DE102015119325A1 - Method for smoothing surfaces of a workpiece - Google Patents

Abstract

The invention relates to a method for smoothing surfaces of a workpiece. The method comprises the steps of: a) generating a plasma beam by microwave induction in a gas medium containing no hydrogen or hydrogen-containing compounds and no halogens or halogen-containing compounds; and b) treating a workpiece surface to be smoothed with the plasma jet.

Description

The invention relates to a method for smoothing surfaces of a workpiece.

Technological background

For smoothing surfaces of hard, brittle and optical materials, various methods are available that are based on different principles. In particular, high-precision optical systems with glass optics represent a key component in numerous applications with high growth rates (measurement and production technology, optical communications, medical and pharmaceutical technology, electronic consumer goods), whose quality significantly contributes to the performance of the overall solution and often paves the way for innovation. For technological reasons, the design of optical components is evolving towards increasingly complex geometries. Despite the technological advantages of aspheres and free-form surfaces, currently a high production outlay, in particular during polishing, and an expensive measuring technique limit the extension of the application. In optics production, polishing is an indispensable step in the technology chain, which crucially determines the quality, but also causes the greatest effort.

State of the conventional technique is the mechanical-abrasive polishing with disc or point tools including magneto-rheological polishing (MRF) and computer aided polishing (CCP) by means of a magnetic polishing paste or a polishing suspension, wherein the smoothing is always associated with a material removal, in particular For very small, strongly curved or free-form surfaces leads to a change in shape.

All plasma-chemical etching processes such as CVM (Chemical Vapor Machining) [see, for example EP 0 300 224 A1 ], PACE (Plasma Assisted Chemical Etching) [see, for example US 5,811,021 ], LDE (Local Dry Etching) [see for example EP 1 054 443 A1 ], PJCE (Plasma Jet Chemical Etching) [see, for example DE 199 25 790 A1 ] and RAPE (Reactive Atom Plasma Etching) [see, for example US 2002/100751 A1 ] have in common that the reactive species are obtained by electromagnetic excitation and dissociation of gases or gas mixtures with halogen-containing components (eg CF ₄ , CHF ₃ , SF ₆ , NF ₃ , CCl ₄ ) and this one caused mainly by chemical reactions material removal at the Generate the surface of the workpiece to be machined. Smoothing effects could only be detected after sufficiently large material removal (up to several 100 μm) (PACE, RAPE).

In DE 42 34 740 C2 describes a method in which a pre-structured glass surface is melted using a complex electron beam process in a vacuum and thereby smoothed.

In the German patent application DE 10 328 250 A1 "Surface Treatment Method" describes a plasma treatment of surfaces that propagates a change in surface shape while maintaining or improving surface roughness. The corresponding method is therefore also suitable in principle for smoothing rough or structured surfaces, but includes both a material removal, as well as hydrogen or hydrogen-containing compounds as plasma gas or plasma gas mixture. Hydrogen can be driven into the material by diffusion and lead to the shattering of near-surface layers and thus to the destruction of the workpiece.

The irradiation of glasses by means of high-power lasers (eg CO ₂ lasers) is also used for polishing or smoothing surfaces or for breaking edges. An example of this is in the European patent specification EP 1 871 566 B1 "Method for fine polishing / structuring of heat-sensitive dielectric materials by means of laser radiation" described. This method is based on purely thermal modification of the surface in the spatially very limited interaction zone with the laser and requires temperatures just below the vaporization point of the glass material, which requires extremely accurate process control and complicates process control. Larger surfaces must be machined by very fine laser scanning, which can lead to the formation of superstructures and consequent ripples due to overlap effects. Therefore, a given surface waviness can not be improved by this process.

There is therefore a continuing need for a process for subaperture surface treatment which overcomes or at least reduces the described disadvantages of the prior art. In particular, the method should be useful for direct, flexible and economical smoothing of surfaces of hard, brittle and optical materials, such as quartz glass and ULE (Ultra-Low Expansion Glass) glass for optically relevant workpieces such as lenses, mirrors, mirror substrates and impression masters ,

Summary of the invention

• a) Erzeugen eines Plasmastrahls durch Mikrowelleninduktion in einem Gasmedium, das keinen Wasserstoff oder wasserstoffhaltige Verbindungen sowie keine Halogene oder halogenhaltige Verbindungen enthält; und
• b) Behandeln einer zu glättenden Werkstückoberfläche mit dem Plasmastrahl.

One or more disadvantages of the prior art are with the help of inventive method for smoothing surfaces of a workpiece fixed or at least mitigated. The method comprises the steps:

• a) generating a plasma beam by microwave induction in a gas medium containing no hydrogen or hydrogen-containing compounds and no halogens or halogen-containing compounds; and
• b) treating a workpiece surface to be smoothed with the plasma jet.

