Fiber-waveguide coupler and method for its manufacture

Abstract

The present invention relates to a method for manufacturing a fiber-waveguide coupler, comprising: applying a mask layer to a first surface on a front side of a substrate, the mask layer having at least one aperture mark; applying an intermediate layer to the mask layer; bonding a first oxide layer to the intermediate layer; applying a stress compensation layer to a second surface on a back side of the substrate, wherein at least one of a material and a thickness of the stress compensation layer is selected to reduce warpage of the substrate; applying a waveguide structure to the first oxide layer, the waveguide structure comprising a core layer and a cladding layer, the core layer having a predetermined width and a predetermined thickness and being aligned with the aperture mark;and locally etching the waveguide structure and the first oxide layer to form an opening above the opening mark; anisotropically etching a V-groove into the substrate at the opening mark of the mask layer such that when a fiber of a predetermined size is inserted into the V-groove, a core of the fiber is centered with respect to the core layer of the waveguide structure. Furthermore, the present invention provides a fiber-waveguide coupler.

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to a fiber-waveguide coupler and a method for producing a fiber-waveguide coupler. TECHNICAL BACKGROUND

[0002] In photonics, fiber-to-waveguide couplers are required to efficiently couple an optical signal into a waveguide of a photonic integrated structure fabricated on a semiconductor platform. For such photonic structures, very thick cladding layers make it difficult to maintain the tight tolerances required for passive alignment of the optical fiber to a waveguide of the photonic integrated structure using a V-groove manufacturing process based on a sequence of deposition and etching steps.

[0003] Wafer-to-wafer fusion bonding can offer a solution for integrating a V-groove with thick cladding layers, but this presents several additional challenges. The technical requirements for high-precision fusion bonding include low surface roughness, typically less than 0.5 nm. Such surface roughness is typically measured using an atomic force microscope. These requirements also include a wafer or substrate curvature of less than 30 µm across the substrate for optimal alignment accuracy. Dielectric materials are often required in such processes to create bondable surfaces with high bond strength. Furthermore, particle contamination must be limited to a certain size; typically, around 50 particles with a size of approximately 90 nm or larger are required on the surface for a high-quality bond.For successful bonding, a relatively flat surface topography or a small gradient of typically less than 10 nm is required.

[0004] US 2024 / 0 053 551 A1 describes a photonic integrated circuit with a V-groove cavity substrate in a substrate.

[0005] JP H10 - 288 717 A describes an optical waveguide substrate and a corresponding method for manufacturing it. The optical waveguide substrate has a V-groove for positioning an optical fiber for coupling into a waveguide. The core of the optical waveguide is formed by patterning and etching away an SiO2 film with a refractive index suitable for a cladding formed on the substrate through photolithography using a photomask. The V-groove is formed in a section where the SiO2 film is patterned and etched away simultaneously with the patterning by the photomask. This is intended to achieve greater positioning accuracy of the optical fiber relative to the waveguide core.

[0006] US 6,865,308 B1 describes a method for eliminating distortions during the manufacture of optical waveguides constructed on a wafer. The method comprises applying a cladding layer and core layer and partially etching them away to produce a structured core layer on the top side of the substrate, as well as applying core material to the underside of the substrate, followed by annealing. Finally, a second cladding layer is applied over the structured core layer to compensate for stresses induced during annealing of the applied core material by stresses induced by annealing the material applied to the underside of the wafer.

[0007] US 2017 / 0 162 522 A1 describes a curvature-compensated semiconductor wafer and a method for compensating a curvature or warpage in a semiconductor wafer. The method involves applying a stress-compensation layer over an adhesion layer disposed on a major surface of the semiconductor wafer. The stress-compensation layer has a high stress state and is formed with a thickness sufficient to substantially reduce the warpage of the wafer.

[0008] US 2023 / 0 142 315 A1 describes a photonic chip and a method for manufacturing a photonic chip. The photonic chip includes an edge coupler comprising a composite conductive structure having multiple parallel layers of optical conductive material. Each layer of this composite conductive structure extends into a cladding material and is aligned with an edge. A waveguide can be applied to the photonic chip in such a way that it overlaps with at least one layer of the composite conductive structure.

[0009] Therefore, there is a need for an improved manufacturing process for V-grooves on a semiconductor substrate with an edge-fiber waveguide coupler for optical coupling at the end. SUMMARY OF THE INVENTION

[0010] According to the invention, this problem is solved by the subject matter of the independent claims.

[0011] According to a first aspect of the invention, a method for manufacturing a fiber-waveguide coupler is provided. The method comprises applying a mask layer to a first surface on a front side of a substrate, the mask layer having at least one aperture mark; applying an intermediate layer to the mask layer; bonding a first oxide layer to the intermediate layer; applying a stress-compensating layer to a second surface on a back side of the substrate, wherein at least one of a material and a thickness of the stress-compensating layer is selected to reduce warpage of the substrate; applying a waveguide structure to the first oxide layer, the waveguide structure comprising a core layer and a cladding layer, the core layer having a predetermined width and a predetermined thickness and being aligned with the aperture mark;locally etching the waveguide structure and the first oxide layer to form an opening above the opening mark; and anisotropically etching a V-groove into the substrate at the opening mark of the mask layer such that when a fiber of a predetermined size is inserted into the V-groove, a core of the fiber is centered with respect to the core layer of the waveguide structure.

[0012] According to a second aspect of the invention, a fiber-waveguide coupler is provided. The fiber-waveguide coupler comprises a substrate having a first surface on a front side and a second surface opposite the first surface on a back side, wherein the first surface of the substrate has a V-groove extending in a Z-direction, wherein the Z-direction is substantially orthogonal to the first surface, a first oxide layer on the front side of the substrate, a waveguide structure deposited on the first oxide layer, wherein the waveguide structure has a core layer and a cladding layer, wherein the core layer has a predetermined width and a predetermined thickness, wherein the V-groove is arranged in an opening of the waveguide structure and the first oxide layer, wherein the core layer is aligned with the V-groove such that when a fiber of a predetermined size is inserted into the V-groove,a core of the fiber is centered with respect to the core layer of the waveguide structure, and a stress compensation layer applied to the second surface of the substrate, wherein at least one of a material of the stress compensation layer and a thickness of the stress compensation layer is selected to reduce warpage of the substrate, a mask layer applied to the substrate, wherein the mask layer has an opening mark arranged above the V-groove, and an intermediate layer applied to the mask layer, wherein the intermediate layer is arranged between the mask layer and the first oxide layer, wherein the opening is arranged above the opening mark.

[0013] A fundamental idea of ​​the present invention is to provide a method and a corresponding fiber coupler in which an oxide layer is bonded, enabling thicker coupler structures for improved photonic structures. To reduce the warpage of the fiber-waveguide coupler, a stress-compensating layer is applied or bonded to the backside of the substrate to compensate for the stress caused by bonding the thick oxide layer.

[0014] In this process, a mask layer, typically a hard mask, is applied to a substrate, which is typically a silicon wafer but is not limited to this material. For example, InP substrates can also be used for this process. The mask layer has an opening that serves to mark the position of the V-groove for an etching process. To form a layer with reduced topography while maintaining the functionality of the mask layer, the mask layer can be embedded in a (first) intermediate layer that is applied after patterning the mask layer. This first intermediate layer can be self-planarizing or substantially reproduce the topography of the patterned mask. In this case, a polishing step may be required to form a bondable surface with low surface topography and low surface roughness.Once the bondable surface is formed, the first oxide layer is bonded to the top surface. This step may require the removal of a substrate that originally supported the first oxide.

