Optical coatings of variable refractive index and high laser-resistance from physical-vapor-deposited perfluorinated amorphous polymer

Variable index optical single-layers, optical multilayer, and laser-resistant coatings were made from a perfluorinated amorphous polymer material by physical vapor deposition. This was accomplished by physically vapor depositing a polymer material, such as bulk Teflon AF2400, for example, to form thin layers that have a very low refractive index (.about.1.10-1.31) and are highly transparent from the ultra-violet through the near infrared regime, and maintain the low refractive index of the bulk material. The refractive index can be varied by simply varying one process parameter, either the deposition rate or the substrate temperature. The thus forming coatings may be utilized in anti-reflectors and graded anti-reflection coatings, as well as in optical layers for laser-resistant coatings at optical wavelengths of less than about 2000 nm.

BACKGROUND OF THE INVENTION 
The present invention is directed to transparent and variable refractive 
index coatings, particularly to the fabrication of such coatings from a 
copolymer of two or more of the following monomers: tetrafluoroethylene, 
2,2-bistrifluoromethyl-4,5 difluoro-1,3 dioxole, perfluoroallyl vinyl 
ether, and perfluorobutenyl vinyl ether, hereafter referred to as a 
"perfluorinated amorphous polymer", and more particularly to variable 
index optical single layers and multilayers, and laser-damage-resistant 
coatings formed by physical-vapor-deposited perfluorinated amorphous 
polymers (PAP). 
Various types of optical coatings have been developed for different 
applications, and numerous processes have been developed over the years. 
These prior efforts are exemplified by U.S. Pat. Nos. 4,545,646 issued 
Oct. 8, 1985 to M. Chern et al.; and U.S. Pat. No. 4,925,259 issued May 
15, 1990 to J. L. Emmett. 
Polymer materials have been widely used for coatings. Perfluorinated 
amorphous polymer coatings have been used as thermal barriers, 
microelectronics insulators, and in doped optical fibers. However, there 
has been a need for alternate optical coating materials for use in the 
ultra-violet (UV), visible, and near-infrared (NIR) regime due to a 
shortage of dielectrics with a low refractive index. Also, with the 
continuing development of high energy laser systems, there is a need for 
high laser-damage-resistant optical coatings operating at optical 
wavelengths of less than 2000 nm. 
This prior need has been satisfied by the present invention by the 
recognition that single layers of polymer materials, such as 
perfluorinated amorphous polymers (PAP), can be physical-vapor-deposited 
from bulk perfluorinated amorphous polymers, which are highly transparent 
in the UV-visible-NIR regime and also has a low refractive index. Also, by 
this invention, optical multilayers can be made by 
physical-vapor-deposited PAP with other physically-vapor-deposited 
dielectric materials. Also, by this invention the refractive index of the 
optical layers may be varied by simply varying the deposition rate. Thus, 
transparent optical coatings having a refractive index in the about 
1.10-1.30 range have been produced by this invention. Thus, multilayered 
optical reflectors have been made by this invention. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an optical coating 
which has a variable index and a high laser-damage-resistance. 
A further object of the invention is to produce such coating from a 
physical-vapor-deposited perfluorinated amorphous polymer. 
Another object of the invention is to produce a highly transparent optical 
coating for use in the ultra-violet, visible, and near infrared regime 
having a refractive index that can be varied by merely varying the 
deposition rate of the perfluorinated amorphous polymer or the temperature 
of the substrate during the deposition process. 
Another object of the invention is to produce high laser-damage-resistant 
optical coatings from an perfluorinated amorphous polymer material. 
Another object of the invention is to produce optical multilayers with 
physically-vapor-deposited perfluorinated amorphous polymer as one of the 
constituent layers, with the other layers being other 
physically-vapor-deposited dielectric materials such as oxides, fluorides, 
sulfides and selenides. 
