Electro-optical element and optical modulation element

Abstract

The invention provides an electro-optical element and an optical modulation element. An electro-optical element includes a single crystal substrate, an optical waveguide (10) formed of a dielectric thin film formed by contacting a principal surface of the single crystal substrate, and an electrode for applying a voltage to the optical waveguide (10), the dielectric thin film is formed of a lithium niobate film, the lithium niobate film is a c-axis oriented epitaxial film, and the c-axis length of the lithium niobate film is # imgabs0 # or more.

Description

Technical Field

[0001] The present invention relates to electro-optical elements and optical modulation elements. Background Art

[0002] Lithium niobate (LiNbO3, hereinafter sometimes referred to as "LN") has a large electro-optic constant and is therefore suitable as a material for electro-optical elements. Electro-optical elements using LN single crystal substrates have excellent high-frequency response characteristics and are therefore used in devices such as optical modulators and optical switches.

[0003] Conventionally, as an electro-optical element using an LN single crystal substrate, there is an optical modulation element having an optical waveguide formed by diffusing Ti (titanium) in the vicinity of the surface of the LN single crystal substrate.

[0004] However, the cross-sectional shape of the optical waveguide of such an optical modulation element is large and the electric field efficiency is poor. Therefore, an optical modulator having an optical modulation element using an LN single crystal substrate has a total length of about 10 cm.

[0005] In recent years, there is a demand for miniaturization of electro-optical elements. As a small electro-optical element, there is an optical modulation element having an optical waveguide formed by an LN film epitaxially grown on a single crystal substrate. This optical modulation element obtains good electric field efficiency by reducing the cross-sectional shape of the optical waveguide, and thus can achieve significant miniaturization.

[0006] For example, Patent Document 1 discloses an optical waveguide element having a waveguide formed of a lithium niobate film which is an epitaxial film formed on a single crystal substrate and having a ridge portion having a ridge shape in cross section.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Publication No. 2015-230466 Summary of the invention

[0010] Problem that the invention aims to solve

[0011] An optical modulation element having an optical waveguide formed of an LN film epitaxially grown on a single crystal substrate can be significantly miniaturized compared to an optical modulation element having an optical waveguide formed by diffusing Ti (titanium) near the surface of an LN single crystal substrate.

[0012] However, an optical modulation element having an optical waveguide formed of an LN film epitaxially grown on a single crystal substrate has a large DC drift.

[0013] In an optical modulation element having an optical waveguide formed of an LN film epitaxially grown on a single crystal substrate, the modulation waveform moves due to the DC (direct current) voltage applied to the electrodes of the optical modulation element. The modulation waveform of the optical modulation element using the LN film changes as the DC (direct current) voltage is applied. This temporal change in the modulation waveform is called DC drift.

[0014] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an electro-optical element having an optical waveguide formed of a lithium niobate film epitaxially grown on a substrate and in which DC drift is suppressed.

[0015] Solutions for solving problems

[0016] An electro-optical element according to one embodiment of the present invention comprises: a single crystal substrate, an optical waveguide formed by a dielectric thin film formed in contact with a main surface of the single crystal substrate, and an electrode for applying a voltage to the optical waveguide, wherein the dielectric thin film is formed by a lithium niobate film, the lithium niobate film is a c-axis oriented epitaxial film, and the c-axis length of the lithium niobate film is above.

[0017] Effects of the Invention

[0018] The electro-optical element of the present invention has an optical waveguide formed by a dielectric thin film formed in contact with the main surface of the single crystal substrate, the dielectric thin film is formed by a lithium niobate film, and the lithium niobate film is a c-axis oriented epitaxial film, and the c-axis length of the lithium niobate film is Therefore, the DC drift when a voltage is applied to the optical waveguide by the electrode is suppressed in the electro-optical element. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 1 is a plan view showing a Mach-Zehnder optical modulation element 200A as an example of the electro-optical element of the present invention.

[0020] Figure 2A yes Figure 1 2 is a cross-sectional view of the optical modulation element 200A taken along line AA′ shown in FIG.

[0021] Figure 2B yes Figure 1 BB′ line cross-sectional view of the optical modulation element 200A shown in FIG.

