Antireflective graded index silica coating, method for making
First Claim
1. A radiation-transmitting non-porous vitreous substrate of predetermined refractive index having coated on at least one surface thereof a substantially non-reflecting coating which functions effectively over a predetermined wide band of radiations, said non-reflecting coating comprising:
- a single silica layer overcoated on said non-porous substrate and having an exposed surface, the thickness of said silica layer as measured between the exposed surface thereof and the silica layer-substrate interface being generally uniform and in the range of from about 150 nm to about 600 nm with the silica layer displaying throughout its thickness at least ome degree of porosity, and the porosity of said overcoated silica layer together with the non-porous nature of said substrate forming a defined interface therebetween;
all of said silica layer consisting essentially of silica except for other glass-forming constituents which may diffuse from said substrate and into said silica layer proximate said layer-substrate interface during the forming-processing of said silica layer on said substrate;
that portion of said silica layer which is immediately proximate the exposed surface thereof displaying the highest degree of porosity and the porosity of said silica layer gradually decreasing from the exposed surface thereof to a depth which is at least about one-quarter of the longest wavelength of said predetermined wide band of radiations to provide a graded degree of porosity which decreases as the distace from the exposed surface thereof increases;
that portion of said silica layer which is immediately proximate the exposed surface thereof having an index of refraction which approaches unity, and that portion of said silica layer which is immediately proximate the interface between said silica layer and said substrate having an index of refraction which approaches that of said substrate; and
the size of the individual pores in said silica layer being sufficiently small that radiations impinging upon the exposed surface of said silica layer are transmitted therethrough without being significantly scattered.
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Accused Products
Abstract
Antireflective silica coating for vitreous material is substantially non-reflecting over a wide band of radiations. This is achieved by providing the coating with a graded degree of porosity which grades the index of refraction between that of air and the vitreous material of the substrate. To prepare the coating, there is first prepared a silicon-alkoxide-based coating solution of particular polymer structure produced by a controlled proportion of water to alkoxide and a controlled concentration of alkoxide to solution, along with a small amount of catalyst. The primary solvent is alcohol and the solution is polymerized and hydrolized under controlled conditions prior to use. The prepared solution is applied as a film to the vitreous substrate and rapidly dried. It is thereafter heated under controlled conditions to volatilize the hydroxyl radicals and organics therefrom and then to produce a suitable pore morphology in the residual porous silica layer. The silica layer is then etched in order to enlarge the pores in a graded fashion, with the largest of the pores remaining being sufficiently small that radiations to be passed through the substrate are not significantly scattered. For use with quartz substrates, extremely durable coatings which display only 0.1% reflectivity have been prepared.
62 Citations
14 Claims
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1. A radiation-transmitting non-porous vitreous substrate of predetermined refractive index having coated on at least one surface thereof a substantially non-reflecting coating which functions effectively over a predetermined wide band of radiations, said non-reflecting coating comprising:
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a single silica layer overcoated on said non-porous substrate and having an exposed surface, the thickness of said silica layer as measured between the exposed surface thereof and the silica layer-substrate interface being generally uniform and in the range of from about 150 nm to about 600 nm with the silica layer displaying throughout its thickness at least ome degree of porosity, and the porosity of said overcoated silica layer together with the non-porous nature of said substrate forming a defined interface therebetween; all of said silica layer consisting essentially of silica except for other glass-forming constituents which may diffuse from said substrate and into said silica layer proximate said layer-substrate interface during the forming-processing of said silica layer on said substrate; that portion of said silica layer which is immediately proximate the exposed surface thereof displaying the highest degree of porosity and the porosity of said silica layer gradually decreasing from the exposed surface thereof to a depth which is at least about one-quarter of the longest wavelength of said predetermined wide band of radiations to provide a graded degree of porosity which decreases as the distace from the exposed surface thereof increases; that portion of said silica layer which is immediately proximate the exposed surface thereof having an index of refraction which approaches unity, and that portion of said silica layer which is immediately proximate the interface between said silica layer and said substrate having an index of refraction which approaches that of said substrate; and the size of the individual pores in said silica layer being sufficiently small that radiations impinging upon the exposed surface of said silica layer are transmitted therethrough without being significantly scattered. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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8. The method of applying to a radiation-transmitting non-porous vitreous substrate of predetermined refractive index a substantially non-reflecting coating which functions effectively over a predetermined wide band of radiations, which method comprises:
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preparing a coating solution comprising a mixture of a predetermined molar amount of silicon alkoxide of the formula Si(OR)4 where R is alkyl having from one to six carbon atoms, water in amount of from about 1.9 moles to about 2.6 moles per mole of alkoxide, organic solvent in which both said alkoxide and said water are miscible and in such amount that the total silicon alkoxide when expressed in terms of equvalent SiO2 content is from about 12% to about 18% by weight of said coating solution, and a small amount of mineral acid catalyst, and heating and then cooling the mixed solution constituents under predetermined conditions to effect a controlled hydrolysis and polymerization thereof; applying the prepared coating solution to the substrate to be coated as a generally uniform liquid film, and rapidly drying the applied film under low humidity conditions to form a residual layer having a thickness of from about 300 nm to about 1200 nm; heating the substrate and the coated layer at a predetermined temperature and for a predetermined time first to drive residual organic material and hydroxyl radicals therefrom and then to heat treat the residual silica to form a thin layer consisting essentially of porous silica except for any minor proportion of glass-forming constituent which may diffuse from the substrate during the heating thereof into the porous silica layer proximate the silica layer-substrate interface, with the size of the individual pores of said porous silica layer being sufficiently small that radiations intended to pass through said substrate are not significantly scattered by the pores in said porous silica layer; and applying to the exposed surface of said formed porous silica layer a relatively weak silica etching solution for a predetermined time sufficient to enlarge in a graded fashion the existing pores in the porous silca layer to a depth, measured from the exposed surface, which is at least about one-quarter of the longest wavelength of radiations of said predetermined wide band of radiations to provide a graded degree of porosity which decreases as the distance from the exposed surface of said silica film increases, with the size of the largest pores in said silica layer being sufficiently small so as not to significantly scatter the radiations intended to be passed through said substrate. - View Dependent Claims (9, 10, 11, 12, 13)
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14. A radiation-transmitting non-porous vitreous substrate of predetermined refractive index having coated on at least one surface thereof a substantially non-reflecting coating which functions effectively over a predetermined wide band of radiations, said non-reflecting coating having been prepared by the method which comprises:
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preparing a coating solution comprising a mixture of a predetermined molar amount of silicon alkoxide of the formula Si(OR)4 where R is alkyl having from one to six carbon atoms, water in amount of from about 1.9 moles to about 2.6 moles per mole of alkoxide, organic solvent in which both said alkoxide and said water are miscible and in such amount that the total silicon alkoxide when expressed in terms of equivalent SiO2 content is from about 12% to about 18% by weight of said coating solution, and a small amount of mineral acid catalyst, and heating and then cooling the mixed solution constituents under predetermined conditions to effect a controlled hydrolysis and polymerization thereof; applying the prepared coating solution to the substrate to be coated as a generally uniform liquid film, and rapidly drying the applied film under low humidity conditions to form a residual layer having a thickness of from about 300 nm to about 1200 nm; heating the substrate and the coated layer at a predetermined temperature and for a predetermined time first to drive residual organic material and hydroxyl radicals therefrom and then to heat treat the residual silica to form a thin layer consisting essentially of porous silica except for any minor proportion of glass-forming constituent which may diffuse from the substrate during the heating thereof into the porous silica layer proximate the porous silica layer-substrate interface, with the size of the individual pores of said porous silica layer being sufficiently small that radiations intended to pass through said substrate are not significantly scattered by the pores in said porous slica layer; and applying to the exposed surface of said formed porous silica layer a relatively weak silica etching solution for a predetermined time sufficient to enlarge in a graded fashion the existing pores in the porous silica layer to a depth, measured from the exposed surface, which is at least about one-quarter of the longest wavelength of radiations of said predetermined wide band of radiations to provide a graded degree of porosity which decreases as the distance from the exposed surface of said silica film increases, with the size of the largest pores in said silica layer being sufficiently small so as not to significantly scatter the radiations intended to be passed through said substrate.
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Specification