Optical waveguide connection structure

Abstract

An optical waveguide connection structure (300), which connects a silicon optical waveguide (310) and a SiO2 optical waveguide (320), is configured to comprise: an underclad layer (302) formed on the upper surface of a support substrate (101); a ridge structure (303) formed on the upper surface of the underclad layer (302); a silicon core (304) being in contact with the ridge structure (303); a pattern structure (305) that is in contact with the silicon core (304), that has a shape and a size matching those of the silicon core (304) in a top view, and that has a refractive index lower than that of the silicon core (304); a SiO2 core (306) that covers the ridge structure (303), the pattern structure (305), and the silicon core (304), and that has a refractive index lower than that of the silicon core (304) and higher than that of the underclad layer (302); and an overclad layer (307) that is in contact with the SiO2 core (306) and that has a refractive index lower than that of the SiO2 core (306).

Description

[Technical Field]

[0001] The present disclosure relates to a connection structure for optical waveguides. [Background technology]

[0002] In recent years, with the increase in communication traffic within datacenters, the importance of optical wiring technology for elements within computer chassis has increased. In particular, silicon photonics technology, which allows for the high-density integration of numerous optical circuits, has attracted attention. Silicon photonics circuits function as the optical transmission medium in silicon photonics technology. They are composed of silicon wire waveguides with a Si core and SiO2 cladding layers. The relative refractive index difference between the core and cladding layers of silicon wire waveguides is approximately 40%, enabling light propagation within an extremely small cross-sectional area of ​​a few hundred nanometers square at wavelengths around 1550 nm, which is the wavelength band used for single-mode communication. In addition, silicon wire waveguides have a small allowable bending radius of a few micrometers, making it possible to form complex wiring patterns within a narrow area.

[0003] Silicon wire waveguides are fabricated using well-known SOI (Silicon on Insulator) substrates. An SOI substrate comprises a silicon support substrate, a buried silicon oxide layer (BOX layer) on the silicon support substrate, and a silicon active layer on the BOX layer. A silicon wire waveguide on such an SOI substrate is fabricated by using the BOX layer as an undercladding layer, processing the silicon active layer into a waveguide shape to form a core, and then forming a silica glass film on top of this core as an overcladding layer. Because silicon wire waveguides can be fabricated on SOI substrates, they can be monolithically integrated with electronic circuits. From a manufacturing perspective, mature semiconductor microfabrication technology can be applied, making it easy to form fine patterns. Therefore, combining silicon photonics technology with semiconductor technology and electronic circuit technology is expected to lead to the realization of integrated optoelectronic devices.

[0004] However, silicon wire waveguides, despite their advantages, present challenges in terms of connection with other optical devices. When connecting optical devices, it is important to match the mode field diameter (MFD) of the light propagating within the optical devices to reduce optical loss at the connection point. When connecting two optical devices butt-to-button, the coupling efficiency of the propagating light is determined by the overlap integral of the MFDs of the two devices. The MFD of a silicon optical circuit is approximately 300 nm. A silicon optical circuit is connected to a single-mode fiber (SMF), which is an external optical transmission medium. The MFD of well-known SMF, which is also used for long-distance transmission, is approximately 9 μm. Furthermore, the MFD of SMF with a high relative refractive index difference (HRR) design, developed for connection to optical waveguides with small MFDs, is approximately 4 μm. Therefore, the MFD of the silicon wire waveguide is about 10 to 30 times smaller than that of the SMF, and there is a risk of large coupling loss occurring when the two are directly connected.

[0005] As a method for solving the above-mentioned problem of connectivity between silicon optical circuits and SMFs, the insertion of a spot size conversion (hereinafter referred to as "SSC") structure has been proposed. FIGS. 1(a), 1(b), and 1(c) are diagrams for explaining a known optical waveguide connection structure, showing an optical waveguide connection structure 600 included in a silicon optical circuit. FIG. 1(a) is a top view of the optical waveguide connection structure 600, FIG. 1(b) is a cross-sectional view taken along the arrows Ib and Ib shown in FIG. 1(a), and FIG. 1(c) is a cross-sectional view taken along the arrows Ic and Ic shown in FIG. 1(a). The optical waveguide connection structure 600 includes a silicon optical waveguide 610 and a planar optical waveguide 620. The silicon optical waveguide 610 includes a silicon core 603, and the planar optical waveguide 620 includes a SiO2 core 604. The optical waveguide connection structure 600 includes an SSC structure 630 to mitigate the effect of differences in MFD between the silicon core 603 and the SiO2 core 604. In all of Figures 1(a), 1(b), and 1(c), the axis along the direction in which an optical signal passes through the silicon optical waveguide 610 and the planar optical waveguide 620 is defined as the Z-axis, the axis perpendicular to the Z-axis and the surface of the support substrate 601 is defined as the Y-axis, and the axis perpendicular to the Z-axis and the Y-axis is defined as the X-axis. In this explanation, the direction of the Y-axis from the support substrate 601 will be referred to as "up."

