Structuring of a surface of an active optical material

Abstract

Method for structuring a surface (32) of an active optical material (12) which is a laser-active material, a Faraday rotator and / or an optically nonlinear material, comprising the steps: Treating (S10) the surface of the active optical material with a plasma etching process in a plasma etching chamber (10); Introducing (S12) a silicon compound into the plasma etching chamber, wherein an etching gas of the plasma etching process comprises a fluorine compound; and A nanostructure is created on the surface of the active optical material using the plasma etching process.

Description

[0001] The present invention relates to a method for structuring a surface of an active optical material. The present invention further relates to an active optical material having a surface structured by the method.

[0002] Laser systems play an important role in materials processing, metrology, and many other applications. Laser micromachining can be used, in particular, for material removal (ablation), surface functionalization, or even surface polishing. For example, distances or material properties can be determined using interferometric or spectroscopic methods with a laser system. Pulsed lasers are used to achieve the highest possible peak power. Continuous laser sources are also used to achieve a high average output power. Different optical systems are used depending on the application.

[0003] In such laser systems, active optical materials are used to generate the laser beam and adjust the properties of the light or laser beam. Commonly used active optical materials include laser materials (e.g., Nd:YAG, Yb:YAG, Nd:YVO4, Ti:sapphire), optically nonlinear materials (e.g., lithium triborate, β-barium borate, BiBO, lithium niobate, lithium tantalate), and Faraday materials or Faraday rotators (e.g., TGG and others). The reflection or transmission properties of the materials are often relevant. In most cases, the highest possible transmission over a broad wavelength range is desired. Furthermore, it is desirable to use surfaces that are as dirt- and water-repellent as possible. Especially at high power levels, dirt or water deposits can lead to material defects, for example, when dirt particles are heated.In addition, performance losses and inaccuracies may occur when using the laser system.

[0004] These material properties are usually achieved through a coating. So-called anti-reflective coatings (AR coatings) can consist of several ultra-thin layers applied using established manufacturing processes and provide excellent anti-reflection properties. The disadvantage of such multi-layer systems is that the anti-reflection properties often depend on the wavelength and angle of incidence of the light. Therefore, an application-specific coating is necessary. Furthermore, thermal cycling can lead to delamination of the coating, particularly at high power levels. The surface's resistance to high thermal power (laser radiation) is reduced.

[0005] Other approaches in the field of passive optical materials are based on surface microstructuring. Carbon compounds are often used for masking. Since mask residues remain after processing, which lead to material defects at high power levels due to residual absorption, such approaches can only be transferred to the field of active optical materials to a limited extent. In particular, limitations may arise with regard to lifetime or potential applications.

[0006] For silicon dioxide, WO 2012 / 032162 A1 discloses a method for reducing the interfacial reflection of a glass surface. The glass surface of the optical element is treated with a plasma etching process without applying a mask material to the glass surface. An etching gas containing a fluorine compound is used in the plasma etching process. Using the maskless plasma etching process, a reflection-reducing nanostructure is created on the surface. This method is based on directly structuring the silicon dioxide of the surface, so that transfer to other materials, in particular active optical materials, is not possible. The method described in WO 2012 / 032162 A1 relates in particular to amorphous materials, which, unlike crystalline active optical materials, do not possess long-range order.This requires a significant change in the mechanisms behind the etching process and the self-organization of structures. Long-range order is the crucial property of active optical materials, as many application properties are related to it and are defined by it. In a self-catalytic process, it cannot be guaranteed that the crystal orientation is maintained. This can lead to a change in the material properties. The same applies to the introduction of an amorphous etching mask in the form of fused silica.

[0007] DE 10 2008 002 193A1 relates to an optical element with a hydrophobic surface and a projection exposure system for immersion lithography. The hydrophobic surface is formed by microstructuring an uncoated area of ​​the element body. The projection exposure system comprises a projection lens having such an optical element as a closure element for partial wetting with an immersion liquid.

[0008] US 2019 / 0109011A1 relates to the formation of superhydrophobic surfaces. Technologies for methods and systems effective for etching nanostructures in a substrate are described. The methods may comprise depositing a structured block copolymer on the substrate. The methods may comprise applying a precursor to the structured block copolymer to create an infiltrated block copolymer. The precursor may infiltrate into the first polymer block domain and create a material.

