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Durable mgo-mgf2 composite film for infrared anti-reflection coatings

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Durable mgo-mgf2 composite film for infrared anti-reflection coatings

This disclosure is directed to an optic having a composited MgO—MgF2 infrared anti-reflective coating that is suitable for use in LWIR, MWIR and SWIR ranges, and is particularly suitable for use in the LWIR range. The coated optic disclosed herein passes the severe abrasion test with a barring force between 2 pounds and 2.5 pounds. The MgO—MgF2 infrared anti-reflective coating has a thickness in the range of 500 nm to 1500 nm and a reflectance value Rx at 12° of less than 2% in the wavelength range of 7.25 nm to 11.75 nm.

Inventors: Horst Schreiber, Jue Wang, Scott J. Wilkinson
USPTO Applicaton #: #20120307353 - Class: 359359 (USPTO) - 12/06/12 - Class 359 

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The Patent Description & Claims data below is from USPTO Patent Application 20120307353, Durable mgo-mgf2 composite film for infrared anti-reflection coatings.

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This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/491,667 filed on May 31, 2011 the content of which is relied upon and incorporated herein by reference in its entirety.


The demands of aerospace and defense has been one of the main driving forces for fast development of infrared (IR) optics, in particular for infrared optics that will operate in a short-wave infrared range (SWIR) of 1-3 μm, a middle-wave infrared range (MWIR) 3-5 μm, and long-wave infrared range (LWIR) of 8-14 μm. The materials that can be used for the LWIR region are limited because the most frequently materials that are used are oxide materials and they are not transparent in the LWIR spectral regime. Germanium (Ge), zinc-selenide (ZnSe) and zinc-sulfide (ZnS) are the most popular optical window materials for use in the LWIR regime, these materials having a transmittance of 47%, 71% and 75%, respectively. An additional consideration of importance is that the IR optic made of the foregoing materials may be exposed to severe environmental condition for various applications. Consequently, environmentally durable antireflection (AR) coatings are necessary for LWIR optics applications.

The optical performance of an AR coating is dominated by the refractive index of outermost layer. A low refractive index of the outermost layer enables one to achieve a broadband AR coating. However, coating durability and environmental stability are mainly affected by the outermost layer in optical coatings. As a result, the material property of the outermost layer plays a critical role not only in optical performance, but also in mechanical strength and environmental stability. An IR-AR coating from 7.7μ to 10.3μ has been established where ytterbium fluoride (YbF3) is used as the outermost layer. There are IR-AR coatings in current use that pass the both the optical specification and a moderate abrasion test with a minimum bearing force of 1 pound. However, recently it has been indicated that a severe abrasion test will be required for future LWIR AR coated products. The existing AR coating, however, could not pass the severe abrasion test with a barring force between 2 and 2.5 lbs (MIL-C-48497A). Consequently, there is a need for a low refractive index coating material with durable mechanical property to ensure both a broadband antireflection spectral performance and to withstand the severe abrasion test. At the present time there are no optical AR coatings that give satisfactory performance, transmission, including abrasion resistance, in all of (1) a short-wave infrared range (SWIR) of 1-3 μm, (2) a middle-wave infrared range (MWIR) 3-5 μm, and (3) a long-wave infrared range (LWIR) of 8-14 μm.


The present disclosure is directed to the formation an optic having a smooth, dense uniform composited MgO—MgF2 coating and a method of forming such composited coating from a MgF2 source material by vaporization of the MgF2 material and fluorine depletion on an oxygen-containing plasma atmosphere that further densifies and smoothes the composited MgO—MgF2 coating. The disclosure is further directed to a low refractive index coating material with durable mechanical properties that provides a broadband antireflection spectral performance in the range of 1-14 μm and can withstand a severe abrasion test. For a selected IR range, for example, SWIR, MWIR or LWIR, the thickness of the coating is dependent on the range in which it will be used. Thus the thickest coatings will be used in the LWIR range, the thinnest coatings will be used in the SWIR range, and coatings of intermediate thickness will be used in the MWIR range. The composited MgO—MgF2 coating described herein can be used in all three ranges. The broad range for the deposited coating is 100 nm to 1500 nm. In an embodiment for the LWIR range the coating thickness is in the range of 600 nm to 900 nm. In an embodiment for the MWIR range the thickness is in the range of 250 nm to 450 nm. In an embodiment for the SWIR range the thickness is in the range of 150 nm to 300 nm.

While MgF2, one of the hardest metal fluoride materials, seems to be a good candidate as low refractive index capping layer because of its transparency up to the LWIR, it has one detriment. It is known that a relative thicker layer is required in the LWIR spectral regime as compared to the VIS and UV spectral regimes. In fact, an MgF2 AR coating must be up to 40 times thicker in the LWIR range than an MgF2 coating in the UV range. However, MgF2 film porosity and surface roughness rise significantly as layer thickness increases, and this in turn reduces the corresponding film durability for LWIR application. The present disclosure solves this problem through the use of modified plasma ion-assisted deposition with in-situ plasma smoothing to provide a technical solution by the production of a chemically and mechanically strengthened the MgO—MgF2 composited coating.

It is shown herein that the goal of durable MgO—MgF22 composited film for infrared AR coatings can be achieved using the following steps: 1. Formation of a MgO—MgF2 composited film by plasma ion-depleting fluorine and replacing the fluorine with oxygen. 2. Densification of MgO—MgF2 composited film using modified PIAD (plasma ion-assisted deposition) with a reversed mask technique. 3. Optimization of optical and mechanical properties of MgO—MgF2 composited film by adjusting the ratio of in-situ plasma smooth and plasma assisted deposition

The result is a smooth and densified MgO—MgF2 composited coating layer that provides for the increased durability of a broadband IR-AR composited MgO—MgF2 coating that can be used in each of the SWIR. MWIR and LWIR range described herein.


FIG. 1 is a schematic drawing of modified PIAD deposition system in an oxygen rich plasma environment using a reversed mask and a side shield that enable the deposition of a thick, densified and smooth MgO—MgF2 composited coating.

FIG. 2 is a graph of the refractive index versus wavelength in the infrared spectral range of the modified MgO—MgF2 composited coating that was obtained using the modified PIAD method.

FIGS. 3a and 3b are AFM images of 200 nm MgO—MgF2 composited film deposited by the modified PIAD process (3a) and a standard 200 nm MgF2 film (3b) that was deposited without using the modified PIAD technique. The films corresponding surface roughness is 0.4 nm of FIG. 3a and 2.4 nm for FIG. 3b.

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