Optical waveguide element, optical modulation device and light transmitter using the same, and manufacturing method of optical waveguide element

Abstract

To provide an optical waveguide element having a structure in which a proper tolerance is ensured in bonding, an optical connection loss between heterogeneous waveguides is small, and a chip size can be reduced even in the optical waveguide element obtained by bonding a low refractive index substrate and a TFLN.SOLUTION: The optical waveguide element comprises: a low refractive index substrate 1 composed of a material having a lower refractive index than that of lithium niobate (LN); a thin film 2 composed of LN and having a thickness of 1 μm or less, arranged on a part of the substrate 1, wherein an optical waveguide 10 having a higher refractive index than the substrate 1 and composed of materials other than LN is arranged on the substrate 1, at least a part of the optical waveguide 10 is continuously arranged from on the substrate 1 to on the thin film 2, the thin film 2 is shaped in slope in a thickness direction in a region (transition region) where the optical waveguide 10 crosses over an outer peripheral edge of the thin film 2, and an inclination (tanθ) of an edge is set to be 0.189 or less.SELECTED DRAWING: Figure 3

Description

Technical Field

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

Background Art

[0002] In recent years, Si waveguides have been used for optical waveguides for optical communication (see Non-Patent Document 1). Since the Si waveguide uses a CMOS process, it is excellent in scalability and cost reduction, and optical reception with a light receiving element (PD) using Ge / Si is possible. On the other hand, as demerits, it cannot be used in the visible light region, and pure phase modulation control is difficult, such as performing phase modulation by flowing a current through the Si waveguide.

[0003] For this reason, as an alternative technology, new platforms using silicon nitride (SiN) or a thin film of lithium niobate (Thin Film LiNbO3 (TFLN)) as an optical waveguide core have been studied (see Non-Patent Documents 2 and 3). For the integration of optical functions, a light source, phase modulation, reception, and optical multiplexing / demultiplexing (power combination / splitting, wavelength combination / separation, polarization combination / splitting, etc.) are essential. Since the optimal materials for these configurations are different, methods for integrating different materials have been developed.

[0004] Among them, an element in which optical multiplexing / demultiplexing and phase modulation are integrated has been developed by loading a silicon nitride (SiN) or amorphous silicon (a-Si) waveguide on TFLN (see Non-Patent Document 4). Compared with a monolithic TFLN modulator (see Non-Patent Document 2), this method has a weak optical confinement of the optical waveguide, so the bending radius is as large as about several hundred μm, and the driving voltage (Vπ) is also large.

[0005] For this reason, as shown in FIG. 1, a method has been studied in which an optical demultiplexing section uses an Si-based waveguide (SiN, a-Si, crystalline silicon (c-Si)), and a phase modulation section uses a rib-type TFLN. In FIG. 1, on a low refractive index substrate 1 made of a material having a lower refractive index than lithium niobate, for example, SiO2, a passive waveguide region A is formed using an Si-based waveguide (10A to 10C, 10C being a ring resonator). Further, in the TFLN 2, an optical control member 200 having a rib-type optical waveguide and a control electrode is formed, and an active waveguide region C is provided.

[0006] The advantage of the optical waveguide element in which the TFLN 2 is loaded on the low refractive index substrate 1 is ensuring the yield by not processing LiNbO3 (LN), which is a difficult-to-process material (difficult to dry-etch). However, a transition region B that connects the two is always formed between the passive waveguide region A formed by the Si-based waveguide and the active waveguide region C using the TFLN.

[0007] FIG. 2 is a cross-sectional view of this transition region B. FIG. 2(a) is an example in which the Si-based waveguide 10 is disposed above the low refractive index substrate 1, and FIG. 2(b) is an example in which the Si-based waveguide 10 is disposed within the low refractive index substrate 1. In the portion indicated by the dotted line frame D, the refractive index change of the optical waveguide is large for the light wave propagating through the Si-based waveguide 10, which causes an increase in the optical connection loss in the transition region B.

[0008] The inventor of the present invention disclosed an efficient and highly productive optical connection method between a silicon nitride waveguide and a rib-type waveguide formed in a TFLN in Patent Document 1. However, it is difficult to say that sufficient productivity is ensured since the wafer on which the silicon nitride waveguide is formed and the wafer on which the TFLN is formed are bonded together to form a single wafer. Further, accuracy is required when attaching the substrate having the passive optical waveguide and the TFLN. When a margin is provided for the bonding accuracy, an excessive space is required and the chip size cannot be reduced.

