U.S. patent application number 11/365528 was filed with the patent office on 2006-07-06 for double silver low-emissivity and solar control coatings.
This patent application is currently assigned to AFG INDUSTRIES, INC.. Invention is credited to Joe Countrywood, Rang Dannenberg, Darin Glenn, Herb Johnson, Peter A. Sieck.
Application Number | 20060147727 11/365528 |
Document ID | / |
Family ID | 25481274 |
Filed Date | 2006-07-06 |
United States Patent
Application |
20060147727 |
Kind Code |
A1 |
Glenn; Darin ; et
al. |
July 6, 2006 |
Double silver low-emissivity and solar control coatings
Abstract
A low-emissivity multilayer coating includes, in order outward
from the substrate, a first layer including a layer containing
titanium oxide, a layer containing silicon nitride, or a sublayer
layer containing titanium oxide in combination with a sublayer
containing silicon nitride; a second layer including Ag; a third
layer including at least one layer selected from titanium oxide
layers and silicon nitride layers; a fourth layer including Ag; and
a fifth layer including silicon nitride. The color of the coatings
can be varied over a wide range by controlling the thicknesses of
the layers of titanium oxide, silicon nitride and Ag. A diffusion
barrier of oxidized metal protects relatively thin, high electrical
conductivity, pinhole free Ag films grown preferentially on zinc
oxide substrates. Oxygen and/or nitrogen in the Ag films improves
the thermal and mechanical stability of the Ag. Dividing the first
layer of titanium oxide, the Ag layers, and/or the third layer with
a sublayer of oxidized metal can provide greater thermal and
mechanical stability to the respective layers.
Inventors: |
Glenn; Darin; (Hampton,
IA) ; Johnson; Herb; (Kingsport, TN) ;
Dannenberg; Rang; (Benicia, CA) ; Sieck; Peter
A.; (Santa Rosa, CA) ; Countrywood; Joe;
(Napa, 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: |
25481274 |
Appl. No.: |
11/365528 |
Filed: |
March 2, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10355018 |
Jan 31, 2003 |
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11365528 |
Mar 2, 2006 |
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09944372 |
Sep 4, 2001 |
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10355018 |
Jan 31, 2003 |
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Current U.S.
Class: |
428/432 ;
427/402; 427/404; 428/428; 428/433; 428/689; 428/698 |
Current CPC
Class: |
C03C 17/3652 20130101;
G02B 5/282 20130101; C03C 17/366 20130101; Y10T 428/24975 20150115;
Y10T 428/265 20150115; C03C 2217/78 20130101; C03C 17/3618
20130101; C03C 17/3639 20130101; C03C 17/36 20130101; C03C 17/3644
20130101; C03C 17/3626 20130101; Y10T 428/12618 20150115 |
Class at
Publication: |
428/432 ;
428/428; 428/433; 428/689; 428/698; 427/404; 427/402 |
International
Class: |
B05D 1/36 20060101
B05D001/36; B32B 17/06 20060101 B32B017/06; B32B 19/00 20060101
B32B019/00; B32B 9/00 20060101 B32B009/00 |
Claims
1-22. (canceled)
23. A low-emissivity coating on a transparent substrate, the
coating comprising, in numerical order outward from the substrate,
a first layer from 5 to 20 nm thick including a silicon nitride; a
second layer from 1 to 20 nm thick including a zinc oxide; a third
layer from 10 to 14 nm thick including Ag; a fourth layer from 2 to
8 nm thick including a partially oxidized Ni--Cr alloy; a fifth
layer from 45 to 90 nm thick including a silicon nitride; a sixth
layer from 1 to 20 nm thick including a zinc oxide; a seventh layer
from 12 to 18 nm thick including Ag; an eighth layer from 2 to 8 nm
thick including a partially oxidized Ni--Cr alloy; and a ninth
layer from 25 to 60 nm thick including a silicon nitride.
24. The coating according to claim 23, wherein the zinc oxide in at
least one of the second layer and the sixth layer is from 6 to 7 nm
thick.
25. The coating according to claim 23, wherein the zinc oxide in at
least one of the second layer and the sixth layer comprises
nitrogen.
26. The coating according to claim 23, wherein the Ag in the third
layer is about 12 nm thick.
27. The coating according to claim 23, wherein the Ag in the
seventh layer is about 16 nm thick.
28. The coating according to claim 23, wherein the Ag in at least
one of the third layer and the seventh layer further includes at
least one of oxygen and nitrogen.
29. The coating according to claim 28, wherein the at least one of
oxygen and nitrogen is distributed homogeneously throughout the
Ag.
