U.S. patent application number 10/809700 was filed with the patent office on 2005-11-24 for protective layers for optical coatings.
This patent application is currently assigned to AFG INDUSTRIES, INC.. Invention is credited to Dannenberg, Rand D., Hukari, Kyle W., Maschwitz, Peter A..
Application Number | 20050260419 10/809700 |
Document ID | / |
Family ID | 27609177 |
Filed Date | 2005-11-24 |
United States Patent
Application |
20050260419 |
Kind Code |
A1 |
Hukari, Kyle W. ; et
al. |
November 24, 2005 |
Protective layers for optical coatings
Abstract
An optical coating on a transparent substrate is provided with a
temporary layer of carbon as protection during manufacturing
against scratches and corrosive environments. When the optical
coating and/or substrate are tempered in an atmosphere reactive to
carbon, such as air, the layer of carbon is removed as a
carbon-containing gas. For an optical coating with a brittle,
glassy, outermost layer furthest from the substrate, additional
protection is provided by a scratch propagation blocker layer
between the outermost layer and the carbon protective layer.
Inventors: |
Hukari, Kyle W.; (Davis,
CA) ; Maschwitz, Peter A.; (Sebastopol, CA) ;
Dannenberg, Rand D.; (Benicia, CA) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
AFG INDUSTRIES, INC.
Kingsport
TN
|
Family ID: |
27609177 |
Appl. No.: |
10/809700 |
Filed: |
March 26, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10809700 |
Mar 26, 2004 |
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10054973 |
Jan 25, 2002 |
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6770321 |
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Current U.S.
Class: |
428/428 ;
428/408; 428/432 |
Current CPC
Class: |
Y10T 428/24975 20150115;
C03C 17/3644 20130101; C03C 17/36 20130101; C03C 2217/78 20130101;
C03C 17/3435 20130101; C03C 17/3634 20130101; Y10T 428/265
20150115; C03C 17/366 20130101; C03C 17/3441 20130101; Y10T
428/24942 20150115; C03C 17/3618 20130101; C03C 2218/355 20130101;
Y10T 428/30 20150115; C03C 17/3626 20130101 |
Class at
Publication: |
428/428 ;
428/432; 428/408 |
International
Class: |
B32B 017/06 |
Claims
1-40. (canceled)
41. A transparent article comprising a substrate; an optical
coating comprising one or more layers on the substrate, where the
one or more layers include furthest from the substrate a
homogeneous outermost layer comprising amorphous silicon nitride;
and a protective coating on the outermost layer, wherein the
protective coating consists of a scratch propagation blocker layer
on the outermost layer, and a layer consisting essentially of
carbon on the scratch propagation blocker layer; and the scratch
propagation blocker layer is a homogeneous layer comprising a
material selected from the group consisting of Ti, Si, Zn, Sn, In,
Zr, Al, Cr, Nb, Mo, Hf, Ta and W; oxides of Ti, Si, Zn, Sn, In, Zr,
Al, Cr, Nb, Mo, Hf, Ta and W; nitrides of Ti, Si, Zn, Sn, In, Zr,
Al, Cr, Nb, Mo, Hf, Ta and W; oxynitrides of Ti, Si, Zn, Sn, In,
Zr, Al, Cr, Nb, Mo, Hf, Ta and W; and mixtures thereof.
42. The transparent article according to claim 41, wherein the
substrate comprises a glass.
43. The transparent article according to claim 42, wherein the
substrate comprises a tempered glass.
44. The transparent article according to claim 41, wherein the
optical coating is a low-emissivity coating.
45. The transparent article according to claim 41, wherein the
optical coating is a tempered coating.
46. The transparent article according to claim 41, wherein the
optical coating contains at least one layer comprising Ag.
47. The transparent article according to claim 41, wherein the
fracture toughness of the scratch propagation blocker layer is
higher than the fracture toughness of the outermost layer.
48. The transparent article according to claim 41, wherein the
scratch propagation blocker layer is from 2 to 8 nm thick.
49. The transparent article according to claim 48, wherein the
scratch propagation blocker layer comprises a material selected
from the group consisting of oxides of Ti, Si, Zn, Sn, In, Zr, Al,
Cr, Nb, Mo, Hf, Ta and W.
50. The transparent article according to claim 48, wherein the
scratch propagation blocker layer comprises a material selected
from the group consisting of oxynitrides of Ti, Si, Zn, Sn, In, Zr,
Al, Cr, Nb, Mo, Hf, Ta and W.
51. The transparent article according to claim 41, wherein the
layer consisting essentially of carbon is doped with nitrogen.
52. The transparent article according to claim 41, wherein the
layer consisting essentially of carbon consists of carbon and
unavoidable impurities.
53. The transparent article according to claim 41, wherein the
carbon in the layer consisting essentially of carbon comprises at
least one form of carbon selected from the group consisting of
diamond-like carbon and graphite.