[0022] Figure 3 It is used to illustrate Figure 1 , Figure 2A and Figure 2B The process diagram of the method for manufacturing the optical modulation element 200A shown in FIG. 1 is a cross-sectional view showing the substrate 1 with a dielectric thin film.

[0023] Figure 4This is a graph showing the relationship between the c-axis length of the lithium niobate film and the stress in the plurality of substrates 1 with dielectric thin films of Experimental Example 1.

[0024] Figure 5 This is a graph showing the relationship between the c-axis length of the lithium niobate film in the substrate 1 with a dielectric thin film used in the optical modulation element 200A of Experimental Example 2 and the DC drift of the optical modulation element 200A of Experimental Example 2 after one hour.

[0025] Figure 6 3 is a graph showing the relationship between the application time of the DC voltage and the DC drift of the optical modulation element 200A of Experimental Example 2.

[0026] Description of Reference Numerals

[0027] 1 substrate with dielectric film, 2 single crystal substrate, 2a main surface, 3 dielectric film, 4 ridge, 5 buffer 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, 200A optical modulation element. DETAILED DESCRIPTION

[0028] The present inventors have solved the above problems by conducting research to suppress the DC drift when a voltage is applied to an optical waveguide in an electro-optical element having an optical waveguide formed of a lithium niobate film epitaxially grown on a single crystal substrate. Specifically, the present inventors determined that the DC drift when the electro-optical element is heated to 120° C. is 50% or less when the application time of the DC (direct current) voltage reaches 1 hour as acceptable, and repeatedly conducted in-depth research focusing on the relationship between the c-axis length of the lithium niobate film forming the optical waveguide and the DC drift.

[0029] As a result, it was found that the DC drift when a voltage is applied to the optical waveguide can be suppressed by manufacturing an electro-optical element having an optical waveguide formed of a lithium niobate film which is a c-axis oriented epitaxial film with a c-axis length of The reason why the DC drift of the electro-optical element is suppressed is not clear, but it can be inferred that the reason is that: The compressive stress of the lithium niobate film is sufficiently large, so microscopic gaps are unlikely to form between crystals in the in-plane direction, and cracks are unlikely to occur.

[0030] More specifically, the inventors investigated the relationship between the c-axis length of a lithium niobate film epitaxially grown on a single crystal substrate and the stress of the lithium niobate film. It was found that the closer the stress of a lithium niobate film used as an optical waveguide is to zero, the better. However, when the c-axis length is The above lithium niobate film will become compressive stress. In addition, it is found that if the c-axis length of the lithium niobate film is less than Sometimes it becomes tensile stress. The greater the tensile stress, the greater the tendency.

[0031] The crystal lattice in the lithium niobate film epitaxially grown on a single crystal substrate tries to maintain a certain volume. Therefore, when the c-axis length of the lithium niobate film is large, the a-axis length becomes small, and when the c-axis length is small, the a-axis length becomes large. In addition, the c-axis length of the bulk lithium niobate single crystal is About, less than

[0032] Therefore, the c-axis length is The lithium niobate film above shows a state where the a-axis length is small, and the lithium niobate film tries to expand along the a-axis direction. As a result, the c-axis length is The above lithium niobate film has compressive stress. In contrast, the c-axis length is less than The lithium niobate film tends to have a larger a-axis length, which easily generates stress that causes the lithium niobate film to shrink along the a-axis direction. As a result, the c-axis length is smaller than The compressive stress of the lithium niobate film is insufficient or it has tensile stress. Therefore, the c-axis length is less than In the lithium niobate film, microscopic gaps are easily formed between crystals in the in-plane direction, and cracks are easily generated. It can be inferred that: in an epitaxial film with a c-axis orientation and a c-axis length less than In an electro-optical element having an optical waveguide formed of a lithium niobate film, a DC drift increases when a voltage is applied to the optical waveguide.

[0033] Furthermore, the present inventors have confirmed that: The present invention was conceived because the above electro-optical device having an optical waveguide formed of a lithium niobate film can suppress a DC drift when a voltage is applied to the optical waveguide.

[0034] The present invention includes the following aspects.