30. The coating according to claim 23, wherein the Ag in at least
one of the third layer and the seventh layer further includes a
means for strengthening the Ag against thermally induced
changes.
31. The coating according to claim 23, wherein the partially
oxidized Ni--Cr alloy in at least one of the fourth and the eighth
layer is from 4 to 6 nm thick.
32. The coating according to claim 23, wherein at least one layer
of the first layer, the third layer, the fifth layer and the
seventh layer is divided by a layer including a means for
strengthening the at least one layer against thermally induced
changes.
33. The coating according to claim 23, wherein the fifth layer has
an index of refraction greater than or equal to 1.9 at a wavelength
of 550 nm.
34. The coating according to claim 23, wherein the silicon nitride
in the fifth layer is about 63 nm thick.
35. The coating according to claim 23, wherein the silicon nitride
in the ninth layer is about 35 nm thick.
36. The coating according to claim 23, wherein the first layer
includes a layer comprising SiN.sub.x, where
0.ltoreq.x.ltoreq.1.34; the fifth layer includes a layer comprising
SiN.sub.x, where 0.ltoreq.x.ltoreq.1.34; and the ninth layer
includes a layer comprising SiN.sub.x, where
0.ltoreq.x.ltoreq.1.34.
37. The coating according to claim 36, wherein the first layer
includes a layer comprising SiN.sub.x, where x=1.34.
38. A method of making a low-emissivity coating on a substrate, the
method comprising depositing at least one layer including Ag on the
substrate; and producing the coating of claim 23.
39. The method according to claim 38, wherein the depositing
comprises sputtering.
40. A method of making a low-emissivity coating on a substrate, the
method comprising a step for depositing at least one layer
including Ag on the substrate; and producing the coating of claim
23.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to low emissivity coatings.
More specifically, the present invention relates to multilayer
coatings for controlling thermal radiation from substrates
transparent to visible light.
[0003] 2. Discussion of the Background
[0004] Solar control coatings on transparent panels or substrates
are designed to permit the passage of visible light while blocking
infrared (IR) radiation. High visible transmittance, low emissivity
coatings on, e.g., architectural glass and automobile windows can
lead to substantial savings in costs associated with environmental
control, such as heating and cooling costs.
[0005] Generally speaking, coatings that provide for high visible
transmittance and low emissivity are made up of a stack of films.
The stack includes one or more thin metallic films, with high IR
reflectance and low transmissivity, disposed between
anti-reflective dielectric layers. The IR reflective metallic films
may be virtually any reflective metal, such as silver, copper, or
gold. Silver (Ag) is most frequently used for this application due
to its relatively neutral color. The anti-reflective dielectric
layers are generally metal oxides selected to minimize visible
reflectance and enhance visible transmittance.
[0006] Conventional low emissivity coatings generally strive to
maintain reflection relatively constant throughout the visible
spectrum so that the coating has a "neutral" color; i.e., is
essentially colorless. However, conventional low-emissivity
coatings fail to provide the extremes of reflected color required
for aesthetic and other reasons by certain applications.
[0007] To achieve the desired properties in a coated substrate, the
composition and thickness of each of the layers of a multilayer
coating must be chosen carefully. For example, the thickness of an
IR reflective layer such as Ag must be chosen carefully. It is well
known that the emissivity of a Ag film tends to decrease with
decreasing Ag sheet resistance. Thus, to obtain a low emissivity Ag
film, the sheet resistance of the Ag film should be as low as
possible. Because film surfaces and pinholes in very thin Ag films
contribute to sheet resistance, increasing Ag film thickness to
separate film surfaces and eliminate pinholes can decrease sheet
resistance. However, increasing Ag film thickness will also cause
visible transmission to decrease. It would be desirable to be able
to increase visible transmission by decreasing Ag film thickness
without increasing sheet resistance and emissivity.
[0008] Thin, transparent metal films of Ag are susceptible to
corrosion (e.g., staining) when they are brought into contact,
under moist or wet conditions, with various staining agents, such
as atmosphere-carried chlorides, sulfides, sulfur dioxide and the
like. To protect the Ag layers, various barrier layers can be
deposited on the Ag. However, the protection provided by
conventional barrier layers is frequently inadequate.
[0009] Coated glass is used in a number of applications where the
coating is exposed to elevated temperatures. For example, coatings
on glass windows in self-cleaning kitchen ovens are repeatedly
raised to cooking temperatures of 120-230.degree. C., with frequent
excursions to, e.g., 480.degree. C. during cleaning cycles. In
addition, when coated glass is tempered or bent, the coating is
heated along with the glass to temperatures on the order of
600.degree. C. and above for periods of time up to several minutes.
