for example, compared with the average value of the domain width. This is because the first crystal 3a, the second crystal 3b, and the LiNb3O8 phase 3c included in the twin crystal structure of LiNbO3 of the stress relaxation layer 31 include those with very narrow domain widths. More specifically, it is not easy to distinguish each of the first crystal 3a, each of the second crystal 3b, and each of the LiNb3O8 phase 3c included in the twin crystal structure of LiNbO3 and measure the domain width of each of them in order to obtain the average value of the domain width of the stress relaxation layer 31.

[0051] In this embodiment, the maximum domain width of the dielectric thin film 3 means the following dimension: That is, the maximum domain width in the direction perpendicular to the thickness direction is measured for each of the first crystals 3a and each of the second crystals 3b included in the twin crystal structure of LiNbO3, which exists in a length range of 200 nm to 800 nm on the interface between the single crystal substrate 2 and the stress relaxation layer 31, and the maximum dimension of the maximum domain width of the multiple first crystals 3a and multiple second crystals 3b, which exists in a length range of 200 nm to 800 nm, is extracted from the results.

[0052] In this embodiment, the maximum domain width of the stress relaxation layer 31 is preferably 20 nm to 70 nm. If the maximum domain width of the stress relaxation layer 31 is 20 nm or more, the domains of the stress relaxation layer 31 epitaxially grown on the main surface 2a of the single crystal substrate 2 are not easily interrupted, and the stress relaxation layer 31 having a uniform thickness is easily formed, which is preferable. If the maximum domain width of the stress relaxation layer 31 is 70 nm or less, the stress relaxation layer 31 can effectively relax the strain and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film forming the dielectric thin film 3. Therefore, the lithium niobate film is more unlikely to crack. The maximum domain width of the stress relaxation layer 31 is more preferably 20 nm to 50 nm, and further preferably 20 nm to 40 nm.

[0053] (Dielectric Thin Film 3) The dielectric thin film 3 is made of a lithium niobate film, which is an epitaxial film oriented along the c-axis. The lithium niobate film forming the dielectric thin film 3 is mainly composed of lithium niobate (LiNbO3). Lithium niobate has a large electro-optic constant and is therefore suitable as a material for the optical waveguide layer of an optical modulation element.

[0054] The composition of the lithium niobate film forming the dielectric thin film 3 is represented by the general formula LixNbAyOz (wherein A is an element other than Li, Nb and O, x is 0.5 to 1.2, y is 0 to 0.5, and z is 1.5 to 4). In the formula, A represents an element other than Li, Nb, and O. Examples of elements represented by A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce. The element represented by A may be one type only, or two or more types. In the formula, x is an integer of 0.5 to 1.2, and preferably 0.9 to 1.05. In the formula, y is 0 to 0.5. In the formula, z is from 1.5 to 4, and preferably from 2.5 to 3.5.

[0055] The lithium niobate film forming the dielectric thin film 3 has a twin crystal structure of LiNbO3 including a first crystal 3a and a second crystal 3b obtained by rotating the first crystal 3a by 180° around the c-axis, similar to the epitaxial film forming the stress relaxation layer 31 described above. The twin crystal structure of LiNbO3 possessed by the lithium niobate film forming the dielectric thin film 3 is similar to the twin crystal structure of LiNbO3 contained in the epitaxial film forming the above-mentioned stress relaxation layer 31, and it is preferable that the half-width of the rocking curve of the (006) reflection measured by X-ray diffraction method of both the first crystal 3a and the second crystal 3b is in the range of 0.3° or more and 0.6° or less.

[0056] In the twin crystal structure of LiNbO3 of the lithium niobate film forming the dielectric thin film 3, the ratio of the first diffraction intensity corresponding to the first crystal 3a to the second diffraction intensity corresponding to the second crystal 3b is preferably 0.5 or more and 2.0 or less in a pole measurement by an X-ray diffraction method. The more uniform the ratio of the first crystal 3a to the second crystal 3b in the twin crystal structure of LiNbO3 of the dielectric thin film 3, the more effectively the above twin crystal structure can relieve the distortion and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film. Therefore, the lithium niobate film is less likely to crack. The above ratio in the dielectric thin film 3 is more preferably 0.8 or more and 1.25 or less, and more preferably closer to 1.0.

[0057] The twin crystal structure of LiNbO3 in the lithium niobate film forming the dielectric thin film 3 is preferably such that the first crystal 3a and the second crystal 3b are bonded to each other without a grain boundary. If a grain boundary exists at the boundary between the first crystal 3a and the second crystal 3b, light scattering occurs at the boundary surface. This is because, when the dielectric thin film 3 of the substrate 1 with the dielectric thin film is used as an optical waveguide layer of an optical modulation element, the optical loss of the optical modulation element increases. In contrast, when the first crystal 3a and the second crystal 3b are bonded to each other and no grain boundary exists at the boundary between them, the refractive indexes of both are the same, so no light scattering occurs. Therefore, the lithium niobate film can obtain optical characteristics equivalent to those of a single crystal.

[0058] The lithium niobate film forming the dielectric thin film 3 is desirably a single phase consisting of a twin crystal phase of LiNbO3. The lithium niobate film forming the dielectric thin film 3 may contain the LiNb3O8 phase 3c to the extent that it does not affect the characteristics such as optical loss of the optical modulation element when the dielectric thin film 3 of the substrate 1 with the dielectric thin film is used as an optical waveguide layer of the optical modulation element.

