U.S. patent application number 15/260748 was filed with the patent office on 2017-05-18 for silica-modified-fluoride broad angle antireflection coatings.
The applicant listed for this patent is Corning Incorporated. Invention is credited to Michael Jerome Cangemi, Paul Gerard Dewa, Joseph D. Malach, Paul Francis Michaloski, Horst Schreiber, Jue Wang.
Application Number | 20170139083 15/260748 |
Document ID | / |
Family ID | 49669946 |
Filed Date | 2017-05-18 |
United States Patent
Application |
20170139083 |
Kind Code |
A1 |
Cangemi; Michael Jerome ; et
al. |
May 18, 2017 |
SILICA-MODIFIED-FLUORIDE BROAD ANGLE ANTIREFLECTION COATINGS
Abstract
The disclosure is directed to a coating consisting of a binary
metal fluoride coating consisting a high refractive index metal
fluoride layer on top of a substrate, a low refractive index metal
fluoride layer on top of the high refractive index layer and layer
of SiO.sub.2 or F--SiO.sub.2 containing 0.2 wt % to 4.5 (2000 ppm
to 45,000 ppm) F on top of the low refractive index layer. In one
embodiment the F content of F--SiO.sub.2 is in the range of 5000
ppm to 10,000 ppm F. The high index and low index materials are
each deposited to a thickness of less than or equal to 0.9 quarter
wave, and the capping material is deposited to a thickness in the
range of 5 nm to 25 nm. The disclosure is also directed to optical
elements having the foregoing coating and a method of making the
coating.
Inventors: |
Cangemi; Michael Jerome;
(Canandaigua, NY) ; Dewa; Paul Gerard; (Newark,
NY) ; Malach; Joseph D.; (Newark, NY) ;
Michaloski; Paul Francis; (Rochester, NY) ;
Schreiber; Horst; (Corning, NY) ; Wang; Jue;
(Fairport, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Corning Incorporated |
Corning |
NY |
US |
|
|
Family ID: |
49669946 |
Appl. No.: |
15/260748 |
Filed: |
September 9, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13834008 |
Mar 15, 2013 |
9482790 |
|
|
15260748 |
|
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|
61653567 |
May 31, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03F 7/7015 20130101;
G02B 1/115 20130101; G02B 1/10 20130101; G02B 1/105 20130101; G03F
7/70958 20130101 |
International
Class: |
G02B 1/115 20060101
G02B001/115 |
Claims
1.-9. (canceled)
10. A method for making an optical element having a coating
thereon, the method comprising providing a substrate selected from
the group consisting of CaF.sub.2, SiO.sub.2 and F--SiO.sub.2,
applying a coating of a high refractive index metal fluoride
material to the surface of the substrate using vacuum deposition at
a temperate greater than or equal to 300.degree. C., the reverse
mask technique, and in-situ or post-deposition plasma ion
treatment; applying a coating of a low refractive index metal
fluoride material to the surface of the high refractive index
material using vacuum deposition at a temperate greater than or
equal to 300.degree. C., the reverse mask technique, and in-situ or
post-deposition plasma ion treatment; and depositing a capping
layer selected from the group consisting of SiO.sub.2 and
F--SiO.sub.2 on top of the low refractive index material using
vacuum deposition at a temperate greater than or equal to
300.degree. C., the reverse mask technique, and in-situ or
post-deposition plasma ion treatment to thereby produce an optical
element having a coating thereon; wherein the high index and low
index materials are each deposited to a thickness of f less than or
equal to 0.9 quarter wave, and the capping material is deposited to
a thickness in the range of 5 nm to 25 nm.
11. The method according to claim 10, wherein the high refractive
index coating material is selected from the group consisting of
GdF.sub.3 and LaF.sub.3
12. The method according to claim 10, wherein the low refractive
index coating material is selected from the group consisting of
AlF.sub.3 and MgF.sub.3,
13. The method according to claim 10, wherein the produced optical
element is a mirror, lens, laser window or prism.
14. The method according to claim 10, wherein the provided
substrate is a F--SiO.sub.2 substrate containing 0.5 wt. % to 4.5
wt. % F.