Prior Art Documents

Patent Documents

[0009] [Patent Document 1] Japanese Patent Application No. 2023-054914 (Filing Date: March 30, 2023) [Non-Patent Document]

[0010] [Non-Patent Document 1] Yikai Su, etc., "Silicon Photonic Platform for Passive Waveguide Devices: Materials, Fabrication, and Applications", Advanced Materials Technologies. 1901153 (2020) [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) [Non-Patent Document 3] Mian Zhang, etc., "Integrated Lithium Niobate Electro-optic Modulators: When performance meets scalability", Optica, Vol.8, No.5, pp652 (2021) [Non-Patent Document 4] Sean Nelan, etc., "Ultra-high Extinction Dual-output Thin-film Lithium Niobate Intensity Modulator", arXiv:2207.02608v1 (Jul. 6, 2022) [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

Problems to be Solved by the Invention

[0011] The problem to be solved by the present invention is to solve the above-described problems, and even for an optical waveguide element using a single wafer in which a low refractive index substrate having a refractive index lower than that of lithium niobate and TFLN are bonded together, an appropriate margin is ensured during bonding, the optical connection loss between different waveguides is small, and further, to provide an optical waveguide element having a structure in which the chip size can be reduced. Furthermore, it is to provide an optical modulation device and an optical transmission device using the optical waveguide element, and a method for manufacturing the optical waveguide element.

Means for Solving the Problems

[0012] In order to solve the above problems, the optical waveguide element of the present invention, an optical modulation device and an optical transmission device using the same, and a method for manufacturing the optical waveguide element have the following technical features.

[0013] (1) A low refractive index substrate made of a material having a refractive index lower than that of lithium niobate, and a thin film having a thickness of 1 μm or less made of lithium niobate is disposed on a part of the low refractive index substrate. An optical waveguide having a refractive index higher 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. Further, at least a part of the optical waveguide is continuously disposed from above the low refractive index substrate to above 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 has a slope shape and the edge inclination is set to 0.189 or less. An optical waveguide element characterized by this.

[0014] (2) A low refractive index substrate made of a material with a refractive index lower than that of lithium niobate, and on a part of the low refractive index substrate, a thin film made of lithium niobate with a thickness of 1 μm or less is disposed. An optical waveguide having a refractive index higher 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. Further, at least a part of the optical waveguide is continuously disposed 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 a region where the optical waveguide crosses the outer peripheral edge of the thin film, the thickness of the thin film has a slope shape and the edge inclination is set to 0.189 or less. An optical waveguide element characterized by this.

[0015] (3) In the optical waveguide element according to the above (1) or (2), a rib-type optical waveguide is formed in the thin film, which is characterized.

[0016] (4) In the optical waveguide element according to the above (1) or (2), the material constituting the low refractive index substrate contains SiO2, which is characterized.

[0017] (5) In the optical waveguide element according to the above (1) or (2), the material constituting the optical waveguide is either a material containing SiN or Si, which is characterized.

[0018] (6) The optical waveguide element according to the above (3) is housed in a housing, and an optical fiber for inputting or outputting light waves to the optical waveguide element is provided. An optical modulation device characterized by this.

[0019] (7) In the optical modulation device according to the above (6), 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, which is characterized.

[0020] (8) An optical transmission device characterized by having the optical modulation device according to the above (7), a light source for inputting light waves to the optical modulation device, and an electronic circuit for outputting a modulation signal to the optical modulation device.

[0021] (9) A manufacturing method for manufacturing the optical waveguide element according to the above (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 water is used as the etching solution.

[0022] (10) In the manufacturing method of the optical waveguide element according to the above (9), when forming the slope shape of the thin film, as the mask material, a soluble mask and an insoluble mask are sequentially laminated and used on the thin plate.

Advantages of the Invention

[0023] The present invention provides, firstly, a low-refractive-index substrate made of a material having a lower refractive index than lithium niobate, and on a part of the low-refractive-index substrate, a thin film made of lithium niobate and having a thickness of 1 μm or less is disposed. An optical waveguide having a refractive index higher 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. Further, at least a part of the optical waveguide is continuously disposed from above the low-refractive-index substrate to above 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 has a slope shape and the edge inclination is set to 0.189 or less. It is an optical waveguide element characterized by this.

