Optical waveguide element, optical modulator and optical transmission device using optical waveguide element, and method for manufacturing optical waveguide element

Abstract

The invention provides an optical waveguide element, an optical modulation device and an optical transmission device using the optical waveguide element, and a method for manufacturing the optical waveguide element. Even if the optical waveguide element is formed by bonding a low-refractive-index substrate and a TFLN, the optical modulator and the optical transmission device are not liable to deform. In addition, the present invention has a structure in which a proper margin is ensured during bonding, optical connection loss between different waveguides is small, and the size of a chip can be reduced. This optical waveguide element is characterized in that a low-refractive-index substrate comprising a material having a refractive index lower than that of lithium niobate (LN) is disposed, a thin film comprising LN and having a thickness of 1 [mu] m or less is disposed on a portion of the substrate, an optical waveguide comprising a material other than LN and having a refractive index higher than that of the substrate is disposed on the substrate, and the thickness of the thin film is 1 [mu] m or less. At least a part of the optical waveguide is continuously disposed on the thin film from above the substrate, and in a region (transition region) in which the optical waveguide traverses the outer peripheral edge of the thin film, the thickness of the thin film is in a slope shape, and the slope (tan [theta]) of the edge is set to 0.189 or less.

Description

Technical Field

[0001] The present invention relates to an optical waveguide element, an optical modulation device and an optical transmission device using the optical waveguide element, and a method for manufacturing the optical waveguide element. In particular, the present invention relates to an optical waveguide element formed by combining an optical waveguide formed on a thin film of lithium niobate and an optical waveguide made of a material other than lithium niobate, an optical modulation device and an optical transmission device using the optical waveguide element, and a method for manufacturing the optical waveguide element. Background Art

[0002] In recent years, Si waveguides have been used in optical communication waveguides (see Non-Patent Document 1). Since Si waveguides use CMOS technology, scalability, low cost, and optical reception with a Ge / Si-based optical receiving element (PD) can be achieved.

[0003] On the other hand, as a disadvantage, it cannot be used in the visible light region, and pure phase modulation control such as flowing a current in the Si waveguide is also difficult for phase modulation.

[0004] Therefore, as an alternative technology, new platforms using silicon nitride (SiN) and thin films of lithium niobate (Thin Film LiNbO3 (TFLN)) as optical waveguide cores have been studied (see Non-Patent Documents 2 and 3). In order to integrate optical functions, a light source, phase modulation, reception, and optical multiplexing / demultiplexing (power combining / splitting, wavelength combining / splitting, polarization combining / splitting, etc.) are required. Since the most suitable materials for these structures are different, methods for integrating different materials have been continuously developed.

[0005] Among them, an element that integrates optical multiplexing / demultiplexing and phase modulation by mounting silicon nitride (SiN) and amorphous silicon (a-Si) waveguides on TFLN has been developed (see Non-Patent Document 4). Compared with a monolithic TFLN modulator (see Non-Patent Document 2), since the optical confinement of the optical waveguide in this method is weak, the bending radius is as large as about several hundred μm, and the driving voltage (Vπ) is also large.

[0006] Therefore, as Figure 1 shown, a method has been studied in which an Si-based waveguide (SiN, a-Si, crystalline silicon (c-Si)) is used for the optical multiplexing / demultiplexing section, and a rib-type TFLN is used for the phase modulation section. In Figure 1 , on a low refractive index substrate 1 using a material having a refractive index lower than that of lithium niobate, such as SiO2, an Si-based waveguide (10A to 10C, 10C is a ring resonator) is used to form a passive waveguide region A. In addition, an optical control member 200 including a rib-type optical waveguide and a control electrode is formed on TFLN2, and an active waveguide region C is provided.

[0007] The advantage of the optical waveguide device that loads TFLN2 on the low refractive index substrate 1 is that the yield is ensured by not processing LiNbO3 (LN), which is a difficult-to-process (difficult to dry-etch) material. However, between the passive waveguide region A based on the Si-based waveguide and the active waveguide region C using TFLN, a transition region B that connects the two must be formed.

[0008] Figure 2A and Figure 2B are cross-sectional views of the transition region B, Figure 2A is an example in which the Si-based waveguide 10 is disposed on the upper side of the low refractive index substrate 1, Figure 2B is an example in which the Si-based waveguide 10 is disposed within the low refractive index substrate 1. In the portion shown by the dashed box D, for the light wave propagating in the Si-based waveguide 10, the refractive index change of the optical waveguide is large, which causes an increase in the optical connection loss of the transition region B.

[0009] The inventors of the present application disclosed an efficient and highly productive optical connection method for a silicon nitride waveguide and a rib waveguide formed on TFLN in Patent Document 1. However, since a wafer having a silicon nitride waveguide formed thereon and a wafer having TFLN formed thereon are bonded to form one wafer, it is difficult to say that sufficient productivity can be ensured. In addition, the accuracy at the time of pasting a substrate having a passive optical waveguide and TFLN is required. When allowing a margin for the bonding accuracy, an excessive space is required, and the chip size cannot be reduced.