[0006] 1(b), the optical waveguide connection structure 600 includes a support substrate 601 made of, for example, silicon, an undercladding layer 602 formed on the support substrate 601, a silicon core 603 formed on the undercladding layer 602, an SiO2 core 604 formed on the silicon core 603, and an overcladding layer 605 that covers all of the above components. The support substrate 601, the undercladding layer 602, and the silicon core 603 are fabricated using an SOI substrate.

[0007] 1(a), the silicon core 603 includes a constant-width portion 603a having a constant length in the X direction (hereinafter also referred to as "width") and a narrow-width portion 603b whose width decreases along the Z direction. The SiO2 core 604 is formed so as to cover the narrow-width portion 603b, and the constant-width portion 603a is exposed from the SiO2 core 604. The over-cladding layer 605 covers the above-mentioned configuration and, together with the under-cladding layer 602, constitutes the cladding layer of the optical waveguide connection structure 600. The relative refractive index difference between the under-cladding layer 602 and the over-cladding layer 605 and the SiO2 core 604 is smaller than the relative refractive index difference between the under-cladding layer 602 and the over-cladding layer 605 and the silicon core 603.

[0008] 1(a) and other figures, the cross-sectional area of ​​the SiO2 core 604 intersecting the XY plane is larger than the cross-sectional area of ​​the silicon core 603 intersecting the XY plane. The MFD of the SiO2 core 604 is also larger than the MFD of the silicon core 603. Therefore, light incident from the constant width portion 603a leaks into the surrounding undercladding layer 602 and SiO2 core 604 as it passes through the narrow width portion 603b and travels in the Z direction. This light transition process is adiabatic, and theoretically no energy loss occurs.

[0009] In the known optical waveguide connection structure 600, the SiO2 core 604 is made of SiO x For example, silica-based optical waveguides with SiO2 cladding layers and polymer optical waveguides with SiO2 core 604 and cladding layers made of polymer materials are used. The relative refractive index difference between these material combinations is approximately 1% to several percent. With this configuration, the cross-sectional area of ​​the silicon core 304, approximately several hundred nanometers squared, can be expanded to approximately several micrometers squared for the SiO2 core 604, improving the coupling efficiency with the SMF. In particular, if the optical waveguide including the SiO2 core 604 is a silica-based optical waveguide made of the same silica-based material as optical fibers, it will have low loss in the communication wavelength band, low temperature dependency, and low polarization dependency, resulting in a highly reliable, high-performance optical device.

[0010] Silicon photonics technology, which combines a silicon optical circuit with a planar optical waveguide and connects two types of optical waveguides with different MFDs with low loss, is described in, for example, Non-Patent Document 1. [Prior art documents] [Non-patent literature]

[0011] [Non-Patent Document 1] R. Marchetti, C. Lacava, L. Carroll, K. Gradkowski, and P. Minzioni, “Coupling strategies for silicon photonics integrated chips,” Photonics Research, Vol. 7, Issue 2, pp. 201-239 (2019). Summary of the Invention

[0012] However, the above-mentioned known configurations have a problem with the connection between the silicon optical waveguide 610 and the planar optical waveguide 620. Specifically, as shown in Figures 1(a) and 1(b), the silicon core 603 is several hundred nanometers thick, and the SiO2 core 604 is several micrometers thick. Because of the difference in their heights, their centers do not align. When adiabatic coupling is used for light transition, perfect coupling is theoretically possible even if the centers of the cores do not align. However, the efficiency of adiabatic coupling depends on the dimensional accuracy of the silicon core 603 and the optical properties of the SiO2 core 604. Therefore, it is not guaranteed that all light energy will be adiabatically coupled in all manufactured components.

[0013] Without adiabatic coupling, the remaining optical energy is butt-coupled with the SiO2 core 604 at the end of the narrow portion 603b of the silicon core 603. In butt-coupling, the coupling efficiency is determined by the overlap integral of the mode fields of the connected optical elements, so if the centers of the cores are not aligned, the butt-coupling efficiency may deteriorate.

[0014] To improve the butt coupling efficiency, it is conceivable to shave off the BOX layer of the SOI substrate used in manufacturing the optical waveguide connection structure 600 by etching or the like, thereby lowering the height of the SiO2 core 604 and aligning the center heights of the silicon core 603 and the SiO2 core 604. However, the BOX layer that becomes the undercladding layer 602 needs to be thick enough (approximately 10 μm) to prevent the mode field of light propagating within the SiO2 core 604 from seeping into the support substrate 601, etc. For this reason, it is difficult to adopt a method of thinning the undercladding layer by etching the BOX layer or the like.

[0015] An SOI substrate having a BOX layer thick enough to withstand etching or other processes can be fabricated, for example, by the method shown in FIGS. 2(a), 2(b), and 2(c). FIGS. 2(a), 2(b), and 2(c) are schematic cross-sectional views illustrating a method for fabricating an SOI substrate having a thick BOX layer. In this method, as shown in FIGS. 2(a) and 2(b), a support substrate 701 is first oxidized for a relatively long time to form a thermal oxide film 702 having a thickness of 10 μm or more. The formed thermal oxide film 702 functions as an undercladding layer for the completed optical waveguide. However, if a thermal oxide film 702 having a thickness of 10 μm or more is formed on the support substrate 701, the stress applied to the front and back of the support substrate 701 becomes non-uniform, resulting in warping of the entire support substrate 701 at the stage shown in FIG. 2(b).