[0035] [1] An electro-optical element comprising:

[0036] Single crystal substrate,

[0037] An optical waveguide formed of a dielectric thin film formed in contact with the main surface of the single crystal substrate, and

[0038] an electrode for applying a voltage to the optical waveguide,

[0039] The dielectric film is formed of a lithium niobate film, wherein the lithium niobate film is a c-axis oriented epitaxial film, and the c-axis length of the lithium niobate film is above.

[0040] [2] The electro-optical element according to [1], wherein the c-axis length of the lithium niobate film is the following.

[0041] [3] The electro-optical element according to [1], wherein the single crystal substrate is a sapphire single crystal substrate whose main surface is a c-plane.

[0042] [4] An optical modulation element formed by the electro-optical element according to any one of [1] to [3].

[0043] Hereinafter, the electro-optical 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 simply enlarge the characteristic parts in order to make the characteristics of the present invention clear and easy to understand. Therefore, the dimensional ratio of each component is sometimes different from the actual one. The materials, dimensions, etc. illustrated in the following description are examples, and the present invention is not limited thereto, and can be implemented by appropriately changing within the scope of not changing its purpose.

[0044] [Optical modulation element]

[0045] Figure 1 1 is a plan view showing a Mach-Zehnder optical modulation element 200A as an example of the electro-optical element of the present invention. Figure 2A yes Figure 1 A cross-sectional view of the optical modulation element 200A taken along line AA′ shown in FIG. Figure 2B yes Figure 1 BB′ line cross-sectional view of the optical modulation element 200A shown in FIG.

[0046] Figure 1 , Figure 2A and Figure 2B The optical modulation element 200A shown in the figure has: a single crystal substrate 2, an optical waveguide 10 formed by a dielectric thin film 3 formed in contact with a main surface of the single crystal substrate 2, and electrodes (first electrodes 7a, 7b, second electrodes 8a, 8b, 8c) for applying voltage to the optical waveguide 10.

[0047] (Single crystal substrate 2)

[0048] As the single crystal substrate 2, any known single crystal substrate may be used as long as it is a substrate capable of growing a c-axis oriented epitaxial film formed of an LN film. As the single crystal substrate 2, for example, a sapphire single crystal substrate, a silicon single crystal substrate, or the like may be used.

[0049] In the optical modulation element 200A of the present embodiment, the single crystal substrate 2 preferably has a refractive index lower than LN. As the single crystal substrate 2, a sapphire single crystal substrate is particularly preferably used. The refractive index of the sapphire single crystal substrate is lower than LN. Therefore, for example, when the dielectric thin film 3 is used as the optical waveguide layer 10, it can play a role as 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 the optical waveguide layer 10 of the optical modulation element 200A between the single crystal substrate 2 and the dielectric thin film 3 without separately providing a cladding layer.

[0050] When a silicon single crystal substrate is used as the single crystal substrate 2 , since the refractive index of silicon is higher than that of LN, a layer having a refractive index lower than that of LN needs to be provided between the single crystal substrate 2 and the dielectric thin film 3 .

[0051] In the optical modulation element 200A of the present embodiment, the dielectric film 3 is formed of an LN film as an epitaxial film oriented along the c-axis, and has a three-dimensional symmetry. Therefore, the crystal orientation of the main surface (the surface on the dielectric film 3 side) of the single crystal substrate 2 is preferably the same symmetry as that of the dielectric film 3. Therefore, when a sapphire single crystal substrate is used as the single crystal substrate 2, the main surface is preferably a c-plane. In addition, when a silicon single crystal substrate is used as the single crystal substrate 2, the main surface is preferably a (111) plane. As the single crystal substrate 2, a substrate with an off angle can be used.

[0052] In the optical modulation element 200A of this embodiment, the dielectric thin film 3 formed on the single crystal substrate 2 can be easily formed as a c-axis oriented epitaxial film for single crystal substrates 2 of various crystal orientations. Therefore, in the optical modulation element 200A of this embodiment, the crystal orientation of the single crystal substrate 2 is not particularly limited.

[0053] (Dielectric film 3)

[0054] The dielectric thin film 3 is formed of an LN film which is a c-axis oriented epitaxial film. LN has a large electro-optical constant and is therefore suitable as a material for the optical waveguide layer 10 of the optical modulation element 200A.

[0055] The LN film forming the dielectric film 3 may contain elements such as K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ta, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce.