[0010] Prior Art Documents

[0011] Patent Documents

[0012] Patent Document 1: Japanese Patent Application No. 2023-054914 (Filing Date: March 30, 2023)

[0013] Non-Patent Documents

[0014] Non-Patent Document 1: Yikai Su, etc., "Silicon Photonic Platform for PassiveWaveguide Devices: Materials, Fabrication, and Applications", AdvancedMaterials Technologies. 1901153 (2020)

[0015] Non-Patent Document 2: Abdul Rahim, etc., "Expanding the Silicon Photonics Portfolio With Silicom Nitride Photonic Integrated Circuits", Journal of Lightwave Technology, Vol.35, No.4, pp639(Feb.15, 2017)

[0016] Non-Patent Document 3: Mian Zhang, etc., "Integrated Lithium Niobate Electro-optic Modulators: When performance meets scalability", Optica, Vol.8, No.5,pp652(2021)

[0017] Non-Patent Document 4: Sean Nellan, etc., "Ultra-high Extinction Dual-output Thin-film Lithium Niobate Intensity Modulator", arXiv: 2207.02608v1(Jul.6,2022)

[0018] Non-Patent Document 5: Di Zhu, etc. "Integrated photonics on thin-film lithium niobate", Advances in Optics and Photonics Vol.13, pp242-352(2021) Summary of the Invention

[0019] Problems to be Solved by the Invention

[0020] The problem to be solved by the present invention is to solve the above problems and provide an optical waveguide element that, even when using a single wafer formed by bonding a low-refractive-index substrate with a refractive index lower than that of lithium niobate to TFLN, has a structure that ensures an appropriate margin during bonding, has a small optical connection loss between different waveguides, and can further reduce the chip size. Furthermore, an optical modulation device, an optical transmission device using the optical waveguide element, and a method for manufacturing the optical waveguide element are provided.

[0021] Means for Solving the Problems

[0022] In order to solve the above problems, the optical waveguide element, the optical modulation device and the optical transmission device using the optical waveguide element, and the manufacturing method of the optical waveguide element of the present invention have the following technical features.

[0023] (1) An optical waveguide element, characterized in that a low refractive index substrate made of a material having a refractive index lower than that of lithium niobate is provided, a thin film made of lithium niobate and having a thickness of 1 μm or less is provided on a part of the low refractive index substrate, an optical waveguide made of a material having a refractive index higher than that of the low refractive index substrate and made of a material other than lithium niobate is provided on the low refractive index substrate, and at least a part of the optical waveguide is continuously provided from the low refractive index substrate onto the thin film. In a region where the optical waveguide crosses the outer peripheral edge of the thin film, the thickness of the thin film forms a ramp shape, and the slope of the edge is set to 0.189 or less.

[0024] (2) An optical waveguide element, characterized in that a low refractive index substrate made of a material having a refractive index lower than that of lithium niobate is provided, a thin film made of lithium niobate and having a thickness of 1 μm or less is provided on a part of the low refractive index substrate, an optical waveguide made of a material having a refractive index higher than that of the low refractive index substrate and made of a material other than lithium niobate is provided inside the low refractive index substrate, and at least a part of the optical waveguide is continuously provided from a region of the low refractive index substrate where the thin film is not provided to a region of the low refractive index substrate where the thin film is provided. In a region where the optical waveguide crosses the outer peripheral edge of the thin film, the thickness of the thin film forms a ramp shape, and the slope of the edge is set to 0.189 or less.

[0025] (3) In the optical waveguide element according to the above (1) or (2), characterized in that a rib-type optical waveguide is formed on the thin film.

[0026] (4) In the optical waveguide element according to the above (1) or (2), characterized in that the material constituting the low refractive index substrate contains SiO2.

[0027] (5) In the optical waveguide element according to the above (1) or (2), characterized in that the material constituting the optical waveguide is any one of a material containing SiN and Si.

[0028] (6) An optical modulation device, characterized in that the optical waveguide element according to the above (3) is housed in a housing, and the optical modulation device includes an optical fiber for inputting or outputting light waves with respect to the optical waveguide element.

[0029] (7) In the optical modulation device according to the above (6), characterized in that the optical waveguide element has a modulation electrode for modulating light waves propagating in the optical waveguide element, and an electronic circuit for amplifying a modulation signal input to the modulation electrode is provided inside the housing.

[0030] (8) An optical transmission device, characterized by comprising: the optical modulation device described in the above (7); a light source that inputs light waves to the optical modulation device; and an electronic circuit that outputs a modulation signal to the optical modulation device.

[0031] (9) A method for manufacturing an optical waveguide element, for manufacturing the optical waveguide element described in the above (1) or (2), characterized in that when forming the ramp shape of the thin film, a mixed solution of an alkali solution and hydrogen peroxide is used as an etching solution.

[0032] (10) In the method for manufacturing an optical waveguide element described in the above (9), characterized in that when forming the ramp shape of the thin film, a soluble mask and an insoluble mask are sequentially stacked on the thin plate as mask materials and used.

[0033] Effects of the Invention

[0034] First, the present invention is an optical waveguide element, characterized in that a low-refractive-index substrate made of a material having a refractive index lower than that of lithium niobate is provided, a thin film made of lithium niobate and having a thickness of 1 μm or less is provided on a part of the low-refractive-index substrate, an optical waveguide made of a material having a refractive index higher than that of the low-refractive-index substrate and made of a material other than lithium niobate is provided on the low-refractive-index substrate, and at least a part of the optical waveguide is continuously provided from the low-refractive-index substrate onto the thin film. In a region where the optical waveguide crosses the outer peripheral edge of the thin film, the thickness of the thin film becomes a ramp shape, and the slope of the edge is set to 0.189 or less.