54. The transparent article according to claim 41, wherein the
layer consisting essentially of carbon is from 1 to 10 nm
thick.
55. A transparent article comprising a substrate; an optical
coating comprising one or more layers on the substrate, where the
one or more layers include furthest from the substrate a
homogeneous outermost layer comprising silicon nitride; and a
protective coating on the outermost layer, wherein the protective
coating consists of a scratch propagation blocker layer on the
outermost layer, and a layer consisting essentially of carbon on
the scratch propagation blocker layer; and the scratch propagation
blocker layer is a homogeneous layer 2 to 8 nm thick comprising a
material selected from the group consisting of Ti, Si, Zn, Sn, In,
Zr, Al, Cr, Nb, Mo, Hf, Ta and W; oxides of Ti, Si, Zn, Sn, In, Zr,
Al, Cr, Nb, Mo, Hf, Ta and W; nitrides of Ti, Si, Zn, Sn, In, Zr,
Al, Cr, Nb, Mo, Hf, Ta and W; oxynitrides of Ti, Si, Zn, Sn, In,
Zr, Al, Cr, Nb, Mo, Hf, Ta and W; and mixtures thereof.
56. The transparent article according to claim 55, wherein the
substrate comprises a glass.
57. The transparent article according to claim 56, wherein the
substrate comprises a tempered glass.
58. The transparent article according to claim 55, wherein the
optical coating is a low-emissivity coating.
59. The transparent article according to claim 55, wherein the
optical coating is a tempered coating.
60. The transparent article according to claim 55, wherein the
optical coating contains at least one layer comprising Ag.
61. The transparent article according to claim 55, wherein the
fracture toughness of the scratch propagation blocker layer is
higher than the fracture toughness of the outermost layer.
62. The transparent article according to claim 55, wherein the
scratch propagation blocker layer comprises a material selected
from the group consisting of oxides of Ti, Si, Zn, Sn, In, Zr, Al,
Cr, Nb, Mo, Hf, Ta and W.
63. The transparent article according to claim 55, wherein the
scratch propagation blocker layer comprises a material selected
from the group consisting of oxynitrides of Ti, Si, Zn, Sn, In, Zr,
Al, Cr, Nb, Mo, Hf, Ta and W.
64. The transparent article according to claim 55, wherein the
layer consisting essentially of carbon is doped with nitrogen.
65. The transparent article according to claim 55, wherein the
layer consisting essentially of carbon consists of carbon and
unavoidable impurities.
66. The transparent article according to claim 55, wherein the
carbon in the layer consisting essentially of carbon comprises at
least one form of carbon selected from the group consisting of
diamond-like carbon and graphite.
67. The transparent article according to claim 55, wherein the
layer consisting essentially of carbon is from 1 to 10 nm
thick.
68. A transparent article comprising a substrate; an optical
coating comprising one or more layers on the substrate, where the
one or more layers include furthest from the substrate a
homogeneous outermost layer comprising silicon nitride; and a
protective coating on the outermost layer, wherein the protective
coating consists of a scratch propagation blocker layer on the
outermost layer; and the scratch propagation blocker layer is a
homogeneous layer comprising a material selected from the group
consisting of Ti, Si, Zn, Sn, In, Zr, Al, Cr, Nb, Mo, Hf, Ta and W;
oxides of Ti, Si, Zn, Sn, In, Zr, Al, Cr, Nb, Mo, Hf, Ta and W;
nitrides of Ti, Si, Zn, Sn, In, Zr, Al, Cr, Nb, Mo, Hf, Ta and W;
oxynitrides of Ti, Si, Zn, Sn, In, Zr, Al, Cr, Nb, Mo, Hf, Ta and
W; and mixtures thereof.
69. The transparent article according to claim 68, wherein the
substrate comprises a glass.
70. The transparent article according to claim 69, wherein the
substrate comprises a tempered glass.
71. The transparent article according to claim 68, wherein the
optical coating is a low-emissivity coating.
72. The transparent article according to claim 68, wherein the
optical coating is a tempered coating.
73. The transparent article according to claim 68, wherein the
optical coating contains at least one layer comprising Ag.
74. The transparent article according to claim 68, wherein the
outermost layer comprises amorphous silicon nitride.
75. The transparent article according to claim 68, wherein the
fracture toughness of the scratch propagation blocker layer is
higher than the fracture toughness of the outermost layer.
76. The transparent article according to claim 68, wherein the
scratch propagation blocker layer is from 2 to 8 nm thick.
77. The transparent article according to claim 76, wherein the
scratch propagation blocker layer comprises a material selected
from the group consisting of oxides of Ti, Si, Zn, Sn, In, Zr, Al,
Cr, Nb, Mo, Hf, Ta and W.
78. The transparent article according to claim 76, wherein the
scratch propagation blocker layer comprises a material selected
from the group consisting of oxynitrides of Ti, Si, Zn, Sn, In, Zr,
Al, Cr, Nb, Mo, Hf, Ta and W.