[0056] The LN film forming the dielectric thin film 3 is preferably a single phase formed of LiNbO3 phase. The LN film forming the dielectric thin film 3 may contain heterogeneous phases such as LiNb3O8 phase and Li3NbO4 phase, but preferably does not contain heterogeneous phases such as LiNb3O8 phase and Li3NbO4 phase in order to obtain good characteristics.

[0057] In the optical modulation element 200A of this embodiment, the dielectric thin film 3 is formed of an LN film which is a c-axis oriented epitaxial film, and the c-axis length is In the optical modulation element 200A of this embodiment, since the c-axis length of the LN film is As described above, the DC drift when a voltage is applied to the optical waveguide 10 is suppressed. In order to form the optical modulation element 200A in which the DC drift is further suppressed, the c-axis length of the LN film is preferably above.

[0058] The LN film in the optical modulation element 200A of this embodiment preferably has a c-axis length of Below. c axis length exceeds The compressive stress of the LN film is very large. As a result, the warping of the single crystal substrate 2 having the epitaxially grown LN film increases. The inventors of the present invention have found that by making a single crystal substrate 2 having a c-axis length of The optical modulation element 200A of the optical waveguide 10 formed of the following LN film can suppress the warpage of the single crystal substrate 2 having the LN film from hindering microfabrication when manufacturing a device using the optical modulation element 200A.

[0059] When the LN film is epitaxially grown, it may be epitaxially grown in a so-called twin crystal state in which crystals sharing the c-axis are bonded. There is no problem even if the LN film forming the dielectric thin film 3 in the optical modulation element 200A of the present embodiment is a twin crystal.

[0060] Figure 1 , Figure 2A and Figure 2B The optical modulation element 200A shown is a device that applies voltage to the Mach-Zehnder interferometer formed by the optical waveguide 10, thereby modulating the light carried in the optical waveguide 10. Figure 1 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.

[0061] like Figure 1 and Figure 2B As shown in FIG. 2 , one first electrode is provided on each of the first optical waveguide 10a and the second optical waveguide 10b, that is, two first electrodes 7a and 7b. Therefore, the optical modulation element 200A has a dual-electrode structure. As the first electrodes 7a and 7b, for example, an electrode formed of an Au film, an electrode formed of a stacked film of a Ti film and an Au film, etc. can be used.

[0062] Figure 1 , Figure 2A and Figure 2BThe optical modulation element 200A shown in the figure has a ridge portion 4 obtained by processing a dielectric thin film 3 into a ridge shape (convex shape). In the optical modulation element 200A, the ridge portion 4 is used to form an optical waveguide 10. Figure 2B As shown in FIG. 1 , 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. Figure 2B As shown, the buffer layer 5 is formed to cover the upper surface and side surfaces of the ridge portion 4. As the buffer layer 5, for example, a buffer layer formed of a SiO2 film, a buffer layer formed of a thin film in which a metal element oxide is added to SiO2, or the like can be used.

[0063] like Figure 1 and Figure 2B 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 formed in contact with the upper surface of the plate portion formed by the dielectric thin film 3. As the second electrodes 8a, 8b, and 8c, for example, an electrode formed of an Au film, an electrode formed of a stacked film of a Ti film and an Au film, etc. can be used. The first electrodes 7a, 7b and the second electrodes 8a, 8b, and 8c apply a voltage that changes the refractive index of the first optical waveguide 10a and the second optical waveguide 10b of the optical waveguide 10 in the in-plane direction from the dielectric thin film 3.

[0064] The plate portion formed of the dielectric thin film 3 is obtained by thinning a portion of the upper surface of the dielectric thin film 3 formed in contact with the main surface of the single crystal substrate 2 by etching or the like.

[0065] like Figure 1 As shown, the first electrodes 7 a and 7 b are connected to the second electrodes 8 a, 8 b and 8 c via a terminating resistor 9 .

[0066] [Method for manufacturing optical modulation element]

[0067] Figure 1 , Figure 2A and Figure 2B The optical modulation element 200A shown can be manufactured by, for example, the manufacturing method shown below. Figure 3 It is used to illustrate Figure 1 , Figure 2A and Figure 2B The process diagram of the method for manufacturing the optical modulation element 200A is a cross-sectional view showing the substrate 1 with the dielectric thin film.

