Electro-optical element, and optical modulation element

Abstract

To provide an electro-optical element which has an optical waveguide made of a lithium niobate film epitaxially grown on a substrate and suppresses DC drift.SOLUTION: An electro-optical element is provided, comprising a single crystal substrate, an optical waveguide 10 made of 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 10, where the dielectric thin film is made of a lithium niobate film that is a c-axis oriented epitaxial film and the lithium niobate film has a c-axial length of 13.88 Å or greater.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an electro-optical element and an optical modulation element. [Background technology]

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

[0003] 2. Description of the Related Art Conventionally, electro-optical elements using an LN single crystal substrate include optical modulation elements having an optical waveguide formed by diffusing Ti (titanium) near the surface of an LN single crystal substrate. However, such optical modulation elements have a large cross-sectional shape of the optical waveguide, and the electric field efficiency is poor. For this reason, optical modulators having optical modulation elements using LN single crystal substrates have a long total length of about 10 cm.

[0004] In recent years, there has been a demand for miniaturizing electro-optical elements. One example of a small electro-optical element is an optical modulation element with an optical waveguide made of an LN film epitaxially grown on a single crystal substrate. In such optical modulation elements, good electric field efficiency can be obtained by making the cross-sectional shape of the optical waveguide small, making it possible to significantly reduce the size.

[0005] For example, Patent Document 1 discloses an optical waveguide element having a waveguide made of a lithium niobate film, which is an epitaxial film formed on a single crystal substrate, and a ridge portion having a ridge-shaped cross section. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2015-230466 A Summary of the Invention [Problem to be solved by the invention]

[0007] An optical modulation element having an optical waveguide made of an LN film epitaxially grown on a single crystal substrate can be significantly smaller than an optical modulation element having an optical waveguide made by diffusing Ti (titanium) near the surface of an LN single crystal substrate. However, optical modulation elements having optical waveguides made of LN films epitaxially grown on single crystal substrates have a large DC drift.

[0008] In an optical modulation element having an optical waveguide made of an LN film epitaxially grown on a single crystal substrate, the modulated waveform moves according to the DC (direct current) voltage applied to the electrodes of the optical modulation element. The modulated waveform in an optical modulation element using an LN film changes with the time that the DC (direct current) voltage is applied. This change in the modulated waveform over time is called DC drift.

[0009] The present invention has been made in consideration of the above-mentioned problems, and aims to provide an electro-optical element having an optical waveguide made of a lithium niobate film epitaxially grown on a substrate, in which DC drift is suppressed. [Means for solving the problem]

[0010] An electro-optical element according to one embodiment of the present invention comprises a single crystal substrate, an optical waveguide made of a dielectric thin film formed on and in contact with a main surface of the single crystal substrate, and electrodes for applying a voltage to the optical waveguide, the dielectric thin film made of a lithium niobate film which is a c-axis oriented epitaxial film, and the c-axis length of the lithium niobate film is 13.88 Å or more. Effect of the Invention

[0011] The electro-optical element of the present invention has an optical waveguide made of a dielectric thin film formed on and in contact with the main surface of a single crystal substrate, the dielectric thin film being made of a lithium niobate film that is an epitaxial film oriented along the c-axis, and the c-axis length of the lithium niobate film is 13.88 Å or more. This results in an electro-optical element in which DC drift is suppressed when a voltage is applied to the optical waveguide by the electrodes. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a plan view showing a Mach-Zehnder type optical modulation element 200A, which is an example of the electro-optical element of the present invention. [Diagram 2] 2(a) is a cross-sectional view of the light modulation element 200A shown in FIG. 1 taken along line AA', and FIG. 2(b) is a cross-sectional view of the light modulation element 200A shown in FIG. 1 taken along line BB'. [Diagram 3] FIG. 3 is a process chart for explaining the method for manufacturing the optical modulation element 200A shown in FIG. 1, FIG. 2(a) and FIG. 2(b), and is a cross-sectional view showing the substrate 1 with the dielectric thin film. [Figure 4] FIG. 4 is a graph showing the relationship between the c-axis length and stress of the lithium niobate film in the multiple dielectric thin film-formed substrates 1 of Experimental Example 1. In FIG. [Diagram 5] FIG. 5 is a graph showing the relationship between the c-axis length of the lithium niobate film on the substrate 1 with the dielectric thin film used in the optical modulation element 200A of the second experimental example and the DC drift of the optical modulation element 200A of the second experimental example after one hour. [Figure 6] FIG. 6 is a graph showing the relationship between the application time of the DC voltage and the DC drift in the light modulation element 200A of the second experimental example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The present inventors have solved the above problems and have studied to suppress DC drift when a voltage is applied to an electro-optical element having an optical waveguide made of a lithium niobate film epitaxially grown on a single crystal substrate. Specifically, the present inventors have determined that an electro-optical element is acceptable if the DC drift when heated to 120°C is 50% or less when the application time of a DC (direct current) voltage reaches 1 hour, and have conducted extensive studies focusing on the relationship between the c-axis length of the lithium niobate film forming the optical waveguide and the DC drift.