79. The transparent article according to claim 68, wherein the
scratch propagation blocker layer is in contact with air.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to protective layers applied to
optical coatings on transparent substrates. In particular, the
invention relates to the use of a temporary protective layer of
carbon. In addition, the invention relates to a scratch propagation
blocker (SPB) protective layer applied to the outermost layer of
various optical coatings.
[0003] 2. Discussion of the Background
[0004] Optical coatings are deposited on transparent substrates to
reflect or otherwise alter the transmission of some or all of the
radiation incident on the substrates. For example, the optical
coating of a mirror is designed to reflect visible light.
Low-emissivity optical coatings are designed to reduce the
transmission of infrared radiation. Optical coatings generally
include two or more different layers each having a thickness in a
range of from less than 1 nm to over 500 nm.
[0005] Optical coatings are frequently damaged during shipping and
handling by scratching and by exposure to corrosive environments.
Silver based low-emissivity coatings in particular have been
plagued with corrosion problems since their introduction into the
fenestration marketplace decades ago. Attempts at improving the
durability of optical coatings have included the application of a
temporary protective layer such as a plastic adhesive backed film.
Other protective layers have been formed by applying and curing
solvent based polymers on glass.
[0006] However, a number of problems are associated with using
adhesive films and polymer films as protective layers on optical
coatings. Expensive, specialized equipment is required to apply the
adhesive films and the polymer films to optical coatings. When an
adhesive film is pulled away from an optical coating, the adhesive
film runs the risk of removing portions of the optical coating.
Even if portions of the optical coating are not removed, the force
on the optical coating associated with removing the adhesive film
can damage the optical coating. A solvent based polymer film
applied to an optical coating must be dried and the solvent removed
in an environmentally friendly manner. Removal of the polymer film
from an optical coating requires specialized washing that can
easily damage the optical coating.
[0007] For protection from corrosion, most silver based
low-emissivity stacks in use today make use of barrier or cladding
layers in direct contact and on one or both sides of the silver
layers. It is well known in the art that various thin film layers
can function as barriers to movement of corrosive fluids such as
water vapor and oxygen. Metals layers are known to be particularly
effective diffusion barriers due to their ability to physically and
chemically inhibit diffusion of corrosive fluids. Metal layers tend
to be more effective physical barriers to diffusion than dielectric
layers such as oxides, because both evaporated and sputtered metal
layers tend to contain fewer pinhole defects than oxide layers.
Metal layers also tend to chemically block diffusion by reacting
with fluids diffusing through a pinhole to stop the movement of all
chemically bound fluid molecules. The bound fluid molecules in turn
restrict the passage of additional fluid through the pinhole. The
more reactive metals are particularly effective for chemically
blocking.
[0008] Tempering greatly reduces the corrosion problems associated
with silver based low-emissivity coatings. Tempering results in an
atomic level restructuring to a lower energy state and renders the
silver far less prone to corrosion. Tempering also improves the
hardness and scratch resistance of optical coatings.
[0009] However, until optical coatings are tempered, the coatings
remain particularly susceptible to damage from scratching and
corrosion. Even after tempering, optical coatings are not immune
from scratching and corrosion.
[0010] Scratches in an optical coating frequently do not become
visible until after the coating is heated and tempered, which can
cause the scratches to grow and propagate.
[0011] Carbon has been used as a protective coating on glass
substrates. For example, U.S. Pat. No. 6,303,226 discloses the use
of an amorphous, diamond-like carbon (DLC), protective layer on a
glass substrate.
[0012] There is a need for improved methods and layers for
protecting optical coatings.
SUMMARY OF THE INVENTION
[0013] The present invention provides a method of making a
transparent article with a reduced number of scratches and other
surface defects. The transparent article includes a optical coating
on a transparent substrate. According to the invention, a
protective coating is formed on the optical coating that improves
the durability and scratch resistance of the optical coating,
particularly during manufacturing.
[0014] The protective coating can include a layer consisting
essentially of carbon. The carbon protective layer is formed on the
optical coating before tempering. During shipping and handling of
the untempered optical coating, the carbon layer serves as a low
friction, protective layer against scratches. Heating and tempering
the optical coating and/or transparent substrate in an atmosphere
reactive to carbon consumes the carbon protective layer, thus
eliminating any scratches or other surface defects in the carbon.
The carbon protective layer is converted into a carbon containing
gas, leaving behind a relatively scratch-free optical coating.