[0016] After forming the thermal oxide film 702, a core layer 703 must be formed on the thermal oxide film 702, as shown in Figure 2(c). However, because the support substrate 701 is warped as described above, it is difficult to bond single-crystal silicon to the thermal oxide film 702 and grind it down to a thickness of several hundred nanometers. Therefore, a promising method for forming the core layer 703 is to bond the core layer of another SOI substrate to the silicon thermal oxide film 702. However, when SOI substrates are bonded together, layers other than the necessary core layer are also integrated with one SOI substrate. In the example shown in Figure 2(c), an oxide film 704 remains on the core layer 703, functioning as an undercladding layer for the other SOI substrate. This oxide film 704 can be removed by grinding, polishing, wet etching, or other methods, but this process may damage the core layer 703. Damage to the core layer 703 can lead to in-plane nonuniformity of the core layer 703 and ultimately to deterioration in the processing accuracy of the silicon core.

[0017] The present disclosure has been made in view of the above points, and relates to an optical waveguide connection structure that can connect two optical waveguides having significantly different mode field sizes with low loss.

[0018] In order to achieve the above object, one embodiment of the present disclosure provides an optical waveguide connection structure that connects a first optical waveguide and a second optical waveguide on a single support substrate, the optical waveguide connection structure including: an underclad layer formed on one surface of the support substrate; a ridge structure formed on a surface of the underclad layer opposite to the surface that contacts the support substrate; a first optical waveguide core in contact with the ridge structure; a pattern structure in contact with the first optical waveguide core, having the same shape and size as the first optical waveguide core in a top view, and made of a material having a lower refractive index than the first optical waveguide core; a second optical waveguide core that covers the ridge structure, the pattern structure, and the first optical waveguide core, and is formed of a material having a lower refractive index than the first optical waveguide core and a higher refractive index than the underclad layer; and an overclad layer in contact with the second optical waveguide core and made of a material having a lower refractive index than the second optical waveguide core. wherein the ridge structure has a thickness approximately equal to half the thickness of the core of the second optical waveguide minus half the thickness of the core of the first optical waveguide, and the height of the center of the core of the first optical waveguide coincides with the height of the center of the core of the second optical waveguide, and by making the refractive indexes of the ridge structure and the pattern structure equal, the refractive indexes of the undercladding layer and the overcladding layer are equal. .

[0019] According to the above embodiment, it is possible to provide an optical waveguide connection structure that can connect two optical waveguides having significantly different mode field sizes with low loss. [Brief explanation of the drawings]

[0020] [Figure 1] 1A and 1B are diagrams for explaining a known SSC structure, in which (a) is a top view of a silicon optical circuit, (b) is a cross-sectional view of the silicon optical circuit of (a), and (c) is another cross-sectional view of the silicon optical circuit of (a). [Figure 2] 1A, 1B, and 1C are schematic cross-sectional views illustrating a method for manufacturing an SOI substrate having a known thick BOX layer. [Figure 3] FIG. 2 is a cross-sectional view illustrating a substrate according to the first embodiment. [Figure 4] 4A to 4C are diagrams for explaining a method for manufacturing the substrate shown in FIG. [Figure 5] FIG. 1A is a top view of the optical waveguide connection structure, and FIG. 1B is a cross-sectional view taken along the arrow shown in FIG. [Figure 6] 5(b), (b) is a cross-sectional view taken along another arrow, and (c) is a cross-sectional view taken along yet another arrow. [Figure 7A] 1(a), 1(b), 1(c), and 1(d) are process drawings for explaining a method for manufacturing the optical waveguide connection structure of the first embodiment. [Figure 7B] 7(e), (f), (g) and (h) are process drawings for explaining the method for manufacturing the optical waveguide connection structure of the first embodiment, which are subsequent to FIG. 7A. DETAILED DESCRIPTION OF THE INVENTION

[0021] Optical waveguide connection structures according to the first and second embodiments of the present disclosure will be described below. The drawings referred to in the first and second embodiments are intended to explain the configuration, arrangement of each part, function, effect, and technical concept of the optical waveguide connection structures according to the first and second embodiments, and do not limit the specific shapes. Furthermore, the drawings referred to in the first embodiment do not necessarily accurately represent the ratios of length, width, and thickness.

[0022] [First embodiment] The optical waveguide connection structure of the first embodiment is manufactured using a substrate 100. In the first embodiment, the substrate 100 will first be described.

[0023] (substrate) FIG. 3 is a cross-sectional view illustrating a substrate 100 according to the first embodiment. The substrate 100 is an SOI substrate and includes a support substrate 101, which is a first support substrate, an undercladding layer 102, a silicon core layer 103, and a glass layer 104, which is an insulating layer. In the first embodiment, the side facing the glass layer 104 from the support substrate 101 side will be referred to as "upper" in the following description. Therefore, the undercladding layer 102 is formed on the support substrate 101, the silicon core layer 103 is formed on the undercladding layer 102, and the glass layer 104 is formed on the silicon core layer 103. In the first embodiment, the length of each layer in a direction perpendicular to the support substrate 101 will hereinafter be referred to as "thickness."

