Patent Description:
Sensing systems that operate in the electromagnetic spectrum are vulnerable to EMI or radio-frequency interference from external sources. EMI can negatively affect performance of the sensor, causing disruption, or can jam the sensor, causing loss of function or failure. As such, many sensitive electronic sensing systems require EMI protection. Current solutions for providing EMI protection include applying sections of metallic or carbon nanotube grids to an optical element. The conductive grid reflects undesired electromagnetic energy while openings in the grid maintain transparency of the optical element. While effective for generally flat windows, the grid structures are not easily applied to curved geometries, such as hemispherical or ogive-shaped domes. Because application of metallic or carbon nanotube grids to curved surfaces is labor intensive and thereby expensive, application is generally limited to covering only portions of a dome-shaped optical element.

New EMI protection systems are needed to provide EMI protection over a full area of dome-shaped optical elements, and more specifically, new EMI protection systems are needed for SWIR-MWIR transparent dome-shaped optical elements.

<CIT> discloses optically transparent electrically conductive semiconductor windows and methods of manufacture.

In one aspect, a shortwave to midwave infrared (SWIR-MWIR) optical window includes a substrate formed from a nanocomposite optical ceramic material and a coating disposed on the substrate to provide electromagnetic interference (EMI) protection. The coating is electrically conductive and SWIR-MWIR transparent and comprises a doped zinc oxide material. The nanocomposite optical ceramic material is a multi-phase composite material formed of a mixture of two or more ceramic phases that are mutually insoluble, thereby forming a multi-phase grain structure having distinct phase separation between two or more constituents.

In another aspect, a method of protecting an EO/IR sensor from electromagnetic interference (EMI) includes depositing a thin film electrically conductive and SWIR-MWIR transparent coating over a surface an optical window of the EO/IR sensor. The optical window is formed from a nanocomposite optical ceramic material and has a curved surface. The thin film electrically conductive and SWIR-MWIR transparent coating comprises an electrically conductive zinc oxide material. The nanocomposite optical ceramic material is a multi-phase composite material formed of a mixture of two or more ceramic phases that are mutually insoluble, thereby forming a multi-phase grain structure having distinct phase separation between two or more constituents.

The present summary is provided only by way of example, and not limitation. Other aspects of the present disclosure will be appreciated in view of the entirety of the present disclosure, including the entire text, claims and accompanying figures.

While the above-identified figures set forth embodiments of the present invention, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the invention by way of representation and not limitation. The figures may not be drawn to scale, and applications and embodiments of the present invention may include features, steps and/or components not specifically shown in the drawings.

Many optical applications, including guidance systems, receive and send energy in the infrared (IR) region of the electromagnetic spectrum. The optical elements used in these systems, such as domes and windows, must be transmissive in the IR spectrum and capable of protecting the electro-optical or infrared (EO/IR) sensors and other components, which they shield, from harsh environmental conditions. Additionally, there is demand for optical elements to protect EO/IR sensors from electromagnetic interference (EMI). Conventional EMI grids used for planar windows are not easily applied to domes and when applied, are generally limited in area, covering only a portion of the optical element. A conductive and shortwave to midwave IR (SWIR-MWIR) transparent thin film coating, as disclosed herein, can provide full EMI protection for SWIR-MWIR windows and domes. The conductive SWIR-MWIR transparent coating can be applied with uniformity to complex and curved geometries.

<FIG> is a cross-sectional view of an electro-optical or infrared (EO/IR) sensor optical element having a hemispherical dome shape and conformal thin film conductive and SWIR-MWIR transparent coating. <FIG> further includes an expanded close-up of this infrared (EO/IR) sensor optical element. <FIG> is a cross-sectional view of an EO/IR sensor optical element having an ogive dome shape and conformal thin film conductive and SWIR-MWIR transparent coating. <FIG> are discussed together herein. The optical elements disclosed herein can be used to protect EO/IR sensors in a variety of applications, including but not limited to, commercial and military applications, such as airborne optical imaging systems for target acquisition, identification, and guidance. Application of the disclosed conformal thin film conductive and SWIR-MWIR transparent coating is not limited to the optical element shapes illustrated in <FIG>. As will be understood by one of ordinary skill in the art, the disclosed conductive and SWIR-MWIR transparent coating can be applied to a variety of optical element shapes, including but not limited to aerodynamic domes, planar windows, and various curved (e.g., convex and concave) geometries.

