Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
  • G02B1/10—Optical coatings produced by application to, or surface treatment of, optical elements
  • G02B1/11—Anti-reflection coatings

Definitions

• the invention relates to a method and to a device for producing an antireflective surface structure (for example for visible light), with a carrier layer on which a light-sensitive material layer is applied, which is obtained with at least two mutually coherent wave fields with a wavelength ⁇ B a stochastically distributed interference field is exposed, whereby the surface structure is formed during or after the exposure by means of targeted material removal.
• an antireflective surface structure for example for visible light
• a carrier layer on which a light-sensitive material layer is applied which is obtained with at least two mutually coherent wave fields with a wavelength ⁇ B a stochastically distributed interference field is exposed, whereby the surface structure is formed during or after the exposure by means of targeted material removal.
• reflective surfaces can be anti-reflective by providing the surface with a suitable roughness. Although no small portion of the incident light is reflected back into the room due to the roughening of the interface surface, light rays incident on the surface are reflected back in different directions due to the surface roughness. In this way, clear mirror images are avoided, that is to say light sources which would normally be reflected at the interface with sharp edges only lead to a fairly homogeneous brightening of the roughened interface. As a result, strong differences in luminance can be avoided and the interference effect that occurs with reflections can be considerably reduced.
• This type of anti-reflective coating is successfully used, for example, on displays with the name Antiglare layer.
• a major advantage of this anti-reflection technique is the ability to mold the structures using inexpensive embossing processes.
• a disadvantage of this type of anti-reflective coating is that the hemispherical reflection, i.e. H. the sum of specular and diffuse reflection in the entire rear area of the room, in the best case not being increased, as a result of which the background brightness of glass surfaces of screens prepared in this way is relatively high. Last but not least, this leads to a considerable reduction in the contrast of an image or display present behind such an anti-glare layer.
• interference layers Another option for anti-reflective optical surfaces is to apply suitable interference layers.
• the surface to be anti-reflective is coated with one or more thin layers with a suitable refractive index and a suitable thickness.
• the interference layer structure is designed in such a way that destructive interference phenomena occur in the reflected radiation field in suitable wavelength ranges, which, for example, greatly reduces the brightness of reflections from light sources.
• their image remains in the reflected beam path, in contrast to the above-mentioned anti-glare Layer, sharp. Even with a residual visual reflection of less than 0.4%, the sharp mirror images sometimes have a more disturbing effect than the relatively high brightness of Antiglare surfaces.
• the contrast ratio is good.
• interference layers are too expensive to manufacture for most screens and other applications.
• a third alternative to the anti-reflective treatment of optical surfaces consists in the introduction of so-called sub-wavelength gratings, which leads to a refractive index gradient on the interface of an optically transparent medium, as a result of which an optical effect is produced, as it were, by interference layers.
• Such a refractive index gradient is realized by surface structures if the structures are smaller than the wavelengths of the incident light.
• the manufacture of periodic structures by means of holographic exposure in a photoresist layer which is applied to the surface of a transparent medium is expediently suitable for this.
• Such sub-wavelength surface gratings with periods of 200 to 300 nm are suitable for broadband reflection reduction.
• Surfaces which are also known under the term “moth eye antireflection surfaces”, are described in an article by MC Hutley, SJ Willson, "The Optical Properties of Moth Eye Antireflection Surfaces", OPTICA ACTA, 1982, Vol 29, No.
• DE 19708776 C1 also discloses a method by means of which a combined surface structure can be obtained by superimposing a coarse-grained speckle pattern and the image of a sub-wavelength grating, which has properties of both an antireflection layer and an antiglare layer. Presentation of the invention
• the invention is based on the object of improving a method for producing a surface structure which is antireflective for visible light in such a way that on the one hand the proportion of light reflected back on the surface structure is considerably reduced and on the other hand the reflected portion of light is specifically reflected back into specific solid angle regions.
• the reflection images on the surface structure that occur in previously known surface structures, although they are greatly reduced in contrast but nevertheless present, are to be avoided as completely as possible, especially since the back-reflected light components are to be diffusely reflected back.
• the method according to the invention should allow the possibility of replicability of the surface structure obtained by means of conventional embossing methods, i.e. any undercuts that may occur within the surface structures that form are to be avoided entirely.
• a method for producing a surface structure which is antireflective for a specific wavelength range and has a smallest wavelength limit ⁇ with a carrier layer, on which a photosensitive material layer is applied, which is obtained with at least two mutually coherent wave fields with a wavelength ⁇ B a stochastically distributed interference field is exposed, as a result of which the surface structure is formed during the exposure or after the exposure by means of targeted material removal, such that the coherent wave fields which are interfering and directed towards the light-sensitive material layer form an angle ⁇ for which:
• the angular relationship is based on the requirement that when producing anti-reflective structures by means of stochastic surface structures, the maximum male lateral dimension of the individual structural elements of the stochastic surface structure should be smaller than the light wavelength that strikes the anti-reflective surface structures.
• the method according to the invention is used in particular for the production of anti-reflective or antireflective surface structures which, for example, should have an anti-reflective effect in the visible spectral range. This means that the individual structural elements in their lateral extension should not be larger than ⁇ M ⁇ approx. 380 nm, which corresponds to the short-wave limit of the visible spectral range.