[0059] When the lithium niobate film forming the dielectric thin film 3 contains the LiNb3O8 phase 3c, the LiNb3O8 phase 3c does not grow in a direction substantially perpendicular to the main surface 2a of the single crystal substrate 2. In contrast, as shown in Fig. 1, the LiNb3O8 phase 3c forming the stress relaxation layer 31 grows substantially perpendicular to the main surface 2a of the single crystal substrate 2. Therefore, even when the lithium niobate film forming the dielectric thin film 3 contains the LiNb3O8 phase 3c, the dielectric thin film 3 and the stress relaxation layer 31 can be easily distinguished.

[0060] The thickness of the dielectric thin film 3 is 0.5 μm or more. Since the thickness of the dielectric thin film 3 is 0.5 μm or more, when the dielectric thin film 3 of the substrate 1 with the dielectric thin film is used as an optical waveguide layer of an optical modulation element, it is applicable to a wide range of light from visible light to infrared light. The thickness of the dielectric thin film 3 is 2 μm or less. Because the thickness of the dielectric thin film 3 is 2 μm or less, the occurrence of cracks in the lithium niobate film forming the dielectric thin film 3 can be effectively suppressed.

[0061] In the substrate 1 with dielectric thin film of this embodiment, the ratio of the film thickness of the stress relaxation layer 31 to the film thickness of the dielectric thin film 3 ((stress relaxation layer 31 / dielectric thin film 3)×100(%)) is 5% or more. Since the ratio of the film thickness of the stress relaxation layer 31 to the film thickness of the dielectric thin film 3 is 5% or more, the stress relaxation layer 31 can effectively relax distortion and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film forming the dielectric thin film 3. Therefore, cracks are less likely to occur in the lithium niobate film.

[0062] The ratio of the thickness of the stress relaxation layer 31 to the thickness of the dielectric thin film 3 is 25% or less. Since the ratio of the thickness of the stress relaxation layer 31 to the thickness of the dielectric thin film 3 is 25% or less, when the dielectric thin film 3 of the substrate 1 with the dielectric thin film is used as an optical waveguide layer of an optical modulation element, the stress relaxation layer 31 does not affect the characteristics such as optical loss of the optical modulation element. The ratio of the thickness of the stress relaxation layer 31 to the thickness of the dielectric thin film 3 is preferably 5% to 20%, more preferably 5% to 15%.

[0063] In the substrate 1 with the dielectric thin film of this embodiment, the stress relaxation layer 31 and the dielectric thin film 3 are epitaxial films formed by epitaxial growth. Therefore, the crystal orientation of the lithium niobate film forming the dielectric thin film 3 is aligned with the crystal orientation of the underlying single crystal substrate 2 and the underlying stress relaxation layer 31. More specifically, when the film plane of the lithium niobate film forming the dielectric thin film 3 is defined as the XY plane and the film thickness direction is defined as the Z axis, the crystals of the single crystal substrate 2, the epitaxial film forming the stress relaxation layer 31, and the epitaxial film forming the dielectric thin film 3 are aligned in the X-axis, Y-axis, and Z-axis directions.

[0064] The fact that the stress relaxation layer 31 and the dielectric thin film 3 are epitaxial films can be proven, for example, by first confirming the peak intensity at the orientation position by 2θ-θ X-ray diffraction, and second confirming the pole by pole measurement.

[0065] Specifically, in order to prove that the stress relaxation layer 31 and the dielectric thin film 3 are epitaxial films, the first condition is that when measured by 2θ-θ X-ray diffraction, all peak intensities other than the target plane must be 10% or less, preferably 5% or less, of the maximum peak intensity of the target plane. In the c-axis oriented epitaxial film forming the stress relaxation layer 31 and the dielectric thin film 3, the peak intensities other than the (00L) plane must be 10% or less, preferably 5% or less, of the maximum peak intensity of the (00L) plane. (00L) is a general designation for equivalent planes such as (001) and (002).

[0066] The above-mentioned conditions for checking the peak intensity at the orientation position by 2θ-θ X-ray diffraction only show the orientation in one direction. Therefore, even if the above-mentioned first condition is met, if the crystal orientation is not uniform within the plane, the intensity of the X-ray will not increase at a specific angle position and no pole will be observed. Therefore, in order to prove that the stress relieving layer 31 and the dielectric thin film 3 are epitaxial films, the second condition is that the poles must be visible in the pole measurement.

[0067] Since LiNbO3 has a trigonal crystal structure, there are three poles of LiNbO3(014) in a single crystal. It is known that when a lithium niobate film is epitaxially grown, it grows epitaxially in a so-called twin state, in which crystals rotated 180° around the c-axis are symmetrically bonded. In this case, two of the three poles are symmetrically bonded, resulting in six poles.

[0068] In the substrate 1 with dielectric thin film of this embodiment, when a sapphire single crystal substrate having a c-plane as the main surface 2a is used as the single crystal substrate 2, the c-axis of the epitaxial film forming the stress relaxation layer 31 and the c-axis of the lithium niobate film forming the dielectric thin film 3 are preferably misaligned by 5° or less among the c-axes of the stress relaxation layer 31, the dielectric thin film 3, and the single crystal substrate 2, and more preferably all coincide with each other. If the c-axis misalignment of the stress relaxation layer 31, the dielectric thin film 3, and the single crystal substrate 2 is 5° or less, no practical problem will arise in the characteristics of the optical modulation element using the substrate 1 with dielectric thin film.