These thermal treatments can cause the optical properties of Ag
coatings to deteriorate irreversibly. This deterioration can result
from oxidation of the Ag by oxygen diffusing across layers above
and below the Ag. The deterioration can also result from reaction
of the Ag with alkaline ions, such as sodium (Na+), migrating from
the glass. The diffusion of the oxygen or alkaline ions can be
facilitated and amplified by the deterioration or structural
modification of the dielectric layers above and below the Ag.
Coatings must be able to withstand these elevated temperatures.
However, multilayer coatings employing Ag as an infrared reflective
film frequently cannot withstand such temperatures without some
deterioration of the Ag film.
[0010] It would be desirable to provide low emissivity, multilayer
coatings exhibiting any of a wide range of colors, along with
improved chemical, thermal and mechanical stability.
SUMMARY OF THE INVENTION
[0011] The present invention provides multilayer coatings that can
reduce the infrared emissivity of a substrate with minimal
reduction in visible transmittance. The inventive coatings can be
designed to exhibit any of a wide variety of different colors in
reflection.
[0012] The multilayer coating includes, in numerical order outward
from the substrate, a first layer including a layer containing
titanium oxide, a layer containing silicon nitride, or a
superlattice of one or more sublayer containing titanium oxide in
combination with one or more sublayer containing silicon nitride; a
second layer including Ag; a third layer including at least one
layer selected from titanium oxide layers and silicon nitride
layers; a fourth layer including Ag; and a fifth layer including
silicon nitride. By varying the thicknesses of the layers of
titanium oxide and silicon nitride the reflected color of the
coating can be "tuned" within any one of the four color coordinate
quadrants in the CIE L*a*b* color space.
[0013] When the first layer is amorphous titanium oxide, the first
layer is particularly dense and provides exceptional barrier
properties against oxygen and alkaline ions migrating from the
substrate. In addition, amorphous titanium oxide provides an
extremely smooth surface, which aids in the deposition of thinner
pin-hole free Ag films with lower emissivity and higher visible
transmission in the second and fourth layers.
[0014] The second and fourth layers can include a sublayer of zinc
oxide, serving as a substrate for a sublayer of the Ag, and
additionally a sublayer of oxidized metal deposited on the Ag
sub-layer. The zinc oxide provides a substrate on which relatively
thin, high electrical conductivity, Ag films preferentially grow.
The sublayer of oxidized metal protects the Ag by acting as a
diffusion barrier against oxygen, water and other reactive
atmospheric gases, and also improves adhesion.
[0015] Incorporating oxygen and/or nitrogen into the Ag sublayers
of the second and fourth layers can improve the strength and
mechanical stability of the Ag sublayers.
[0016] Dividing a first layer of titanium oxide and/or silicon
nitride, the Ag sublayers, and/or the third layer with a sublayer
of oxidized metal can provide greater strength and mechanical
stability to the divided layers during heat treatments.
[0017] The fifth layer of silicon nitride provides enhanced
resistance to scratching.
[0018] In embodiments, multilayer coatings according to the present
invention can undergo heat treatments, suitable to temper or bend
glass, with minimal mechanical or optical degradation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 shows bright field transmission electron micrographs
comparing Ag deposited directly on amorphous TiO.sub.x with Ag
deposited directly on ZnO (5 nm thick) resting on amorphous
TiO.sub.x. In both cases the amorphous TiO.sub.x was deposited on
50 nm thick, amorphous silicon nitride membranes.
[0020] FIG. 2 shows dark field transmission electron micrographs
comparing Ag deposited directly on amorphous TiO.sub.x with Ag
deposited directly on ZnO (5 nm thick) resting on amorphous
TiO.sub.x.
[0021] FIG. 3 is a transmission electron micrograph showing a
discontinuous layer of Ag, containing pinholes, deposited on
amorphous TiO.sub.x.
[0022] FIG. 4a shows CIE 1976 L*a*b* (CIELAB) transmitted color
variations from multilayer coatings on glass substrates resulting
from changes in layer thicknesses.
[0023] FIG. 4b shows CIE 1976 L*a*b* (CIELAB) reflected glass side
color variations from multilayer coatings on glass substrates
resulting from changes in layer thicknesses.
[0024] FIG. 4c shows CIE 1976 L*a*b* (CIELAB) reflected coating
side color variations from multilayer coatings on glass substrates
resulting from changes in layer thicknesses.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] The present invention provides a low emissivity multilayer
coating in which the color in reflection can be varied to lie in
any of the four color quadrants of the CIE L*a*b* color space. The
coating can be provided with a normal emissivity of
0.02.ltoreq..epsilon..ltoreq.0.10, a solar transmission (T.sub.sol)
of less than 45%, a solar reflection (R.sub.sol) from either the
coating or glass substrate side of greater than 28%, and CIE 1931
Yxy (Chromaticity) transmission and reflection (from either the
coating or glass side) greater than 75% and less than 7%,
respectively.
