EP1399312A1

EP1399312A1 - Robust highly reflective optical construction

Info

Publication number: EP1399312A1
Application number: EP02706051A
Authority: EP
Inventors: Robert J. Saccomanno; Gary A. West
Original assignee: Honeywell International Inc
Current assignee: Honeywell International Inc
Priority date: 2001-01-29
Filing date: 2002-01-29
Publication date: 2004-03-24
Also published as: IL157094A0; JP2004532423A; NZ527275A; EP1399312A4; IL157094A; CA2436584A1; AU2002240158B2; AU2002240158C1; WO2002060686A1

Abstract

An optical construction (20) comprises an optically transmissive substrate (12), an optional adhesion-promoting layer (18) comprising an oxide form of at least one metal or metalloid deposited onto the surface of the optically transmissive substrate (12), a reflective layer (14) composed of a highly reflective metal overlaying the adhesion-promoting layer (18), and a protective layer (16) composed of a parylene polymer film bonded to the reflective m etal layer (14).

Description

ROBUST HIGHLY REFLECTIVE OPTICAL CONSTRUCTION

Field of the Invention

The present invention relates to optical constructions, and more

particularly to optical constructions having an optically transmissive substrate

material coated with a robust highly reflective optical layer.

Background of the Invention

Optical components such as waveguides are generally designed to confine

and direct the propagation of light waves for many applications. In applications

that rely on the reflection and transmission of light, significant gains in

performance can be made when highly reflective materials are used in

combination with optically transmissive materials. For example, a step-index

fiber optic is composed of a thin strand of concentric layers of optically

transmissive materials- a central optical medium (i.e., the core) and a surrounding

optical medium (i.e., the cladding), the latter having a lower index of refraction.

Light is channeled through the core. During transmission, the light often travels to

the boundary of the core and cladding, where it is reflected back towards the core by total internal reflection. However, total internal reflection is not total, as some

of the light is lost, for example, due to scatter induced by imperfections within the

core or at the core/cladding boundary.

To reduce this loss, a reflective layer can be applied over the surface of the

cladding along the length of the fiber optic. The reflective layer significantly

increases the amount of light directed back to the core and improves the overall

light transmission through the fiber optic.

Ideally, the reflective layer used in optical components should possess a

high reflectance characteristic over a broad spectrum of light and over all

incidence angles of reflectance. Silver is one metal known to possess a high

reflectance value. Silver has a reflectance of about 98% over the entire visible

light spectrum at normal incidence. Silver also sustains a high reflectance of

about 96%) for off-normal light at near grazing incidence angles. In comparison,

aluminum, a more commonly used reflective-layer material, possesses a

reflectance of about 93% at normal incidence. The reflectance of aluminum drops

precipitously to 75% for light at grazing incidence angles.

Although silver possesses excellent optical characteristics, there are several

problems associated with the use of the reflective metal. Silver has a tendency to

undesirably tarnish when exposed to the atmosphere, especially in the presence of corrosive gases and contaminants, including sulfur dioxide, hydrogen sulfide,

nitrogen dioxide, ozone, hydrogen chloride, chlorine, and organic acids. It is

known that long-term performance of silver coatings is rarely, if ever, guaranteed

by commercial coating facilities based on the aggressive nature of silver tarnishing

brought on by ordinary exposure to the environment, along with the lack of

suitably available protective measures which have been successfully tested under

corrosive conditions.

Further, silver's adherence to optically transmissive substrate materials,

including glass or polymeric materials such as polymethyl methacrylate, is

moderate at best. Polymethyl methacrylate is a low-cost acrylic resin frequently

used in the fabrication of optical components.

For the foregoing reasons, there is a need for an optical construction

having a highly reflective coating that adheres favorably to a range of optically

transmissive materials and that possesses improved resistance against corrosion

and tarnishing to provide improved optically effective performance and longer

lasting operating life. Summary of the Invention

The present invention is generally directed to an optical construction for

optical components such as hollow and solid waveguides, solid and hollow light

pipes, fiber optics, prisms, microstructured sheets, curved mirrors (ellipsoidal,

parabolic, etc.), piano mirrors, and other optics having topographic forms. The

optical construction of the present invention is designed to maintain high optical

performance and light transmission through the optical component in the presence

of potentially corrosive substances including sulfur dioxide, hydrogen sulfide,

nitrogen dioxide, ozone, hydrogen chloride, chlorine, organic acids and the like,

which are present in the atmosphere at least in trace amounts.

The optical construction of the present invention is especially useful in

optical components where a highly reflective surface composed of a metal such as

silver is desired. The optical construction is further adapted to provide favorable

durability and preservation of the highly reflective surface in the optical

component without measurably degrading the total reflectance qualities of the

optical component.

In one aspect of the invention, the optical construction generally comprises

an optically transmissive substrate adapted for efficiently channeling light

therethrough with a highly reflective layer composed of a highly reflective metal deposited on the surface of the substrate, and bonded thereto. Overlying the

highly reflective metal layer and firmly adherently bonded thereto is a protective

layer comprised of a parylene polymer film.

