Substrate with dielectric thin film, optical waveguide element, and optical modulation element

Abstract

The present invention addresses the problem of providing a substrate with a dielectric thin film, said substrate being capable of forming an optical waveguide element and an optical modulation element that are not susceptible to cracking in a lithium niobate film, can be applied to a wide range of light from visible light to infrared light, and have little optical loss. This substrate (1) with a dielectric thin film is provided with a single crystal substrate (2), a stress relaxation layer (31), and a dielectric thin film (3), and the stress relaxation layer (31) comprises a c-axis oriented epitaxial film and comprises: a LiNbO3 bimorph structure comprising a first crystal (3a) and a second crystal (3b) obtained by rotating the first crystal (3a) by 180 degrees about the c-axis; and a LiNb3O8 phase, the dielectric thin film (3) is composed of a lithium niobate film that is a c-axis oriented epitaxial film, has a bicrystal structure of LiNbO3, has a film thickness of 0.5 [mu] m to 2 [mu] m, and has a maximum domain width greater than the maximum domain width of the stress relaxation layer (31), and the ratio of the film thickness of the stress relaxation layer (31) to the film thickness of the dielectric thin film (3) is 5% to 25%.

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 substrate. Background Art

[0002] Currently, there are optical waveguide elements and optical modulation elements that use a lithium niobate film epitaxially grown on a substrate.

[0003] For example, Patent Document 1 describes a substrate with a dielectric thin film, which includes a dielectric thin film composed of a c-axis oriented lithium niobate film epitaxially formed on a main surface of a single crystal substrate. Patent Document 1 describes that the dielectric thin film has a double crystal structure including a first crystal and a second crystal obtained by rotating the first crystal 180° around the c-axis, and in a pole measurement by an X-ray diffraction method, a ratio of a first diffraction intensity corresponding to the first crystal to a second diffraction intensity corresponding to the second crystal is greater than 0.5 and less than 2.0.

[0004] Patent Document 2 describes an optical modulator having a lithium niobate film, which is an epitaxial film formed on the main surface of a single crystal substrate and has a ridge-shaped portion. Patent Document 2 also describes that the lithium niobate film is epitaxially grown on a silicon single crystal substrate via an epitaxial buffer layer that promotes epitaxial growth. Patent Document 2 describes that ZrO is preferably 2 As an epitaxial buffer layer.

[0005] In addition, Patent Document 3 describes a method in which a 2 O 3 An optical element having a structure in which a ruthenium lower electrode, a lithium niobate thin film as an electro-optical effect material heteroepitaxially formed thereon, and an upper electrode of a metal material are stacked in sequence on a hexagonal sapphire substrate.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: International Publication No. 2018 / 016428

[0009] Patent Document 2: Japanese Patent Application Publication No. 2014-142411

[0010] Patent Document 3: Japanese Patent Application Publication No. 2003-15098 Summary of the invention

[0011] Problems to be solved by the invention

[0012] Optical waveguide elements and optical modulators using a substrate with a dielectric thin film having a lithium niobate film obtained by epitaxial growth on the substrate have better production efficiency than optical waveguide elements and optical modulators using a lithium niobate single crystal substrate. This is because, in a substrate with a dielectric thin film having a lithium niobate film obtained by epitaxial growth on the substrate, a lithium niobate film of a desired film thickness can be formed by adjusting the film forming conditions. That is, when using a substrate with a dielectric thin film having a lithium niobate film obtained by epitaxial growth on the substrate, it is not necessary to adjust the thickness by cutting or grinding 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 obtained by epitaxial growth on the substrate is cheaper than a lithium niobate single crystal substrate.

[0013] However, in optical waveguide elements and optical modulation elements using a lithium niobate film obtained by epitaxial growth on a substrate, the thickness of the lithium niobate film sometimes becomes a limitation in the design of the optical waveguide elements and optical modulation elements. More specifically, it is preferable that the lithium niobate film obtained by epitaxial growth on a substrate has a sufficient film thickness in order to obtain an optical waveguide element and optical modulation element that can be applied to a wide range of light from visible light to infrared light. However, the lithium niobate film obtained by epitaxial growth on a substrate is prone to cracking, and there is a disadvantage that cracking is more likely to occur as the film thickness increases.

[0014] Therefore, as a substrate with a dielectric thin film having a lithium niobate film obtained by epitaxial growth on a substrate, it is required that cracks are unlikely to occur in the lithium niobate film.

[0015] The present invention has been completed in view of the above-mentioned problems, and its purpose is to provide a substrate with a dielectric thin film, which has a lithium niobate film obtained by epitaxial growth on the substrate, and can form an optical waveguide element and an optical modulation element that are not prone to cracks in the lithium niobate film and can be applied to a wide range of light from visible light to infrared light and have low optical loss.

[0016] In addition, an object of the present invention is to provide a substrate with a dielectric thin film having a dielectric thin film composed of a lithium niobate film, in which cracks are not easily generated in the lithium niobate film, and an optical waveguide element and an optical modulation element can be applied to a wide range of light from visible light to infrared light and have low optical loss.

[0017] Means used to solve problems

[0018] A substrate with a dielectric film according to one embodiment of the present invention comprises: a single crystal substrate; a stress relaxation layer formed in contact with a main surface of the single crystal substrate; and a dielectric film formed in contact with the stress relaxation layer, wherein the stress relaxation layer is composed of a c-axis oriented epitaxial film and comprises: LiNbO 3A double crystal structure comprising a first crystal and a second crystal obtained by rotating the first crystal 180° around the c-axis; and LiNb 3 O 8 phase, the dielectric thin film is composed of a lithium niobate film as a c-axis oriented epitaxial film, and has the LiNbO 3 The double crystal structure has a film thickness of 0.5 μm to 2 μm, a maximum domain width greater 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 film is 5% to 25%.

[0019] Effects of the Invention

[0020] The substrate with dielectric thin film of the present invention comprises: a stress relaxation layer and a dielectric thin film formed in contact with the stress relaxation layer, wherein the stress relaxation layer is composed of a c-axis oriented epitaxial film formed in contact with the main surface of the single crystal substrate, and comprises: LiNbO including a first crystal and a second crystal obtained by rotating the first crystal by 180 degrees around the c-axis. 3 The twin crystal structure of LiNb 3 O 8 phase, the dielectric thin film is composed of a lithium niobate film as a c-axis oriented epitaxial film, and has the LiNbO 3 The double crystal structure has a film thickness of 0.5 μm to 2 μm, a maximum domain width greater than the maximum domain width of the stress relief layer, and a ratio of the film thickness of the stress relief layer to the film thickness of the dielectric film is 5% to 25%. Therefore, in the substrate with a dielectric film of the present invention, the strain and stress caused by the difference in lattice constant and linear expansion coefficient between the single crystal substrate and the lithium niobate film are absorbed and relieved by the stress relief layer. Therefore, the substrate with a dielectric film of the present invention is not prone to cracks in the lithium niobate film.

[0021] Furthermore, the dielectric thin film of the substrate with a dielectric thin film of the present invention is composed of a lithium niobate film as a c-axis oriented epitaxial film, and is not prone to cracking. Therefore, for example, the thickness can be 0.5 μm or more, which is suitable for manufacturing an optical waveguide element and an optical modulation element using the substrate with a dielectric thin film. Therefore, the substrate with a dielectric thin film of the present invention can be preferably used for manufacturing an optical waveguide element and an optical modulation element that can be applied to a wide range of light from visible light to infrared light.

[0022] Furthermore, the ratio of the film thickness of the stress relief layer of the substrate with dielectric film of the present invention to the film thickness of the dielectric film is less than 25%. Therefore, the stress relief layer does not affect the characteristics of the optical waveguide element and the optical modulator element, and can form an optical waveguide element and an optical modulator element with less optical loss.

[0023] The optical waveguide element and the optical modulation element of the present invention include a substrate with a dielectric thin film having a lithium niobate film that is not prone to cracking. Therefore, in the manufacturing process of the optical waveguide element and / or the optical modulation element, even if annealing is performed, for example, the lithium niobate film on the substrate with a dielectric thin film is not prone to cracking, and the production efficiency is excellent. In addition, since the lithium niobate film on the substrate with a dielectric thin film is not prone to cracking, the optical waveguide element and the optical modulation element are excellent in durability.

[0024] In addition, the base with dielectric film of the present invention has: a dielectric film composed of a lithium niobate film with a sufficiently thick film thickness, and the thickness of the stress relief layer is appropriately proportional to the thickness of the dielectric film. Therefore, 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 low optical loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 1 is a schematic cross-sectional view showing a substrate 1 with a dielectric thin film according to an embodiment of the present invention.

[0026] Figure 2 Indicates the use Figure 1 FIG. 1 is a plan view of an example of an optical waveguide device 100 having a substrate 1 with a dielectric thin film.

