A Graphene Based EMI Shielding Optical Coating

Field of the Invention

This invention relates to making windows having an infrared transparent substrate. More particularly it relates to infrared transmitting windows with an electrically conductive, infrared transparent continuous thin film coating for electromagnetic interference shielding in electro-optical systems. The invention further relates to making such infrared transparent windows which are not only protected from electromagnetic interference shielding but also from hostile environmental conditions.

Background of the Invention

Electro-optical systems used for surveillance are made up of a number of different parts contained within a housing. Apart from the optical assembly, typically an electro-optical system is composed of parts such as sensors, electronic systems which control the sensors, analyze and interpret signals received by sensors. Such an electro-optical system receives signals from the target through its external window.

While transmitting the signals to the sensors this external window also served to protect these sensors and other electronic component from harsh environmental conditions.

Not only must the window possess sufficient physical strength to withstand rapidly changing pressure and temperature differentials, the external surface of the window must withstand abrasion by rain, air and dust. Furthermore, unwanted electro-magnetic radiation may also enter through this window beside the signals from the target. If this is a radar or radio frequency then it may interfere with the electronic components of the system in a negative way. For this reason, in military applications, the window must also provide a shield against longer wavelength electromagnetic radiation such as radar and radio.

Depending on the surveillance wavelength a number of different choices become available as substrates materials for window and dome applications. Compositions of zinc selenide, zinc sulfide for example are commonly used as window materials for applications requiring transparency from near to long wave infrared (800- 12000 nm) range of the spectrum. Other materials such as aluminum oxynitride, spinel, sapphire, yttria make good window materials for wavelength within the near- to mid- infrared range. Yet for application of mid-infrared range (3000-5000 nm) silicone makes a good substrate and germanium is the choice for mid (3000- 5000 nm) - and long (8000-12000 nm) applications

However, all these window substrates, even though have high transparency in the infrared, are also transparent to most radar and radio wavelengths. Thus, unless special shielding is provided, they do not shield the interior aircraft electronics from longer wavelength penetration.

The term "shielding" as applied to electromagnetic radiation can be defined as the reduction of the amount of EM radiation originating from a specific point in space. In order to shield EM radiation, the shielding medium needs to be conductive promoting the utilization of metals for EM interference (EMI) shielding purposes. Electromagnetic interference (EMI) shielding of windows and domes in electro— -optical (EO) systems can be accomplished by several different ways. Among the common approaches is applying a transparent electrically conductive coating. For systems operating in the visible spectrum of light, glass and/or polymer windows coated with ΠΌ are widely used. ITO coatings have high transparency from 0,40um up to l,06um. As a result of the band restrictions of ΓΓΌ coatings this method cannot be used for electro- optical systems operating in the Near/Mid and Long wave infrared regions of the spectrum. For these regions, there are mainly two alternative EMI shielding methods to apply. One requires the utilization of patterned opaque electrically conductive structures embedded on/into the outer most lens of the optical path.

In US patent 20120037803 (1) a conductive mesh having appropriately chosen dimensions and spacing is embedded in a transparent medium is provided for electro-magnetic shielding. To minimize the impact of the mesh on the effective aperture of the medium the strands of the mesh had to be made relatively narrow and to provide sufficient shielding despite the narrow strand width the mesh had to be embedded relatively deeply in the medium.

Although widely used, this method has issues in terms of ease of application and cost. The application process includes steps like mesh structure design, lithography, ion etching and vapor evaporation. All these steps are time consuming and result in increased costs.

Infrared EO systems often use windows or domes made of semiconductive material. These semiconductive materials also provide some EMI shielding for the system. The shielding efficiency of these systems can further be increased by increasing the conductivity of these semiconductor materials by increasing their doping content. In a study (2) infrared transparent conductive diffused layers have been integrated into germanium windows using an ion— implantation/diffusion technique. These layers are nominally 25 microns thick with sheet resistance of 5-10 ohms/square. However the increase in the number of dopant atoms within the semiconducting crystal results in a decrease in the transparency up to %5 due to scattering and introduction of possible color centers. It is possible to optimize the doping density so that a good conductivity allowing for the required EMI shielding and high transmission of light is obtained. The main restriction of this method is that it can only be applied to semiconducting crystal lenses.

A need thus remains for new and improved a electro-magnetic shielding techniques that improve electro-magnetic interference filtering efficiency in the desired wavelength ranges.

The use of graphene as an EMI shielding material is well documented (3, 4). However in none of these reports graphene was utilized in optical applications.