[0035] Second, the present invention is an optical waveguide element, characterized in that a low-refractive-index substrate made of a material having a refractive index lower than that of lithium niobate is provided, a thin film made of lithium niobate and having a thickness of 1 μm or less is provided on a part of the low-refractive-index substrate, an optical waveguide made of a material having a refractive index higher than that of the low-refractive-index substrate and made of a material other than lithium niobate is provided in the low-refractive-index substrate, and at least a part of the optical waveguide is continuously provided from a region of the low-refractive-index substrate where the thin film is not provided to a region of the low-refractive-index substrate where the thin film is provided. In a region where the optical waveguide crosses the outer peripheral edge of the thin film, the thickness of the thin film becomes a ramp shape, and the slope of the edge is set to 0.189 or less.

[0036] Thus, in the outer peripheral edge portion (transition region) of the lithium niobate thin film disposed on the low-refractive-index substrate, the thickness of the thin plate becomes a ramp shape, and the slope of the edge is set to 0.189 or less. Therefore, an optical waveguide element having the following structure can be provided: an appropriate margin is ensured when the low-refractive-index substrate is bonded to TFLN, the optical connection loss between different waveguides is small, and the chip size can be reduced. Moreover, an optical modulation device and an optical transmission device using the optical waveguide element can be provided.

[0037] Furthermore, in the method for manufacturing an optical waveguide element, when forming a ramp shape on a thin film of lithium niobate, by using a mixed solution of an alkaline solution and hydrogen peroxide as an etchant, and further using a soluble mask and an insoluble mask as mask materials and laminating them in sequence on the thin plate, a ramp shape as designed can be easily obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 FIG. is a plan view showing an example of an optical waveguide element obtained by combining a passive waveguide and an active waveguide.

[0039] Figure 2A and Figure 2B is Figure 1 a cross-sectional view of the transition region B of the optical waveguide element of Figure 2A showing the case where the optical waveguide 10 is disposed on the TFLN2, Figure 2B showing the case where the optical waveguide 10 is disposed in the low refractive index substrate 1.

[0040] Figure 3 FIG. is a cross-sectional view showing an example of the optical waveguide element of the present invention, showing an example where the optical waveguide 10 is disposed on the TFLN2.

[0041] Figure 4 FIG. is a cross-sectional view showing an example of the optical waveguide element of the present invention, showing an example where the TFLN2 is bonded after the optical waveguide 10 is disposed in the low refractive index substrate 1.

[0042] Figure 5 FIG. is a view showing Figure 3 a part of the manufacturing process of the optical waveguide element shown.

[0043] Figure 6 FIG. is a view showing Figure 5 a part of the subsequent manufacturing process of

[0044] Figure 7A , Figure 7B and Figure 7C FIG. are views for explaining a method of controlling the shape of the edge portion of the TFLN. Figure 7A FIG. shows the case where the etched surface is concave, Figure 7B FIG. shows the case where the etched surface is convex, Figure 7C FIG. shows the case where the etched surface is linear.

[0045] Figure 8 FIG. is a view for explaining a sample for evaluating the shape change caused by etching of the LN.

[0046] Figure 9 FIG. is a chart showing the etched shape of a sample using Figure 8 of

[0047] Figure 10It represents the relationship between the slope of the ramp of the sample using Figure 8 and the thickness of the soluble mask in a graph.

[0048] Figure 11 It represents the relationship between the slope of the ramp of the sample using Figure 8 and the etching shape of the sample, showing the shape during agitation during etching in a graph.

[0049] Figure 12A and Figure 12B are respectively a three-dimensional view and three orthographic views (top view, side view, front view) showing the case where the slope of the ramp at the edge of TFLN is constant.

[0050] Figure 13 It represents the relationship between the height data and the fitting curve during etching using the method of Figure 7B in a graph.

[0051] Figure 14A and Figure 14B are respectively a three-dimensional view and three orthographic views showing the case where the slope of the ramp at the edge of TFLN is in the shape of a fitting function (the etched surface is convex).

[0052] Figure 15 It represents the result of calculating the optical loss in the case of changing the slope of the ramp (changing the length of the slope part (Slope_L)) for the edge part of TFLN having the shapes shown in Figure 12A and Figure 12B and Figure 14A and Figure 14B in a graph.

[0053] Figure 16 It represents the manufacturing process of the optical waveguide element shown in Figure 4 in a graph.

[0054] Figure 17A and Figure 17B are respectively a three-dimensional view and three orthographic views of an optical waveguide element in which an optical waveguide is arranged in a low-refractive-index substrate and a TFLN is integrated on the low-refractive-index substrate.

[0055] Figure 18 It is Figure 12A and Figure 12B an application example of the optical waveguide element, showing a structure for further suppressing optical loss by continuously changing the shape of the optical waveguide loaded on the TFLN in a graph.

[0056] Figure 19 It is Figure 17A and Figure 17B an application example of the optical waveguide element, showing a part of the manufacturing process for forming a rib-type optical waveguide on the TFLN in a graph.

[0057] Figure 20is a view showing a part of the manufacturing process following Figure 19 of the manufacturing process.

[0058] Figure 21 is a perspective view of an optical waveguide element formed by the manufacturing process of Figure 19 and Figure 20 of the manufacturing process.

[0059] Figure 22 is a view showing an example of the optical transmission device of the present invention.

[0060] Reference Numeral Explanation

[0061] 1 Low refractive index substrate

[0062] 10 Optical waveguide (such as SiN)

[0063] 2 TFLN

[0064] 20 Rib waveguide

[0065] F Optical fiber

[0066] LD Light source

[0067] CA Housing

[0068] MD Optical modulation device

[0069] DRV Driver circuit

[0070] DSP Digital signal processor

[0071] OTA Optical transmission device Detailed Description of the Invention

[0072] Hereinafter, the optical waveguide element of the present invention will be described in detail using preferred examples.