[0027] Figure 3 yes Figure 2 A cross-sectional view of the optical waveguide element 100 taken along line AA′ is shown.

[0028] Figure 4 It means use Figure 1 FIG. 1 is a plan view of an example of a Mach-Zehnder type light modulation element 200A having a substrate 1 with a dielectric thin film.

[0029] Figure 5 yes Figure 4 A cross-sectional view of the light modulation element 200A taken along line BB′ is shown.

[0030] Fig. 6A This is a photograph showing an image of the interface between the single crystal substrate 2 and the stress relaxation layer 31 of Example 5 obtained by mapping analysis. Fig. 6A LiNbO contained in the stress relaxation layer 31 is formed 3 A photograph of the first crystal 3a and the second crystal 3b of the twin crystal structure.

[0031] Figure 6B This is a photograph showing an image of the interface between the single crystal substrate 2 and the stress relaxation layer 31 of Example 5 obtained by mapping analysis. Figure 6B LiNbO contained in the stress relaxation layer 31 is formed 3A photograph of the first crystal 3a and the second crystal 3b of the twin crystal structure.

[0032] Figure 6C LiNb contained in the stress relaxation layer 31 3 O 8 Photo of phase 3c.

[0033] Fig. 7A This is a photograph showing an image of the interface between the single crystal substrate 2 and the dielectric thin film 3 of Comparative Example 1 obtained by mapping analysis. Fig. 7A The LiNbO contained in the dielectric thin film 3 is formed 3 A photograph of the first crystal 3a and the second crystal 3b of the twin crystal structure.

[0034] Figure 7B This is a photograph showing an image of the interface between the single crystal substrate 2 and the dielectric thin film 3 of Comparative Example 1 obtained by mapping analysis. Figure 7B The LiNbO contained in the dielectric thin film 3 is formed 3 A photograph of the first crystal 3a and the second crystal 3b of the twin crystal structure.

[0035] Figure 7C LiNb contained in the dielectric thin film 3 3 O 8 Photo of phase 3c. DETAILED DESCRIPTION

[0036] The inventors of the present invention have repeatedly studied, as described below, to solve the above problems and to suppress the occurrence of cracks in a substrate with a dielectric thin film having a lithium niobate film epitaxially grown on a single crystal substrate.

[0037] That is, in a substrate with a dielectric thin film on which a lithium niobate film obtained by epitaxial growth is formed in contact with a single crystal substrate, large strain and stress are generated at the interface between the single crystal substrate and the lithium niobate film due to the difference in lattice constant and linear expansion coefficient between the single crystal substrate and the lithium niobate film.

[0038] Therefore, in the prior art, in the manufacturing process of the substrate with dielectric film, when the lithium niobate film is epitaxially grown, cracks are likely to be generated in the lithium niobate film. Furthermore, the thicker the lithium niobate film is, the greater the strain and stress are. Therefore, the thicker the lithium niobate film is, the more likely it is that cracks will be generated in the lithium niobate film during and after the manufacturing of the substrate with dielectric film. In addition, if annealing and other processes are performed when manufacturing optical modulators and the like using the substrate with dielectric 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. Therefore, when manufacturing optical modulators and the like using the substrate with dielectric film, cracks are likely to be generated in the lithium niobate film during and after the manufacturing.

[0039] The inventors of the present invention have proposed the following method as a method for suppressing cracks in a lithium niobate film: as a lithium niobate film, a c-axis oriented epitaxial film is used, which has: a LiNbO film including a first crystal and a second crystal obtained by rotating the first crystal by 180 degrees around the c-axis. 3 In this case, it is speculated that the strain and stress generated by the difference in lattice constant and linear expansion coefficient between the single crystal substrate and the lithium niobate film are alleviated by the above-mentioned twin crystal structure, so that cracks are less likely to occur in the lithium niobate film.

[0040] However, even when a c-axis oriented epitaxial film having the above-mentioned twin crystal structure is used as a lithium niobate film, the generation of cracks in the lithium niobate film obtained by epitaxial growth on a single crystal substrate cannot be sufficiently suppressed. Therefore, a substrate with a dielectric thin film that further suppresses the generation of cracks in the lithium niobate film is required.

[0041] Therefore, the inventors of the present invention believed that a c-axis oriented lithium niobate film having the above-mentioned twin crystal structure could be epitaxially grown on a single crystal substrate via a stress relaxation layer, and repeatedly conducted studies focusing on film formation conditions of the lithium niobate film.

[0042] As a result, it was found that, before epitaxially growing a c-axis oriented lithium niobate film having the above twin crystal structure on a single crystal substrate, a material having a higher Nb content and a lower Li content than the material used in forming the lithium niobate film having the above twin crystal structure was used to form a single crystal substrate including the above twin crystal structure and LiNb. 3 O 8 The stress relaxation layer may be a c-axis oriented epitaxial film having a phase and a maximum domain width smaller than the maximum domain width of the lithium niobate film having the above-mentioned twin crystal structure.

[0043] In the following case, i.e., in contact with the stress relief layer formed in this way, a c-axis oriented lithium niobate film having the above-mentioned twin crystal structure with a film thickness of less than 2 μm is epitaxially grown, and the ratio of the film thickness of the stress relief layer to the film thickness of the lithium niobate film obtained by epitaxial growth is more than 5%, it is speculated that: the strain and stress generated at the interface between the single crystal substrate and the lithium niobate film are relieved by the stress relief layer having a sufficient film thickness relative to the film thickness of the lithium niobate film obtained by epitaxial growth, and the generation of cracks in the lithium niobate film is suppressed.

[0044] In more detail, when the single crystal substrate and the lithium niobate film are connected via a stress relief layer, strain and stress caused by the difference in lattice constant and linear expansion coefficient between the single crystal substrate and the lithium niobate film are also generated. However, when the single crystal substrate and the lithium niobate film are connected via a stress relief layer, before cracks are generated in the lithium niobate film due to the above-mentioned strain and stress, fine nanocracks as the initial stage of cracks are generated in the stress relief layer. The extension of the nanocracks generated in the stress relief layer is limited within the range of the stress relief layer. Therefore, it is speculated that the above-mentioned strain and stress are absorbed and relieved by the stress relief layer, thereby suppressing the generation of cracks in the lithium niobate film.

[0045] In addition, the inventors of the present invention have discovered that by making the thickness of the lithium niobate film grown in contact with the stress relief layer be greater than 0.5 μm, a substrate with a dielectric thin film can be preferably used to manufacture optical waveguide elements and optical modulator elements that can be applied to a wide range of light from visible light to infrared light. Furthermore, by making the ratio of the film thickness of the stress relief layer to the film thickness of the lithium niobate film be less than 25%, a substrate with a dielectric thin film can be formed that can form optical waveguide elements and optical modulator elements with very little light loss.

[0046] In addition, the stress relaxation layer is composed of a c-axis oriented epitaxial film having a maximum domain width smaller than the maximum domain width of the lithium niobate film having the twin crystal structure, and includes LiNbO 3 The twin crystal structure and LiNb 3 O 8 phase, and therefore, the bonding with the c-axis oriented lithium niobate film having the above-mentioned twin crystal structure obtained by epitaxial growth thereon is good, and the influence on the characteristics of the light modulation element having the substrate with the dielectric thin film is small.

[0047] On the other hand, for example, when the stress relaxation layer is replaced with ZrO2, which is a material completely different from lithium niobate, 2 In the case of an epitaxial buffer layer formed of a dielectric thin film, poor bonding with the lithium niobate film is likely to occur, and the optical loss and other characteristics of the optical modulation element having a substrate with a dielectric thin film are greatly affected.

[0048] Furthermore, the inventor of the present invention confirmed that even if a substrate with a dielectric thin film in which a single crystal substrate and a lithium niobate film are connected via the above-mentioned stress relief layer is annealed, cracks are not easily generated in the lithium niobate film, and optical waveguide elements and optical modulation elements with low optical loss can be manufactured with a high yield, and the inventor of the present invention was conceived.

[0049] The present invention includes the following aspects.

[0050] [1] A substrate with a dielectric film, wherein:

[0051] include:

[0052] Single crystal substrate;

[0053] A stress relaxation layer formed in contact with the main surface of the single crystal substrate; and

[0054] a dielectric film formed in contact with the stress relaxation layer,

[0055] The stress relaxation layer is composed of a c-axis oriented epitaxial film and includes: LiNbO 3 A double crystal structure comprising a first crystal and a second crystal obtained by rotating the first crystal 180° around the c-axis; and LiNb 3 O 8 Mutually,

[0056] The dielectric thin film is composed of a lithium niobate film as a c-axis oriented epitaxial film and has the LiNbO 3 The double crystal structure has a film thickness of 0.5 μm to 2 μm, and the maximum domain width is greater than the maximum domain width of the stress relaxation layer.

[0057] The ratio of the film thickness of the stress relaxation layer to the film thickness of the dielectric thin film is 5% to 25%.