• At least one of the interfering coherent wave fields should preferably have a stochastic amplitude and phase distribution. The more wave fields impinging on the light-sensitive material layer under the above angular relationship, the amplitudes of which are preferably of the same size, the better exposure results can be achieved.
• Wavelengths in the UV range are preferably suitable for producing such stochastic surface structures, so that, for example at an exposure wavelength of 364 nm (Ar-ion laser), there is an angular range of> 57 °, which consists of at least two mutually interfering wave fields to form the stochastic interference pattern is to be included.
• a reasonable upper limit for the angular range for is 180 °.
• the angular range When using shorter-wave exposure waves, for example ⁇ e of 266 nm (quadrupled Nd-YAG wavelength), the angular range already begins at 41 °.
• the stochastically distributed surface structures produced with the method according to the invention have high-frequency structure components as are known in analogy from telecommunications using Fourier formalism for interpreting signals which vary over time.
• the signals which vary locally for example the surface relief structures, can be spectrally analyzed in analogy. If periodic surface relief structures are involved, such as a sub-wavelength grating, only discrete spatial frequencies occur in the analysis.
• a stochastic surface relief structure, as obtained with the method according to the invention is characterized by a continuous spatial frequency spectrum.
• a special property of the stochastically distributed surface structures produced with the method according to the invention is the formation of such surface structures with spatial frequencies which are approximately of the same order of magnitude or larger than the inverse of the wavelength of the incident radiation.
• the greatest structural depths in the stochastic surface structure correspond to at least the order of magnitude of the smallest wavelength of the light striking the surface structure.
• the original design of such a stochastic surface structure requires a radiation source which emits light with a coherence required for the formation of a stochastic interference pattern.
• Particularly suitable light sources for this are UV light-emitting lasers, for example Ar-ion lasers, the light beams of which are brought into interference in a suitable manner with or without an upstream filter.
• the exposure waves ⁇ B should be equal to or less than those light wavelengths which are applied to the in a later application Hit the anti-reflective surface.
• a light-sensitive layer for example a photoresist layer
• the stochastic interference pattern is exposed to the stochastic interference pattern, as a result of which relief structures arise after or during the exposure due to the intensity distribution in the light-sensitive layer.
• the intensity distribution can cross-link low molecular weight polymers within the light-sensitive layer, which results in targeted deformations on the surface of the layer.
• surface structures are formed by the exposure of a photoresist layer and a subsequent development step or etching process.
• Electroplated matrices can be used as an embossing stamp or tool for large-area replication of microstructures, as a result of which many embossing stamps can advantageously be obtained from an original surface structure by copying.
• a structure can also be brought into a stamp by an etching process.
• the light waves of which strike the material layer can be exposed in a suitable manner.
• a single light source for example an excimer laser
• the light beam is preferably expanded divergently in order to illuminate the entire surface of a diffuser, the central region of which is made opaque.
• the diffuser is designed in such a way that light can only pass in its edge regions, as a result of which the light beams change in the beam direction Superimpose the diffuser in the manner specified according to the invention.
• the carrier layer with the corresponding light-sensitive material layer is arranged at a suitable point downstream of the diffuser.
• radiation sources with a defined intensity profile can be used. Additional masks, filters with speckle patterns or similar, beam-shaping optical means can be introduced into the beam path in order to generate the desired interference pattern.
• Fig. 1 radiation structure for producing a stochastic surface structure.
• FIG. 1 shows an irradiation setup with a light source 1, preferably an excimer laser, for example an Ar ion laser, which emits a coherent beam 2.
• a lens 3 is provided, which widens the light beam 2 to a diffuser unit 4, which provides an optically diffuse, transparent ring area 5 and is otherwise designed to be opaque.
• a carrier plate 6, on which a photoresist layer 7 is applied, is provided in the beam path downstream of the diffuser unit 4.
• the individual waves emanating from the diffuser unit 4 interfere on the side facing away from the light source in such a way that partial waves from opposite sectors of the diffuser unit, which is preferably designed as a ring diffuser, enclose the large angle ⁇ which results from the geometric specification of the ring region 5 and the distance determined between the diffuser unit 4 and the carrier plate 6. Due to the geometric specification, mainly light waves hit the Photoresist layer 7, which enclose a high angle of incidence relative to the plane of the photoresist layer 7, which results in surface relief structures on the photoresist layer by the corresponding exposure with subsequent development in the photoresist layer, which have high spatial frequencies with high amplitudes. This achieves the anti-reflective effect and a targeted redistribution of the back reflections.
• the stochastic surface structure has high-frequency structural components with amplitudes which ideally lie in the same order of magnitude as the typical lateral dimensions of these structural components.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Surface Treatment Of Optical Elements (AREA)

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• 2000
  • 2000-08-09 DE DE10038749A patent/DE10038749A1/de not_active Ceased
• 2001
  • 2001-08-09 WO PCT/EP2001/009233 patent/WO2002012927A2/de not_active Application Discontinuation
  • 2001-08-09 EP EP01976065A patent/EP1373943A2/de not_active Withdrawn
  • 2001-08-09 US US10/344,132 patent/US20040021948A1/en not_active Abandoned
  • 2001-08-09 JP JP2002517557A patent/JP2004506928A/ja active Pending

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