Durable silver-based mirror coating employing nickel oxide

Abstract

A reflective optical coating has a thin film of silver as the primary reflecting material, a thin protective anti-oxidation layer of nickel oxide (NiO) deposited directly on top of the silver layer, and one or more thin transparent barrier layers deposited on top of the NiO, where each barrier layer is composed of a fluoride, a metal oxide, or a nitride. Optionally, a thin protective layer of NiO or Ni may be included, directly beneath the silver layer. Optionally, one or more thin barrier underlayer(s) may be included below the silver (and below the Ni or NiO protective layer, if present), where each of the barrier underlayers is a fluoride, a metal oxide, a metal nitride, or a bare metal.

Claims

1. A reflective optical coating deposited on a top surface of a substrate, the reflective optical coating comprising: a silver reflective layer consisting essentially of silver, disposed above the substrate; a protective nickel oxide layer 1-10 nm in thickness consisting essentially of NiO, disposed above the silver reflective layer relative to the substrate m and in direct contact with the silver reflective layer; a transparent barrier layer consisting essentially of one of a fluoride, a metal oxide, or a transparent nitride, where the transparent barrier layer is disposed above the protective nickel oxide layer relative to the substrate and in direct contact with the protective nickel oxide layer.

2. The reflective optical coating of claim 1 where the transparent barrier layer consists essentially of one of YF.sub.3, YbF.sub.3, TiO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, Al.sub.2O.sub.3, or Si.sub.3N.sub.4.

3. The reflective optical coating of claim 1 where the silver reflective layer is in direct contact with the substrate.

4. The reflective optical coating of claim 1 further comprising a protective underlayer consisting essentially of NiO or Ni disposed below the silver reflective layer and in direct contact with the silver reflective layer.

5. The reflective optical coating of claim 4 where the protective underlayer is in direct contact with the substrate.

6. The reflective optical coating of claim 4 further comprising a barrier underlayer disposed below the protective underlayer and where the barrier underlayer is in direct contact with the substrate and the protective underlayer.

7. The reflective optical coating of claim 6 where the barrier underlayer consists essentially of a fluoride, a metal oxide, a metal nitride, or a bare metal.

8. The reflective optical coating of claim 7 where the barrier underlayer consists essentially of YF.sub.3, YbF.sub.3, TiO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, Al.sub.2O.sub.3, TiN, CrN, Ni, m Cr, or Ti.

9. The reflective optical coating of claim 1 further comprising a barrier underlayer layer disposed below the silver reflective layer and where the barrier underlayer is in direct contact with the substrate and the silver reflective layer.

10. The reflective optical coating of claim 9 where the barrier underlayer consists essentially of a fluoride, a metal oxide, a metal nitride, or a bare metal.

11. The reflective optical coating of claim 10 where the barrier underlayer consists essentially of YF.sub.3, YbF.sub.3, TiO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, Al.sub.2O.sub.3, TiN, CrN, Ni, Cr, or Ti.

12. The reflective optical coating of claim 1 further comprising a second transparent barrier layer consisting essentially of one of a fluoride, a metal oxide, or a transparent nitride, where the transparent barrier layer is disposed above the transparent barrier layer relative to the substrate and in direct contact with the transparent barrier layer.

13. The reflective optical coating of claim 12 where the second transparent barrier layer consists essentially of one of YF.sub.3, YbF.sub.3, TiO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, Al.sub.2O.sub.3, or Si.sub.3N.sub.4.

14. The reflective optical coating of claim 1 where the silver reflective layer, the protective nickel oxide layer, and/or the transparent barrier layer is deposited by e-beam, ion-assisted e-beam, sputter, cathodic arc physical vapor deposition, or atomic layer deposition.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0017] FIG. 1A is a schematic cross-sectional view of a reflective optical coating 120 deposited on a top surface of a substrate 100, according to an embodiment of the m invention.

[0018] FIG. 1B is a schematic cross-sectional view of a reflective optical coating 122 deposited on a top surface of a substrate 100, according to another embodiment of the invention.

[0019] FIG. 1C is a schematic cross-sectional view of a reflective optical coating 124 deposited on a top surface of a substrate 100, according to another embodiment of the invention.

[0020] FIG. 1D is a schematic cross-sectional view of a reflective optical coating 126 deposited on a top surface of a substrate 100, according to another embodiment of the invention.

[0021] FIG. 2 is a graph of measured reflectivity vs wavelength comparing the improved NiO silver reflective coating of the invention with a nickel-chromium nitride (NiCrN) silver reflective coating and a freshly-deposited bare aluminum mirror.

DETAILED DESCRIPTION OF THE INVENTION

[0022] FIG. 1A is a schematic cross-sectional view of a reflective optical coating 120 deposited on a top surface of a substrate 100, according to an embodiment of the invention. The reflective optical coating 120 in this embodiment has a silver reflective layer 102 consisting essentially of silver, disposed above and in direct contact with the substrate 100. A 1-10 nm thick protective nickel oxide layer 104 consisting essentially of NiO is disposed above and in direct contact with the silver reflective layer 102. A transparent barrier layer 106 consisting essentially of one of YF.sub.3, YbF.sub.3, TiO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, Al.sub.2O.sub.3, or Si.sub.3N.sub.4 is disposed above and in direct contact with the protective nickel oxide layer 104. Transparent barrier layer 106 may consist of multiple transparent barrier layers, each consisting essentially of one of YF.sub.3, YbF.sub.3, TiO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, Al.sub.2O.sub.3, or Si.sub.3N.sub.4.