[0014] As a result, it was found that by forming an electro-optical element having an optical waveguide made of a lithium niobate film that is a c-axis oriented epitaxial film and has a c-axis length of 13.88 Å or more, it is possible to suppress DC drift when a voltage is applied to the optical waveguide. Although the reason why the DC drift of the electro-optical element is suppressed has not been clearly elucidated, it is presumed that this is because the lithium niobate film with a c-axis length of 13.88 Å or more has a sufficiently large compressive stress, so that microscopic gaps are unlikely to form between crystals in the in-plane direction and cracks are unlikely to occur.

[0015] More specifically, the inventors have 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. As a result, it has been conventionally believed that the stress of a lithium niobate film used as an optical waveguide is better when it is closer to 0, but it has been found that a lithium niobate film with a c-axis length of 13.88 Å or more has compressive stress. In addition, it has been found that if the c-axis length of a lithium niobate film is less than 13.88 Å, it may have tensile stress, and that the tensile stress tends to increase as the c-axis length becomes smaller than 13.88 Å.

[0016] The crystal lattice in the lithium niobate film epitaxially grown on the single crystal substrate tries to maintain a constant volume. Therefore, the larger the c-axis length of the lithium niobate film, the smaller the a-axis length, and the smaller the c-axis length, the larger the a-axis length. The c-axis length of the lithium niobate single crystal in the bulk state is about 13.864 Å, which is less than 13.88 Å.

[0017] For these reasons, in a lithium niobate film with a c-axis length of 13.88 Å or more, the a-axis length is small, and the lithium niobate film is trying to expand in the a-axis direction. As a result, a lithium niobate film with a c-axis length of 13.88 Å or more has compressive stress. In contrast, a lithium niobate film with a c-axis length of less than 13.88 Å is likely to have a large a-axis length, and stress that tends to shrink in the a-axis direction is likely to occur in the lithium niobate film. As a result, a lithium niobate film with a c-axis length of less than 13.88 Å is insufficient in compressive stress or has tensile stress. For this reason, in a lithium niobate film with a c-axis length of less than 13.88 Å, microscopic gaps are likely to form between the crystals in the in-plane direction, and cracks are likely to occur. From these findings, it is estimated that in an electro-optical element having an optical waveguide made of a lithium niobate film, which is a c-axis oriented epitaxial film with a c-axis length of less than 13.88 Å, DC drift becomes large when a voltage is applied to the optical waveguide.

[0018] Furthermore, the inventors have confirmed that in an electro-optical element having an optical waveguide made of a lithium niobate film having a c-axis length of 13.88 Å or more epitaxially grown on a single crystal substrate, DC drift can be suppressed when a voltage is applied to the optical waveguide, and have arrived at the present invention.

[0019] The present invention includes the following aspects. [1] A single crystal substrate; an optical waveguide made of a dielectric thin film formed on and in contact with a main surface of the single crystal substrate; an electrode for applying a voltage to the optical waveguide; The electro-optical element, wherein the dielectric thin film is made of a lithium niobate film that is a c-axis oriented epitaxial film, and the c-axis length of the lithium niobate film is 13.88 Å or more.

[0020] [2] The electro-optical element according to [1], wherein the c-axis length of the lithium niobate film is 13.92 Å or less. [3] The electro-optic element according to [1], wherein the single crystal substrate is a sapphire single crystal substrate, the main surface of which is a c-plane.