In step a) of the method, therefore, a microwave-induced plasma jet is generated, for example with the aid of a suitably modified MPT (Microwave Plasma Torch). The gas or gas mixture used to generate the plasma jet contains inert components (in particular Ar and He), but no hydrogen or hydrogen-containing compounds and no halogens or halogen-containing compounds. In step a), the generation of the plasma jet is preferably atmospheric pressure.

By excluding halogen- and hydrogen-containing compounds to produce the plasma jet, there is no material removal on direct contact with the surface, especially in the processing (smoothing) of quartz glass or quartz-like materials (e.g., ULE).

Furthermore, by the combination of convective and microwave heating, a significant depth effect, which in addition to the superficial smoothing especially near-surface defects can be cured, resulting in an improvement of the Laserzerstörschwelle or reduced roughening by subsequent etching (plasmachemisches- or wet-chemical etching).

The structure is chosen so that the plasma jet is in direct contact with the surface to be processed (step b)) and the surface is traversed by a linear, spiral and / or rotating movement of the plasma jet relative to the surface. Upon direct contact of the plasma jet with the surface, the plasma jet can act as a microwave conductor. This can lead to a coupling of microwave power into the surface and thus to an additional, local temperature increase by conversion of dielectric power loss. As a result of the heating in conjunction with the convective influence of the plasma jet and the interfacial tension, the material is subject to redistribution and thus smoothness, which, in addition to the high-frequency and medium-frequency components, detects the roughness and waviness. Plasma jet and workpiece surface can be moved relative to each other at a constant or changing speed.

In step b) of the method, a surface temperature is preferably set which is above the melting point, but below the evaporation point of the workpiece. The surface temperatures should be well below the evaporation point, e.g. for quartz glass in the range 1500 to 2000 ° C. Preferably, the workpiece is preheated to a temperature> 100 ° C. The plasma jet can strike the surface perpendicularly or at an angle.

According to a further preferred variant of the method, a surface is processed simultaneously with a plurality of plasma jets.

By setting suitable plasma jet parameters, the smoothing factor as well as the smoothness rate can be influenced. Typical values for the smoothing factor, defined as the ratio of the roughness before and after the smoothing, are between 100 and 1000. The smoothness rates are typically 0.1-10 cm ² / min depending on the smoothing factor. After step b), the surface preferably has a roughness value of Ra <1 nm at spatial wavelengths of less than 10 μm.

In addition, under suitable conditions, an effective smoothing (roughness after processing <0.5 nm rms) of rough starting surfaces without material removal results.

• • eine Oberflächenglättung erfolgt durch Materialumverteilung ohne Materialabtrag;
• • die Ausgangsrauheit des Werkstücks wird verbessert;
• • die räumliche Ausdehnung des Plasmastrahl erlaubt eine lokale Glättung mit lateraler Auflösung < 5 mm;
• • die axiale Ausdehnung des Plasmastrahls lässt eine Bearbeitung stark gekrümmter Oberflächen zu;
• • das Verfahren ist anwendbar auf Mikrooptiken (< 2mm Durchmesser);
• • das Verfahren ist anwendbar auf Werkstückkanten; und
• • oberflächennahe Defekte werden durch die Plasmabehandlung ausgeheilt.

In summary, the method according to the invention has the following advantages:

• • surface smoothing is achieved by material redistribution without material removal;
• • the starting roughness of the workpiece is improved;
• • the spatial extent of the plasma jet allows a local smoothing with lateral resolution <5 mm;
• • the axial extent of the plasma jet allows processing of strongly curved surfaces;
• • the method is applicable to micro-optics (<2mm diameter);
• • the method is applicable to workpiece edges; and
• • Near-surface defects are healed by the plasma treatment.

The method is particularly suitable for smoothing a surface of an optical workpiece made of optical glasses or glass ceramic, preferably quartz glass, or comparable materials used for optics. The plasma-assisted method can be used in particular for the direct smoothing of the surface of optical workpieces, such as lenses, mirrors, mirror substrates or mastermasters of quartz glass or ULE.

Following the plasma smoothing, the workpiece may be suitably annealed to reduce or eliminate existing stresses in the workpiece.

Further preferred embodiments of the invention will become apparent from the following description.

Brief description of the figures

The invention will be explained in more detail with reference to an embodiment and associated drawings. The figures show:

1 a schematic representation of the plasma slab according to the invention in one embodiment;

2 a schematic representation of the movement of a contact point between the plasma jet and the workpiece surface; and

3 a profile measurement and evaluation of the roughness and waviness of the cross section through a partially smoothed with the inventive method surface of quartz glass.

Detailed description of the invention

1 schematically illustrates the machining of an optical workpiece 1 made of quartz glass with free-form surface. It is done with the help of a plasma source 5 a microwave-induced plasma jet 4 over the workpiece surface 2 guided. To generate the plasma, a gas mixture of argon and helium is used under atmospheric pressure. The microwave excitation takes place with a frequency of 2.45 GHz.