[0024] Secondly, a low-refractive-index substrate made of a material having a lower refractive index than lithium niobate, and on a part of the low-refractive-index substrate, a thin film made of lithium niobate and having a thickness of 1 μm or less is disposed. An optical waveguide having a refractive index higher 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. Further, at least a part of the optical waveguide is continuously disposed 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 a region where the optical waveguide crosses the outer peripheral edge of the thin film, the thickness of the thin film has a slope shape and the edge inclination is set to 0.189 or less. It is an optical waveguide element characterized by this.

[0025] Thus, at the outer peripheral edge (transition region) of the lithium niobate thin film disposed on the low refractive index substrate, the thickness of the thin plate has a slope shape, and since the edge inclination is set to 0.189 or less, an appropriate margin is ensured when bonding the low refractive index substrate and the TFLN, the optical connection loss between different waveguides is small, and furthermore, it is possible to provide an optical waveguide element having a structure in which the chip size can be reduced. Furthermore, it is possible to provide an optical modulation device and an optical transmission device using the optical waveguide element.

[0026] Furthermore, in the method for manufacturing the optical waveguide element, when forming a slope shape on the lithium niobate thin film, a mixed solution of an alkaline solution and hydrogen peroxide water is used as the etching solution, and furthermore, as the mask material, a soluble mask and an insoluble mask are sequentially laminated on the thin plate and used, so that it is possible to easily obtain a slope shape as designed.

Brief Description of the Drawings

[0027]

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Embodiments for Carrying Out the Invention

[0028] Hereinafter, the optical waveguide element of the present invention will be described in detail using preferred examples. As shown in FIG. 3, the optical waveguide element of the present invention includes a low refractive index substrate 1 made of a material having a lower refractive index than lithium niobate, and a thin film (TFLN) 2 made of lithium niobate and having a thickness of 1 μm or less is disposed on a part of the low refractive index substrate 1. An optical waveguide 10 having a higher refractive index than the low refractive index substrate 1 and made of a material other than lithium niobate is disposed on the low refractive index substrate 1. Further, at least a part of the optical waveguide 10 is continuously disposed from above the low refractive index substrate 1 to above the thin film 2. 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 has a slope shape and the edge inclination (tanθ) is set to 0.189 or less. It is an optical waveguide element characterized by this.

[0029] Also, as shown in FIG. 4, the optical waveguide device of the present invention includes a low refractive index substrate 1 made of a material having a lower refractive index than lithium niobate, and a thin film (TFLN) 2 made of lithium niobate and having a thickness of 1 μm or less is disposed on a part of the low refractive index substrate 1. 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 disposed in the low refractive index substrate 1. Further, at least a part of the optical waveguide 10 is continuously disposed from a region of the low refractive index substrate 1 where the thin film 2 is not disposed to a region of the low refractive index substrate 1 where the thin film 2 is disposed. 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 has a slope shape and the edge inclination (tan θ) is set to 0.189 or less. The optical waveguide device is characterized by this.

[0030] In the optical waveguide device of the present invention, in the transition region B (outer peripheral edge of TFLN) connecting the passive waveguide region A and the active waveguide region C in FIG. 1, an inclined surface (slope shape) is formed on the TFLN 2. Specifically, a slope shape in which the thickness of the TFLN 2 gradually changes is formed. The slope shape is not limited to a linear shape as shown in FIGS. 3 and 4, and can be set to a slope shape using a curved surface such as a shape that changes exponentially as described later. In the optical waveguide device of the present invention, by reducing the angle θ of the edge of this slope shape, specifically, by setting the inclination (tan θ) to 0.189 or less, it is possible to suppress the optical connection loss in the transition region B.

[0031] Next, the manufacturing process of the optical waveguide device shown in FIG. 3 will be described with reference to FIGS. 5 and 6. (STEP1) The TFLN 2 is directly bonded and attached to a low refractive index substrate 1 made of a material such as SiO2 to prepare a TFLN wafer. Reference numeral 3 is a holding substrate, which is for increasing the mechanical strength of the entire TFLN wafer, and Si, SiO2, etc. can be used.

[0032] (STEP2) On TFLN2, a film (etching mask) M1 and M2 composed of appropriate two layers are patterned. For example, the material of the first layer (the layer in contact with TFLN) is Ti or Al, and the material of the second layer is Au, Ni, or a-Si.

