ENHANCED, DURABLE SILVER COATING STACKS FOR HIGHLY REFLECTIVE MIRRORS

FIELD

The disclosure is directed to Enhanced, Durable Innovative Silver-containing (“EDIS”) coating stacks for highly reflective mirrors, and to a method of making such stacks, for use in ISR detectors.

BACKGROUND

There has been constant improvements in Intelligence-Surveillance-Reconnaissance (ISR) detector technology, for example, increasing the wavelength detection range, decreasing detector footprint and reducing pixel size, all of which enable the system size and weight to be reduced. These improvements created an increased demand for ISR broad -multi-band optical systems; specifically including a high definition visible band (VIS, 0.34 μm to ˜0.75 μm) as well as the near infrared (NIR, 0.75 μm to 1.4 μm), the short wavelength infrared (SWIR, 1.4 μm to 3 μm), the mid-wavelength infrared (MWIR, 3 μm to 8 μm), and long wavelength infrared (LWIR, 8 μm to 15 μm) bands. A key component of such systems are reflective optics that have a silver coating thereon which enables them systems to achieve this spectral performance. However, historically the silver coatings have been a source of system failures due to the propensity of the silver coating to “break-down” or “corrode” over time. It may not be possible to completely protected future systems from some of the harsh environments in which they will operate, particularly environments that are hot, humid and contain salt. It is thus desirable to have a system having highly durable broad band silver coated optics.

There are several test procedures that are used to evaluate the durability performance of thin film coated optical components. Examples include military specification documents such as MIL-C-48497, MIL-F-48616 and MIL-PRF-13830B, which include tests that involve exposure to humidity, salt fog, salt solutions, temperature cycling, abrasion, and other test procedures others. The harshest of these tests is the “24-hour salt fog” test. While at the present time there are two groups that claim highly reflective mirror stacks, made by specific processes, that can pass the 24-hour salt fog test (Quantum Coating Inc. using a Denton Vacuum process (not described) and Lawrence Livermore Laboratories using a process described in U.S. Pat. No.7,838,134), neither of these stacks meet the entire ISR spectral requirements, specifically they do not meet them for the LWIR range. The Quantum/Denton silver coating is called “X-1 Silver” by both parties; and the published information shows performance in the 0.4 μm to 0.7 μm range (2000 Society of Vacuum, Coaters 505/856-7188; 43^rdAnnual Technical Conference Proceedings (2000) ISSN 0737-5921), with no information for longer wavelengths into the infrared out to the LWIR range being given for this coating. This article also suggests the use of ion beam assisted deposition and substrate heating might further improve performance. However, substrate heating is not desirable when using some metallic substrates, for example 6061-Al substrates, because if the temperature is too high the mechanical strength and corrosion resistance of the substrate is decreased. Consequently, it is preferred that the substrate temperature be below the heat treating (˜415° C.) and stress relief (˜350° C.) temperatures of the 6061-Al substrates. Lawrence Livermore Laboratories, U.S. Pat. No.7,838,134, claims the use of nitrides for the silver adhesion-interface layers while using a Si₃N4 protective cap layer. The patent states that the “silver mirror is characterized by high reflectance in a broad spectral range of 300 nm in the UV to the far infrared (˜10000nm)”, this range being 0.3 μm to 10 μm.

However, despite the advances made in the art, further extension of the silver mirror's reflectance properties, including the wavelength range into the LWIR and mirror durability, is desirable.

SUMMARY

The disclosure is directed to a highly reflective multiband mirror that is reflective in the VIS-NIR-SWIR-MWIR-LWIR bands, the mirror being a complete thin film stack that consists of six (6) types of layers on a substrate. The multiband mirror is more durable than existing mirrors on light weight metal substrates, for example 6061-Al, designed for similar applications. In each of the layer types methods and materials are used to process each layer so as to achieve the desired layer characteristics, which aid to enhancing the durability performance of the stack. While any given layer may improve stack durability, it is the combination of these five layer types that results in the higher, enhanced level of performance exhibited by the mirror of this disclosure. Table 1 illustrates these 5 types of layers, which are a barrier layer, at least one interface layer, a reflective layer, a tuning layer and a protective layer, and defines their properties. Multiple materials can be used to meet the characteristics needed for each of the different layer types.

TABLE 1

Coating Design and Materials

Layer
Layer Characteristics
Exemplary Materials

Protective
The layer possesses a high level of both chemical and
YbF₃, YF₃, Si₃N₄,

Layer(s)
mechanical durability
Yb_xF_yO_z

Tuning
A low refractive index, high refractive index design is
YbF₃, YF₃, GdF₃,

Layer(s)
used for layer tuning. The Materials must be low
Bi₂O₃Yb_xF_yO_z, Bi₂O₃,

absorbing in the wavelength range of from 0.4 μm to
ZnS

15.0 μm, and possess a medium to high level of

chemical and mechanical durability.

Second
This layer is used to promote adhesion of the tuning
Bi₂O₃, ZnS, Nb₂O₅,

Interface
layers to the reflective layer, must have low
TiO₂, Ta₂O₅, Al₂O₃

Layer
absorption and be galvanically compatible with the

(2^ndlayer)
reflective layer

Reflective
Silver (Ag) is used as the reflective layer to provide
Ag, Au, Al, Rh, Cu,

Layer
high reflectance (high % R) in the wavelength range of
Pt, Ni

layer possesses a high level of both chemical and

mechanical durability

First Interface
This interface layer is used to promote adhesion
Al₂O₃, TiO₂, Bi₂O₃

Layer
between the barrier layer and the reflectance layer,
ZnS, Ni, Bi, Monel

(1st Layer)
and must have galvanic compatibility with the barrier
(Ni—Cu), Ti, Pt

and Reflectance Layers.