[0026] An embodiment of the low-emissivity coating of the present
invention appears in Table 1: TABLE-US-00001 TABLE 1 Layer Material
5 silicon nitride 4 Ag 3 titanium oxide; silicon nitride; or
superlattice of titanium oxide and silicon nitride 2 Ag 1 titanium
oxide and/or silicon nitride layers 0 substrate
The coating is deposited on a substrate, and includes, in numerical
order outward from the substrate, a first layer including a layer
containing titanium oxide, a layer containing silicon nitride, or a
superlattice of one or more sublayer containing titanium oxide in
combination with one or more sublayer containing silicon nitride; a
second layer including Ag; a third layer including at least one
layer selected from titanium oxide layers and silicon nitride
layers; a fourth layer including Ag; and a fifth layer including
silicon nitride. The multiple layers of silver in the low
emissivity coating of the present invention provide greater
efficiency in reflecting IR radiation, and a sharper cut-off
between transmitted and reflected wavelengths, than is possible
with a single layer of silver.
[0027] Layer 0 is the substrate. The multilayer coating of the
present invention is deposited on and is mechanically supported by
the substrate. The substrate surface serves as a template for the
coating, and influences the surface topography of the coating. To
maximize transmission of visible light, preferably the surface of
the substrate has a roughness less than the wavelength of the
light. Such a smooth surface can be formed by, e.g., solidifying a
melt of the substrate. The substrate can be any material having an
emissivity that can be lowered by the multilayer coating of the
present invention. For architectural and automotive applications,
the substrate is preferably a material which has superior
structural properties and minimum absorption in the visible and
near-infrared spectra regions where the solar energy is
concentrated. Crystalline quartz, fused silica, soda-lime silicate
glass and plastics, e.g., polycarbonates and acrylates, are all
preferred substrate materials.
[0028] Layer 1 promotes adhesion between the coating and the
substrate; serves as a barrier to oxygen and alkaline ions (e.g.,
Na.sup.+) migrating from the substrate to the coating; influences
the surface roughness of the coating; and promotes the transmission
of visible light through the coating. The present inventors have
discovered that titanium oxide and silicon nitride are both well
suited to these functions.
[0029] Titanium oxide is particularly well suited for layer 1. The
titanium oxide is preferably a dielectric and electrically
insulating. The titanium oxide of layer 1 can be TiO.sub.x, where x
ranges from 1 to 2. The titanium oxide can be sputtered in a
variety of phases: e.g., as rutile and anatase polycrystalline
phases, and as an amorphous phase. Anatase and rutile layers
provide higher indices of refraction, making it possible to attain
higher visible transmission. However, preferably the titanium oxide
is amorphous, because amorphous titanium oxide forms a denser layer
than other metal oxides and provides a superior barrier to oxygen
and alkaline ions diffusing from the substrate. In addition,
because an amorphous layer of titanium oxide is smoother than a
polycrystalline layer, the amorphous layer of titanium oxide
permits thinner continuous films of infrared reflective Ag to be
deposited than does a polycrystalline film. An amorphous titanium
oxide layer can be formed by DC, AC, or RF magnetron sputtering
under conditions well known in the art.
[0030] The silicon nitride of layer 1 can be SiN.sub.x, where x
varies from greater than 0 to 1.34. When x=1.34 in SiN.sub.x, the
silicon nitride is stoichiometric Si.sub.3N.sub.4.
[0031] The titanium oxide of layer 1 has a higher index of
refraction (approximately 2.4 at 550 nm) compared with silicon
nitride (greater than 1.9 at 550 nm) and many other oxides. Thus,
the titanium oxide promotes transmission and reduces reflection of
light to a greater extent than these other materials. As a result
of titanium oxide's higher index of refraction, a similar optical
behavior in layer 1 can be achieved using a thinner layer of
titanium oxide than of the other materials. Alternatively, by
replacing a conventional oxide in layer 1 with titanium oxide of
equal thickness the thickness of subsequent IR reflective silver
layers in a coating can be increased without reducing visible
transmittance of the coating.
[0032] When present in layer 1, the titanium oxide can have a
thickness in the range of about 5 to 30 nm, preferably 5 to 20 nm,
more preferably 5 to 15 nm. If the titanium oxide film is less than
5 nm thick, then the film fails to block migration of oxygen and
alkaline ion impurities from the substrate. If the titanium oxide
film is thicker than 30 nm, then the film tends to block
transmission of visible light. Most preferably, the titanium oxide
of layer 1 is about 10 nm thick.