[0033] (STEP3) Using the patterned films (M1, M2) as an etching mask, TFLN2 is wet-etched. As the wet etching solution, a mixed solution of aqueous ammonia and hydrogen peroxide (APM solution) or the like is appropriate. Here, the etching rate of the material of the first layer (M1) is greater than that of the material of the second layer and LN. Therefore, the etching shape of TFLN becomes a shape with an inclination. When the low refractive index substrate 1 is SiO2, since SiO2 is insoluble in the APM solution, the low refractive index substrate 1 functions as an etching stop layer.

[0034] (STEP4) The mask materials (M1, M2) are removed with an appropriate chemical solution or the like. For example, iodine potassium iodide aqueous solution can be used for Au, APM solution for Ti, and KOH solution for Al and a-Si.

[0035] (STEP5) On the upper surfaces of the low refractive index substrate 1 and TFLN2, a SiN film (11) that becomes the optical waveguide 10 is formed. As the material of the optical waveguide 10, SiN, a-Si, etc. can be used, and hereinafter, the description will be centered on SiN.

[0036] (STEP6) Using an appropriate dry etching mask m, a SiN waveguide (10) is formed. At this time, the height of the SiN waveguide in the passive waveguide region and the active waveguide region may be adjusted to be optimal in each region, and it is also possible to perform it by multiple dry etching processes.

[0037] (STEP7) After removing the dry etching mask m, if necessary, a clad layer is formed, heat treatment is performed, electrodes are formed, and the like. In FIG. 6, the optical waveguide (SiN waveguide) 10 is illustratively shown as being cut, but it can also be formed in a continuous pattern.

[0038] Next, the wet etching of TFLN will be described in detail. Generally, since LN is chemically stable, when processing LN by wet etching, a chemical solution using hydrofluoric acid is used. This method has crystal orientation and crystal defect dependencies, and moreover, since the etching rate is slow, it is not used in device manufacturing. In order to reduce the chemical stability of LN, chemical etching methods using hydrofluoric acid or KOH have been developed by performing proton exchange or ion implantation (see Non-Patent Document 5). When hydrofluoric acid or high-concentration (about 50 wt%) KOH is used for the wet etching of LN, a new problem arises that the SiO2 of the low refractive index substrate 1 is also etched at the same time. In addition, the slope shape of the TFLN edge depends on the distribution of proton exchange or ion implantation, and it is difficult to form an arbitrary shape of the slope.

[0039] To solve this problem, the present inventor investigated the etching rates of LN with respect to a number of chemical solutions. As a result, it was found that LN can be etched by using a mixed solution of an alkaline solution and hydrogen peroxide water. It was also found that SiO2 is not etched by adopting ammonia water as the alkaline solution.

[0040] The etching rate of LN in the X-axis direction was about 55 nm / hour with respect to a 40°C solution prepared by mixing 29 wt% aqueous ammonia and 30 wt% hydrogen peroxide solution at a volume ratio of 1:3. The mixed solution of aqueous ammonia and hydrogen peroxide solution is known as a chemical solution used for semiconductor cleaning as APM (Ammonia-hydrogen Peroxide Mixture cleaning) or SC-1, but it was not known that LN could be etched. Here, although there may be those skilled in the art who feel that the etching rate of LN is slow, since the film thickness of TFLN is 1 μm or less and it can be processed by a batch process, it can be used for device manufacturing.

[0041] Next, with reference to FIG. 7, a method of making the TFLN edge portion into a slope shape using the above-described chemical solution will be described. The dotted lines indicated by S1 to S3 in each figure show the shape of the etching surface formed as the etching progresses. FIG. 7(a) shows a case where the etching mask M is not dissolved in the chemical solution (insoluble mask). In this case, the wet-etched TFLN2 has a circularly scooped shape (a concave shape with respect to the etching surface).

[0042] Next, in FIG. 7(b), the etching mask is composed of two layers (M1, M2), and the first layer mask M1 is composed of a "soluble mask" that is soluble in the chemical solution. Also, when the second layer mask M2 is insoluble and has high rigidity as an "insoluble / high-rigidity mask", the chemical solution not only etches LN from 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 first layer soluble mask M1 is faster than the etching rate of LN, the side dissolution from the first layer becomes slower with time, so the edge shape of TFLN becomes a convex shape with respect to the etching surface. On the other hand, when the etching rate of the first layer soluble mask is slower than the etching rate of LN, the edge shape of TFLN becomes a concave shape with respect to the etching surface.