The parylene polymer protective layer as used in the present invention

serves to isolate the reflective layer from exposure to external elements such as

ambient atmosphere, corrosive substances, salt, humidity and the like. Such

external elements can cause the destruction and degradation of the metal reflective

layer over time through tarnishing, breakdown, delamination, or discoloration,

resulting in the loss of its reflectivity. The parylene polymer protective layer

further improves the reflective layer's resistance to mechanical deformation and

delamination as indicated by a tape-pull test described hereinafter.

Optionally, the optical construction of the present invention can further

include an adliesion-promoting layer applied between the surface of the substrate

and the reflective layer to strengthen the bond therebetween. The adhesion-

promoting layer as used in the present invention significantly improves the

adhesion between the functional reflective metal layer and the optically

transmissive substrate for improved resistance against delamination where the

reflective layer physically separates from the optically transmissive substrate

resulting in degraded performance and reduction in reflectivity. Further, the adhesion-promoting layer promotes uniformity and consistency in reflective

properties of the reflective layer along the substrate/reflective layer interface.

In an alternative form of the invention, a waveguide structure such as a

fiber optic, comprising an optically transmissive glass or polymer material, is

coated with an adhesion-promoting layer of the oxide form of a metal or

metalloid. A silver reflective layer is applied in contact with the adhesion-

promoting layer. A protective layer of a parylene polymer film is applied over the

silver reflective layer to prevent the silver from losing its high reflective luster or

from delaminating or degrading due to corrosive agents in the environment such

as ambient air. The preferred form of the invention forms a robust highly

reflective parylene/silver/metal-oxide/waveguide structure with improved

performance qualities including longer operating life.

Brief Description of the Drawings

Various embodiments of the invention are described in detail below with

reference to the drawings, in which like items are identified by the same reference

designation, wherein:

Figure 1 is a cross sectional view of an optical construction having a

highly reflective layer illustrative of one embodiment of the present invention; Figure 2 depicts a schematic diagram of a parylene vacuum evaporation

deposition reactor system for depositing a parylene polymer film to make an

optical construction in accordance with the principles of the present invention;

Figure 3 is a cross sectional view of an optical construction having a

highly reflective layer illustrative for a second embodiment of the present

invention;

Figure 4 is a cross sectional view of an optical construction having a

highly reflective layer illustrative for a third embodiment of the present invention;

Figure 5 is a cross sectional view of a fiber optic waveguide comprising

the optical construction in accordance with the present invention;

Figure 6 is a graph plotting the silver corrosions rates for various samples

exposed in the presence of ambient air; and

Figure 7 is a graph plotting the silver corrosion rates for various samples

exposed in the presence of an ammonium sulfide solution. Description of the Invention and Preferred Embodiments

The present invention is generally directed to an optical construction and a

method of making such optical constructions. The optical construction of the

present invention includes a substrate, a highly reflective layer, an optional

adliesion-promoting layer in contact between the substrate and the reflective layer,

and a protective layer comprising a parylene polymer film overlaying the

reflective layer. The optical construction of the present invention provides

favorable optical qualities with improved adherence of the reflective layer to the

substrate and improved resistance to corrosion and tamisliing for a longer

operating life. The substrate material can be selected from the group consisting of

glass and organic polymer-based materials such as polymethyl methacrylate

(PMMA), for example.

In the present invention, the parylene polymer film, useful as a protective

layer, has the following polymer repeat unit structure:

where "n" indicates the number of repeating units in the structure. The parylene

polymer coating may be exemplified in three forms or variations, with each

comprising varying degrees of chlorination. The three forms include parylene N

as shown above with no chlorine atoms, parylene C which is produced from the

same monomer as parylene N and is further modified by the substitution of a

chlorine atom for one of the aromatic hydrogens, and parylene D which is

produced from the same monomer as parylene N and is further modified by the

substitution of two chlorine atoms for two of the aromatic hydrogens.

With reference to Figure 1, there is depicted an optical construction

illustrative for one embodiment of the present invention. We note that the

thickness of the corresponding elements in the construction are not drawn to scale,

and is shown for illustrating the general structure and relationships thereof. The

optical construction denoted herein by reference numeral 10, can be applied for

the fabrication of a range of optical components where a highly reflective surface

composed of a metal such as silver, is desired.

The optical construction 10 generally comprises an optically transmissive

substrate 12 for efficiently transmitting and directing light therethrough, a

reflective layer 14 preferably composed of a highly reflective metal such as silver

vapor-deposited on the surface of the optically transmissive substrate 12, and a

protective layer 16 preferably composed of a parylene polymer film. Preferably, the surface of the substrate 12 is optically- smooth and substantially free from

optical imperfections to provide the highest specular reflectance. The surface of

the substrate 12 can be optionally treated to promote adhesion with the reflective

layer 14 including, but not limited to, plasma treatment as described in U.S. Pat.

No. 5,982,546, the content of which is incorporated herein by reference to the

extent that there is no conflict.