[0024] The thickness of the undercladding layer 102 is preferably sufficiently thicker than that of known undercladding layers. In the first embodiment, the thickness of the undercladding layer 102 is set to 15 μm. The undercladding layer 102 is formed of a material having a refractive index smaller than that of the silicon core layer 103. Such a material is preferably, for example, a material containing silica glass containing SiO2 as the main component, and specifically, for example, SiO2, SiO x , polymers, etc.

[0025] The thickness of the silicon core layer 103 may be within the range of the thickness of the core layer of a known silicon photonics circuit. This thickness may be, for example, about 0.2 μm to 1 μm. The silicon core layer 103 is made of a material with a higher refractive index than the undercladding layer 102. Examples of such materials that can be used include Si, SiN, and SiON.

[0026] The thickness of the glass layer 104 may be, for example, approximately the same as the thickness of the silicon core layer 103, and may be, for example, approximately 0.1 μm to 2 μm. The material of the pattern structure 305 (FIG. 5(a), etc.) formed by the glass layer 104 only needs to satisfy the following requirements: it has a refractive index lower than that of the silicon core layer 103, it is not removed in the step of removing the silicon core layer 103, and it can be a material that can serve as an etching mask when etching the silicon core layer 103 to form a silicon core. Examples of the material for such a glass layer 104 include SiO2, SiO x SiO2, SiO etc. can be used. x The glass layer 104 made of SF6, i.e., the pattern structure 204 (FIG. 5(a) and the like), can serve as a mask when etching the silicon core layer 103 of Si using SF6. Here, "can serve as an etching mask" means that the glass layer 104 is a material that is not removed from above the silicon core layer 103 until etching of the silicon core layer 103 is completed, and does not damage the silicon core layer 103 below the pattern structure 305 (FIG. 5(a) and the like). For such a pattern structure 305 (FIG. 5(a) and the like), the thickness as well as the material are taken into consideration.

[0027] 4 is a diagram illustrating a method for manufacturing the substrate 100 shown in FIG. In this explanation, an example is given in which the undercladding layer 102 is made of SiO2, the silicon core layer 103 is made of Si, and the glass layer 104 is made of SiO2. The manufacturing of the substrate 100 includes the steps of forming the undercladding layer 102, the silicon core layer 103, and the glass layer 104. The support substrate 101 on which the undercladding layer 102 is formed is preferably a silicon substrate, but may also be a glass substrate.

[0028] The process for forming the undercladding layer 102 may be carried out by any method capable of forming an undercladding layer 102 having uniformity and smoothness sufficient for forming the silicon core layer 103 directly thereon. Examples of such methods include flame deposition. Alternatively, the support substrate 101 may be thermally oxidized to form the undercladding layer 102 as a thermal oxide film. However, if an oxide film having a thickness of 10 μm or more is formed on the support substrate 101, stress is applied to the support substrate 101 due to unevenness in the amount of film formed on the front and back surfaces. The support substrate 101 will warp overall. It is difficult to bond single-crystal silicon to the undercladding layer 102 of the warped support substrate 101 and then grind it down to the desired thickness (approximately several hundred nanometers). Therefore, in the first embodiment, the silicon core layer 103 is formed as follows.

[0029] The step of forming the silicon core layer 103 on the undercladding layer 102 of the first embodiment is performed by bonding a substrate 31 constituted by a support substrate 101 and the undercladding layer 102 to an SOI substrate 32. The SOI substrate 32 is a substrate including a support substrate 109 that is a second support substrate, the silicon core layer 103, and a glass layer 104 formed between the support substrate 109 and the silicon core layer 103 and made of a material having a smaller refractive index than the silicon core layer 103. The substrate 31 and the SOI substrate 32 are bonded together so that the silicon core layer 103 is in contact with the undercladding layer 102.

[0030] Alternatively, bonding may be performed by room temperature bonding, checking the bonding state, and then performing annealing at 1000°C or higher to ensure bonding strength. Immediately after bonding, the silicon core layer 103, glass layer 104, and support substrate 109 of the SOI substrate 32 are integrated with the substrate 31. In the first embodiment, the support substrate 109 is removed by, for example, polishing.

[0031] After removing the support substrate, the glass layer 104 may be removed by, for example, grinding / polishing or wet etching. However, removing the glass layer 104 carries the risk of damaging or peeling off the silicon core layer 103, and such damage or peeling may impair the in-plane uniformity of the silicon photonics circuit. In consideration of this, in the first embodiment, at least a portion of the glass layer 104 is left unremoved during the fabrication of the substrate 100. In the first embodiment, it is sufficient that a portion of the glass layer 104 remains on the silicon core layer 103, and the glass layer 104 may be removed to a desired thickness by wet etching or the like.