[0069] [Method of manufacturing substrate with dielectric thin film] Next, a method for producing the dielectric thin film-coated substrate 1 of this embodiment will be described with an example. To manufacture the substrate 1 with a dielectric thin film of this embodiment, first, the stress relaxation layer 31 is formed on and in contact with the main surface 2a of the single crystal substrate 2 (stress relaxation layer forming step), and then the dielectric thin film 3 is formed on and in contact with the stress relaxation layer 31 (dielectric thin film forming step).

[0070] (Stress relaxation layer deposition process) In the stress relaxation layer formation step, the stress relaxation layer 31 is formed by epitaxial growth on the main surface 2a of the single crystal substrate 2. The stress relaxation layer 31 can be formed by, for example, sputtering, vacuum deposition, pulsed laser ablation (PLD), chemical vapor deposition (CVD), sol-gel method, or the like.

[0071] Of the above methods, it is preferable to use the sputtering method as the film formation method for the stress relaxation layer 31. By forming the stress relaxation layer 31 using the sputtering method, a single domain structure can be obtained in the as-formed state without performing any special processing after film formation. This is because the heat applied during sputtering and the electric field due to the self-bias also serve as a polarization process. If there is a distribution in the polarization, this will cause the electro-optic effect to decrease. With a single domain structure, it is possible to obtain an electro-optic coefficient similar to that of a single crystal.

[0072] When a sputtering method is used as a method for depositing the stress relaxation layer 31, a target having a composition that will be a target film composition is used. Specifically, for example, a target having a composition in the range of Li / (Li+Nb)=48% to 51% can be used. The target can be produced, for example, by the following method. As the raw material, for example, a sintered body mainly composed of Li2CO3 and Nb2O5 with a purity of 3N or more is prepared. Next, the raw material is pulverized and mixed using a ball mill using balls made of ZrO2 to obtain a target powder material. The obtained target powder material is sintered using a known method to obtain a target.

[0073] In the target manufacturing process, when the raw material is pulverized using the ball mill, balls made of ZrO2 are cut off. Therefore, several hundred ppm of Zr is mixed into the target powder obtained after the raw material is pulverized and mixed. Therefore, Zr is also mixed into the target obtained by sintering the target powder. However, since the amount of Zr contained in the target is small, the stress relaxation layer 31 can be epitaxially grown on the main surface 2a of the single crystal substrate 2 by sputtering using the target containing Zr without any problems. Therefore, no problems occur due to Zr being mixed into the target.

[0074] The shape of the target used for depositing the stress relaxation layer 31 is not particularly limited. The target is preferably circular with a planar area at least twice that of the single crystal substrate 2, so that the stress relaxation layer 31 having a uniform thickness can be obtained. Moreover, the stress relaxation layer 31 is preferably deposited by arranging the target coaxially with the single crystal substrate 2, so that the stress relaxation layer 31 having a uniform thickness can be obtained.

[0075] When a sputtering method is used as a deposition method for the stress relaxation layer 31, for example, a mixed gas of Ar and O2 is used as the sputtering gas, the O2 ratio in the sputtering gas is set to 35% to 65%, the gas pressure is set to 0.08 Pa to 0.3 Pa, the temperature of the single crystal substrate 2 is set to 450 to 700°C, and a power of 1500 to 2000 W is applied, so that the deposition rate is 300 to 1800 nm / h. It is preferable that the stress relaxation layer 31 is formed in a so-called single step without changing the film formation conditions midway.

[0076] (Dielectric thin film deposition process) In the dielectric thin film formation process, the dielectric thin film 3 is formed by epitaxial growth on the stress relaxation layer 31. The dielectric thin film 3 can be formed by, for example, a sputtering method, a vacuum deposition method, a pulsed laser ablation (PLD) method, a chemical vapor deposition (CVD) method, a sol-gel method, or the like.

[0077] Of the above methods, sputtering is preferably used as the method for forming the dielectric thin film 3. By forming the dielectric thin film 3 using the sputtering method, a single domain structure can be obtained in the as-formed state without any special treatment after film formation. This is because the heat applied during sputtering and the electric field due to the self-bias also serve as a polarization process. Distribution of polarization is a factor in reducing the electro-optic effect. With a single domain structure, it is possible to obtain an electro-optic coefficient similar to that of a single crystal.

[0078] When sputtering is used as a method for depositing the dielectric thin film 3, a target having a composition in the range of Li / (Li+Nb)=48% to 51%, for example, can be used. The target used to deposit the dielectric thin film 3 can be manufactured by a known method. For example, the target can be manufactured by a method similar to that used to deposit the stress relaxation layer 31.

[0079] There is no particular limitation on the shape of the target used to deposit the dielectric thin film 3. The target is preferably circular with a planar area at least twice that of the single crystal substrate 2, so that a dielectric thin film 3 having a uniform thickness can be obtained. Furthermore, the deposition of the dielectric thin film 3 is preferably performed by arranging the target coaxially with the single crystal substrate 2, so that a dielectric thin film 3 having a uniform thickness can be obtained.

[0080] When a sputtering method is used as a method for depositing the dielectric thin film 3, for example, a mixed gas of Ar and O2 is used as the sputtering gas, the O2 ratio in the sputtering gas is set to 35% to 65%, the gas pressure is set to 0.08 Pa to 0.5 Pa, the temperature of the single crystal substrate 2 is set to 450 to 700°C, and a power of 1500 to 2000 W is applied, so that the deposition rate is 300 to 1800 nm / h. It is preferable that the dielectric thin film 3 is formed in a so-called single step without changing the film forming conditions midway.