[0068] In this embodiment, first, Figure 3As shown, the substrate 1 with a dielectric thin film is manufactured by forming a dielectric thin film 3 in contact with the main surface 2a of the single crystal substrate 2 (dielectric thin film film forming step). In the dielectric thin film film forming step, the dielectric thin film 3 is formed on the main surface 2a of the single crystal substrate 2 by epitaxial growth.

[0069] The film thickness of the dielectric thin film 3 epitaxially grown on the main surface 2a of the single crystal substrate 2 is preferably 0.2 μm to 2 μm. If the film thickness of the dielectric thin film 3 is 0.2 μm or more, when the dielectric thin film 3 of the substrate 1 with a dielectric thin film is used as the optical waveguide layer 10 of the optical modulation element 200A, it can be applied to a wide range of light from visible light to infrared light. If the film thickness of the dielectric thin film 3 is 2 μm or less, the generation of cracks in the LN film forming the dielectric thin film 3 can be effectively suppressed.

[0070] The dielectric thin film 3 may be formed by any method capable of epitaxial growth on the main surface 2a of the single crystal substrate 2, and may be formed by sputtering, vacuum evaporation, pulsed laser ablation (PLD), chemical vapor deposition (CVD), sol-gel method, etc.

[0071] 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.

[0072] The target can be produced using the method shown below. As a raw material, for example, a raw material formed of a sintered body with a purity of 3N or more and Nb2O5 as the main raw materials is prepared. Then, using a ball mill using balls formed of ZrO2, the raw materials are crushed and mixed to produce a target powder material. The obtained target powder material is sintered using a known method to obtain a target.

[0073] It should be noted that, in the manufacturing process of the target, if the raw material is crushed using the ball mill, the balls formed of ZrO2 are cut, and Zr of several hundred ppm or less is mixed into the target. However, the amount of Zr mixed into the target is small, so the dielectric thin film 3 can be epitaxially grown on the main surface 2a of the single crystal substrate 2 by sputtering using the target mixed with Zr.

[0074] The shape of the target used for forming the dielectric thin film 3 is not particularly limited. In order to obtain the dielectric thin film 3 having a uniform film thickness, the target preferably has a flat surface area that is twice or more that of the single crystal substrate 2 .

[0075] As film forming conditions when sputtering is used as a film forming method for the dielectric thin film 3, for example, a mixed gas of Ar and O2 can be used as a sputtering gas, the O2 ratio in the sputtering gas can be set to 20% to 60%, the gas pressure can be set to 0.1Pa to 2Pa, the temperature of the single crystal substrate 2 can be set to 400°C to 700°C, a power of 500W to 2000W can be applied, and the film forming rate can be set to 2nm / h to 15nm / h. In this way, the dielectric thin film 3 formed of an LN film as a c-axis oriented epitaxial film can be formed.

[0076] When sputtering is used as the film forming method of the dielectric thin film 3, by appropriately adjusting the above film forming conditions, a dielectric thin film having a c-axis length of Above and However, the situation varies depending on the equipment used to form the dielectric thin film 3, so even if the above film forming conditions are met, it is not necessarily possible to obtain a LN film with a c-axis length of Above and The following LN membrane.

[0077] Next, using a known method such as etching, Figure 3 The dielectric film 3 in the substrate 1 with the dielectric film is processed into a ridge shape (convex shape), such as Figure 2A and Figure 2B As shown, the optical waveguide 10 formed of the ridge portion 4 is formed, and the slab portion formed of the dielectric thin film 3 is formed.

[0078] Next, the buffer layer 5 is formed so as to cover the upper surface and side surfaces of the ridge portion 4 by using a known method such as sputtering, vacuum deposition, pulsed laser ablation (PLD), or chemical vapor deposition (CVD).

[0079] Thereafter, the second electrodes 8 a , 8 b , 8 c are formed in contact with the upper surface of the plate portion formed of the dielectric thin film 3 by a known method such as sputtering or vacuum deposition, and the first electrodes 7 a , 7 b are formed on the buffer layer 5 .

[0080] Through the above process, we can get Figure 1 , Figure 2A and Figure 2B The optical modulation element 200A is shown.