[0021] [4] An optical modulation element comprising the electro-optical element according to any one of [1] to [3].

[0022] The electro-optical element and the light modulation element of the present embodiment will be described in detail below with reference to the drawings as appropriate. The drawings used in the following description may show characteristic parts enlarged for the sake of convenience in order to make the features of the present invention easier to understand. Therefore, the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto, and may be appropriately modified and implemented within the scope of the present invention.

[0023] [Light modulation element] Fig. 1 is a plan view showing a Mach-Zehnder type light modulation element 200A which is an example of an electro-optical element of the present invention. Fig. 2(a) is a cross-sectional view of the light modulation element 200A shown in Fig. 1 taken along line AA', and Fig. 2(b) is a cross-sectional view of the light modulation element 200A shown in Fig. 1 taken along line BB'.

[0024] The optical modulation element 200A shown in Figures 1, 2(a) and 2(b) has a single crystal substrate 2, an optical waveguide 10 consisting of a dielectric thin film 3 formed on and in contact with the main surface of the single crystal substrate 2, and electrodes (first electrodes 7a, 7b and second electrodes 8a, 8b, 8c) that apply a voltage to the optical waveguide 10.

[0025] (Single crystal substrate 2) As the single crystal substrate 2, any known single crystal substrate can be used as long as it is capable of growing a c-axis oriented epitaxial film made of an LN film. As the single crystal substrate 2, for example, a sapphire single crystal substrate, a silicon single crystal substrate, etc. can be used.

[0026] In the optical modulation element 200A of this embodiment, the single crystal substrate 2 preferably has a lower refractive index than LN. It is particularly preferable to use a sapphire single crystal substrate as the single crystal substrate 2. The sapphire single crystal substrate has a lower refractive index than LN. For this reason, for example, when the dielectric thin film 3 is used as the optical waveguide layer 10, it can play the role of a cladding layer. Therefore, when the single crystal substrate 2 is a sapphire single crystal substrate, the dielectric thin film 3 can be suitably used as the optical waveguide layer 10 of the optical modulation element 200A without providing a separate cladding layer between the single crystal substrate 2 and the dielectric thin film 3. When a silicon single crystal substrate is used as the single crystal substrate 2, it is necessary to provide a layer between the single crystal substrate 2 and the dielectric thin film 3, the refractive index of which is lower than that of LN, since the refractive index of silicon is higher than that of LN.

[0027] In the light modulation element 200A of this embodiment, the dielectric thin film 3 is made of an LN film, which is an epitaxial film oriented along the c-axis, and has three-fold symmetry. For this reason, it is desirable that the crystal orientation of the main surface (the surface on the dielectric thin film 3 side) of the single crystal substrate 2 has the same symmetry as that of the dielectric thin film 3. Therefore, when a sapphire single crystal substrate is used as the single crystal substrate 2, for example, it is preferable that the main surface is a c-plane. Furthermore, when a silicon single crystal substrate is used as the single crystal substrate 2, for example, it is preferable that the main surface is a (111) plane. The single crystal substrate 2 may have an off-angle.

[0028] In the light modulation element 200A of this embodiment, the dielectric thin film 3 formed on the single crystal substrate 2 is likely to be formed as a c-axis oriented epitaxial film for single crystal substrates 2 of various crystal orientations. Therefore, in the light modulation element 200A of this embodiment, the crystal orientation of the single crystal substrate 2 is not particularly limited.

[0029] (Dielectric Thin Film 3) The dielectric thin film 3 is made of an LN film, which is an epitaxial film oriented along the c-axis. Since LN has a large electro-optic constant, it is suitable as a material for the optical waveguide layer 10 of the optical modulation element 200A.

[0030] The LN film forming the dielectric thin 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.

[0031] The LN film forming the dielectric thin film 3 is preferably a single phase consisting of the LiNbO3 phase. The LN film forming the dielectric thin film 3 may contain a different phase such as the LiNb3O8 phase or the Li3NbO4 phase, but in order to obtain good characteristics, it is preferable that the LN film does not contain a different phase such as the LiNb3O8 phase or the Li3NbO4 phase.