Barrier Layer
This layer is used to create galvanic compatibility
Si₃N₄, SiO₂, TiAlN,

between the Ag reflectance layer (0.15v) and the Al
TiAlSiN, TiO₂, DLC,

substrate (0.9v). While Al and CrN can be a barrier
Al, CrN, Si_xN_yO_z

layers, an interface layer is needed to achieve galvanic

compatibility with the reflective layer.

Substrate
A light weight diamond turned optic/substrate, for
silica, fused silica and

example 6061 aluminum (6061-Al) and other Al
F-doped fused; and

alloys, Mg alloys, Ti Alloys and ceramic alloys. A
6061-Al alloy, other

barrier layer is needed for metal alloys and its
light weight Al alloys

thickness is substrate dependent.
Mg alloys, Ti alloys

1. The interface layers are also referred to as “adhesion” layers.

2. Yb_xF_yO_zis formed when oxygen is admitted during the depositions of YbF₃. Electronically, nYb⁺³= yF⁻¹+ zO⁻²so that the sum of the positive and the negative valances balance and there is no net charge to the coating layer. The same is true for Si_xN_yO_z.

Thus, in order from substrate to the final layer, the mirror consists of (a) substrate, (b) barrier layer, (c) first interface layer, (d) reflective layer, (e) second interface layer, (f) tuning layer and (g) protective layer. In some embodiments the tuning layer and the protective are combined into a single layer using a single coating material.

Barrier Layer

The thickness of the barrier layer can be in the range of 100 nm to 50 μm. In one embodiment the barrier layer has a thickness in the range of 500 nm to 10 μm. In another embodiment the barrier layer has a thickness in the range of 1 μm to 5 μm. One criterion for determining the thickness of the barrier is the number of hours the article will have to withstand the salt fog test. The longer the duration of the salt fog test the thicker the barrier layer. For a salt fog test of 24 hours a barrier layer of 10 μm has been found sufficient. In many applications, if the barrier layer is too thick it will cause distortion of the finished part with changes in temperature, but since typically the operational temperature are given in the specification the thickness of the barrier layer can be adjusted to prevent distortion. The differences in the thermal expansion coefficients of the barrier layer and the substrate will cause the optical figure, power and irregularity, to change ΔT (the change in temperature). In some embodiments the barrier layer is sufficiently thick so that it will cover or smooth out any high and irregular substrate peak-to-valley variations. Smoothing out such variations aids in polishing the surface to optimize surface quality. The surface quality is important in promoting adhesion on entire surface and minimizing localized defect sites that may be caused by the peak-to-valley variations.

The First and Second Interface Layers, also Known as “Adhesion Layers”

The thickness of these layers is dependent of factors including the material used for the layers, whether the layer the 1^stor 2^ndinterface or adhesion layer, and whether it is on the front surface (the surface of the reflecting layer) or back surface (the layer on which the reflecting layer is deposited) of the mirror. When Ni, Cr and Ti materials are used as the interface layer only a thin layer of material, on the order of angstroms “A”, is used. For a front surface mirror the interface layer on top of the reflecting layer, that is, the second interface layer, needs to be thick enough to promote adhesion, but also thin enough so that it does not absorb any of the reflected radiation. In general, the thickness of the first interface layer is in the range of 2 A to 250 A (0.2 nm to 25 nm). For metallic interface layers, for example Ni and Cr, the thickness is in the range of 2 A to <25 A (0.2 nm to <2.5 nm). In one embodiment the metallic first interface layer thickness is in the range of 2 A to 10 A (0.2 nm to 1 nm). When a metal oxide or sulfide, for example Al₂O₃or ZnS, is used as the first interface layer the thickness is greater than 25 A (>2.5 nm). In an embodiment the first interface layer is a metal oxide or sulfide the thickness is in the range of 50 A to 250 A (5 nm to 25 nm). In another embodiment the first interface layer thickness is in the range of 10 nm to 20 nm.

The interface-adhesion layer used under the reflective layer, that is, the first interface layer, is present only to promote adhesion so that optical considerations such as absorbing reflection radiation is not a consideration. Consequently, the thickness of the first interface or adhesion layer is determined needs only to be concerned with adhesion and not optical considerations. As a result the first interface layer can a minimum thickness to provide the adhesion, but no maximum thickness because there are no absorption or optical concerns. However, the thickness of the second interface layer has to be carefully controlled so the reflection losses are minimized. For the second interface layer the thickness is in the range of 5 nm to 20 nm. In an embodiment the thickness of the second interface layer is in the range of 8 nm to 15 nm. In another embodiment where reflective article is intended for use in the wavelength range of 9.5 μm to 15 μm, the thickness of the second interface layer is in the range of 8 μm to 12 μm to maximize the reflectance of the final reflective article.

Reflective Layer

For the reflective metal layer the thickness must be sufficient to provide optimum reflection properties. If the reflective layer is too thin the film is not continuous and/or transmitting and if it is too thick this can create durability concerns. The thickness of the reflective layer is in the range of 75 nm to 350 nm. In an embodiment the thickness of the reflective layer is in the range of 80 nm to 150 nm. In another embodiment the thickness of the reflective layer is in the range of 90 nm to 120 nm.

In addition to using Ag as the reflective material, the stack configuration can also be used with other reflective materials, for example Au, Al, Rh, Cu, Pt, Ni, to provide an enhanced durable, chemical and mechanical performance, but with a change in spectral range and reflection intensity.

Tuning and Protective Layers:

The thicknesses of these layers depends the optimization necessary to achieve the required spectral performance while simultaneously optimizing the protection necessary to pass the required tests, for example, the salt fog and humidity tests. The thickness of these layers can vary significantly depending on application and materials used.

An Ag mirror prepared using the scheme and materials described in Table 1 will have the following advantages 1-7.