[0009] US 2004 / 0 056 271 A1 relates to a nanotip structure created by exposing a substrate to a process gas that simultaneously creates nanomasks on a surface of the substrate and etches exposed portions of the surface to create the nanotip structure. Components of the process gas mixture form nanocrystal structures on the surface of the substrate and mask portions of the substrate from other components of the process gas mixture, which in turn etch other portions of the substrate.

[0010] Based on this, the present invention seeks to provide an approach for improving the reflection properties of an active optical material. In particular, the transmission should be increased as broadly as possible. Furthermore, a surface that is as water- and dirt-repellent as possible should be created.

[0011] To achieve this object, the present invention relates in a first aspect to a method for structuring a surface of an active optical material which is a laser-active material, a Faraday rotator and / or an optically non-linear material, comprising the steps: - Treating the surface of the active optical material with a plasma etching process in a plasma etching chamber; - Introducing a silicon compound into the plasma etching chamber, whereby - an etching gas of the plasma etching process comprises a fluorine compound; and - a nanostructure is created on the surface of the active optical material using the plasma etching process.

[0012] In a further aspect, the present invention relates to an active optical material having a surface structured in the method described above.

[0013] Preferred embodiments of the invention are described in the dependent claims. It is understood that the features mentioned above and those to be explained below can be used not only in the respective combinations specified, but also in other combinations or alone, without departing from the scope of the present invention. In particular, the active optical material can have a surface that has been structured using a method as described in one of the dependent claims.

[0014] According to the invention, a surface of an active optical material is structured. The active optical material is introduced as a substrate into a plasma etching chamber and treated therein in a plasma etching process. A silicon compound is introduced into the plasma etching chamber. An etching gas containing a fluorine compound is used. During the plasma etching process, silicon dioxide is used to mask the surface, creating a nanostructure on the surface of the active optical material (so-called etching grass). The process parameters of the plasma etching process (applied voltage, temperature, duration, etc.) are selected according to the intended application. In particular, the process is terminated when the desired surface nanostructure is achieved. The process parameters depend on the plasma etching chamber used.

[0015] The process according to the invention can significantly improve the transmission compared to untreated active optical materials.

[0016] In addition, a water- and dirt-repellent surface can be created, which offers a longer service life because, for example, no dust can accumulate as an absorption nucleus. Compared to previous processes in which a coating is applied, the process according to the invention offers the advantage that the desired transmission properties can be guaranteed even at higher power levels. It prevents the coating from flaking off due to different expansion coefficients of the coating material and thus leading to material defects (delamination due to thermal effects). A long service life of the active optical material is achieved. In addition, higher power levels can be used due to the improved damage threshold, which leads to greater efficiency.Compared to previous approaches that use carbon compounds as mask material, the method according to the invention offers the advantage that no carbon residues remain. This improves durability and reduces the susceptibility to defects. The desired optical properties of the active optical material can be created without any unwanted foreign substances remaining on the surface. While it is impossible to prevent foreign substances from being applied during the process, the invention ensures that this occurs without any negative effects. The use of a silicon compound or silicon dioxide has the advantage that the optical properties of the active optical material are not changed. The desired reduction in reflection is achieved without changing the properties of the active optical material.Reduced absorption can improve the efficiency of a component, as higher achievable power densities also favor nonlinear processes. The achieved reflection reduction is broadband and independent of the wavelength of the incident light. The reflection reduction is independent of the angle of incidence of the light or the viewing angle. This allows for good anti-reflection properties even when using tilted components. Furthermore, manufacturing costs are reduced.

[0017] In a preferred embodiment, the active optical material is a laser-active material, a Faraday rotator, and / or an optically nonlinear material. Such materials are used in laser systems. At high power levels, conventional coating or structuring methods cannot be used without restrictions, as suboptimal optical properties can lead to performance losses or reduced lifetimes.

[0018] In a preferred embodiment, the etching gas contains no fluorocarbon compounds and preferably no carbon compounds. By eliminating carbon, carbon residues on the surface can be avoided. Carbon residues can be a source of material defects, particularly at high power levels. By avoiding the introduction of carbon, the durability of the active optical material is extended while improving transmission properties.