Microwave induced plasma sources (MIP) have been used for several decades. These plasma sources are generally composed of various high frequency components such as resonators, matching elements, etc. For the purposes of the method according to the invention are particularly suitable so-called plasma torches (MPT Microwave plasma torch). Simple transformation circuits allow the MPTs to achieve very good power matching to the RF generator for broad ranges of microwave power and gas flow rate. MPT can also be used at atmospheric pressure. With a suitable guide attachment, the plasma can then be guided over the surface of the material.

The distance and angle of the plasma source 5 For the material surface normal, it is appropriate to choose such that a) the contact point 6 between plasma jet 4 and material surface 2 remains and b) a temperature in the material surface is sufficiently large. For quartz glass, the temperature entry should be controlled so that at the contact point 6 a temperature in the range of 1500 to 2000 ° C sets.

In addition, the workpiece can 1 during processing via a hotplate 3 To avoid stresses in the material to be tempered. The heating temperature depends on the specific choice of material and dimensioning of the workpiece 1 from.

The movement of a contact point 6 between plasma jet 4 and workpiece surface 2 can, as in 2 represented, according to a predetermined raster path 7 respectively. The movement of the contact point 6 is not on the grid path shown 7 limited, but may for example also have the shape of a spiral or the contact point 6 can by arbitrary path over the material surface 2 be guided. The movement can be done with constant or variable, but for the purposes of smoothing sufficiently low speed.

3 is a height profile measurement using laser profiler to take the evaluation of the roughness and waviness of a partially smoothed surface of a workpiece made of quartz glass. On the left side of the profile is the area of the material surface without plasma jet smoothing and on the right side of the profile the area of the material surface with plasma jet smoothing is shown. The plasma smoothing was carried out according to the procedure described above. The temperature in the right-hand area of the material surface was approximately 1500 to 2000 ° C. at the point of contact between the plasma jet and the material surface.

The action of a microwave-induced plasma jet from the inert Ar / He gas mixture under atmospheric pressure resulted in a local transformation of the finely ground material surface (initial roughness Ra> 100 nm) into an optically smooth surface with a roughness Ra <1 nm. A significant improvement in the surface ripple was also observed. The redistribution and thus the smoothing of the substrate material takes place without material removal. White light microscope images of the smoothed material surface (measurement area of 160 × 120 μm) provided a roughness value of Ra <0.3 nm.

In the classical production of optical surfaces, the workpiece is formed by lapping and grinding processes and the surface is then smoothed by polishing processes using abrasive fluids. In this case, near-surface defects can occur (so-called sub- Surface Damage). Processing of such a faulty surface with the method according to the invention heals these disturbances and thus prevents the roughening of the surface during subsequent chemical or plasma-chemical etching processes.

Thus, in a series of experiments, conventionally smoothed quartz glass surfaces smoothed by the process according to the invention were plasma-chemically etched (erosion 21 μm) and further investigated. The conventionally smoothed quartz glass surface showed a roughness value of Ra> 10 nm in white light microscope images, while the surface pretreated with the method according to the invention had a significantly lower roughness value of Ra <1 nm.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been 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.

Cited patent literature

• EP 0300224 A1 [0004]
• US 5811021 [0004]
• EP 1054443 A1 [0004]
• DE 19925790 A1 [0004]
• US 2002/100751 A1 [0004]
• DE 4234740 C2 [0005]
• DE 10328250 A1 [0006]
• EP 1871566 B1 [0007]

Claims (8)

Method for smoothing surfaces of a workpiece ( 1 ), comprising the steps of: a) generating a plasma jet ( 4 by microwave induction in a gas medium containing no hydrogen or hydrogen-containing compounds and no halogens or halogen-containing compounds; and b) treating a workpiece surface to be smoothed ( 2 ) with the plasma jet ( 4 ). Method according to Claim 1, in which, in step a), the generation of the plasma jet ( 4 ) Atmospheric pressure prevails. A method according to claim 1, wherein in step b) a surface temperature above the melting point, but below the evaporation point of the workpiece ( 1 ) is given. Method according to Claim 1, in which the workpiece ( 1 ) is preheated to a temperature> 100 ° C. Method according to claim 1, wherein in step b) plasma jet ( 4 ) and workpiece surface ( 2 ) are moved relative to each other at a constant or changing speed. The method of claim 1, wherein the workpiece is an optical workpiece made of optical glass or glass ceramic. The method of claim 1, wherein the surface after step b) has a roughness value of Ra <1 nm at spatial wavelengths of less than 10 microns. The method of claim 1, wherein simultaneously with multiple plasma jets the surface ( 2 ) of the workpiece ( 1 ) is processed.

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