[0015] The protective coating can also include a thin protective
layer of a scratch propagation blocker (SPB) material. The SPB
material inhibits the propagation of scratches into the brittle,
glassy, outermost layer of various optical coatings during
tempering. SPB materials such as Ti, Si, Zn, Sn, In, Zr, Al, Cr,
Nb, Mo, Hf, Ta and W, and oxides and nitrides thereof, are suitable
for use on an outermost layer of silicon nitride (e.g.,
Si.sub.3N.sub.4). The SPB layer can be formed by depositing on the
outermost layer of an optical coating a diffusion barrier layer of
at least one metal, metal sub-oxide or metal sub-nitride of Ti, Si,
Zn, Sn, In, Zr, Al, Cr, Nb, Mo, Hf, Ta or W; and then reacting the
diffusion barrier layer with an oxygen containing atmosphere such
as air to form a metal oxide SPB layer including at least one of
TiO.sub.2, SiO.sub.2, ZnO, SnO.sub.2, In.sub.2O.sub.3, ZrO.sub.2,
Al.sub.2O.sub.3, Cr.sub.2O.sub.3, Nb.sub.2O.sub.5, MoO.sub.3,
HfO.sub.2, Ta.sub.2O.sub.5 and WO.sub.3. The SPB layer can be used
with or without a carbon protective layer on the SPB layer.
[0016] Use of the temporary carbon protective layer when
manufacturing a transparent article having an optical coating
significantly reduces the number and severity of scratches
introduced into the optical coating by the manufacturing process.
Because the carbon layer is removed during tempering, the carbon
layer does not affect the optical properties of the optical
coating. While the SPB layer is not removed during tempering and
may affect the optical properties of an optical coating, the SPB
layer, by inhibiting scratch propagation, is particularly useful in
protecting a brittle, glassy, outermost layer of an optical coating
from the formation of visible scratches. A metal, metal sub-oxide
or metal sub-nitride layer is particularly useful in providing
corrosion protection before tempering and can be converted by
tempering in an atmosphere containing oxygen to a metal oxide SPB
layer that is essentially transparent to visible light.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The preferred embodiments of this invention will be
described in detail with reference to the following figures.
[0018] FIGS. 1A-1C show the deposition of a carbon protective layer
on an optical coating on a glass substrate and the subsequent
removal of the carbon protective layer.
[0019] FIG. 2 shows a glass substrate coated with an optical
coating, a scratch propagation blocker layer and a carbon
protective layer.
[0020] FIG. 3. shows the propagation of a scratch through a layer
of Si.sub.3N.sub.4.
[0021] FIGS. 4A-4C show the deposition of a metal layer on an
optical coating on a glass substrate and the subsequent conversion
of the metal layer to a metal oxide scratch propagation blocker
layer.
[0022] FIG. 5 compares glass substrates, having the same optical
coating but with and without a carbon protective layer, when
scratched.
[0023] FIG. 6 compares glass substrates, having the same optical
coating but with and without a carbon protective layer, when
scratched.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0024] The present invention provides a protective coating on an
optical coating deposited on a transparent substrate to inhibit the
formation of scratches on and corrosion of the optical coating.
[0025] The transparent substrate can be a plastic or a glass.
Preferably, the transparent substrate is a glass that can be
tempered by heating and quenching.
[0026] In embodiments, the protective coating includes a carbon
protective layer. Carbon is a classic low-friction material. Even
if an abrasive succeeds in initially scratching carbon, the
abrasive often becomes coated with carbon. Subsequent contact
between the carbon coated abrasive and carbon is characterized by
one of the lowest coefficients of friction,
.mu..sub.static=.mu..sub.kinetic=0.1 to 0.2. Thus, the carbon
coated abrasive tends to slide off of the carbon, doing no further
damage to the carbon. Carbon is also inert in many corrosive
environments and exhibits good resistance to alkalies and most
acids. Thus, a carbon layer on an optical coating can protect the
optical coating from scratches and environmental corrosion during
handling.
[0027] FIGS. 1A-1C illustrate embodiments of the invention in which
a temporary carbon layer is formed on an optical coating to protect
the optical coating from scratching and environmental corrosion
during manufacturing. FIG. 1A shows a glass substrate 1 coated with
an optical coating 2. FIG. 1B shows that to protect the optical
coating from scratches and environmental corrosion during shipping
and handling, a carbon protective layer 3 is deposited on the
optical coating 2. FIG. 1C shows that after tempering the optical
coating 2 and/or the glass substrate 1 at elevated temperatures in
an atmosphere reactive to carbon, the carbon protective layer 3 is
converted into a carbon containing gas, eliminating any scratches
or other defects that had been present in the carbon protective
layer 3.
[0028] The carbon protective layer is a layer consisting
essentially of carbon. The term "consisting essentially of", while
not excluding unavoidable impurities, excludes other unspecified
elements and compounds that would be left behind as a solid residue
when the carbon is reacted to completion with a reactive atmosphere
to form a carbon containing gas. In embodiments, the carbon layer
consists of carbon and unavoidable impurities.
[0029] The carbon layer can be deposited on the optical coating by
a vapor deposition process. Techniques and processes for vapor
depositing carbon are well known in the art. Suitable vapor
deposition processes include evaporation and plasma deposition
processes such as plasma chemical vapor deposition, ion
implantation and sputtering. The sputtering can be DC or RF. An
inert gas such as Ar, with or without small amounts of additional
gases such as hydrogen and nitrogen, can be used in the plasma
deposition processes to form the carbon layer. The presence of 1 to
10% nitrogen in the inert gas favors the deposition of graphitic
carbon. The nitrogen in the inert gas can be used to dope the
carbon with nitrogen.