[0032] In the light modulation element 200A of this embodiment, the dielectric thin film 3 is made of an LN film that is an epitaxial film with a c-axis orientation, and the c-axis length is 13.88 Å or more. In the light modulation element 200A of this embodiment, the c-axis length of the LN film is 13.88 Å or more, so that DC drift is suppressed when a voltage is applied to the optical waveguide 10. The c-axis length of the LN film is preferably 13.89 Å or more, so that the light modulation element 200A has even more suppressed DC drift.

[0033] The LN film in the light modulation element 200A of this embodiment preferably has a c-axis length of 13.92 Å or less. The LN film having a c-axis length of more than 13.92 Å has a very large compressive stress. As a result, the warping of the single crystal substrate 2 having the epitaxially grown LN film becomes large. As a result of the study by the present inventors, it was found that the light modulation element 200A having the optical waveguide 10 made of an LN film having a c-axis length of 13.92 Å or less can suppress the warping of the single crystal substrate 2 having the LN film from interfering with the microfabrication when manufacturing a device using the light modulation element 200A.

[0034] When an LN film is epitaxially grown, it may grow epitaxially in a so-called twin state in which crystals sharing a 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 this embodiment is a twin crystal.

[0035] 1, 2(a) and 2(b) is a device that applies a voltage to a Mach-Zehnder interferometer formed of an optical waveguide 10 to modulate light propagating through the optical waveguide 10. As shown in Fig. 1, the optical waveguide 10 has a first optical waveguide 10a and a second optical waveguide 10b branched from a single input optical waveguide, and an output optical waveguide 10c in which the first optical waveguide 10a and the second optical waveguide 10b are combined.

[0036] As shown in Fig. 1 and Fig. 2(b), two first electrodes 7a and 7b are provided on the first optical waveguide 10a and the second optical waveguide 10b, respectively. Therefore, the light modulation element 200A has a dual electrode structure. The first electrodes 7a and 7b may be made of, for example, an Au film or a laminated film of a Ti film and an Au film.

[0037] The optical modulation element 200A shown in FIG. 1, FIG. 2(a) and FIG. 2(b) has a ridge portion 4 formed by processing a dielectric thin film 3 into a ridge shape (convex shape). In the optical modulation element 200A, the ridge portion 4 forms an optical waveguide 10. As shown in FIG. 2(b), a first electrode 7a is formed on the ridge portion 4 constituting the first optical waveguide 10a of the optical waveguide 10 via a buffer layer 5. Also, a first electrode 7b is formed on the ridge portion 4 constituting the second optical waveguide 10b of the optical waveguide 10 via a buffer layer 5. As shown in FIG. 2(b), the buffer layer 5 is formed so as to cover the upper surface and the side surface of the ridge portion 4. As the buffer layer 5, for example, one made of a SiO2 film, one made of a thin film in which an oxide of a metal element is added to SiO2, or the like can be used.

[0038] As shown in FIG. 1 and FIG. 2(b), the second electrodes 8a, 8b, 8c are provided spaced apart from each other via the first electrodes 7a, 7b. The second electrodes 8a, 8b, 8c are formed in contact with the upper surface of the slab portion made of the dielectric thin film 3. The second electrodes 8a, 8b, 8c may be made of, for example, an Au film or a laminated film of a Ti film and an Au film. The first electrodes 7a, 7b and the second electrodes 8a, 8b, 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 above the dielectric thin film 3. The slab portion made of the dielectric thin film 3 is obtained by thinning a part 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. As shown in FIG. 1, the first electrodes 7a, 7b and the second electrodes 8a, 8b, 8c are connected via a termination resistor 9.

[0039] [Method of manufacturing optical modulation element] The light modulation element 200A shown in Figures 1, 2(a) and 2(b) can be manufactured, for example, by the manufacturing method shown below. Figure 3 is a process diagram for explaining the manufacturing method of the light modulation element 200A shown in Figures 1, 2(a) and 2(b), and is a cross-sectional view showing the substrate 1 with the dielectric thin film.

[0040] 3, a dielectric thin film 3 is formed on and in contact with a main surface 2a of a single crystal substrate 2 to manufacture a substrate 1 with a dielectric thin film (dielectric thin film formation step). In the dielectric thin film formation step, the dielectric thin film 3 is formed on the main surface 2a of the single crystal substrate 2 by an epitaxial growth method.