[0019] In a preferred embodiment, the etching gas comprises sulfur hexafluoride (SF 6 The use of sulfur hexafluoride results in a rapid plasma etching process. The desired surface structuring can be achieved in a comparatively short time, increasing cost-effectiveness.

[0020] In a preferred embodiment, the etching gas comprises tetrafluorosilane (SiF4). The silicon compound is introduced into the plasma etching chamber in the form of tetrafluorosilane. Tetrafluorosilane, particularly in a gas mixture of sulfur hexafluoride and tetrafluorosilane, enables an efficient and rapid etching process. The surface of the active optical material is structured to achieve the desired effect.

[0021] In a preferred embodiment of the method, the silicon compound is introduced into the plasma etching chamber in the form of a silicon dioxide sacrificial glass. The use of a sacrificial glass allows for cost-effective introduction of the silicon compound. The amount of silicon dioxide (SiO 2 ) used in the process.

[0022] Simple control of the process parameters and efficient process control is achieved.

[0023] In one embodiment, the plasma etching process is terminated when the nanostructure has a structure height of 250 nm to 1,500 nm, preferably 500 nm to 1,000 nm. The structure height determines the wavelength range of the improved transmission of the active optical material. The structure height must be on the order of the maximum wavelength to achieve reflection reduction. Preferably, a nanostructure (nanocluster) with a comparatively high structure height is created to achieve broadband reflection reduction.

[0024] In a preferred embodiment, the active optical material is cooled during the plasma etching process. Cooling the substrate during the process allows the mobility of the free particles (molecules or atoms) to be controlled. This allows the plasma etching process to be controlled. The plasma etching process is carried out in such a way that the desired structure is achieved. Cooling can influence other process parameters to achieve a structure optimized for a specific application.

[0025] In a preferred embodiment, the method comprises a step of applying octafluorocyclobutane (OFCB) and / or polymerization products of OFCB, in particular plasma-polymerized OFCB (PP-OFCB), to the surface of the active optical material after completion of the plasma etching process. Applying an OFCB layer does not change the optical properties or changes them only to a very small extent. Dirt and water repellency are improved. The applicability of the surface-structured active optical material is further enhanced.

[0026] In a preferred embodiment, the plasma etching process is a combined physical and chemical etching process. Preferably, both physical and chemical effects are utilized in the plasma etching process to create the desired nanostructure of the surface. A physical etching process involves bombardment of the substrate surface with (ionized) particles. A chemical process involves the removal of particles through a chemical reaction at the surface of the substrate. A combined physical and chemical etching process results in an improved etching effect and a faster process. The cost-effectiveness of the process is improved. Furthermore, it results in improved controllability and process monitoring, allowing for fine-tuning of the etching effect.

[0027] A surface of an active optical material is understood to mean, in particular, its uppermost layer. For example, the surface can comprise an area up to a depth of 1 µm or 2 µm. A plasma etching process is understood to mean a material-removing, plasma-assisted etching process. The plasma etching process can, in particular, have a chemical and a physical component. A nanostructure is understood to mean, in particular, an irregular, self-organizing structure. A nanostructure can also be referred to as a nanocluster and, in particular, can have structures on the order of a few hundred µm. An active optical material is a material that causes amplification, a rotation of the polarization, a frequency conversion and / or a nonlinear phase change of the light as it passes through.

[0028] The invention is described and explained in more detail below using selected embodiments in conjunction with the accompanying drawings. They show: Fig. 1 is a schematic representation of a plasma etching chamber for performing a plasma etching process; Fig. 2 a schematic representation of an active optical material with a structured surface; Fig. 3 a schematic representation of the achievable improvement in transmission; Fig. 4 a schematic representation of the influence of an OFCB coating on the transmission; and Fig. 5 a schematic representation of the process steps of the process according to the invention.

[0029] The method according to the invention is used to structure the surface of laser-active materials, Faraday rotators, and optically nonlinear materials. The structure improves transmission across a broad range. Furthermore, a dirt- and water-repellent surface is created. Possible applications include, in particular, optical isolators, frequency converters, laser systems and optical amplifiers, as well as micro- and macro-laser processing systems. The method can be used in particular in the following application areas or for structuring the following materials: laser crystals (in laser resonators and amplifiers), frequency converters (optically nonlinear crystals), e.g., for generating the second harmonic, Faraday rotators in optical isolators, and fast switches (acousto-optical or electro-optical crystals).