[0032] According to the above method, a flat SOI substrate 32 is bonded to a substrate 31 that has warped due to the formation of the undercladding layer 102, so that the warpage of the substrate 31 is corrected by the SOI substrate 32, and it becomes possible to form the silicon core layer 103 on the flat undercladding layer 102.

[0033] (Optical waveguide connection structure) Next, an optical waveguide connection structure manufactured using the above-described substrate 100 will be described.

[0034] 5(a) and 5(b) are diagrams for explaining the optical waveguide connection structure of the first embodiment, showing a silicon optical circuit including an optical waveguide connection structure 300. FIG. 5(a) is a top view of the optical waveguide connection structure 300, and FIG. 5(b) is a cross-sectional view taken along the arrows Vb and Vb shown in FIG. 5(a). In the following descriptions of the figures, the axis along the direction in which an optical signal passes through the silicon optical waveguide 310 and the SiO2 optical waveguide 320 is referred to as the Z-axis, the axis perpendicular to the Z-axis and the surface of the support substrate 101 is referred to as the Y-axis, and the axis perpendicular to the Z-axis and the Y-axis is referred to as the X-axis. In this specification, the direction of the Y-axis from the support substrate 101 will be referred to as "up."

[0035] The optical waveguide connection structure 300 is an optical waveguide connection structure that connects a silicon optical waveguide 310, which is a first optical waveguide, and a SiO2 optical waveguide 320, which is a second optical waveguide, on one support substrate 101. The silicon optical waveguide 310 is an optical waveguide whose core is formed using single crystal silicon as a material. The SiO2 optical waveguide 320 is an optical waveguide whose core is made of a material containing silica-based glass with SiO2 as a base material. The optical waveguide connection structure 300 includes an undercladding layer 302 formed on one side of the support substrate 101, a ridge structure 303 formed on the side of the undercladding layer 302 opposite to the side in contact with the support substrate 101, a silicon core 304 which is a first optical waveguide core in contact with the ridge structure 303, and a pattern structure 305 in contact with the silicon core 304, having the same shape and size as the silicon core 304 when viewed from above, and made of a material having a lower refractive index than the silicon core 304.

[0036] The silicon core 304 includes a constant width portion 304a with a constant width and a narrow width portion 304b whose width decreases in the Z direction. As the width of the narrow width portion 304b decreases, light passing through the narrow width portion 304b leaks into the SiO2 core 306, which is the core of the SiO2 optical waveguide 320, and an optical signal circulates between the silicon optical waveguide 310 and the SiO2 optical waveguide 320. This configuration constitutes an SSC structure 330.

[0037] The optical waveguide connection structure 300 also has an SiO2 core 306, which is a second optical waveguide core that covers the ridge structure 303, the pattern structure 305, and the silicon core 304. The SiO2 core 306 is made of a material that has a refractive index lower than that of the silicon core 304 and a refractive index higher than that of the undercladding layer 302. The optical waveguide connection structure 300 also has an overcladding layer 307 that is in contact with the SiO2 core 306 and is made of a material that has a refractive index lower than that of the SiO2 core 306.

[0038] 5(a) and 5(b) show only a cut-out portion of an optical circuit in which one silicon optical waveguide 310 and one SiO2 optical waveguide 320 are integrated on a support substrate 101. The number of silicon optical waveguides 310 and SiO2 optical waveguides 320 is not limited to this, and more silicon optical waveguides 310 and SiO2 optical waveguides 320 may be included. Furthermore, the optical waveguides are not limited to the silicon optical waveguides 310 and SiO2 optical waveguides 320, and may include optical waveguides of other configurations.

[0039] Here, the refractive index of the silicon core 304 is n1, the refractive index of the SiO2 core 306 is n2, and the refractive index of the undercladding layer 302 is n3. The materials constituting the silicon optical waveguide 310 and the SiO2 optical waveguide 320 are: n1>n2>n3...Equation (1) It is sufficient if the following relationship is satisfied.

[0040] In the description of the first embodiment, the silicon core 304 is Si, the SiO2 core 306 is SiO2, and the undercladding layer 302 is SiO2, which has a lower refractive index than the SiO2 core 306. However, the first embodiment is not limited to using such materials. For example, the silicon core 304 may be made of SiN or SiON, and the SiO2 core 306 may be made of SiO x The core of the second optical waveguide may be a polymer. In the first embodiment, the over cladding layer 307 and the under cladding layer 302 may be made of the same material and have the same refractive index, but they do not have to be exactly the same. In other words, when the refractive index of the over cladding layer 307 is n4, the over cladding layer 307 is made of a material that satisfies the following formula (2) as well as the above formula (1).

[0041] n1>n2>n4...Equation (2)

[0042] In the first embodiment, the pattern structure 305 is formed by etching the glass layer 104. However, the material of the pattern structure 305 may be any material that has a lower refractive index than the silicon core 304 and is not removed when the silicon core 304 is removed. The material of the pattern structure 305 may be SiO2, SiO x etc. When the material of the silicon core 304 is Si, such a material can serve as a mask in etching using SF6. Here, "can serve as an etching mask" means that the pattern structure 305 is a material that is not removed from above the silicon core 304 until etching for forming the silicon core 304 is completed, and that does not damage the silicon core 304 below the pattern structure 305. For such a pattern structure 305, the thickness as well as the material are taken into consideration.

