Method for producing a waveguide structure, and waveguide structure

Abstract

The invention relates to a method for producing a waveguide structure (1), comprising the following steps: a) providing a waveguide layer (2) which comprises lithium niobate or lithium tantalate; b) applying a hard mask layer (3) and a mask layer (4) to the waveguide layer (2); c) lithographically structuring the mask layer (4); d) performing dry-chemical etching in order to structure the hard mask layer (3) by means of the mask layer (4); e) performing dry-chemical etching in order to structure the waveguide layer (2) into a waveguide (20) by means of the hard mask layer (3); and f) performing a post-treatment on the waveguide layer (2), with the hard mask layer (3) being removed in a first etching step and redeposits on the side flanks being removed in a second etching step (4). The invention furthermore relates to a waveguide structure (1).

Description

[0001] Description

[0002] METHOD FOR PRODUCING A WAVEGUARD STRUCTURE AND WAVEGUARD STRUCTURE

[0003] The present application relates to a method for manufacturing a waveguide structure and a waveguide structure.

[0004] For the production of photonic integrated circuits (PICs), low-loss waveguide transmission is of great importance.

[0005] For example, high optical losses limit the efficiency of feasible assemblies such as modulators, detectors, or non-linear optical elements.

[0006] One task is to specify a method for efficiently and reliably fabricating a waveguide structure with low optical losses. Furthermore, a waveguide structure characterized by low optical losses should be specified.

[0007] These tasks are solved, among other things, by a method and a waveguide structure according to the independent patent claims. Further embodiments and advantages are the subject of the dependent patent claims.

[0008] A method for fabricating a waveguide structure is described. The waveguide structure is, for example, part of a photonic integrated circuit or intended for use in a photonic integrated circuit. According to at least one embodiment of the method, the method comprises a step of providing a waveguide layer, wherein the waveguide layer comprises lithium niobate (LiNbOg) or lithium tantalate (LiTaOg).

[0009] These materials have a comparatively high refractive index, making it easy to achieve a relatively large refractive index difference with an adjacent cladding or protective layer, for example, based on silicon oxide. This simplifies the fabrication of low-loss waveguides with small radii of curvature, for example, with a radius of curvature of at most 1 mm, 100 pm, or 10 pm.

[0010] This allows photonic components with high density to be integrated on a single chip, as it enables more compact and complex circuit designs.

[0011] In particular, the integrated optical elements of the photonic integrated circuit can also be based on lithium niobate or lithium tantalate. For example, the integrated optical elements and the waveguide structure are fabricated on a common wafer.

[0012] According to at least one embodiment of the method, the process includes a step in which a mask layer is applied to the waveguide layer. For example, the mask layer may contain a photoresist. The term photoresist generally refers to an exposed, photosensitive material. The terms "photoresist," "photosensitive," and "exposed" do not imply a restriction to exposure by electromagnetic radiation such as visible light. Rather, exposure can also be achieved, for example, by a particle beam, such as an electron beam.

[0013] According to at least one embodiment of the method, the method comprises a step in which a hard mask layer is applied to the waveguide layer. In particular, the hard mask layer is applied in front of the mask layer, so that the hard mask layer is arranged between the waveguide layer and the mask layer.

[0014] According to at least one embodiment of the process, the method comprises a step in which the mask layer is lithographically structured. After lithographic structuring, the mask layer only partially covers the hard mask layer. Exposure is achieved, for example, via electromagnetic radiation, such as radiation in the visible spectral range, the ultraviolet spectral range, the extreme ultraviolet spectral range, or by X-rays, or via a particle beam, such as an electron beam.

[0015] According to at least one embodiment of the process, the method comprises a step in which the hard mask layer is dry-chemically etched to create the structure. This allows the structure of the mask layer to be transferred into the hard mask layer. For example, the dry-chemical etching is carried out with the addition of Cl₂ and O₂.