The optically transmissive substrate used for fabricating optical

components such as fiber optic waveguides can be selected from a range of

materials depending, for example, on the application, the desired performance

characteristics, the cost, and the characteristics of the transmitted light. The

optically transmissive substrate 12 can be composed of glass or polymer material.

The polymer materials can include organic polymers such as polyhydrocarbons,

polyoxyhydrocarbons, polysulfohydrocarbons, and fluorocarbon and

fluorohydrocarbon materials, as well. Representative organic polymers include

polyesters such as poly(ethyleneterephthalate) and poly(butyleneterephthalate),

polyacrylates and methacrylates such as poly(metlιylmethacrylate) (PMMA),

poly(methacrylate), and poly(ethylacrylate), copolymers such as

poly(methylmethacrylate-co-ethylacrylate) and polycarbonates. Fluorocarbon

polymers such as TEFLON and the various fluorohydrocarbon polymers known in

the art can be used as well. More preferably, the polymer material is PMMA. Other polymers can be used as optically transmissive substrate materials,

particularly in applications where low birefringence is desired. Such polymers

include CR-39 allyl diglycol carbonate resin marketed by PPG Industries of

Pittsburgh, Pennsylvania; OZ-1000 cycloaliphatic acrylic resin marketed by

Hitachi Chemical Co., Ltd. of Tokyo, Japan; CALIBRE 1080 DVD polycarbonate

resin marketed by Dow Engineering Plastics of Midland, Michigan;

MAKROLON DPI -1265 polycarbonate resin marketed by Bayer Corporation of

Pittsburgh, Pennsylvania; PLEXIGLAS VOD-100 acrylic molding resin marketed

by ATOFINA Chemicals, Inc. of Philadelphia, Pennsylvania, TOP AS cyclo-olefin

copolymer resin marketed by Ticona of Summit, New Jersey; ZEONEX cyclo-

olefin polymer resin marketed by Nippon Zeon Co., Ltd of Tokyo, Japan; and the

like.

Although not a limitation to the application of this invention, the plastic or

polymer material can be clear, transparent, and optically transmissive. When used

in context of plastic or polymer materials, the terms "clear", "transparent", and

"optically transmissive" means a plastic or polymer that, in its configuration of

use, exhibits transmission over a desired range of wavelengths. The polymer-

based substrates themselves are commercially available or can be prepared by

various art-known processes and do not, in and of themselves, constitute an aspect

of this invention. The polymer substrates can be formed into solid bodies, sheets, films, or coatings applied or laminated onto nonpolymeric surfaces such as metal

and glass.

The reflective layer 14 of the optical construction 10 shown in Figure 1 is

preferably made up of one or more functional metals that possess high reflectance

values such as silver, copper, gold, palladium, iridium, rhodium, combinations in

the form of alloys thereof, and the like. Among these metals, copper, silver, and

gold are preferred, with silver being the most preferred metal for the visible range

of light. The reflective layer 14 comprising a metal or an alloy of metals, can be

deposited onto the optically smooth surface of the optically transmissive substrate

12 through conventionally known deposition methods such as cathode sputtering,

vacuum evaporation or vapor-phase deposition techniques for a thickness ranging

from about 100 to 10,000 A, preferably 500 to 3,000 A, and more preferably from

about 1000 to 3,000 A. Individual metals can be used, or a plurality of layers of

different metals or layers of alloys of these metals can be used, if desired.

In another embodiment of the present invention, the reflective layer 14 of

the optical construction 10 is enclosed and sealed from ambient by the protective

layer 16 for optimal protection against corrosion and tarnishing. The protective

layer 16, in the form of a parylene polymer film, is vapor deposited on the surface

of the reflective layer 14 distal from the optically transmissive substrate 12. The parylene polymer protective layer 16, as applied, forms a continuously uniform

coating as will be further described.

The parylene polymer film of the protective layer 16 can be composed of

parylene N, parylene C, parylene D, or combinations or mixtures thereof. The

parylene polymer film can be composed of an interpolymer of monomers of

parylene variants of varying mixture ratios. The thickness of the parylene

polymer film of the protective layer 16 is preferably at least 0.0001", more

preferably in the range of from about 0.001 to 0.0001". We note that the actual

thickness of and the mixture ratios of the variants in the parylene polymer

protective layer can be adjusted according to the application, requirements, the

reflective layer metal used, the desired effect, the duration of effect, and the types

of expected contaminant exposures and the like, and may be readily determined by

one skilled in the art.

The parylene polymer film can be optionally processed using suitable

annealing or heat-treatment techniques to improve the chemical resistance and

durability of the coating as will be described. The term "annealing" or "heat-

treating" as used herein refers to any processes for treating a substance or material

with heat followed by cooling to modify or alter the structural properties of the

treated substance or material. In accordance with the present invention, the parylene polymer film is

applied through a coating process using conventionally known vapor phase

deposition or vacuum evaporation deposition techniques. It is understood that the

present invention can utilize any suitable commercially available method for

applying parylene polymer on a surface as known by one skilled in the art.