[0015] Furthermore, a stress-compensating layer is applied or bonded to the backside or rear surface (second surface) of the substrate. The stress-compensating layer is adapted such that the stress of the material or the thickness of the stress-compensating layer is selected to reduce the warpage of the substrate caused by bonding the (first) oxide layer on the front side. A suitable material for the stress-compensating layer is characterized by high compressive strength, so that it can counteract the stress caused by the manufacturing process. In particular, the compressive strength of such a material can be higher than the stress value of the first oxide layer. The material of the stress-compensating layer can be an oxide, such as SiO2, or even a metal, as described further below.

[0016] Furthermore, a waveguide structure is applied to the oxide layer. The waveguide structure is characterized by a core layer and a cladding layer that supports the propagation of an optical mode. Typically, the core layer has a higher refractive index than the cladding layer that typically surrounds the core layer. In this way, an optical mode coupled in the core layer propagates in the core layer. The waveguide structure can have a core layer consisting of multiple core and multiple cladding layers. The core layer has a predetermined width and thickness to support only a limited number of modes, typically a single mode. The dimensions of the waveguide also determine the mode area or mode field diameter of the mode. When using silicon nitride (SiN) as the core material, typical values ​​for the width are about 50 nm to several µm and the thickness are about 50 nm to 800 nm.The position of the core layer in cross-section orthogonal to the wafer surface is aligned with the aperture marking of the mask layer. Typically, the core layer is located directly above the aperture of the mask layer. In this sense, "above" means a direction orthogonal to the first surface of the substrate. However, an offset can also be applied when using special fibers.

[0017] Finally, an anisotropic etching of a V-groove is performed into the substrate at the location of the mask layer opening. For this purpose, the overlying layers, i.e., the waveguide structure and the oxide layer above the opening mark, are also removed. Such a cavity exposes the substrate surface on which the V-groove is formed. In crystalline silicon with <100> For surface orientation, etching can be performed with KOH or TMAH or by plasma etching techniques. This allows the formation of a suitable V-groove with a trench along the crystallographic plane (110) and surfaces in the <111> plane. However, the process is not limited to this specific case.

[0018] The V-groove is etched so that a fiber of a predetermined size inserted into the V-groove is centered with respect to the core layer of the waveguide structure. For example, a standard single-mode fiber, such as an SMF-28 (e.g., SMF-28 ULL from Corning) with a core diameter of approximately 8 µm and a cladding diameter of 125 µm, inserted into the V-groove with its surfaces mechanically supporting the fiber, is centered with respect to the core layer of the waveguide structure to achieve optimal coupling efficiency between the fiber and the waveguide. Of course, the fiber-waveguide coupler is not limited to this type of single-mode fiber and can also support single-mode fibers with a large mode area, fibers with few modes, and even multimode fibers.

[0019] A bonding-based manufacturing process according to the present invention offers the advantage of being faster and more reliable with regard to the alignment accuracy of the final V-groove relative to the optical waveguide. Furthermore, only a relatively low alignment accuracy of even more than 100 nm is required for the bonding process to achieve a satisfactory final product. Nevertheless, the bonding process enables higher accuracies of less than 100 nm than are required at this development stage, so the inventive method has the potential to meet more stringent requirements in the future. These relatively low requirements lead to a significant cost reduction for the manufacturing process and the wafers.

[0020] In this document, applying a first layer to a second layer describes the general approach for applying the first layer to a second layer. This can be done by any suitable method, for example, by chemical vapor deposition, CVD, such as low-pressure CVD or plasma-enhanced CVD, by bonding, such as wafer-to-wafer bonding, or by any other suitable method. Furthermore, applying or arranging a first layer to a second layer by bonding is understood to mean the more specific form of directly bonding the first layer to the second layer.

[0021] According to some further aspects of the invention, the intermediate layer has a bondable top surface. Bonding the first oxide layer to the mask layer comprises bonding the first oxide layer to the bondable top surface of the intermediate layer. A bondable top surface is understood to be a surface that promotes bonding to another semiconductor or another oxide layer. For a suitable bondable top surface, dielectric materials such as SiO2, SiCxNy, SiOxNy, Al2O3 can be used for the underlying material, in this case the intermediate layer. As previously described, the intermediate layer further serves to planarize the mask layer, which also promotes bonding of the first oxide layer. In this way, bonding of the first oxide layer can be performed with higher accuracy and yield, resulting in a planar and more stable layer stack.

[0022] According to some further aspects of the invention, bonding the first oxide layer comprises bonding a second substrate having the first oxide layer applied on a front side to the bondable top side of the intermediate layer, and etching the second substrate from the first oxide layer. This wafer-to-wafer bonding process is particularly suitable for meeting the high requirements for flatness and warpage, since only high-quality surfaces are affected, namely the bondable top side of the intermediate layer and the top side of the first oxide layer, which is connected to the bondable top side. In some embodiments, an additional oxide layer is applied to a back side of the second substrate. Typically, the first and additional oxide layers can be applied to the second substrate using the same process, for example, by chemical vapor deposition (CVD).Therefore, the additional oxide layer is identical in material and thickness to the first oxide layer. This reduces the warpage of the second substrate, facilitating wafer-to-wafer bonding. The second oxide layer is then etched in the same way as the second substrate, leaving behind the layer stack containing the substrate, the mask layer, the interlayer, the first oxide layer, and, optionally at this stage, the stress-compensation layer.

[0023] According to some further aspects of the invention, the waveguide structure comprises: the core layer on the first oxide layer; and a first cladding layer on the core layer and the first oxide layer, the first cladding layer having a bondable top surface; and a second cladding layer on the bondable top surface of the first cladding layer. This represents a simple waveguide structure in which the cladding of the core layer is formed by the first oxide layer on the bottom surface and the first cladding layer on the top surface and sides of the first core layer. In this case, the refractive index of the first oxide layer and the first cladding layer is lower than the refractive index of the core layer. The core layer can be formed from Si3N4 or another suitable high refractive index material. A second cladding layer is deposited above the first cladding layer to improve the confinement of the optical mode in the core layer.

[0024] According to some further aspects of the invention, the method further comprises providing a third substrate comprising a second oxide layer on a front side of the third substrate, the second oxide layer having a bondable top surface, bonding the second oxide layer to the waveguide structure, and etching the substrate to expose the second oxide layer. In this way, a high-quality second oxide layer is applied to the waveguide structure, thereby improving propagation mode confinement and reducing fiber-to-chip coupling loss. Furthermore, the second oxide layer provides physical protection for the waveguide structure.

[0025] According to some further aspects of the invention, the method further comprises applying a second stress-compensating layer to the backside cladding structure. This allows the stress caused by the third oxide layer to be compensated and substrate warpage to be reduced.

[0026] According to some further aspects of the invention, the aperture mark has a rectangular shape. This shape of the aperture mark is suitable for etching the V-groove.

[0027] According to some further aspects of the invention, the mask layer is structured with a tapered width such that the V-groove has a depth that varies along the Z-direction, with the Z-direction being substantially orthogonal to the top surface. This can be used to adjust the height of the fiber relative to the waveguide core.