[0016] According to at least one embodiment of the process, the method comprises a dry chemical etching step to structure the waveguide layer into a waveguide using the hard mask layer. In this step, the structure of the hard mask layer is transferred to the waveguide layer. In particular, the dry chemical etching can be performed using an inductively coupled plasma (ICP). With an ICP process, the energy of the ions and the density of the plasma can be largely controlled independently of each other. This allows the etching process to be adjusted so that comparatively little damage occurs to the waveguide layer being structured. For example, a radio frequency (RF) power for adjusting the ion energy is between 300 W and 500 W inclusive. An ICP power is, for example, between 600 W and 1200 W inclusive.The higher the ICP power, the higher the plasma density can be.

[0017] At the time of structuring the waveguide layer, the mask layer may already have been removed, so that the waveguide layer is only partially covered by the hard mask layer during dry chemical etching.

[0018] For example, dry chemical etching of the waveguide layer is carried out using a fluoride-containing gas, for example fluoroform (CHFg).

[0019] According to at least one embodiment of the process, the method includes a post-treatment step of the waveguide layer. This post-treatment can be divided into several sub-steps. In particular, the hard mask layer can be removed in a first etching step. For example, the first etching step is a wet chemical etching step. Furthermore, in a second etching step, redeposits on the sidewalls can be removed. For example, these redeposits contain lithium. In particular, when using a fluoride-containing gas for dry chemical etching, crystalline lithium fluoride (LiF) can be formed on the sidewalls of the waveguide layer during the dry chemical etching process.This post-treatment step also makes it possible to achieve smooth surfaces of the waveguide to be produced, even when redeposits are formed on the side flanks, for example when using a fluoride-containing gas for the dry chemical etching of the waveguide layer.

[0020] The terms first etching step and second etching step are used for simplified reference only and do not imply any restriction regarding the order of the etching steps or the total number of etching steps performed for post-treatment.

[0021] In at least one embodiment of the process, a waveguide layer containing lithium niobate or lithium tantalate is provided. A hard mask layer and a mask layer are applied to the waveguide layer. The mask layer is lithographically structured. Dry chemical etching is performed using the mask layer to structure the hard mask layer. Dry chemical etching using the hard mask layer is also performed to structure the waveguide layer into a waveguide. The waveguide layer is then post-treated, with the hard mask layer being removed in a first etching step and redeposits being removed from the side faces in a second etching step. The described process allows the fabrication of a waveguide with a comparatively high refractive index and high-quality side faces.

[0022] In particular, the side faces can be manufactured with exceptionally low roughness and large flank angles. This minimizes stray losses within the waveguide and improves its performance. For a straight waveguide based on lithium niobate, extremely low transmission losses of only 2 dB / m have been experimentally demonstrated. Such a waveguide is therefore particularly suitable for achieving photonic integrated circuits with high efficiency and performance.

[0023] Furthermore, the described method can achieve a high refractive index contrast between the waveguide and the adjacent material, for example silicon oxide, so that efficient waveguiding can be achieved even for small bending radii of the waveguide.

[0024] In contrast, di f fusion-based waveguides, which are produced in lithium niobate bulk material by di f fusion or proton exchange, can only achieve comparatively low refractive index contrasts, which means that the radius of curvature of the waveguide is in the millimeter range and cannot be easily reduced.

[0025] In comparison to mechanical methods for manufacturing waveguides, for example by using a precision saw that removes material through depth-limited cuts offset by the width of the waveguide, there is greater freedom in the shape of the waveguide structures to be manufactured, since curved shapes are also possible.

[0026] While curved waveguides can in principle be produced using a purely mechanical method with an ultra-precise diamond scraping tool, the minimum distance between two waveguides is limited by the width of the diamond tool, making small distances between waveguides, such as those used for waveguide directional couplers in rings, impossible.

[0027] Furthermore, the described method is characterized by improved scalability compared to mechanical methods and is therefore better suited for mass production.