As an illustrative example, one process for applying a parylene polymer

coating is described in U.S. Pat. No. 3,342,754, the disclosure of which is hereby

incorporated by reference in its entirety to the extent that no conflict exists. It is

understood that the invention is not limited to the use of this process.

With reference to Figure 2, a general schematic diagram of a basic

parylene vacuum evaporation deposition reactor system 40 for carrying out the

vacuum evaporation deposition process described in U.S. Pat. No. 3,342,754, is

shown. As noted above, there are many known systems and processes lαiown in

the art for applying a polymer film on a substrate. The following description of

system 40 provides an illustration of the process that may be used for coating a

substrate with a parylene polymer layer. The system 40 can be constructed using

commercially available components and parts as known by those skilled in the art.

With further reference to Figure 2, the system 40 comprises a vaporization

chamber 42, a cracking chamber 44, a deposition chamber 46, and a vacuum pump 48. The vacuum pump 48 operates to evacuate the air from the interior of the

system 40. The vaporization chamber 42 is adapted to heat a sample of the di-p-

xylylene dimer under vacuum at an elevated temperature sufficient to vaporize the

dimer. Under vacuum conditions, the vaporized dimer radiates in all directions

within the chamber 42.

The vaporized dimer proceeds to the cracking chamber 44 where the

dimmer is heated to a temperature of less than 700°C, preferably between 450°C

and 700°C, and more preferably at about 680°C for a sufficient time at a pressure

such that the vapor pressure is below 1.0 mm Hg, to form a parylene diradical

monomer of parylene.

The parylene diradical monomer proceeds to the deposition chamber 46

where the diradical monomer condenses and polymerizes at a temperature of less

than 200°C, preferably below the ceiling condensation temperature of the parylene

diradical monomer, and more preferably at room temperature on the cooler surface

of the reflective metal-coated optically transmissive substrate. The condensation

of the diradical monomer yields a tough, linear, non-fluorescent polymer. The

vacuum pump 48 is connected to the system 40 to ensure that the process is

carried out in an evacuated atmosphere for optimal processing. The vacuum evaporation technique of depositing parylene polymer

provides several advantages. The first is that the room temperature deposition

process enables a range of substrates to be coated with parylene polymer films.

The second is the formation of a highly conforming and uniformly continuous

coating on substrates with complex shapes. The third is the capability to form

very thin coating layers while remaining continuous and uniform for precise

coating control.

With particular reference to Figures 1 and 2, the overall process of making

the optical construction of the present invention will now be described. In a

preferred form of the optical construction 10, the construction is formed by vapor

depositing a silver layer 14 onto the optically smooth surface of a PMMA-based

optically transmissive substrate 12. The reflective metal-coated optically

transmissive substrate is placed into the deposition chamber 46 of the reactor 40,

and suitably positioned for exposing the outer surface of the reflective silver metal

14 to the parylene diradical monomer flow. The parylene vacuum evaporation

process produces a parylene polymer protective layer 16 of sufficient thickness on

the surface of the silver metal layer 14. The thickness of the deposited parylene

polymer protective layer 16 can be determined while in the deposition chamber 46

using any one of various optical methods known in the art. Alternatively, the

thickness of the parylene polymer protective layer 16 can be determined after the

article is removed from the deposition chamber 46. The above deposition process can be repeated at least once using the same

or a different parylene variant (i.e., parylene N, parylene C, parylene D, and/or

mixtures thereof) to produce a multilaminate parylene polymer coating on the

surface of the reflective silver layer 14 as will be further described hereinafter.

The deposition chamber 46 is sealed from ambient air and the atmosphere of the

chamber 46 is evacuated with the vacuum pump 48. Alternatively, the atmosphere

in the deposition chamber 46 can be substituted at ambient pressure with an inert

gas such as helium, argon or nitrogen.

We have discovered that by annealing the deposited parylene polymer

protective film in the protective layer at an elevated temperature for a sufficient

time, and allowing them to cool, a substantially improved chemically resistant

parylene polymer barrier is formed. We have also discovered that the physical

barrier and mechanical properties of the parylene polymer coating are greatly

improved after the annealing thermal treatment. The annealing temperature can be

at least 120°C, preferably from about 120°C to 220°C and the annealing time may

range from about 1 hour to five (5) days. The annealing process can be carried out

under suitable atmospheric conditions including, but not limited to vacuum, inert

gas, and normal ambient atmosphere. The annealing conditions can be varied as

required by the thermal mass of the substrate, the maximum substrate temperature

rating, and the like, as may be determined by those skilled in the art. The parylene polymer film can be annealed immediately after the

completion of the parylene deposition process. The annealing process is

preferably conducted in a vacuum, or in the presence of at least one inert gas such

as helium, argon, nitrogen, and the like, at atmospheric pressure. The optimal

annealing conditions may differ slightly between each variant of the parylene

polymer. We further note that the annealing process may be utilized on each

parylene polymer protective layer individually as applied during the vapor

deposition process, or on the parylene polymer protective layer as a whole after

applying more than one parylene polymer layer.