Description
PRIORITY
[0001] This application claims the priority and benefit of U.S.
Provisional Application No. 61/653,567 titled
"Silica-Modified-Fluoride Broad Angle Antireflection Coatings"
filed May 31, 2012 in the name of inventors Michael Jerome Cangemi,
Paul Gerard Dewa, Joseph D. Malach, Paul Francis Michaloski, Horst
Schreiber and Jue Wang.
FIELD
[0002] The disclosure is directed to environmentally stable and
laser durable optical coatings optics that will be used lithography
systems operating in deep ultraviolet ("DUV") wavelength range.
BACKGROUND
[0003] As semiconductor processing progresses 45 nm node and
beyond, the applications of ArF excimer lasers with increasing
power and repetition rate require low loss, environmentally stable
and laser durable coatings for optical components and systems for
laser optics and precision optics. Surface and coating technologies
will play a critical role in supporting the use precision optics
and laser optics in DUV spectral regime. Wide band-gap fluoride
thin films are generally preferred as coating for DUV uses.
[0004] At 193 nm wavelength, a well-prepared substrate surface is
one of the dominant preconditions for good optical coatings, which
includes surface finishing and cleaning prior to optical coatings,
various methods such as optical polishing (Jue Wang, Robert L.
Maier, John H. Burning, "Surface characterization of optically
polished CaF.sub.2 crystal by quasi-Brewster angle technique," SPIE
5188, 106-114(2003)); magnetorheological finishing (MRF) (Jue Wang,
Robert L. Maier, "Quasi-Brewster angle technique for evaluation the
quality of optical surface," SPIE 5375, 1286-1294(2004)); diamond
turning (Eric R. Marsh et al, "Predicting surface figure in diamond
turned calcium fluoride using in-process force measurement," J.
Vac. Sci. Technol. B 23(1), p 84-89(2005)); Ultrasonic/megasonic
and ultraviolet ozone cleaning (Jue Wang, Robert L. Maier, "Surface
assessment of CaF.sub.2 DUV and VUV optical components by
quasi-Brewster angle technique," Applied Optics 45(22),
5621-5628(2006)); in-situ plasma ion cleaning (Jue Wang et al.,
"Color center formation of CaF.sub.2 (111) surface investigated by
low-energy-plasma-ion," Frontier in Optics, 88th OSA annual meeting
(2004)). Improved optical surface quality extends component
lifetime (U.S. Pat. No. 7,128,9847, "Improved surfacing of metal
fluoride excimer optics" and U.S. Pat. No. 7,242,843, "Extended
lifetime excimer laser optics"). Optical coating development has
focused on fundamental understanding of film growth mechanism and
plasma ion interaction (Jue Wang et al., "Correlation between
mechanical stress and optical properties of
SiO.sub.2/Ta.sub.2O.sub.5 multilayer UV NBF deposited by plasma
ion-assisted deposition," SPIE 5870, 58700E1-9(2005); Jue Wang et
al., "Elastic and plastic relaxation of densified SiO.sub.2 films,"
Applied Optics 47(13), C131-134(2008); Jue Wang et al, "Crystal
phase transition of HfO2 films evaporated by plasma ion-assisted
deposition," Applied Optics 47(13), C189-192(2008); Jue Wang et
al., "Wavefront control of SiO.sub.2-based ultraviolet narrow band
pass filters prepared by plasma ion-assisted deposition", Applied
Optics Vol. 46(2), pp 175-179(2007); and Jue Wang et al.,
"Nanoporous structure of a GdF.sub.3 thin film evaluated by
variable angle spectroscopic ellipsometry", Applied Optics Vol.
46(16), 3221-3226(2007)). This understanding has led to new optical
thin film design and coating process improvements; from oxide
materials to fluoride materials and ultimately oxide-fluoride
hybrids (U.S. Pat. No. 7,961,383, Jue Wang et al., "Extended
lifetime of fluoride optics," Boulder Damage Symposium, SPIE
6720-24 (2007); Jue Wang et al., "Structural comparison of
GdF.sub.3 films grown on CaF.sub.2 (111) and SiO.sub.2 substrates,"
Applied Optics Vol. 47(23), 4292 (2008); and Jue Wang et al.,
"Optical coatings with ultralow refractive index SiO.sub.2 films,"
SPIE 7504, 75040F(2009)).