[0058] [2] The substrate with a dielectric thin film as described in [1], wherein:

[0059] The maximum domain width of the stress relaxation layer is 20 nm to 70 nm.

[0060] [3] The substrate with a dielectric thin film as described in [1], wherein:

[0061] The single crystal substrate is a sapphire single crystal substrate whose main surface is a c-plane.

[0062] [4] The substrate with a dielectric thin film as described in [1], wherein:

[0063] The dual 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 extreme point measurement by an X-ray diffraction method.

[0064] [5] An optical waveguide element comprising: the substrate with a dielectric thin film according to any one of [1] to [4].

[0065] [6] A light modulation element comprising: the substrate with a dielectric thin film according to any one of [1] to [4].

[0066] Hereinafter, the substrate with dielectric thin film, the optical waveguide element, and the optical modulation element of the present embodiment will be described in detail with reference to the accompanying drawings as appropriate. The accompanying drawings used in the following description sometimes enlarge the characteristic parts for the purpose of making the characteristics of the present invention easier to understand. Therefore, the dimensional ratios of the various components may be different from the actual ones. The materials, dimensions, etc. illustrated in the following description are only examples, and the present invention is not limited to these, and can be appropriately changed and implemented without changing the scope of the subject matter.

[0067] [Substrate with dielectric film]

[0068] Figure 1 FIG. 1 is a schematic cross-sectional view of a substrate 1 with a dielectric thin film according to an embodiment of the present invention. Figure 1 As shown, the substrate 1 with dielectric thin film of this embodiment includes a single crystal substrate 2, a stress relaxation 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 relaxation layer 31.

[0069] The stress relaxation layer 31 of the substrate 1 with a dielectric thin film of the present embodiment is formed of a c-axis oriented epitaxial film and includes: LiNbO 3 The twin crystal structure of LiNb 3 O 8 The dielectric thin film 3 of the substrate 1 with a dielectric thin film of the present embodiment is composed of a lithium niobate film as a c-axis oriented epitaxial film, and has a LiNbO phase 3c with the stress relaxation layer 31. 3 The twin crystal structure of the same LiNbO 3 The double crystal structure has a film thickness of 0.5 μm to 2 μm, and the maximum domain width is greater than the maximum domain width of the stress relaxation layer 31.

[0070] In addition, the ratio of the film thickness of the stress relaxation layer 31 of the substrate with dielectric thin film 1 of the present embodiment to the film thickness of the dielectric thin film 3 is 5% to 25%.

[0071] (Single crystal substrate 2)

[0072] As the single crystal substrate 2, any substrate that can form an epitaxial film including LiNbO 3 The twin crystal structure and LiNb 3 O 8 The stress relaxation layer 31 of the phase 3c may be formed on a substrate, and a known single crystal substrate may be used. The single crystal substrate 2 preferably has a refractive index higher than that of lithium niobate (LiNbO 3 ) is low, for example, a sapphire single crystal substrate, a silicon single crystal substrate, etc. can be used.

[0073] In the substrate 1 with a dielectric thin film of the present embodiment, a sapphire single crystal substrate is preferably used as the single crystal substrate 2. The refractive index of the sapphire single crystal substrate is higher than that of LiNbO. 3 Therefore, for example, when the dielectric thin film 3 of the substrate 1 with a dielectric thin film is used as an optical waveguide layer of an optical modulator, it can function as a cladding layer. Therefore, when the single crystal substrate 2 is a sapphire single crystal substrate, it is not necessary to separately provide a cladding layer between the single crystal substrate 2 and the dielectric thin film 3, and the dielectric thin film 3 can be preferably used as the optical waveguide layer of the optical modulator.

[0074] The stress relaxation layer 31 of the substrate 1 with a dielectric film of the present embodiment and the dielectric film 3 formed on the single crystal substrate 2 via the stress relaxation layer 31 can be easily formed into a c-axis oriented epitaxial film with respect to the single crystal substrate 2 having various crystal orientations. Therefore, the crystal orientation of the single crystal substrate 2 is not particularly limited.

[0075] In the substrate 1 with a dielectric thin film of the present embodiment, the stress relaxation layer 31 is composed of a c-axis oriented epitaxial film including LiNbO 3 The twin crystal structure and LiNb 3 O 8 Phase 3c, the dielectric thin film 3 formed on the single crystal substrate 2 via the stress relaxation layer 31 is composed of a lithium niobate film as a c-axis oriented epitaxial film, having LiNbO 3 The double crystal structure has a three-fold symmetry. Therefore, it is preferred that the crystal orientation of the main surface 2a of the single crystal substrate 2 has the same symmetry as the dielectric film 3. Therefore, as the single crystal substrate 2, for example, when a sapphire single crystal substrate is used, it is preferred that the main surface 2a is a c-plane. In addition, as the single crystal substrate 2, for example, when a silicon single crystal substrate is used, it is preferred that the main surface 2a is a (111) plane.

[0076] (Stress Relaxation Layer 31)

[0077] The stress relaxation layer 31 is composed of an epitaxial film oriented in the c-axis direction. The epitaxial film forming the stress relaxation layer 31 is made of lithium niobate (LiNbO 3 ) is the main component. Lithium niobate has a large electro-optical constant and is therefore preferably used as a material for the optical waveguide layer of an optical modulation element.

[0078] The composition of the epitaxial film forming the stress relaxation layer 31 is LiNbO 3 The composition of the twin crystal structure is similar to that of LiNb 3 O 8 Therefore, the composition of the epitaxial film forming the stress relaxation layer 31 is different from that of LiNbO 3 The composition of the double crystal structure is based on LiNb 3 O 8The proportion of phase 3c content corresponding to the reduction of Li 2 Specifically, the composition of the epitaxial film forming the stress relaxation layer 31 is LiNbO 3 The composition of the twin crystal structure is preferably reduced by 35 to 48 mol% of Li 2 The amount of O.

[0079] The stress relaxation layer 31 includes LiNbO 3 The twin crystal structure and LiNb 3 O 8 Phase 3c. LiNbO forming the stress relaxation layer 31 3 The double crystal structure includes: a first crystal 3a, and a second crystal 3b obtained by rotating the first crystal 3a by 180° about the c-axis. Figure 1 As shown, the first crystals 3a and the second crystals 3b, LiNb 3 O 8 The phases 3 c all grow substantially vertically with respect to the main surface 2 a of the single crystal substrate 2 .

[0080] In this embodiment, the LiNbO forming the stress relaxation layer 31 3 The first crystal 3a included in the twin crystal structure is LiNbO having the same in-plane orientation as that of the single crystal substrate 2. 3 The second crystal 3b is a LiNbO crystal whose in-plane orientation is rotated 180° relative to the in-plane orientation of the single crystal substrate 2. 3 of crystals.

[0081] 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 strain and stress caused by the difference in lattice constant and linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film forming the dielectric thin film 3 by the above-mentioned twin crystal structure is sometimes reduced, and it becomes easy to generate cracks in the lithium niobate film. Therefore, the first crystal 3a and the second crystal 3b preferably have a half-value width of the rocking curve (locking curve) of the (006) reflection measured by the X-ray diffraction method in the range of 0.3° or more and 0.6° or less. When the half-value 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 optically regarded as a single crystal. Moreover, the first crystal 3a and the second crystal 3b are not too close to a complete single crystal, and the generation of cracks in the lithium niobate film can be effectively suppressed.

[0082] In the cross-sectional view, LiNbO contained in the epitaxial film forming the stress relaxation layer 31 3 The area of ​​the twin crystal structure is relatively large compared to LiNbO3 The twin crystal structure of LiNb 3 O 8 The ratio of the total area of ​​the phase 3c can be, for example, in the range of 0.6 to 0.95, preferably in the range of 0.7 to 0.9. 3 O 8 The ratio of the area of ​​the phase 3 c to the above-mentioned 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.

[0083] In the cross-sectional view, the LiNbO contained in the epitaxial film forming the stress relaxation layer 31 3 The twin crystal structure of LiNb 3 O 8 The total area of ​​phase 3c is LiNbO 3 The ratio of the area of ​​the twin crystal structure is 0.6 or more (in other words, the ratio of the area of ​​the LiNb 3 O 8 When the area ratio of the phase 3c is less than 0.4), the crystal orientation of the lithium niobate film of the dielectric thin film 3 formed on the stress relaxation layer 31 is more easily aligned with the crystal orientation of the stress relaxation layer 31. Therefore, when the dielectric thin film 3 of the substrate 1 with a dielectric thin film is used as an optical waveguide layer of an optical modulator, the stress relaxation layer 31 is less likely to affect the characteristics of the optical modulator, such as the optical loss.

