US20080223295A1

US20080223295A1 - METHOD FOR PRODUCING A TOOL WHICH CAN BE USED TO CREATE OPTICALLY ACTIVE SURFACE STRUCTRES IN THE SUB-nuM RANGE AND A CORRESPONDING TOOL

Info

Publication number: US20080223295A1
Application number: US12/104,467
Authority: US
Inventors: Frank Burmeister; Walter Doll; Gunter Kleer
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2000-08-10
Filing date: 2008-04-17
Publication date: 2008-09-18
Also published as: DE10039208A1; EP1307604B1; US7390531B2; ATE266106T1; US20040052946A1; WO2002012586A1; EP1307604A1; DE50102217D1

Abstract

The invention is a method for producing a tool which can be used to create optically active surface structures in the sub-μm range, having a support surface onto which relief surface structures are applied over the support surface by means of material deposition. The invention is distinguished by the support surface being directly contacted with a mask in which openings with diameters in the sub-μm range are provided or can be provided, by the support surface including the mask being subjected to a coating process in which the coating material deposits through the openings of the mask onto the support surface, and the mask is removed from the support surface when a partial amount of an average end structure height of the surface structures is reached and the coating procedure is then continued without the mask using the same coating material or different coating materials.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a method for producing a tool which can be used to create optically active surface structures in the sub-μm range, having a support surface onto which relief surface structures are applied by means of the deposition of materials. Furthermore, a corresponding tool is described.
2. Description of the Prior Art
In many light-optical applications, reflections of light have a disturbing and energy-diminishing effect on technical and optical surfaces and interfaces. For example, the readability of displays is distinctly impaired by the reflection of external light sources. Furthermore, in the case of glazed solar cell modules, reflection losses occur on the cover thereby considerably reducing the solar cell's efficiency.
To decrease reflection and increase transmission, technically complicated single layer or multi-layer systems have hitherto been applied to the surfaces in order to diminish reflection from them. These surface finishes are, however, not only expensive but also unsatisfactory, because their effect depends largely on the angle of incidence and the wavelength of the light impinging on the surfaces.
Most recent antireflective methods for reducing reflection pursue a different approach, notably to design the transition in the refractive index at the surface of each optical component and to not vary abruptly but rather vary continuously. In order to do this, each surface is “roughened” and structured in order to copy the “moth-eye effect” known from nature. In designing such type surface structures, the average lateral dimensions of each structure must be distinctly smaller than the wavelength of the incident light in order to prevent the light from scattering and the increased diffuse reflection accompanying it. Furthermore, for an effective reduction of reflection, it is necessary that the structures have an aspect relationship, that is the relationship between the vertical to the lateral dimensions of the surface structures, in the magnitude of 1 or higher.
The production of such type surface structures with suitably shaped tool surfaces using hot stamping or injection molding processes by, for example, heating an optical element for surface coating by means of hot stamping until above the glass melting temperature of the material of which it is composed and structuring it at its surface accordingly with a stamping tool bearing a negative image of the microstructuring. Such types of stamping tools with a microstructured stamp can be made of nickel using galvanic methods, in which periodic surface microstructures can be obtained using photolithographic techniques. Such types of periodic surface structures are very efficient in their optical effect and far superior to the prior art stochastically distributed microstructure surfaces. They, however, have the drawback that they are produced with nickel. However, such types of stamps, also called “masters”, possess low high-temperature resistance due to the material they are made of, and can therefore only be utilized for shaping the surface of organic glasses, such as for example optical elements made of PMMA. Moreover, the lifetime of such nickel master tools is short as distinct wear can be observed with repeated use due to their material. Finally, the production method of periodic structures using photolithography has the disadvantage that open surfaces or aspherical surfaces, often needed in technology, cannot be structured.
Milled steel tools coated with a layer of mechanically resistant, high-temperature resistant, ceramic material are prior art for the aforedescribed tools for transferring periodic structures onto preferably optical surfaces. With a suitable selection of layer deposition parameters, these layers fulfill basically the aforeset requirements with regard to the aspect relationship and the lateral dimensions of the microstructures but not with regard to the regular, periodic arrangement of the surface structures as is the case with the aforementioned nickel master stamps.
Due to the non-periodicity of the microstructures applicable in the layers of mechanically resistant ceramic materials, these surface structures are called stochastic surface structures. In these stochastic surface structures not only the lateral arrangement of the single elements of the structure in the mechanically resistant ceramic material are irregular, but the quite varied growth of the single elements of the structure can be observed. The presence of large structural elements within the microstructure surface, however, leads to an undesiderable increase in diffuse reflection and at the same time to a decrease in direct reflection when being formed. With reference to this see: A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, V. Wittwer, “Subwavelength-Structured Antireflective Surfaces on Glass”, Thin Solid Films 351 (1999), No. 1-2, pp. 73-78.
The article “Fabrication of a Micro-Patterned Diamond Film by Site-Selective Plasma Chemical Vapor Deposition”, by Sakamoto et al., Thin Solid Films, Elsevier-Sequoia S. A. Lausanne, CH, vol. 334, No. 1-2, 4 Dec. 1998 (1998-12-04), pp. 161-164, ISSN:0040-6090, describes a method of selective coating of substrates with a diamond film. Such types of substrates provided with diamond film structures are used in promising applications in technology, such as for example in micro-electronics for field effect transistors. In order to obtain selective diamond depositions with dimensions of the structures in the sub-μm range, a property of diamond is utilized that it deposits on a platinum layer with approximately a 10⁴greater affinity than, for example, on a SiO₂surface. If suited platinum structures are provided on a substrate surface, all that is needed is full-surface PVD coating of such a type of pre-structured SiO₂substrate with diamond, which only deposits significantly on the Pt-structured areas. The selective, diamond coated substrate yielded by this method meets microelectronic needs, but it cannot be used as a heat-resistant tool, in the sense described in the introduction, due to the purposely selective coating.