[0005] Wide band-gap fluoride thin films are generally preferred as
coating for DUV uses. Using energetic deposition process is
restricted for fluoride materials, because of fluorine depletion.
The porous nature of thermal-evaporated fluoride coatings leads to
measureable scatter loss and environmentally unstable. To overcome
the porous nature of the metal fluoride coatings (MF.sub.x where
x=2 or 3), hybrid oxide-fluoride coating were developed in which
fluoride doped silica (F--SiO.sub.2) layers was inserted into
stacks of MF.sub.x coating layers (U.S. Pat. No. 7,961,383). In
addition, and outermost F--SiO.sub.2 layer was applied as a top of
capping layer. However, these coating are relatively thick, and the
problem still remains with regard to providing both environmental
protection and anti-reflections properties to optics suitable for
use in the DUV region. The present disclosure is directed to
overcoming the shortcoming of the current environmental AR coatings
for fluoride optics.
SUMMARY
[0006] The disclosure is directed to silica-modified-fluoride AR
coatings for use in the DUV region and process for making them. The
attributes of these coatings include: [0007] (1) A reflectance less
than 0.5% over a broad angle of incidence which is necessary for
lens surfaces operating in the DUV, this broad angle AR performance
to ensures a high system throughput; [0008] (2) The coating
prevents environmental substances from penetrating into the
underneath fluoride layers and reducing optical performance; and
[0009] (3) The coating surface is chemically non-reactive to
environmental substances. The coating of the of disclosure consists
of a binary metal fluoride coating consisting of a high refractive
index MF.sub.2 species and a low refractive index species, and a
capping or last layer of SiO.sub.2 or F--SiO.sub.2 containing 0.2
wt. % to 4.5 wt. % (2000 ppm to 45,000 ppm) F in the capping layer.
In one embodiment the F content is in the range of 5000 ppm to
10,000 ppm F.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIGS. 1A and 1B are graphs illustrating the DUV spectral
reflectance of an oxide-fluoride hybrid AR coating, FIG. 1A, and an
all fluoride AR coating, FIG. 1B
[0011] FIGS. 2A and 2B are cross-sectional images of an
oxide-fluoride hybrid AR coating, FIG. 2A, and an all fluoride AR
coating, FIG. 2B
[0012] FIG. 3 is a graph illustrating the spectral reflectance of
an oxide-fluoride hybrid AR coating 30 and a standard 2-layer
fluoride AR coating 32 on CaF.sub.2 substrates as a function of an
angle of incidence.
[0013] FIG. 4 is a graph illustrating the spectral reflectance of
an oxide-fluoride hybrid AR coating 40 and the AR coating with a 2%
thickness falloff as numeral 42.
[0014] FIG. 5 is a graph illustrating the spectral reflectance
shift of a standard 2-layer quarter-wave AR coating 50 and the AR
coating with a 2% thickness falloff as numeral 52.
[0015] FIG. 6 is a graph of the spectral reflectance of an
oxide-modified 3-layer AR coating 60 and the AR coating with a 2%
falloff as numeral 62; the 3-layer coating consisting of 2-layers
of fluorides with an additional 5 nm thick SiO.sub.2 coating as the
outermost surface modification for laser use.
[0016] FIG. 7 is a graph illustrating the spectral reflectance of
an oxide-modified 3-later AR coating 70 and the AR coating with a
2% thickness falloff illustrated as numeral 72; the 3-layer Ar
coating consisting of 2 layer of fluorides with an additional 10 nm
thick coating as the outermost surface layer for laser use.