In another embodiment of the present invention as shown in Figure 3, there

is provided an optical construction 20 which is not drawn to scale, comprising an

optically transmissive substrate 12 as described above and a thin adhesion-

promoting layer 18 comprising the oxide form of at least one metal or metalloid

that is applied to the substrate surface using conventional deposition processes

such as vacuum evaporation, cathode sputtering, electron beam evaporation, and

the like. The adliesion-promoting layer 18 is applied to the substrate 12 prior to

the application of the reflective layer 14. Details describing the use of aluminum

oxides for enhancing the adhesion of silver to glass substrates, is found in Hass et

al., Applied Optics, 14, 2639 (1975), the content of which is incorporated herein

by reference. The reflective layer 14 comprising a highly reflective metal such as silver

is deposited, using methods described above including electron beam evaporation,

onto the surface of the adhesion-promoting layer 18 for a thickness sufficient to

form an opaque, highly reflective surface at the interface between the substrate 12

and the reflective layer 14. Finally, the surface of the reflective layer 14 is coated

with a protective layer 16 comprising a parylene polymer film preferably using the

vacuum evaporation deposition or suitable process as described above.

As noted above, the adhesion-promoting layer 18 preferably comprises the

oxide form of at least one metal or metalloid that is sufficient to bond the metal

atoms of the reflective layer 14 to the smooth surface of the optically transmissive

substrate 12. Preferably, the thickness of the adhesion-promoting layer 18 can

range from about 10 to 1000 A, and more preferably about 300 A. The use and

application of metal- and metalloid-based oxides (collectively referred hereinafter

as "metal oxides") as adhesion promoting materials between a metal and a

polymer substrate is further described in U.S. Pat. Nos. 5,589,280 and 5,902,634,

the pertinent teachings of both are incorporated herein by reference to the extent

that there is no conflict.

For most applications, any of the adhesion-promoting materials selected

should be as nearly colorless as possible, at least in the amounts found effective to provide reliable adhesion. An adhesion-promoting material that imparts a visually

detectable color to the substrate 12 under the desired illuminant not only reduces

the efficiency of reflection by absorbing light passing to and from the reflective

layer 14 but also changes the color value of the light rays directed at the reflective

layer 14 through the substrate 12. We note that the adhesion-promoting material,

in addition to promoting adhesion of the metallic reflective layer 14 to the

substrate 12, must resist corrosion to maintain its optical qualities. We further

note that the selection of the materials for the adhesion-promoting layer must also

take into account the effects of the relative expansion coefficients in order to

preclude undesirable effects including delamination resulting from cyclic

temperature changes.

In one embodiment of the present invention, the adhesion-promoting layer

18 which is positioned between the optically transmissive substrate 12 and the

reflective layer 14, is composed of the oxide form of one or more metals

including, but not limited to, hafnium, zirconium, tantalum, titanium, niobium,

silicon, tungsten, aluminum, vanadium, molybdenum, chromium, tin, antimony,

indium, zinc, bismuth, cadmium, nickel and the like.

Generally, the method for producing the adhesion-promoting layer 18 is to

deposit the metal oxide via cathode sputter deposition, electron beam evaporation

deposition or any suitable process for depositing metal oxides. The metal oxides are preferably deposited in the oxidized mode, which may be achieved for

example by sputtering in the presence of an excess of oxygen so that the metal is

fully oxidized, to attain the desired adhesion promotion.

Since some of the metals considered here for the adhesion-promoting layer

18 exhibit substantial absorption in their metal state (i.e., >3% absorption at

thicknesses less than 20 A), it is advantageous to deposit them as oxides.

Similarly, it may also be advantageous to up-oxidize the metal layers fully or

partially after their deposition.

Referring to Figure 4, an optical construction is depicted for a third

embodiment of the invention. The optical construction denoted herein as

reference numeral 30 is similar to the optical construction 20 of Figure 3

previously described above. We again note that the thickness of the

corresponding elements in the construction are not drawn to scale, and is shown

for illustrating the general structure and relationships thereof. In the present

embodiment, the optical construction 30 includes a protective layer 16 that is

composed of a multilaminate structure with each layer being composed of a

distinct parylene polymer selected from the group consisting of parylene N,

parylene C, parylene D and combinations or mixtures thereof. The multilaminate

form of the protective layer 16 provides benefits of each parylene variant and/or mixtures of parylene variants for improved compatability with the reflective metal

layer, chemical resistance and the like.

The protective layer 16 includes first parylene film 17 composed of a first

parylene variant or mixtures of parylene variants. The first parylene film 17 is

deposited on the reflective layer 14 using one of the suitable deposition methods

described above. The protective layer 16 further includes a second parylene film

19 composed of a second parylene variant or mixtures of parylene variants

overlaying the surface of the first parylene film 17 distally from the reflective

layer 14. The actual thickness of each parylene variant layer can be adjusted

according to the application, requirements, the reflective layer metal, the desired

effect, the duration of effect, and the types of expected contaminant exposures and

the like, and may be readily determined by one skilled in the art.