[0028] According to some further aspects of the invention, the stress-compensation layer comprises a metal. The stress-compensation layer may also comprise a material that has high compressive strength and high thermal conductivity to dissipate heat. Furthermore, the material of the stress-compensation layer is designed to withstand temperatures of approximately 1300°C, which occur during the fabrication of a photonic integrated circuit comprising the fiber-waveguide coupler of the present invention. Metals, particularly tungsten and tungsten alloys, are suitable materials for the stress-compensation layer.

[0029] According to some further aspects of the invention, a back cladding structure comprising a cladding layer is applied to the stress compensation layer, wherein a material and a thickness of the back cladding structure and the waveguide structure are substantially the same. As described above, this embodiment makes it possible to compensate for the stresses caused by the application of the waveguide structure on the first oxide layer and the substrate. The back cladding structure can also be designed to have a thickness and materials of all cladding layers applied to the front side of the structure. By choosing the same material as for the front side cladding layers, the back first cladding layer causes a stress similar to that caused by the application of the waveguide structure. This significantly reduces the warpage of the substrate.

[0030] According to some further aspects of the invention, the waveguide structure comprises: a first cladding layer on the first oxide layer, a first core layer on the first cladding layer, a second cladding layer on the first core layer and the first cladding layer, a second core layer on the second cladding layer, wherein the second core layer has a predetermined second width and a second thickness; a third cladding layer on the second core layer and the second cladding layer, wherein a core of a fiber having a predetermined size, disposed within the V-groove, is centered with respect to one of the first core layer and the second core layer of the waveguide structure. In this embodiment, the waveguide structure comprises a second core layer, which may be disposed proximate the first core layer.In this way, it is possible to manipulate the optical mode to provide a wider range of modes for improved coupling efficiency. This embodiment also allows for the fabrication of a multilayer photonic structure. It is understood that the waveguide structure can also comprise more than two core layers and more than three cladding layers, allowing even greater flexibility for photonic circuits integrated into the waveguide structure.

[0031] According to some further aspects of the invention, wherein the back cladding structure comprises: a back first cladding layer deposited on the stress compensation layer, a back second cladding layer deposited on the back first cladding layer, and a back third cladding layer deposited on the back second cladding layer, wherein a material and a thickness of the respective back first, second, and third cladding layers and the respective first, second, and third cladding layers are substantially the same. This structure compensates for the stress caused by depositing the aforementioned waveguide structure with two core layers and three cladding layers on the first oxide layer. As a result, the warpage of the substrate is significantly reduced. It is understood that for a waveguide structure with more than three cladding layers, a similar number of corresponding back cladding layers can be deposited.

[0032] According to the invention, the fiber-waveguide coupler further comprises a mask layer deposited on the substrate, wherein the mask layer has an aperture mark located above the V-groove. The intermediate layer is disposed between the mask layer and the first oxide layer. The opening of the waveguide structure and the first oxide layer are located above the aperture mark. Although the mask layer and the intermediate layer are not directly involved in the fiber coupling and the photonic waveguides in the waveguide structure, these layers are useful for the manufacturing process, as described above.

[0033] According to some further aspects of the invention, the fiber-waveguide coupler further comprises a second oxide layer deposited on top of the waveguide structure. This provides a high-quality additional oxide layer on top of the waveguide structure, improving propagation mode confinement and reducing coupling loss.

[0034] According to some further aspects of the invention, the fiber-waveguide coupler further comprises a second stress-compensating layer deposited on the backside cladding layer structure. This allows the stress caused by the third oxide layer to be compensated and substrate warpage to be reduced, as described above.

[0035] According to a third aspect of the invention, another method for manufacturing a fiber-waveguide coupler is provided. The method comprises etching a pair of alignment markers into a first surface on a front side of a substrate; depositing a mask layer having at least one aperture mark spaced a predetermined distance from the pair of alignment markers; anisotropically etching a reference V-groove into the substrate through the aperture mark; etching the mask layer to expose the first surface of the substrate; depositing a first oxide layer on the first surface of the substrate; depositing a backside first oxide layer on a second surface on a backside of the substrate, the backside of the substrate being opposite the front side of the substrate, wherein a material and a thickness of the backside first oxide layer and the first oxide layer are substantially the same;Applying a waveguide structure comprising at least a cladding layer and a core layer to the first oxide layer; applying a back cladding structure to the back oxide layer, wherein a thickness of the back cladding structure and the waveguide structure are substantially the same; locally etching the waveguide structure and the first oxide layer to form an opening above the opening mark; and anisotropically etching a second V-groove spaced a predetermined distance from the reference V-groove such that when a fiber of a predetermined size is inserted into the V-groove, a core of the fiber is centered with respect to the core layer of the waveguide structure.

[0036] According to a fourth aspect of the invention, another fiber-waveguide coupler is provided. The fiber-waveguide coupler comprises a substrate having a first surface and a second surface opposite the first surface, the first surface of the substrate comprising a pair of alignment marks, a reference V-groove spaced a predetermined distance from the alignment marks, a V-groove spaced a predetermined distance from the reference groove and extending in a Z-direction, the Z-direction being substantially orthogonal to the first surface;a first oxide layer on the first surface of the substrate, a backside first oxide layer deposited on the backside of the substrate, a waveguide structure deposited on the first oxide layer, the waveguide structure comprising a core layer and a cladding layer, the core layer having a predetermined width and a predetermined thickness, the V-groove being arranged in an opening of the waveguide structure and the first oxide layer, and a backside cladding structure deposited on the backside first oxide layer, a thickness of the backside cladding structure and the waveguide structure being substantially the same;

[0037] Another basic idea of ​​the present invention is to use a crystalline <100> -Substrate to compensate for the <100> The crystal plane is not exactly parallel to the surface of the substrate, which is typically a wafer. Therefore, the substrate includes a pair of alignment marks and a reference V-groove to determine the crystal axis of the substrate. As described in German patent application 102023136803.6, filed on December 28, 2023, these allow the determination of the crystal planes so that the core layer can be aligned with the final V-groove etched into the substrate.

[0038] Another idea behind this further process is to provide identical first oxide layers on the front and back of the substrate. The back-side first oxide layer serves as a stress-compensation layer. However, due to the etched mask layer, the front and back of the substrate are identical at this stage of the manufacturing process, allowing for optimal compensation of substrate warpage. Furthermore, the deposition of the first oxide layers on the front and back can typically be performed simultaneously using the same process, which is usually a low-pressure CVD process. Thus, these layers can be reliably deposited in identical thicknesses and from identical materials.

[0039] According to some further aspects of the invention, the further method comprises bonding a second oxide layer to the waveguide structure and bonding a second stress-compensating layer to the back cladding structure. Accordingly, according to some further aspects of the invention, the fiber-waveguide coupler further comprises a second oxide layer applied to the waveguide structure and a second stress-compensating layer applied to the back cladding structure. In this way, a high-quality second oxide layer is applied to the waveguide structure, so that the limitation of the propagation modes is improved and the coupling loss is reduced. In this embodiment, this second stress-compensating layer thus corresponds to the second stress-compensating layer of the first method and the fiber-waveguide coupler.In this way, the stress caused by the second oxide layer can be compensated and warpage of the substrate can be reduced.

[0040] The above embodiments and further developments can be combined with one another as desired. In particular, all features of the fiber-waveguide coupler are transferable to the method for producing the fiber-waveguide coupler as well as to the further method for producing the fiber-waveguide coupler and the resulting further fiber-waveguide coupler, and vice versa. Further possible aspects, further developments, and embodiments of the invention also include combinations of features of the invention described above or below with reference to the exemplary embodiments that are not expressly mentioned. In particular, the person skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the present invention.