[0043] As shown in Fig. 7(c), when the second-layer mask M2 is an "insoluble and low-rigidity mask" that is insoluble and has low rigidity, the second-layer etching mask M2 (M2' indicates the floating mask) floats due to the dissolution of the first-layer etching mask M1. As a result, the lateral dissolution of the first layer M1 is approximately constant with respect to time, and the edge shape of the TFLN becomes a straight line.

[0044] It is also possible to change the edge shape of the TFLN by changing the thickness of the first-layer etching mask M1. Also, as will be described later with reference to Fig. 11, when the insoluble and low-rigidity mask M2 in Fig. 7(c) does not float, the mask can be floated by using stirring of the chemical solution or a stress-applying film (such as curing shrinkage of the photoresist).

[0045] Furthermore, it was also confirmed that the etching shape changes when using an APM solution and an etching mask. Specifically, a sample shown in Fig. 8 was fabricated, and it was confirmed that the slope of the slope changes depending on the etching conditions. Fig. 8 shows a stack of an etching mask M1 (soluble mask) and an etching mask M2 (insoluble mask) on TFLN2. The upper part of Fig. 8 is a plan view, and the lower part is a side view. The arrow on the left side of the plan view indicates the crystal axis direction, and it is understood that Fig. 8 is an X-cut LN. Al is used for the etching mask M1, and a-Si is used for the etching mask M2 to form a pattern. Photolithography technology is used for pattern formation, and an RF sputtering apparatus is used when laminating Al and a-Si. The thicknesses of Al and a-Si are four types: 0, 10, 20, and 30 nm for Al, and the thickness of a-Si is 100 nm.

[0046] The X-cut LN with an etching mask in FIG. 8 was immersed in APM (a mixed solution of 29 wt% aqueous ammonia and 30 wt% hydrogen peroxide solution (volume ratio 1:3)) at 40° C. for 8 hours. Then, after removing the etching mask with a 1N KOH solution of alkalinity, the surface profile of LN was obtained with a stylus thickness gauge. FIG. 9 shows the shape change of the etched surface when the thickness of the etching mask M1 (using Al) was changed. Also, the relationship between the slope of the etched surface (slope) in FIG. 9 and the thickness of the etching mask M1 is shown in FIG. 10. As a result, it can be seen that as the soluble mask (M1) becomes thicker, the slope length (Slope_L) becomes longer and the slope of the slope becomes smaller.

[0047] Furthermore, by the same method, the results when stirring was performed during the etching of LN by APM for a sample with an Al thickness of 20 nm are shown in FIG. 11. When stirring is performed, the shape of the slope part is formed in a more planar (linear) shape. By changing the material of the soluble mask M1 from Al to Ti or W with a high etching rate, the slope of the slope can be made even smaller. The slope and shape of the slope are in a trade-off relationship with the dimensions and optical loss of the optical transition region and are selected according to the design. Also, in this example, XcutLN was used, but the same effect has been confirmed with ZcutLN.

[0048] 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, a simulation of the optical connection loss was performed using the models in FIGS. 12 and 14.

[0049] In the model of FIG. 12, the edge part (outer peripheral edge part) of the TFLN2 has a planar slope (the thickness of the TFLN changes linearly). Also, in the model of FIG. 14, the edge part of the TFLN2 has a convex curved slope (the thickness of the TFLN changes exponentially). Note that (a) in each figure is a perspective view, and (b) is a three-view drawing showing a plan view in the center, a front view on the left side, and a side view on the lower side.

[0050] Figure 13 shows the measured data of a sample in which TFLN is etched to form a convex slope and the result of fitting its surface shape with an exponential function. When using the exponential function expressed by the following formula, the fitting parameters were α = 3, Slope_L = 60 μm, and LN_t = 0.25 μm.

[0051]

Equation

[0052] In the model of Figure 14, the shape of the convex curved slope was set using the exponential function (the above formula) used for the shape of Figure 13. As parameters for Figures 12 and 14, the width (SiN_w) of the optical waveguide (SiN waveguide) 10 was set to 0.8 μm, the thickness (SiN_t) of the optical waveguide 10 was set to 0.5 μm, the thickness (LN_t) of TFLN2 was set to 0.3 μm, and the optical loss of the light wave propagating through the optical waveguide 10 was simulated with the length (Slope_L) of the slope portion as a variable.

[0053] Specifically, the length (Slope_L) of the slope portion of TFLN was changed 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)) was derived. The simulation results are shown in Figure 15.

