Enhanced, protected silver coatings on aluminum for optical mirror and method of making same

Abstract

The disclosure is directed to enhanced silver coated aluminum substrates for use as optical mirrors in which galvanic corrosion between the silver and aluminum is prevented and a method of making such silver coating and mirrors. The optical mirror according to the disclosure has an in-situ formed barrier layer inserted between the aluminum substrate and the silver layer. In addition, selected layers are densified by carrying out their deposition using a high power RF ion source during their deposition.

Claims

1. A silver mirror having enhanced galvanic corrosion resistance, said mirror consisting essentially of an aluminum substrate having an in-situ formed chromium nitride layer having a thickness in the range of 50-150 nm on the aluminum substrate, a silver layer having a thickness in the range of 75-150 nm on the chromium nitride layer, a formed chromium oxide layer having a thickness in the range of 0.1-0.6 nm on the top of the silver layer, and one or a plurality of enhancing layers formed on the chromium oxide layer; wherein when a plurality of enhancing layers is used, different materials are used in alternating adjacent enhancing layers and each individual enhancing layer has a thickness in the range of 10-100 nm.

2. The silver mirror according to claim 1, wherein the chromium nitride layer thickness is in the range of 75-125 nm.

3. The silver mirror according to claim 1, wherein the silver layer thickness is in the range of 100-130 nm.

4. The silver mirror according to claim 1, wherein the enhancing layers are individually selected from the group consisting of silicon dioxide, silicon monoxide, niobium pentoxide, ytterbium fluoride, titanium dioxide, tantalum pentoxide, magnesium fluoride, yttrium fluoride, aluminum oxide, yttrium oxide, zirconium oxide, hafnium oxide or combinations thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic drawing illustrating the film structure according to the disclosure on an aluminum substrate.

(2) FIG. 2 is a process flow chart illustrating steps of the disclosure for preparing an enhanced protected silver coating on an aluminum substrate.

DETAILED DESCRIPTION

(3) The process for making a silver mirror using an aluminum substrate as described herein utilizes a commercial RF ion gun for surface treatment before deposition of any materials and during deposition of the of the barrier, binding, and certain enhancing layers to enable high deposition rates and increase the film packing density which also aids in improving the life time of the product.

(4) FIG. 2 is a process flow chart illustrating the method of the present disclosure. Prior to using the following process the substrates are meticulously cleaned, inspected, and placed into a nitrogen purged container until such a time that they are ready to be loaded into the vacuum chamber.

(5) The process as illustrated in FIG. 2 has the steps of:

(6) loading the product tooling (substrate) to be coated into the coating chamber [100];

(7) evacuating the coating chamber to a pressure of <310.sup.6 Torr [102];

(8) performing an RF pre-clean @300-500 W with approximately 10 sccm argon flow through the chamber for in the range of 7-15 minutes [104];

(9) depositing an in situ formed chromium nitride barrier layer to a thickness in the range of 50-150 nm [106] by deposition of chromium (Cr), also called chrome in some arts, in a flow of approximately 30 sccm N.sub.2 and 5 sccm Ar (approximately 6/1 V/V N.sub.2/Ar at a RF power of 400 W [108] that thereby converts the chromium to chromium nitride;

(10) turning off the RF power and depositing a silver layer to a thickness in the range of 75-150 nm [108];

(11) with the RF source off and no gas flowing, deposit a chromium (Cr) layer having a thickness in the range of 0.1-0.6 nm [110];

(12) turning on the RF power to a setting of 200-300 W and providing an oxygen flow of approximately 50 sccm, and maintaining these conditions to fully oxidize the chrome layer [112] into a chromium oxide layer, also called the formed chromium oxide layer;

(13) depositing one or a plurality of enhancing layers [114]; and

(14) venting and clearing the chamber, for example, using nitrogen or air, and removing the coated mirror from the chamber [116].

(15) An example of a plurality of enhancing layers is illustrated in FIG. 1 as three layers that have been deposited in the following order as (i) a first silicon dioxide layer 18 having a thickness in the range of 40-60 nm, (ii) a niobium pentoxide layer 20 having a thickness in the range of 15-30 nm, and (iii) a second silicon dioxide layer 22 having a thickness in the range of 40-60 nm. Other materials can be used alone or in combination as enhancing layer materials as is described below.