[0084] In addition, if the LiNbO contained in the epitaxial film forming the stress relaxation layer 31 is 3 The twin crystal structure of LiNb 3 O 8 The total area of ​​phase 3c is LiNbO 3 The area ratio of the twin crystal structure is 0.95 or less (in other words, the total area of ​​LiNb 3 O 8 The area ratio of phase 3c is greater than 0.05), because LiNb is not fully contained. 3 O 8 Phase 3c is formed, and therefore, the maximum domain width of the stress relaxation layer 31 tends to be small. As a result, the stress relaxation layer 31 significantly relaxes the strain and stress generated by the difference in lattice constant and linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film forming the dielectric thin film 3. Therefore, the generation of cracks in the lithium niobate film can be more effectively suppressed.

[0085] In the cross-sectional view, the LiNbO contained in the epitaxial film forming the stress relaxation layer 31 3 The twin crystal structure and LiNb 3 O 8The total area of ​​phase 3c, LiNbO 3 The twin crystal structure and LiNb 3 O 8 The ratio of the area of ​​the phase 3c can be calculated by, for example, the following method.

[0086] The cross section of the substrate 1 with the dielectric thin film was observed at a magnification of 100,000 to 400,000 times using a scanning transmission electron microscope (manufactured by FEI Corporation). By using the scanning transmission electron microscope, it was possible to clearly distinguish the LiNbO 3 The twin crystal structure of LiNb 3 O 8 In the obtained image, the LiNbO phase 3c and the LiNbO phase 3c existing in the interface between the single crystal substrate 2 and the stress relaxation layer 31 in the range of 200 nm to 800 nm in length were measured. 3 The area of ​​the twin crystal structure and the LiNb 3 O 8 The area of ​​phase 3c is calculated using the results relative to LiNbO 3 The area of ​​the twin crystal structure is similar to that of LiNb 3 O 8 The total area of ​​phase 3c, LiNbO 3 The ratio of the area of ​​the twin crystal structure and the LiNb 3 O 8 The ratio of the area of ​​phase 3c.

[0087] The LiNbO of the stress relaxation layer 31 of this embodiment 3 In the double crystal structure, in the extreme point measurement by the X-ray diffraction method, 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, more preferably 0.8 or more and 1.25 or less, and the closer to 1.0, the more preferred. In the extreme point measurement by the X-ray diffraction method, 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.

[0088] It is preferred that the ratio of the first crystals 3a and the second crystals 3b in the stress relaxation layer 31 be as uniform as possible. This is because the LiNbO 3 The more even the ratio of the first crystal 3a and the second crystal 3b of the twin crystal structure is, the greater the LiNbO 3 The ratio of the first crystal 3a and the second crystal 3b of the twin crystal structure is more likely to be equal. The LiNbO 3The more even the ratio of the first crystal 3a and the second crystal 3b of the twin crystal structure is, the more effectively the strain and stress generated by the difference in lattice constant and linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film can be alleviated by the twin crystal structure. Therefore, cracks are less likely to occur in the lithium niobate film.

[0089] The epitaxial film forming the stress relaxation layer 31 may be composed only of LiNbO 3 The twin crystal structure of LiNb 3 O 8 Phase 3c is formed and may also contain LiNbO 3 The twin crystal structure and LiNb 3 O 8 Phases other than phase 3c are preferably composed only of LiNbO 3 The twin crystal structure of LiNb 3 O 8 Phase 3c is formed.

[0090] The maximum domain width of the stress relaxation layer 31 is smaller than the maximum domain width of the dielectric film 3 (in other words, the maximum domain width of the dielectric film 3 is larger than the stress relaxation layer 31). Therefore, in the substrate 1 with a dielectric film of the present embodiment, the strain and stress generated at the interface between the single crystal substrate 2 and the dielectric film 3 are relaxed by the stress relaxation layer 31.

[0091] In this embodiment, the maximum domain width of the stress relaxation layer 31 refers to the following dimensions. That is, for a LiNbO layer with a length ranging from 200 nm to 800 nm existing at the interface between the single crystal substrate 2 and the stress relaxation layer 31, 3 The first crystals 3a and the second crystals 3b, LiNb 3 O 8 Phase 3c, the maximum domain width in the direction perpendicular to the thickness direction is measured, and from the results, a plurality of first crystals 3a and a plurality of second crystals 3b, a plurality of LiNb 3 O 8 The maximum dimension of the maximum domain width of phase 3c.

[0092] In this embodiment, the maximum domain width is used as an indicator of the domain width of the stress relaxation layer 31. This is because the maximum domain width of the stress relaxation layer 31 can be measured more easily and accurately than the average value of the domain width, for example. 3 The first crystal 3a and the second crystal 3b, LiNb 3 O 8The phase 3c includes a phase with a very narrow domain width. More specifically, this is because it is not easy to distinguish the LiNbO 3 The first crystals 3a and the second crystals 3b, the LiNb 3 O 8 Phase 3c, determination of the respective domain widths.

[0093] In this embodiment, the maximum domain width of the dielectric thin film 3 refers to the following dimensions. That is, for a LiNbO layer with a length ranging from 200 nm to 800 nm existing at the interface between the single crystal substrate 2 and the stress relaxation layer 31, 3 The maximum domain widths of each first crystal 3a and each second crystal 3b contained in the twin crystal structure in a direction perpendicular to the thickness direction are measured respectively, and the maximum size of the maximum domain widths of multiple first crystals 3a and multiple second crystals 3b existing in the length range of 200nm to 800nm ​​is extracted from the results.

[0094] In the present embodiment, the maximum domain width of the stress relief layer 31 is preferably 20nm to 70nm. If the maximum domain width of the stress relief layer 31 is greater than 20nm, the domain of the stress relief layer 31 obtained by epitaxial growth on the main surface 2a of the single crystal substrate 2 is not easily interrupted, and it is easy to form a stress relief layer 31 with uniform thickness, so it is preferred. In addition, if the maximum domain width of the stress relief layer 31 is less than 70nm, the strain and stress generated by the difference in lattice constant and linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film forming the dielectric film 3 can be effectively relieved by the stress relief layer 31. Therefore, it is further difficult to generate cracks in the lithium niobate film. The maximum domain width of the stress relief layer 31 is more preferably 20nm to 50nm, and more preferably 20nm to 40nm.

[0095] (Dielectric film 3)

[0096] The dielectric thin film 3 is composed of a lithium niobate film which is a c-axis oriented epitaxial film. The lithium niobate film forming the dielectric thin film 3 is composed of lithium niobate (LiNbO 3 ) is the main component. Since lithium niobate has a large electro-optical constant, it is preferably used as a material for an optical waveguide layer of an optical modulation element.

[0097] 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).

[0098] A in the formula represents an element other than Li, Nb, and O. Examples of the element 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 only one or more.

[0099] In the formula, x is 0.5 to 1.2, preferably 0.9 to 1.05.

[0100] In the formula, y is 0 to 0.5.

[0101] In the formula, z is 1.5 to 4, preferably 2.5 to 3.5.

[0102] The lithium niobate film forming the dielectric thin film 3 has, similarly to the epitaxial film forming the stress relaxation layer 31, a LiNbO film including a first crystal 3a and a second crystal 3b obtained by rotating the first crystal 3a by 180° about the c-axis. 3 The twin crystal structure.

[0103] The LiNbO 3 The twin crystal structure of LiNbO contained in the epitaxial film forming the stress relaxation layer 31 is 3 Similarly to the twin crystal structure, preferably, the half-value widths of the rocking curves of the (006) reflections of the first crystal 3a and the second crystal 3b measured by X-ray diffraction are both in the range of 0.3° to 0.6°.

[0104] Preferably, the LiNbO 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 in the extreme point measurement by the X-ray diffraction method is 0.5 or more and 2.0 or less. The LiNbO dielectric thin film 3 3 The more equal the ratio of the first crystal 3a and the second crystal 3b of the twin crystal structure is, the more effective it is to alleviate the strain and stress generated by the difference in lattice constant and linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film through the twin crystal structure. Therefore, cracks are less likely to occur in the lithium niobate film. The above ratio of the dielectric thin film 3 is more preferably greater than 0.8 and less than 1.25, and the closer it is to 1.0, the more preferably.

[0105] Preferably, the LiNbO 3The first crystal 3a and the second crystal 3b of the twin crystal structure are bonded to each other without a grain boundary. This is because 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. Therefore, when the dielectric film 3 of the substrate 1 with a dielectric film is used as an optical waveguide layer of an optical modulator, the optical loss of the optical modulator increases. In contrast, when the first crystal 3a and the second crystal 3b are bonded to each other and there is no grain boundary at the boundary between the two, since the refractive index of the two is the same, no light scattering occurs. Therefore, the lithium niobate film can obtain the same optical properties as those of a single crystal.

[0106] Preferably, the lithium niobate film forming the dielectric thin film 3 is made of LiNbO 3 The lithium niobate film forming the dielectric thin film 3 may contain LiNb in a range that does not affect the optical loss and other characteristics of the optical modulator 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 modulator. 3 O 8 Phase 3c.