[0043] The ridge structure 303 is made of the same material as the undercladding layer 302, i.e., SiO2, SiO x , polymer, etc. The width of the ridge structure 303 may be equal to or greater than the width of the silicon core 304, but less than the width of the SiO2 core 306. The thickness of the ridge structure 303 is preferably approximately equal to half the thickness of the SiO2 core 306 minus half the thickness of the silicon core 304. Here, the degree of "approximately" depends on the controllability of the deposition of the SiO2 core 306 and the silicon core 304, and a difference of, for example, about ±1 μm is permitted.

[0044] (Core size, MFD for single mode) The silicon optical waveguide 310 and the SiO2 optical waveguide 320 have no upper limit to the cross-sectional size (hereinafter simply referred to as "size") intersecting the XY plane, and can be used as a multi-mode optical waveguide that propagates multiple modes of light in the wavelength band of the light used as a signal. Furthermore, by reducing the core cross-sectional size, they can also be used as a single-mode optical waveguide that propagates only the lowest-order mode. In the silicon optical waveguide 310, the silicon core 304 functions as the core, and the SiO2 core 306 and ridge structure 303 function as cladding layers. The pattern structure 305 can be considered a remnant of etching the silicon core 305, but in the silicon waveguide 310, it functions as part of the overcladding layer. In such a silicon optical waveguide 310, the refractive index difference between the core and the cladding layer is relatively large. When the silicon optical waveguide 310 is single-mode, the size of the silicon core 304 can be reduced to several hundred nanometers square.

[0045] In the SiO2 optical waveguide 320, the SiO2 core 306 functions as the core, and the undercladding layer 302 and overcladding layer 307 function as cladding layers. In this configuration, SiO2 is used for both the core and the cladding, so the refractive index difference between the core and the cladding layer is smaller than that of the silicon optical waveguide 310. The cross-sectional size of such an SiO2 core 306 is several μm square to approximately 10 μm square in the case of a single mode.

[0046] As described above, the cross-sectional size of the core differs by up to 100 times between the silicon optical waveguide 310 and the SiO2 optical waveguide 320. Therefore, the MFD of light propagating in the SiO2 core 306 is significantly larger than the MFD of light propagating in the silicon core 304.

[0047] (spot size conversion) To connect a silicon optical waveguide 310 and a SiO2 optical waveguide 320 with different MFDs, the first embodiment includes an SSC structure 330, as shown in FIG. 5(a), which gradually expands the MFD propagating within the silicon core 304. This function of the SSC structure 330 is realized by the narrow portion 304b of the silicon core 304. The narrow portion 304b is not limited to a structure that narrows in the Z direction. For example, it may narrow in the Y direction, i.e., it may become lower in the Z direction. A structure that narrows in the Z direction is also called a tapered structure. The SSC structure can also be realized by a segmented structure in which the silicon core 304 is divided in the light propagation direction, i.e., regions with and without cores alternate. The SSC structure of the first embodiment may be a structure that combines a tapered shape and a segmented structure.

[0048] (Connection loss) Next, a configuration for reducing coupling loss in the first embodiment will be described. As shown in FIG. 5(a), the first embodiment includes an SSC structure 330, which adiabatically transfers light passing through the silicon core 304 to the SiO2 core 306. However, in this type of coupling, some of the optical energy may not be adiabatically coupled. The optical energy that is not adiabatically coupled propagates through the silicon core 304 and reaches the interface between the silicon optical waveguide 310 and the SiO2 optical waveguide 320, where it is butt-coupled to the SiO2 core 306. The butt-coupling efficiency increases as the butt-coupling efficiency, which is defined as the overlap integral of the MFD of the silicon core 304 and the MFD of the SiO2 core 306 at the boundary between the silicon optical waveguide 310 and the SiO2 optical waveguide 320, increases. In the first embodiment, attention is paid to this point, and a ridge structure 303 is provided to adjust the height of the silicon core 304 to match the center of the SiO2 core 306, thereby increasing the overlapping portion of the MFDs of both.

[0049] 6(a), 6(b), and 6(c) are cross-sectional views taken along the arrows in FIG. 5(b). FIG. 6(a) is a cross-sectional view taken along the arrows VIa and VIa, FIG. 6(b) is a cross-sectional view taken along the arrows VIb and VIb, and FIG. 6(c) is a cross-sectional view taken along the arrows VIc and VIc. As shown in FIGS. 6(a), 6(b), and 6(c), the silicon optical waveguide 310 has the widest silicon core 304 in the cross-section taken along the arrows VIa and VIa, and the silicon core 304 has the narrowest width in the cross-section taken along the arrows VIb and VIb. Furthermore, in the cross-section taken along the arrows VIc and VIc, the waveguide of the optical waveguide connection structure 300 is a SiO2 optical waveguide 320. As shown in FIGS. 6(a) and 6(b), the silicon core 304 is formed on the ridge structure 303, thereby being positioned near the center of the SiO2 core 306.