[0081] [Operation principle of optical modulation element]

[0082] Next, the operating principle of the optical modulation element 200A will be described.

[0083] like Figure 1As shown in FIG. 1 , two first electrodes 7a, 7b and second electrodes 8a, 8b, 8c are connected with a terminal resistor 9 to function as traveling wave electrodes. The first electrodes 7a, 7b are used as signal electrodes, and the second electrodes 8a, 8b, 8c are used as ground electrodes. In addition, so-called complementary signals having the same absolute value and different positive and negative phases and not staggered 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.

[0084] In this embodiment, when a signal is input from the input side 15a, 15b, a DC (direct current) voltage is applied in a superimposed manner along the in-plane direction of the dielectric film 3 from the first electrodes 7a, 7b toward the second electrodes 8a, 8b, 8c. As a result, the refractive index of the first optical waveguide 10a and the second optical waveguide 10b of the optical waveguide 10 changes in proportion to the DC voltage, and an output light having a modulated waveform is output from the output optical waveguide 10c. The modulated waveform of the output light output from the optical modulation element 200A changes as the application time of the DC (direct current) voltage passes. The temporal change of the modulated waveform is called DC drift.

[0085] The LN film forming the dielectric thin film 3 in the substrate 1 with a dielectric thin film has an electro-optical effect. Therefore, due to the DC (direct current) voltage applied 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 like +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 having a modulated waveform modulated in intensity by the phase difference change 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.

[0086] Figure 1 , Figure 2A and Figure 2B The optical modulation element 200A of the present embodiment shown in the figure has an optical waveguide 10 formed by a dielectric thin film 3 formed in contact with the main surface 2a of the single crystal substrate 2. The dielectric thin film 3 is formed by an LN film as a c-axis oriented epitaxial film. The c-axis length of the LN film is As a result, the optical modulation element 200A is formed in which the DC drift is suppressed when a DC (direct current) voltage is applied along the in-plane direction from the optical waveguide 10 by using the first electrodes 7a, 7b and the second electrodes 8a, 8b, 8c. Therefore, the optical modulation element 200A of this embodiment has excellent reliability and can be suitably used as a device for optical communication, for example.

[0087] The optical modulation element 200A of this embodiment is heated to 120°C and a signal is input from the input sides 15a and 15b of the first electrodes 7a and 7b. When a DC (direct current) voltage is applied in a superimposed manner from the first electrodes 7a and 7b toward the second electrodes 8a, 8b and 8c along the in-plane direction of the dielectric film 3, the DC drift is preferably 50% or less when the application time of the DC (direct current) voltage reaches 1 hour. The DC drift is a value calculated using the following formula (I).

[0088] DC drift (%) = (drift voltage (V) / applied DC voltage (V)) × 100···(I)

[0089] (In formula (I), the drift voltage (V) is a DC (direct current) voltage that represents the shift (drift) of the phase of the modulation waveform relative to the time when the DC (direct current) voltage is applied.)

[0090] The optical modulation element 200A of this embodiment shows a sufficiently practical characteristic in the device studied by the present inventors when the DC drift when heated to 120° C. is less than 50% when the application time of the DC (direct current) voltage reaches 1 hour. This is because: by providing a feedback driver (control circuit) that controls the DC (direct current) voltage applied from the first electrodes 7a, 7b to the second electrodes 8a, 8b, 8c in the optical modulation element 200A, and applying a DC voltage corresponding to the drift voltage (V) by the feedback driver, the phase shift amount of the modulation waveform can be easily compensated.

[0091] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. The present invention can be modified in various ways without departing from the gist of the present invention, and these modifications are naturally included in the scope of the present invention.

[0092] For example, in the above embodiment, an optical modulation element is described as a preferred example of the electro-optical element of the present invention, but the electro-optical element of the present invention is not limited to the optical modulation element. Any electro-optical element that controls the operating point by applying a DC voltage can be used, such as an optical switch.

[0093] Example

[0094] "Experimental Example 1"

[0095] By the method shown below, multiple pieces were manufactured Figure 3 The substrate 1 with the dielectric film is shown.

[0096] First, as the single crystal substrate 2 , a 4-inch sapphire single crystal substrate whose main surface 2 a is the c-plane is prepared.