[0081] It is preferable that the dielectric thin film 3 is formed by successively depositing the stress relaxation layer 31 by a sputtering method in the same manner as the deposition of the stress relaxation layer 31, except that the O2 ratio and gas pressure conditions in the sputtering gas are changed. Specifically, after the stress relaxation layer 31 is formed, the O2 ratio in the sputtering gas is lowered and the gas pressure during sputtering is increased, whereby the dielectric thin film 3 can be formed. Through the above steps, the substrate 1 with the dielectric thin film of this embodiment is obtained.

[0082] In the substrate 1 with the dielectric thin film of this embodiment, the strain and stress caused by the difference in lattice constant and the difference in linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film forming the dielectric thin film 3 are absorbed and relaxed by the stress relaxation layer 31. Therefore, in the substrate 1 with the dielectric thin film of this embodiment, cracks are less likely to occur in the lithium niobate film.

[0083] [Optical waveguide element] Fig. 2 is a plan view showing an example of an optical waveguide element 100 using the substrate 1 with the dielectric thin film shown in Fig. 1. Fig. 3 is a cross-sectional view of the optical waveguide element 100 shown in Fig. 2 taken along the line AA'. In the optical waveguide element 100 shown in FIGS. 2 and 3, the same members as those of the substrate 1 with the dielectric thin film shown in FIG. 1 are denoted by the same reference numerals, and the description thereof will be omitted.

[0084] 2 and 3 has a ridge portion 4 obtained by processing the dielectric thin film 3 in the dielectric thin film-coated substrate 1 shown in Fig. 1 into a ridge shape (convex shape). The ridge portion 4 of the optical waveguide element 100 is a portion through which the target light propagates in the TM fundamental mode. The optical waveguide element 100 shown in Figures 2 and 3 can be manufactured by processing the dielectric thin film 3 in the dielectric thin film-coated substrate 1 shown in Figure 1 into a ridge shape (convex shape). The dielectric thin film 3 can be processed into a ridge shape by a known method such as an etching method.

[0085] The optical waveguide element 100 shown in Fig. 2 and Fig. 3 includes the substrate 1 with the dielectric thin film shown in Fig. 1. Therefore, the lithium niobate film forming the dielectric thin film 3 of the substrate 1 with the dielectric thin film is less likely to crack, resulting in excellent productivity. In addition, since the lithium niobate film forming the dielectric thin film 3 of the substrate 1 with the dielectric thin film is less likely to crack, the optical waveguide element 100 has excellent durability. In addition, the optical waveguide element 100 shown in Fig. 2 and Fig. 3 has a sufficiently thick dielectric thin film 3 and an appropriate ratio of the thickness of the stress relaxation layer 31 to the thickness of the dielectric thin film 3, so that the optical waveguide element 100 is applicable to a wide range of light from visible light to infrared light and has little optical loss.

[0086] [Light modulation element] Fig. 4 is a plan view showing an example of a Mach-Zehnder type optical modulation element 200A using the substrate 1 with the dielectric thin film shown in Fig. 1. Fig. 5 is a cross-sectional view taken along line BB' of the optical modulation element 200A shown in Fig. 4. The cross-sectional view taken along line AA' of the optical modulation element 200A shown in Fig. 4 is the same as the cross-sectional view of the optical waveguide element 100 shown in Fig. 3. In the light modulation element 200A shown in FIGS. 4 and 5, the same reference numerals are used to designate the members of the dielectric thin film-formed substrate 1 shown in FIG. 1, and the description thereof will be omitted.

[0087] 4 and 5 is a device that applies a voltage to a Mach-Zehnder interferometer formed by the optical waveguide 10 to modulate light propagating through the optical waveguide 10. As shown in Fig. 4, the optical waveguide 10 has a first optical waveguide 10a and a second optical waveguide 10b branched from a single input optical waveguide, and an output optical waveguide 10c in which the first optical waveguide 10a and the second optical waveguide 10b are combined.

[0088] 4 and 5, two first electrodes 7a and 7b are provided on the first optical waveguide 10a and the second optical waveguide 10b, respectively. Therefore, the light modulation element 200A has a dual electrode structure.

[0089] The optical modulation element 200A shown in Figures 4 and 5 has a ridge portion 4 formed by processing the dielectric thin film 3 in the dielectric thin film-attached substrate 1 shown in Figure 1 into a ridge shape (convex shape). In the optical modulation element 200A, the ridge portion 4 forms an optical waveguide 10. As shown in Figure 5, a first electrode 7a is formed on the ridge portion 4 constituting the first optical waveguide 10a of the optical waveguide 10, with a buffer layer 5 interposed therebetween. In addition, a first electrode 7b is formed on the ridge portion 4 constituting the second optical waveguide 10b of the optical waveguide 10, with the buffer layer 5 interposed therebetween.

[0090] As shown in FIG. 4 and FIG. 5, the second electrodes 8a, 8b, 8c are provided at a distance from each other via the first electrodes 7a, 7b. The second electrodes 8a, 8b, 8c are formed in contact with the upper surface of the slab portion made of the dielectric thin film 3. The slab portion made of the dielectric thin film 3 is formed by thinning a part of the upper surface of the dielectric thin film 3 in the substrate 1 with the dielectric thin film shown in FIG. 1 by etching or the like. As shown in FIG. 4, the first electrodes 7a, 7b and the second electrodes 8a, 8b, 8c are connected by a termination resistor 9. Also, as shown in FIG. 5, a dielectric layer 6 is formed in contact with the lower surface of the buffer layer 5 and the side surface of the ridge portion 4.