[0028] Furthermore, dry chemical structuring of the waveguide layer, compared to methods using wet chemical etching, allows for structuring independent of the crystal orientation of the waveguide layer. In contrast, wet chemical manufacturing processes are limited due to the crystallographic dependence of the etching rates.

[0029] This not only reduces the number of crystal cuts that can be used with this fabrication method, but can also lead to undesirable preferential etching along crystal faces. This can result in abrupt changes in sidewall orientation along waveguide bends and cause increased scattering losses.

[0030] According to at least one embodiment of the process, the second etching step during the post-treatment of the waveguide layer is performed with megasound assistance. For example, the megasound frequency is between 0.1 MHz and 10 MHz, inclusive, for example, 1 MHz.

[0031] According to at least one embodiment of the process, an etching mixture for the second etching step for post-treatment of the waveguide layer contains or consists of the following liquids: nitric acid

[0032] (HNOg ) , in particular with a proportion between 1% and 5% inclusive , phosphoric acid (H3PO4 ) , in particular with a proportion between 65% and 75% inclusive , acetic acid (CH3COOH) , in particular with a proportion between 5% and 10% inclusive , and de-ionized water, in particular with a proportion between 10% and 30% inclusive .

[0033] It has been shown that this etching mixture, especially in combination with megasound assistance, can remove redeposits on the waveguide's flanks particularly efficiently, thus creating smooth waveguide flanks. In this way, low transmission losses in the waveguide can be reliably achieved.

[0034] According to at least one embodiment of the process, the hard mask layer contains chromium. Specifically, the chromium is deposited by adding nitrogen such that the average grain size of the hard mask layer is at most 50 nm. It has been found that the grain size can be significantly reduced by adding nitrogen. For example, the nitrogen addition ranges from 1 sccm (standard cm⁻¹) to 10 sccm. It has been shown that the small grain size of the structured hard mask layer simplifies the production of smooth side faces of the waveguide to be manufactured.

[0035] According to at least one embodiment of the process, the hard mask layer has a thickness between 100 nm and 150 nm. Below a layer thickness of 100 nm, there is a risk that the hard mask will not adequately protect the waveguide being manufactured during the dry chemical structuring of the waveguide layer. Layer thicknesses above 150 nm can lead to a reduction in resolution during the structuring of the waveguide layer.

[0036] According to at least one embodiment of the process, dry chemical etching for structuring the waveguide layer is carried out at a temperature between -40°C and -80°C. Such low temperatures can be achieved by using special cryogenic equipment. It has been shown that the formation of redeposits on the side surfaces is significantly reduced by etching in this temperature range.

[0037] According to at least one embodiment of the process, the post-treatment of the waveguide layer includes a step in which the waveguide layer is heated to a temperature between 450 °C and 700 °C, particularly under a controlled gas atmosphere. This can induce thermal annealing of the waveguide layer. In particular, the crystalline properties of the waveguide layer near its surfaces can be restored after dry chemical structuring. The optical properties of the waveguide can thus be improved. The heating is carried out, for example, with a temperature ramp of at most 5 K / min.

[0038] According to at least one embodiment of the process, the mask layer has a chemically enhanced photoresist (CAR).

[0039] Chemically enhanced photoresists utilize acid catalysis, which is released during exposure. This acid catalyzes chemical reactions that modify the photoresist, enabling higher sensitivity and resolution. This allows for shorter exposure times and more precise and controlled photoresist development.

[0040] According to at least one implementation of the process, the mask layer is exposed to several scripts using an electron beam. For example, exposure is carried out using a VSB EBL (variable shaped beam electron beam lithography) process, which utilizes a shaped electron beam that produces rectangular or polygonal beam shapes.