[0073] As Figure 3 shown, the optical waveguide element of the present invention is characterized in that a low refractive index substrate 1 made of a material having a refractive index lower than that of lithium niobate is provided, and a thin film (TFLN) 2 made of lithium niobate and having a thickness of 1 μm or less is provided on a part of the low refractive index substrate 1, and an optical waveguide 10 having a refractive index higher than that of the low refractive index substrate 1 and made of a material other than lithium niobate is provided on the low refractive index substrate 1, and at least a part of the optical waveguide 10 is continuously provided from the low refractive index substrate 1 to the thin film 2, and in a region (transition region) where the optical waveguide 10 crosses the outer peripheral edge portion of the thin film 2, the thickness of the thin film 2 has a ramp shape, and the slope (tanθ) of the edge is set to 0.189 or less.

[0074] In addition, as Figure 4As shown, the optical waveguide element of the present invention is characterized in that a low refractive index substrate 1 made of a material with a refractive index lower than that of lithium niobate is provided, a thin film (TFLN) 2 made of lithium niobate with a thickness of 1 μm or less is provided on a part of the low refractive index substrate 1, an optical waveguide 10 with a refractive index higher than that of the low refractive index substrate 1 and made of a material other than lithium niobate is provided in the low refractive index substrate 1, and at least a part of the optical waveguide 10 is continuously provided from a region of the low refractive index substrate 1 where the thin film 2 is not provided to a region of the low refractive index substrate 1 where the thin film 2 is provided. In a region (transition region) where the optical waveguide 10 crosses the outer peripheral edge of the thin film 2, the thickness of the thin film 2 becomes a ramp shape, and the slope (tanθ) of the edge is set to 0.189 or less.

[0075] In the optical waveguide element of the present invention, in the transition region B (the outer peripheral edge of TFLN) connecting the passive waveguide region A and the active waveguide region C of Figure 1 , an inclined surface (ramp shape) is formed in the TFLN2. Specifically, a ramp shape in which the thickness of the TFLN2 gradually changes is formed. The ramp shape is not limited to Figure 3 and Figure 4 shown linear shape, and can also be set to a shape that changes according to a fitting function as described later, such as a ramp shape using a curved surface. And in the optical waveguide element of the present invention, by reducing the angle θ of the edge of the ramp shape, specifically, by setting the slope (tanθ) to 0.189 or less, the optical connection loss in the transition region B can be suppressed.

[0076] Next, use Figure 5 and Figure 6 to illustrate Figure 3 the manufacturing process of the optical waveguide element shown.

[0077] (STEP1)

[0078] On a low refractive index substrate 1 made of a material such as SiO2, a TFLN wafer is prepared by directly bonding and pasting the TFLN2. The label 3 is a holding substrate, which is used to improve the mechanical strength of the entire TFLN wafer, and Si, SiO2, etc. can be used.

[0079] (STEP2)

[0080] On the TFLN2, a film (etching mask) M1 and M2 composed of an appropriate two layers are patterned. For example, the material of the first layer (the layer in contact with the TFLN) is Ti, Al, and the material of the second layer is Au, Ni, a-Si.

[0081] (STEP3)

[0082] The patterned films (M1, M2) are used as an etching mask to wet-etch TFLN2. As the wet-etching solution, a mixture of ammonia water and hydrogen peroxide (APM solution) etc. is appropriate. Here, the etching rate of the material of the first layer (M1) is higher than that of the material of the second layer, LN. Therefore, the etched shape of TFLN becomes a shape with a slope. When the low-refractive-index substrate 1 is SiO2, SiO2 is insoluble in the APM solution, so the low-refractive-index substrate 1 functions as an etching stop layer.

[0083] (STEP4)

[0084] Remove the mask materials (M1, M2) with an appropriate chemical solution etc. For example, for Au, an aqueous solution of iodine-potassium iodide can be used, for Ti, the APM solution can be used, and for Al and a-Si, the KOH solution can be used.

[0085] (STEP5)

[0086] A SiN film (11) that becomes the optical waveguide 10 is formed on the upper surfaces of the low-refractive-index substrate 1 and TFLN2. As the material of the optical waveguide 10, SiN, a-Si etc. can be used, and the following will be described mainly with SiN.

[0087] (STEP6)

[0088] Use an appropriate dry-etching mask m to form the SiN waveguide (10). At this time, the height of the SiN waveguide in the passive waveguide region and the active waveguide region can be adjusted to be optimal in each region, or it can be carried out by multiple dry-etching processes.

[0089] (STEP7)

[0090] After removing the dry-etching mask m, film formation of the cladding layer, heat treatment, electrode formation etc. are carried out as needed. In addition, although the optical waveguide (SiN waveguide) 10 is exemplified as being cut in Figure 6 it can also be formed in a continuous pattern.

[0091] Next, the wet-etching of TFLN will be described in detail.

[0092] Generally, LN is chemically stable. Therefore, in the case of processing LN by wet etching, a chemical solution using hydrofluoric acid is used. This method has a dependence on crystal orientation and crystal defects, and the etching rate is slow, so it is not used in device manufacturing. In order to reduce the chemical stability of LN, chemical etching methods using hydrofluoric acid and KOH by proton exchange and ion implantation have been developed (see Non-Patent Document 5). When hydrofluoric acid or high-concentration (about 50 wt%) KOH is used in the wet etching of LN, there is also a problem that the newly generated SiO2 of the low-refractive-index substrate 1 is etched at the same time. In addition, the ramp shape at the edge of TFLN depends on the distribution of proton exchange and ion implantation, and it is difficult to form an arbitrary shape of the ramp.