[0030] In the Fig. 1 schematically shows a plasma etching chamber 10 in which the steps of the method according to the invention are carried out. In the example shown, a parallel plate reactor is shown. In the plasma etching chamber 10, the active optical material 12 to be structured is located on a substrate holder 14. Between two electrodes 16a, 16b, an electric field is generated by means of a frequency generator 18, in which plasma formation occurs. The plasma 20 is generated between the electrodes 16a, 16b. An inert gas 24, in particular oxygen, is supplied via an inlet 22. The reactive ions 26 in the plasma 20 act on the active optical material 12 in a combined physical and chemical etching process and structure its surface. Additional control of the plasma etching process can be achieved via an outlet 28, which can be closed with a valve 30.

[0031] The plasma etching chamber 10 is part of a plasma system, which preferably operates according to the principle of an electron cyclotron resonance (ECR) system. The plasma etching chamber 10 is pressurized, for example, to a pressure of 2.8 to 3.0 * 10 -2 Millibars are pumped out. For example, a power of 1 to 10 watts can be applied via the frequency generator. A further power of 700 to 800 watts can be applied with a MW generator. The structured surface can be created, for example, with a bias voltage between 60 volts and 150 volts. It is understood that the selected process parameters depend on the system. Furthermore, depending on the process parameter settings, a differently structured surface can be achieved.

[0032] The etching gas or plasma 20 in the plasma chamber comprises a fluorine compound, in particular sulfur hexafluoride. It is understood that the use of an etching gas comprising a different fluorine compound is also possible. The use of sulfur hexafluoride achieves good results in terms of process speed. It is particularly advantageous that no fluorocarbon compounds are present in the etching gas. This ensures that no carbon can be deposited in the active optical material 12, which would alter the optical properties or, at high power levels, could lead to material defects due to a different expansion coefficient.

[0033] According to the invention, a silicon compound is additionally introduced into the plasma etching chamber 10. The silicon compound can be introduced, for example, in the form of a sacrificial glass (in particular, silicon dioxide sacrificial glass), which is placed into the etching chamber as a solid. It is also possible for the etching gas to comprise tetrafluorosilane, so that the silicon compound is introduced into the plasma etching chamber 10 in the form of tetrafluorosilane.

[0034] In the plasma etching chamber 10, it is also advantageous if the substrate can be cooled by means of a suitable cooling device. This allows the mobility of the compounds involved to be reduced in order to adjust the distribution of the generated silicon dioxide nanoclusters.

[0035] In the plasma etching chamber 10, chemical dry etching is realized using sulfur hexafluoride. The process is based on the formation of free fluorine atoms within the plasma 20 in combination with the supplied inert gas 24. SF 6 +O→SOF 5 +F

[0036] The free fluorine atoms can react with the additionally introduced silicon compound to form tetrafluorosilane. This can either occur with a silicon dioxide sacrificial glass or directly with the additional tetrafluorosilane. The resulting or additionally added tetrafluorosilane reacts with water (H) in a catalytic process within the etching plasma. 2 O) to silicon dioxide. 3SiF 4 +2H 2 O→SiO 2 +2H 2 SiF 6

[0037] The silicon dioxide produced in this reaction deposits on the surface of the active optical material 12 and thus acts as a mask for the physical component of the etching process. This newly formed nanostructure (nanocluster) is deposited, among other things, in a statistically distributed manner on the substrate surface. The deposition occurs randomly, resulting in an essentially random nanostructure. The deposition creates unevenness on the surface homogeneously removed by chemical etching, which acts as a mask for the physical etching component. Physical etching, achieved by accelerating ions onto the surface, exhibits a lower etching effect on steep flanks and tips of the formed nanostructures. This allows a deeper effect to be achieved. A two-stage passivation and etching process is preferably not required. The process preferably takes place in a single step within the plasma etching chamber.