[0041] Advantageous embodiments and further developments emerge from the description with reference to the figures. BRIEF SUMMARY OF THE DRAWINGS

[0042] The present invention will be explained in more detail below with reference to the embodiments shown in the schematic figures, where Fig. 1 shows a flowchart for a method of manufacturing a fiber-waveguide coupler according to an embodiment of the invention; Fig. 2 shows a flowchart for a method of manufacturing a fiber-waveguide coupler according to another embodiment of the invention; Fig. 3a-d show cross-sections of fiber-waveguide couplers according to an embodiment of the invention; Fig. 4a-b show cross sections of intermediate products of a fiber-waveguide coupler in the manufacturing process according to another embodiment of the invention; Fig. 5 shows a schematic representation of an intermediate product of a fiber-waveguide coupler in the method of manufacturing according to an embodiment of the invention; Fig. 6 shows a schematic representation of an intermediate product of a fiber-waveguide coupler in the method of manufacture according to another embodiment of the invention; Fig. 7a-c show a schematic representation of fiber-waveguide couplers according to another embodiment of the invention; Fig. 8 shows a flowchart for a method of manufacturing a fiber-waveguide coupler according to another embodiment of the invention; and Fig. 9a-e show a schematic representation of a fiber-waveguide coupler according to another embodiment of the invention;

[0043] The accompanying figures are intended to provide a further understanding of embodiments of the invention. They illustrate embodiments and, in conjunction with the description, are used to explain principles and concepts of the invention. Other embodiments and many of the noted advantages will be apparent from the drawings. Elements of the drawings are not necessarily drawn to scale. Directional designations such as "top," "bottom," "left," "right," "upper," "down," "horizontal," "vertical," "front," "rear," and similar designations are for purposes of illustration only and are not intended to limit the generality of the embodiments shown in the figures.

[0044] In the figures of the drawing, like elements, features and components that have the same function and the same effect have the same reference numerals unless otherwise indicated. DESCRIPTION OF EMBODIMENTS

[0045] Fig. 1 shows a flowchart for a method of manufacturing a fiber-waveguide coupler according to an embodiment of the invention.

[0046] In this procedure, as in Fig. 3a, in step M1, a mask layer 11 is applied to a first surface 10a on a front side F of a substrate 10. The mask layer 11 has at least one opening marking 11a, which is used in an etching step for the V-groove. In step M2, an intermediate layer 12 is applied to the mask layer 11. This application can be carried out by CVD or any other suitable method. As also shown in Fig. 3b, in step M3, a first oxide layer 13 is bonded to the intermediate layer 12. The thickness of this oxide layer is typically relatively thick compared to the mask or the intermediate layer. In step M4, a stress compensation layer 21 is applied to a second surface 10b on a backside B of the substrate 10. At least one of a material and a thickness of the stress compensation layer 21 is selected to reduce warpage of the substrate 10. The stress compensation layer 21 can be selected to compensate for additional stresses from further processing. For example, the thickness of the compensation layer 21 can be selected to compensate for the combination of stresses from a group of layers forming a waveguide structure 14, as shown in Fig. 3c.

[0047] In step M5, a waveguide structure 14 is applied to the first oxide layer 13. In further embodiments, other functional structures can be applied to the first oxide layer 13, in particular before the waveguide structure 14 is applied to these structures. The waveguide structure 14 has a core layer 141, 143, 145 and a cladding layer 142, 144, 146, 148, 149. The core layer 141, 143, 145 has a predetermined width 141a, 143a, 145a and a predetermined thickness 141b, 143b, 145b and is aligned with the opening marking 11a (see also Fig. 3a). It is understood that the waveguide structure supports optical modes propagating in the core layer through a photonic integrated circuit formed in the waveguide structure. Thus, the refractive index of the core layer is typically higher than the refractive index of the cladding layer, which typically at least partially surrounds the core layer.

[0048] The waveguide structure 14 and the first oxide layer 13 are locally etched in step M6 to form an opening 160 above the opening mark 11a. In this step, all layers above the opening mark 11a are etched, resulting in the opening 160.

[0049] In step M7, a V-groove 16 is etched into the substrate 10 at the opening mark 11a of the mask layer 11 by an anisotropic etching process (see Fig. 3d). Typically, KOH or TMAH are used as the etchant for silicon substrates. In some embodiments, the aperture mark 11a has a rectangular shape optimized for the V-groove 16. In further embodiments, the mask layer 11 is patterned with a tapered width such that the V-groove 16 has a depth that varies along a Z-direction Z, wherein the Z-direction is substantially orthogonal to the top surface. The etching is performed such that, when a fiber 50 with a predetermined size 50a is inserted into the V-groove 16, a core 51 of the fiber 50 is centered with respect to the core layer 141, 143, 145 of the waveguide structure 14.

[0050] Fig. 2 shows a flowchart for a method for manufacturing a fiber-waveguide coupler according to another embodiment of the invention.

[0051] The Fig. 2 is based on the embodiment of the previously described with reference to Fig. 1. In the following explanations, only the differences compared to the embodiment described above are highlighted.

[0052] In this embodiment, the intermediate layer 12 has a bondable top surface. A bondable top surface is defined as a surface that facilitates bonding to another semiconductor or dielectric layer. Materials such as SiO2, SiCxNy, SiOxNy, or Al2O3 can be used for the underlying material, in this case the intermediate layer, for a suitable bondable top surface.

[0053] Consequently, the connection M3 of the first oxide layer 13 on the mask layer 11 is made by connecting M3 of the first oxide layer 13 on the bondable top surface 12a of the intermediate layer.

[0054] In addition, as in Fig. 2, in a practical application, the bonding M3 of the first oxide layer 13 is typically carried out by bonding M3a to a second substrate 30, on the front side of which the first oxide layer 13 is applied, and then etching M3b the second substrate 30 from the first oxide layer 13. Such bonding is typically carried out by a wafer-to-wafer bonding process, which is particularly suitable for meeting the high requirements for flatness and warpage, since only high-quality surfaces are affected, ie the bondable top side of the intermediate layer and the top side of the first oxide layer, which is bonded to the bondable top side.

[0055] As will be described in more detail below, in some embodiments such a second substrate 30 has an additional oxide layer 31 applied to a backside of the second substrate 30 (as also shown in Fig. 4a). During the etching step M3b, the second substrate 30 is etched together with the additional oxide layer 31 from the first oxide layer 13 (as also shown in Fig. 4b). Typically, the first oxide layer 13 and the additional oxide layer 31 are applied to the second substrate using the same process, for example, chemical vapor deposition (CVD). Therefore, the additional oxide layer is identical to the first oxide layer in terms of material and thickness.

[0056] Fig. 3a-d show cross sections of a fiber-waveguide coupler according to an embodiment of the invention.

[0057] Fig. 3a to Fig. 3d describe the different steps in the manufacturing process of the fiber-waveguide coupler 1. This manufacturing process is based on the method for manufacturing a fiber-waveguide coupler 1 as described above with reference to Fig. 1 and Fig. 2 was described.

