Etching a substrate: processes – Forming or treating optical article
Reexamination Certificate
2000-12-19
2003-08-12
Alanko, Anita (Department: 1765)
Etching a substrate: processes
Forming or treating optical article
C216S041000, C216S049000, C216S056000, C427S248100, C427S255370, C427S243000, C427S162000, C427S407100, C427S340000, C521S064000
Reexamination Certificate
active
06605229
ABSTRACT:
The present invention relates to a process for producing an element comprising a substrate and at least one antireflection coating having pores, the dimensions of the antireflection coating being below the wavelength of visible light or the neighboring spectral ranges, and also an element produced according to this process having at least one antireflection coating, in particular optical lenses, mirrors or other optical components, such elements having an essentially improved optical antireflection coating.
The use of thin coatings for the antireflection coating of optical components is well-known. To reduce the reflected light at the interface of two optical media having different refractive indices, an optical coating whose refractive index is between that of the two optical media is applied to this interface.
It is further known that this antireflection coating can be improved by using several coatings instead of one coating and alternating coatings having high and low refractive indices. For example, an antireflection coating having three layers reduces the reflectivity of a glass-air interface from 4.3% of the total intensity to less than 1% above the total visible spectrum. Optical antireflection coatings are in particular important for lens systems having several air-glass transitions, since the reflection losses add up at each interface.
Calculating the reflectivity of the interface between two media having the refractive indices n
0
and n
1
allows one to determine the optical properties of an antireflection coating. A coating whose refractive index equals the root of the product of n
1
×n
0
is optimum. For monochromatic light the reflectivity drops to 0 when the optical thickness of the coating is one-fourth the wavelength of the radiated light. For air and glass having n
0
=1 and n
1
=1.52, for example, the optimum refractive index of an antireflection coating is 1.23. However, the magnesium fluoride material frequently used for antireflection coatings has a refractive index of 1.38 and thus affects a reduction in the reflectivity at the glass-air interface by merely 1.3-2%. Materials having refractive indices lower than 1.3 in the visible wavelength range or in the neighboring infrared or ultraviolet ranges are unknown.
Applying an antireflection coating optimally reduces the reflectivity of the interface at the wavelength corresponding to one-fourth the optical thickness of the antireflection coating. Other wavelengths produce a higher reflectivity of the incident light. The reflectivity can be reduced uniformly in a broader wavelength range with the use of several antireflection coatings. In this case the optical properties of a series of antireflection coatings can also be determined via calculations. Several coatings, each of whose optical thickness equals one-fourth a so-called reference wavelength, are optimum. The refractive index of multicoating systems should ideally vary progressively between the refractive indices of the two media. For a glass-air interface this is not currently feasible with known media, since materials having refractive indices below 1.3 are not available.
A more recent innovation to the described technology consists of the use of so-called nanoporous materials. Materials having pores or air pockets whose dimensions are below the wavelength of visible light have an effective refractive index indicated by the average of the refractive index of the material and that of air. By varying the number of pores per volume and/or by varying the total percent by volume of pores, the refractive index can thus be set continuously by the refractive index of the substrate at a refractive index that is approximately 1. At this time two processes of the prior art which utilize this more recent innovation are known, namely a process that uses the sol-gel method, and a so-called embossing process. However, both processes have the decisive drawback that producing coatings having the desired properties is very expensive and moreover permits only the production of single coatings.
Thus, the problem underlying the present invention is to provide a process which is as simple, fast and affordable as possible for producing such elements comprising a substrate and at least one reflection-reducing coating, such as for example optical lenses, mirrors and other optical components, so that such elements have an essentially improved optical antireflection coating. A further problem of the present invention is to provide an element which in the case of a substrate-(e.g. glass)-air interface has a refractive index below 1.3.
These problems are solved by the embodiments characterized in the claims. In particular, there is provided a process for producing an element comprising a substrate and at least one antireflection coating having pores, the dimensions of the antireflection coating being below the wavelength of visible light or neighboring spectral ranges, comprising the steps
preparing a substrate,
applying a solution of at least two mutually incompatible polymers which are dissolved in a common solvent in such a manner that a common intermixed phase is produced, to a substrate, phase separation on its surface producing a coating having essentially laterally alternating polymer phases, and
exposing this coating to another solvent so that at least one polymer remains undissolved.
When the solution of at least two mutually incompatible polymers is applied in accordance with the process of the present invention, a phase separation takes place in a two-component or multicomponent (“incompatible”) mixture of macromolecular substances which are essentially immiscible, i.e. incompatible. Macromolecular substances suitable for this process are in particular polymers or oligomers, hereafter referred to collectively as polymers. Fundamentally, no restrictions of any kind are placed on the polymers that can be used in the scope of the process in accordance with the present invention, except that in the polymer combinations thus utilized the polymers used are essentially not mutually miscible, i.e. incompatible. Using known polymers in the polymer combinations or mixtures such as e.g. polystyrene, polymethyl methacrylate, polymethacrylate, polyacrylate, polyvinyl pyridine or polyvinyl chloride, all of which have refractive indices around 1.5, antireflection coatings having refractive indices below 1.3, for example even below 1.2, can be produced in accordance with the process of the present invention. Examples of mutually incompatible polymers that can be used are polystyrene and polymethyl methacrylate or polystyrene and polyvinyl chloride. The refractive index of the antireflection coating is the root of the product of the refractive index of the optical substrate and of the neighboring medium.
The process in accordance with the present invention makes it possible to obtain elements of, for example, glass, Plexiglas and polycarbonate having a refractive index less than 1.3 in the case when the neighboring medium is air. For antireflection coatings which consist of several layers or coatings, if one or more layers having a refractive index greater than 1.3 are present, one or more of the additional coatings must have a refractive index less than 1.2 or less than 1.1. If fluorinated polymers, such as e.g. DuPont's commercially available Teflon AF, having a refractive index of approximately 1.3 are used, antireflection coatings having refractive indices below 1.1 are obtainable according to the process in accordance with the present invention.
The selected polymers are dissolved in a substance that acts as a solvent for all components, such as for example toluene, benzene, tetrahydrofuran, ethanol, acetone and methanol, in such a manner that a common intermixed phase is produced. A coating or film is then produced from this solution on a suitable substrate via spin coating, dip coating, spray coating or other type of application. Assuming that the selected polymers are essentially not mutually miscible, the phase separation occurs while the coating is
Eggert Stefan
Mlynek Jürgen
Schäffer Erik
Steiner Ullrich
Walheim Stefan
Alanko Anita
Burns Doane Swecker & Mathis L.L.P.
Universitat Konstanz
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