Another object of the invention is to produce a broadband anti-reflection 
coating on non-absorbing substrates having refractive indices between 1.35 
and 1.69 using physically-vapor-deposited perfluorinated amorphous 
polymer. 
Other objects and advantages will become apparent from the following 
description and accompanying drawing. The present invention involves the 
formation of variable index optical single-layer and multilayered 
coatings, and other laser-resistant coatings by physical-vapor-deposition 
of a polymer material, such as a perfluorinated amorphous polymer, such as 
bulk Teflon AF. Also, by use of physical-vapor deposition of the 
perfluorinated amorphous polymer, the process parameters may be varied to 
produce coatings that are less dense and therefore have an even lower 
refractive index than the bulk perfluorinated amorphous polymer. High 
transparency coatings have been produced with a refractive index in the 
range of about 1.10-1.30. During experimental verification of this 
invention, single layers of perfluorinated amorphous polymer, having a 
thickness of .about.1500.ANG. for use in the visible regime, were 
deposited in a vacuum chamber with a simple resistance heater. The 
adhesion, transmittance, and refractive indices of the coatings were 
determined as a function of the deposition rate, substrate temperature, 
and glow-discharge bias potential. The coatings produced by this invention 
may be used as optical coatings in the UV-visible-NIR regimes, as well as 
in applications requiring a variable refractive index, such as rugate 
filters and graded anti-reflection coatings, as well as for 
laser-damage-resistant coatings such as reflectors, polarizers, and 
filters, in operating wavelength regimes for less than 2000 nm. Thus, by 
this invention, perfluorinated amorphous polymer coatings, primarily 
utilized in numerous non-optical applications, have been made into optical 
and laser-damage-resistant coatings, thus greatly expanding the use 
capability of polymer materials, such as Teflon AF.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is directed to the formation of variable index 
optical single-layer and multilayers, and laser-damage-resistant coatings 
from physical-vapor-deposited polymer material, such as perfluorinated 
amorphous polymer (PAP) material. The perfluorinated amorphous polymer 
material utilized in verifying the invention was Teflon AF2400, a bulk 
perfluorinated amorphous polymer, made by E. I. Du Pont and Co., and the 
bulk material was physically-vapor-deposited to form thin layers (100 to 
3000.ANG.) that were characterized optically and mechanically. While 
Teflon is known by the generic term tetrafluoroethylene, no generic term 
is known for Teflon AF, made by Du Pont, but is an amorphous fluoropolymer 
(AF). Bulk perfluorinated amorphous polymers are highly transparent in the 
ultra-violet (UV), visible, and near infrared (NIR) regime, and they also 
have a low refractive index (.about.1.31). The optical properties of the 
coatings produced by the physical-vapor-deposition process are similar to 
that of the bulk perfluorinated amorphous polymer material. The coatings 
are transparent from the UV (200 nm) through to the NIR (1200 nm), and the 
majority of coatings have a 1.30 refractive index, similar to that of the 
bulk material. However, for the lower substrate temperature range, the 
refractive indices of the coatings noticably decreased with increasing 
deposition rate, and a coating with a refractive index of as low as 1.16 
was obtained. The refractive index variation was also observed at the 
higher substrate temperature range. The thus produced coatings adhered to 
fused silicon and silicon wafers under normal handling conditions. By the 
process of this invention, variation of the refractive index can be 
achieved simply by varying a process parameter, the deposition rate. 
During experimental verification of the physical-vapor-deposition process 
using bulk perfluorinated amorphous polymer, coatings with thicknesses 
(.about.1500.ANG.) used in the visible regime were fabricated and the 
characteristics measured. A Box-Behnkem experimental strategy (3-factor 
uniform shell design for quadratic interpolation) was used to examine the 
relationship between the process parameters and the material properties. 