In patent WO2013096036 A9 (5) a transparent conductive article includes a transparent substrate, a thin electrically conductive grid and a carbon nanolayer. The carbon nanolayer has a morphology that includes graphite platelets embedded in nano-crystalline carbon. The said article have a visible light transmission of at least 80% and a sheet resistance less than 500 or 100 ohm/square. Further the transparent substrate may comprise a flexible polymer film.

However for electro-optical applications transparency at least one of the near- infrared, mid-wave infrared and long-wave infrared radiations is required. Moreover for proper EMI shielding sheet resistance less than 10 ohm/square between 100 MHz to 20 GHz range is needed. References

[1] US Patent # 20120037803, William B. Strickland, "Electromagnetic

Interference Shielding"

[2] Michael E. Borden, "Infrared Transparent Conductive Diffused Layers In Germanium Windows", SPIE Vol. 1326 Window and Dome Technologies and Materials II (1990), p. 99.

[3] A.P. Singh, M. Mishra, A. Chandra and S.K. Dhawan, " Graphene

oxide/ferrofluid/cement composites for electromagnetic interference shielding application", Nanotechnology, Vol. 22 (2011).

[4] Seul Ki Hong; Ki Yeong Kim; Taek Yong Kim; Jong Hoon Kim; Seong Wook Park; Joung Ho Kim; Byung Jin Cho, "Electromagnetic Interference Shieldinig Effectiveness of Manolayer Graphene", Nanotechnology, V.23 (2012)

[5] WO2013096036 A9, Divigalpitiya, Ranjityh; Pellerite, Mark J.; Baetzold, John P.; Korba, Gary A.; Mazurek, Mieczyslaw H., "Electrically Conductive Article With High Optical Transmission".

Summary of the Invention

The general object of present invention to provide an electro-optical window having and electrically conductive continuous thin film coating substantially transparent to at least one of the near-infrared, mid-wave infrared and long-wave infrared radiation. Here near-, mid-wave and long-wave infrared radiations being the 800-2000 nm, 3000-5000 nm and 8000-12000 nm wavelength ranges, respectively.

A yet further object of the present invention is to provide an electro-optical window with high resistance to environmental damage.

Such a multi-functional window can be achieved through layered structure as depicted in Figure 1. According to this figure the electro-optical window of the present invention comprises of a substrate material [11] transparent to at least one of the near-infrared, mid-wave infrared and long-wave infrared radiation. The substrates of the said electro-optical windows of the invention can be selected from a range of materials including zinc selenide, zinc sulfide, magnesium fluoride, calcium fluoride, sapphire, aluminum oxynitride, spinel, yttria, germanium, silicon depending on the desired wavelengths. Further the substrates may be manufactured in the form of a window, dome or a lens. This window is forming a conducting first layer [12] on top of which lies a second multi-layered coating protecting the whole structure from environmental damage. This coating also serves as an antireflection (AR) coating providing a high transparency through the electro-optical window. Multi-layered AR coatings are composed of high and low refractive index alternating layers. Low refractive index layers of the AR coating can be selected from a range of materials including Si0 ₂ , YbF ₃ , MgF ₂ , Y ₂ 0 ₃ , Sc ₂ 0 ₃ , Lu ₂ 0 ₃ , A1 ₂ 0 ₃ , ZnSe, Zns and high refractive index layers of the AR coating can be selected from Ta ₂ 0 ₅ , Ti0 ₂ , Zr0 ₂ , Si, Ge, Nb0 ₂ , Hf0 ₂ , Zr0 ₂ , ZnO, ZnS, ZnSe, Ge and Si.

Brief Description of Drawings

A system and method realized to fulfill the objective of the present invention is illustrated in the accompanying figures, in which:

Figure 1 shows An EMI shielding AR coated lens/window/dome

Figure 2 shows transmission spectra for NIR-MWIR dual band EMI shielding lens/window/dome

Figure 3 shows transmission spectra for NIR-MWIR Tri-Band EMI shielding lens/window/dome

Figure 4 shows transmission spectra for MWIR-LWIR Dual Band EMI shielding lens/window/dome Detailed Description of the Invention

An EMI shielding external window for an electro-optic system located at the outer most position in an optical path. Even though there are various ways, as described in the Background, to construct such an optical element such as the used of ITO or a metal mesh structure; in this description, a graphene based approach is assumed. In this invention, an electro-optical window, Fig I, comprises of a substrate material [11] transparent to at least one of the near- infrared, mid-wave infrared and long-wave infrared radiation forms a base for an electrically conductive continuous thin film coating [12] made up of graphene is protected against environmental conditions through a multi layered anti-reflection coating [13]. Both graphene based electrically conductive continuous thin film and anti- reflection coating are also at least 90% transparent to at least one of the near- infrared, mid-wave infrared and long-wave infrared radiation. The structure is designed such that transparency is increased while maintaining a good EMI shielding and environmental protection.