[0097] (Dielectric thin film forming process)

[0098] In the dielectric thin film forming step, the dielectric thin film 3 composed of an LN film is formed by epitaxial growth on the main surface 2 a of the single crystal substrate 2 using a sputtering method.

[0099] As the target, a target having a circular shape with a diameter of 8 inches and a composition of Li / (Li+Nb)=50% was used.

[0100] The target is produced by the method shown below. As a raw material, a raw material formed by a sintered body with Li2CO3 and Nb2O5 having a purity of 3N or more as the main raw materials is prepared. Then, the raw material is crushed and mixed using a ball mill using balls formed of ZrO2 to prepare a target powder material. The obtained target powder material is sintered to obtain a target.

[0101] The dielectric thin film 3 was formed by arranging the target thus obtained coaxially with the single crystal substrate 2 so that the distance between the target and the main surface 2 a of the single crystal substrate 2 was 70 mm.

[0102] In addition, the formation of the dielectric thin film 3 is carried out as follows: a mixed gas of Ar and O2 is used as a sputtering gas, the O2 ratio in the sputtering gas is set to 35% to 60%, the gas pressure is set to 0.1Pa to 0.5Pa, the temperature of the single crystal substrate 2 is set to 450°C to 700°C, a power of 1500W to 2000W is applied, and the film formation speed is carried out in a manner of 2nm / h to 15nm / h.

[0103] Through the above steps, a plurality of substrates 1 with dielectric thin films of Experimental Example 1 were obtained.

[0104] The plurality of substrates 1 with dielectric thin films obtained in Experimental Example 1 were subjected to 2θ-θ scanning of out-of-plane measurement using an X-ray diffraction measuring apparatus (manufactured by Rigaku Corporation) to investigate whether they had c-axis orientation according to the criteria shown below. As a result, it was confirmed that the dielectric thin film 3 was an epitaxial film with c-axis orientation for all the substrates 1 with dielectric thin films in Experimental Example 1.

[0105] "Benchmark"

[0106] In the 2θ-θ scan of the out-of-plane measurement, the peak of the LN (006) plane with c-axis orientation was significantly observed, and when the peak intensity other than the (006) plane was less than 10% of the maximum peak intensity of the (006) plane, it was considered to have c-axis orientation.

[0107] In addition, for each of the plurality of substrates 1 with dielectric thin films of Experimental Example 1, the c-axis length of the LN film and the stress (film stress) of the dielectric thin film 3 formed of the LN film were investigated by the following method. The results are shown in Figure 4 .

[0108] “c-axis length”

[0109] The c-axis length was defined as a value six times the d value (interplanar spacing) of LN (006) observed in the 2θ-θ scan of the Out-of-plane measurement.

[0110] "Stress in dielectric films"

[0111] The warpage of each of the plurality of substrates 1 with dielectric films in Experimental Example 1 was measured using a stylus-type height difference meter (manufactured by KLA-Tenchore), and the stress of the dielectric film 3 formed of the LN film was calculated using the Stoney equation. When measuring the warpage, the direction was defined so that the calculated value of the stress was a tensile stress, which was positive, and the calculated value of the stress was a compressive stress, which was negative.

[0112] Figure 4 1 is a diagram showing the relationship between the c-axis length of the LN film and the stress in the plurality of substrates 1 with dielectric thin films of Experimental Example 1. FIG. Figure 4 When the stress value in is "positive (+)", the LN film has tensile stress. Figure 4 When the stress value in is "negative (-)", the LN film has compressive stress.

[0113] like Figure 4 As shown in the figure, it can be confirmed that the c-axis length of the LN film in the substrate 1 with a dielectric thin film is When the value is above , there is compressive stress. Figure 4 As shown in the figure, it can be confirmed that the c-axis length of the LN film in the substrate 1 with a dielectric thin film is When the compressive stress is -700 or more, the warping can be suppressed.

[0114] "Experimental Example 2"

[0115] [Manufacturing of optical modulation element]

[0116] Thirteen substrates 1 with dielectric thin films having different c-axis lengths of the LN film were manufactured by the following manufacturing method using thirteen substrates 1 with dielectric thin films selected from the plurality of substrates 1 with dielectric thin films of Experimental Example 1. Figure 1 , Figure 2A and Figure 2B The optical modulation element 200A is shown.