[0093] To solve this problem, the inventors of the present application investigated the etching rate of LN with respect to a large number of chemical solutions. As a result, it was found that LN can be etched by using a mixed solution of an alkali solution and hydrogen peroxide. In addition, it was also found that by using ammonia water as the alkali solution, SiO2 is not etched.

[0094] The etching rate of LN in the X-axis direction with respect to a solution at 40 °C obtained by mixing 29 wt% ammonia water and 30 wt% hydrogen peroxide in a volume ratio of 1:3 is about 55 nm / h. A mixed solution of ammonia water and hydrogen peroxide is known as APM (Ammonia-hydrogen Peroxide Mixture cleaning) or SC-1, which is a chemical solution used for semiconductor cleaning, but it is not known that it can etch LN. Here, there may be those skilled in the art who feel that the etching rate of LN is slow, but since the film thickness of TFLN is 1 μm or less and it can be processed by a batch process, it can be used in device manufacturing.

[0095] Next, use Figures 7A - 7C , a method of making the edge portion of TFLN into a ramp shape using the above chemical solution will be described. In addition, the dotted lines shown as S1 to S3 in each figure represent the shape of the etched surface formed as the etching progresses.

[0096] In Figure 7A , a case where the etching mask M is insoluble in the chemical solution (insoluble mask) is shown. In this case, the wet-etched TFLN2 becomes a shape that is dug into a circle (a shape that is concave with respect to the etched surface).

[0097] Next, in Figure 7BIn this case, the etching mask is composed of two layers (M1, M2). The first-layer mask M1 is composed of a "soluble mask" that is soluble in the chemical solution. Additionally, when the second-layer mask M2 is an "insoluble / high-rigidity mask" that is insoluble and has high rigidity, the chemical solution not only etches the LN through the opening of the etching mask but also dissolves the first layer M1 of the etching mask from the side. When the etching rate of the soluble mask M1 in the first layer is faster than the etching rate of the LN, the dissolution from the side of the first layer slows down over time. Therefore, the edge shape of the TFLN becomes convex with respect to the etching surface. On the other hand, when the etching rate of the soluble mask in the first layer is slower than the etching rate of the LN, the edge shape of the TFLN becomes concave with respect to the etching surface.

[0098] As Figure 7C shown, when the second-layer mask M2 is an "insoluble / low-rigidity mask" that is insoluble and has low rigidity, the second-layer etching mask M2 (M2' represents the floating state of the mask) floats up due to the dissolution of the first-layer etching mask M1. As a result, the dissolution from the side of the first layer M1 is approximately constant over time, and the edge shape of the TFLN becomes a straight line.

[0099] By changing the thickness of the first-layer etching mask M1, the edge shape of the TFLN can also be changed. Additionally, as described later, when the Figure 11 insoluble / low-rigidity mask M2 does not float up, the mask can be floated by stirring the chemical solution and applying stress to the film (such as curing shrinkage of the photoresist). Figure 7C

[0100] Furthermore, it was also confirmed that the etching shape changes when using an APM solution and an etching mask. Specifically, the sample shown in Figure 8 was fabricated, and it was confirmed that the slope of the ramp changes according to the etching conditions. Figure 8 is a structure in which an etching mask M1 (soluble mask) and an etching mask M2 (insoluble mask) are stacked on the TFLN2. Figure 8 The upper side is a top view, and the lower side is a side view. The arrow on the left side of the top view indicates the crystal axis direction, Figure 8 and it is understood as X-cut LN. The etching mask M1 uses Al, and the etching mask M2 uses a-Si to form a pattern. Lithography technology is used in the pattern formation, and an RF sputtering device is used when stacking Al and a-Si. Regarding the thickness of Al and a-Si, there are 4 types of Al with thicknesses of 0, 10, 20, and 30 nm, and the thickness of a-Si is 100 nm.

[0101] Put Figure 8The X-cut LN with an etching mask was immersed in APM (a mixed solution of 29 wt% ammonia water and 30 wt% hydrogen peroxide (volume ratio 1:3)) at 40 °C for 8 hours. Then, after removing the etching mask with a 1 normal (1N) KOH solution of alkalinity, the surface distribution of the LN was obtained using a stylus thickness gauge. Figure 9 shows the shape change of the etched surface when the thickness of the etching mask M1 (using Al) is changed. In addition, Figure 9 the relationship between the slope of the etched surface (ramp) and the thickness of the etching mask M1 is shown in Figure 10 . As a result, it can be seen that as the soluble mask (M1) becomes thicker, the ramp length (Slope_L) becomes longer and the slope of the ramp becomes smaller.

[0102] Furthermore, the results in the case of stirring during the etching of LN based on APM for a sample with an Al thickness of 20 nm using the same method are shown in Figure 11 . In the case of stirring, the shape of the ramp part is formed into a more planar (linear) shape.

[0103] By changing the material of the soluble mask M1 from Al to Ti or W with a higher etching rate, the slope of the ramp can be further reduced. The slope and shape of the ramp become a trade-off relationship between the size of the optical transition region and the optical loss, and are selected according to the design.

[0104] In addition, in this embodiment, X-cut LN is used, but the same effect is also confirmed in Z-cut LN.

[0105] Next, in order to confirm the conditions for low-loss optical transition when the optical waveguide (SiN waveguide) 10 is arranged from the low refractive index substrate 1 to the TFLN2, Figure 12A and Figure 12B as well as Figure 14A and Figure 14B models were used to simulate the optical connection loss.