[0091] Next, the operating principle of the light modulation element 200A will be described. 4, the two first electrodes 7a, 7b and the second electrodes 8a, 8b, 8c are connected by a termination resistor 9 to function as traveling wave electrodes. The second electrodes 8a, 8b, 8c are ground electrodes. So-called complementary signals, which have the same absolute value, different positive and negative phases, and are not shifted, are input to the two first electrodes 7a, 7b from the input sides 15a, 15b of the first electrodes 7a, 7b of the optical modulation element 200A.

[0092] The lithium niobate film forming the dielectric thin film 3 in the substrate 1 with the dielectric thin film has an electro-optic effect. Therefore, the refractive indexes of the first optical waveguide 10a and the second optical waveguide 10b change to +Δn and -Δn, respectively, due to an electric field applied to the first optical waveguide 10a and the second optical waveguide 10b. As a result, the phase difference between the first optical waveguide 10a and the second optical waveguide 10b changes. The signal light intensity-modulated by this change in phase difference is output to the output side 12 from the output optical waveguide 10c in which the first optical waveguide 10a and the second optical waveguide 10b are multiplexed.

[0093] The light modulation element 200A of this embodiment includes a substrate 1 with a dielectric thin film shown in Fig. 1. Therefore, the light modulation element 200A of this embodiment is less susceptible to cracks in the lithium niobate film forming the dielectric thin film 3 of the substrate 1 with a dielectric thin film, is applicable to a wide range of light from visible light to infrared light, has excellent productivity and durability, and has little optical loss.

[0094] Although the preferred embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. The present invention can be modified in various ways without departing from the spirit of the present invention, and it goes without saying that such modifications are also included within the scope of the present invention. EXAMPLES

[0095] "Examples 1 to 8, Comparative Examples 2 to 5" The substrate 1 with the dielectric thin film shown in FIG. 1 was produced by the method described below. As the single crystal substrate 2, a 4-inch sapphire single crystal substrate having a c-plane as the main surface 2a was prepared.

[0096] (Stress relaxation layer deposition process) In the stress relaxation layer formation step, the stress relaxation layer 31 was formed on the main surface 2a of the single crystal substrate 2 by epitaxial growth using a sputtering method. The target used was a circle having a diameter of 8 inches and a composition of Li / (Li+Nb)=50%. The target was produced using the following method. The raw materials were prepared as sintered bodies mainly composed of Li2CO3 and Nb2O5 with a purity of 3N or higher. The raw materials were then pulverized and mixed in a ball mill using balls made of ZrO2 to produce target powder. The obtained target powder was sintered to obtain the target.

[0097] The stress relaxation layer 31 was formed by arranging the target thus obtained coaxially with the single crystal substrate 2 so that the distance from the main surface 2a of the single crystal substrate 2 was 110 mm. The stress relaxation layer 31 was formed using a mixed gas of Ar and O2 as the sputtering gas, with the O2 ratio in the sputtering gas set to 45% to 65%, the gas pressure set to 0.08 Pa to 0.4 Pa, the temperature of the single crystal substrate 2 set to 550 to 700°C, and a power of 1500 to 2000 W applied, so that the film formation rate was 500 nm / h to 600 nm / h. The film formation time was changed so that the film thickness of the stress relaxation layer 31 was set to a predetermined thickness. The stress relaxation layer 31 was formed in a so-called single step without changing the film formation conditions midway.

[0098] (Dielectric thin film deposition process) In the dielectric thin film formation step, the dielectric thin film 3 was formed on the stress relaxation layer 31 by epitaxial growth using a sputtering method. The target used was a circle having a diameter of 8 inches and a composition of Li / (Li+Nb)=50%. The target was produced using the following method. The raw materials were prepared as sintered bodies mainly composed of Li2CO3 and Nb2O5 with a purity of 3N or higher. The raw materials were then pulverized and mixed in a ball mill using balls made of ZrO2 to produce target powder. The obtained target powder was sintered to obtain the target.

[0099] The dielectric thin film 3 was formed by positioning the target thus obtained coaxially with the single crystal substrate 2 so that the distance from the stress relaxation layer 31 formed on the main surface 2a of the single crystal substrate 2 was 110 mm. The dielectric thin film 3 was formed continuously after the stress relaxation layer 31 was formed by sputtering.

[0100] The dielectric thin film 3 was formed by sputtering in the same manner as the stress relaxation layer 31, except that the O2 ratio in the sputtering gas and the gas pressure conditions were changed. Specifically, after forming the stress relaxation layer 31, the O2 ratio in the sputtering gas was lowered and the gas pressure during sputtering was increased to form the dielectric thin film 3. Also, the film formation time was changed to make the dielectric thin film 3 have a predetermined thickness. The film forming conditions for the dielectric thin film 3 were not changed midway through the process, and the film was formed in a so-called single step. By the above steps, the substrates 1 with dielectric thin films of Examples 1 to 8 and Comparative Examples 2 to 5 were obtained.

[0101] "Comparative Example 1" A substrate with a dielectric thin film was produced in the same manner as in Example 5, except that the dielectric thin film 3 was formed on the main surface 2a of the single crystal substrate 2 without forming the stress relaxation layer 31 thereon.