[0041] The 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 thickness of the dielectric thin film 3 is 0.2 μm or more, the dielectric thin film 3 of the substrate 1 with the dielectric thin film is applicable to a wide range of light from visible light to infrared light when used as the optical waveguide layer 10 of the optical modulation element 200A. If the thickness of the dielectric thin film 3 is 2 μm or less, the occurrence of cracks in the LN film forming the dielectric thin film 3 can be effectively suppressed.

[0042] The method for forming the dielectric thin film 3 may be any method that allows epitaxial growth on the main surface 2a of the single crystal substrate 2, and may be, for example, a sputtering method, a vacuum deposition method, a pulsed laser ablation (PLD) method, a chemical vapor deposition (CVD) method, a sol-gel method, or the like.

[0043] When sputtering is used as a method for depositing the dielectric thin film 3, a target having a composition in the range of Li / (Li+Nb)=48% to 51%, for example, can be used. The target can be produced, for example, by the following method. As the raw material, for example, a sintered body mainly composed of Li2CO3 and Nb2O5 with a purity of 3N or more is prepared. Next, the raw material is pulverized and mixed using a ball mill using balls made of ZrO2 to obtain a target powder material. The obtained target powder material is sintered using a known method to obtain a target.

[0044] In the target manufacturing process, when the raw material is pulverized using the ball mill, the balls made of ZrO2 are scraped off, and several hundred ppm or less of Zr is mixed into the target. However, since the amount of Zr mixed into the target is small, 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 containing Zr.

[0045] There is no particular limitation on the shape of the target used for forming the dielectric thin film 3. In addition, the target preferably has a planar area twice or more the single crystal substrate 2 so that the dielectric thin film 3 having a uniform thickness can be obtained.

[0046] When sputtering is used as the method for depositing the dielectric thin film 3, the deposition conditions can be, for example, a mixture of Ar and O2 gas as the sputtering gas, with the O2 ratio in the sputtering gas set to 20% to 60%, the gas pressure set to 0.1 Pa to 2 Pa, the temperature of the single crystal substrate 2 set to 400°C to 700°C, and a power of 500 W to 2000 W applied, so that the deposition rate is 2 nm / h to 15 nm / h. This allows the deposition of the dielectric thin film 3 made of an LN film, which is an epitaxial film with a c-axis orientation.

[0047] When sputtering is used as a method for depositing the dielectric thin film 3, an LN film having a c-axis length of 13.88 Å or more and 13.92 Å or less can be obtained by appropriately adjusting the above deposition conditions. However, since the circumstances differ depending on the equipment used to deposit the dielectric thin film 3, even if the above deposition conditions are satisfied, it does not necessarily mean that an LN film having a c-axis length of 13.88 Å or more and 13.92 Å or less can be obtained.

[0048] Next, the dielectric thin film 3 in the substrate 1 with the dielectric thin film shown in FIG. 3 is processed into a ridge shape (convex shape) using a known method such as an etching method, to form an optical waveguide 10 consisting of a ridge portion 4 and a slab portion consisting of the dielectric thin film 3, as shown in FIGS. 2(a) and 2(b).

[0049] Next, a buffer layer 5 is formed so as to cover the top and side surfaces of the ridge portion 4 using a known method such as sputtering, vacuum deposition, pulsed laser ablation (PLD) or chemical vapor deposition (CVD). Thereafter, using a known method such as sputtering or vacuum deposition, second electrodes 8a, 8b, and 8c are formed in contact with the upper surface of the slab portion made of the dielectric thin film 3, and first electrodes 7a and 7b are formed on the buffer layer 5. Through the above steps, the light modulation element 200A shown in FIG. 1, FIG. 2(a) and FIG. 2(b) is obtained.

[0050] [Operation principle of light modulation element] Next, the operating principle of the light modulation element 200A will be described. 1, the two first electrodes 7a, 7b and the second electrodes 8a, 8b, 8c are connected by a termination resistor 9 to function as traveling wave electrodes. The first electrodes 7a, 7b are used as signal electrodes, and the second electrodes 8a, 8b, 8c are used as ground electrodes. So-called complementary signals, which have the same absolute value, different positive and negative phases, and are not shifted, are input to the two first electrodes 7a, 7b from the input sides 15a, 15b of the first electrodes 7a, 7b of the optical modulation element 200A.