[0030] The carbon layer can include one or more phases of carbon,
such as graphite, diamond and amorphous phases of carbon. The
carbon layer can also include diamond-like carbon. The carbon in
graphite has sp2 bonding. The carbon in diamond has sp3 bonding.
Amorphous carbon generally includes both sp2 and sp3 bonding, but
has no long range order. Diamond-like carbon also includes both sp2
and sp3 bonding, and exhibits a hardness resembling that of
diamond.
[0031] The carbon layer can be from 1 to 10 nm thick. A carbon
layer less than 1 nm thick does not provide adequate scratch
resistance. A carbon layer more than 10 nm thick becomes difficult
to remove completely in a atmosphere reactive to carbon.
[0032] The reactive atmosphere used to convert the carbon
protective layer into a carbon containing gas can include various
gases known in the art to be reactive with carbon. For example, the
reactive atmosphere can include hydrogen, which can convert the
carbon into methane gas. A halogen, such as fluorine or chlorine,
can be used to form at elevated temperatures a tetrahalomethane gas
such as CF.sub.4 or CCl.sub.4. Oxygen in a reactive atmosphere can
be used to form carbon monoxide and carbon dioxide gases. Because
optical coatings and glasses generally contain various oxides that
are inert in oxygen, the atmosphere reactive with carbon preferably
contains oxygen. Because air, which contains O.sub.2 is inexpensive
and readily available, more preferably the reactive atmosphere is
air.
[0033] Tempering is a process which involves heating a material to
elevated temperatures and then quenching. Tempering is known to
significantly increase the strength and toughness of glass and of
optical coatings on glass. Glass can be tempered by heating to a
temperature in the range of 400 to 650.degree. C. followed by
quenching to room temperature. Optical coatings including Ag layers
can be tempered by heating to a temperature in a range below the
960.degree. C. melting point of Ag followed by quenching to room
temperature. For example, a low-emissivity optical coating
including an Ag layer can be tempered by heating to about
730.degree. C. for a few minutes following by quenching.
Preferably, the glass and optical coatings are tempered at a
temperature of at least 400.degree. C. In embodiments of the
invention, both the glass and the optical coating are tempered in
an oven held at an elevated temperature. In other embodiments, to
avoid having to heat the entire mass of the glass, only the optical
coating is tempered. For example, instead of being heated in an
oven, the optical coating can be heated by a flame or high
intensity lamp to a temperature sufficient to both temper the
optical coating and burn away the protective carbon layer.
[0034] Thus, tempering an optical coating covered with a carbon
protective layer in an atmosphere reactive with carbon can cause
the carbon to form a carbon containing gas and leave the surface of
the optical coating. Any scratches in the carbon layer disappear
along with the carbon layer. Preferably, the reactive atmosphere
tempering removes all of the carbon protective layer from the
optical coating.
[0035] The carbon protective layer can protect an optical coating
from scratches caused during the manufacture of the coating by,
e.g., shipping and handling. In addition, the carbon protective
layer can protect an optical coating from corrosive environments
that might develop when the optical coating with the carbon
protective layer is stored in air for one or more days or is
washed. Preferably, the number of scratches in the optical coating
immediately after the carbon protective layer is removed is no more
than 110% of the number of scratches in the optical coating
immediately before the carbon was deposited on the optical
coating.
[0036] In embodiments of the present invention, between the carbon
protective layer and the optical coating, an SPB layer can be
formed. Preferably, the SPB layer has a uniform composition and is
homogeneous throughout. An SPB layer is made from a material having
the property of inhibiting during tempering the propagation of
scratches and cracks into the outermost layer of an optical
coating. Different outermost layers require different materials in
an SPB layer. The material forming the SPB layer should be less
brittle and glass-like than the outermost layer of the optical
coating. Preferably, the fracture toughness of the SPB material is
higher than that of the outermost layer.
[0037] FIG. 2 shows embodiments of the invention in which a SPB
layer 4 is sandwiched between a carbon protective layer 3 and an
outermost Si.sub.3N.sub.4 layer 2a of an optical coating 2. Both
the SPB layer 4 and the carbon protective layer 3 provide scratch
protection to the optical coating 2. In particular, the SPB layer 4
inhibits the propagation of scratches in the carbon protective
layer 3 down to and into the Si.sub.3N.sub.4 layer 2a.
[0038] Preferably the silicon nitride outermost layer has a uniform
composition and is homogeneous throughout.
[0039] An outermost layer of amorphous silicon nitride (e.g.,
amorphous Si.sub.3N.sub.4) is preferred in an optical coating on
glass subject to tempering. Amorphous silicon nitride does not
undergo a phase change upon heating to the temperatures necessary
to temper glass. Furthermore, the density of amorphous silicon
nitride is the same before and after the tempering, so the
tempering does not leave stresses at the interface of the silicon
nitride and the rest of the optical coating that could lead to
delamination.