1. It will meet a high reflectivity specification in all desired wavelength bands, VIS-NIR-SWIR-MWIR-LWIR, for angle on incidence (AOI) of 0 to 45 degrees and in some cases >45 degrees. The materials and thicknesses used to prepare an Ag mirror as described herein has resulted in minimum absorption in LWIR region where other stacks and prior art have absorption issues. The materials used in the present disclosure include YbF₃, YF3, ZnS, Bi₂O₃, GdF₃that have no absorption bands in the defined LWIR band, 8 μm tol5 μm.

2. YbF₃and YF₃have demonstrated high resistance to the salt fog environment and pass specification moderate abrasion tests, so they are alternative materials that can be used as a protective cap layer

3. The Al₂O₃, Bi₂O₃, ZnS and TiO₂transparent materials, used as an interface-adhesion layer, can be thicker on the front end interface; thus it is easier to control layer termination during the deposition process compared to ultra-thin layers of Cr or Ni, or related alloys. These materials, Al₂O₃, ZnS and TiO₂, have considerably less absorption in all bands compared to the metals typically used for this purpose, for example, Ni and Cr.

4. Replacing a metal and/or conductive interface-adhesion layer with non-conductive materials results in ideal galvanic compatibility.

5. Replacing a metal and/or conductive barrier layer with dielectric material results in ideal galvanic compatibility.

6. Thick barrier layers have increased overall stack resistance to salt fog and extended humidity environments. Materials successfully used in achieving this resistance are Si₃N₄, SiO₂, Si_xN_yO_z, DLC (diamond-like carbon), TiO₂, CrN, Si_xN_yO_z, TiAlN or TiAlSiN or similar composite films. In addition, if needed to optimize galvanic compatibility, an optional multi-layer barrier could be configured and applied as a non-conducting layer at the metal/substrate surface with the addition of a thicker conductive layer, for example, SiO₂—CrN. For example, CrN is a very good barrier layer because of its resistance to alkali and (possibly) low stress, as well as ease of deposition process (control and deposition rate), but the galvanic compatibility of CrN with the Al or metal alloy is not optimum because it is conductive. Using a thinner dielectric layer like SiO₂or Si₃N₄, that has high intrinsic stress, will isolate the CrN from the metal substrate, thus creating optimum galvanic compatibility. In addition, one can also design and use a combination of different materials to create a “barrier stack” so as to have minimum stress. For example, a highly compressive stress film layer stacked with a highly tensile stress film layer will cancel each other resulting on zero or minimal stress.

7. A thick Al layer on top of the 6061-Al diamond turned substrate with a thin barrier layer increased the overall stack resistance to salt fog and extended humidity environments. In addition, one can consider the Al layer deposited on top of the 6061-Al substrate as being a barrier layer in its own right or as the first later of a barrier stack. In addition, these thick layers can be polished prior to the deposition of additional layers. The polishing will result in improved surface quality that cannot be achieved with the bare Al-6061 substrate. The use of a thick SiO₂layer for this purpose is known (U.S. Pat. No. 6,921,177).

As mentioned above, mirrors prepared using other reflective metals will have similar advantages though with changes in spectral range and reflection intensity.

The barrier layer, the first and the second interface layers, the silver layer and the tuning layer can each, independently, be deposited using ion assistance. In some embodiment ion assistance is mot used or is used for only part of the deposition process. In all embodiments the protective layer is deposited using ion assistance. However, these materials can also be deposited using other processes and will perform well over the 0.34 μm to 15 μm wavelength range, with the provision that the process will optimized for the characteristics desire. Ion assist under the correct conditions optimizes stoichiometry and density and possibly structure. An example, different techniques were used to prepare a mirror having the 0.34 μm to 15 μm wavelength range characteristics described herein. The techniques selected were dependent on the materials being used and what was the coal that was to be achieved.

TABLE 2

Exemplary Materials and Techniques

Material
Process Comments

DLC
RF-PECVD process

Thick Al
Ultra-high rate thermal deposition or

sputtering. No IAD. Substrate source

distance critically close.

Top Al₂O₃
(a) No Initial IAD, then deposit with IAD

after first 2-4 nm; or

(b) No IAD at all

YbF₃or YbF_xO_y
IAD with or without the use of O₂

Nb₂O₅
E-beam IAD or U.S. Pat. No. 5,851,365

Si₃N₄or SiO₂
E-beam IAD or U.S. Pat. No. 5,851,365

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a graph illustrating the effect of ion-assisted deposition (IAD) of the Ag layer on the performance of a substrate/CrN/Ag mirror both before and after a 2 hour salt fog (SF) test compared to a mirror in which IAD was not used.

FIG. 1B is an optical microscope image (magnification 374×) illustrating the surface deterioration, the dark spots indicated by the arrows 18, the after 2 hours exposure to salt fog of a mirror in which the silver coating was deposited without IAD assistance.

FIG. 1C is an optical microscope image (magnification 374×) illustrating the surface deterioration, the dark spots indicated by (arrows 18, after 2 hours exposure to salt fog of a mirror in which the silver coating was deposited with IAD assistance, the deterioration being retarded relative to that in FIG. 1B as a result of using ion assistance during the deposition of the silver layer.

FIG. 2A a graph of Reflectance versus Wavelength of Ag/Cr/Si₃N₄and Ag/Ni/Si₃N₄mirrors after 0, 4, 6 or 10 hours of salt fog (SF) exposure; numerals 20 and 22 being the Cr-containing mirror after 0 and 4 hours SF exposure, respectively, and numerals 24, 26 and 28 being the Ni-containing mirror after 0, 6 and 10 hours SF exposure.

FIG. 2B is a photograph of mirror illustrating the effects of galvanic potential difference on controlled stack of coating on a glass substrate, the stacks being Ag/Cr/Si₃N₄(upper photograph) and Ag/Ni/Si₃N₄(lower photograph) after 4 hours and 5 hours salt fog exposure, respectively.

FIG. 3 is a graph illustrating the AL-o absorption band at approximately 10.7 μm for an Al₂O₃greater than 100 nm thick.