[0051] 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 in the in-plane direction of the dielectric thin film 3 from the first electrodes 7a, 7b toward the second electrodes 8a, 8b, 8c. This causes the refractive indexes of the first optical waveguide 10a and the second optical waveguide 10b of the optical waveguide 10 to change in proportion to the DC voltage, and 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 with the application time of the DC (direct current) voltage. This change in the modulated waveform over time is called DC drift.

[0052] The LN film forming the dielectric thin film 3 in the substrate 1 with the dielectric thin film has an electro-optic effect. Therefore, the refractive indexes of the first optical waveguide 10a and the second optical waveguide 10b change to +Δn and -Δn, respectively, depending on a DC (direct current) voltage applied to the first optical waveguide 10a and the second optical waveguide 10b. As a result, the phase difference between the first optical waveguide 10a and the second optical waveguide 10b changes. Signal light having a modulated waveform that is intensity-modulated by this change in phase difference is output to the output side 12 from the output optical waveguide 10c in which the first optical waveguide 10a and the second optical waveguide 10b are multiplexed.

[0053] The optical modulation element 200A of this embodiment shown in Figures 1, 2(a) and 2(b) has an optical waveguide 10 made of a dielectric thin film 3 formed on the main surface 2a of a single crystal substrate 2, the dielectric thin film 3 being made of an LN film which is an epitaxial film with a c-axis orientation, and the c-axis length of the LN film is 13.88 Å or more. Therefore, the optical modulation element 200A is one in which DC drift is suppressed when a DC (direct current) voltage is applied in the in-plane direction from above the optical waveguide 10 by the first electrodes 7a, 7b and the second electrodes 8a, 8b and 8c. Therefore, the optical modulation element 200A of this embodiment is highly reliable and can be suitably used, for example, as an optical communication device.

[0054] When the optical modulation element 200A of this embodiment is heated to 120°C and a signal is input from the input sides 15a, 15b of the first electrodes 7a, 7b, a DC (direct current) voltage is applied from the first electrodes 7a, 7b toward the second electrodes 8a, 8b, 8c in the in-plane direction of the dielectric thin film 3 in a superimposed manner. It is preferable that the DC drift is 50% or less when the application time of the DC (direct current) voltage reaches 1 hour. The DC drift is a numerical value calculated using the following formula (I).

[0055] DC drift (%) = (shift voltage (V) / applied DC voltage (V)) x 100 (I) (In formula (I), the shift voltage (V) is a DC (direct current) voltage that indicates the amount of phase shift (amount of drift) of the modulated waveform relative to the modulated waveform at the time when the DC (direct current) voltage is applied.)

[0056] When the above DC drift of the light modulation element 200A of this embodiment when heated to 120° C. is 50% or less when the application time of the DC (direct current) voltage reaches 1 hour, the inventors consider that the characteristic is sufficiently practical for the device they are considering. This is because the light modulation element 200A is provided with 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, and the amount of shift in the phase of the modulation waveform can be easily compensated by applying a DC voltage corresponding to the shift voltage (V) by the feedback driver.

[0057] Although the preferred embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. The present invention can be modified in various ways without departing from the spirit of the present invention, and it goes without saying that such modifications are also included within the scope of the present invention. For example, in the above-described embodiment, an optical modulation element has been 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 an optical modulation element and may be any electro-optical element that performs operating point control by applying a DC voltage, such as an optical switch. EXAMPLES

[0058] "Experimental Example 1" A plurality of substrates 1 with a dielectric thin film as shown in FIG. 3 were manufactured by the method described below. First, as the single crystal substrate 2, a 4-inch sapphire single crystal substrate having a c-plane main surface 2a was prepared.

[0059] (Dielectric thin film deposition process) In the dielectric thin film formation step, the dielectric thin film 3 made of an LN film was formed on the main surface 2a of the single crystal substrate 2 by epitaxial growth using a sputtering method. The target used was a circle having a diameter of 8 inches and a composition of Li / (Li+Nb)=50%.