[0058] In Fig. 3a shows a substrate 10 with a first surface 10a on a front side F and a second surface 10b opposite the first surface 10a on a back side B. A mask layer 11 is applied to the substrate 10, wherein the mask layer 11 has an opening marking 11a. An intermediate layer 12 is applied to the mask layer 11, which fills the opening marking 11a. The intermediate layer 12 has a bondable surface 12a, as described in the previous embodiment of the method.

[0059] In Fig. 3b, a first oxide layer 13 is applied to the bondable top side 12a of the intermediate layer 11 by a bonding process. Furthermore, a stress compensation layer 21 is applied to the second surface 10b on the back side B of the substrate 10. This stress compensation layer 21 is designed such that at least one of a material of the stress compensation layer 21 and a thickness of the stress compensation layer 21 is selected such that warping of the substrate 10 is reduced. A suitable material for the stress compensation layer 21 is characterized by high compressive strength, so that it can absorb the stress caused by the bonding process of the oxide layer 13 to the intermediate oxide 11. In particular, the compressive strength of such a material can be higher than that of the material of the substrate. In some embodiments, the material of the stress compensation layer 21 is an oxide, for example, SiO2.In further embodiments, the stress-compensating layer 21 is made of a metal such as tungsten. A metal increases thermal conductivity, which is suitable for cooling the waveguide structure 14, especially when heating elements are used. Tungsten or a tungsten alloy as a material can withstand temperatures of up to 1300°C during manufacturing processes.

[0060] In Fig. 3c, a waveguide structure 14 is applied to the first oxide layer 13. The waveguide structure 14 comprises a core layer 141 and a cladding layer 142. In this particular embodiment, the waveguide structure 14 has an additional cladding layer 149 on the cladding layer 142 to improve optical mode confinement. The core layer 141 has a predetermined width 141a and a predetermined thickness 141b, both of which are adapted to a desired mode field diameter or a desired shape of the optical mode (not shown) that can be coupled into the core layer 141.

[0061] In Fig. 3d, the waveguide structure 14 and the first oxide layer 13 have been locally etched to form an opening 160 above the opening mark 11a. The first surface of the substrate 10 has a V-groove etched into the first surface 10a of the substrate 10 by an anisotropic etching process. The V-groove 16 is thus arranged in the opening 160 of the waveguide structure 14 and the first oxide layer 13.

[0062] The V-groove 16 extends in a Z-direction. The Z-direction is substantially orthogonal to the first surface 10a. The first oxide layer 13 on the front side of the substrate 10 is located above the V-groove 16. The core layer 141 is aligned with the V-groove 16 such that when a fiber 50 with a predetermined size 50a is inserted into the V-groove 16, a core 51 of the fiber 50 is centered with respect to the core layer 141 of the waveguide structure 14, as shown in Fig. 3d. The predetermined size 50a, in this case, is specified as the diameter of the circular-cross-section fiber 50. For example, a standard single-mode fiber such as an SMF-28 (e.g., SMF-28 ULL from Corning) with a core diameter of approximately 8 µm and a cladding diameter of 125 µm, inserted into the V-groove so that its surfaces mechanically support the fiber, can have its core centered with respect to the core layer of the waveguide structure to achieve optimal coupling efficiency between the fiber and the waveguide. The fiber-waveguide coupler 1 is not limited to this specific single-mode fiber type and can support single-mode fibers with a large mode area, fibers with few modes, and even multimode fibers.

[0063] In some embodiments, the aperture mark 11a has a rectangular shape to facilitate etching of the V-groove 16. Furthermore, in some embodiments, the mask layer 11 is patterned with a tapered width so that the V-groove 16 has a depth that varies along the Z-direction. This can be used to adjust the height of the fiber relative to the waveguide core.

[0064] The Fig. 4a-b show cross sections of intermediate products of a fiber-waveguide coupler 1 in the manufacturing method according to another embodiment of the invention.

[0065] The Fig. 4a and Fig. 4b describe various steps of the manufacturing process of the fiber waveguide coupler 1. This manufacturing process is based on the method for manufacturing a fiber waveguide coupler 1 as described above with reference to the Fig. 1 and Fig. 2 was described.

[0066] In Fig. 4a, the intermediate layer 12 has a bondable top side 12a. Bonding M3 of the first oxide layer 13 on the intermediate layer 12 is achieved by bonding M3 of the first oxide layer 13 to the bondable top side 12a of the intermediate layer.

[0067] Furthermore, the bonding step M3 is performed by providing a second substrate 30, on the front side of which the first oxide layer 13 is applied. In this embodiment, the first oxide applied to the second substrate 30 also has a bondable surface 13. In addition, the second substrate 30 has an additional oxide layer 31 applied to the back side of the second substrate 30. Typically, the first oxide layer 13 and the additional oxide layer 31 are applied to the second substrate 30 using the same process, for example, by chemical vapor deposition (CVD). Therefore, the additional oxide layer 31 is identical to the first oxide layer 13 in terms of material and thickness.

[0068] In some embodiments, substrate 10 and second substrate 30 are wafers, typically made of silicon. In some embodiments, these substrates 10, 30 are made of InP. Bonding of the first oxide layer 13 to the intermediate layer 12 is performed by wafer-to-wafer bonding.

[0069] In Fig. 4b, the first oxide layer 13 is bonded to the intermediate layer 11. The second substrate 30 is etched together with the additional oxide layer 31, so that the first oxide layer 13 is exposed as the topmost layer of the layer stack.

[0070] Fig. 5 shows a schematic representation of an intermediate product of a fiber-waveguide coupler 1 in the manufacturing method according to an embodiment of the invention.

[0071] The embodiment of an intermediate product of a fiber-waveguide coupler 1 shown in Fig. 5 is based on the fiber-waveguide couplers 1 and / or manufacturing processes described above. Only specific differences from the previously described intermediate products and fiber-waveguide couplers 1 are described.

[0072] In this embodiment, the waveguide structure 14 consists of a first cladding layer 142 deposited on the first oxide layer 13, a first core layer 141 deposited on the first cladding layer 142, and a second cladding layer 144 deposited on the first core layer 141 and the first cladding layer 142.

[0073] A second core layer 143 is deposited on the second cladding layer 144. The second core layer 143 has a predetermined second width 143a and a second thickness 143b. In this embodiment, the second width 143a is smaller than a first width 141a of the first core layer 141. The second thickness 143a is greater than the first thickness 141b of the first core layer 141.

[0074] Furthermore, a third cladding layer 146 is applied to the second core layer 143 and the second cladding layer 144. Additionally, a third core layer 145 having a predetermined third width 145a and a predetermined third thickness 145b is applied to the third cladding layer 146. The predetermined third width 145a is greater than the predetermined first width 141a, and the predetermined third thickness 145b is less than the predetermined first and second thicknesses 141b, 143b.

[0075] A fourth cladding layer 148 is deposited on the third core layer 145 and the third cladding layer 146. An additional cladding layer 149 is deposited above the fourth cladding layer 148. All predetermined thicknesses 141b, 143b, 145b are smaller than a thickness of one of the cladding layers 142, 144, 146, 148, 149. It is understood that this waveguide structure 14 is an example of a more complex arrangement of the waveguide structure. In further embodiments, the arrangement of the core layers 141, 143, 145 and their predetermined widths 141a, 143a, 145a and predetermined thicknesses 141b, 143b, 145b are designed for a specific application, for example, for optical mode converters.