The deposition rates were set at 2, 11 and 20.ANG./S. The substrate 
temperatures were set at 20.degree., 110.degree. and 200.degree. C. The 
substrate platen or glow-discharge potential was biased at -1500, zero, 
and +1500 volts in a pre-coating glow discharge procedure, attempting to 
vary the adhesion of the coatings. Single layers of perfluorinated 
amorphous polymer as desibed above were deposited in a vacuum chamber with 
a simple resistance heater. The thickness of the coatings in this series 
ranged from 1000 to 3000.ANG.. The deposition rate may vary from 
2-200.ANG./S. The transmittances, adhesion, and refractive indices of the 
coatings were determined as a function of deposition rate, substrate 
temperature, and glow discharge. The transmittances were measured on a 
Cary spectrophotometer. The refractive index and thickness were determined 
on a Rudolf Research Auto El II-NIR-3 ellipsometer. 
By this series of experiments, it was determined that the optical 
properties of the thus formed coatings were similar to that of the bulk 
material. These coatings were found to be transparent from the 
ultra-violet (200 nm) through the near infrared (1200 nm). The coatings 
adhered to the substrates under normal conditions, but could be pulled off 
the fused silica substrates by using a tape with a 12.6 gr/mm tension. The 
majority of the coatings had a 1.30 refractive index, similar to that of 
the bulk material. However, for the lower substrate temperature range, the 
refractive indices of the coating decreased with increasing deposition 
rate, and a coating with a refractive index as low as 1.16 was obtained, 
thus verifying that coatings with a variable refractive index can be 
produced by this invention by varying the deposition rate. Therefore, 
highly transparent, variable index optical single layers and multilayers 
can be made using only one material. 
The following table sets forth the above-referenced experiment runs using 
bulk Teflon AF2400, and sets forth the measured refractive indices and 
thicknesses: 
TABLE I 
______________________________________ 
Refractive Index 
Thickness (.ANG.) 
Glow Measured At Measured At 
Temp Rate Disch 4050 6330 8300 4050 6330 8300 
.degree.C. 
A/s Volts .ANG. .ANG. 
.ANG. .ANG. 
.ANG. .ANG. 
______________________________________ 
110 11 0 1.308 1.307 
1.305 1353 1354 1349 
20 2 0 1.263 1.257 
1.263 1489 1497 1489 
200 11 1500 1.173 1.294 
1.3 2296 1817 1819 
110 2 1500 1.23 1.306 
1.305 1451 1432 1431 
20 11 1500 1.199 
1.216 2440 2332 
20 20 0 1.097 1.157 
1.168 2729 2274 2235 
110 2 -1500 1.295 1.309 
1.308 1237 1207 1200 
200 20 0 1.308 1.307 
1.305 1322 1338 1308 
20 11 1500 1.184 1.216 
1.219 1829 1720 1724 
200 2 0 1.305 1.303 
1.303 1244 1248 1241 
110 20 -1500 1.283 1.298 
1.298 1372 1321 1317 
110 20 1500 1.292 1.302 
1.302 1547 1527 1525 
20 1.1 0 1.288 1.277 
1.286 1000 1077 1030 
______________________________________ 
FIG. 1 illustrates the iso-refractive index (at 6330.ANG.) contour as a 
function of the deposition rate (.ANG./s) and substrate temperature 
(.degree.C.). The surface was determined from a quadratic fit of the data 
using regression analysis. 
Utilizing this invention, high laser-damage-resistant anti-refractive 
coatings were made from a perfluorinated amorphous polymer (Teflon AF2400) 
material by physical vapor deposition. As in the above experimental 
description, single layers of perfluorinated amorphous polymer were 
thermally deposited in a vacuum chamber. The transmittance and refractive 
indices were determined as set forth above. It was found that an 
anti-reflective coating of the physical-vapor-deposited perfluorinated 
amorphous polymer had a laser-damage-resistance of &gt;47j/cm.sup.2 (1.06 
.mu.m, 3-ns pulselength). Single surface reflections as low as 0.5% or 
less were obtained on these anti-reflection coatings. These coatings were 
also transparent from 200 nm to 1200 nm. Based on these initial tests, it 
appears that the coatings of this invention may be transparent at other 
optical wavelengths greater than 1200 nm, possibly about 2000 nm, but such 
has not yet been experimentally verified. Scanning electron microscopy and 
nuclear magnetic resonance observations indicate that morphological 
changes causes the variations in the refraction index rather than 
compositional changes. As pointed out above, the thus fabricated high 
laser-damage-resistant anti-reflective coatings adhered to fused silica 
and silicon wafers under normal handling conditions. 