[0060] The target was produced using the following method. The raw materials were prepared as sintered bodies mainly composed of Li2CO3 and Nb2O5 with a purity of 3N or higher. The raw materials were then pulverized and mixed in a ball mill using balls made of ZrO2 to produce target powder. The obtained target powder was sintered to obtain the target.

[0061] The dielectric thin film 3 was formed by arranging the target thus obtained coaxially with the single crystal substrate 2 so that the distance from the main surface 2a of the single crystal substrate 2 was 70 mm. In addition, the dielectric thin film 3 was formed by using a mixed gas of Ar and O2 as the sputtering gas, setting the O2 ratio in the sputtering gas to 35% to 60%, the gas pressure to 0.1 Pa to 0.5 Pa, setting the temperature of the single crystal substrate 2 to 450°C to 700°C, and applying a power of 1500 W to 2000 W so that the film formation rate was 2 nm / h to 15 nm / h. By the above steps, a plurality of substrates 1 with dielectric thin films of Experimental Example 1 were obtained.

[0062] For each of the multiple dielectric thin film-attached substrates 1 of Experimental Example 1 obtained in this manner, an out-of-plane 2θ-θ scan was performed using an X-ray diffraction measuring device (manufactured by Rigaku Corporation) to check whether or not they had a c-axis orientation according to the following criteria. As a result, it was confirmed that the dielectric thin film 3 of all of the dielectric thin film-attached substrates 1 of Experimental Example 1 was an epitaxial film having a c-axis orientation.

[0063] "standard" In the 2θ-θ scan of the out-of-plane measurement, the peak of the LN(006) plane, which is c-axis oriented, was strongly observed, and the peak intensity of the LN plane other than the (006) plane was 10% or less of the maximum peak intensity of the (006) plane, so it was determined to have c-axis orientation.

[0064] Furthermore, for each of the multiple dielectric thin film-attached substrates 1 in Experimental Example 1, the c-axis length of the LN film and the stress (film stress) of the dielectric thin film 3 made of an LN film were examined by the method described below. The results are shown in FIG.

[0065] "c-axis length" The c-axis length was determined as six times the d value (planar spacing) of LN(006) observed in the 2θ-θ scan of the out-of-plane measurement described above.

[0066] "Stress in thin dielectric films" The amount of warpage of each of the multiple dielectric thin film-attached substrates 1 in Experimental Example 1 was measured using a needle-type step gauge (manufactured by KLA-Tenchore), and the stress of the dielectric thin film 3 made of an LN film was calculated using the Stoney formula. When measuring the amount of warpage, the direction was specified so that the calculated stress was positive when it was tensile stress, and negative when it was compressive stress.

[0067] Fig. 4 is a graph showing the relationship between the c-axis length and stress of the LN film in the multiple dielectric thin film-coated substrates 1 of Experimental Example 1. When the stress value in Fig. 4 is "plus (+)", the LN film has tensile stress, and when the stress value in Fig. 4 is "minus (-)", the LN film has compressive stress.

[0068] As shown in Fig. 4, it was confirmed that the LN film in the substrate 1 with the dielectric thin film, which has a c-axis length of 13.88 Å or more, has compressive stress. Also, as shown in Fig. 4, it was confirmed that the LN film in the substrate 1 with the dielectric thin film, which has a c-axis length of 13.92 Å or less, has compressive stress of -700 or more, and warpage can be suppressed.

[0069] "Experimental Example 2" [Manufacturing of light modulation elements] Using the manufacturing method described below, thirteen dielectric thin film substrates 1 each having a different c-axis length of the LN film were selected from the multiple dielectric thin film substrates 1 of Experimental Example 1, and thirteen optical modulation elements 200A shown in Figures 1, 2(a) and 2(b) were manufactured.

[0070] First, the dielectric thin film 3 in the substrate 1 with the dielectric thin film was processed into a ridge shape (convex shape) by etching to form an optical waveguide 10 consisting of a ridge portion 4, and a slab portion consisting of the dielectric thin film 3. Next, a buffer layer 5 made of a thin film containing SiO 2 as a main component and an oxide of In was formed so as to cover the upper and side surfaces of the ridge portion 4 by using a sputtering method.