SUMMARY OF THE INVENTION

The present invention is a method for producing a tool and to provide the tool which can be utilized for fabricating optically active surface structures in the sub-μm range and which has a support surface on which relief surface structures are applied over the support structure by means of material deposition that, on the one hand, prevents the drawbacks arising with the aforedescribed stochastic surface structures, that is undesirable high diffuse and direct reflection, and, on the other hand, nonetheless ensures utilizing the tool under high temperatures in order to be able to provide antireflective optical or technical surfaces made of not only organic materials but also of inorganic materials with it.
According to the present invention, a method for producing a tool is disclosed, which can be used to create optically active surface structures in the sub-μm range and which has a support surface formed or processed to a desired macroscopic contour on which relief surface structures are applied by means of material deposition over the support surface, is further developed in such a manner that the support surface is directly contacted with a mask which is provided or can be provided with openings with diameters in the sub-μm range. Then the support surface with the mask is subjected to a coating process in which the coating material is also deposited through the openings of the mask onto the support surface. Upon reaching a partial amount of an average height of an end structure of the desired surface structures, the mask is removed from the support structure and the coating procedure is then continued without the mask. The partial surface structures formed directly after removal of the mask are used as growth or crystallization seeds for the continued coating procedure by means of which material deposition and further crystal growth occurs preferably at the crystallization seeds.
A particularly favorable time to remove the mask from the support surface is reached when the developing elements of the surface structures have reached approximately 0.5% to 80% of their desired end height.
By dividing the coating procedure into two parts, single surface structure elements are obtained whose lateral structure dimensions diminish with increasing height of the structure due to the subsequent, preferred material deposition on the coating seeds. In this manner, column-like formations with diminishing diameters are formed as surface structures on the support surface. In this manner protrusions can be completely avoided so that transfer of the surface structure onto optical or technical surfaces is readily possible in a molding procedure.
The arrangement with which the individual surface structure elements are deposited on the support surface is determined by the arrangement of the openings in the mask. In the simplest case, a layer of conventional photoresist varnish applied uniformly onto the support surface serves as the mask material. With the aid of suitable photolithographic exposure techniques, openings of any desired size and distribution can be exposed into the photosensitive varnish layer in dependence on the exposure patterns.
Another possible way to produce the desired mask uses an aqueous suspension in which the colloidal, bead-shaped particles are dispersed and which are applied to the support surface. After a drying process, the colloidal particles remaining on the support surface form the mask layer with wide gaps between the particles corresponding to the openings through which material deposition forming the aforedescribed surface elements can occur. Further details for producing such a mask layer are given in the following sections of the description with reference to the accompanying drawings.
Especially suitable as materials for forming the surface structure are the following elements: Ti, Al, Zr, Cr, Mo, Ni, W, Si B, and C as well as mixtures of elements of the just listed examples of elements. Furthermore, at least one of the layer-forming elements may be at least partially oxidized or nitrated.
Suitable as materials for developing crystallization seeds in the first coating step are, in addition to the aforementioned elements, particularly also Ag, Au, Pd, Pt.
With the aid of such a coating, it is possible to produce a high-temperature resistant layer which can withstand even temperatures up to over 850° C. and permits molding of the surface structure on optical or technical surfaces made not only of organic, preferably, light-transparent plastics but, in particular, also structures composed of inorganic materials, such as glass.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is made more apparent in the following using preferred embodiments with reference to the accompanying drawings by way of example without the intention of limiting the scope or spirit of the overall inventive idea.