External windows for electro-optic systems vary in material, size and shape according to the system and the particular application. Windows as large as ten inches diameter are sometimes required. Electro-optic windows can be fabricated in different ways depending on the material of choice. Common polycrystalline windows such as zinc selenide and zinc sulfide are fabricated by means of chemical vapor deposition. Other polycrystalline materials such as spinel, aluminum oxynitride and yttria are manufactured by means of ceramic processing techniques such as hot pressing, hot isostatic pressing or sintering. Single crystal optical windows such as sapphire, germanium, silicone, calcium fluoride and magnesium fluoride on the other hand, are grown from the melt. Processes for forming such windows are well-known to a person skilled in the art and form no part of this invention The substrate [11] material utilized in this invention shall be transparent to at least one of the near-infrared, mid-wave infrared and long-wave infrared spectrum of light. Depending on the desired surveillance wavelength the substrate of the window of the invention is made of a material selected from zinc selenide, zinc sulfide, magnesium fluoride, calcium fluoride, sapphire, aluminum oxynitride, spinel, yttria, germanium, silicon.

The electrically conductive, infrared transparent continuous thin film coating [12], made up of graphene, is capable of dampening the amplitude of the interfering electro-magnetic radiation may be fabricated by several different approaches. In one approach the electrically conductive, infrared transparent continuous thin film coating of the present invention is comprised of a single or multi-layer graphene film manufactured by means of a technique selected from chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD) and molecular beam epitaxy (MBE) techniques. Using at least one of the above mentioned techniques graphene films are grown on carrier substrates and then transferred onto the window substrate of choice. The transferred graphene can either be single or multi-layer depending on the required conductivity and level of EMI shielding.

In a second approach, electrically conductive, infrared transparent continuous thin film coating is formed dispersing graphene in a metal-oxide solution prepared by means of sol-gel processes. In the latter approach, a graphene based composite can be prepared by different chemical routes for different metal-oxide matrix materials used. This graphene/metal-oxide sol mixture forms a composite layer and applied on the substrate window by means of spin coating, spray coating dipping techniques.

The thickness and the conductance of the graphene containing conducting layer to act as an EMI shield is determined from the strength and frequency spectrum of the electromagnetic field and the tolerance of the device or components which is to be shielded. An AR layer [13] to enhance the light gathering capacity of the window shall be applied on the conducting layer [12]. The said layer shall also be responsible for protecting the underlying layers from the effects of environment due to the fact that it is the outer most layers on window. The AR coating comprises alternating low and high index layers for the desired protection and transparency. The layers of the low refractive index can be selected from a range of materials including Si0 ₂ , YbF ₃ , MgF ₂ , Y ₂ 0 ₃ , Sc ₂ 0 ₃ , Lu ₂ 0 ₃ , A1 ₂ 0 ₃ , ZnSe, Zns. Similarly high refractive index layers of the AR coating can be selected from Ta ₂ C>5, Ti0 ₂ , Zr0 ₂ , Si, Ge, Nb0 ₂ , Hf0 ₂ , Zr0 ₂ , ZnO, ZnS, ZnSe, Ge and Si. These layers may be applied by means of vacuum deposition techniques such as chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), plasma echaced physical deposition (PEPVD), sputtering, atomic layer deposition (ALD).

The window comprising the said substrate [11], the said conducting layer [12] and the said AR layer [13] shall be capable of transmitting at least 90% of the incoming IR light onto the system. The optical EMI shielding system shall be constructed to work with either single, dual or tri-band applications. Typical transmission curves for dual and tri-band applications are shown in Figures 2 through 4. Figure 2 is a transmission curve for window that can be used in the NIR (800-2000 nm ) and MWIR (3000 - 5000 nm) dual band applications, Figure 3 is a typical example for tri-band applications and Figure 4 shows the curve for a dual band application in the MWIR and LWIR (8000-12000 nm) range.

Optical EMI shielding system shall, also, be able to cope with the environmental conditions forced upon the system by the platform on which the electro-optical system to be used. In order to satisfy this requirement, optical EMI shielding system shall be subjected to MIL-C-675 and/or MIL-M- 13508 (whichever is applicable) for adherence, humidity, abrasion and salt spray tests.