[0054] Due to the difference in the slope shapes of Figures 12 and 14, there is a slight difference in optical loss, but as Slope_L increases, the optical loss 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 exponential function at this time is 0.189 (~10°). Similar results can be obtained even when LN_t, SiN_w, and SiN_t are changed. From this result, it is preferable that the change in the thickness of the thin film (the slope of the edge of the slope shape, tanθ) at the edge portion (outer peripheral edge portion) of TFLN2 is set to 0.189 or less.

[0055] Also, as shown in FIG. 15, since the optical loss is significantly reduced when Slope_L is 5 μm or more with respect to LN_t of 0.3 μm, it can be said that a lower-loss optical waveguide element can be obtained by setting LN_t (thickness) / Slope_L (length of slope) ≦ 0.06, which is the "average slope" of the slope portion.

[0056] Above, an example in which TFLN2 is formed on a thermal oxidation Si substrate or SiO2 substrate, which is the low refractive index substrate 1, and an optical waveguide (SiN waveguide) is further formed on the upper surfaces of the low refractive index substrate 1 and TFLN2 has been mainly described. FIG. 16 is a diagram for explaining a method of integrating sloped TFLN2 on a substrate 1 with an optical waveguide in which an optical waveguide 10 is formed in the low refractive index substrate 1 as shown in FIG. 4.

[0057] (STEP1) An optical waveguide 10 is formed in the low refractive index substrate 1 made of a material such as SiO2 with SiN, amorphous Si, crystalline Si, etc., and a low refractive index substrate 1 with the upper surface (upper cladding) of the low refractive index substrate 1 flattened is prepared. Note that reference numeral 3 indicates a holding substrate.

[0058] (STEP2) TFLN2 is directly bonded and attached to the low refractive index substrate 1. When bonding TFLN2, a three-layer structure in which an intermediate layer (a material such as Ti or WOx (oxygen-deficient tungsten oxide) that is easy to remove is appropriate) is interposed on the Si substrate and TFLN is disposed on the outermost surface is used. The bonding between the low refractive index substrate 1 and TFLN2 uses plasma-activated bonding. When removing the Si substrate from TFLN2, a 5% TMAH solution is used, and when further removing the intermediate layer, an APM solution is used. In the case of the above material configuration, the TMAH solution can only remove the Si substrate. Similarly, since the etching rates of Ti and WOx by the APM solution are sufficiently higher than the etching rates of LN, Si, SiO2, and SiN, the influence on the remaining materials (LN, Si, SiO2, SiN) can be ignored.

[0059] (STEP3) Form a two-layer mask at the necessary locations. In this embodiment, Al was used as the soluble mask M1, and a-Si was used as the insoluble mask M2.

[0060] (STEP4) Etch the TFLN2 not protected by the mask using the APM solution.

[0061] (STEP5) Remove the etching masks (M1, M2) using a 1N aqueous potassium hydroxide solution.

[0062] FIG. 17 shows an optical waveguide element formed by the manufacturing method of FIG. 16. Note that FIG. 17(a) is a perspective view of the optical waveguide element, and (b) is a three-view. In FIG. 17, the optical waveguide 10 in the low refractive index substrate 1 is shown as a single continuous optical waveguide. However, as shown in FIG. 16, there may be a portion where the optical waveguide 10 is discontinuous under the TFLN2 as needed.

[0063] In FIGS. 3, 12, and 14, when forming the optical waveguide 10 on the upper side of the low refractive index substrate 1 and the TFLN2, the optical waveguide 10 of the same thickness is arranged on both the upper side of the low refractive index substrate 1 and the upper side of the TFLN2. Generally, an optical circuit with a channel waveguide has an optimal film thickness depending on the application. On the other hand, when forming a channel waveguide of the same thickness as that on the low refractive index substrate 1 on the TFLN2, it is likely to become a multimode waveguide. Also, when adding an electrode for optical control, the efficiency (driving voltage) changes due to the overlap of the electric field distribution and the optical distribution caused by voltage application. Therefore, the optimal SiN film thickness and width are different between the passive waveguide region and the active waveguide region.

[0064] For example, when prioritizing performance improvement, as shown in Fig. 18, it is necessary to reduce the thickness of the SiN film in the active waveguide region (on TFLN2). This can be achieved by lithography and dry etching processes. In the transition region (the edge of TFLN2), as shown in Fig. 18, by continuously changing the width and thickness of the SiN waveguide, the effective refractive index of the optical waveguide 10 changes continuously, enabling low-loss optical transitions.