DETAILED DESCRIPTION
[0017] Herein the terms "half wave," quarter wave" and "less than
quarter wave" are used in reference to the thickness of the layers
of a coating materials deposited on an optic. Such terms are
dependent on the wavelength of light, for example 193 nm laser
light, with which they are used. Consequently, the thicknesses can
vary. For example, for 193 nm light the quarter-wave thickness is
48.25 nm, .+-.1% for variations in coating techniques and
equipment. Hence, the terms are to be understood in relationship to
the laser system with which the optics are to be used. Also herein,
while the examples are given in terms of CaF.sub.2 optics because
these are the preferred optics for DUV applications, it is to be
understood that that in a general sense the applications can be
used with MF.sub.2 optics where M is Ca, Ba, Mg or Sr, or mixtures
thereof, or with SiO.sub.2 substrates
[0018] The disclosure is directed to silica-modified-fluoride AR
coatings for use in the DUV region and process for making them. The
attributes of these coating include: [0019] (1) A reflectance less
than 0.5% over a broad angle of incidence which is necessary lens
surfaces operating in the DUV, this broad angle AR performance to
ensures a high system throughput; [0020] (2) The coating is
prevents environmental substances from penetrating into the
underneath fluoride layers and reducing optical performance; and
[0021] (3) The coating surface is chemically non-reactive to
environmental substances.
[0022] FIGS. 1A and 1B are graphs of DUV spectral reflectance (R)
versus wavelength and compare the spectral of the oxide-fluoride
hybrid anti-reflection (AR) coating (FIG. 1A) to a standard
fluoride A coating (FIG. 1B). Numeral 10 in each Figure represents
the measurement performed right after deposition of the coating
film. After a five (5) month laboratory exposure, the center
wavelength shift of the hybrid AR denoted by numeral 12 in FIG. 1A
is negligible, whereas the environmental effect is significant for
the standard fluoride AR of FIG. 1B in which after a 7-day lab
exposure to the environment there is a center wavelength shift to
206 nm from 193 nm. FIGS. 2A and 2B are SEM cross-sectional images
of the oxide-fluoride hybrid AR (FIG. 2A) and the standard fluoride
AR (FIG. 2B). The hybrid AR coating of FIG. 2A with improved film
structure is clearly revealed by SEM. The oxide-fluoride hybrid AR
coating consists of a quarter-wave high refractive index fluoride
layer, for example without limitation GdF.sub.3 or LaF.sub.3, and a
quarter-wave low refractive index fluoride layer, for example
without limitation AlF.sub.3 or MgF.sub.2, followed by a half-wave
silica layer, for example SiO.sub.2 or F--SiO.sub.2 as the
outermost sealant or capping layer. In contrast, a standard
fluoride AR coating comprises a quarter-wave high refractive index
fluoride layer followed by a quarter-wave low refractive index
fluoride layer with no oxide capping layer.
[0023] The hybrid approach enables low loss and environmentally
stable DUV coatings on flat or near flat surfaces for a specific
angle of incidence. For optical systems, especially inspection
objectives, an AR coating with a reflectance less than 0.5% over a
broad angle of incidence is desired for every surface. This
presents some challenges for the standard hybrid AR coatings which
are: [0024] 1. The oxide-fluoride hybrid AR coating does not
provide a broad angle spectral performance. FIG. 3 is a graph
illustrating plots spectral reflectance of an oxide-fluoride hybrid
AR coating 30 along with a standard 2-layer fluoride AR coating 32
as a function of angel of incidence. The standard 2-layer fluoride
AR coating consists of one quarter-wave GdF.sub.3 as the first
layer deposited on a CaF.sub.2 substrate, followed by one
quarter-wave AlF.sub.3 as the second layer. The oxide-fluoride
hybrid AR coating has an additional half-wave silica capping layer
on top of the standard 2-layer fluoride AR coating. The broadness
of the angle of incidence with reflectance less than 0.5% is
reduced to 22.degree. for the hybrid coating from 36.degree. for
the standard coating at a wavelength of 193 nm. [0025] 2. The
oxide-fluoride hybrid AR coating is more sensitive to coating
thickness fall-off when compared to the standard fluoride AR
coating. FIG. 4 is a graph illustrating the spectral reflectance
shift of the oxide-fluoride hybrid AR coating 40 with 2% thickness
falloff (curve 42). The broadness of the angle of incidence with
reflectance less than 0.5% is reduced to 14.degree. from 23.degree.
at a wavelength of 193 nm. For comparison, FIG. 5 plots the
spectral reflectance shift of the standard 2-layer fluoride AR 50
coating with 2% thickness falloff (curve 52). The broadness of the
angle of incidence with reflectance less than 0.5% is reduced to
33.degree. from 36.degree..