[0050] In order to increase the overlapping area of ​​the MFDs of the silicon core 304 and the SiO2 core 306, the first embodiment sets the thickness of the ridge structure 303 to a thickness obtained by subtracting half the thickness of the silicon core 304 from half the thickness of the SiO2 core. In this way, the center of the silicon core 304 formed on the upper surface of the ridge structure 303 coincides with the center of the SiO2 core.

[0051] As described above, in the first embodiment, the center heights of the silicon core 304 and the SiO2 core 306 are made to match each other, thereby increasing the butt coupling efficiency defined by the overlap integral of the mode field and enabling coupling of optical energy with low loss.

[0052] (Manufacturing method) Next, a method for manufacturing the optical waveguide connection structure 300 described above will be described. FIGS. 7A(a) to 7B(h) are cross-sectional views illustrating the method for manufacturing the optical waveguide connection structure 300. In each figure, (i) is a cross-sectional view taken along the arrows VIa and VIa in FIG. 5(b), and (ii) is a cross-sectional view taken along the arrows Vb and Vb in FIG. 5(a). In manufacturing the optical waveguide connection structure 300, first, the substrate 100 shown in FIG. 3 is fabricated as shown in FIG. 7A(a). Next, in the first embodiment, a protective film pattern 108 is formed directly on the glass layer 104 as shown in FIG. 7A(b). The protective film pattern 108 may be formed by a known photolithography technique using an electron beam lithography apparatus, a reduction projection exposure apparatus, or the like.

[0053] Next, in the first embodiment, as shown in FIG. 7A(c), the glass layer 104 is etched using the protective film pattern 108 as a mask to form a pattern structure 305. Then, in the first embodiment, as shown in FIG. 7A(d), the silicon core layer 103 is etched using the pattern structure 305 as a mask. As a result of the etching, a silicon core 304 capable of propagating light is formed. By forming the silicon core 304 in this manner, the shape and size of the pattern structure 305 and the silicon core 304 match when viewed from above. However, "matching the shape and size when viewed from above" may be determined visually using a microscope or the like, and differences such as corners of the pattern structure 305 being more rounded than the corners of the silicon core 304 due to over-etching are acceptable. Furthermore, in the first embodiment, a silicon photonics optical circuit may be formed in conjunction with the formation of the silicon core 304.

[0054] In the process described above, in the first embodiment, it is not necessary to remove all of the glass layer 104, which is the uppermost layer of the substrate 100 shown in Fig. 3, and it can be used as a hard mask when processing the silicon core 304. This makes it possible to prevent deterioration in the in-plane uniformity of the silicon core layer 103 due to the removal of the glass layer 104, and ultimately allows the silicon core 304 to be processed with high precision.

[0055] Next, in the first embodiment, as shown in FIG. 7B(e), the undercladding layer 102 is processed to form a ridge structure 303 and an undercladding layer 302. The undercladding layer 102 has a thickness of approximately 15 μm. Therefore, even after the ridge structure 303 is formed, the undercladding layer 302 maintains a thickness sufficient to function as an undercladding layer, preventing leakage of the core mode field of approximately several μm. Next, in the first embodiment, as shown in FIG. 7B(f), an SiO layer 506 is formed on the ridge structure 303, silicon core 304, and pattern structure 305. The SiO layer 506 is processed to be capable of propagating light as a waveguide core, forming the SiO core 306, as shown in FIG. 7B(g). At this time, it is desirable that the SiO core 306 be wider than the previously processed silicon core 304 and pattern structure 305. This is to avoid the sidewalls of the silicon core 304 and the pattern structure 305, which have been processed beforehand, being affected when the SiO2 core 306 is processed.

[0056] 7B(h), the first embodiment further includes an overcladding layer 307 made of SiO2 having a lower refractive index than the SiO2 core 306. Through the steps described above, the optical waveguide connection structure 300 of the first embodiment is completed.

[0057] Next, other effects of the first embodiment will be described with reference to FIGS. 2 and 3. The connection loss between the silicon optical waveguide 310 and the SiO2 optical waveguide 320 depends on the dimensional accuracy of the silicon core 304 in the SSC structure 330. For example, when the SSC structure 330 has a tapered shape that narrows in the Z-axis direction as shown in FIG. 5(a), it is desirable that the width of the tip of the silicon core 304 is sufficiently narrow. In order to process the tip of the silicon core 304 to be sufficiently narrow, there is a method in which the protective film pattern 108 shown in FIG. 7A(b) is formed so that its width is the minimum width, and then the protective film pattern is further processed so that the width is below the lower limit of the width of the protective film pattern 108 that can be formed.

[0058] The above-mentioned technique includes, for example, reactive ion etching using oxygen gas. In this case, in the first embodiment, the silicon core layer 103 is covered with the glass layer 104, so the surface of the silicon core layer 103 is not damaged by the spray of etching gas. In other words, the silicon core 304 in the SSC structure 330 can be finally processed with high precision without impairing the in-plane uniformity of the silicon core layer 103, and the connection loss between optical waveguides with different mode field sizes can be reduced.

