Wide-band two-layer antireflection coating for optical surfaces

A process for forming a two-layer coating on glass or another substrate involves applying first a layer of light metal fluoride on the substrate and subsequently a layer of a porous metal oxide e.g. silica. The metal fluoride layer may be applied by using the sol-gel technique or by other methods e.g. vacuum deposition. The metal oxide layer is preferably applied by a sol-gel technique, by either a dip coating or spin coating. Each layer is preferably about one quarter-wavelength thick. The refractive index of the first layer is lower than that of the substrate, and the second layer has lower refractive index than the first layer. This dependence minimizes reflection of the coating over a broad wavelength range.

FIELD OF THE INVENTION 
This invention relates to broadband antireflective coatings applied to the 
surface of optical elements such as lenses or CRT (cathode ray tube) 
display panels. 
BACKGROUND OF THE INVENTION 
It is desirable to reduce as much as possible the amount of light reflected 
at the air-glass interface of an optical element e.g. an optical lens, to 
increase the light throughput and, in some instances, to minimize ghost 
images. In some applications, the coating must perform over a wide 
wavelength range. 
Several types of antireflective coatings are known already. U.S. Pat. No. 
5,572,086 to Tong et al. proposes an antireflective and antistatic coating 
for a CRT display panel. The multi-layer coating comprises a first 
conductive grounded inner coating including a metal salt such as 
antimony-tin oxide, and an outer coating disposed on the first coating and 
comprising a water soluble organic or inorganic salt or a polymer soluble 
in an organic solvent. The outer coating has a plurality of pores of 
various depths for providing the outer coating with a range of light 
refractive indexes determined by the depth of the pores. 
Boulos et al, U.S. Pat. No. 5,208,181 proposes an antireflective coating 
having a metal oxide and a graded concentration of light metal fluoride. 
The process for forming the coatings is also described in Boulos et al, 
U.S. Pat. No. 5,268,196. 
Both these patents describe an approach in which the fluorination of the 
fluoride layer is controlled such that the concentration of the fluoride 
varies across the thickness of the fluoride layer. 
There is still a need for efficient, durable, easily-applied and relatively 
inexpensive antireflective (AR) coatings having a passband from about 360 
to about 1400 nanometers. 
SUMMARY OF THE INVENTION 
In accordance with the invention, there is provided an antireflective 
coating on the surface of a substrate. The coating comprises: 
a layer of a light metal fluoride applied on the surface of the substrate, 
and 
a layer of porous metal oxide applied over the metal fluoride layer. 
Preferably, the optical thickness of each layer is about one quarter 
wavelength at the wavelength where a reflection minimum is desired. 
The substrate may be glass, plastic or a crystalline material. 
The light metal is at least one metal selected from the group consisting of 
magnesium, lithium, sodium, potassium, calcium, strontium or barium. 
The metal oxide is one or more of the oxides of silicon, aluminum and 
titanium. Preferably, the metal oxide is silica. 
The process of forming the antireflective coatings of the invention 
comprises the steps of: 
applying a coating of a light metal fluoride on the surface of a substrate, 
applying a porous metal oxide coating over the light metal fluoride 
coating. 
Preferably, the porous metal oxide layer is applied by a sol-gel technique. 
The light metal fluoride may be applied by vacuum deposition. 
Alternatively, it may be applied by fluorination of a metal oxide layer 
deposited by a sol-gel technique. 
Following the above steps, the total coating may be hardened by any 
suitable method e.g. by treatment with gaseous ammonia or ammonium 
hydroxide vapor. 
Optionally, the total two-layer coating may be rendered hydrophobic e.g. by 
treating it with a silane compound.