[0033] When present in layer 1, the silicon nitride can have a
thickness in a range from 5 to 30 nm, preferably 5 to 20 nm, more
preferably 5 to 15 nm. The silicon nitride can enhance the barrier
properties and also influence the optical properties of the coating
when a sufficient thickness of silicon nitride is present.
[0034] Layer 2 is designed to reflect IR radiation. To accomplish
this task, while retaining the possibility of a relatively neutral
color in reflection, layer 2 is formed primarily from Ag. The Ag of
layer 2 can have a thickness in the range of about 8 to 16 nm,
preferably 8 to 14 nm, more preferably 10 to 14 nm, most preferably
about 12 nm.
[0035] Layer 3 includes one or more anti-reflective layers to
enhance visible transmission. The anti-reflective layers are
dielectric materials and electrically insulating. Preferably, the
dielectric materials are selected from titanium oxide and silicon
nitride. The titanium oxide can be TiO.sub.x, where x varies from
greater than 1 to 2, and is preferably amorphous. The silicon
nitride can be SiN.sub.x, where x varies from greater than 0 to
1.34. When x=1.34 in SiN.sub.x, the silicon nitride is
stoichiometric Si.sub.3N.sub.4. Preferably, layer 3 is
Si.sub.3N.sub.4. Because titanium oxide has a higher index of
refraction than silicon nitride, the same optical behavior can be
obtained using a thinner layer of titanium oxide than silicon
nitride. On the other hand, silicon nitride provides greater
mechanical stability than titanium oxide during heat treatments,
and thus greater heat treatability. The combination of silicon
nitride with titanium oxide in a superlattice provides both the
optical advantages of the higher index of refraction of titanium
oxide and the thermal and mechanical stability advantages
associated with silicon nitride. The higher average index of
refraction of the titanium oxide/silicon nitride superlattice
relative to silicon nitride alone permits a higher visible,
photopic, transmission for the same Ag thickness, or a similar
photopic transmission for an increased number of stabilizing
barrier layers. The thickness of layer 3 can be from 45 to 90 nm,
and is preferably about 63 nm. When layer 3 includes a superlattice
of titanium oxide and silicon nitride, the layers in the
superlattice can each have a thickness of from 1 to 45 nm.
[0036] Layer 4 is designed to reflect IR radiation. To accomplish
this task, while retaining the possibility of a relatively neutral
color in reflection, layer 4 is formed primarily from Ag. The Ag of
layer 4 can have a thickness in the range of about 8 to 24 nm,
preferably 10 to 20 nm, more preferably 12 to 18 nm, most
preferably about 16 nm thick.
[0037] If the sum of the Ag thicknesses in layers 2 and 4 is less
than about 16 nm, insufficient infrared radiation will be reflected
by the multilayer coating. If the sum of the Ag layer thicknesses
in layers 2 and 4 is more than about 40 nm, the visible
transmission will be reduced to unacceptable levels.
[0038] Layer 5 serves to protect the multilayer coating of the
invention from scratches and abrasion; improves heat treatability
of the coating; acts as a barrier to oxygen and other chemicals in
the environment; and influences the optical properties of the
low-emissivity coating. Preferably, layer 5 is silicon nitride. The
silicon nitride can be SiN.sub.x, where x varies from greater than
0 to 1.34. The thickness of the silicon nitride of layer 5 is from
25 to 60 nm, and is preferably about 35 nm.
[0039] In embodiments of the present invention, layer 1 can
include, in addition to a sublayer of titanium oxide, a sublayer of
silicon nitride, thus forming a superlattice of titanium oxide and
silicon nitride. The term "superlattice" as used herein refers to
any number of alternating titanium oxide and silicon nitride
layers, including a titanium oxide/silicon nitride bilayer.
Suitable structures are shown in Tables 2-3. The silicon nitride
can enhance the barrier properties achieved using titanium oxide
and also influence the optical properties of the coating when a
sufficient thickness of silicon nitride is present. In the
superlattice each of the titanium oxide sublayers and the silicon
nitride sublayers can be from 1 to 30 nm thick. TABLE-US-00002
TABLE 2 Sub-layer Material 1b silicon nitride 1a titanium oxide
[0040] TABLE-US-00003 TABLE 3 Sub-layer Material 1b titanium oxide
1a silicon nitride
[0041] In other embodiments of the present invention, the Ag of one
or more of layers 2 and 4 can include oxygen and/or nitrogen. The
incorporation of oxygen and/or nitrogen in the Ag improves the
thermal and mechanical stability of the Ag. The oxygen and/or
nitrogen can be distributed homogeneously throughout the Ag of a
layer, or can be segregated to a portion of the Ag of a layer. The
oxygen and/or nitrogen can incorporated into the Ag by adding
oxygen and/or nitrogen to the inert gas used to sputter deposit the
Ag. When the Ag including the oxygen and/or nitrogen is DC, AC or
RF reactively sputtered, the amount of oxygen and/or nitrogen in
the inert gas can range from greater than 0 to 20%.