[0040] The amorphous silicon nitride also inhibits the formation of
haze in the optical coating. Haze develops when materials mix
together to form a two phase system causing the index of refraction
to vary as a function of position throughout a layer. Because the
phase stability of silicon nitride prevents mixing, the haze in
optical coatings with an outermost silicon nitride is low after
tempering.
[0041] Since the silicon nitride remains amorphous, there is less
atomic motion at the interfaces between layers of the optical
coating than there would be if there were a phase change, which
results in better retention of the initial adhesion between
layers.
[0042] A problem with an outermost layer of amorphous silicon
nitride in an optical coating is that the covalent bonding and
amorphous structure of the silicon nitride results in a stiff
material with crack propagation properties similar to those of
glass. Small cracks propagate easily through stiff, glassy
materials.
[0043] FIG. 3 illustrates a possible mechanism by which cracks can
propagate through an optical coating 2 having an outermost layer of
silicon nitride. Initially small scratches are shallow and not
detectable by the "naked eye" inspection methods used on most
tempering lines. This is because the scratches do not penetrate
completely through the outermost silicon nitride. However, upon
heating the small cracks propagate through the silicon nitride to
underlying layers of, e.g., Ag. Once exposed by the crack, the Ag
can agglomerate at its unconstrained surface. When the Ag
agglomerates, the crack becomes visible and the part must be
rejected.
[0044] In the embodiments shown in FIG. 2, cracks in tempered
optical coatings with silicon nitride outermost layers are
minimized by depositing before tempering an SPB layer on the
silicon nitride and a C layer on the SPB layer. The same sputtering
equipment can be used to deposit the SPB/C combination and the
optical coating onto glass.
[0045] As discussed above, carbon provides a classic low-friction
surface. Even when an abrasive initially scratches carbon, the
abrasive becomes coated with carbon, leading to carbon-on-carbon
sliding with extremely low friction.
[0046] If an abrasive succeeds in puncturing the protective carbon
layer, then the abrasive will encounter the SPB layer. However,
most scratches or cracks formed by the abrasive will not propagate
through the SPB layer upon tempering. Although, unlike the carbon
protective layer, the SPB layer remains after tempering, most
scratches in the SPB remain invisible to the naked eye.
[0047] Suitable materials for forming an SPB layer include metals
such as Ti, Si, Zn, Sn, In, Zr, Al, Cr, Nb, Mo, Hf, Ta and W;
oxides of these metals; and nitrides of these metals.
[0048] The term "oxides" as used herein includes stoichiometric
oxides; superoxides, containing more than a stoichiometric amount
of oxygen; and suboxides, containing less than a stoichiometric
amount of oxygen. The term "metal suboxide" as used herein includes
metals doped with small amounts, e.g, 0.1-10 atomic %, of
oxygen.
[0049] The term "nitrides" as used herein includes stoichiometric
nitrides; supernitrides, containing more than a stoichiometric
amount of nitrogen; and subnitrides, containing less than a
stoichiometric amount of nitrogen. The term "metal subnitride" as
used herein includes metals doped with small amounts, e.g., 0.1-10
atomic %, of nitrogen.
[0050] Suitable stoichiometric oxides for forming an SPB layer
include TiO.sub.2, SiO.sub.2, ZnO, SnO.sub.2, n.sub.2O.sub.3,
ZrO.sub.2, Al.sub.2O.sub.3, Cr.sub.2O.sub.3, Nb.sub.2O.sub.5,
MoO.sub.3, HfO.sub.2, Ta.sub.2O.sub.5, WO.sub.3. Suitable
stoichiometric nitrides for forming an SPB layer include TiN.
TiO.sub.2 in particular is very good at inhibiting scratches. The
SPB layer can be formed by vapor deposition techniques known in the
art.
[0051] The SPB layer can be from 2 to 8 nm thick. When the SPB
layer is a stoichiometric oxide or nitride, the SPB layer is
preferably from 2 to 8 nm, more preferably from 3 to 6 nm, thick.
When the SPB layer is a metal, the SPB layer is preferably from 4
to 8 nm, more preferably from 4 to 6 nm, thick. If the
stoichiometric oxide or nitride SPB layer is thinner than 2 nm, or
the metal SPB layer is thinner than 4 nm, then the SPB material
exhibits a decreased tendency to inhibit the propagation of
scratches. There is little advantage to an SPB layer thickness of
greater than 8 nm, because the scratch propagation inhibition
resulting from the SPB layer saturates at a thickness of about 8 nm
and the influence of the SPB layer on the optical characteristics
of an optical coating, which must be taken into account, increases
with SPB layer thickness. However, as discussed below, metals,
metal suboxides and metal subnitrides can be used as diffusion
barrier layers greater than 2 nm thick that, after being oxidized
during tempering, can form metal oxide SPB layers that can be
substantially invisible.