[0065] Similarly, when forming the optical waveguide 10 in the low refractive index substrate 1 as shown in Figs. 4 and 17, a lower-loss optical waveguide element can be realized by changing the waveguide shape depending on the presence or absence of TFLN2 loading.

[0066] Figs. 19 to 21 show an optical waveguide element with a rib-type optical waveguide formed on the upper surface of TFLN2. Figs. 19 and 20 are diagrams explaining the manufacturing process, and Fig. 21 is a perspective view of the optical waveguide element. In each step of Figs. 19 and 20, the left figure shows a cross-sectional view seen from the extending direction of the optical waveguide 10, and the right figure shows a plan view of the optical waveguide element.

[0067] (STEP1) Attach TFLN2 to the low refractive index substrate 1 having the optical waveguide 10 inside. Generally, the size of TFLN2 is larger than the size of the active waveguide region where a rib-type optical waveguide, control electrodes, etc. are formed.

[0068] (STEP2) Form a mask material m1 for processing TFLN2 by dry etching. Generally, a UV resist is used when using UV exposure, and an EB resist is used when using EB exposure.

[0069] (STEP3) Process TFLN2 by dry etching and remove the mask material to form a rib-type optical waveguide 20 in TFLN2.

[0070] (STEP4) Cover the vicinity of the rib-type optical waveguide of TFLN2 to be retained 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.

[0071] (STEP5) Etch TFLN2 using an APM solution. At this time, SiO2, Si, and SiN are not etched. After dissolving the unnecessary TFLN2, remove the mask materials Al and a-Si with potassium hydroxide.

[0072] In the optical waveguide element having a rib-type optical waveguide shown in Patent Document 1, there are protrusions at the edge portions of TFLN, and the manufacturing process is also complicated. However, in the present invention, such protrusions are unnecessary, and low-loss optical transitions are possible only by two-dimensional pattern formation.

[0073] In the present invention, although mainly SiN and SiO2 are used as the materials for the passive waveguide region, the present invention is not limited to only those materials. In addition, in the present invention, a configuration in which a cladding material having a low refractive index is deposited on the upper surfaces of the passive waveguide and TFLN can also be adopted together. Regarding the above-described holding substrate, it may be composed of a single cladding material or a plurality of cladding materials. Furthermore, electrodes may be formed in the active waveguide region.

[0074] Also, regarding the shape of the edge portion of TFLN2 parallel to the optical waveguide 10, there are no particular restrictions. When X-cut LN is used, the Z-axis of the crystal is orthogonal to the waveguide. On the +Z plane and the -Z plane, the etching rates are different, so the edges of TFLN become asymmetric. In FIG. 20 (STEP5), the etching shape of the edge portion of TFLN2 is shown as symmetric, but it may be asymmetric. This is because if the edge portion of TFLN is away from the optical waveguide 10, there is no optical influence.

[0075] Since the refractive index of the cladding (SiO2) of the passive waveguide region is low (~1.45), the optical waveguide can be sharply bent, which is advantageous for miniaturization. On the other hand, in the active waveguide region, since LN having an electro-optic effect exists, phase control of light becomes possible. In the boundary (transition region) between the passive waveguide region and the active waveguide region, since the thickness of TFLN changes continuously, the optical connection loss can be reduced. Note that the optical waveguide in the active waveguide region is not naturally limited to a straight line.

[0076] Next, an example in which the optical waveguide element of the present invention is applied to an optical modulation device or an optical transmission device will be described. Hereinafter, an example of a high bandwidth-coherent driver modulator (HB-CDM) will be used for the description, but the present invention is not limited thereto, and it can also be applied to an optical phase modulator, an optical modulator having a polarization combining function, an optical waveguide element in which more or fewer Mach-Zehnder type optical waveguides are integrated, a bonding device with an optical waveguide element made of other materials such as silicon, and a device for sensor applications.

[0077] As shown in FIG. 22, the optical waveguide element employs a substrate in which TFLN 2 is bonded to a low refractive index substrate 1, and further includes an optical waveguide composed of a SiN waveguide 10 and a rib type optical waveguide 20, and control electrodes (not shown) such as a modulation electrode that modulates light waves propagating through the rib type optical waveguide 20. The optical waveguide element is housed in a housing CA. Furthermore, an optical fiber (F) for inputting and outputting light waves to and from the optical waveguide can be provided to constitute an optical modulation device MD.