[0059] [Second embodiment] Next, a second embodiment of the present disclosure will be described. In the second embodiment, the refractive indexes of the pattern structure 305 and the ridge structure 303 are made equal to each other, thereby eliminating the asymmetry of the mode field. The second embodiment differs from the first embodiment in this respect, but its shape is similar to that of the first embodiment. Therefore, the second embodiment will be described using FIGS. 5(a) and 5(b).

[0060] In the second embodiment, the ridge structure 303 and the pattern structure 305 are made of the same material, such as SiO2, SiO x The ridge structure 303 and the pattern structure 305 have refractive indices that are approximately equal to each other, where "approximately equal" refractive indices allows for differences in refractive indices that result from manufacturing of similar materials.

[0061] Since the ridge structure 303 and the pattern structure 305 have almost the same refractive index, the overcladding layer and the undercladding layer of the silicon core 304 also have almost the same refractive index. As a result, the silicon core 304 has a mode field that is line-symmetric in the up-down direction (direction perpendicular to the substrate) with respect to an imaginary plane passing through the central axis of the silicon core 304. Note that, since both sides of the silicon core 304 on the XZ plane are SiO2 cores 306, the refractive indexes of the silicon core 304 on the left and right (direction horizontal to the substrate) are also symmetric. That is, in the second embodiment, the mode field of the silicon core 304 can be line-symmetric in the left-right direction (direction horizontal to the substrate) with respect to the central axis.

[0062] Furthermore, since the surrounding cladding (undercladding layer 302 and overcladding layer 307) all have the same refractive index, the SiO2 core 306 has a mode field that is line-symmetric in both the up-down direction (vertical to the substrate) and the left-right direction (horizontal to the substrate). As described above, in the second embodiment, the mode field of the silicon core 304 is line-symmetric, which increases the butt coupling efficiency defined by the overlap integral of both mode fields, thereby achieving optical energy coupling with even lower loss than in the first embodiment. [Explanation of symbols]

[0063] 31. Circuit board 32 SOI substrate 100 boards 101, 109, 601 Support substrate 102 Underclad layer 103 Silicon core layer 104 Glass Layer 108 Protective film pattern 300, 600 optical waveguide connection structure 302, 602 undercladding layer 303 Ridge Structure 304, 603 Silicon Core 304a, 603a Constant width section 304b, 603b narrow part 305 Pattern Structure 306, 604 SiO2 core 307, 605 overclad layer 310, 610 Silicon optical waveguide 320 SiO2 optical waveguide 330, 630 Spot Size Converter 506 SiO2 layer 620 Planar optical waveguide

Claims

1. An optical waveguide connection structure that connects a first optical waveguide and a second optical waveguide in one support substrate, an undercladding layer formed on one surface of the support substrate; a ridge structure formed on a surface of the undercladding layer opposite to the surface that contacts the support substrate; a first optical waveguide core in contact with the ridge structure; a pattern structure that is in contact with the first optical waveguide core, has the same shape and size as the first optical waveguide core in a top view, and is made of a material that has a lower refractive index than the first optical waveguide core; a second optical waveguide core that covers the ridge structure, the pattern structure, and the first optical waveguide core and is formed of a material that has a refractive index lower than that of the first optical waveguide core and a refractive index higher than that of the undercladding layer; an overclad layer in contact with the second optical waveguide core and formed of a material having a refractive index lower than that of the second optical waveguide core; the ridge structure has a thickness approximately equal to half the thickness of the core of the second optical waveguide minus half the thickness of the core of the first optical waveguide; the height of the center of the core of the first optical waveguide is the same as the height of the center of the core of the second optical waveguide; The optical waveguide connection structure, wherein the refractive index of the undercladding layer is equal to that of the overcladding layer by making the refractive index of the ridge structure equal to that of the pattern structure.

2. 2. The optical waveguide connection structure according to claim 1, wherein the material of the pattern structure is a material that is not removed during etching to form the first optical waveguide core and can serve as a mask when forming the first optical waveguide core.

3. 3. The optical waveguide connection structure according to claim 1, wherein the first optical waveguide and the second optical waveguide are single-mode waveguides with respect to the wavelength of the optical signal to be guided.

4. 2. The optical waveguide connection structure according to claim 1, further comprising a spot size converter at an interface between the first optical waveguide and the second optical waveguide, the spot size converter changing a mode field diameter of an optical signal propagating through the first optical waveguide.

5. The spot size converter 5. The optical waveguide connection structure according to claim 4, comprising at least one of a structure in which the diameter of the first optical waveguide changes in a tapered shape in a horizontal plane or a vertical plane, and a structure in which the core of the first optical waveguide is formed intermittently.

6. The core of the first optical waveguide is made of single crystal silicon, and the undercladding layer, the ridge structure, the pattern structure, the core of the second optical waveguide, and the overcladding layer are made of SiO 2 3. The optical waveguide connection structure according to claim 1, comprising silica-based glass having a base material of:

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