[0052] As discussed above, in embodiments the SPB layer can be
combined with a carbon protective layer on top of the SPB layer. In
other embodiments, the SPB layer can form the only protective layer
on an optical coating. An SPB layer can help to prevent scratching
and scratch propagation on handling, even without a protective
carbon layer.
[0053] In embodiments of the invention, the SPB layer can be formed
by oxidizing a diffusion barrier layer used to provide corrosion
protection to an optical coating before tempering. The diffusion
barrier layer is a metal, metal suboxide or metal subnitride
material including an metal element selected from Ti, Si, Zn, Sn,
In, Zr, Al, Cr, Nb, Mo, Hf, Ta and W. The diffusion barrier layer
is deposited on the outermost layer of an optical coating before
tempering the optical coating. Tempering the optical coating in an
atmosphere containing oxygen converts the diffusion barrier layer
into a metal oxide SPB layer. Preferably, the diffusion barrier
layer contains Ti, Zr or Al, which upon heating in air can be
converted to SPB layers of the metal oxides TiO.sub.2, ZrO.sub.2 or
Al.sub.2O.sub.3, respectively. Preferably, the metal suboxide
contains about 80% or less of the oxygen present in the most fully
oxidized stoichiometric oxide of the metal. Metal suboxide films
deposited with about 80% or less of full oxidation tend to form
better diffusion barriers than films reactively deposited with more
than about 80% of full oxidation.
[0054] As discussed above, metal layers are known to be
particularly effective barriers to diffusive movement of corrosive
fluids. Metals suboxides and metal subnitrides function similarly
to metals as diffusion barriers. Metal suboxides and metal
subnitrides tend to form dense layers when sputtered or evaporated
and chemically inhibit diffusion of oxygen and water vapor to a
greater extent than the corresponding fully oxidized metals.
[0055] Metal suboxides and metal subnitrides can be formed by vapor
deposition methods known in the art. For example, metal suboxides
and metal subnitrides can be formed by vapor depositing a metal in
an atmosphere containing a controlled amount of oxygen and
nitrogen.
[0056] Metal suboxides and subnitrides tend to be optically
absorbing and reduce visible transmission of an optical coating
until they are heated and reacted to a fully oxidized state.
[0057] The bonding between nitrogen and metal in a metal subnitride
is typically not as strong as the bonding between oxygen and metal
in a metal suboxide. Heating a metal subnitride in an atmosphere
containing oxygen will generally convert the metal subnitride to
the corresponding metal oxide or at least to a metal oxynitride
that is substantially transparent.
[0058] The diffusion barrier layer can be between about 4 and 8 nm
thick, preferably between 4 and 6 nm thick. Typically, reactive
metal layers will fully oxidize in room temperature air if the
metal is 2 nm or less in thickness. Thicker metal layers will often
oxidize to a depth of 2 nm while the remainder of the layer remains
metallic. The oxidation process can be driven deeper if the metal
is exposed to an energy source such as heat or a more chemically
reactive environment than air. In embodiments of the invention, the
diffusion barrier layer is deposited thicker than the thickness
which allows complete oxidation in room temperature air. In this
way, the layer remains metallic and functions as an effective
corrosion barrier prior to tempering. To provide the scratch
propagation resistance discussed above before oxidation, preferably
the diffusion barrier layer is deposited to a thickness of 4 nm or
more. To ensure that the diffusion barrier layer is fully oxidized
during the tempering process, the diffusion barrier layer is
deposited to a thickness of 8 nm or less, preferably 6 nm or
less.
[0059] When a metal, metal suboxide or metal subnitride layer 4 to
6 nm thick is fully oxidized, it tends to have little optical
effect on the optical stack. Because metal oxides are more
transparent to visible light than metals, metal suboxides and metal
subnitrides, fully oxidizing the diffusion barrier layer results in
a metal oxide SPB layer that is effectively optically
invisible.
[0060] Using the tempering process to form a metal oxide SPB layer
from a diffusion barrier on a temperable low-emissivity optical
coating both protects the coating from corrosion before tempering
and eliminates many undesirable optical effects associated with
having a diffusion barrier layer as the SPB layer on the
low-emissivity optical coating after tempering.
[0061] In further embodiments, a carbon layer can be deposited onto
the diffusion barrier layer on the temperable low-emissivity
optical coating as additional protection for the optical coating.
Tempering the optical coatings by heating in air can then both burn
away the carbon layer and convert the diffusion barrier layer into
a transparent metal oxide SPB layer.
[0062] FIGS. 4A-4C illustrate embodiments of the invention in which
a metal oxide SPB layer is formed by depositing a metal layer onto
an optical coating and then reacting the metal in an atmosphere
containing oxygen to form the oxide. FIG. 4A shows a glass
substrate 1 provided with an optical coating 2. FIG. 4B shows a
metal layer 5 deposited on the optical coating 2. FIG. 4C shows
that upon heating the metal layer 5 in an atmosphere containing
oxygen, such as air, the metal layer 5 is converted to a metal
oxide scratch propagation blocker layer 4.
