DE102021003426A1 - Method for producing an optical waveguide, an optical waveguide produced using this method, and a medical implant with such an optical waveguide - Google Patents

Abstract

The invention relates to a method for producing an optical waveguide. The object of the invention is therefore to create a method for producing optical waveguides based on the material combination PMMA/SU8/PMMA, with which lateral periodicities of less than 50 μm can be achieved. Furthermore, the optical waveguide should have high mechanical flexibility, with bending radii of less than 1 mm being possible. This object is achieved with a method having the following steps: - formation of a first PMMA layer to form the lower cladding layer (4) of the optical waveguide (1), - formation of a protective layer (5) on the first PMMA layer, - application an SU8 layer on the protective layer (5), photolithographic structuring of the SU8 layer by exposing the areas of the SU8 layer intended for the core layer (6, 6a, 6b, 6c) of the optical waveguide (1) and subsequent removal of unexposed areas the SU8 layer using a developer liquid suitable for SU8,- formation of a second PMMA layer on surfaces of the SU8 layer and the protective layer (5) that are exposed after the photolithographic structuring to form the upper cladding layer (7) of the optical waveguide (1), wherein the protective layer (5) is resistant to the developer liquid and the protective layer (5) inside the optical waveguide (1) is essentially o is optically inactive.

Description

The invention relates to a method for producing an optical waveguide, an optical waveguide produced using this method, and a medical implant with such an optical waveguide.

Optical waveguides are structures made of transparent materials in which light can propagate in the form of modes. An optical waveguide consists of a core material and a cladding material, with the core material having a higher refractive index than the cladding material. The greater the difference between the refractive indices of both materials, the smaller the bending radii that can be achieved for the optical waveguides. In optogenetics in particular, flexible waveguides with the smallest possible bending radii (less than 1 mm) are required. Since larger differences in the refractive indices lead to increased scattering losses at rough interfaces, the combination of materials must be chosen appropriately. Furthermore, the lateral periodicity of the waveguides (sum of waveguide width and spacing) should be less than 50 µm in order to be able to integrate a sufficient number of waveguides side by side on one component.

Various methods for producing optical waveguides are known from the prior art.

the EP 0 445 527 B1 describes a photolithographic process in which fine channels approx. 7 µm wide are etched into a glass substrate and the bottom of the channel is then provided with a PMMA layer. Then the channels are filled with a non-linear optical polymer and covered with another PMMA layer. The disadvantage, however, is that the glass substrate is rigid and the optical waveguides produced using this method are not flexible. Furthermore, the introduction of the PMMA layer on the channel base of the fine channels is technologically demanding.

Another photolithographic process for the production of optical waveguides is presented in EP 1 674 905 B1 described. In this case, a double layer made of a lower cladding material and a core material is first provided. The core material layer is then structured photolithographically and then coated with an upper cladding material. Organosilicon compounds are used as cladding material and core material.

In WO 2018/089929 A1 describes a flexible optical waveguide and a method for producing flexible optical waveguides. The process initially provides for the application of a first cladding layer and a core layer by means of spin coating. The core layer is then structured photolithographically and then covered with a second cladding layer, which is also applied using spin coating. The core material can include SU8. A fluoropolymer is used as the jacket material. SU8 is a photoresist consisting essentially of three components: base resin, solvent and photosensitive component, with the base resin being an epoxy resin formed from a glycidyl ether derivative of bisphenol A.

Theoretical studies on the suitability of the material combination PMMA/SU8/PMMA for waveguide structures are known from "SU8 Based Waveguide for Optrodes", S. Pimenta, Proceedings 2, 814 MDPI (2018). The disadvantage, however, is that photolithographic structuring of the SU8 core layer using methods known from the prior art causes cracking in the PMMA due to the low resistance of PMMA to solvents typical of the process (or SU8 developer solution). This formation of cracks prevents the production of defect-free layers and thus low-loss waveguides. To produce defect-free layers, only mechanical structuring of the SU8 core layer can be carried out, which in turn can only be used to produce significantly coarser structures compared to photolithographic structuring. In particular, lateral periodicities of less than 50 µm cannot be achieved with mechanical structuring with the material combination PMMA/SU8/PMMA.

The object of the invention is therefore to create a method for producing optical waveguides based on the material combination PMMA/SU8/PMMA, with which lateral periodicities of less than 50 μm can be achieved. Furthermore, the optical waveguide should have high mechanical flexibility, with bending radii of less than 1 mm being possible.