[0042] In still other embodiments of the present invention, layers
2 and 4 can include, in addition to a sublayer of Ag, a sublayer of
zinc oxide and a sublayer of an oxidized metal. As shown in Tables
4-5, the zinc oxide sublayer serves as a substrate for the sublayer
of Ag, and the sublayer of Ag serves as a substrate for the
sublayer of an oxidized metal. The sublayer of an oxidized metal
protects the Ag from reactive materials such as oxygen in the
environment. TABLE-US-00004 TABLE 4 Sub-layer Material 4c oxidized
metal 4b Ag 4a zinc oxide 3 titanium oxide; silicon nitride; or
superlattice of titanium oxide and silicon nitride
[0043] TABLE-US-00005 TABLE 5 Sub-layer Material 2c oxidized metal
2b Ag 2a zinc oxide 1 titanium oxide and/or silicon nitride
layers
[0044] The sublayer of zinc oxide that can be in layers 2 and 4 is
generally polycrystalline. The zinc oxide can be ZnO. The present
inventors have discovered that, when deposited on amorphous
titanium oxide, zinc oxide is particularly useful as a substrate
for growing low sheet resistance, strongly adherent Ag layers. The
amorphous titanium oxide, as discussed above, provides an extremely
smooth surface on which to grow subsequent layers. The zinc oxide
grows with the {0001} orientation, which orients the Ag to
preferentially grow with a {111} orientation. The epitaxial lattice
match between Ag {111} and ZnO {0001} leads to lower sheet
resistance and improved adhesion of the Ag. The use of zinc oxide
as a substrate for Ag instead of another material lowers the Ag
sheet resistance by approximately 1 .OMEGA./.quadrature.. The net
result of using zinc oxide as a substrate for Ag is a decrease in
emissivity without lowering the visible, photopic transmission. The
zinc oxide provides a means for forming a high conductivity,
strongly adherent Ag layer with a thickness as low as 8 nm.
[0045] Another interesting finding associated with the use of a
sublayer of zinc oxide is that the transmitted a* value increases
by about one color point (e.g., from a*=-3.0 to a*=-2.0) and the
photopic transmission increases about +1.5% per zinc oxide layer
added up to a maximum enhancement of up to +3%. These effects vary
as the thickness of the zinc oxide changes.
[0046] In embodiments, the zinc oxide can include nitrogen and can
be represented by the formula ZnO.sub.xN.sub.y. The nitrogen
containing zinc oxide can be formed by sputtering a Zn target in a
sputtering gas including 33 to 84%, preferably 43 to 80%, O.sub.2;
1 to 25%, preferably 3 to 14%, N.sub.2; and a remainder of argon. A
coater manufactured by Leybold Systems GmbH with model number Typ A
2540 Z 5 H/20-29 is suitable for sputter depositing the nitrogen co
ntaining zinc oxide, using gas flows of 200 to 600 sccm, preferably
300 to 450 SCCM O.sub.2; 10 to 100 sccm, preferably 25 to 50 sccm
N.sub.2; and 100 to 300 sccm Ar. The addition of nitrogen to the
zinc oxide improves the thermal stability of the layered coatings
of the present invention.
[0047] The sublayer of zinc oxide can have a thickness in the range
of about 1 to 20 nm, preferably about 6-7 nm. If the zinc oxide is
too thick, the sheet restant of the Ag will begin to increase. By
limiting zinc oxide underlayer thickness to 20 nm and less, the
zinc oxide allows for the deposition of pinhole-free, low sheet
resistance Ag films at lower thicknesses than are possible with
other substrates, while minimizing the undesirable characteristic
of thick zinc oxide. Because thin zinc oxide enables thinner Ag
films to be used, which enhances visible transmission, use of thin
zinc oxide leads to enhancements in the visible transmission of low
emissivity coatings.