[0078] In Fig. 22, the optical fiber F is optically coupled to the SiN waveguide 10 in the optical waveguide element using an optical block with an optical lens, a lens barrel, a polarization multiplexing section OB, etc. Not limited to this, the optical fiber may be introduced into the housing through a through-hole penetrating the side wall of the housing, and the optical component or substrate and the optical fiber may be directly joined, 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. Further, in order to stably perform the joining with the optical fiber or the optical block, it is also possible to stack a reinforcing member (not shown) along the end face of the substrate including the SiN waveguide (including the low refractive index substrate 1). The polarization combining section OB can replace the spatial system with a waveguide by applying the waveguide structure described in Non-Patent Document 3 to the SiN waveguide, and can suppress the manufacturing and component costs.

[0079] An optical transmission device OTA can be configured 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. 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. For this reason, in Fig. 22, a driver circuit DRV is used to amplify the modulation signal. The driver circuit DRV and the digital signal processor DSP can 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, it is possible to further reduce the propagation loss of the modulation signal from the driver circuit.

[0080] The input light L1 to the optical modulation device MD may be supplied from outside the optical transmission device OTA, but as shown in Fig. 22, it is also possible to use a semiconductor laser (LD) as the light source. The output light L2 modulated by the optical modulation device MD is output to the outside by the optical fiber F.

Industrial Applicability

[0081] As described above, according to the present invention, even in an optical waveguide element using a single wafer in which a low refractive index substrate having a lower refractive index than lithium niobate and TFLN are bonded together, an appropriate margin is ensured during bonding, the optical connection loss between different waveguides is small, and furthermore, it is possible to provide an optical waveguide element having a structure in which the chip size can be reduced. Furthermore, an optical modulation device, an optical transmission device using the optical waveguide element, and a method for manufacturing the optical waveguide element can be provided.

Explanation of Signs

[0082] 1 Low refractive index substrate 10 Optical waveguide (SiN, etc.) 2 TFLN 20 Rib-type optical waveguide F Optical fiber LD Light source CA Housing MD Optical modulation device DRV Driver circuit DSP Digital signal processor OTA Optical transmission device

Claims

1. A low refractive index substrate made of a material having a lower refractive index than lithium niobate, A thin film having a thickness of 1 μm or less made of lithium niobate is disposed on a part of the low refractive index substrate, An optical waveguide having a higher refractive index than 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 part of the optical waveguide is continuously disposed from above the low refractive index substrate to above the thin film, An optical waveguide element, wherein in a region where the optical waveguide crosses the outer peripheral edge of the thin film, the thickness of the thin film has a slope shape and the inclination of the edge is set to 0.189 or less.

2. A low refractive index substrate made of a material having a lower refractive index than lithium niobate, A thin film having a thickness of 1 μm or less made of lithium niobate is disposed on a part of the low refractive index substrate, An optical waveguide having a higher refractive index than 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 part of the optical waveguide is continuously disposed 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, An optical waveguide element, wherein in a region where the optical waveguide crosses the outer peripheral edge of the thin film, the thickness of the thin film has a slope shape and the inclination of the edge is set to 0.189 or less.

3. The optical waveguide element according to claim 1 or 2, wherein a rib-type optical waveguide is formed in the thin film.

4. In the optical waveguide device according to claim 1 or 2, the material constituting the low refractive index substrate contains SiO ２ and is characterized by an optical waveguide device.

5. The optical waveguide element according to claim 1 or 2, wherein the material constituting the optical waveguide is either a material containing SiN or Si.

6. The optical waveguide element according to claim 1 or 2 is housed in a housing, An optical modulation device comprising an optical fiber for inputting or outputting light waves to or from the optical waveguide element.

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

8. The optical modulation device according to claim 7, A light source for inputting light waves to the optical modulation device, An optical transmission device comprising an electronic circuit for outputting a modulation signal to the optical modulation device.

9. A manufacturing method for manufacturing the optical waveguide device according to claim 1 or 2, when forming the slope shape of the thin film, a mixed solution of an alkali solution and hydrogen peroxide water is used as an etching solution, and the manufacturing method of the optical waveguide device is characterized by this.

10. In the manufacturing method of the optical waveguide device according to claim 9, when forming the slope shape of the thin film, as a mask material, a soluble mask and an insoluble mask are sequentially laminated and used on the thin plate, and the manufacturing method of the optical waveguide device is characterized by this.

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