[0106] Figure 12A and Figure 12B In the models, the edge part (outer peripheral part) of the TFLN2 becomes a planar ramp (linear change in the thickness of the TFLN). In addition, in Figure 14A and Figure 14B the models, the edge part of the TFLN2 is a convex curved ramp (the thickness of the TFLN changes according to a fitting function).

[0107] In addition, Figure 12A and Figure 14A are three-dimensional views, Figure 12B and Figure 14B are three-view drawings with a top view in the center, a front view on the left, and a side view on the bottom.

[0108] Figure 13 This is a graph showing the measured data of a sample with a convex slope formed by etching TFLN and the result of fitting its surface shape with a fitting function. If the fitting function represented by the following formula is used, each fitting parameter is α = 3, Slope_L = 60 μm, and LN_t = 0.25 μm.

[0109] [Formula 1]

[0110] In Figure 14A and Figure 14B of the model, the shape of the convex curved slope is set using the fitting function (the above formula) used in the shape of Figure 13 .

[0111] As Figure 12A and Figure 12B and Figure 14A and Figure 14B parameters, it is set that the width (SiN_w) of the optical waveguide (SiN waveguide) 10 = 0.8 μm, the thickness (SiN_t) of the optical waveguide 10 = 0.5 μm, the thickness (LN_t) of TFLN2 = 0.3 μm, and with the length (Slope_L) of the slope portion as a variable, the optical loss of the light wave propagating in the optical waveguide 10 is simulated.

[0112] Specifically, the length (Slope_L) of the slope portion of TFLN is varied in the range of 0 to 30 μm, and the intensity ratio of the output light to the input light in the optical waveguide 10 (optical loss [dB] = -10·log10(output light / input light)) is derived. The simulation results are as Figure 15 shown.

[0113] According to Figure 12A and Figure 12B and Figure 14A and Figure 14B differences in the slope shapes, some differences are observed in the optical loss, but when Slope_L becomes longer, the optical loss gradually approaches 0. In Figure 15 , when Slope_L is 5 μm or more, the optical loss in the transition region can be ignored, and the maximum slope of the fitting function at this time is 0.189 (~10°). The same result can be obtained even if LN_t, SiN_w, and SiN_t are changed.

[0114] Based on this result, the change in the thickness of the thin film at the edge portion (outer peripheral edge portion) of TFLN2 (the slope (tanθ) at the edge of the slope shape) is preferably set to 0.189 or less.

[0115] In addition, observe Figure 15It can be seen that, compared with LN_t being 0.3 μm, when Slope_L is 5 μm or more, the optical loss is significantly reduced. Therefore, by setting the "average slope", that is, LN_t (thickness) / Slope_L (length of the slope) ≤ 0.06 for the ramp portion, an optical waveguide element with lower loss can be obtained.

[0116] Above, the example of forming TFLN2 on a thermally oxidized Si substrate or SiO2 substrate as the low refractive index substrate 1, and then forming an optical waveguide (SiN waveguide) on the upper surfaces of the low refractive index substrate 1 and TFLN2 has been mainly described.

[0117] Figure 16 It is to illustrate Figure 4 a method of integrating a ramped TFLN2 on a waveguide substrate 1 having an optical waveguide 10 formed therein in a low refractive index substrate 1 as shown

[0118] (STEP1)

[0119] An optical waveguide 10 is formed in a low refractive index substrate 1 made of a material such as SiO2 using SiN, amorphous Si, crystalline Si, etc. Further, a low refractive index substrate 1 whose upper surface (upper cladding) is flattened is prepared. In addition, reference numeral 3 denotes a substrate holder.

[0120] (STEP2)

[0121] TFLN2 is bonded to the low refractive index substrate 1 by direct bonding. When bonding TFLN2, a three-layer structure in which an intermediate layer (suitable materials are those easily removable such as Ti, WOx (oxygen-deficient tungsten oxide), etc.) is interposed on a Si substrate and TFLN is disposed on the top is used. The bonding of the low refractive index substrate 1 and TFLN2 uses plasma-activated bonding. A 5% TMAH solution is used to remove the Si substrate from TFLN2, and further, an APM solution is used to remove the intermediate layer. In the case of the above material composition, the TMAH solution can only remove the Si substrate. Similarly, the etching rate of the APM solution for Ti and WOx is large enough compared with the etching rates of LN, Si, SiO2, and SiN, so the influence on the remaining materials (LN, Si, SiO2, SiN) can be ignored.

[0122] (STEP3)

[0123] Two-layer masks are formed at necessary positions. In this embodiment, Al is used as the soluble mask M1, and a-Si is used as the insoluble mask M2.

[0124] (STEP4)

[0125] The APM solution is used to etch the unmasked TFLN2.

[0126] (STEP 5)

[0127] Remove the etching masks (M1, M2) using 1N aqueous potassium hydroxide solution.

[0128] Figure 17A and Figure 17B shows an optical waveguide element formed by Figure 16 the manufacturing method. In addition, Figure 17A is a perspective view of the optical waveguide element, Figure 17B and

[0129] In Figure 17A and Figure 17B the optical waveguide 10 in the low refractive index substrate 1 is represented by a single continuous optical waveguide, but as Figure 16 shown, there may also be a discontinuous portion of the optical waveguide 10 below the TFLN2 as needed.