[0117] First, the dielectric thin film 3 in the dielectric thin film-attached substrate 1 is processed into a ridge shape (convex shape) by etching to form the optical waveguide 10 formed by the ridge portion 4 and the plate portion formed by the dielectric thin film 3 .

[0118] Next, the buffer layer 5 composed of a thin film containing SiO 2 as a main component and indium oxide is formed by sputtering so as to cover the upper surface and side surfaces of the ridge portion 4 .

[0119] Thereafter, a Ti film and an Au film are sequentially formed on the upper surface of the plate portion formed by the dielectric thin film 3 to form the second electrodes 8a, 8b, and 8c, and a Ti film and an Au film are sequentially formed on the buffer layer 5 to form the first electrodes 7a and 7b. The obtained Ti film and Au film are used as seed layers, and an Au film is further formed by plating to form the second electrodes 8a, 8b, and 8c and the first electrodes 7a and 7b formed by a stacked film of the Ti film and the Au film. Through the above steps, 13 sheets of Figure 1 , Figure 2A and Figure 2B An optical modulation element 200A of Experimental Example 2 is shown.

[0120] “DC Drift Evaluation”

[0121] The DC drift evaluation was performed on the 13 optical modulation elements 200A of Experimental Example 2 by the following method.

[0122] That is, the optical modulation element 200A is heated to 120° C., and a signal is input from the input sides 15a and 15b of the first electrodes 7a and 7b. At this time, a DC (direct current) voltage of 8 V is superimposedly applied from the first electrodes 7a and 7b to the second electrodes 8a, 8b and 8c along the in-plane direction of the dielectric film 3. Then, from the time when the DC (direct current) voltage is applied along the in-plane direction of the dielectric film 3 to the time when 1 hour has passed, the modulation waveform of the output light output from the output optical waveguide 10c of the optical modulation element 200A to the output side 12 is observed using an oscilloscope.

[0123] Then, the DC drift (DC drift after 1 hour) is calculated using the above formula (I) based on the shift amount (drift amount) of the phase of the modulation waveform relative to the modulation waveform at the time when the DC (direct current) voltage application time reaches 1 hour. The results are shown in Figure 5 .

[0124] Figure 5 This is a graph showing the relationship between the c-axis length of the LN film in the substrate 1 with a dielectric thin film used in the optical modulation element 200A of Experimental Example 2 and the DC drift of the optical modulation element 200A of Experimental Example 2 after one hour. Figure 5The DC drift after 1 hour was evaluated as unacceptable when it exceeded 50%, and as acceptable when it was 50% or less.

[0125] in addition, Figure 6 3 is a graph showing the relationship between the application time of the DC voltage and the DC drift of the optical modulation element 200A of Experimental Example 2. Figure 6 In Example 1, the c-axis length of the LN film in the substrate 1 with a dielectric thin film used in the optical modulation element 200A of Experimental Example 2 is In Example 2, the c-axis length is In Example 3, the c-axis length is

[0126] like Figure 5 and Figure 6 As shown in the figure, the c-axis length of the LN film is The DC drift of the optical modulation element 200A described above after one hour is less than 50%. The DC drift of the optical modulation element 200A after 1 hour exceeds 50%.

[0127] It was confirmed that the optical waveguide 10 was formed of an LN film as an epitaxial film with a c-axis orientation and a c-axis length of The optical modulation element 200A described above can suppress DC drift. The optical waveguide 10 is formed of the dielectric thin film 3 formed in contact with the main surface 2 a of the single crystal substrate 2 .

Claims

1. An electro-optical element, comprising: Single crystal substrate, an optical waveguide formed of a dielectric thin film formed in contact with the main surface of the single crystal substrate, and an electrode for applying a voltage to the optical waveguide, The dielectric film is formed by a lithium niobate film, and the lithium niobate film is a c-axis oriented epitaxial film, and the c-axis length of the lithium niobate film is above.

2. The electro-optical element according to claim 1, wherein The c-axis length of the lithium niobate film is the following.

3. The electro-optical element according to claim 1, wherein The single crystal substrate is a sapphire single crystal substrate whose main surface is a c-plane. 4 . An optical modulation element, comprising the electro-optical element according to claim 1 .

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