[0076] Furthermore, a back cladding structure 22 comprising similar first, second, third, fourth, and additional back cladding layers 222, 224, 246, 248, 249 is deposited on the stress compensation layer 21. A material and thickness of the back cladding structure 22 and the cladding layers 142, 144, 146, 148, and 149 of the waveguide structure 14 are substantially the same.

[0077] The rear cladding structure 22 thus comprises a rear first cladding layer 222 applied to the stress compensation layer 21, a rear second cladding layer 224 applied to the rear first cladding layer 222, a rear third cladding layer 226 applied to the rear second cladding layer 224, a rear fourth cladding layer 228 applied to the rear third cladding layer 226 and a rear additional cladding layer 229 applied to the rear fourth cladding layer 228. The material and the thickness of the rear first, second, third, fourth and additional cladding layers 222, 224, 246, 248, 2 and an additional back cladding layer 229 applied to the rear fourth cladding layer 228.A material and a thickness of the respective backside first, second, third, fourth, and additional cladding layers 222, 224, 226, 228, 229 and the respective first, second, third, fourth, and additional cladding layers 142, 144, 146, 148, 149 are substantially the same. In further embodiments, the number of cladding layers and core layers of the frontside waveguide structure 14 and the backside cladding structure 22 may vary depending on the application requirements.

[0078] Fig. 6 shows a schematic representation of an intermediate product of a fiber-waveguide coupler 1 in the manufacturing method according to another embodiment of the invention.

[0079] The embodiment of an intermediate product of a fiber-waveguide coupler 1, which is shown in Fig. 6 is based on the fiber-waveguide couplers 1 and / or manufacturing processes described above. Only specific differences from the previously described intermediate products and fiber-waveguide couplers 1 are described.

[0080] In the manufacturing step shown, a third substrate 40 is provided, which has a second oxide layer 17 on a front side of the third substrate 40. The second oxide layer 17 has a bondable top surface 17a, which is brought together with the top surface 149a of the additional cladding layer 149. The top surface 149a is also a bondable surface 149a, which promotes bonding. In this embodiment, the third substrate 40 also comprises, on an opposite side, an additional oxide layer 41, which is identical in thickness and material to the second oxide layer 41. The second oxide layer 17 is then bonded to the waveguide structure 14 by a wafer-to-wafer bonding step, which provides high-quality surfaces.

[0081] The substrate 40 is then etched together with the additional oxide layer 41 to form the second oxide layer 17 on top of the Fig. 6 to expose the layer stack shown.

[0082] Fig. 7a-c show a schematic representation of fiber-waveguide couplers 1 according to another embodiment of the invention.

[0083] Fig. 7a to Fig. 7c describe various steps in the manufacturing process of the fiber-waveguide coupler 1. This manufacturing process is based on the method for manufacturing a fiber-waveguide coupler 1 as described above with reference to Fig. 1 and Fig. 2, and is at least connected to the fiber-waveguide couplers 1 or the respective intermediate products in the Fig. 3 to 6 compatible.

[0084] In Fig. 7a, the waveguide structure 14 comprises a first core layer 141 comprising two first cores 141c, 141d deposited on the first cladding layer 142. A first width 141a and a first thickness 141b of the two first cores 141c, 141d are substantially identical. A second core layer 145 comprising only a single core 145 is deposited on the second cladding layer 144, which is deposited on the first cladding layer 142 and the first cores 141c, 141d. A second width 143a of the second core layer 143 is smaller than the first width 141a, and a second thickness 143b of the second core layer 143 is greater than the first thickness 141b.A third core layer 145 with a large third width 145a and a small thickness 145b compared to the first and second core layers 141, 143 is deposited on a fourth cladding layer 148, which is deposited on a third cladding layer 146, which is deposited on the second core layer 145 and the second cladding layer 144. An additional cladding layer 149 is deposited above the third core layer 145 and the fourth cladding layer 148. The core layers are designed to support only a limited number of optical modes, typically a single mode, and to determine the mode range or mode field diameter of the mode. For SiN as the core material, typical values ​​for the width are approximately 500 nm to several µm and the thickness approximately 200 nm to 350 nm.

[0085] In this embodiment, a second oxide layer 17 is also applied to the waveguide structure 14. Furthermore, a second stress-compensation layer 23 is applied to the rear cladding structure 22. In some embodiments, the second stress-compensation layer 23 is also applied by bonding. In this example, the second stress-compensation layer 23 is also designed such that, due to its material and thickness, it compensates for the stresses caused by the third oxide layer, thus reducing warpage of the substrate 10. For this purpose, the second stress-compensation layer 23 can also comprise a material that has high compressive strength and high thermal conductivity in order to be able to dissipate heat.Furthermore, in some embodiments, the material of the second stress-compensating layer 23 is configured to withstand temperatures of approximately 1300°C, which occur during the fabrication of a photonic integrated circuit including the fiber-waveguide coupler 1 of this embodiment. In some embodiments, a metal is selected as the material of the second stress-compensating layer. In some of these embodiments, tungsten was selected as the material of the second stress-compensating layer.

[0086] In Fig. 7b, a V-groove 16 was etched through the opening mark 11a into the first surface of the substrate 10. The position of the V-groove 16 is aligned with the position of the second core layer 143 in the cross-section orthogonal to the first surface 10a of the substrate, as shown in Fig. 7b. The position of the core layer 143 is thus aligned with the opening marking 11a of the mask layer 11. The second core layer 143 is thus located directly above the opening marking 11a of the mask layer 11a.

[0087] In Fig. Figure 7c shows a fiber inserted into the V-groove 16. A core 51 of the fiber 50 is centered with respect to the second core 143 in this embodiment. Thus, an optical mode coupled to the second core layer 143 propagates in the second core layer 143 by experiencing optical coupling between the core 51 of the fiber 50 and the core layer 143 of the photonic integrated circuit. The mode range can be influenced by the first and third core layers 141, 145, particularly if dopants are applied to one of these layers 141, 145.

[0088] In further embodiments, the waveguide structure 14 has a different number of core layers 141, 143, 145 with multiple cores and cladding layers 142, 144, 146, 148, 149.

[0089] The V-groove 16 is etched such that when a fiber 50 of a predetermined size is inserted into the V-groove 16, a core 51 of the fiber 50 is centered with respect to the core layer 143 of the waveguide structure 14. In some embodiments, a standard single-mode fiber, such as an SMF-28 (e.g., SMF-28 ULL from Corning) with a core diameter of approximately 8 µm and a cladding diameter of 125 µm, is inserted into the V-groove 16 such that its surfaces support the fiber 50 and have its core 51 centered with respect to the second core layer 143 of the waveguide structure 14 to achieve optimal coupling efficiency between the fiber and the optical fiber. Of course, the fiber-waveguide coupler 1 is not limited to this specific single-mode fiber type and can also support single-mode fibers with a large mode area, few-mode fibers, and even multimode fibers.

[0090] Fig. 8 shows a flowchart for a method for manufacturing a fiber-waveguide coupler 1 according to another embodiment of the invention.

[0091] This further process serves to balance a crystalline <100> -substrate, in which the <100> The crystal plane is not exactly parallel to the surface of the substrate, which is typically a wafer. Therefore, the substrate includes a pair of alignment marks and a reference V-groove to determine the crystal axis of the substrate. As described in German patent DE102023136803B3, filed on December 28, 2023, these allow the determination of the crystal planes so that the core layer can be aligned with the final V-groove etched into the substrate.