[0107] The lithium niobate film forming the dielectric thin film 3 contains LiNb 3 O 8 Phase 3c case of LiNb 3 O 8 Phase 3c does not grow in a direction substantially perpendicular to the main surface 2a of the single crystal substrate 2. Figure 1 As shown, the LiNb forming the stress relaxation layer 31 3 O 8 The phase 3c grows substantially vertically with respect to the main surface 2a of the single crystal substrate 2. Therefore, the lithium niobate film forming the dielectric thin film 3 contains LiNb 3 O 8 In the case of the phase 3 c as well, the dielectric thin film 3 and the stress relaxation layer 31 can be easily distinguished.

[0108] The dielectric thin film 3 has a thickness of 0.5 μm or more. Therefore, when the dielectric thin film 3 of the substrate 1 with dielectric thin film is used as an optical waveguide layer of an optical modulation element, it can be applied to a wide range of light from visible light to infrared light.

[0109] The film thickness of the dielectric thin film 3 is 2 μm or less. Since the film 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.

[0110] In the substrate 1 with a dielectric film of the present embodiment, the ratio of the film thickness of the stress relaxation layer 31 to the film thickness of the dielectric film 3 ((stress relaxation layer 31 / dielectric 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 film 3 is 5% or more, the strain and stress generated by the difference in lattice constant and linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film forming the dielectric film 3 can be effectively relaxed by the stress relaxation layer 31. Therefore, cracks are less likely to occur in the lithium niobate film.

[0111] The ratio of the film thickness of the stress relaxation layer 31 to the film thickness of the dielectric thin film 3 is 25% or less. Since the ratio of the film thickness of the stress relaxation layer 31 to the film thickness of the dielectric thin film 3 is 25% or less, when the dielectric thin film 3 of the substrate 1 with a 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 film thickness of the stress relaxation layer 31 to the film thickness of the dielectric thin film 3 is preferably 5% to 20%, and more preferably 5% to 15%.

[0112] In the substrate 1 with a dielectric thin film of the present embodiment, the stress relief 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 single crystal substrate 2 as the base and the stress relief layer 31 as the base film. More specifically, when the film surface of the lithium niobate film forming the dielectric thin film 3 is set as the XY plane and the film thickness direction is set as the Z axis, the crystals of the single crystal substrate 2, the epitaxial film forming the stress relief layer 31, and the epitaxial film forming the dielectric thin film 3 are aligned in the X-axis, Y-axis, and Z-axis directions.

[0113] That the stress relaxation layer 31 and the dielectric thin film 3 are epitaxial films can be verified by, for example, firstly, confirming the peak intensity at the orientation position by 2θ-θ X-ray diffraction and secondly, confirming the pole by pole measurement.

[0114] Specifically, when the stress relaxation layer 31 and the dielectric thin film 3 are epitaxial films, as a first condition, when measuring by 2θ-θ X-ray diffraction, all peak intensities other than the target surface need to be less than 10% of the maximum peak intensity of the target surface, preferably less than 5%. In the c-axis oriented epitaxial film forming the stress relaxation layer 31 and the dielectric thin film 3, the peak intensity other than the (00L) surface is less than 10% of the maximum peak intensity of the (00L) surface, preferably less than 5%. (00L) is a general term for equivalent surfaces such as (001) and (002).

[0115] In the above-mentioned conditions for confirming the peak intensity at the orientation position by 2θ-θ X-ray diffraction, only the orientation in one direction is indicated. Therefore, even if the above-mentioned first condition is met, when the in-plane crystal orientation is not consistent, the intensity of the X-ray at a specific angle position is not high and the pole cannot be seen.

[0116] Therefore, when proving that the stress relaxation layer 31 and the dielectric thin film 3 are epitaxial films, it is necessary to see a pole in pole measurement as a second condition.

[0117] Since LiNbO 3 The crystal structure of single crystal LiNbO is trigonal. 3 There are three poles of (014). When epitaxially growing a lithium niobate film, it is known that epitaxial growth is achieved in a so-called twin state in which crystals obtained by rotating 180° around the c-axis are symmetrically bonded. In this case, since the three poles are symmetrically bonded in pairs, there are six poles.

[0118] In the substrate 1 with a dielectric thin film of the present embodiment, when a sapphire single crystal substrate whose main surface 2a is a c-plane 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 preferably deviate from the c-axis of the stress relaxation layer 31, the dielectric thin film 3 and the single crystal substrate 2 by 5° or less, and more preferably all coincide with each other. If the deviation of the c-axis of the stress relaxation layer 31, the dielectric thin film 3 and the single crystal substrate 2 is 5° or less, there will be no practical problems with the characteristics of the optical modulation element using the substrate 1 with a dielectric thin film.

[0119] [Method for manufacturing a substrate with a dielectric thin film]

[0120] Next, the manufacturing method of the substrate 1 with a dielectric thin film of the present embodiment is described by way of example. When manufacturing the substrate 1 with a dielectric thin film of the present embodiment, first, a stress relaxation layer 31 is formed in contact with the main surface 2a of the single crystal substrate 2 (stress relaxation layer film forming step). Then, a dielectric thin film 3 is formed in contact with the stress relaxation layer 31 (dielectric thin film film forming step).

[0121] (Stress relaxation layer film forming process)

[0122] In the stress relaxation layer film forming step, the stress relaxation layer 31 is formed by epitaxial growth on the main surface 2a of the single crystal substrate 2. As a film forming method of the stress relaxation layer 31, for example, a sputtering method, a vacuum evaporation method, a pulsed laser ablation (PLD) method, a chemical vapor deposition method (CVD), a sol-gel method, etc. can be used.

[0123] As a film forming method of the stress relief layer 31, sputtering is also preferably used among the above. This is because, by forming the stress relief layer 31 using the sputtering method, no special treatment can be performed after the film is formed, and a single domain structure can be obtained by maintaining the state of the film. This is because the heat applied during sputtering and the electric field generated by the self-bias (self-bias) have both polarization treatment. If there is a distribution in the polarization, it becomes an important reason for the decrease in the electro-optical effect. If it is a single domain structure, the same electro-optical coefficient as the case of a single crystal can be obtained.

[0124] When sputtering is used as a method for forming the stress relaxation layer 31, a target having a composition of a target film composition is used as a target. Specifically, for example, a target having a composition in the range of Li / (Li+Nb)=48% to 51% can be used.

[0125] The target can be prepared, for example, by the following method. As a raw material, for example, Li with a purity of 3N or more is prepared. 2 CO 3 and Nb 2 O 5 Next, a ball mill is used, and a sintered body made of ZrO is used. 2 The raw materials are crushed and mixed to form a target powder material. The target powder material is sintered using a known method to obtain a target.

[0126] In the target manufacturing process, when the raw material is pulverized using the above-mentioned ball mill, the ZrO 2 Therefore, hundreds of ppm of Zr are mixed into the target powder obtained by crushing and mixing the raw materials. Therefore, Zr is also mixed into the target formed 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 mixed with Zr without any obstacles. Therefore, there will be no problem caused by the mixing of Zr in the target.

[0127] The shape of the target used in forming the stress relaxation layer 31 is not particularly limited. In order to obtain a stress relaxation layer 31 having a uniform film thickness, the target is preferably circular with a flat surface area that is twice or more that of the single crystal substrate 2. In addition, in order to obtain a stress relaxation layer 31 having a uniform film thickness, the target is preferably arranged coaxially with the single crystal substrate 2 to form the stress relaxation layer 31.

[0128] When sputtering is used as a method for forming the stress relaxation layer 31, for example, Ar and O can be used. 2 The mixed gas is used as the sputtering gas, and the O 2The ratio 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., a power of 1500 to 2000 W is applied, and the film formation rate is set to 300 to 1800 nm / h.

[0129] It is preferable that the stress relaxation layer 31 is formed in a so-called single step without changing the film forming conditions during the process.

[0130] (Dielectric thin film forming process)

[0131] In the dielectric thin film forming step, the dielectric thin film 3 is formed by epitaxial growth on the stress relaxation layer 31. As a method for forming the dielectric thin film 3, for example, a sputtering method, a vacuum evaporation method, a pulsed laser ablation (PLD) method, a chemical vapor deposition method (CVD), a sol-gel method, etc. can be used.

[0132] As a film forming method of the dielectric thin film 3, sputtering is also preferably used among the above. This is because, by forming the dielectric thin film 3 using the sputtering method, no special treatment can be performed after the film is formed, and a single domain structure can be obtained by maintaining the film-formed state. This is because the heat applied during sputtering and the electric field generated by the self-bias voltage have both polarization treatment. If there is a distribution in the polarization, it becomes an important reason for the decrease in the electro-optical effect. If it is a single domain structure, the same electro-optical coefficient as the case of a single crystal can be obtained.

[0133] When sputtering is used as the film forming method of the dielectric thin film 3 , a target having a composition in the range of Li / (Li+Nb)=48% to 51% can be used as a target.