[0048] The sublayer of oxidized metal in layers 2 and 4 protects
the Ag sublayer from corroding by acting as a diffusion barrier
against oxygen, water and other reactive atmospheric gases. In
addition, the sublayer of oxidized metal improves adhesion between
layers in the multilayer coating. Preferably, the sublayer of
oxidized metal is an oxidized metal such as oxidized Ti, oxidized
W, oxidized Nb, and oxidized Ni--Cr alloy. Different advantages and
disadvantages are associated with each of the barrier layers. Some
of the barrier layers provide particularly high thermal and
mechanical durability, while others particularly benefit color
and/or photopic transmission and reflection. For example, an at
least partially oxidized Ni--Cr alloy (e.g., NiCrO.sub.y, where
0<y<2) provides particularly good heat treatability
characteristics to a multilayer coating, enhancing the thermal and
mechanical durability of a coating during heat treatments above
700.degree. C., such as those necessary for bending and tempering a
glass substrate. Preferably the oxidized metal is a suboxide near
the metal insulator transition. Such a suboxide will generally have
an oxygen content less than the stoichiometric amount of oxygen in
the fully oxidized metal. The suboxide will be able to react with,
and thus block diffusion of, additional oxygen and other reactive
gases. The oxidized metal sublayer can have a thickness in the
range of 2 to 8 nm, more preferably 4 to 6 nm, most preferably
about 5 nm thick. The sublayer of oxidized metal is preferably
formed by reactively sputtering a metal target in a sputtering gas
including an inert gas and 10 to 75%, preferably 20 to 55%,
oxygen.
[0049] In further embodiments of the present invention, the thermal
and mechanical stability of various layers can be improved by
dividing each of the layers with a layer of the oxidized metal. The
layer of oxidized metal strengthens the layers against thermally
induced changes. For example, the titanium oxide and/or silicon
nitride layers of layer 1 can be divided by a layer of oxidized
metal. In addition, at least one of the Ag sublayers in layers 2
and 4 can be divided by a layer of oxidized metal. Furthermore, at
least a portion of the titanium oxide in layer 3 can be divided by
a layer of oxidized metal. Preferably, the layer of oxidized metal
is an at least partially oxidized Ni--Cr alloy (e.g., NiCrO.sub.y,
where 0<y<2) . The oxidized metal provides improved
mechanical stability to the divided layers during heat
treatments.
[0050] The layers in the multilayer coatings of the present
invention can be deposited by conventional physical and chemical
vapor deposition techniques. The details of these techniques are
well known in the art and will not be repeated here. Suitable
deposition techniques include sputtering methods. Suitable
sputtering methods include DC sputtering, using metallic targets,
and AC and RF sputtering, using metallic and non-metallic targets.
All can utilize magnetron sputtering. The sputtering can be in an
inert gas, or can be carried out reactively in reactive gas. The
total gas pressure can be maintained in a range from
5.times.10.sup.-4 to 8.times.10.sup.-2 mbar, preferably from
1.times.10.sup.-3 to 1.times.10.sup.-2 mbar. Sputtering voltages
can be in a range from 200 to 1200 V, preferably 250 to 1000 V.
Dynamic deposition rates can be in a range of from 25 to. 700
nm-mm.sup.2/W-sec, preferably 30 to 700 nm-mm.sup.2/W-sec. Coaters
manufactured by Leybold Systems GmbH with model numbers Typ A 2540
Z 5 H/13-22 and Typ A 2540 Z 5 H/20-29 are suitable for sputter
depositing the multilayer coatings of the present invention.
EXAMPLES
[0051] To further illustrate the invention, the following
non-limiting examples are provided:
Example 1
[0052] As discussed above, a sublayer of zinc oxide deposited on
amorphous titanium oxide promotes the wetting of Ag on the zinc
oxide and the formation of thinner layers of pin-hole free Ag.
[0053] To demonstrate this, Ag films 16 nm thick were planar DC
magnetron sputter deposited onto amorphous TiO.sub.x (a-TiO.sub.x)
underlayers 25 nm thick, and also onto ZnO (5 nm)/a-TiO.sub.x (25
nm) under(bi)layers. Transmission electron diffraction micrographs
of the amorphous TiO.sub.x showed only broad diffuse rings,
indicating that the TiO.sub.x was amorphous. The ZnO and
a-TiO.sub.x dielectric layers were reactively sputtered from metal
targets. The substrates for the a-TiO.sub.x layers included glass,
and transmission electron microscopy (TEM) grids each having a 50
nm thick, amorphous, silicon nitride, electron transparent membrane
peripherally supported by Si. The membrane was formed in a manner
well known in the art by depositing silicon nitride by LPCVD
(liquid phase chemical vapor deposition) onto a Si wafer, and then
back-etching the Si.