[0092] Unless otherwise stated, the properties and functions of the respective layers and other elements are the same as in the previously described method and the fiber-waveguide couplers 1.

[0093] In this method, first, in step M21, a pair of alignment marks 101 are etched into a first surface 10a on a front side F of a substrate 10. In step M22, a mask layer 11 having at least one opening mark 11a is deposited at a predetermined distance from the pair of alignment marks. In step M23, a reference V-groove 102 is etched into the substrate 10 through the opening mark 11a by anisotropic etching. The mask layer 11 is etched in step M24 to expose the first surface of the substrate 10. In step M25, a first oxide layer 13 is deposited on the first surface of the substrate 10. In step M26, a back-side first oxide layer 21' is deposited on a second surface 10b on a back side B of the substrate 10. The back side B of the substrate 10 faces the front side F of the substrate 10.A material and a thickness 141b of the backside first oxide layer 21' and the first oxide layer 13 are substantially the same.

[0094] Furthermore, in step M27, a waveguide structure 14 comprising at least one cladding layer 142, 144, 146, 148, 149 and one core layer 141, 143, 145 is applied to the first oxide layer 13. In step M28, a back cladding structure 22 is applied to the back oxide layer. The thickness of the back cladding structure 22 and the waveguide structure 14 are substantially the same.

[0095] The waveguide structure 14 and the first oxide layer 13 are locally etched in step M29 to form an opening 160 above the opening mark 11a. In step M30, a second V-groove is etched by anisotropic etching at a predetermined distance from the reference V-groove, so that when a fiber 50 with a predetermined size 50a is inserted into the V-groove 16, a core 51 of the fiber 50 is centered with respect to the core layer 141, 143, 145 of the waveguide structure.

[0096] The method further comprises the two optional steps, ie a step of bonding a second oxide layer 17 to the waveguide structure 14 in step M31 and a step of bonding a second stress compensation layer 23 to the back cladding structure 22 in step M32.

[0097] Fig. 9a-e show a schematic representation of a fiber-waveguide coupler 1 according to another embodiment of the invention.

[0098] Fig. 9a to Fig. 9c describe various steps in the manufacturing process of the fiber waveguide coupler 1. This manufacturing process is based on the further method for manufacturing a fiber waveguide coupler 1 as described above with reference to Fig. 8 was described.

[0099] In Fig. Figure 9a shows a substrate 10 having a first surface 10a and a second surface 10b opposite the first surface 10a. The first surface 10a of the substrate 10 includes a pair of alignment marks 101 that are parallel to each other.

[0100] In Fig. 9b, the first surface 10a of the substrate further includes a reference V-groove 102 spaced a predetermined distance from the alignment marks 101. A mask layer 11 having at least one aperture mark 11a is deposited at a predetermined distance from the pair of alignment marks 101. A reference V-groove 102 is etched into the first surface 10a of the substrate 10 through this aperture mark 11a.

[0101] In Fig. 9c, the mask layer is etched so that only the substrate 10 with the first surface 10a with the alignment marks 101 and the reference V-groove 102 from the previous step of Fig. 9b remains. A first oxide layer 13 is deposited on the first surface 10a of the substrate 10, and a backside first oxide layer 21' is deposited on the backside B of the substrate 10. The first oxide layer 13 and the backside first oxide layer 21' are typically formed in the same process step, resulting in identical materials and identical thicknesses.

[0102] In Fig. 9d, a waveguide structure 14 is applied to the first oxide layer 13, wherein the waveguide structure 14 comprises three core layers 141, 143, 145 and five cladding layers 142, 144, 146, 148, 149. In this embodiment, too, the core layers 141, 143, 145 have predetermined widths 141a, 143a, 145a and predetermined thicknesses 141b, 143b, 145b (not shown in this figure, similar use as in Fig. 5 and Fig. 7a), which are designed for the special application of a photonic integrated circuit formed in the waveguide structure.

[0103] Additionally, a back cladding structure 22 is applied to the back first oxide layer 21'. The back cladding structure 22 comprises substantially identical cladding layers 222, 224, 226, 228, 229 as the respective cladding layers 142, 144, 146, 148, 149 of the waveguide structure on the front side of the substrate 10, as previously described. Therefore, the thickness of the back cladding structure 22 and the waveguide structure 14 are substantially the same.

[0104] In Fig.9e, the waveguide structure 14 and the first oxide layer 13 have been locally etched to form an opening 160 above the opening mark 11a. A V-groove 16, spaced a predetermined distance from the reference groove 102 and extending in a Z direction (not shown here), is etched into the first surface 10a of the substrate 10 by an anisotropic etching process. In this etching process, all layers above the substrate 10, i.e., the first oxide layer 13, the waveguide structure 14, and the second oxide layer 17, are also etched, creating an opening 160.

[0105] The Z-direction is essentially orthogonal to the first surface. However, in this scenario, if the <100> -crystal plane is inclined with respect to the first surface 10 of the substrate, the extension of the V-groove is typically not exactly parallel to the first surface 10a. In fact, a valley or trench 16a follows the V-groove of the <110> -Crystal plane.

[0106] In this embodiment, a second oxide layer 17 is also applied to the waveguide structure 14 by bonding. For this purpose, both the upper additional cladding layer 149 and the second oxide layer 17 have bondable surfaces 149a, 17a, which are brought together during the bonding process. To reduce substrate warpage, a second stress-compensation layer 23 is applied to the rear cladding structure 22.

[0107] In the above detailed description, various features have been combined in one or more examples for the purpose of clarity. However, it should be understood that the above description is merely illustrative and not restrictive in any way. It is intended to cover all alternatives, modifications, and equivalents of the various features and embodiments.

[0108] The embodiments were chosen and described in order to best illustrate the principles underlying the invention and their possible practical applications. This will enable one skilled in the art to optimally modify and utilize the invention and its various embodiments with regard to the intended use. In the claims and the description, the terms "including" and "having" are used as neutral language terms for the corresponding term "comprising." Furthermore, the use of the terms "a," "an," and "another" does not generally exclude the majority of the features and components so described.

[0109] Although at least one embodiment of the present inventions is disclosed herein, it should be understood that modifications, substitutions, and alternatives will be obvious to one skilled in the art and may be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiments. Furthermore, throughout this disclosure, the terms "comprise" or "comprising" do not exclude other elements or steps, the terms "a" or "an" do not exclude a plurality, and the term "or" means either or both. Furthermore, described features or steps may also be used in combination with other features or steps and in any order, unless the disclosure or context dictates otherwise.This disclosure hereby incorporates by reference the complete disclosure of all patents or applications to which it claims claim or priority. LIST OF REFERENCE NUMERALS USED 1 fiber-waveguide coupler 10 Substrat 10a first surface 10b second surface 11 mask layer 11a Opening marking 12 Intermediate layer 12a, 17a, 142a bondable top 13 first oxide layer 14 Waveguide structure 16 V-groove 17 second oxide layer 21, 21' stress compensation layer 22 rear shell structure 23 second stress compensation layer 30 second substrate 31 additional oxide layer 40 third substrate 50 fiber 50a predetermined size of the fiber 51 core 101 alignment marks 102 Reference V-groove 141 (first) core layer 141a predetermined (first) width of the (first) core layer 141b predetermined (first) thickness of the (first) core layers 142 first mantle layer 143 second core layer 143a predetermined second width of the second core layer 143b predetermined second thickness of the second core layer 144 second cladding layer 145 third core layer 145a predetermined third width of the third core layer 145b predetermined third thickness of the third core layer 146 third top layer 148 fourth top layer 149 additional top layer 160 opening 222 back first cladding layer 224 rear second cladding layer 226 rear third cladding layer 228 back fourth cladding layer 229 additional back sheath layer B Back of the substrate F Front of the substrate M1-M31 process steps