[0102] "Example 9, Example 10, Comparative Examples 6 to 8" Substrates 1 with dielectric thin films of Examples 9, 10, and Comparative Examples 6 to 8 were obtained in the same manner as in Example 5, except that the deposition time of the stress relaxation layer 31 in the stress relaxation layer deposition process and the deposition time of the dielectric thin film 3 in the dielectric thin film deposition process were changed.

[0103] The substrates with dielectric thin film of Examples 1 to 10 and Comparative Examples 1 to 8 obtained in this manner were examined for the "presence or absence of a stress relaxation layer 31" by the method described below, and by firstly confirming the peak intensity at the orientation position by 2θ-θ X-ray diffraction and secondly confirming the pole by pole measurement using the above-mentioned method, it was confirmed that the stress relaxation layer 31 and the dielectric thin film 3 (or the dielectric thin film 3 when the stress relaxation layer 31 was not present) were epitaxial films.

[0104] "Presence or absence of stress relaxation layer 31" The cross section of the substrate 1 with the dielectric thin film was observed at a magnification of 400,000 times using a scanning transmission electron microscope (STEM) (manufactured by FEI). In the obtained image, it was confirmed whether or not the twin crystal structure of LiNbO3 and the LiNb3O8 phase 3c were present in a range of 200 nm or more on the interface between the single crystal substrate 2 and the stress relaxation layer 31.

[0105] Furthermore, for the substrates with dielectric thin films of Examples 1 to 10 and Comparative Examples 1 to 8, it was examined by the method described below whether the stress relaxation layer 31 and the dielectric thin film 3 (dielectric thin film 3 when there is no stress relaxation layer 31) have a twin crystal structure of LiNbO3. As a result, it was confirmed that the stress relaxation layer 31 and the dielectric thin film 3 (dielectric thin film 3 when there is no stress relaxation layer 31) of the substrates with dielectric thin films of Examples 1 to 10 and Comparative Examples 1 to 8 all have a twin crystal structure of LiNbO3.

[0106] "How to confirm that LiNbO3 has a twin crystal structure" A cross section of the substrate with the dielectric thin film cut in the thickness direction was observed using a transmission electron microscope (TEM) (manufactured by FEI) to obtain a dark field (DF) image. At this time, the beam incidence conditions were adjusted so that the image of either the first crystal 3a or the second crystal 3b contained in the twin crystal structure of LiNbO3 forming the stress relaxation layer 31 and the dielectric thin film 3 (or the dielectric thin film 3 when the stress relaxation layer 31 is not present) was in a high contrast (bright) state.

[0107] In the dark-field image obtained by adjusting the beam incidence conditions as described above, when the stress relaxation layer 31 and the dielectric thin film 3 (the dielectric thin film 3 when there is no stress relaxation layer 31) have a twin crystal structure of LiNbO3, one of the images of the first crystal 3a and the second crystal 3b has a high contrast (bright) state and the other image has a low contrast (dark) state, and the first crystal 3a and the second crystal 3b can be clearly distinguished. This confirmed that the stress relaxation layer 31 and the dielectric thin film 3 (the dielectric thin film 3 when there is no stress relaxation layer 31) have a twin crystal structure of LiNbO3.

[0108] Furthermore, for the thus obtained substrates with dielectric thin films of Examples 1 to 10 and Comparative Examples 1 to 8, the "maximum domain width of the stress relaxation layer 31," "maximum domain width of the dielectric thin film 3," "film thickness of the stress relaxation layer 31," "film thickness of the dielectric thin film 3," "ratio of the film thickness of the stress relaxation layer 31 to the film thickness of the dielectric thin film 3," and "ratio of the first diffraction intensity corresponding to the first crystal 3a to the second diffraction intensity corresponding to the second crystal 3b" were examined by the methods described below. The results are shown in Tables 1 and 2. Tables 1 and 2 show the types of single crystal substrates 2 used in the substrates with dielectric thin films in Examples 1 to 10 and Comparative Examples 1 to 8. Additionally, Example 5 is listed in both Tables 1 and 2 for comparison.

[0109] [Table 1]

[0110] [Table 2]

[0111] "Maximum domain width of stress relaxation layer 31" The cross section of the substrate 1 with the dielectric thin film was observed at a magnification of 400,000 times using a scanning transmission electron microscope (STEM) (manufactured by FEI). In the obtained image, the maximum domain width in the direction perpendicular to the thickness direction was measured for each of the first crystals 3a, the second crystals 3b, and the LiNb3O8 phase 3c included in the twin crystal structure of LiNbO3, which exist in a length range of 200 nm to 800 nm on the interface between the single crystal substrate 2 and the stress relaxation layer 31. From the results, the maximum dimension in the maximum domain width of the multiple first crystals 3a, the multiple second crystals 3b, and the multiple LiNb3O8 phases 3c, which exist in a length range of 200 nm to 800 nm, was extracted and defined as the "maximum domain width of the stress relaxation layer 31."

[0112] "Maximum domain width of dielectric thin film 3" The cross section of the substrate 1 with the dielectric thin film was observed at a magnification of 400,000 times using a scanning transmission electron microscope (STEM) (manufactured by FEI). In the obtained image, the maximum domain width in the direction perpendicular to the thickness direction was measured for each of the first crystals 3a and each of the second crystals 3b included in the twin crystal structure of LiNbO3, which exist in a length range of 200 nm to 800 nm on the interface between the single crystal substrate 2 and the stress relaxation layer 31. From the results, the maximum dimension in the maximum domain width of the multiple first crystals 3a and multiple second crystals 3b existing in a length range of 200 nm to 800 nm was extracted and defined as the "maximum domain width of the dielectric thin film 3."