In one embodiment, the first parylene film 17 is composed of parylene C,

and the second parylene film 19 is composed of parylene D. We have determined

from experimental results that when parylene C was deposited as a protective

layer directly on the silver reflective layer, the change in silver reflectance at the

parylene/silver interface, was observed to be within the noise of the experimental

data. The findings indicated that there is little or no reactivity between parylene C

and silver. We have further determined from experimental results that when parylene

D was deposited on the silver layer as a protective layer, the silver reflectance at

the parylene/silver interface, was measurably diminished or degraded. Since

parylene D is known to possess an average chlorine content of two chlorine atoms

per monomer unit, we theorize that the presence of unbonded or trapped chlorine

in the parylene polymer film may be reacting with the silver. Although the

findings indicated that there may be some reactivity between parylene D and

silver, parylene D is a suitable candidate for use as part of the protective layer.

Parylene D is known to have a lower gas permeability value than parylene C for

better exposure protection of the silver reflective layer. The silver/parylene

C/parylene D laminate combination provides an effective protective layer, which

possesses the low reactivity with silver of parylene C, and the low gas

permeability of parylene D.

In yet another embodiment, the transitioning of the deposition of parylene

films from one parylene variant to another, can be made gradually to form a

transitional interlayer (not shown) between the first and second parylene polymer

layers. As the deposition of the parylene variants transitions, the vapor flow of the

first parylene polymer is gradually reduced while the vapor flow of the second

parylene polymer is ramped up in proportion to the corresponding reduction of the

first parylene polymer vapor flow. This action produces a graded interface

between the pure parylene polymer layers and forms an interpolymer with improved adhesion therebetween. We note that the resulting parylene polymer

layer can be annealed or heat-treated as desired to modify the properties of layer as

described above.

It is understood that the actual thickness of the interlayer can be adjusted

according to the application, requirements, the desired effect, the duration of

effect, and the types of expected contaminant exposures and the like, and may be

readily determined by one skilled in the art.

Referring to Figure 5, a fiber optic waveguide is depicted for one

illustrative embodiment of the present invention. The fiber optic waveguide

denoted generally by reference numeral 50, generally comprises an elongated

cylindrical body having concentric layers of glass for channeling light

therethrough. The fiber optic waveguide 50 of Figure 5 comprises a core 52

composed of an optically transmissive glass or polymer material, a cladding 54

composed of an optically transmissive glass or polymer material with a lower

refractive index than the core 52, a reflective layer 58 with an optional adhesive-

promoting layer 56 interposed between the reflective layer 58 and the cladding 54,

and a parylene polymer protective layer 60 overlaying the reflective layer 56. The

fiber optic 50 includes the optical construction of the present invention where the

cladding 54 establishes the optically transparent substrate. The fiber optic 50 can

be fabricated from any commercially available fiber optic waveguide while using the above-described techniques for applying the reflective layer, the optional

adhesion-promoting layer, and parylene polymer layer, all onto the surface of the

cladding 54.

EXAMPLE 1

Experimental Tests

We obtained samples of optical quality polymethyl methylacrylate

(PMMA) substrates with a reflective index of 1.49 for testing. An aluminum

oxide coating was evaporatively applied to one set of samples using conventional

electron beam evaporation deposition process to form an adhesion-promoting

layer. The aluminum oxide source having a purity of 99.999 %, was obtained

from Cerac of Milwaukee, WI. The aluminum oxide was deposited using a flow

of 21.8 % O₂/Ar at a total pressure of 2x10^"4 Torr. The deposition rate was set at

approximately 1 A per second to produce a final thickness of about 300 A.

A layer of silver metal was applied to the surface of each sample substrate

using a conventional electron beam evaporation deposition process. The silver

metal source having a purity of 99.999 %, was obtained from Cerac. The silver

layer was applied at a thickness of 1,000 A at a deposition rate of from about 1.2

to 7.3 A per second. The average deposition rate was about 3 A per second. Parylene D and C were each obtained from Paratronix, Inc. of Attleboro,

MA. The parylene polymers were applied to the samples using chemical

deposition processes resulting in a coating of about 0.0005". The degree of

protection the parylene polymer layer provided was measured by the changes in

reflectance of the silver layer through the substrate. Reflectance measurements

were made using a MacBeth Color-Eye 7000 spectrometer with a spectral range of

from about 360 to 750 mn. Measurements at the interface were made through the

PMMA substrate and will include any absorption due to the PMMA or

interference effects from the first surface reflectance.