[0023] FIG. 1B is a schematic cross-sectional view of a reflective optical coating 122 deposited on a top surface of a substrate 100, according to another embodiment of the invention. This embodiment is identical to the embodiment of FIG. 1A except that the reflective optical coating 122 in this case includes a protective underlayer 108 consisting essentially of NiO or Ni disposed below and in direct contact with the silver reflective layer 102. The protective underlayer is also in direct contact with the substrate 100.

[0024] FIG. 1C is a schematic cross-sectional view of a reflective optical coating 124 deposited on a top surface of a substrate 100, according to another embodiment of the invention. This embodiment is identical to the embodiment of FIG. 1B except that the reflective optical coating 124 in this case includes a barrier underlayer 110 disposed below and in direct contact with the protective underlayer 108. The barrier underlayer 110 is also in direct contact with the substrate 100. Preferably, the barrier underlayer consists essentially of one of YF.sub.3, YbF.sub.3, TiO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, Al.sub.2O.sub.3, TiN, CrN, Ni, Cr, or Ti. Barrier underlayer 110 may consist of multiple barrier underlayers, each consisting essentially of one of YF.sub.3, YbF.sub.3, TiO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, Al.sub.2O.sub.3, TiN, CrN, Ni, Cr, or Ti.

[0025] FIG. 1D is a schematic cross-sectional view of a reflective optical coating 126 deposited on a top surface of a substrate 100, according to another embodiment of the invention. This embodiment is identical to the embodiment of FIG. 1C except that the reflective optical coating does not include the protective underlayer of Ni or NiO. Instead, the barrier underlayer 110 is in direct contact with the silver reflective layer 102.

[0026] The embodiments described above may be generically characterized as a reflective optical thin film coating deposited on a substrate, where the coating includes: [0027] a thin film of silver as the primary reflecting material (preferably 60-180 nm thick, more preferably, 120 nm thick); [0028] a thin protective anti-oxidation layer of nickel oxide (preferably 1-10 nm thick, more preferably, 2 nm thick) deposited directly on top of the silver layer; [0029] one or more thin transparent barrier layers (preferably 8-240 nm thick in total, more preferably 80 nm thick) deposited on top of the NiO, where each of these layers is composed of a fluoride (e.g., YF.sub.3 or YbF.sub.3), a metal oxide (e.g., TiO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, or Al.sub.2O.sub.3), or a transparent nitride (e.g., Si.sub.3N.sub.4); [0030] Optionally, a thin protective layer of NiO or Ni (preferably 1-10 nm thick, more preferably 2 nm thick) directly beneath the silver layer; [0031] Optionally, one or more thin barrier underlayer(s) (preferably 10-50 nm thick, more preferably 40 nm thick) below the silver (and below the Ni or NiO protective layer, if present), where each of the barrier underlayers consists of a fluoride such as YF.sub.3 and YbF.sub.3, a metal oxide such as TiO.sub.2, Ta.sub.2O.sub.5, Y.sub.2O.sub.3 and Al.sub.2O.sub.3, a metal nitride such as TiN and CrN, or a bare metal such as Ni, Cr and Ti.

[0032] The coating may have any size up to 10 m in diameter. The substrate material may be optical glass including fused silica; or ultra-low-expansion glass/ceramic mixtures.

[0033] The silver reflective layer, the protective nickel oxide layer, and/or the transparent barrier layer may be deposited by e-beam, ion-assisted e-beam, sputter, cathodic arc physical vapor deposition, or atomic layer deposition (ALD).

[0034] In one illustrative example, the layers of a coating are deposited on the substrate in the following order: a 22 nm thick Y.sub.2O.sub.3 adhesion-barrier underlayer, a 120 nm thick silver film, a 51 nm thick protective layer of NiO, and a 72 nm thick transparent barrier layer of Al.sub.2O.sub.3. The NiO is deposited by a slow evaporation of nickel metal in a background pressure of oxygen to reactively form the oxide; this oxide is most likely sub-stoichiometric. The transparent barrier of Al.sub.2O.sub.3 is deposited by thermal ALD using trimethylaluminum and water for the Al and oxidizer, respectively. It is highly-desired to keep substrate temperatures as low as possible during the coating process, both to reduce risk of damaging expensive substrate optics with thermal cycling, and especially to avoid damaging epoxy bonds in mounting hardware, which is becoming widely practiced. The ALD-barrier layer may be deposited at a common process temperature of 150 C or at a more practical temperature (for epoxy bonds) of 60 C.

[0035] The measured reflectivity of an example of the improved coating is shown in FIG. 2, comparing it with nickel-chromium nitride (NiCrN) silver reflective coating and a freshly-deposited bare aluminum mirror. The improved coating with NiO has comparable reflectivity to the NiCrN coating in the infrared portion of the spectrum, but significantly superior reflectivity at wavelengths in the 350-550 nm range. It also has comparable or significantly greater reflectivity everywhere with respect to aluminum except below 340 nm. In the thermal IR, the reflectivity of the silver-based coatings is 99%, whereas bare Al is 97%, meaning that the emissivity of the silver-based coatings is that of aluminum; this is extremely important for observations of faint sources in the infrared.