[0113] "Thickness of the stress relaxation layer 31" "Thickness of the dielectric thin film 3" "Ratio of the thickness of the stress relaxation layer 31 to the thickness of the dielectric thin film 3" The thicknesses of the stress relaxation layer 31 and the dielectric thin film 3 in the substrate 1 with the dielectric thin film were determined by measuring the thicknesses at three points within the field of view and calculating the average value by performing high-resolution analysis using a scanning transmission electron microscope (STEM) (manufactured by FEI). The ratio of the thickness of the stress relaxation layer 31 to the thickness of the dielectric thin film 3 was calculated using the following formula, using the thicknesses of the stress relaxation layer 31 and the dielectric thin film 3 calculated by the above method. Ratio of film thickness of stress relaxation layer 31=(stress relaxation layer 31 / dielectric thin film 3)×100(%)

[0114] "Ratio of the first diffraction intensity corresponding to the first crystals 3a of the dielectric thin film 3 to the second diffraction intensity corresponding to the second crystals 3b" For the dielectric thin film 3 of the substrate 1 with the dielectric thin film, pole measurement was performed by X-ray diffraction using an X-ray diffractometer (manufactured by Rigaku Corporation) to measure a first diffraction intensity corresponding to the first crystals 3a and a second diffraction intensity corresponding to the second crystals 3b of the dielectric thin film 3. Using the results, the ratio of the first diffraction intensity to the second diffraction intensity was calculated.

[0115] Furthermore, the substrates with dielectric thin films of Examples 1 to 18 and Comparative Example 1 were examined for "stress in the dielectric thin film," "presence or absence of cracks in the dielectric thin film 3," "presence or absence of cracks caused by annealing," and "optical loss in the dielectric thin film." The results are shown in Tables 3 and 4.

[0116] [Table 3]

[0117] [Table 4]

[0118] "Stress in thin dielectric films" The amount of warping of the dielectric thin film 3 in the substrate 1 with the dielectric thin film was measured using a needle-type step gauge (manufactured by KLA-Tenchore), and the stress of the dielectric thin film was calculated by the Stoney formula. When the stress value of the obtained dielectric thin film was "plus (+)", it was evaluated as tensile stress. When the stress value of the obtained dielectric thin film was "minus (-)", it was evaluated as compressive stress.

[0119] "Presence or absence of cracks in dielectric thin film 3" The substrate with the dielectric thin film was observed using an optical microscope (manufactured by Olympus) with an objective lens magnification of 20x and a field of view diameter of approximately 0.5 mm, and an objective lens magnification of 100x and a field of view diameter of approximately 0.1 mm, to check for the presence or absence of cracks in the dielectric thin film 3. When no cracks were found, the specimen was rated as "absent." When even one crack was found, the specimen was rated as "present."

[0120] "Whether or not cracks occur in the dielectric thin film 3 due to annealing" The substrate 1 with the dielectric thin film was annealed in an oxygen atmosphere at 1 atmospheric pressure at 600° C. for 1 hour. Thereafter, the dielectric thin film 3 was examined for the presence or absence of cracks and evaluated in the same manner as in "Presence or absence of cracks in the dielectric thin film 3" described above.

[0121] "Optical loss in dielectric thin films" Infrared light of 1550 nm was propagated through the dielectric thin film of the substrate with the dielectric thin film, and the propagation loss in the dielectric thin film was measured using a prism coupler (manufactured by Metricon). A propagation loss of less than 1 dB / cm was evaluated as pass, and a propagation loss of 1 dB / cm or more was evaluated as fail.

[0122] As shown in Tables 3 and 4, the substrates with dielectric thin film of Examples 1 to 10 all had a "negative (-)" value for "stress in dielectric thin film," which was compressive stress, and there were no cracks in the dielectric thin film 3 or cracks caused by annealing. Furthermore, the substrates with dielectric thin film of Examples 1 to 10 all had a propagation loss in the dielectric thin film of less than 1 dB / cm, and the optical loss of the dielectric thin film was low.

[0123] In contrast, as shown in Tables 3 and 4, the substrate with dielectric thin film of Comparative Example 1, which did not have a stress relaxation layer 31, had a numerical value for "stress of dielectric thin film" that was "plus (+)" and was tensile stress, and cracks in the dielectric thin film 3 and cracks caused by annealing were [present]. From this, it was confirmed that by forming a substrate with a dielectric thin film in which the single crystal substrate 2 and the dielectric thin film 3 are in contact with each other via the stress relaxation layer 31, cracks are less likely to occur in the dielectric thin film 3.

[0124] Furthermore, as shown in Table 3, in Comparative Example 2 and Comparative Example 3, in which the ratio of the film thickness of the stress relaxation layer 31 to the film thickness of the dielectric thin film 3 was less than 5%, there were no cracks in the dielectric thin film 3, but there were cracks that occurred due to annealing. Furthermore, as shown in Table 4, in Comparative Examples 7 and 8 in which the thickness of the stress relaxation layer 31 exceeded 2 μm, there were no cracks in the dielectric thin film 3, but there were cracks caused by annealing.