• - Ausbildung einer ersten PMMA-Schicht zur Bildung der unteren Mantelschicht des optischen Wellenleiters,
• - Ausbildung einer Schutzschicht auf der ersten PMMA-Schicht,
• - Auftragen einer SU8-Schicht auf die Schutzschicht,
• - photolithographische Strukturierung der SU8-Schicht durch Belichtung der für die Kernschicht des optischen Wellenleiters vorgesehenen Bereiche der SU8-Schicht und anschließende Entfernung unbelichteter Bereiche der SU8-Schicht mittels für SU8 geeigneter Entwicklerflüssigkeit,
• - Ausbildung einer zweiten PMMA-Schicht auf nach der photolithographischen Strukturierung freiliegenden Oberflächen der SU8-Schicht und der Schutzschicht zur Bildung der oberen Mantelschicht des optischen Wellenleiters,

This object is achieved with a method that includes the following steps,

• - formation of a first PMMA layer to form the lower cladding layer of the optical waveguide,
• - formation of a protective layer on the first PMMA layer,
• - Application of a SU8 layer on the protective layer,
• - Photolithographic structuring of the SU8 layer by exposure to light for the core layer areas of the SU8 layer provided for the optical waveguide and subsequent removal of unexposed areas of the SU8 layer using a developer liquid suitable for SU8,
• - formation of a second PMMA layer on surfaces of the SU8 layer and the protective layer that are exposed after the photolithographic patterning to form the upper cladding layer of the optical waveguide,

The material of the protective layer is selected in such a way that on the one hand it is resistant to the developer liquid and on the other hand it is essentially optically inactive within the optical waveguide.

The protective layer within the optical waveguide is essentially optically inactive when the refractive index of the protective layer and the refractive index of the core layer are approximately the same and the protective layer has a layer thickness of less than or equal to 5% of the layer thickness of the core layer. Alternatively, the refractive index of the protective layer and the refractive index of the lower cladding layer may be about the same. In one possible embodiment, the protective layer between the waveguides is removed after the waveguides have been structured, e.g. B. by dry chemical or wet chemical etching. In this case, the protective layer is optically inactive, regardless of its refractive index.

The first PMMA layer is protected by the protective layer when the developer liquid is used during the photolithographic structuring of the SU8 core layer. This prevents cracking in the first PMMA layer caused by the developer liquid.

In an advantageous embodiment, the protective layer is formed by applying an SU8 layer to the first PMMA layer and then exposing this SU8 layer to light. As a result of the exposure, this SU8 layer becomes resistant to the photolithographic structuring of the further SU8 layer (applied subsequently to build up the core layer). Furthermore, the protective layer designed as an SU8 layer and the core layer have the same refractive index.

One embodiment of the method provides that the formation of the first cladding layer and/or the formation of the second cladding layer takes place by layer application using spin coating and subsequent thermal curing.

A further embodiment provides that the SU8 layer is also applied to the protective layer and/or the SU8 layer is applied to the first PMMA layer by means of spin coating.

Furthermore, an optical waveguide is proposed which has a lower cladding layer made of PMMA, a core layer made of SU8 and an upper cladding layer made of PMMA, the core layer being formed by means of photolithographic structuring of an SU8 layer. A protective layer is arranged between the lower cladding layer and the core layer. This protective layer is resistant to the photolithographic developer liquid and is essentially optically inactive within the optical waveguide.

In an advantageous embodiment, the protective layer of the optical waveguide is an SU8 layer.

One embodiment provides that the optical waveguide has a plurality of core layers arranged next to one another. The optical waveguide thus forms a component which contains several channels in the core layer for light propagation, which can also be designed with changes in direction, connected to one another via switches, changed in their lateral dimensions or equipped with coupling and decoupling structures.

An advantageous embodiment provides that the lower cladding layer has a layer thickness of 5 μm to 10 μm, the upper cladding layer has a layer thickness of 2 μm to 5 μm, the protective layer has a layer thickness of 0.1 μm to 0.2 μm and the core layer has a layer thickness from 5 µm to 15 µm. A waveguide designed in this way is characterized by high mechanical flexibility. The PMMA/SU8/PMMA material system enables the waveguide to have bending radii of less than 1 mm. In the red spectral range (638 nm to 660 nm), PMMA has a refractive index of 1.49 and SU8 has a refractive index of 1.57. Due to this large difference in the refractive indices, only very small optical losses occur when a corresponding light wave passes through the waveguide, even with such small bending radii. The dimensioning of the layer thickness of the protective layer also ensures that it is essentially optically inactive in the optical waveguide.

In one embodiment, the core layers arranged next to one another have a width of 5 μm to 15 μm and a spacing of 10 μm to 20 μm transverse to the direction of light travel. With such a dimensioning, lateral periodicities of less than or equal to 50 μm are generated. For example, with a width of 10 μm and a spacing of 15 μm, a periodicity of 25 μm is achieved and with little smaller width and/or smaller distance corresponding to a lateral periodicity of less than 25 μm. A high packing density of multiple channels in the core layer for light propagation can thus be achieved in one component.