DETAILED DESCRIPTION OF THE INVENTION 
The invention provides a two-layer anti-reflective coating on a substrate 
such as glass, plastic or a crystalline material. The first layer, in 
direct contact with the substrate, is a light metal fluoride layer that 
may be applied by a well-known vacuum deposition technique or by a sol-gel 
technique. The second layer is a porous oxide layer, e.g. a silica layer, 
that is preferably applied with a sol-gel technique, also commonly known 
in the art. 
According to the invention, the concentration of fluoride is substantially 
constant throughout the fluoride layer. To this effect, where sol-gel and 
subsequent fluorination is chosen as the method of applying the light 
metal fluoride, the fluorination is continued for a period of time 
sufficient to substantially completely fluorinate the light metal which 
has been applied by the sol-gel technique. 
The vacuum deposition method is preferred where it is desired to achieve 
the refractive index of the bulk metal fluoride in the layer. The sol-gel 
technique combined with fluorination produces a layer of higher index 
which is preferred for substrates having a refractive index higher than 
1.5. 
The second layer is preferably applied using a sol-gel technique to produce 
a porous metal oxide layer, preferably silica (SiO.sub.2). The porosity of 
the layer causes the refractive index of that layer to be approximately an 
average of the index of the bulk material and air. 
It is the stepped decrease in refractive index occurring between the 
substrate and the second (metal oxide) layer that results in the enhanced 
broad-band performance of the coating of the invention. 
Each of the two layers has a thickness of approximately one 
quarter-wavelength at the wavelength where a reflection minimum is 
desired. However, the thickness of the layers can be changed and the 
refractive indexes can be varied over a small range to minimize the 
reflectivity at a chosen wavelength and to maximize the bandwidth. 
In a specific example, the antireflection coating was applied to an optical 
element working in the visible and near-infrared range from 400 to 1400 
nanometers. As seen in FIG. 2, the substrate 10 was Schott BK7 optical 
glass having a refractive index of about 1.5. 
The first layer 12 was MgF.sub.2 applied by vacuum deposition in a known 
manner. The thickness of the layer was one quarter-wavelength at 600 nm. 
The layer had a very low porosity and a refractive index near that of the 
bulk magnesium fluoride. It is known that the refractive index of bulk 
magnesium fluoride is 1.38 (Modern Optical Engineering, McGraw-Hill 1966, 
page 170). 
For the second layer 14, a sol-gel solution was prepared from the following 
ingredients (in weight percent): 
______________________________________ 
Anhydrous ethyl alcohol 
93.2% 
Tetra-ethyl orthosilicate (TEOS) 5.2% 
Ammonium hydroxide (50%) 1.6% 
______________________________________ 
The TEOS was stirred into the alcohol and ammonium hydroxide was added. The 
mixture was allowed to stand in a closed container at room temperature for 
three days to allow the growth of silica spheres, about 20 nm in diameter, 
which formed a suspension in the alcohol. The proportions given here 
produced a silica concentration of 1.5 % which can be decreased by 
dilution with ethyl alcohol to control the thickness of the coating. In 
the present example, the silica concentration was reduced to 0.85 %. The 
coating was applied by spinning the substrate at 250 RPM on a horizontal 
turntable, and pouring the sol-gel solution onto the center of the 
substrate. The solution was poured at a rate of 1 or 2 millilitres per 
second until the entire surface had been wetted and liquid was being flung 
from the edge. The turntable was allowed to spin for another 2 minutes to 
dry the coating. Because the coating was porous, the refractive index was 
less than that of the bulk silica. In the present example, the index was 
about 1.22. 
The second, external layer 14 of the two-layer coating of the invention is 
typically very fragile and is easily damaged by wiping. The layer was 
hardened by enclosing the glass in an airtight container with an open 
vessel containing 50% ammonium hydroxide. The duration of the exposure to 
ammonia vapor was 24 hours. This step improved the hardness and durability 
of the coating to the point where the coating was tolerant of mild wiping 
and cleaning with solvents. 
The hardening also caused some shrinkage of the sol-gel layer, resulting in 
a decrease in the wavelength of minimum reflectivity of about 7%. This 
should be taken into account when the solution concentration is chosen. 
It is known that porous silica is hygroscopic and will absorb water if 
placed in a humid atmosphere for an extended time. The air volume in the 
porous layer will eventually be displaced by water, causing the index of 
the layer to rise. As a result, the reflection characteristics of the 
coating are degraded. It is therefore preferred that the silica layer be 
made hydrophobic. In the specific example, the coating was immersed for a 
few minutes in a solution of 5 ppm of dichlorodimethylsilane in anhydrous 
ethyl alcohol. The substrate was then dried while standing on edge to 
allow the solution to run off. The treated coating did not show any 
degradation of antireflection properties after immersion in water or 
long-term exposure to a humid environment. 
The reflectance of the coating 12, 14 obtained in the above example 
compares favourably, as shown in FIG. 1, with that of uncoated Schott BK7 
optical glass over a range from 400 to 1400 nanometers.