FIGS. 1 a) to d) show a schematic for producing surface structures on a support surface;

FIG. 2 shows a representation of surface structure elements on the support surface;

FIG. 3 shows a cross section of a coated support surface bearing surface structure elements; and

FIGS. 4 a) to c) show the production of a mask of colloidal particles and their use as a mask for applying coating seeds.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 a) to d) show on the left a lateral view and on the right a top view of the respective steps of the method. FIG. 1 a depicts a lateral and a top view of the support of a tool, with a support surface 1 which is to be selectively coated. In FIG. 1 b), a mask 2, which is provided with a multiplicity of openings 3 where the support surface 1 is surrounded free of the material of mask 2, is placed directly on the support surface 1. The mask material 2 can, for example, comprise a photoresist, which is removed locally and selectively at the sites of openings 3 after corresponding exposure and the subsequent etching process. Depending on the manner of exposure, the openings 3 form in a specific configuration within the mask 2. A top view of FIG. 1 b) shows an example of opening geometry and a corresponding configuration. The openings 3 represent dark areas on the support surface with the remaining white areas being covered by the mask material 2.
The mask 2 provided with openings 3 is then coated in a subsequent coating procedure in which particularly high-temperature resistant materials are deposited, preferably with the aid of a PVD process. In FIG. 1 c), the mask 2 and the openings 3 are covered by a coating material 4, which deposits directly onto the support surface 1 particularly inside the openings 3. The coating of support surface 1 and the mask 2 can be interrupted as soon as the height of the coating inside the openings 3 reaches a level corresponding to approximately 0.5% to 80% of the average projected end structure height of the individual elements of the surface structure. When such a layer thickness is reached, mask 2 and the layer 4 are removed from support surface 1 with the aid of selective etching techniques. The material deposits 5 remain as single surface structure elements which have deposited directly on the support surface inside the openings 3, as FIG. 1 d) shows both in a lateral view and a top view. Then the coating process is continued and the surface structure elements 5 remaining on the support surface 1 after removal of the mask serve as coating seeds as shown in FIG. 2. In the further course of the coating process, the coating material is deposited on the entire support surface but deposited with preferred growth of the single structures in the region of the seeds.
The surface structures created with the aid of the aforedescribed method usually have, depending on the openings 3 inside the mask 2, 6 to 400 surface structure elements per μm². Moreover, the single elements of the structure are usually spaced between 50 and 400 nm apart.
Especially good properties can be obtained if the surface structure has single surface structure elements whose respective size and shape can be divided into at least two different groups of surface structure elements. Fundamentally, the structure elements of one group are larger than those of the other group. Furthermore, the lateral spaces of the centers of the single structure elements are distributed approximately at least two characteristic, average distances M1 and M2. The following relationships have proven particularly advantageous for these two average distances: the value M₁should lie in a region between 50 nm and 180 nm, whereas the average distance M₂has values between 200 nm and 400 nm. Especially good results are yielded, for example, if M₁=170 nm and M₂=300 nm. Furthermore, it was discovered that the relationship of M₂and M₁should preferably be greater than 1.1 and smaller than 8, in particular 1.72.
FIG. 3 shows a cross section of a tool with a support surface 1, which can preferably be created out of a metallic material, in particular out of a steel or out of ceramic materials or out of silicon. A multiplicity of surface structure elements 5 illustrated in FIG. 2, which are shaped column-like and taper upward, are deposited onto the support surface 1. Provided between the single surface structure elements 5 of FIG. 2 are material deposits also in different coating heights due to the continued coating procedure. The surface structure elements preferably have heights whose highest heights h.H. in relation to the respective surrounding lowest layer height differs by the factor 1.6 from the lowest surface structure elements n.H. in the layer. Furthermore, the single surface structure elements 5 illustrated in FIG. 2 preferably have structure heights H lying between 50 and 1000 nm.
FIG. 4 shows an alternative masking and coating method to the one described in FIG. 1. In this case, the masking of the support structure 1 occurs before the coating process by means of applying an aqueous suspension of colloidal particles 6. The suspension 6 is applied onto support surface 1 with the aid of conventional microtitter devices 7. The a drying process according to FIG. 4 b occurs resulting in a densely packed two-dimensional arrangement of particles on the support surface. This is followed by the coating procedure 8 required for placing crystallization seeds. The colloidal particles 9 remaining on the support surface 1 cover the support surface in such a manner that gaps form between the immediately adjacent colloidal particles 9 corresponding to the aforedescribed openings 3. The size of the particles 9 determines the spacing as well as the size of the openings 3. Preferably, the particles 9 can be composed of polymer materials or inorganic materials, such as for example silicon oxide and usually have a diameter of 50 nm to 50 μm. FIG. 4 c shows a coating pattern yielded after the colloidal particles have been coated and correspondingly removed from the support surface 1.
The colloidal particles 9 can be selected in such a manner that their sizes are distributed by an average value (with a monomode size distribution being reached) or by at least two concrete values (with a bimode distribution being reached).
The invention provides novel tools for surface structuring technical and optical surfaces. These tools are, in particular, high-temperature resistant. Furthermore, the applied new, submicrostructured mechanically resistant layers, preferably on steel, have the following advantages: the suitable selection of the material to-be-applied onto the support surface permits obtaining a high-temperature resistance that allows using such molding tools not only for organic glasses but also for inorganic glasses, for example silicate glass.
The coating procedure is not restricted to plane support surfaces. With the aid of the method of the invention practically any desired contour can be coated permitting, in particular producing an antireflective, non-plane optical components, such as for example spheres.
A special variant of the tool designed according to the invention provides structuring the support surface before it is subjected to the coating procedure in the aforedescribed manner. In this case, the support surface is provided with indentations partially or full-surface. At the surface, the diameter of these indentations lies in the range between 1 μm and 30 μm. The indentations reach with approximately the same dimensions into the interior of the support substrate in such a manner that an aspect relationship of approximately 1 is present. The indentations are evenly distributed over the support surface and border each other directly with their on-the-support-surface-lying peripheral edges.
If the preceding measures are met, based on the method of the invention, after fabrication, a tool is obtained for producing technical surfaces can be produced which possess dirt-repelling and or oil-repelling properties.