FIG. 2 illustrates the use of a physical-vapor-deposited perfluorinated 
amorphous polymer and another dielectric material in an optical 
multilayer, more specifically an optical reflector using the reflector 
design: BK-7 (HL).sup.3 H Air, where H=ZnS and L=Teflon AF2400. The layers 
in the reflector adhered to the substrate and to each other. Therefore, 
other optical multilayers can be made by physical-vapor-deposited of 
perfluorinated amorphous polymer with other dielectric materials. 
To exemplify the invention in greater detail, the following sets forth a 
brief description of a specific apparatus utilized in the 
physical-vapor-deposition technique in carrying out this invention, and a 
specific operational sequence, using exemplified vacuum conditions, 
materials, deposition times, temperatures, energies, etc., which produce a 
coating having an exemplified thickness and refractive index. 
The apparatus, while well known in the field of physical vapor deposition, 
may comprise a stainless steel bell jar connected by a pumping manifold to 
a liquid-nitrogen baffled diffusion pump. The diffusion pump is backed by 
a mechanical roughing pump. This vacuum coating chamber routinely had a 
base pressure in the mid 10.sup.-7 Torr range. The chamber is equipped 
with quartz lamps for heating the substrates, a vibrating crystal head for 
monitoring the rate and coating thickness, and a tungsten filament for 
heating the crucible. A crucible containing the charge of perfluorinated 
amorphous polymer was resistance heated until the perfluorinated amorphous 
polymer boiled. The heater power was then adjusted to give the proper 
deposition rate, as determined by a crystal rate monitor. The shutter, 
between the crucible and the substrates, was opened to allow the 
evaporated perfluorinated amorphous polymer to reach the substrate. 
An example of the operation sequence for producing a coating of specified 
thickness and refractive index on a selected substrate, is as follows: 
1. Select a substrate composed of polished fused silica, silicon wafer, or 
another suitably polished material. 
2. Clean the substrate with alcohol in a class 1000 environment. 
3. Load the substrates into the vacuum chamber and pump down to a base 
pressure below 1.times.10.sup.-6 Torr. 
4. Set the heat lamps to obtain the proper substrate temperature. 
5. Boil the perfluorinated amorphous polymer with the shutter closed. 
6. Adjust the power to the crucible heater to obtain a specified 
evaporation rate. 
7. Open the shutter and monitor the thickness. 
8. When the coating reaches a given thickness, the shutter closes over the 
crucible. 
It has thus been shown that the present invention enables the use of 
polymer materials, such as perfluorinated amorphous polymers, to be 
utilized as optical coatings for use in the ultra-violet, visible, and 
near infrared regime, thereby greatly expanding the use of perfluorinated 
amorphous polymer materials for highly transparent, low refractive index 
applications. In addition, the invention enables the formation of such 
coatings having a variable refraction index that remains highly 
transparent. Also, the coatings formed by this invention may be utilized 
as high laser-damage-resistant anti-reflective coatings and are 
transparent at optical wavelengths less than about 2000 nm. The coatings 
produced by this invention may be utilized in ultra-violet regime 
applications, such anti-reflectors and graded anti-reflection coatings. 
While particular materials, parameters, apparatus, etc. has been described 
to illustrate the principle features of this invention, such are not 
intended to limit the scope of the invention. Modifications and changes 
will become apparent to those skilled in the art, and it is intended that 
the invention be limited only by the scope of the appended claims.