FIG. 4 is a graph of the absorption band for Si—N and Al—o at approximately 9.1 μm and 10.7 μm, respectively, for Si₃N₄and Al₂O₃thin films that are less than 100 nm thick.

FIG. 5A is a graph of a partial stack illustrating the reduction in reflectivity when a thin protective of Si₃N₄is placed on top of an Al₂O₃second interface layer covering the Ag layer, the graph showing a reduction in reflectivity not only from 0.5 μm to 0.8 μm, but also out to 1.6 μm. This is not due to absorption part (k) of the dispersion properties but to refractive index n and matching these indices in the stack design. FIG. 5B is a graph illustrating a theoretical stack and showing how adjusting the materials affects the reflectivity.

FIG. 5B, instead of having just Si₃N₄top of an Al₂O₃second interface layer as in FIG. 5A, has a three part coating consisting of Si₃N₄—SiO₂—Si₃N₄placed on top of the Al₂O₃layer. This illustrates that the protective layer has to be designed to not only protect the coating and make it more durable, but also to optimize or enhance the reflectivity of the desired wavelength bands.

FIG. 6A is a graph illustrating the absorption band for Nb₂O₅and YF₃when the coating thickness of these materials is >100 nm.

FIGS. 6B and 6C are graphs illustrating the optical performance of an actual stack designed for VIS-SWIR-MWIR-LWIR band performance, with adjustment to the Nb₂O₅layer to minimize absorption in the LWIR band.

FIG. 7 is graph of Reflectance versus wavelength illustrating that the use of a high refractive index material, for example YF₃, in the desired MWIR and LWIR bands which results in a higher % R performance.

FIG. 8A is provides graphs and an illustration of a mirror, fused silica substrate, with an EDIS coating having a barrier layer after greater than 23 hours exposure to salt fog showing that there was no measurable change in performance after salt fog exposure; the configuration being, from substrate to top layer, fused silica, Si₃N₄or CrN, Al₂O₃, Ag, Al₂O₃, YbF₃, Nb₂O₅,YbF₃.

FIG. 8B provides a graph and an illustration of a mirror, 6061-Al substrate, with an EDIS coating and a thick barrier layer after greater than 23 hours exposure to salt fog; the graph illustrating that there is a potential for causing out of specification Δ-figure and Δ-temperature performance; the configuration being, from substrate to top layer, 6061-Al, ultra thick Ni barrier, Al₂O₃, Ag, Al₂O₃, YbF₃, Nb₂O₅,YbF₃. No blemishes were observed on the mirror after the salt fog testing. The Ni was deposited by electroless plating on the 6061-1 substrate. Regarding the performance of a coated mirror with regard to temperature changes, this is particularly related to the CTE of both the substrate and barrier layer. The difference between the two will cause distortion to the optic or a change in figure with changes in temperature. The thicker the barrier layer the more of a change in figure, and the larger difference in the CTE of the substrate and barrier layer the more the increase in figure change. Consequently, it is necessary to design-in the CTE and the thickness in order to minimize Δ-figure and Δ-temperature.

FIG. 8C is an illustration of a mirror similar to that of in FIG. 8B except that the mirror has a thin barrier layer after 6 hours salt fog testing; the mirror showing a blemish on the left at the arrow.

FIG. 8D is a complete stack formed on turned 6061 aluminum that passed all tests after exposure to 120 hours of humidity exposure (3.4.1.2), moderate abrasion (3,4,1,3 and adhesion testing (3.4.1.1).

FIG. 9 are interferometer results showing precipitate peaks or nodules on the surface of diamond turned and polished 6061-Al before coating; the being characterized using EDS and identified as being impurities in the 6061-Al alloy. These large precipitates create a non-homogenous surface which makes it difficult to obtain a surface finish less than 60 A rms, the best results being about 30 A rms, which was obtained with difficulty.

DETAILED DESCRIPTION

Herein the term “high reflectance” means a reflectance of at least a reflectivity of at least 95% over the wavelength range of 0.4 μm to 15 μm. Also herein the phrase “salt fog” is abbreviated as “SF”. The 6061-Al aluminum substrate, or other metallic substrates, is diamond turned and polished before the application of any coating materials. Glass, glass-ceramic or ceramic substrates are ground, lapped and polished before the application of any coating. The abbreviation “AOI” means Angle of Incidence” and is in degrees, and the term “pol” means a “polished aluminum substrate.” In the process described herein it is desirable that the substrate temperature be minimized during the deposition of the coating materials. When 6061-Al substrates are used the temperature should be below the heat treating and stress relief temperatures of 6061-Al which are 415° C. and 350° C., respectively.

The Reflective Layer

Due to the multiband reflection requirements, ranging from visible (VIS) through the long wave infrared (LWIR), 0.40 μm out through 15 μm, a thin film layer of silver is used for the reflective layer. Silver is known to have the highest reflectivity, lowest polarization splitting, and lowest emissivity through this entire wavelength range. [See S. A. Kovalenko and M. P. Lisita, “Thickness dependences of optical constants for thin layers of some metals and semiconductors,” Semiconductor Physics, Quantum Electronics and Optoelectronics Vol.4, No. 4, pages 352-357 (2001); Chang Kwon Hwangbo, et al, “Ion assisted deposition of thermally evaporated Ag and Al films”, Applied Optics Vol.28, No 14, (Jul. 15, 1989); and N. Thomas et al, “Protected Silver Coating for Flashlamp-Pumped Nd: glass Amplifiers,” 30th Annual Symposium on Optical Materials for High Power Lasers; Boulder, Colo. Sep. 30-Oct. 2, 1998; (preprint from Lawrence Livermore Laboratories; site location

http://library.llnl.gov/uhtbin/cgisirsi/mgYv2G09Sa/MAIN/103110005/60/502/X;

search term “30^thannual symposium,” Paper #1, 236354 (UCRL-JC-135179, preprint). The following characteristics are critical to many multiband imaging systems.