[0026] The technical challenge is to achieve appropriate
environmental protection while maintaining a reasonable AR
performance including low reflectance over a broad angle of
incidence. In general, there are 2 potential technical solutions.
One is a chemical method, such as sol-gel derived broad angle AR
coatings with chemical modification to enhance coating durability
and environmental stability (see Jue Wang et al., "Optical coatings
with ultralow refractive index SiO.sub.2 films," SPIE 7504,
75040F(2009); Hitoshi Ishizawa et al, "Preparation of
MgF.sub.2--SiO.sub.2 thin films with a low refractive index by a
sol-gel process," Applied Optics 47(13), C200(2008); and U.S.
Patent Application Publication No. 2010/0297430). The other is a
physical method that has been addressed with quarter-wave capping
for high reflective coatings and antireflective coatings at 193 nm
(U.S. Patent Application Publication Nos. 20100215932A1 and
2009.0297821, and U.S. Pat. No. 7,961,383). The results from
analysis of the foregoing Figures clearly indicate that
quarter-wave silica capping is too thick to provide a broad angle
performance. The present disclosure reveals that a thin silica
modified non-quarter-wave fluoride AR coating will provide the
desired protection.
[0027] In the present disclosure a 3-step approach is employed to
deal with the technical challenges mention above. The first step is
to reduce the capping layer thickness from a half-wave (.about.60
nm) down to less than 20 nm. The second step is to replace the
quarter-wave fluoride AR coating with a non-quarter-wave fluoride
AR coating. The third step is to densify the thin silica layer,
using for example SiO.sub.2 or F--SiO.sub.2, without introducing
additional absorption of the underneath fluoride layers. The
densification process of the thin silica layer includes high
temperature deposition (T.gtoreq.300.degree. C.), reversed mask
deposition technique, and in-situ or post-deposition plasma-ion
treatment.
[0028] The advantages of the silica-modified-fluoride AR coating as
described in the paragraph above include: [0029] 1. Providing
appropriate physical capping so that less environmental substances
can penetrate into the underneath fluoride layers [0030] 2.
Offering surface chemical modification so that the surface is
chemically inactive to environmental substances [0031] 3.
Maintaining a broad-angle AR performance to ensure a high system
throughput [0032] 4. Recovering optical performance by enabling
surface cleaning when contamination occurs [0033] 5. Providing
appropriate surface protection for handling and mounting coated
optical elements
[0034] Fluorides are generally the materials of choice in optical
coatings for laser optics and precision optics operating at 193 nm.
However, the usage of energetic deposition process is restricted
for fluoride materials. The porous nature of thermally-evaporated
fluoride coatings leads to measureable scatter loss and the
coatings are environmentally unstable. In order to cope with the
challenges that fluoride coatings have presented, oxide-fluoride
hybrid ArF laser optic coatings have been developed and
commercialized in laser optics business. The basic concept of the
oxide-fluoride hybrid coatings is to insert F--SiO.sub.2 layers
into fluoride stacks for interface smoothing and to apply an
additional F--SiO.sub.2 layer on top of HR (highly reflective)
coatings for capping, whereas for PR (partially reflective) and AR
coatings, an outermost F--SiO.sub.2 layer is deposited of top of
fluoride multilayers to seal the porous structure of the fluoride
coatings. This technical approach results in low scatter loss and
environmentally stable DUV coatings on flat or near flat surfaces
for a specific angle of incidence.