[0125] Furthermore, as shown in Table 3, in Comparative Example 4 and Comparative Example 5, in which the ratio of the film thickness of the stress relaxation layer 31 to the film thickness of the dielectric thin film 3 was more than 25%, there were no cracks in the dielectric thin film 3 and no cracks caused by annealing. However, in both Comparative Example 4 and Comparative Example 5, the propagation loss in the dielectric thin film was more than 1 dB / cm, and the optical loss of the dielectric thin film was high. Furthermore, as shown in Table 4, in Comparative Example 6 in which the thickness of the stress relaxation layer 31 was less than 0.5 μm, there were no cracks in the dielectric thin film 3 and no cracks caused by annealing. However, in Comparative Example 6, the propagation loss in the dielectric thin film exceeded 1 dB / cm, and the optical loss of the dielectric thin film was high.

[0126] [Mapping analysis of substrates with thin dielectric films] For the substrates with the dielectric thin film of Example 5 and Comparative Example 1, a scanning transmission electron microscope (STEM) (manufactured by FEI) was used to perform mapping analysis of the interface between the single crystal substrate 2 and the stress relaxation layer 31 or the dielectric thin film 3 in the cross section of the substrate with the dielectric thin film. The results are shown in Figs. 6 and 7.

[0127] Fig. 6 is a photograph showing an image obtained by mapping analysis of the interface between the single crystal substrate 2 and the stress relaxation layer 31 of Example 5. In Fig. 6, the lower region consisting of a single phase is the single crystal substrate 2. Figs. 6(a) and 6(b) are photographs of the first crystal 3a and the second crystal 3b forming a twin structure of LiNbO3 contained in the stress relaxation layer 31. Fig. 6(c) is a photograph of the LiNb3O8 phase 3c contained in the stress relaxation layer 31.

[0128] Fig. 7 is a photograph showing an image obtained by mapping analysis of the interface between the single crystal substrate 2 and the dielectric thin film 3 of Comparative Example 1. In Fig. 7, the lower region consisting of a single phase is the single crystal substrate 2. Figs. 7(a) and 7(b) are photographs of the first crystal 3a and the second crystal 3b forming a twin structure of LiNbO3 contained in the dielectric thin film 3. Fig. 7(c) is a photograph of the LiNb3O8 phase 3c contained in the dielectric thin film 3.

[0129] As shown in Figures 6(a) to 6(c), it was confirmed that the stress relief layer 31 of Example 5 had a significantly smaller maximum domain width than the dielectric thin film 3 of Comparative Example 1 shown in Figures 7(a) to 7(c). As shown in FIG. 6(c), the LiNb3O8 phase 3c contained in the stress relaxation layer 31 of Example 5 has grown approximately perpendicular to the main surface of the single crystal substrate 2, and coexists in the stress relaxation layer 31 with the first crystals 3a and the second crystals 3b which form a twin structure of LiNbO3 grown approximately perpendicular to the main surface of the single crystal substrate 2 shown in FIG. 6(a) and FIG. 6(b). In contrast, the LiNb3O8 phase 3c contained in the dielectric thin film 3 of Comparative Example 1 is not grown from the single crystal substrate 2, as shown in FIG. Therefore, the LiNb3O8 phase 3c contained in the stress relaxation layer 31 of Example 5 and the LiNb3O8 phase 3c contained in the dielectric thin film 3 of Comparative Example 1 can be easily distinguished. [Explanation of symbols]

[0130] 1 substrate with dielectric thin film, 2 single crystal substrate, 2a main surface, 3 dielectric thin film, 3a first crystal, 3b second crystal, 3c LiNb3O8 phase, 4 ridge portion, 5 buffer layer, 6 dielectric layer, 7a, 7b first electrode, 8a, 8b, 8c second electrode, 9 termination resistor, 10 optical waveguide, 10a first optical waveguide, 10b second optical waveguide, 10c output optical waveguide, 12 output side, 15a, 15b input side, 31 stress relaxation layer, 100 optical waveguide element, 200A optical modulation element.

Claims

1. A single crystal substrate; a stress relief layer formed on and in contact with a main surface of the single crystal substrate; a dielectric thin film formed on and in contact with the stress relaxation layer, The stress relaxation layer is made of a c-axis oriented epitaxial film and includes a first crystal and a second crystal obtained by rotating the first crystal by 180° about the c-axis. ３ and the twin structure of LiNb ３ O ８ phase, The dielectric thin film is made of a lithium niobate film, which is an epitaxial film oriented along the c-axis. ３ a film thickness of 0.5 μm to 2 μm and a maximum domain width greater than the maximum domain width of the stress relaxation layer; A substrate with a dielectric thin film, wherein the ratio of the thickness of the stress relaxation layer to the thickness of the dielectric thin film is 5% to 25%.

2. 2. The substrate with a dielectric thin film according to claim 1, wherein the maximum domain width of the stress relaxation layer is 20 nm to 70 nm.

3. 2. The substrate with a dielectric thin film according to claim 1, wherein the single crystal substrate is a sapphire single crystal substrate, the main surface of which is a c-plane.

4. 2. The substrate with the dielectric thin film according to claim 1, wherein the twin crystal structure of the dielectric thin film has a ratio of a first diffraction intensity corresponding to the first crystal to a second diffraction intensity corresponding to the second crystal of 0.5 or more and 2.0 or less in a pole measurement by an X-ray diffraction method.

5. An optical waveguide element comprising a substrate with a dielectric thin film according to any one of claims 1 to 4.

6. 5. An optical modulation element comprising a substrate with a dielectric thin film according to claim 1.

Images