(1) The silver layer must have a minimum thickness to obtain optimum reflectivity. It is suggested in the literature that the thickness be on the order of 150 nm, depending on the process used to deposit the silver. A silver layer thickness in the range of 135 nm to 175 nm is beneficial.

(2) The process that is used to deposit the silver layer influences the durability of the silver layer.

(3) FIGS. 1A-1C illustrate how ion-assisted deposition (IAD) of silver improves it chemical durability. It was noted that there was some reflection loss is seen at the 400 nm range of the pretest IAD scan, probably due to the trapping of gas atoms used for bombardment.

In FIGS. 1A-1C the substrate was silica glass having a CrN barrier layer on top of the glass and a silver layer deposited on top the barrier layer. No coating layers were applied on top of the silver layer. Numeral 10 designates an article where the silver layer was deposited on top of the barrier layer without ion-assistance and the reflectance measured after deposition, but before salt fog testing. Numeral 12 designates the article of numeral 10 after it has been exposed to salt fog for 2 hours. Numeral 14 represents an article where the silver was deposited on top of the barrier layer with ion-assistance and the reflectance measure after deposition, but before salt fog testing. Numeral 16 represents the article of numeral 14 after it has been exposed to salt fog for 2 hours. The graph clearly indicates that that without ion assistance the reflectivity of the silver coating deteriorates much more quickly then the coating with ion assisted deposition after just 2 hours of salt fog testing. In FIG. 1B, taken at 374× optical magnification, dark “spots” indicated by arrows are the corrosion that has taken place on the silver coating surface. In FIG. 1C there are much fewer corrosion spots and those that are present are much smaller which indicates the clear advantage of ion assistance during the deposition of the silver reflecting layer.

Barrier Layer

Since Al-6061 is the substrate material used in these applications (though other light weight, diamond machined alloys, silica, fused silica and F-doped fused silica can also be used), a barrier layer must be used between the silver layer and the substrate, or an Al layer deposited on any of the foregoing substrates, substrate, to create galvanic compatibility. The military standards for the use of dissimilar metals is defined in MIL-STD-889B and MIL-STD-1250. These documents suggest, for systems that are expected to be exposed to harsh environments such as hot and humid and/or containing salts, that dissimilar metals should not be joined or interfaced if they exceed a galvanic potential difference of 0.25 v (in a high humid environment with no salts the potential difference can be >0.45V). In some of the engineering literature on corrosion a potential difference of 0.15V is suggested for harsh salt environments. Al-6061 is considered an anodic material with a potential of 0.90 v while silver, a cathodic material, has a potential of 0.15 v, resulting in a potential difference of 0.75 v. Interfacing anodic metals to Al as the barrier material, for example cadmium, iron, and carbon, results in a low potential difference of <0.25 v. The galvanic potential difference is >0.25V for tin, 0.33 v for chromium, 0.33 v for zinc, 0.63 v for nickel 0.83 v for magnesium. We have also effectively used [diamond like carbon (DLC) remove and add with the dielectrics], TiAlN (this TiAlN can be made to behave like metal or like a dielectric depending on the Ti—Al ratio), and non-metallic (dielectric to replace non-metallic) coatings such as diamond like carbon (DLC), Al₂O₃, Si₃N₄, Si_xN_yO_z, SiO₂,and TiO₂. While CrN has been used with some success, care must be exercised in view of the intended application because its galvanic compatibility is border-line.

The surface quality of the 6061-Al also plays an important role. Large precipitate sites are formed by the “impurities” in the 6061-Al, some of which come from the controlled addition of materials required in order to meet material specifications for strength characteristics, and other impurities are simply contaminants. The large precipitates make it difficult to achieve a smooth surface, <30 Å rms, and some of the high peaks or nodules may result in poor adhesion (or cracking from stresses or voids) between the substrate and the coating stack (silver layer, or silver layer covers differently), resulting in a defect site once the completed mirror is exposed to the environmental testing using both salt fog and long term humidity conditions. An approach to manage this condition is to deposit a very thick barrier layer that result in effectively coating over these sites. FIG. 9, illustrating the finished surface of a 6061-Al substrate before is it coated with any material, shows the presence of these nodules. Barrier layer materials are selected from the group consisting of Si₃N₄, Si_xN_yO_z, SiO₂, TiAlN, TiAlSiN, TiO₂, Si_xN_yO, and DLC, and additionally Al or Al₂O₃.

These presence of large precipitates create a non-homogenous surface which makes it difficult to obtain a surface finish less than 60 A rms, the best results being about 30 A rms, which was obtained with difficulty. The presence of the nodules serves to illustrate why the presence of the barrier works to improve reflectivity. Without being held to any particular theory, the nodules can become defect sites where localized corrosion occurs when exposed to these harsh environments. They may result in poor adhesion , so coating cracks or falls off at sites exposing areas or creating pathways. A sufficiently thick barrier layer can smooth out this surface and create a continuous film with good adhesion across the entire surface. If this barrier layer surface is sufficiently thick, polishing the layer prior to the placement of additional coating layers would result in better surface finish in the approximate range of 5 A to 15 A.

The thickness of the barrier coating can be in range of 10 nm to 100 μm. When the nodules or other surface defects are present on the substrate and cannot be removed the barrier coating is in the higher end of the range and must be sufficient to cover the nodules. If the substrate is substantially free of the nodules then the barrier coating can be at the lower end of the range. In addition, the use of ion assistance during the deposition of the barrier layer will densify the barrier coating and aid in providing a smooth surface.