[0035] For optical systems, especially inspection objectives, AR
coatings with a reflectance less than 0.5% over a broad angle of
incidence is anticipated for lens surfaces which are curved. This
presents some challenges for the existing hybrid AR coatings. The
first challenge is that the oxide-fluoride hybrid AR coating works
only for a specific angle of incidence. In other words, the
oxide-fluoride hybrid AR coating has a limited angular broadness. A
comparison of spectral reflectance an oxide-fluoride hybrid
coating, FIG. 1A, and an all fluoride coating, FIG. 1B, as a
function of angle of incidence of a oxide-fluoride hybrid AR
coating and a standard 2-layer fluoride AR coating is presented in
FIG. 1. The broadness of the angle of incidence with reflectance
less than 0.5% is reduced to 22.degree. from 36.degree. at a
wavelength of 193 nm. Second, the oxide-fluoride hybrid AR coating
is more sensitive to coating thickness falloff when compared to the
standard fluoride AR coating. As plotted in FIG. 4, the spectral
reflectance shift of the oxide-fluoride hybrid AR coating with 2%
thickness falloff. A 61% reduction of angular broadness appears at
a wavelength of 193 nm when a 2% thickness falloff is considered.
For comparison shown in FIG. 5, a 92% reduction of angular
broadness of the standard fluoride AR coating with the same amount
of thickness falloff is considered.
[0036] A 3-step approach is employed to deal with the technical
challenge in order to achieve an appropriate environmental
protection while maintaining low reflectance over a broad angle of
incidence: [0037] 1. The first step is to reduce capping layer
thickness from a half-wave (.about.60 nm) down to less than 25 nm.
[0038] 2. The second step is to replace a quarter-wave fluoride AR
with a non-quarter-wave fluoride AR. [0039] 3. The third step is to
densify the thin silica layer without introducing additional
absorption of the underneath fluoride layers. The densification
process of the thin silica layer includes high temperature
deposition, reversed mask technique and in-situ or post-deposition
plasma-ion treatment..sup.28
[0040] For example, FIG. 6 plots spectral reflectance as a function
of angle of incidence of an oxide-modified fluoride AR coating. The
modified AR coating 60 consists of 3 layers. Starting from the
CaF.sub.2 substrate there are 2 layers of non-quarter-wave
fluorides (0.9 quarter-wave thick GdF.sub.3 and AlF.sub.3) and an
addition of 5 nm-thick silica layer for surface modification. The
shift of a oxide-modified 3-layer AR coating 60 and the AR with 2%
thickness falloff, curve 62. The 3-layer oxide-modified-fluoride AR
coating provides a broad angle of incidence up to 40.degree. (curve
60 in FIG. 6). The angular broadness maintains when a 2% thickness
falloff is considered (curve 62 in FIG. 6).
[0041] Another example of the oxide-modified-fluoride AR coating 70
is presented in FIG. 7, showing spectral reflectance as a function
of angle of incidence. The modified AR coating consists of 3
layers. Starting from CaF.sub.2 substrate there are 2 layers of
non-quarter-wave fluorides (0.78 quarter-wave thick GdF.sub.3 and
AlF.sub.3) and an addition of 10 nm-thick silica layer for surface
modification. The 3-layer oxide-modified-fluoride AR coating
provides a broad angle of incidence up to 38.degree. (curve 70 in
FIG. 7). When a 2% thickness falloff is considered (curve 72 in
FIG. 7), an angular broadness of 40.degree. is achieved.
[0042] There are several advantages of the silica modified fluoride
AR coatings: [0043] 1. To provide appropriate physical capping so
that few environmental substances can penetrate into the underneath
fluoride layers. The top surface contaminates can be cleaned up via
appropriate cleaning methods [0044] 2. To offer surface chemical
modification so that the surface is chemically inactive to
environmental substances [0045] 3. To maintain a broad-angle AR
performance to ensure a high system throughput [0046] 4. To provide
appropriate surface protection for handling and mounting coated
optical elements
[0047] As an added benefit, the oxide-fluoride hybrid DUV coatings
of this disclosure can reduce the risk of laser-induced
contamination. Trace amounts of volatile organic substances are
omnipresent in DUV laser systems from metal housings and organic
potting compound to purge gas lines. Photo-decomposition of the
organic substances under 193 nm laser irradiation is the potential
source of selective contaminations on the optical surfaces.