[0071] Thereafter, a sputtering method was used to form a Ti film and an Au film in this order to become second electrodes 8a, 8b, and 8c in contact with the upper surface of the slab portion made of the dielectric thin film 3, and a Ti film and an Au film in this order to become first electrodes 7a and 7b were formed on the buffer layer 5. Using the obtained Ti film and Au film as seed layers, an Au film was further formed by plating, to form second electrodes 8a, 8b, and 8c and first electrodes 7a and 7b made of a laminated film of the Ti film and the Au film. Through the above steps, thirteen light modulation elements 200A of Experimental Example 2 shown in FIG. 1, FIG. 2(a) and FIG. 2(b) were obtained.

[0072] "DC drift evaluation" The DC drift evaluation was performed on each of the 13 light modulation elements 200A of Experimental Example 2 by the method described below. That is, the optical modulation element 200A was heated to 120°C, and a signal was input from the input sides 15a, 15b of the first electrodes 7a, 7b. At that time, a DC (direct current) voltage of 8 V was superimposed and applied from the first electrodes 7a, 7b to the second electrodes 8a, 8b, 8c in the in-plane direction of the dielectric thin film 3. Then, for one hour from the time when the DC (direct current) voltage was applied in the in-plane direction of the dielectric thin film 3, the modulated waveform of the output light output from the output optical waveguide 10c of the optical modulation element 200A to the output side 12 was observed by an oscilloscope.

[0073] Then, when the application time of the DC voltage reached 1 hour, the DC drift (DC drift after 1 hour) was calculated using the above formula (I) from the phase shift (drift amount) of the modulated waveform relative to the modulated waveform at the time of application of the DC voltage. The results are shown in Figure 5.

[0074] 5 is a graph showing the relationship between the c-axis length of the LN film in the substrate 1 with the dielectric thin film used in the optical modulation element 200A of Experimental Example 2 and the DC drift after one hour of the optical modulation element 200A of Experimental Example 2. Those whose DC drift after one hour in FIG. 5 exceeded 50% were evaluated as failing, and those whose DC drift was 50% or less were evaluated as passing. Fig. 6 is a graph showing the relationship between the application time of the DC voltage and the DC drift in the light modulation element 200A of Experimental Example 2. Example 1 in Fig. 6 is the LN film in the substrate 1 with the dielectric thin film used in the light modulation element 200A of Experimental Example 2, whose c-axis length is 13.884 Å, Example 2 is the LN film whose c-axis length is 13.889 Å, and Example 3 is the LN film whose c-axis length is 13.902 Å.

[0075] 5 and 6, the DC drift after one hour of all the light modulation elements 200A in which the c-axis length of the LN film was 13.88 Å or more was 50% or less. In contrast, the DC drift after one hour of the light modulation elements 200A in which the c-axis length of the LN film was less than 13.88 Å was more than 50%.

[0076] From these findings, it was confirmed that DC drift can be suppressed by making the optical waveguide 10 consisting of the dielectric thin film 3 formed on and in contact with the main surface 2a of the single crystal substrate 2 into an LN film, which is an epitaxial film with a c-axis orientation, and by making the optical modulation element 200A into one having a c-axis length of 13.88 Å or more. [Explanation of symbols]

[0077] 1 substrate with dielectric thin film, 2 single crystal substrate, 2a main surface, 3 dielectric thin film, 4 ridge portion, 5 buffer layer, 7a, 7b first electrode, 8a, 8b, 8c second electrode, 9 termination resistor, 10 optical waveguide, 10a first optical waveguide, 10b second optical waveguide, 10c output optical waveguide, 12 output side, 15a, 15b input side, 200A optical modulation element.

Claims

1. A single crystal substrate; an optical waveguide made of a dielectric thin film formed on and in contact with a main surface of the single crystal substrate; an electrode for applying a voltage to the optical waveguide; The electro-optical element, wherein the dielectric thin film is made of a lithium niobate film that is a c-axis oriented epitaxial film, and the c-axis length of the lithium niobate film is 13.88 Å or more.

2. 2. The electro-optical element according to claim 1, wherein the c-axis length of the lithium niobate film is 13.92 Å or less.

3. 2. The electro-optic element according to claim 1, wherein the single crystal substrate is a sapphire single crystal substrate, the main surface of which is a c-plane.

4. 4. An optical modulation element comprising the electro-optical element according to claim 1.

Images