[0041] This technique allows for the rapid and efficient creation of complex, high-resolution patterns, as the electron beam can be precisely controlled and focused onto the photoresist. Exposure in multiple pass passes, also known as the multi-pass writing strategy, exposes the same pattern several times. This results in more uniform and precise structures. In particular, this method minimizes thermal distortion and improves edge sharpness by compensating for beam current fluctuations.

[0042] Furthermore, this exposure method can be carried out relatively quickly, so that the process can also be carried out efficiently on large wafers.

[0043] According to at least one embodiment of the process, a protective layer is applied to the waveguide after post-treatment. This protective layer has a lower refractive index than the waveguide layer and can therefore also function as a cladding layer for the waveguide. Furthermore, the protective layer simplifies the achievement of a symmetrical mode field distribution within the waveguide.

[0044] According to at least one embodiment of the method, the waveguide layer is provided on a support substrate. In particular, the waveguide layer borders on a side facing the support substrate a material that has a lower refractive index than the waveguide layer. This allows wave guidance by total internal reflection to be achieved even on the side of the cladding layer facing the support substrate.

[0045] According to at least one embodiment of the method, an oxide layer is arranged as a cladding layer between the support substrate and the waveguide layer. In particular, the waveguide layer, the oxide layer, and the support layer can together form a substrate, for the fabrication of which the oxide layer acts as an interface between the waveguide layer and the support substrate. In this case, the support substrate can be made of the same material as the waveguide layer or of a different material. For example, a silicon wafer is suitable for the support substrate.

[0046] Furthermore, a waveguide structure is specified. The method described above is particularly suitable for manufacturing the waveguide structure. Therefore, features mentioned in connection with the method can also be applied to the waveguide structure, and vice versa.

[0047] According to at least one embodiment of the waveguide structure, the waveguide exhibits a transmission loss of at most 10 dB / m, 5 dB / m, or 2 dB / m, at least in certain areas. The transmission loss refers particularly to straight sections of the waveguide. These low transmission losses can be achieved, in particular, through exceptionally smooth and relatively steep flanks of the waveguide.

[0048] According to at least one embodiment of the waveguide structure, a side flank of the waveguide has a flank angle of at least 60°, for example a flank angle between 65° inclusive and 75° inclusive.

[0049] The described method and the waveguide produced in this way can be used in various areas of photonics and optics, especially for integrated photonic circuits, for example for telecommunications, data transmission or quantum communication or quantum processing, as well as in areas of nonlinear integrated optics.

[0050] For example, due to lower optical losses, more optical power can be available for interactions within the components of a photonic integrated circuit. This is particularly important for optically nonlinear processes, whose efficiency depends strongly on the optical intensities.

[0051] For applications of integrated quantum optics, waveguides can be used to achieve signal transmission at the single-photon level due to their low losses.

[0052] Furthermore, reduced optical losses result in less heat generation within the waveguide material. This can increase the stability and reliability of photonic integrated circuits.

[0053] Furthermore, lower optical losses can lead to improved optical signal quality with reduced noise and / or signal distortion. This enables applications where high signal fidelity is of particular importance, such as telecommunications or quantum photonics.

[0054] Features mentioned above in connection with at least one execution form of fenbart can also be combined with other features in connection with other execution forms of fenbart, as long as these are not mutually exclusive.

[0055] Further designs and advantages will become apparent from the following description of the exemplary designs in conjunction with the figures.

[0056] They show:

[0057] Figures 1A to 1H show an exemplary embodiment of a method for manufacturing a waveguide structure by means of intermediate steps shown in schematic sectional view, with figure 1H showing an exemplary embodiment of a completed waveguide structure.

[0058] The figures are schematic representations and therefore not to scale. Individual elements, and especially layer thicknesses, may be exaggerated for clarity.

[0059] Identical, similar, or similarly effective elements are provided with the same reference symbols in the figures.