The invention is not limited to linear core layers. In principle, photolithographic structuring allows any geometry in the formation of the channels serving as the core layer. For example, the channels within the first PMMA layer can also be designed with changes in direction or be connected to one another via switches. With photolithographic structuring, changes in direction of the core layer with radii of curvature of less than 1 mm can also be produced in high quality, which in turn are passed through by a light wave with low losses due to the large difference in the refractive indices of PMMA and SU8.

Furthermore, an optogenetic implant is proposed which has an optical waveguide according to the invention with the dimensions mentioned above. The high mechanical flexibility of the optical waveguide according to the invention, combined with the low optical losses, enables integration into such an implant. With the high packing density of the channels in the core layer, a high density of transmittable light pulses is possible, which in turn enables high-resolution optical excitation of organic cell material.

In particular, an optogenetic implant designed as a cochlear implant is proposed, which has an optical waveguide according to the invention and is inserted in the inner ear of a human being. Due to the high-resolution optical stimulation, the auditory nerves in the inner ear can be stimulated in a more spatially targeted manner in comparison to electrically acting implants.

An exemplary embodiment of the invention is explained below with reference to the drawings. the 1a until 1g show a sequence of the method according to the invention for producing an optical waveguide 1 ( 1g) . The starting point is a carrier substrate, which 1a is designed as a silicon wafer 2 with a sacrificial layer 3, without being limited to this. A first PMMA layer 4 is formed on the sacrificial layer 3 on this carrier substrate. This first PMMA layer is applied to the carrier substrate using spin coating and then thermally cured ( 1b) . After the formation of the first PMMA layer 4, a protective layer 5 is formed on it ( 1c ). In the embodiment shown, an SU8 layer is formed as the protective layer 5, which is first applied to the first PMMA layer 4 by means of spin coating and then exposed. According to 1d A further SU8 layer 6 is then applied to the exposed SU8 protective layer 5 by means of spin coating. This is structured in a subsequent step using photolithography ( 1e) , thereby forming the core layer 6a, 6b, 6c of the optical waveguide 1. In the photolithographic structuring of the SU8 layer 6, after it has been applied, the areas which are to form the core layer 6a, 6b, 6c of the optical waveguide 1 are first exposed to light. In the exposed areas, she polymerizes the SU8 layer in a subsequent baking step. Thereafter, the partially exposed SU8 layer 6 is treated with a developer liquid suitable for SU8, unexposed areas of the SU8 layer 6 being dissolved and removed therewith. The exposed areas, on the other hand, remain intact. the 1e 12 shows the remaining exposed areas which form the core layer 6a, 6b, 6c of the optical waveguide 1. FIG. In the embodiment shown, three channels 6a, 6b, 6c arranged next to one another are provided in the core layer 6 for guiding the light, but without the invention being restricted to this embodiment. Likewise, fewer or more channels 6a, 6b, 6c can be provided. A second PMMA layer 7 is then formed on the surfaces of the SU8 layer 6 and protective layer 5 that are exposed after the photolithographic structuring, with PMMA being applied for this purpose by means of spin coating and then cured ( 1f) . After the second PMMA layer has been formed, the sacrificial layer 3 is removed using suitable solvents, so that the optical waveguide 1 can be removed from the silicon wafer 2 .

1g shows the optical waveguide 1 produced with the described method with a lower cladding layer made up of the first PMMA layer 4, an upper cladding layer made up of the second PMMA layer 7 and a core layer made up of the SU8 layer 6 with the three channels 6a, 6b, 6c. Furthermore, the protective layer 5 arranged between the lower cladding layer and the core layer and in areas adjoining the core layer between the upper cladding layer and the lower cladding layer is visible. The protective layer 5 thus forms a continuous layer on the first PMMA layer 4.

In the illustrated embodiment, the lower cladding layer has a layer thickness of 5 μm to 10 μm, the upper cladding layer has a layer thickness of 2 μm to 5 μm, the protective layer 5 has a layer thickness of 0.1 μm to 0.2 μm and the core layer 6 has a layer thickness from 5 µm to 15 µm. The core layers 6a, 6b, 6c or the channels 6a, 6b, 6c of the core layer 6 point transversely to the direction of the light direction (the direction of light travel is perpendicular to the image plane) has a width of 5 μm to 15 μm and the core layers 6a, 6b, 6c arranged next to one another have a spacing of 10 μm to 20 μm. With a width of 10 μm and a distance of 15 μm, a lateral periodicity L of 25 μm and thus less than or equal to 50 μm is achieved. A lateral periodicity L of less than 25 μm is correspondingly achieved with a smaller width and/or smaller spacing.