Claims

[1] A method for producing a fiber-waveguide coupler (1), comprising: Applying (M1) a mask layer (11) on a first surface (10a) on a front side (F) of a substrate (10), wherein the mask layer (11) has at least one opening marking (11a); Applying (M2) an intermediate layer (12) to the mask layer (11); Bonding (M3) a first oxide layer (13) to the intermediate layer (12); Applying (M4) a stress compensation layer (21) on a second surface (10b) on a back side (B) of the substrate (10), wherein at least one of a material and a thickness (141b) of the stress compensation layer (21) is selected to reduce warpage of the substrate (10); Applying (M5) a waveguide structure (14) on the first oxide layer (13), wherein the waveguide structure (14) has a core layer (141, 143, 145) and a cladding layer (142, 144, 146, 148, 149), wherein the core layer (141, 143, 145) has a predetermined width (141a, 143a, 145a) and a predetermined thickness (141b, 143b, 145b) and is aligned with the opening mark (11a); locally etching (M6) the waveguide structure (14) and the first oxide layer (13) to form an opening (160) above the opening mark (11a); and anisotropic etching (M7) of a V-groove (16) into the substrate (10) at the opening mark (11a) of the mask layer (11) such that when a fiber (50) having a predetermined size (50a) is inserted into the V-groove (16), a core (51) of the fiber (50) is centered with respect to the core layer (141, 143, 145) of the waveguide structure (14). [2] The method of claim 1, wherein the intermediate layer (12) has a bondable top surface, wherein bonding (M3) the first oxide layer (13) on the mask layer (11) comprises bonding (M3) the first oxide layer (13) on the bondable top surface (12a) of the intermediate layer (12). [3] The method of claim 2, wherein bonding (M3) of the first oxide layer (13) comprises: Bonding (M3a) a second substrate (30) with the first oxide layer (13) applied on a front side, preferably with an additional oxide layer (31) applied on a back side of the second substrate (30), to the bondable top side of the intermediate layer (12), and Etching (M3b) the second substrate (30) and preferably etching the additional oxide layer (31) from the first oxide layer (13). [4] Method according to one of the preceding claims, wherein the waveguide structure (14) comprises: the core layer (141) on the first oxide layer (13); and a first cladding layer (142) on the core layer (141) and the first oxide layer (13), the first cladding layer (142) having a connectable top surface (142a); and a second cladding layer (144) on the connectable top surface (142a) of the first cladding layer (142). [5] Method according to one of the preceding claims, wherein the application (M5) of the waveguide structure (14) comprises the application (M5) of a back cladding structure (22) comprising a back cladding layer (222, 224, 226, 228, 229) on the stress compensation layer (21), wherein a thickness (141b) of the back cladding structure (22) is substantially equal to a thickness (141b) of the waveguide structure (14). [6] A method according to any one of the preceding claims, further comprising: Providing a third substrate (40) comprising a second oxide layer (17) on a front side of the third substrate (40), wherein the second oxide layer (17) has a bondable top side (17a), Bonding the second oxide layer (17) on the waveguide structure (14) and Etching the substrate (40) to expose the second oxide layer (17). [7] Method according to claims 5 and 6, further comprising: applying a second stress compensation layer (23) to the rear shell structure (22). [8] Method according to one of the preceding claims, wherein the opening mark (11a) has a rectangular shape and / or wherein the mask layer (11) is structured with a tapered width such that the V-groove (16) has a depth that changes along a Z-direction (Z), the Z-direction being substantially orthogonal to the top side. [9] Fiber-waveguide coupler (1), comprising: a substrate (10) having a first surface (10a) on a front side (F) and a second surface (10b) opposite the first surface (10a) on a back side (B), wherein the first surface of the substrate (10) has a V-groove extending in a Z-direction, wherein the Z-direction is substantially orthogonal to the first surface, a first oxide layer (13) on the front side (F) of the substrate (10), a waveguide structure (14) applied to the first oxide layer (13), wherein the waveguide structure (14) has a core layer (141, 143, 145) and a cladding layer (142, 144, 146, 148, 149), wherein the core layer (141, 143, 145) has a predetermined width (141a, 143a, 145a) and a predetermined thickness (141b, 143b, 145b), wherein the V-groove (16) is arranged in an opening (160) of the waveguide structure (14) and the first oxide layer (13), wherein the core layer (141, 143, 145) is aligned with the V-groove (16) such that when a fiber (50) with a predetermined size (50a) is inserted into the V-groove (16) is used, a core (51) of the fiber (50) is centered with respect to the core layer (141, 143, 145) of the waveguide structure (14), a stress compensation layer (21) is applied to the second surface of the substrate (10), wherein at least one of a material of the stress compensation layer (21) and a thickness of the stress compensation layer (21) is selected to reduce warpage of the substrate (10), a mask layer (11) applied to the substrate (10), the mask layer (11) having an opening mark (11a) arranged above the V-groove (16), and an intermediate layer (12) which is applied to the mask layer (11), wherein the intermediate layer (12) is arranged between the mask layer (11) and the first oxide layer (13), wherein the opening (160) is arranged above the opening mark (11a). [10] Fiber-waveguide coupler (1) according to claim 9, wherein a back cladding structure (22) having a cladding layer (222, 224, 246, 248, 249) is applied to the stress compensation layer (21), wherein a material and a thickness of the back cladding structure (22) and the waveguide structure (14) are substantially the same. [11] Fiber-waveguide coupler (1) according to one of claims 9 or 10, wherein the waveguide structure (14) comprises: a first cladding layer (142) on the first oxide layer (13), a first core layer (141) on the first cladding layer (142), a second cladding layer (144) on the first core layer (141) and the first cladding layer (142), a second core layer (143) on the second cladding layer (144), the second core layer (143) having a predetermined second width (143a) and a second thickness (143b); a third cladding layer (146) on the second core layer (143) and the second cladding layer (144), wherein a core (51) of a fiber (50) having a predetermined size (50a) disposed within the V-groove (16) is centered with respect to one of the first core layer (141) and the second core layer (143) of the waveguide structure (14). [12] Fiber waveguide coupler (1) according to claim 10 and 11, wherein the rear cladding structure (22) comprises: a rear first cladding layer (222) applied to the stress compensation layer (21), a rear second cladding layer (224) applied to the rear first cladding layer (222) and a rear third cladding layer (226) deposited on the rear second cladding layer (224), wherein a material and a thickness of the respective rear first, second and third cladding layers (222, 224, 226) and the respective first, second and third cladding layers (142, 144, 146) are substantially the same. [13] Fiber waveguide coupler (1) according to one of claims 9 to 12, further comprising a second oxide layer (17) applied to the waveguide structure (14). [14] Fiber waveguide coupler (1) according to one of claims 10 and 13, further comprising a second stress compensation layer (23) applied to the rear cladding structure (22). [15] Fiber waveguide coupler (1) according to one of claims 9 to 14, wherein the stress compensation layer (21) comprises a metal, in particular tungsten.

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