[0054] FIG. 1 shows bright field transmission electron micrographs
comparing Ag deposited directly on the a-TiO.sub.x with Ag
deposited directly onto the ZnO resting on a-TiO.sub.x. The Ag
grown directly on the a-TiO.sub.x has an abnormal microstructure
with irregular grains. The Ag grown directly on the ZnO has a more
normal microstructure with regular grains. The average normal grain
size of the Ag directly on the ZnO is about 25 nm, while that of
the Ag directly on the a-TiO.sub.x is about 15 nm.
[0055] FIG. 2 shows dark field transmission electron negative
micrographs comparing the Ag deposited directly on the a-TiO.sub.x
with the Ag deposited directly on the ZnO resting on TiO.sub.x. The
dark field images were obtained using {220} Ag reflections. The
images show that {111} oriented Ag grains giving rise to the strong
220 reflections have a significantly larger average grain size (two
to three times larger) when deposited directly on the 5 nm thick
ZnO than when deposited directly on a-TiO.sub.x.
[0056] FIG. 3 shows is a bright field transmission electron
micrograph of Ag deposited directly the a-TiO.sub.x underlayer near
the center of the TEM grid. The Ag film near the center of the TEM
grid is clearly discontinuous. A grayish haze was observed by eye
near the center of the grid from the scattering of light from the
rough surface. In contrast, the Ag film near the membrane
supportive, back-etched Si was free of pinholes and continuous. The
discontinuous Ag film containing pinholes is believed to result
from increased deposition temperatures at the center of the
membrane due to thermal isolation. Remarkably, the Ag deposited
directly on 5 nm thick ZnO was continuous over the entire TEM grid,
even in places where Ag deposited directly on a-TiO.sub.x was
discontinuous.
[0057] The sheet resistance of the Ag films, measured when
deposited on substrates of bulk glass, was found to be 5.68
.OMEGA./.quadrature. with the ZnO/a-TiO.sub.x under(bi)layer and
7.56 .OMEGA./.quadrature. with the a-TiO.sub.x, underlayer. Since
there was no visual haze, and the films deposited on glass were on
a heat sink even larger than the TEM grid edge, it is expected that
the Ag films were continuous and pinhole free on the glass.
[0058] Thus, zinc oxide provides an underlayer on which Ag
preferentially grows as a pinhole free, continuous film.
Furthermore, the sheet resistance of the Ag film can be reduced
without an increase in Ag thickness. The addition of zinc oxide was
observed to decrease the Ag sheet resistance by approximately 1
.OMEGA./.quadrature..
Example 2
[0059] A complex structure incorporating many of the features of
the present invention appears in Table 6. TABLE-US-00006 TABLE 6
Layer Material* 5 SiN.sub.x 4c(2) NiCrO.sub.x 4b(2) Ag 4c(1)
NiCrO.sub.x 4b(1) Ag 4a ZnO.sub.x 3c TiO.sub.x 3b NiCrO.sub.x 3a
TiO.sub.x, SiN.sub.x, or superlattice 2c(2) NiCrO.sub.x 2b(2) Ag
2c(1) NiCrO.sub.x 2b(1) Ag 2a ZnO.sub.x 1a(2) TiO.sub.x 1b
NiCrO.sub.x 1a(1) TiO.sub.x, SiN.sub.x, or superlattice 0 glass
substrate *In Table 6, the subscript "x" indicates both
stoichiometic and sub-stoichiometric compositions.
[0060] Various multilayer coatings including all, or a portion, of
the layers shown in Table 6 were made by DC magnetron
sputtering.
[0061] It was found that by varying the thicknesses of the silicon
nitride and titanium oxide layers the reflected color of the
coating can be positioned in any of the four color coordinate
quadrants of the CEE 1976 L*a*b* (CIELAB) and CIE 1931 Yxy
(Chromaticity) color spaces. Techniques and standards for
quantifying the measurement of color are well known to the skilled
artisan and will not be repeated here.
[0062] FIGS. 4a-4c show transmitted, reflected glass side and
reflected film side color variance for the various multilayer
coatings. As with conventional structures, color neutrality
(colorless) was achieved with some of the coatings. FIG. 4a shows
that the transmitted color varied dramatically in the second
quadrant. FIGS. 4b and 4c show that the coatings can produce
reflected color in any of the four color coordinate quadrants of
the CIE 1976 L*a*b* (CIELAB) and CIE 1931 Yxy (Chromaticity) color
spaces.
[0063] The photopic transmission and reflection of the various
coatings varied with changes in the thickness of the silicon
nitride and titanium oxide. The photopic transmission varied from
about 50 to 80%. The reflection from the glass side varied from
about 5% to 22%. The reflection from the coated side varies from
about 3% to about 20%.
[0064] 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.
* * * * *