In an embodiment that is not shown, the optical waveguide is according to FIG 1g integrated in an optogenetic implant designed as a cochlear implant. An optogenetic implant is a form of medical implant.

In a further embodiment of the optical waveguide 1 (not shown), the protective layer 5 has been removed in the areas adjoining the core layer by means of a further process step (between photolithographic structuring and application of the second PMMA layer 7). This further method step can be a dry-chemical or wet-chemical etching step, for example. As a result, the protective layer 5 in this embodiment of the optical waveguide 1 is then only present between the lower cladding layer and the core layers 6a, 6b, 6c. In the areas adjoining the core layers 6a, 6b, 6c, the upper cladding layer rests on the lower cladding layer.

Reference List

1

optical waveguide

2

silicon wafer

3

sacrificial layer

4

first PMMA layer, lower cladding layer

5

protective layer

6

SU8 layer, core layer

6a

core layer, channel of the core layer

6b

core layer, channel of the core layer

6c

core layer, channel of the core layer

7

second PMMA layer, upper cladding layer

L

Lateral periodicity

QUOTES INCLUDED IN DESCRIPTION

This list of the documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• EP 0445527 B1 [0004]
• EP 1674905 B1 [0005]
• WO 2018089929 A1 [0006]

Claims (12)

A method of manufacturing an optical waveguide (1) comprising a PMMA undercladding layer (4), a SU8 core layer (6, 6a, 6b, 6c) and a PMMA overcladding layer (7), the method comprising the following steps includes, - formation of a first PMMA layer to form the lower cladding layer (4) of the optical waveguide (1), - formation of a protective layer (5) on the first PMMA layer, - Application of a SU8 layer on the protective layer (5), - photolithographic structuring of the SU8 layer by exposing the areas of the SU8 layer intended for the core layer (6, 6a, 6b, 6c) of the optical waveguide (1) and subsequent removal of unexposed areas of the SU8 layer using a developer liquid suitable for SU8, - Formation of a second PMMA layer on surfaces of the SU8 layer and the protective layer (5) that are exposed after the photolithographic structuring to form the upper cladding layer (7) of the optical waveguide (1), the protective layer (5) being resistant to the developer liquid and the protective layer (5) within the optical waveguide (1) is essentially optically inactive. procedure after claim 1 , characterized in that the protective layer (5) is formed by applying an SU8 layer to the first PMMA layer and then exposing this SU8 layer to light. procedure after claim 1 , characterized in that the formation of the first PMMA layer and/or the formation of the second PMMA layer takes place by layer application by means of spin coating and subsequent thermal curing. Method according to one of the preceding claims, characterized in that the SU8 layer is applied to the protective layer (5) and/or the SU8 layer is applied to the first PMMA layer by means of spin coating. Optical waveguide (1) having a lower cladding layer (4) made from PMMA, a core layer (6, 6a, 6b, 6c) made from SU8 and an upper cladding layer (7) made from PMMA, characterized in that the core layer (6, 6a, 6b, 6c) is formed by photolithographic structuring of an SU8 layer and that a protective layer (5) is arranged between the lower cladding layer (4) and the core layer (6, 6a, 6b, 6c) and that this protective layer (5) is opposite to the is stable in the photolithographic developer liquid and the protective layer (5) within the optical waveguide (1) is essentially optically inactive. Optical waveguide (1) according to claim 5 , characterized in that the protective layer (5) is a SU8 layer. Optical waveguide (1) according to one of Claims 5 until 6 , characterized in that it comprises a plurality of core layers (6, 6a, 6b, 6c) arranged next to one another. Optical waveguide (1) according to one of Claims 5 until 7 , characterized in that the lower cladding layer (4) has a layer thickness of 5 µm to 10 µm, the upper cladding layer (7) has a layer thickness of 2 µm to 5 µm, the protective layer (5) has a layer thickness of 0.1 µm to 0. 2 µm and the core layer (6, 6a, 6b, 6c) have a layer thickness of 5 µm to 15 µm. optical waveguide claim 7 and 8th , characterized in that the core layer (6, 6a, 6b, 6c) has a width of 5 µm to 15 µm transversely to the direction of light travel and the core layers (6, 6a, 6b, 6c) arranged next to one another have a spacing of 10 µm to 20 µm . Optical waveguide (1) according to one of Claims 7 until 9 , characterized in that it has a lateral periodicity (L) of less than or equal to 50 µm, preferably less than or equal to 25 µm. Medical implant, characterized in that this has an optical waveguide (1) according to claim 8 or 9 having. Medical implant after claim 11 , characterized in that this is a cochlear implant.

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