[0038] The deposited silicon dioxide is removed over time by the physical etching component. Depending on the selected process parameters, the structuring process is terminated when the surface is largely free of silicon dioxide. In exceptional cases, however, it is also possible to adjust the properties of the active optical material 12 by deliberately depositing silicon dioxide on the surface. For example, hygroscopic materials such as lithium triborate (LBO) or β-barium borate (BBO) can be made water-repellent by leaving a thin silicon dioxide layer after the process and sealing the surface.

[0039] In the Fig. Figure 2 schematically shows an active optical material 12 with a surface 32 structured according to the invention. The plasma etching process is terminated when the structure height h exceeds a desired minimum height. In particular, the method according to the invention makes it possible to achieve structure heights of 250 µm to 1,000 µm. Such large structure heights make it possible to realize a spectrally broadband antireflection effect. The nanostructure of the surface 32 is random.

[0040] On the left side of the Fig. Figure 2 schematically shows an active optical material 12 with a nanostructured surface 32 according to the invention. The structure height h of the nanostructure can, in particular, be less than half the wavelength λ / 2 of the light.

[0041] On the left side of the Fig. 2 shows a transition of the refractive index n upon penetrating the active optical material 12. The vertical x-axis is the height axis of the active optical material 12. The refractive index n has a first value of 34 in air and gradually changes to a second value of 36 upon traversing the nanostructured surface 32. The first value can be 1 in air, for example. The second value 36 can be 1.45, for example.

[0042] In the Fig. Figure 3 schematically shows an example of the anti-reflection effect that can be achieved using the method according to the invention. The wavelength λ is plotted in nanometers on the horizontal axis. The transmission is plotted in percent on the vertical axis. For an untreated active optical material, for example, the characteristic curve shown below results over the different wavelengths. For an average height of the generated nanostructure on the surface of the active optical material of approximately 40 nm, the curve running in the middle results. For an average height of approximately 230 nm, the curve running above results. As shown, a transmission of over 98 percent can be achieved if an active optical material is treated using the method according to the invention.

[0043] In the Fig. Figure 4 shows the effect of an octafluorocyclobutane layer applied in an additional process step on the same scale. The lower curve shows the transmittance of an untreated material. The two upper curves show measurement results for a nanostructured surface of an active optical material with and without an additional OFCB layer applied. The two upper curves are essentially congruent. The additional OFCB layer therefore causes no or only a minimal change in the transmittance.

[0044] In the Fig.Figure 5 schematically illustrates the sequence of the method according to the invention. The method comprises steps of treating S10 the surface of the active optical material and introducing S12 a silicon compound into the plasma etching chamber. Optionally, a further step of applying S14 OFCB or polymerization products of OFCB to the surface of the active optical material is provided. The method can be carried out, in particular, in a plasma etching chamber.

Claims

[1] A method for structuring a surface (32) of an active optical material (12) which is a laser-active material, a Faraday rotator and / or an optically non-linear material, comprising the steps: Treating (S10) the surface of the active optical material with a plasma etching process in a plasma etching chamber (10); Introducing (S12) a silicon compound into the plasma etching chamber, wherein an etching gas of the plasma etching process comprises a fluorine compound; and A nanostructure is created on the surface of the active optical material using the plasma etching process. [2] A method according to any one of the preceding claims, wherein the etching gas does not contain fluorocarbon compounds. [3] Method according to one of the preceding claims, wherein the etching gas is SF 6 includes. [4] Method according to one of the preceding claims, wherein the etching gas SiF 4and the silicon compound in the form of SiF 4 is introduced into the plasma etching chamber (10). [5] Method according to one of the preceding claims, wherein the silicon compound is introduced into the plasma etching chamber (10) in the form of a silicon dioxide sacrificial glass. [6] Method according to one of the preceding claims, wherein the plasma etching process is terminated when the nanostructure has a structure height of 250 nm to 1500 nm. [7] A method according to any one of the preceding claims, wherein the active optical material (12) is cooled during the plasma etching process. [8] Method according to one of the preceding claims, comprising a step of applying (S14) octafluorocyclobutane, OFCB, and / or polymerization products of OFCB to the surface (32) of the active optical material (12) after completion of the plasma etching process. [9] A method according to any one of the preceding claims, wherein the plasma etching process is a combined physical and chemical etching process. [10] Active optical material (12) which is a laser-active material, a Faraday rotator and / or an optically non-linear material, having a surface (32) structured by a method according to one of claims 1 to 9.

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