Improvements in or relating to antireflective silica coatings

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, characterized in that said non-reflecting coating comprises 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 150 nm to 600 nm with the silica layer displaying throughout its thickness at least some degree of porosity; 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 distance from the exposed surface thereof increases; said silica layer portion having an index of refraction which approaches unity while that portion of said silica layer which is immediately proximate the interface between said silica layer and said substrate has 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.

Claim 13

A 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, characterized in that 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 1.9 moles to 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 Si0 2 content is from 12% to 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 300 nm to 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 significantly to scatter the radiations intended to be passed through said substrate.

Claim 14

A 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, characterized in that 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 1.9 moles to 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 Si0 2 content is from 12% to 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 300 nm to 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 significantly to scatter the radiations intended to be passed through said substrate.

Claim 18

A method according to any of claims 11 to 14, characterized in that the etching solution includes HF in a concentration equivalent to 7.5 grams of 48% HF diluted with water in amount of from 7.5 kg to 15 kg, with the etching time being from 8 minutes to 40 minutes, with the weaker the etching solution, the longer the etching time.