[0130] In Figure 3 , Figure 12A and Figure 12B and Figure 14A and Figure 14B when the optical waveguide 10 is formed on the upper sides of the low refractive index substrate 1 and the TFLN2, the optical waveguides 10 of the same thickness are arranged on both the upper side of the low refractive index substrate 1 and the upper side of the TFLN2. Generally, an optical circuit having a channel waveguide has an optimal film thickness depending on its use.

[0131] On the other hand, when a channel waveguide of the same thickness as that on the low refractive index substrate 1 is formed on the TFLN2, it is likely to become a multimode waveguide. In addition, when an electrode is added for optical control, the efficiency (driving voltage) changes according to the overlap between the electric field distribution caused by voltage application and the light distribution. Therefore, the optimal SiN film thickness and width are different in the passive waveguide region and the active waveguide region.

[0132] For example, when giving priority to improving characteristics, as Figure 18 shown, it is necessary to make the SiN film thickness in the active waveguide region (on the TFLN2) thinner. This can be achieved by photolithography and dry etching processes. In the transition region (the edge portion of the TFLN2), by continuously changing the width and thickness of the SiN waveguide as Figure 18 shown, the effective refractive index of the optical waveguide 10 changes continuously, and a low-loss optical transition can be achieved.

[0133] Similarly, when the optical waveguide 10 is formed in the low refractive index substrate 1 as Figure 4 and Figure 17A and Figure 17B shown, a lower-loss optical waveguide element can also be achieved by changing the waveguide shape according to the presence or absence of the loading of the TFLN2.

[0134] Figures 19 - 21 An optical waveguide element in which a rib-type optical waveguide is formed on the upper surface of TFLN2. Figure 19 and Figure 20 is a diagram illustrating its manufacturing process, Figure 21 is a perspective view of the optical waveguide element. In Figure 19 and Figure 20 for each step, the left figure shows a cross-sectional view observed from the extending direction of the optical waveguide 10, and the right figure shows a top view of the optical waveguide element.

[0135] (STEP1)

[0136] Adhere TFLN2 to the low-refractive-index substrate 1 having the optical waveguide 10 inside. The size of TFLN2 is generally larger than the size of the active waveguide region where the rib-type optical waveguide, control electrodes, etc. are formed.

[0137] (STEP2)

[0138] Form a mask material m1 for processing TFLN2 by dry etching. Generally, a UV resist is used in the case of using UV exposure, and an EB resist is used in the case of using EB exposure.

[0139] (STEP3)

[0140] Process TFLN2 by dry etching and remove the mask material, thereby forming a rib-type optical waveguide 20 on TFLN2.

[0141] (STEP4)

[0142] Cover the vicinity of the rib-type optical waveguide of TFLN2 that is desired to remain with a laminated film of a soluble mask M1 and an insoluble mask M2. For example, Al is used for the soluble mask M1 and a-Si is used for the insoluble mask M2.

[0143] (STEP5)

[0144] Etch TFLN2 using an APM solution. At this time, SiO2, Si, and SiN are not etched. After dissolving the unnecessary TFLN2, remove the Al and a-Si of the mask material with potassium hydroxide.

[0145] In the optical waveguide element having a rib-type optical waveguide shown in Patent Document 1, there are protrusions at the edge portion of TFLN, and the manufacturing process is also complicated. However, in the present invention, such protrusions are not required, and low-loss optical transition can be achieved only by two-dimensional pattern formation.

[0146] In the present invention, SiN and SiO2 are mainly used as the materials for the passive waveguide region, but the present invention is not limited to these materials.

[0147] In addition, in the present invention, a structure in which a cladding material with a low refractive index is stacked on the upper surfaces of the passive waveguide and the TFLN can also be adopted.

[0148] The above-mentioned holding substrate can be composed of a single cladding material or multiple cladding materials.

[0149] Furthermore, electrodes can also be formed in the active waveguide region.

[0150] In addition, there is no particular limitation on the shape of the edge portion of the TFLN2 parallel to the optical waveguide 10. When X-cut LN is used, the Z-axis of the crystal is orthogonal to the waveguide. The etching rates are different on the +Z plane and the -Z plane, so the edges of the TFLN are asymmetric. In Figure 20 (STEP5), the etched shape of the edge portion of the TFLN2 is described as symmetric, but it can also be asymmetric. This is because if the edge portion of the TFLN is separated from the optical waveguide 10, there is no optical impact.

[0151] Since the refractive index of the cladding (SiO2) in the passive waveguide region is low (about 1.45), the optical waveguide can be bent sharply, which is beneficial to miniaturization.

[0152] On the other hand, since there is LN with an electro-optic effect in the active waveguide region, optical phase control can be performed.

[0153] At the boundary (transition region) between the passive waveguide region and the active waveguide region, the thickness of the TFLN changes continuously, so optical connection loss can be reduced. In addition, the optical waveguide in the active waveguide region is of course not limited to a straight line.

[0154] Next, an example of applying the optical waveguide element of the present invention to an optical modulator and an optical transmission device will be described. Hereinafter, an example of a high-bandwidth coherent driver modulator (HB-CDM) will be used for description, but the present invention is not limited thereto, and it can also be applied to an optical phase modulator, an optical modulator with a polarization synthesis function, an optical waveguide element integrated with more or fewer Mach-Zehnder type optical waveguides, a bonding device bonded to an optical waveguide element made of other materials such as silicon, a device for sensor applications, etc.