[0060] Figures 1A to 1H show a simplified representation of a waveguide structure with a waveguide to be fabricated. However, the method can be used to fabricate a large number of such waveguides simultaneously on a single wafer. Furthermore, active optical elements for a photonic integrated circuit can be fabricated on the same wafer or combined with it. To fabricate the waveguide structure, a waveguide layer 2, comprising lithium niobate or lithium tantalate, is provided. For example, the waveguide layer 2 has a thickness between 200 nm and 2 pm.

[0061] Figure 1A shows a stage of the process in which a hard mask layer 3 and a mask layer 4 have already been applied to the waveguide layer 2.

[0062] The waveguide layer 2 is part of a substrate, in which the waveguide layer is connected to a support substrate 6 via a cladding layer 7. For example, the cladding layer 7 is an oxide layer, such as a silicon oxide layer, through which the waveguide layer 2 is connected to the support substrate 6 during the fabrication of the substrate.

[0063] For example, the thickness of the mantle layer 7 is between 1 pm and 10 pm.

[0064] Silicon, lithium niobate or lithium tantalate are suitable as carrier substrate 6, for example.

[0065] A large refractive index step can be achieved between the waveguide layer 2 and the cladding layer 7, so that radiation propagating in the waveguide 2 towards the substrate 6 can be effectively deflected at the cladding layer 7 by total internal reflection. For example, the refractive index step between the waveguide layer 2 and the cladding layer 7 is at least 0.5. However, the method is also suitable for a bulk substrate where the cladding layer 7 is formed by a portion of the bulk substrate. In this case, the refractive index in the region of the cladding layer 7 can be reduced by treating this portion of the substrate compared to the refractive index of the waveguide layer 2.

[0066] The hard mask layer 3 is formed, for example, by chromium, whereby the chromium is applied in such a way that the grain sizes are as small as possible. This can be achieved in particular by adding nitrogen during the coating process, for example with a gas flow between 1 sccm and 10 sccm.

[0067] The thickness of the hard mask layer 3 is between 100 nm and 150 nm, for example 120 nm.

[0068] The mask layer 4, as shown in Figure 1B, is then lithographically structured. A chemically strengthened photoresist is particularly suitable for mask layer 4. This allows exposure to be carried out with a comparatively low exposure dose.

[0069] The areas of mask layer 4 to be exposed are repeatedly exposed using a VSB EBL process with a multi-pass writing strategy, resulting in more uniform structures. For example, the exposure is carried out in at least 4 or at least 8 writing styles, each exposing the same areas of the mask layer. Alternatively, other exposure methods, particularly those using electromagnetic radiation, can be employed.

[0070] After exposure, the mask layer 4 is developed and partially removed according to the exposure pattern, so that the hard mask layer 3, as shown in Figure 1B, is only partially covered by the mask layer 4.

[0071] In the process step shown in Figure IC, the structure of the mask layer 4 is transferred to the hard mask layer 3 using a dry chemical etching process. For this purpose, the areas of the hard mask layer's surface not covered by the mask layer 4 can first be etched by an oxygen plasma. Subsequently, the dry chemical etching can be carried out under a gas flow of Clg and Og.

[0072] In a subsequent dry chemical etching step, the waveguide layer 2 is structured into a waveguide 20, whereby the structure of the hard mask layer is transferred to the waveguide layer 2. This can be done by an ICP etching process, in particular using fluoroform.

[0073] Dry chemical etching is performed, for example, with an RF power between 300 W and 500 W (e.g., 350 W) and an ICP power between 600 W and 1200 W (e.g., 700 W).

[0074] This step is preferably carried out at a temperature between -40 °C and -80 °C inclusive, for example -50 °C. This minimizes redeposits on the side faces 201 of the waveguide 20.

[0075] Figure ID shows the process stage after the dry chemical structuring of the waveguide layer 2 .