[0134] As a method for producing the target used for forming the dielectric thin film 3 , a known method can be used, and for example, the same method as the method for producing the target used for forming the stress relaxation layer 31 can be used.

[0135] The shape of the target used in forming the dielectric thin film 3 is not particularly limited. In order to obtain a dielectric thin film 3 having a uniform film thickness, the target is preferably circular with a flat surface area that is twice or more that of the single crystal substrate 2. In addition, in order to obtain a dielectric thin film 3 having a uniform film thickness, the target is preferably arranged coaxially with the single crystal substrate 2 to form the dielectric thin film 3.

[0136] When sputtering is used as the film forming method of the dielectric thin film 3, for example, Ar and O can be used. 2 The mixed gas is used as the sputtering gas, and the O 2The ratio 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., a power of 1500 to 2000 W is applied, and the film formation rate is set to 300 to 1800 nm / h.

[0137] The dielectric thin film 3 is preferably formed in a so-called single step without changing the film forming conditions during the process.

[0138] Preferably, the formation of the dielectric thin film 3 includes the following steps: forming the stress relaxation layer 31 by sputtering and then changing the amount of O in the sputtering gas. 2 Except for the conditions of the ratio and the gas pressure, the film is continuously formed by the sputtering method in the same manner as the film forming method of the stress relaxation layer 31 .

[0139] Specifically, after the stress relaxation layer 31 is formed, the O content in the sputtering gas can be reduced. 2 The ratio is increased and the gas pressure during sputtering is increased to form the dielectric thin film 3.

[0140] Through the above steps, the substrate 1 with a dielectric thin film according to the present embodiment is obtained.

[0141] In the substrate 1 with a dielectric film of the present embodiment, strain and stress caused by the difference in lattice constant and linear expansion coefficient between the single crystal substrate 2 and the lithium niobate film forming the dielectric film 3 are absorbed and relaxed by the stress relaxation layer 31. Therefore, the substrate 1 with a dielectric film of the present embodiment is less likely to have cracks in the lithium niobate film.

[0142] [Optical waveguide components]

[0143] Figure 2 It means use Figure 1 FIG. 1 is a plan view of an example of an optical waveguide device 100 having a substrate 1 with a dielectric thin film. Figure 3 yes Figure 2 The optical waveguide element 100 is a cross-sectional view taken along the line AA'. Figure 2 and Figure 3 In the optical waveguide element 100 shown in FIG. Figure 1 The components of the substrate 1 with a dielectric thin film shown are denoted by the same reference numerals, and description thereof will be omitted.

[0144] Figure 2 and Figure 3 The optical waveguide element 100 shown has: Figure 1 The dielectric thin film 3 of the dielectric thin film-attached substrate 1 shown is processed into a ridge-shaped (convex shape) ridge portion 4. The ridge portion 4 of the optical waveguide element 100 is a portion where the target light propagates in the TM fundamental mode.

[0145] Figure 2 and Figure 3 The optical waveguide element 100 shown can be Figure 1 The dielectric thin film 3 of the substrate with dielectric thin film 1 shown is manufactured by processing it into a ridge shape (convex shape). As a method of processing the dielectric thin film 3 into a ridge shape, a well-known method such as an etching method can be used.

[0146] Figure 2 and Figure 3 The optical waveguide element 100 shown has Figure 1 The substrate 1 with dielectric thin film shown. Therefore, cracks are not likely to occur in the lithium niobate film forming the dielectric thin film 3 of the substrate 1 with dielectric thin film, and the production efficiency is excellent. In addition, since cracks are not likely to occur in the lithium niobate film forming the dielectric thin film 3 of the substrate 1 with dielectric thin film, the optical waveguide element 100 is excellent in durability. In addition, Figure 2 and Figure 3 The dielectric film 3 of the optical waveguide element 100 shown has a sufficiently thick film thickness, and the ratio of the film thickness of the stress relaxation layer 31 to the film thickness of the dielectric film 3 is appropriate, so that it can be applied to a wide range of light from visible light to infrared light and has low optical loss.

[0147] [Light modulation element]

[0148] Figure 4 It means use Figure 1 FIG. 1 is a plan view of an example of a Mach-Zehnder type light modulation element 200A having a substrate 1 with a dielectric thin film. Figure 5 yes Figure 4 A cross-sectional view of the light modulation element 200A taken along line BB′ is shown. Figure 4 The cross-sectional view of the light modulation element 200A taken along the line AA' is similar to Figure 3 The cross-sectional views of the optical waveguide components 100 shown are identical.

[0149] exist Figure 4 and Figure 5 In the light modulation element 200A shown in FIG. Figure 1 The components of the substrate 1 with a dielectric thin film shown are denoted by the same reference numerals, and description thereof will be omitted.

[0150] Figure 4 and Figure 5 The optical modulation element 200A shown is a device that applies voltage to the Mach-Zehnder interferometer formed by the optical waveguide 10 to modulate the light propagating in the optical waveguide 10. Figure 4 As shown, the optical waveguide 10 includes a first optical waveguide 10a and a second optical waveguide 10b branched from one input optical waveguide, and an output optical waveguide 10c obtained by combining the first optical waveguide 10a and the second optical waveguide 10b.

[0151] like Figure 4 and Figure 5 As shown in FIG. 1 , two first electrodes 7a and 7b are provided on the first optical waveguide 10a and the second optical waveguide 10b, respectively. Therefore, the optical modulation element 200A has a two-electrode structure.

[0152] Figure 4 and Figure 5 The light modulation element 200A shown has: Figure 1 The dielectric thin film 3 of the substrate 1 with dielectric thin film shown in FIG. 1 is processed into a ridge portion 4 having a ridge shape (convex shape). In the optical modulation element 200A, an optical waveguide 10 is formed by the ridge portion 4. Figure 5 As shown, a first electrode 7a is formed on the ridge 4 of the first optical waveguide 10a constituting the optical waveguide 10 via a buffer layer 5. Also, a first electrode 7b is formed on the ridge 4 of the second optical waveguide 10b constituting the optical waveguide 10 via a buffer layer 5.

[0153] like Figure 4 and Figure 5 As shown in FIG. 1 , the second electrodes 8a, 8b, and 8c are provided separately from each other via the first electrodes 7a and 7b. The second electrodes 8a, 8b, and 8c are respectively formed in contact with the upper surface of the slab portion formed of the dielectric film 3. The slab portion formed of the dielectric film 3 is formed by etching or the like. Figure 1 The upper surface of the dielectric film 3 of the substrate 1 with dielectric film shown in FIG. Figure 4 As shown in FIG. 1 , the first electrodes 7a, 7b and the second electrodes 8a, 8b, 8c are connected via a terminal resistor 9. Figure 5 As shown, a dielectric layer 6 is formed so as to be in contact with the lower surface of the buffer layer 5 and the side surfaces of the ridge portion 4 .

[0154] Next, the operating principle of the light modulation element 200A will be described.

[0155] like Figure 4 As shown in FIG. 1 , the two first electrodes 7a, 7b and the second electrodes 8a, 8b, 8c are connected via a terminal resistor 9 to function as traveling wave electrodes. The second electrodes 8a, 8b, 8c are ground electrodes. Then, a so-called complementary signal having the same absolute value but different positive and negative phases without deviation is input from the input sides 15a, 15b of the first electrodes 7a, 7b of the optical modulation element 200A to the two first electrodes 7a, 7b.

[0156] The lithium niobate film forming the dielectric thin film 3 of the substrate 1 with dielectric thin film has an electro-optical effect. Therefore, by applying an electric field to the first optical waveguide 10a and the second optical waveguide 10b, the refractive index of the first optical waveguide 10a and the second optical waveguide 10b changes in the manner of +Δn and -Δn, respectively. 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 the change in the 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 combined.

[0157] The light modulation element 200A of this embodiment includes Figure 1 The substrate with dielectric film 1 is shown. Therefore, the light modulator 200A of this embodiment is not prone to cracks in the lithium niobate film forming the dielectric film 3 of the substrate with dielectric film 1, and is applicable to a wide range of light from visible light to infrared light, has excellent production efficiency and durability, and has low optical loss.

[0158] Although preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments. The present invention can be modified in various ways without departing from the gist of the present invention, and such modifications are naturally included in the scope of the present invention.

[0159] [Example]

[0160] "Example 1 to Example 8, Comparative Example 2 to Comparative Example 5"

[0161] The following method is used to produce Figure 1 The substrate 1 with dielectric film is shown.

[0162] As the single crystal substrate 2 , a 4-inch sapphire single crystal substrate whose main surface 2 a is the c-plane is prepared.

[0163] (Stress relaxation layer film forming process)

[0164] In the stress relaxation layer film forming step, the stress relaxation layer 31 is formed on the main surface 2 a of the single crystal substrate 2 by epitaxial growth using a sputtering method.