Accelerated silver tarnishing was induced by placing the sample in a

sealed 200 mm diameter Pyrex glass desiccator containing normal ambient air and

a evaporation dish holding 2 cc of ammonium sulfide (20 % aqueous solution) in

18 cc of deionized water. The ammonium sulfide was obtained from Strem

Chemicals of Newburyport, MA. The samples of substrates were positioned 4 cm

above the solution with the silver layer side exposed to the solution. The silver

reflectance was measured as a function of the exposure time in the desiccator

chamber. The ammonium sulfide solution generated hydrogen sulfide as the

primary corrosion agent. We had observed that elemental sulfur had deposited on

the desiccator walls after long exposure times. Ammonium sulfide solution is

lαiown to be one of the most aggressive tarnishing agent of silver. See, Dar-Yuan

Song et al, Applied Optics 24 (8), 1164 (1985). Ambient Air Results

In order to estimate the rate of silver corrosion in ambient air for an

unprotected sample, the reflectance of a silver coated PMMA sample was

measured periodically when exposed to the ambient air of the laboratory. The

change in reflectance of the silver surface and the silver/PMMA interface as

measured through the optically transmissive PMMA substrate was recorded for

each sample. The reflectance was measured using light with a wavelength of

about 550 nm extending over a period of about 70 days. The points were plotted

and linear regression analysis was executed to generate a graph depicted in Figure

6.

With reference to Figure 6, the graph shows that the ambient air exposure

resulted in tarnish rates of about 6.3 x 10^"2 %/day for the silver surface, and about

2.2 x 10^"2 %/day for the silver/PMMA interface. We believe that the lower tarnish

rate at the interface as compared to the silver surface can be explained in that the

diffusion of con-osion agents through the silver layer, or less likely, through the

much thicker PMMA substrate was slower. Included in the graph are reflectance

measurements for samples (control) that had been stored in 3M Corrosion Control

Absorber Paper (CPAP), an anticorrosion paper product of Minnesota Mining and

Manufacturing Co. of St. Paul, Minnesota. The anticorrosion paper is designed to prevent tarnishing from the presence of air contaminants that cause oxidation and

corrosion. When the corrosive elements were removed from the air by the

anticorrosion paper, both the silver surface and the silver/PMMA interface showed

no measurable change in reflectance. The change of reflectance was less than 3 x

10^"4 %/day over the 70 day measurement period. Comparing the two sets of data,

we can conclude that the changes in silver reflectance were produced by air

corrosion alone, and there appeared to be no perceptible interaction of the silver

mirror with the PMMA substrate at the interface.

Ammonium sulfide

To test the ability of parylene coatings to inhibit the tarnish of silver,

several silver coated PMMA samples were prepared in the manner as described

above. The PMMA samples were encapsulated with films of both C and D

variants of parylene. The parylene polymer coated PMMA samples were obtained

from Paratronix. The film thickness of the parylene coatings was measured to be

on average of about 0.00043 of an inch.

Using the test procedure described above, the effectiveness of parylenes

coatings C and D were evaluated. Changes in silver reflectance as a function of

exposure time in the corrosion chamber were measured and the results are shown

in Figure 7. Referring to Figure 7, the samples were each exposed to ammonium sulfide solutions. The corrosion rates were determined from data analyses using

linear least-square fits. The corresponding corrosion rates for exposure to ambient

air and ammonium sulfide are listed below in Table 1.

Table 1

Silver Tarnish Rates Determined from Reflectance Measurements at 550 nm

Comparing the tarnish rates through the parylene C and D films, we had

observed that the tarnish rate for the parylene C was fifteen times higher than the

rate for parylene D. Comparing the tarnish rates for parylene protected samples to

the unprotected silver samples, we had observed that the tarnish rate was reduced

by a factor of 6.9 x 10^"5 for the parylene C coating and a factor of 4.6 x 10^"6 for the

parylene D coating. Assuming that similar corrosion agents are responsible for

the ambient air tarnish results, the above tarnish reduction factors can be used to

estimate a tarnish rate for parylene polymer protected silver in normal atmospheric

air. Applying the tarnish reduction factors to the ambient air data results in an

estimated air tarnish rate of about 4.3 xlO^"6 %/day for a parylene C protected silver

film and an estimated air tarnish rate of about 2.9 x 10^"7 %/day for parylene D. Based on this analysis, either of the parylene variants would protect silver for 50

years with less than a 0.1% change in reflectance.

The measured tarnish rates at the silver/PMMA interface listed in Table 1

are at all times lower that those from the silver surface. This result is expected

since there is the added requirement for the corrosion gases to diffuse through the

silver layer to reach the silver/PMMA interface.

Silver Adhesion

Parylene C and D films were deposited directly onto PMMA to test the

adhesion of these films. Several samples of each variant were tested with

SCOTCH tape marketed by Minnesota Mining and Manufacturing. Co., and were

observed to be adherent to the substrate with no instances of the parylene film

removal by the tape pulls.

Although silver appears to be compatible with PMMA when in direct

contact, the adhesion to this material is marginal. SCOTCH tape tests of silver

coatings on PMMA consistently removed all of the silver film. Encapsulation of

the silver coated PMMA substrates with parylene, as would be done for the final

silver coated waveguide structure, does improve the robustness of the silver

coating. Due to the high tensile strengths of the parylene films, silver films on

PMMA that have been coated with either parylene C or parylene D will usually

pass the SCOTCH tape test without any film delamination. However, in some

instances blisters can be seen in the film after the pull test indicating areas where

the silver film has detached from the PMMA substrate. The parylene film,

however, remains intact and well-bonded to the underlying silver film. These

failures confirmed the need to improve the silver/PMMA interfacial bond.