Accumulated absorption on multiple optical surfaces over a time
period of laser exposure may lead to inacceptable transmission loss
and system degradation. The length of such time period and is
dependent on the power at which the laser operates and the amount
of contaminants to which the optics are exposed. Using thin oxide
films as disclosed herein as a top layer can reduce the risk of
laser-induced contamination on the optical surfaces when compared
to that of fluoride surfaces. Similar effects have been also
reported in space optics where the optics are exposed to
contaminants within the spacecraft and solar radiation unfiltered
by earth's atmosphere.
[0048] It is also here noted that although only 2-layer fluoride AR
coatings are used as examples in this disclosure, silica modified
fluoride AR coating approach described herein can apply to other
multilayer fluoride AR coatings to further broad angular
performance or cover a strong surface curvature with large
thickness falloff, although only 2-layer fluoride AR coatings are
used as examples in this document. It is further noted that the
teaching disclosed herein can be used with having flat surfaces
such as prisms and some mirrors, the teaching can also be applied
to optics having a curvature such as lenses and curved mirrors.
[0049] Thus, in one embodiment the disclosure is directed to an
optical coating for laser optics, the coating consisting of a high
refractive index metal fluoride material, a low refractive index
metal fluoride material and a capping coating selected from the
group consisting of SiO.sub.2 and F--SiO.sub.2 on a substrate,
wherein the high and low index metal fluorides are applied to a
less than quarter wave thickness and the capping coating is applied
to a thick in the range of 5 nm to 25 nm. Each of the high
refractive index and low refractive index metal fluorides are
applied to a thickness less than or equal to 0.9 quarter wave. The
high refractive index material is selected from the group
consisting of GdF.sub.3 and LaF.sub.3, and the low refractive index
material is selected from the group consisting of AlF.sub.3 and
MgF.sub.3.
[0050] In another embodiment the disclosure is directed to an
optical element consisting of a substrate selected from the group
consisting of CaF.sub.2, SiO.sub.2 and F--SiO.sub.2; a first
coating directly on the substrate, the coating being a high
refractive index metal fluoride coating material; a low refractive
index coating on top of the high refractive index material; and a
capping layer on top of the low refractive index material; wherein
each of the high refractive index and low refractive index metal
fluorides have a thickness of less than or equal to 0.9 quarter
wave, and the capping material has a thickness in the range of 5 nm
to 25 nm. The high refractive index material on the element is
selected from the group consisting of GdF.sub.3 and LaF.sub.3 and
the low refractive index material on the element is selected from
the group consisting of AlF.sub.3 and MgF.sub.3, The elements can
be a mirror, lens, laser window or prism. When the substrate is a
F--SiO.sub.2 substrate the substrate contains 0.5 wt. % to 4.5 wt.
% F.
[0051] The disclosure is also directed to a method for making an
optical element having a coating thereon, the method comprising
providing a substrate selected from the group consisting of
CaF.sub.2, SiO.sub.2 and F--SiO.sub.2, applying a coating of a high
refractive index metal fluoride material to the surface of the
substrate using vacuum deposition at a temperate greater than or
equal to 300.degree. C., the reverse mask technique, and in-situ or
post-deposition plasma ion treatment; applying a coating of a low
refractive index metal fluoride material to the surface of the high
refractive index material using vacuum deposition at a temperate
greater than or equal to 300.degree. C., the reverse mask
technique, and in-situ or post-deposition plasma ion treatment; and
depositing a capping layer selected from the group consisting of
SiO.sub.2 and F--SiO.sub.2 on top of the low refractive index
material using vacuum deposition at a temperate greater than or
equal to 300.degree. C., the reverse mask technique, and in-situ or
post-deposition plasma ion treatment; wherein the high index and
low index materials are each deposited to a thickness off less than
or equal to 0.9 quarter wave, and the capping material is deposited
to a thickness in the range of 5 nm to 25 nm.
[0052] It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention
without departing from the spirit or scope of the invention. Thus,
it is intended that the present invention cover modifications and
variations of this invention provided they come within the scope of
the appended claims and their equivalents.
* * * * *