[0165] As the target, a circular target with a diameter of 8 inches and a composition of Li / (Li+Nb)=50% was used.

[0166] The target was prepared by the following method. As a raw material, Li with a purity of 3N or higher was prepared. 2 CO 3 and Nb 2 O 5 Next, a ball mill is used, and a sintered body made of ZrO is used. 2The raw materials are crushed and mixed to form a ball, and the target powder is obtained by sintering the obtained target powder to obtain a target.

[0167] The stress relaxation layer 31 was formed by arranging the target obtained in this way coaxially with the single crystal substrate 2 so that the distance from the main surface 2 a of the single crystal substrate 2 was 110 mm.

[0168] The stress relaxation layer 31 was formed by using Ar and O as sputtering gases. 2 The mixed gas is O 2 The ratio is set to 45% to 65%, the gas pressure is set to 0.08 Pa to 0.4 Pa, the temperature of the single crystal substrate 2 is set to 550 to 700° C., the power is applied to 1500 to 2000 W, and the film forming rate is 500 nm / h to 600 nm / h. In addition, by changing the film forming time, the film thickness of the stress relaxation layer 31 becomes a predetermined film thickness.

[0169] The stress relaxation layer 31 is formed in a so-called single step without changing the film forming conditions during the process.

[0170] (Dielectric thin film forming process)

[0171] In the dielectric thin film forming step, the dielectric thin film 3 is formed on the stress relaxation layer 31 by epitaxial growth using a sputtering method.

[0172] As the target, a circular target with a diameter of 8 inches and a composition of Li / (Li+Nb)=50% was used.

[0173] The target was prepared by the following method. As a raw material, Li with a purity of 3N or higher was prepared. 2 CO 3 and Nb 2 O 5 Next, a ball mill is used to form a sintered body of ZrO. 2 The raw materials are crushed and mixed to form a ball, and the target powder is sintered to obtain a target.

[0174] The dielectric thin film 3 was formed by arranging the target obtained in this way coaxially with the single crystal substrate 2 so that the distance from the stress relaxation layer 31 formed on the main surface 2 a of the single crystal substrate 2 was 110 mm.

[0175] The dielectric thin film 3 is formed continuously after the stress relaxation layer 31 is formed by sputtering.

[0176] The film formation of the dielectric thin film 3 is performed by changing the O in the sputtering gas. 2 The stress relaxation layer 31 is formed by sputtering in the same manner as the film formation method except for the ratio and the gas pressure.

[0177] Specifically, after the stress relaxation layer 31 is formed, the O content in the sputtering gas is reduced. 2 The ratio is set and the gas pressure during sputtering is increased to form the dielectric thin film 3. In addition, by changing the film forming time, the film thickness of the dielectric thin film 3 becomes a predetermined film thickness.

[0178] The film formation conditions of the dielectric thin film 3 were not changed during the process, and the process was performed in a so-called single step. Through the above steps, the substrates 1 with dielectric thin films of Examples 1 to 8 and Comparative Examples 2 to 5 were obtained.

[0179] "Comparative Example 1"

[0180] A substrate with a dielectric thin film was manufactured in the same manner as in Example 5 except that the dielectric thin film 3 was formed on the main surface 2 a of the single crystal substrate 2 without forming the stress relaxation layer 31 .

[0181] "Example 9, Example 10, Comparative Example 6 to Comparative Example 8"

[0182] 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 film formation time of the stress relaxation layer 31 in the stress relaxation layer film formation step and the film formation time of the dielectric thin film 3 in the dielectric thin film film formation step were changed.

[0183] For the substrates with dielectric films of Examples 1 to 10 and Comparative Examples 1 to 8 obtained in this way, the "presence or absence of stress relaxation layer 31" is investigated according to the method shown below. By confirming the peak intensity at the orientation position by 2θ-θ X-ray diffraction in the first pass and the pole by pole measurement in the second pass by the above-mentioned method, it is confirmed that the stress relaxation layer 31 and the dielectric film 3 (or the dielectric film 3 in the case of no stress relaxation layer 31) are epitaxial films.

[0184] "Presence or absence of stress relaxation layer 31"

[0185] 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 image obtained, it was confirmed whether LiNbO 3 The twin crystal structure and LiNb 3 O 8 Phase 3c.

[0186] In addition, for the substrates with dielectric films of Examples 1 to 10 and Comparative Examples 1 to 8, the stress relaxation layer 31 and the dielectric film 3 (or the dielectric film 3 in the case of no stress relaxation layer 31) were investigated by the following method to see whether the stress relaxation layer 31 and the dielectric film 3 had LiNbO 3 As a result, it can be confirmed that the stress relaxation layer 31 and the dielectric film 3 (or the dielectric film 3 in the case of no stress relaxation layer 31) of the substrate with dielectric film in Examples 1 to 10 and Comparative Examples 1 to 8 all have LiNbO 3 The twin crystal structure.

[0187] “With LiNbO 3 "Method for confirming the twin crystal structure of

[0188] A transmission electron microscope (TEM) (manufactured by FEI) was used to observe a cross section of the substrate with the dielectric film cut in the thickness direction to obtain a dark field (DF) image. At this time, the LiNbO 3 The incident conditions of the electron beam are adjusted so that the image of either the first crystal 3a or the second crystal 3b included in the twin crystal structure becomes a high-contrast (bright) state.

[0189] In the dark field image obtained by adjusting the incident conditions of the electron beam as described above, the stress relaxation layer 31 and the dielectric thin film 3 (the dielectric thin film 3 in the case where there is no stress relaxation layer 31) have LiNbO 3 In the case of a twin crystal structure, the image of one of the first crystal 3a and the second crystal 3b becomes a high-contrast (bright) state, and the image of the other becomes a low-contrast (dark) state, so that the first crystal 3a and the second crystal 3b can be clearly distinguished. Therefore, it is confirmed that the stress relaxation layer 31 and the dielectric film 3 (or the dielectric film 3 in the case of no stress relaxation layer 31) have LiNbO 3 The twin crystal structure.

[0190] In addition, for the substrates with dielectric thin films of Examples 1 to 10 and Comparative Examples 1 to 8 obtained in this way, 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 investigated by the methods shown below. The results are shown in Tables 1 and 2.

[0191] Tables 1 and 2 show the types of the single crystal substrates 2 used in the substrates with dielectric thin films of Examples 1 to 10 and Comparative Examples 1 to 8.

[0192] In addition, Example 5 is described in both Table 1 and Table 2 for comparison.

[0193] [Table 1]

[0194]

[0195] [Table 2]

[0196]

[0197] "Maximum domain width of the stress relaxation layer 31"

[0198] The cross section of the substrate 1 with the dielectric thin film was observed by a scanning transmission electron microscope (STEM) (manufactured by FEI) at a magnification of 400,000 times. In the image obtained, the LiNbO 3 The first crystals 3a and the second crystals 3b, LiNb 3 O 8 The maximum domain width in the direction perpendicular to the thickness direction of the phase 3c was measured. From the results, a plurality of first crystals 3a and a plurality of second crystals 3b, a plurality of LiNb 3 O 8 The maximum dimension of the maximum domain width of the phase 3 c is taken as the “maximum domain width of the stress relaxation layer 31 ”.

[0199] "Maximum domain width of dielectric film 3"

[0200] The cross section of the substrate 1 with the dielectric thin film was observed by a scanning transmission electron microscope (STEM) (manufactured by FEI) at a magnification of 400,000 times. In the image obtained, the LiNbO 3 The maximum domain width of each first crystal 3a and each second crystal 3b included in the twin crystal structure is measured in a direction perpendicular to the thickness direction. From the result, the maximum size of the maximum domain width of the plurality of first crystals 3a and the plurality of second crystals 3b existing in the range of 200nm to 800nm ​​in length is extracted as the "maximum domain width of the dielectric thin film 3".

[0201] “Thickness of stress relaxation layer 31 ” “Thickness of dielectric thin film 3 ” “Ratio of thickness of stress relaxation layer 31 to thickness of dielectric thin film 3 ”

[0202] The film thickness of the stress relaxation layer 31 in the substrate 1 with dielectric film and the film thickness of the dielectric film 3 are obtained by performing high-resolution analysis using a scanning transmission electron microscope (STEM) (manufactured by FEI Corporation), measuring the film thickness of three locations within the field of view, and calculating the average value thereof. The ratio of the film thickness of the stress relaxation layer 31 to the film thickness of the dielectric film 3 is calculated using the film thickness of the stress relaxation layer 31 and the film thickness of the dielectric film 3 calculated by the above-mentioned method, using the following formula.