As detailed previously, metal- or metalloid-oxides are lαiown to enhance

the adhesion of silver to glass substrates. Alumina was chosen since it is also an

excellent candidate for the silver coated waveguide application due to its high

transparency throughout the visible spectrum. In order to test alumina as an

adhesion layer for silver on PMMA, a 300 Angstrom-thick layer was deposited on

PMMA prior to deposition of the silver mirror. SCOTCH tape tests indicate that

the alumina interfacial layer improves the silver adhesion. Approximately 80%) of

the tape pulls resulted in no loss of silver film with 20%) of the pulls removing a

portion of the silver mirror from the PMMA substrate. Once alumina-bonded

silver films were overcoated with parylene C, no removal or delamination of the

silver mirror from the substrate was observed from tape test pulls. Although various embodiments of the invention have been shown and

described, they are not meant to be limiting. Those of skill in the art may

recognize various modifications to these embodiments, which modifications are meant to be covered by the spirit and scope of the appended claims.

Claims

What is Claimed is:

1. An optical construction comprising:

an optically transmissive substrate;

a reflective layer composed of a highly reflective metal overlaying

the optically transmissive substrate, and bonded thereto; and

a protective layer composed of a parylene polymer film bonded to

the reflective metal layer.

2. The optical construction of Claim 1, further comprising an

adhesion-promoting layer deposited between the optically transmissive substrate

and the reflective layer for increasing the strength of the bond therebetween.

3. The optical construction of Claim 2, wherein the adhesion-

promoting layer is composed of the oxide form of a metal or metalloid.

4. The optical construction of Claim 3, wherein the metal or metalloid

of the adhesion-promoting layer is selected from the group consisting of

aluminum, hafnium, zirconium, tantalum, titanium, niobium, silicon, tungsten,

vanadium, molybdenum, chromium, tin, antimony, indium, zinc, bismuth,

cadmium, and nickel.

5. The optical construction of Claim 3, wherein the metal of the

adhesion-promoting layer is aluminum.

6. The optical construction of Claim 1, wherein the parylene polymer

film comprises at least one layer of a parylene polymer variant.

7. The optical construction of Claim 6, wherein the parylene polymer

variant is selected from the group consisting of parylene N, paiylene C, parylene

D, and combinations thereof.

8. The optical construction of Claim 1 wherein the highly reflective

metal is selected from the group consisting of silver, copper, gold, palladium,

iridium, rhodium, and combinations in the form of alloys thereof.

9. The optical construction of Claim 1, wherein the highly reflective

metal is silver.

10. The optical construction of Claim 1, wherein the optically

transmissive substrate is composed of glass or a polymer material.

11. The optical construction of Claim 10, wherein the polymer material

of the optically transmissive substrate is selected from the group consisting of polyhydrocarbons, polyoxyhydrocarbons, polysulfohydrocarbons, fluorocarbons

and fluorohydrocarbons, polyesters, poly(ethyleneterephthalate),

poly(butyleneterephthalate), polyacrylates methacrylates,

poly(methylmethacrylate) (PMMA), poly(methacrylate), poly(ethylacrylate),

copolymers, poly(methylmethacrylate-co-ethylacrylate), and polycarbonates, and

CR-39 allyl diglycol carbonate resin, OZ-1000 cycloaliphatic acrylic resin,

CALIBRE 1080 DVD polycarbonate resin, MAKROLON DP1-1265

polycarbonate resin, PLEXIGLAS VOD-100 acrylic molding resin, TOP AS

cyclo-olefin copolymer resin, ZEONEX cyclo-olefin polymer resin, and

combinations thereof.

12. The optical construction of Claim 10, wherein the polymer material

of the optically transmissive substrate is poly (methylmethacry late).

13. The optical construction of Claim 1, wherein the highly reflective

layer comprises a thickness of from about 100 to 10,000 A.

14. The optical construction of Claim 2, wherein the adhesion-

promoting layer comprises a thickness of from about 10 to 1000 A.

15. The optical construction of Claim 6, wherein the parylene polymer

film comprises a layer of parylene C in contact with the reflective layer, and a

layer of parylene D in contact with the layer of parylene C.

16. The optical construction of Claim 15, wherein the parylene

polymer film further comprises an interlayer of parylene C and parylene D

between the layer of parylene D and the layer of parylene C.

17. The optical construction of Claim 1, wherein the protective layer

comprises a thickness of from about 0.001 to 0.0001 of an inch.

18. The optical construction of Claim 1, wherein the substrate

comprises a fiber optic waveguide.

19. The optical construction of Claim 1, wherein the protective layer

composed of the parylene film is annealed or heat-treated.

20. An optical construction comprising:

an optically transmissive substrate;

an adhesion-promoting layer comprising the oxide form of at least

one metal or metalloid deposited onto the surface of the optically transmissive

substrate; a reflective layer composed of a highly reflective metal; and

a protective layer composed of a parylene polymer film in bonded

contact with the reflective metal layer.