(16) In the process as described above in 106 chrome or chromium (Cr) is the metal being deposited. However, when the deposition is carried out using a mixture of pure nitrogen (99.999%) and argon (99.999%) by flowing through the RF on source (power level of 400 W) at a rate of 28/3.5 sccm, respectively, the chrome is altered to form a layer of chromium nitride. In similar fashion, in 110 chrome is the metal being deposited to a target thickness in the range of 4-6 nm, and after the metal has been deposited step 112 is carried out whereby the chrome is oxidized to chromium oxide. The time required to oxidize the chromium metal to chromium oxide is in the approximate range of 7-8 minutes. Further, in one embodiment the RF pre-clean step is carried to a time in the range of 8-12 minutes.

(17) In an example, a silver coated mirror was prepared using an aluminum substrate. The coating on the mirror, prepared as described herein, from the aluminum substrate to the last applied coating layer (which is an enhancement layer), had measured thicknesses of 100 nm CrN, 120 nm Ag, 0.45 nm (4.5 Angstroms) chrome oxide, 55.3 nm first layer silicon dioxide, 20.9 nm niobium pentoxide and 53.8 nm second layer silicon dioxide. The variation in each thickness was 5% except for the chrome oxide layer which can have a variation of 10%.

(18) The final phase of the process, as represented in FIG. 2 by 114, is the additional depositions of what are frequently referred to as is referred to as the enhancing layers. These layers can be any combination of metals, typically as oxides or fluorides, which are deposited on top of the oxidized chromium layer 112 to increase reflectivity in different wavelength regions. Materials utilized in this process can include silicon dioxide, silicon monoxide, niobium pentoxide, ytterbium fluoride, titanium dioxide, tantalum pentoxide, yttrium fluoride, hafnium oxide, magnesium fluoride, aluminum oxide, yttrium oxide, zirconium oxide, or combinations thereof, for example without limitation, silicon aluminum oxide, etc. While the enhancing can be carried out using only a single layer, it is typically carried out using a plurality of layers in the range of three to nine alternating layers. When a plurality of enhancing layers is used, different materials are used in alternating adjacent layers; for example without limitation. A-B-A and A-B-C-A, A-B-A-B, et cetera.

(19) TABLE-US-00001 TABLE 1 Galvanic Table From MIL-STD-889 Active (Anodic) Chromium (Plated) Monel 400 Magnesium Tantalum Stainless steel 201 (active) Mg alloy AZ-31B AM350 (active) Carpenter 20 (active) Mg alloy HK-31A Stainless steel 310 (active) Stainless steel 321 (active) Zinc (hot-dip, die cast, or Stainless steel 301 (active) Stainless steel 316 (active) plated) Stainless steel 304 (active) Stainless steel 309 (active) Beryllium (hot pressed) Stainless steel 430 (active) Stainless steel 17-7PH Al 7072 clad on 7075 Stainless steel 410 (active) (passive) Al 2014-T3 Stainless steel 17-7PH Silicone Bronze 655 Al 1160-H14 (active) Stainless steel 304 (passive) Al 7079-T6 Tungsten Stainless steel 301 (passive) Cadmium (plated) Niobium (columbium) 1% Zr Stainless steel 321 (passive) Uranium Brass, Yellow, 268 Stainless steel 201 (passive) Al 218 (die cast) Uranium 8% Mo. Stainless steel 286 (passive) Al 5052-0 Brass, Naval, 464 Stainless steel 316L (passive) Al 5052-H12 Yellow Brass AM355 (active) Al 5456-0, H353 Muntz Metal 280 Stainless steel 202 (passive) Al 5052-H32 Brass (plated) Carpenter 20 (passive) Al 1100-0 Nickel-silver (18% Ni) AM355 (passive) Al 3003-H25 Stainless steel 316L (active) A286 (passive) Al 6061-T6 Bronze 220 Titanium 5Al, 2.5 Sn Al A360 (die cast) Copper 110 Titanium 13V, 11Cr, 3Al Al 7075-T6 Red Brass (annealed) Al 6061-0 Stainless steel 347 (active) Titanium 6Al, 4V (solution Indium Molybdenum, Commercial treated and aged) Al 2014-0 pure Titanium 6Al, 4V (anneal) Al 2024-T4 Copper-nickel 715 Titanium 8 Mn Al 5052-H16 Admiralty brass Titanium 13V, 11Cr 3Al Tin (plated) Stainless steel 202 (active) (solution heat treated and Stainless steel 430 (active) Bronze, Phosphor 534 (B-1) aged) Lead Monel 400 Titanium 75A Steel 1010 Stainless steel 201 (active) AM350 (passive) Iron (cast) Carpenter 20 (active) Silver Stainless steel 410 (active) Stainless steel 321 (active) Gold Copper (plated, cast, or Stainless steel 316 (active) Graphite wrought)) End - Noble (Less Active, Nickel (plated) Cathodic) Chromium (Plated) Tantalum AM350 (active) Stainless steel 310 (active)

(20) 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.