[0155] As Figure 22As shown, the optical waveguide element uses a substrate formed by bonding TFLN2 onto a low-refractive-index substrate 1, and also has an optical waveguide composed of a SiN waveguide 10 and a rib-type optical waveguide 20, and control electrodes such as a modulation electrode (not shown) for modulating the light wave propagating in the rib-type optical waveguide 20. The optical waveguide element is housed in a housing CA. Moreover, by providing optical fibers (F) for inputting and outputting light waves in the optical waveguide, an optical modulation device MD can be formed.

[0156] In Figure 22 , the optical fiber F is optically coupled to the SiN waveguide 10 in the optical waveguide element using an optical block having an optical lens, a lens barrel, a polarization multiplexing section OB, etc. Not limited thereto, the optical fiber may also be introduced into the housing through a through-hole penetrating the side wall of the housing, the optical member or the substrate may be directly joined to the optical fiber, or an optical fiber having a lens function at the end of the optical fiber may be optically coupled to the optical waveguide in the optical waveguide element. In addition, in order to stably perform the joining with the optical fiber and the optical block, a reinforcing member (not shown) may be arranged to overlap along the end face of the substrate (including the low-refractive-index substrate 1) including the SiN waveguide. By applying the waveguide structure described in Non-Patent Document 3 to the SiN waveguide, the polarization multiplexing section OB can replace the spatial system with a waveguide and can suppress the manufacturing / component cost.

[0157] By connecting an electronic circuit (digital signal processor DSP) that outputs a modulation signal So for causing the optical modulation device MD to perform a modulation operation to the optical modulation device MD, an optical transmission device OTA can be formed. In order to obtain the modulation signal S applied to the optical waveguide element, it is necessary to amplify the modulation signal So output from the digital signal processor DSP. Therefore, in Figure 22 , a driver circuit DRV is used to amplify the modulation signal. The driver circuit DRV and the digital signal processor DSP can also be arranged outside the housing CA, but can also be arranged inside the housing CA. In particular, by arranging the driver circuit DRV inside the housing, the propagation loss of the modulation signal from the driver circuit can be further reduced.

[0158] The input light L1 to the optical modulation device MD can be supplied from outside the optical transmission device OTA, but as shown in Figure 22 , a semiconductor laser (LD) can be used as the light source. The output light L2 modulated by the optical modulation device MD is output to the outside through the optical fiber F.

[0159] [Industrial Applicability]

[0160] As described above, according to the present invention, it is possible to provide an optical waveguide element which, even when using a single wafer optical waveguide element formed by bonding a low refractive index substrate having a refractive index lower than that of lithium niobate and TFLN, has a structure that ensures an appropriate margin during bonding, has a small optical connection loss between different waveguides, and can further reduce the chip size. Moreover, it is possible to provide an optical modulation device, an optical transmission device using the optical waveguide element, and a method for manufacturing the optical waveguide element.

Claims

1. An optical waveguide component, characterized in that: A low-refractive-index substrate made of a material having a lower refractive index than lithium niobate is provided. A thin film made of lithium niobate and having a thickness of 1 μm or less is disposed on a portion of the low refractive index substrate. An optical waveguide having a higher refractive index than that of the low refractive index substrate and made of a material other than lithium niobate is disposed on the low refractive index substrate. Furthermore, at least a portion of the optical waveguide is continuously arranged from the low refractive index substrate to the film, In the region where the optical waveguide crosses the outer peripheral edge portion of the film, the thickness of the film is sloped, and the slope of the edge is set to be 0.189 or less.

2. An optical waveguide component, characterized in that: A low-refractive-index substrate made of a material having a lower refractive index than lithium niobate is provided. A thin film made of lithium niobate and having a thickness of 1 μm or less is disposed on a portion of the low refractive index substrate. An optical waveguide having a higher refractive index than that of the low refractive index substrate and made of a material other than lithium niobate is disposed in the low refractive index substrate. Furthermore, at least a portion of the optical waveguide is disposed continuously from a region of the low-refractive-index substrate where the thin film is not disposed to a region of the low-refractive-index substrate where the thin film is disposed. In the region where the optical waveguide crosses the outer peripheral edge portion of the film, the thickness of the film is sloped, and the slope of the edge is set to be 0.189 or less.

3. The optical waveguide element according to claim 1 or 2, characterized in that: A rib-type optical waveguide is formed in the thin film.

4. The optical waveguide element according to claim 1 or 2, characterized in that: The material constituting the low refractive index substrate includes SiO2.

5. The optical waveguide element according to claim 1 or 2, characterized in that: The material constituting the optical waveguide is any one of a material containing SiN and Si.

6. An optical modulation device, characterized in that ， The optical waveguide element according to claim 1 or 2 is accommodated in a housing, The optical modulation device includes an optical fiber for inputting or outputting a light wave to or from the optical waveguide element.

7. The optical modulation device according to claim 6, characterized in that: The optical waveguide element has a modulation electrode for modulating the light wave propagating in the optical waveguide element. An electronic circuit for amplifying a modulation signal input to the modulation electrode is provided inside the housing.

8. An optical transmitting device, characterized in that: have: The light modulation device according to claim 7; a light source for inputting a light wave into the light modulation device; and The electronic circuit outputs a modulation signal to the optical modulation device.

9. A method for manufacturing an optical waveguide element, comprising manufacturing the optical waveguide element according to claim 1 or 2, characterized in that: When forming the slope shape of the thin film, a mixed solution of an alkaline solution and hydrogen peroxide is used as an etching solution.

10. The method for manufacturing an optical waveguide element according to claim 9, wherein: When forming the slope shape of the thin film, a soluble mask and an insoluble mask are sequentially stacked on the thin plate and used as mask materials.

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