Claims

1. A tool for surface structuring and fabrication of optically active surface structures on a support surface in a sub-μm range, comprising:

a layer of high-temperature resistant material applied onto the surface support having surface structures over the support surface, the surface structures including dimensions in the sub-μm range having heights between 50 and 1000 nm and including distances apart between 50 and 400 nm.

2. The tool according to claim 1, wherein:

the surface structures comprise substantially at least one identical discrete shape and identical discrete size.

3. The tool according to claim 2, wherein:

the surface structures are divided into at least two discrete shape and discrete size groups which are arranged in order on the support surface.

4. The tool according to claim 3, wherein:

centers of the surface structures each have mutual lateral spacing distributed by at least average distances M₁and M₂, wherein for M₁and M₂:

M₁=M₂or

M₂>M₁, with 50 nm<M₁<180 nm and 200 nm<M₂<400 nm or

with M₁=170 nm and M₂=300 nm or

with 1.1<M₂/M₁<8 or

with M₂/M₁=1.72.

5. The tool according to claim 1, wherein:

a magnitude of a greatest difference in height between a highest surface structure and a surrounding indentation on the support surface is less than 1.6 times the magnitude of a smallest difference in height between a smallest surface structure and an indentation surrounding the smallest surface structure.

6. The tool according to claim 1, wherein:

the surface structures have different structural heights and elevations measured from respective indentations surrounding the surface structures, which range between 50 and 1000 nm.

7. The tool according to claim 1, wherein:

the support surface is made from one of metal, ceramic material, quartz glass or silicon.

8. The tool according to claim 1, wherein:

the support surface, prior to application of the layer, is smooth or structured.

9. The tool according to claim 7, wherein:

the support surface, prior to application, has indentations with a diameter at the support surface and depth within the range from 1 μm to 30 μm.

10. The tool according to claim 9, wherein the indentations have aspect relationships in an order of a magnitude of 1.

11. A tool as defined in claim 1 comprising antireflective surfaces.

12. A tool to claim 11, wherein:

the antireflective optical surfaces comprise organic or inorganic materials.