Interface Layer A and B

Silver and gold have considerably lower oxide formation energies compared to other metals like titanium, aluminum, chromium, and nickel, and because of this silver and gold do not adhere well to many materials. It has been known for some time that ultra-thin films of Cr and Ni, or alloys of these metals, are excellent adhesion promoting layers for silver due to the metal-to-metal diffusion with Ag (or gold), along with metallic bonding strengths between Ag or Au and Cr or Ni. Because of the environments the mirrors disclosed herein will be exposed to, galvanic compatibility is critical and therefore must be considered when choosing the interface material. The galvanic potential difference at the interface of silver-Cr and silver-Ni are 0.45 v and 0.15 v respectively. Table 1 illustrates the significant role the galvanic potential difference has in the performance of the coating stack when it is exposed to a salt fog environment. Nickel or Al₂O₃have been used as the first interface layer between the barrier and the silver layers because the two materials are compatible.

Al₂O₃has been discussed in the literature as an adhesion promoting material for certain metals; specifically the discussion involves Ag—Al₂O₃and Al—Al₂O₃non-stoichiometric interfaces, and how they influence adhesion (W. Zhang and J. R. Smith, Nonstoichiometric interfaces and Al₂O₃adhesion with Al and Ag, Physical Review Letters, Vol 85, No 15, Oct. 9, 2000, pages 3225-3228; Jiwei Feng, et al., Ab initio study of Ag/Al₂O₃and Au/Al₂O₃interfaces, Physical Review B, 72, 115423, Sep. 21, 2005). The data in these papers show deviations of Al₂O₃from stoichiometry at the interface can significantly affect adhesion with the either Ag or Al metal, two metals chosen for their oxide heats of formation being at opposite end of the range. M. A. Scobey, U.S. Pat. No. 5,851,365 titled “Low Pressure Reactive Magnetron sputtering apparatus and method,” describes the conditions for two types of deposition processes: ion-assisted deposition (IAD) and e-beam deposition, and a low pressure reactive magnetron sputtering process, that produce optimum adhesion between Al₂O₃—Ag, and Al₂O₃—Al. Due to the first interface being on the backside of the reflective layer, between the substrate and the Ag layer, it's upper limit of thickness is not limited by absorption, but should be monitored for stress considerations. On the front side of the reflective layer, the second interface layer, the layer deposited on top of the Ag layer and the thickness of interface must be limited to minimize its absorption band in the LWIR band while obtaining optimum adhesion to Ag. This absorption band is illustrated in FIG. 3 which appears as the peak at approximately 10.7 μm. ZnS is an additional non-conducting material that can be used as an adhesion layer, avoiding galvanic compatibility issues. For example, ZnS has been found to be a successful interface material, for example, at a gold interface Au—ZnS—YbF₃or Ag-ZnS—YbF₃.

The Protective Layer and Tuning Layer(s)

Silver can react with various substances that may be present in the atmosphere, for example salts, acids, and sulfur compounds. Well known examples are silver tarnishing which is the formation of black silver sulfide (Ag₂S) by the reaction of Ag⁰with sulfur containing compounds and silver corrosion which results from the reaction of Ag⁰with halogen-containing substances in the atmosphere, the most common of which is probably NaCl and HCl (T. E. Graedel, Corrosion Mechanisms for Silver Exposed to the Atmosphere, J. Electrochemical Society Vol. 139, No. 7, pages 1963 -1969 (1992), and D. Liang et al, Effects of Sodium Chloride Particles, Ozone, UV, and Relative Humidity on Atmospheric Corrosion of Silver, J. Electrochemical Society Vol. 157, No. 4, pages C146-C156 (2010)). Both corrosion and tarnishing can be accelerated by the presence of humidity and ozone in the atmosphere.

The Vickers hardness (HV) of silver is 100 HV (electro-deposited), which is low compared to the other end of the HV spectrum where diamond has a value of 10,000 HV. As a result of the relative softness of silver compared to other materials, the handling of a silver coated optics for system assembly, or cleaning the optics which include mirrors, will often result in damaging the silver surface. As a result a protective layer is needed to minimize damaging the silver surface. In order to be effective the protective layer must be (1) sufficiently dense such that no pathways are provided from the optic's surface to silver and interface layers, (2) insoluble in alkali and acidic environments, (3) mechanically hard to provide scratch resistance, and (4) have either (a) only minimal absorption throughout the entire wavelength range of interest, 0.34 μm to 15 μm in the present case, or (b) no absorption over the 0.34 μm to 15 μm wavelength range. Silicon nitride, Si₃N₄, was tested for its alkali diffusion properties, solubility in alkali solution and in for its mechanical hardness properties, and was found to provide a very durable and chemically resistant coating. However, while this material was unfortunately found to have an absorption band at approximately 9.1 μm, this disclosure shown that if the Si₃N₄is sufficient thin it can be used. The exact thickness depends on the throughput of the system for the band range. For some applications this band is of no interest so the thickness is of limited consequence.

A single protective layer along with the ultra-thin interface layer reduces the stack reflection performance in the VIS range out into the SWIR bands, as observed in FIG. 5. Because different applications of the mirrors disclosed herein require the application of the tuning layers in order to optimize reflection in defined wavelength regions, these tuning layers need to have characteristics similar to those of the protective layer, but some minimal trade-offs can be made in the durability of these materials. To tune for the desired reflectance bands the thickness of the tuning layer(s) will have to be varied and a combination of low, medium and/or high index materials are used. The thickness of the tuning layer(s) is in the range of 75 nm to 300 nm. The protective layer applied on top of the tuning layer(s) has a thickness in the range of 60 nm to 200 nm.