[0076] During dry chemical structuring, the waveguide layer 2 can be completely etched through in certain areas, so that the side flanks 201 of the waveguide 20 extend to the cladding layer 7. The thickness of the waveguide 20, i.e., the extent of the waveguide perpendicular to a principal plane of extension of the waveguide layer 2, therefore essentially corresponds to the original thickness of the waveguide layer 2. However, this is not strictly necessary. Alternatively, material from the waveguide layer 2 can remain laterally of the waveguide 20.

[0077] The waveguide 20 produced in this way is subjected to a multi-stage post-treatment.

[0078] In the process step schematically depicted in Figure IE, the hard mask layer 3 is removed in a first wet chemical etching step using an etching mixture for the first etching step 81. For example, the etching mixture is based on cerium ammonium nitrate and perchloric acid.

[0079] Subsequently, as shown in Figure 1F, a second wet chemical etching step is carried out using an etching mixture 82 for the second etching step to remove redeposits 9 from the side faces 201 of the waveguide 20. The redeposits 9 on the side faces 201 are shown schematically in Figure 1F by means of an enlarged section. In particular, the etching mixture 82 for the second etching step for the post-treatment of the waveguide layer 2 comprises the following liquids: nitric acid (HNOg), in particular in a proportion between 1% and 5% inclusive; phosphoric acid (H3PO4), in particular in a proportion between 65% and 75% inclusive; acetic acid (CH3COOH), in particular in a proportion between 5% and 10% inclusive; and deionized water, in particular in a proportion between 10% and 30% inclusive.

[0080] The second etching step is preferably carried out at a temperature above room temperature, approximately 50 °C. The duration of the second etching step is typically between 30 and 90 minutes, for example, 60 minutes. It has been shown that by megasonical assistance, for example at a frequency of 1 MHz, redeposits 9 can be removed particularly efficiently, thus producing particularly smooth side faces 201 of the waveguide 20.

[0081] In particular, lithium-containing redeposits 9 such as crystalline LiF redeposits formed during the use of fluoride-containing gases during dry chemical etching can be removed so efficiently that sufficiently smooth side faces 201 can be obtained for optical waveguide applications.

[0082] Furthermore, particularly smooth side flanks 201 can be achieved by the specially adapted deposition and subsequent structuring of the hard mask layer 3 described above in conjunction with the dry chemical etching of the waveguide layer 2 at low temperatures, especially in conjunction with the post-treatment described.

[0083] The waveguide 20 produced in this way can then be thermally cured, for example at a temperature between 450 °C and 700 °C (Figure IG).

[0084] This can be done, for example, in an oxygen atmosphere. Preferably, the heating of the waveguide 20 is carried out with a temperature ramp of at most 5 K / min.

[0085] As shown in Figure 1H, the waveguide 20 can subsequently be coated with a protective layer 5. For example, silicon oxide can be deposited for this purpose, such as by chemical vapor deposition, in particular by plasma-enhanced chemical vapor deposition (PECVD). For example, the thickness of the protective layer 5 is between 1 pm and 10 pm inclusive.

[0086] In addition to providing protection against external environmental influences or mechanical damage, the protective layer 5 can also serve to achieve a more symmetrical mode field distribution within the waveguide 20, particularly in conjunction with a silicon oxide waveguide layer 7 between the waveguide 20 and the support substrate 6. In a cross-section through the waveguide 20, the waveguide 20 can thus be surrounded by silicon oxide along its entire circumference. However, other materials, in particular different materials, can be used for the cladding layer 7 and the protective layer 5, preferably materials that exhibit a refractive index step of at least 0.5 with respect to the waveguide 20.

[0087] Figure 1H schematically represents the completed waveguide structure 1. Using the described method, side flanks 201 of the waveguide 20 can be produced whose flank angle 202 can approach the schematically depicted ideal case of 90° compared to other manufacturing methods. For example, the flank angle 202 is at least 60°. For example, the flank angle 202 lies in a range between 65° and 75° inclusive.

[0088] Using the described method, an attenuation of 2 dB / m was achieved for lithium niobate as a waveguide material for the straight waveguide 20. The method can be applied analogously to lithium tantalate by adjusting the process parameters.