[0203] Thickness ratio of stress relaxation layer 31 = (stress relaxation layer 31 / dielectric film 3) × 100 (%)

[0204] "Ratio of the first diffraction intensity corresponding to the first crystal 3a of the dielectric thin film 3 to the second diffraction intensity corresponding to the second crystal 3b"

[0205] The dielectric thin film 3 of the dielectric thin film-attached substrate 1 was subjected to extreme point measurement by the X-ray diffraction method using an X-ray diffraction apparatus (manufactured by Rigaku Corporation) to measure the first diffraction intensity corresponding to the first crystal 3a and the second diffraction intensity corresponding to the second crystal 3b of the dielectric thin film 3. Using the result, the ratio of the first diffraction intensity to the second diffraction intensity was calculated.

[0206] In addition, the “stress of the dielectric film”, “presence of cracks in the dielectric film 3”, “presence of cracks caused by annealing”, and “optical loss of the dielectric film” were investigated for the substrates with dielectric films of Examples 1 to 18 and Comparative Example 1. The results are shown in Tables 3 and 4.

[0207] [Table 3]

[0208]

[0209] [Table 4]

[0210]

[0211] "Stress in dielectric films"

[0212] The warpage of the dielectric film 3 in the dielectric film-attached substrate 1 was measured using a stylus height difference meter (manufactured by KLA-Tenchore Corporation), and the stress of the dielectric film was calculated based on Stoney's equation.

[0213] When the obtained numerical value of the stress of the dielectric film is a positive number (+), it is evaluated as tensile stress. When the numerical value of the stress of the dielectric film is a negative number (-), it is evaluated as compressive stress.

[0214] "Presence or absence of cracks in dielectric thin film 3"

[0215] The substrate with dielectric thin film was observed using an optical microscope (manufactured by Olympus Corporation) with a field diameter of about 0.5 mm at an objective lens magnification of 20 times and a field diameter of about 0.1 mm at an objective lens magnification of 100 times to check whether there were cracks in the dielectric thin film 3 .

[0216] When no crack was observed, it was evaluated as "no" crack. When even one crack was observed, it was evaluated as "presence" of crack.

[0217] “Presence or absence of cracks in dielectric thin film 3 caused by annealing”

[0218] The substrate 1 with the dielectric thin film was annealed in an oxygen atmosphere at 1 atmosphere and 600° C. for 1 hour.

[0219] Thereafter, the presence or absence of cracks in the dielectric thin film 3 is checked and evaluated in the same manner as in the above-mentioned “presence or absence of cracks in the dielectric thin film 3 ”.

[0220] "Optical Losses in Dielectric Thin Films"

[0221] Infrared light of 1550 nm was propagated in the dielectric thin film of the substrate with dielectric thin film, and the propagation loss in the dielectric thin film was measured using a prism coupler (manufactured by Metricon).

[0222] The case where the propagation loss was less than 1 dB / cm was evaluated as acceptable, and the case where the propagation loss was 1 dB / cm or more was evaluated as unacceptable.

[0223] As shown in Table 3 and Table 4, for all the substrates with dielectric films of Examples 1 to 10, the value of "stress of dielectric film" was "negative (-)", indicating compressive stress, and the cracks in the dielectric film 3 and the cracks caused by annealing were "none". In addition, for all the substrates with dielectric films of Examples 1 to 10, the propagation loss in the dielectric film was less than 1 dB / cm, and the optical loss of the dielectric film was small.

[0224] On the other hand, as shown in Tables 3 and 4, in the substrate with a dielectric film of Comparative Example 1 without the stress relaxation layer 31, the value of "Stress of dielectric film" is "positive (+)" indicating tensile stress, and the cracks in the dielectric film 3 and the cracks caused by annealing are "present". Therefore, it can be confirmed that cracks are less likely to occur in the dielectric film 3 by making the single crystal substrate 2 and the dielectric film 3 in contact with each other via the stress relaxation layer 31.

[0225] As shown in Table 3, in Comparative Examples 2 and 3 where the ratio of the thickness of the stress relaxation layer 31 to the thickness of the dielectric thin film 3 was less than 5%, the dielectric thin film 3 had no cracks but had cracks caused by annealing.

[0226] As shown in Table 4, in Comparative Examples 7 and 8 in which the film thickness of the stress relaxation layer 31 exceeded 2 μm, cracks in the dielectric thin film 3 were “absent”, but cracks generated by annealing were “present”.

[0227] Furthermore, as shown in Table 3, in Comparative Examples 4 and 5, in which the ratio of the film thickness of the stress relaxation layer 31 to the film thickness of the dielectric film 3 exceeded 25%, there were no cracks in the dielectric film 3 or cracks caused by annealing. However, in both Comparative Examples 4 and 5, the propagation loss in the dielectric film exceeded 1 dB / cm, and the optical loss of the dielectric film was large.

[0228] As shown in Table 4, in Comparative Example 6 in which the film thickness of the stress relaxation layer 31 was less than 0.5 μm, cracks in the dielectric thin film 3 and cracks caused by annealing were “none”. 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 large.

[0229] [Mapping analysis of substrates with dielectric films]

[0230] Using a scanning transmission electron microscope (STEM) (manufactured by FEI), 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 a dielectric thin film of Example 5 and Comparative Example 1 was performed. The results are shown in Fig. 6A ~C and Fig. 7A ~C.

[0231] Fig. 6A Figures 1 to 3 are photographs showing images of the interface between the single crystal substrate 2 and the stress relaxation layer 31 of Example 5 obtained by mapping analysis. Fig. 6A In ∼C, the lower region composed of a single phase is the single crystal substrate 2 . Fig. 6A and Figure 6B LiNbO contained in the stress relaxation layer 31 is formed3 A photograph of the first crystal 3a and the second crystal 3b of the twin crystal structure. Figure 6C LiNb contained in the stress relaxation layer 31 3 O 8 Photo of phase 3c.

[0232] Fig. 7A Figures 1 to 3 are photographs showing images of the interface between the single crystal substrate 2 and the dielectric thin film 3 of Comparative Example 1 obtained by mapping analysis. Fig. 7A In ∼C, the lower region composed of a single phase is the single crystal substrate 2 . Fig. 7A and Figure 7B The LiNbO contained in the dielectric thin film 3 is formed 3 A photograph of the first crystal 3a and the second crystal 3b of the twin crystal structure. Figure 7C LiNb contained in the dielectric thin film 3 3 O 8 Photo of phase 3c.

[0233] like Figure 6A to Figure 6C As shown, it can be confirmed that the stress relaxation layer 31 of Example 5 is Figure 7A to Figure 7C Compared with the dielectric thin film 3 of Comparative Example 1 shown, the maximum domain width is extremely small.

[0234] In addition, if Figure 6C As shown, the LiNb contained in the stress relaxation layer 31 of Example 5 3 O 8 The phase 3c grows approximately vertically with respect to the main surface of the single crystal substrate 2, forming Fig. 6A and Figure 6B The LiNbO layer grown substantially vertically with respect to the main surface of the single crystal substrate 2 is shown. 3 The first crystal 3 a and the second crystal 3 b of a twin crystal structure coexist in the stress relaxation layer 31 .

[0235] In contrast, Figure 7C As shown, the LiNb contained in the dielectric thin film 3 of Comparative Example 1 3 O 8 The phase 3 c does not grow from the single crystal substrate 2 .

[0236] Therefore, it can be easily identified that the LiNb contained in the stress relaxation layer 31 of Example 5 3 O 8 phase 3c, and LiNb contained in the dielectric thin film 3 of Comparative Example 1 3 O 8 Phase 3c.

[0237] Description of Reference Numerals

[0238] 1Substrate with dielectric film; 2Single crystal substrate; 2aMain surface; 3Dielectric film; 3aFirst crystal; 3bSecond crystal; 3cLiNb 3 O 8 phase; 4 ridge; 5 buffer layer; 6 dielectric layer; 7a, 7b first electrode; 8a, 8b, 8c second electrode; 9 terminal 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 substrate with a dielectric film, in, include: Single crystal substrate; A stress relaxation layer formed in contact with the main surface of the single crystal substrate; and a dielectric film formed in contact with the stress relaxation layer, The stress relaxation layer is composed of a c-axis oriented epitaxial film and includes: a LiNbO3 double crystal structure including a first crystal and a second crystal obtained by rotating the first crystal 180 degrees around the c-axis; and a LiNb3O8 phase, The dielectric film is composed of a lithium niobate film as a c-axis oriented epitaxial film and has a twin crystal structure of the LiNbO3, 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. The ratio of the film thickness of the stress relaxation layer to the film thickness of the dielectric thin film is 5% to 25%.

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. The substrate with a dielectric thin film according to claim 1, wherein: The single crystal substrate is a sapphire single crystal substrate whose main surface is a c-plane.

4. The substrate with a dielectric thin film according to claim 1, wherein: The dual 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 extreme point measurement by an X-ray diffraction method.

5. An optical waveguide component, wherein: A substrate with a dielectric thin film according to any one of claims 1 to 4.

6. A light modulation element, wherein: A substrate with a dielectric thin film according to any one of claims 1 to 4.

Images