Corning has developed thin film deposition processes, for example the process described in U.S. Pat. No. 7,242,843, which can be used for ytterbium fluoride (YbF₃) and yttrium fluoride (YF₃), both of which are low refractive index materials. When the process is used to coat highly reflective silver mirrors the results indicate that the resulting mirrors are highly resistant to alkali solutions while also providing scratch resistance that meets the military specification moderate abrasion testing procedures; properties that will aid in protecting the silver layer. The low refractive index materials were used in combination with high refractive index materials, for example, niobium pentoxide (Nb₂O₅) and zinc sulfide (ZnS). FIG. 6A shows the reflectance of (a) Ag (only), (b) Ag—Nb₂O₅and (c) Ag—YbF₃coatings over the wavelength range of 3 μm to 19 μm. In FIG. 6A the Ag(only) film has a reflective of at least 98% over the wavelength range of the graph, 3 μm to 19 μm, except for a small decrease at approximately 18.6 μm, and is substantially 99% reflective over the wavelength range of 3 μm to approximately 17 μm. The Ag—Nb₂O₅coating shows an Nb—O absorption with the reflectivity dipping below 96% over the approximate wavelength range of 10 μm to 13 μm. The Ag—YbF₃coating shown Yb—F absorption with reflectivity of greater than 98% in the wavelength range of 3 μm to 16.5 μm. The Reflectance for all three coating was measured at AOI=45°.

Using YbF₃as an exemplary low refractive index fluoride material, a coating combination of YbF₃—Nb₂O₅—YbF₃was tuned for high reflectivity in the VIS range, 0.34 μm to 0.75 μm, and also in a MWIR-LWIR range of 3 μm to 11.3 μm. FIG. 6B, Ag—Al₃O₃—YbF₃—Nb₂O₅—YbF₃on 6061-Al having a barrier layer, shows that when measured in VIS-SWIR range of 0.4 μm to 1.7 μm at an AOI of 45° the coating has a reflectivity of greater than 96%, and when measured at an AOI 12° the reflectivity is substantially 97%. FIG. 6C, Ag—Al₃O₃—YbF₃—Nb₂O₅—YbF₃on 6061-Al having a barrier layer, shows that in the MWIR-LWIR range of 4 μm to 15 μm, (a) at an of AOI 12° the reflectivity is substantially 99% over the wavelength range and (b)at an AOI 45° the reflectivity was greater than 96% over the wavelength range and greater than 98.5% over substantially the entire range of 3 μm to 15 μm. When oxygen is used during the deposition of the YbF₃material the deposited layer becomes a ytterbium oxyfluoride materials that is designate herein as Yb_xF_yO_z, and the thickness of this layer is the in the same range as that for YbF₃.

The tuning layer and the protective layer can also be combined into a single layer using a single material which is Yb_xF_yO_z, When only a single tuning/protective layer is used the thickness of the layer is in the range of 150 nm to 350 nm.

It was found that the combination YbF₃(low refractive index) and ZnS (high refractive index) provides minimum absorption throughout the desired wavelength range. FIG. 7 is a graph of reflectance versus wavelength of an Ag—Al₂O₃—YbF₃—ZnS—YbF₃stack of coatings on a 6061-Al substrate having a barrier layer. The graph shows the LWIR absorption at approximately 11 μm from the Al₂O₃interface layer. The minimum Al₂O₃thickness required to achieve optimum adhesion still results in observable absorption, unlike using an ultra-thin Ni layer as the interface layer, which would results in less absorption.

Materials that found to be useful as protective layers are YbF₃, YF₃, and Si₃N₄. YbF₃and YF₃(low refractive index), GdF₃(medium refractive index in VIS range) , and ZnS and Bi₂O₃(both high refractive index) are materials that can be used for tuning and that have minimum absorption in all bands including the LWIR. In addition, Si₃N₄(medium refractive index), and Nb₂O₅, TiO₂and Ta₂O₅(all three high refractive index) can be used as a tuning layer materials, but their LWIR absorption bands need to be considered in view of the application in which the mirror will be used.

Combining the Layers Together, and Testing for Durability and Spectral Performance

Different combinations can be used to meet various military specification environmental tests. The most difficult test to successfully pass is the 24 hour salt fog test. The coating stacks used for FIGS. 6A and 6B were deposited on both=fused silica substrates and diamond turned 6061-Al substrates. The test results are shown in FIGS. 8A and 8D after >23 hours salt fog testing and 120 hours humidity testing (respectively) relative humidity, RH, of approximately 98% in accord with the Mil-C-48497 specification. No measurable change was detected for(the spectral performance for both tests, salt fog and humidity. FIG. 8A indicates that the coating stack on fused silica substrate resulted in passing >23 hours exposure to salt fog in accordance per the Mil-C-48497 specification and . In FIG. 8D, the identical stack and barrier layer used for FIG. 8A was deposited on a 6061-Al substrate and the resulting mirror was exposed to 120 hours of humidity in accord with the same Mil-C-48497 specification.

FIGS. 8B and 8C illustrate the role of the barrier layer when a coating stack is deposited on 6061-Al and the resulting mirror is exposed to the salt fog environment. The stack break-down resistance is low for a thin barrier layer and increases with a the barrier layer becomes thicker. The mirror of FIG. 8B, which has a thick barrier layer, passed the salt for test whereas the mirror of FIG. 8C, which has a thin barrier layer, developed defects which are pointed out by the arrows in the figure. As indicated above, different materials can be for the barrier layer, including Si₃N₄, SiO₂, DLC, and CrN. These materials can be deposited using different processes that have included IAD e-beam, low pressure DC magnetron sputtering (U.S. Pat. No. 5,851,365, Corning Incorporated), CVD, sol-gel, and metal plating.

Process considerations for depositing the stack must be taken into consideration and these considerations are material and interface dependent. Because film density and stoichiometry are critical, Ion beam bombardment is used during the deposition. Ion energies and densities must be adjusted appropriately so as to densify but not damage the film. Gas ratios of Ar, N₂, O₂are adjusted to control desired stoichiometry, with the warning that O₂should either be: (a) not be used during deposition of the Ag layer or the second interface layer afterwards, or (b) not used at the beginning of the deposition of the second interface layer, but added into the deposition process after a very thin, 3-5 nm second interface layer has been applied to the Ag layer, The objective is to have the second interface layer, for example an oxide such as Al₂O₃, adhere to the silver layer while not exposing the silver surface to excessive O₂before the Al₂O₃deposition, while maintaining substantially all of the Al₂O₃at a stoichiometric or near stoichiometric Al:O ratio. The following are some process criteria used for the stack.