[0089] In a top view of the substrate 6, the orientation of the waveguide structures to be fabricated can be varied within wide limits. In particular, low-loss curved waveguides with comparatively small radii of curvature can also be produced. Small distances between adjacent waveguides can also be achieved. This simplifies the fabrication of complex and / or compact photonic integrated circuits.

[0090] Furthermore, the process is characterized by good scalability, enabling cost-effective mass production. This patent application claims priority from German patent application 10 2024 119 458.8, the disclosure of which is hereby incorporated by reference. The invention is not limited to the description by means of the exemplary embodiments. Rather, the invention encompasses every new feature as well as every combination of features, which in particular includes every combination of features in the claims, even if that feature or combination itself is not explicitly stated in the claims or the exemplary embodiments.

[0091] Reference character list

[0092] 1 Waveguide structure

[0093] 2 waveguide layers 20 waveguides

[0094] 201 Sidewall

[0095] 202 flank angles

[0096] 3 hard mask layers

[0097] 4 mask layer 5 protective layer

[0098] 6 Carrier substrate

[0099] 7. Mantle layer

[0100] 81 Etching mixture for the first etching step

[0101] 82 Etching mixture for second etching step 9 Redeposit

Claims

Patent claims 1. Method for fabricating a waveguide structure (1) comprising the steps of: a) providing a waveguide layer (2) comprising lithium niobate or lithium tantalate; b) applying a hard mask layer (3) and a mask layer (4) to the waveguide layer (2); c) lithographically structuring the mask layer (4); d) dry chemical etching to structure the hard mask layer (3) using the mask layer (4); e) dry chemical etching to structure the waveguide layer (2) into a waveguide (20) using the hard mask layer (3); and f) post-treatment of the waveguide layer (2), wherein in a first etching step the hard mask layer (3) is removed and in a second etching step (4) redeposits on the side flanks are removed.

2. Method according to claim 1, wherein the second etching step in step f) is carried out with megasonic assistance.

3. A method according to claim 1 or 2, wherein an etching mixture for the second etching step in step f) comprises the following liquids: nitric acid, phosphoric acid, acetic acid and deionized water.

4. Method according to any of the preceding claims, wherein the hard mask layer (3) comprises chromium which is deposited by admixture of nitrogen such that the mean grain size of the hard mask layer (3) is at most 50 nm.

5. Method according to any of the preceding claims, wherein the hard mask layer (3) has a thickness between 100 nm inclusive and 150 nm inclusive.

6. Method according to any of the preceding claims, wherein step e) is carried out at a temperature between and including -40°C and -80°C.

7. Method according to any of the preceding claims, wherein step f) comprises a sub-step in which the waveguide layer (20) is heated to a temperature between 450°C inclusive and 700°C inclusive.

8. Method according to any of the preceding claims, wherein the mask layer (4) in step c) comprises a chemically enhanced photoresist.

9. Method according to one of the preceding claims, wherein the mask layer (4) is exposed in step c) in several scripts by an electron beam.

10. Method according to one of the preceding claims, wherein after step f) a protective layer is applied to the waveguide.

11. Method according to one of the preceding claims, wherein the waveguide layer is provided on a support substrate (6) in step a), wherein the waveguide layer (2) is adjacent on a side facing the support substrate to a material which has a lower refractive index than the waveguide layer (2).

12. Method according to claim 11, wherein an oxide layer is arranged as a sheath layer (7) between the support substrate (6) and the waveguide layer (2).

13. Waveguide structure manufactured according to a method according to any one of claims 1 to 11.

14. Waveguide structure according to claim 13, wherein the waveguide (2) has a transmission loss of at most 2 dB / m at least in certain locations.

15. Waveguide structure according to claim 13 or 14, wherein a side flank (201) of the waveguide (2) has a flank angle (202) of at least 65°.

Images