EXAMPLES
[0063] The following examples are intended to illustrate the
invention further but not to limit the field of use as defined in
the attached patent claims.
Example 1
[0064] FIGS. 5(1)-5(4) are optical microscope photographs showing
the significant decrease in scratches that results according to the
present invention by depositing a temporary carbon protective layer
on an optical coating before tempering, and then removing the
carbon protective layer by tempering in a reactive atmosphere. Each
sample had the same optical coating. The optical coating included
multiple layers of Zn, Ag, and NiCr, along with an outermost layer
of 36 nm thick Si. A carbon protective layer 1 nm thick was
deposited on the optical coatings of the samples shown in FIGS.
5(1) and 5(2), but not on the optical coatings of the samples shown
in FIGS. 5(3) and 5(4). The samples were then scratched under the
same conditions using the same commercial abrasion wheel (a
TABER.RTM. wheel). FIGS. 5(1) and 5(2) show different areas of
carbon protected samples representative of the worst scratching.
The scratch in FIG. 5(1) is about 10-15 nm wide. FIGS. 5(1) and
5(3) show scratched samples before tempering. FIGS. 5(2) and 5(4)
show scratched samples after tempering in air at 730.degree. C. for
four minutes. During the tempering in air, the width of the
scratches roughly doubled. The carbon protective layer on the
sample shown in FIG. 5(2) burned away during the tempering along
with most of the scratches.
[0065] FIG. 5 shows that the presence of a carbon protective layer
on an optical coating before tempering greatly reduces the number
of scratches appearing on the optical coatings after tempering in
air when the carbon layer has burned away.
Example 2
[0066] FIG. 6 shows nine samples (numbered 1 through 9) comparing
the effect of different carbon protective layer thicknesses on
scratches remaining on optical coatings after tempering. Each
sample had the same optical coating. The optical coating included
multiple layers of Zn, Ag, and NiCr, along with an outermost layer
of 36 nm thick Si. Carbon protective layers of various thicknesses
were deposited on the samples as shown in the following Table 1.
Samples 1-2 contained no carbon protective layer.
1 TABLE 1 SAMPLE CARBON THICKNESS (nm) 1 none 2 none 3 1 4 1.2 5
1.8 6 5 7 5 8 10 9 15
[0067] The samples were scratched under the same conditions using
the same commercial abrasion wheel (a TABER.RTM. wheel). The nine
samples were each tempered in air at 730.degree. C. for four
minutes. FIG. 6 shows Samples 1-9 after the tempering.
[0068] As shown in FIG. 6, Samples 3-9, which included temporary
carbon protective layers, had significantly fewer scratches after
tempering in air than did Samples 1-2, which did not include carbon
protective layers. The color of Samples 3-8 after tempering was the
same as the color of Samples 1-2 before tempering, indicating that
the carbon layer on Samples 3-8 was completely removed. A trace of
carbon remained on Sample 9 after the tempering.
Example 3
[0069] Individual protective layers of various SPB materials and
carbon were deposited onto identical optical coatings on glass. The
protective layers were scratched under the same conditions using
the same commercial abrasion wheel (a TABER.RTM. wheel). Table 2
shows the relative abilities of individual SPB materials and of
carbon to lessen scratch damage.
2TABLE 2 PROTECTIVE LAYER (SPB or C) THICKNESS (nm) DAMAGE (%)
unprotected (standard) -- 100 SiO.sub.2 2 60 TiN 2 30 TiO.sub.2 2
30 ZnO 2 10 C 1 10 C 10 2
[0070] In Table 1, the "% Damage" is the approximate number of
scratches per unit length perpendicular to the direction of the
abrasive tool.
[0071] Table 2 shows that an SPB layer can help to prevent
scratching and scratch propagation on handling, even without a
protective carbon layer. Combined, the SPB and C layers can have an
even greater effect in inhibiting scratches. The thicknesses of
each SPB and C layer can be varied as needed.
Example 4
[0072] Zr layers of different thicknesses are deposited onto
identical silver based low-emissivity optical coatings on glass
substrates. The Zr coated optical coatings are exposed to room
temperature air having a relative humidity of 80% for 24 hours. The
optical coatings are then tempered at 730.degree. C. in air. Zr
layers 2 mm and 3 mm thick are found to provide no corrosion
protection to the silver based low-emissivity coatings. In
contrast, Zr layers 4 nm and 8 nm thick are found to provide
substantial corrosion protection to the silver based low-emissivity
coatings.
[0073] While the present invention has been described with respect
to specific embodiments, it is not confined to the specific details
set forth, but includes various changes and modifications that may
suggest themselves to those skilled in the art, all falling within
the scope of the invention as defined by the following claims.
* * * * *