1. If the Al₂O₃is used as a barrier layer, then its initial partial pressures of Ar—O₂gases must be adjusted to create the desired AlxOy stoichiometry needed to optimize adhesion at the Al—Al_xO_yinterface. The Al_xO_y—Ag interface partial pressures are different than the Al—AlxOy interface to achieve optimum adhesion so the process must be adjusted towards the end of this barrier layer. The Al_xO_ystoichiometry needed for optimum adhesion at either the Al or Ag interfaces are discussed in references 4 and 5 above. Their partial pressures or gas flow ratios will be dependent on deposition rates, pumping speeds and deposition volumes.

2. Stoichiometry is also critical at the oxide-fluoride interfaces to obtaining optimum adhesion. In the case of oxide material, terminating the layer so that it is a stoichiometric oxide is important; while the fluoride at the interface should be an oxy-fluoride.

3. There are considerations to take into account when using IAD during silver deposition; it is important not to exceed certain Ion energies and densities because it can result in trapping Ar into the film. These defects can act as scattering centers which will reduce reflectivity at the lower visible wavelength bands.

4. Bombardment energies and gas ratios should be adjusted to obtain optimum film density. When densifying fluoride materials one has to consider ion energies that will not dissociate the fluorine atoms of the growing film. If this occurs the film will become very unstable and spectral shifting will be observed.

Thus, in one aspect the disclosure is directed to a highly reflective mirror having a for use in the wavelength range of 0.4 μm to 15 μm, the mirror comprising a substrate, a barrier layer on the substrate, a first interface layer on top of the barrier layer, a reflective layer on top of the first interface layer, a second interface layer on top of the reflective layer, at least one tuning layer on top of the second interface layer and at least one protective layer on top of the tuning layer, said mirror have a reflectivity of at least 96% over the wavelength ranges of 0.4 μm to 1.8 μm and 3 μm to 15 μm at an AOI 45°. The mirror has a reflectivity is at least 97% over the wavelength range of 0.4 μm to 1.8 μm and a reflectivity of greater than 98% over the wavelength range 0.4 μm to 1.8 μm at AOI 12°. The substrate that can be used in making the mirror can be selected from the groups consisting of fused silica, fluorine doped fused silica and diamond turned aluminum alloys In one embodiment the substrate is 6061-Al alloy. In another embodiment the substrate is fused silica. The barrier layer is selected from the group consisting of Si₃N₄, SiO₂, TiAlN, TiAlSiN, TiO₂and DLC. The first interface layer is selected from the group consisting of Al₂O₃, TiO₂, Bi₂O₃and ZnS, and the metallic materials Ni, Bi, Monel (Ni—Cu), Ti and Pt. The reflective layer is selected from the group consisting of zero valent Ag, Au, Al, Rh, Cu, Pt and Ni. The second interface layer is selected from the group consisting of SiO₂, Si₃N₄, Nb₂O₅, TiO₂, Ta₂O₅, Bi₂O₃, ZnS and Al₂O₃. The tuning layer is at least one material selected from the group consisting of YbF₃, YF₃, GdF₃and Bi₂O₃. The protective layer is at least one material selected from the group consisting of YbF₃, YF₃and Si₃N₄.

An exemplary mirror according to the disclosure consists of, from substrate to the top protective layer, fused silica, Si₃N₄or CrN, Al₂O₃, Ag, Al₂O₃, YbF₃, Nb₂O₅and YbF₃.

The disclosure is also directed to a method for making a highly reflective mirror, the method comprising the steps of:

providing a substrate selected from the group consisting of aluminum alloys, silica, fused silica, F-doped fused silica, magnesium alloys and titanium alloys;

polishing the substrate to a roughness of less than 10 nm;

applying a barrier layer having a thickness in the range of 10 nm to 100 μm to the surface of the substrate

applying a first interface layer having a thickness in the range of 0.2 nm to 50 nm on top of the barrier layer, said thickness being dependent on the interface layer material;

applying a reflecting layer having a thickness in the range of 100 nm to 300 nm on top of the adhesion layer;

applying a second interface layer of at least one selected material, the second interface layer having a thickness in the range of 0.2 nm to 50 nm on top of the reflecting layer in top of the reflecting layer;

applying a tuning layer consisting comprising at least one selected material, the thickness of the tuning layer being dependent on the at least one selected material(s); and

applying at least protective layer on top of the tuning layer to thereby form a highly reflective mirror having a reflectance of at least 96% over the wavelength range of 0.4 μm to 15 μm.

In the foregoing method the barrier layer material is selected from the group consisting of Si₃N₄, SiO₂, TiAlN, TiAlSiN, TiO₂, DLC, Al and CrN; the first interface layer material is selected from the group consisting of Al₂O₃, TiO₂, Bi₂O₃and ZnS, and the metallic materials Ni, Monel (Ni—Cu), Ti and Pt; the reflective layer is a material selected from the group consisting of Ag, Au, Al, Rh, Cu, Pt and Ni; the second interface layer is at least one material selected from the group consisting of SiO₂, Si₃N₄, Nb₂O₅, TiO₂, Ta₂O₅and Al₂O₃; the tuning layer is at least one material selected from the group consisting of YbF₃, YF₃, GdF₃and Bi₂O₃; and the at least one protective layer is selected from the group consisting of YbF₃, YF₃, Si₃N₄. In an embodiment the reflective layer is silver. In an embodiment the second interface layer comprises Si₃N₄—SiO₂—Si₃N₄.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

ENHANCED, DURABLE SILVER COATING STACKS FOR HIGHLY REFLECTIVE MIRRORS

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