<FIG> shows optical element <NUM> having substrate <NUM>, conductive and SWIR-MWIR transparent coating <NUM>, optional intermediate layer <NUM>, and optional anti-reflective coating <NUM>. <FIG> shows optical element <NUM> having substrate <NUM> and conductive and SWIR-MWIR transparent coating <NUM>. Intermediate layer <NUM> and anti-reflective coating <NUM> are omitted in the illustration of optical element <NUM> but can be optionally included as described with respect to dome <NUM> of <FIG>. Substrate <NUM> has inner side <NUM> and outer side <NUM>. Substrate <NUM> has inner side <NUM> and outer side <NUM>. Conductive and SWIR-MWIR transparent coating <NUM> can be disposed over outer side <NUM> of optical element <NUM>. Conductive and SWIR-MWIR transparent coating <NUM> can be disposed over outer side <NUM> of optical element <NUM>. Optional intermediate layer <NUM> can be disposed between substrate <NUM> and conductive and SWIR-MWIR transparent coating <NUM>. Optional anti-reflective coating <NUM> can be disposed on inner side <NUM> of substrate <NUM>. Optical elements <NUM> and <NUM> shield EO/IR sensor <NUM>.

Substrate <NUM>, <NUM> can be a SWIR or MWIR nanocomposite optical ceramic (NCOC). NCOC materials have been shown to offer enhanced mechanical strength and thermal shock resistance over conventional optical element materials including sapphire, spinel, polycrystalline alumina, aluminum oxynitride, and have been developed for use in military optical imaging systems. NCOC is a multi-phase composite material formed of a mixture of two or more ceramic phases that are mutually insoluble, thereby forming a multi-phase grain structure having distinct phase separation between two constituents. The multi-phase grain structure remains distinct after processing such that separation between the phases can be observed.

NCOC optical elements, including domes and windows, have been successfully manufactured using near-net shape powder processing techniques. Nano-sized ceramic powders are packed into a mold and pressed to produce a green body having a general shape of the optical element but with increased thickness. The green bodies are then sintered to remove any organics added during powder processing and to achieve a high density (><NUM>%). Finally, hot isostatic pressing (applying pressure and heat) to the sintered body forms a fully densified blank having a near-net shape of the optical element. Final shape finishing, including precision grinding and polishing, can be provided to achieve a final shape of the optical element.

In one embodiment, substrate <NUM>, <NUM> is a NCOC formed from first and second oxide nanograin materials selected from yttria (Y<NUM>O<NUM>), magnesia (MgO), aluminum oxide (Al<NUM>O<NUM>), magnesium aluminum oxide (MgAl<NUM>O<NUM>), zirconia (ZrOz), calcium oxide (CaO), beryllium oxide (BeO), silica (SiOz), and germanium oxide (GeOz). For example, the first oxide nanograin material can be yttria and the second oxide nanograin material can be magnesia. The NCOC material can be composed of two or more of any of the disclosed ceramic phases provided the ceramic phases are mutually insoluble. Dopants can be added to one or both phases to change the refractive index. The NCOC material composition can be selected to provide desired optical and mechanical properties for a particular application. For example, two or more nanograin materials can be selected based on optical transmittance in a particular portion of the IR spectrum. Specifically, NCOC material composition is selected to provide transmittance in the SWIR-MWIR spectrum with a targeted wavelength range of about <NUM> to about <NUM>. In one embodiment, an MWIR NCOC optical element can be formed from a mixture of Y<NUM>O<NUM>:MgO (e.g., approximately <NUM>:<NUM> mixture by volume). In another embodiment, an SWIR NCOC optical element can be formed from a mixture of Y<NUM>O<NUM>:MgO with a nickel oxide dopant in the MgO phase.

Conductive and SWIR-MWIR transparent coating <NUM>, <NUM> is an SWIR-MWIR transparent and electrically conductive coating that can be applied as a thin film conformal coating over outer side <NUM> of substrate <NUM> and outer side <NUM> of substrate <NUM>. Conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can be applied directly to an outer surface of substrate <NUM>, <NUM> or can be applied to an outer surface of one or more intermediate layers, such as intermediate layer <NUM>. Conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can be applied to fully cover outer side <NUM>, <NUM> with a substantially uniform thickness, thereby providing substantially uniform EMI protection over all portions of optical element <NUM>, <NUM>.

In alternative embodiments, conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can be applied as a thin film conformal coating over inner side <NUM>, <NUM> of substrate <NUM>, <NUM>. As described above, conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can be applied directly to an outer surface of substrate <NUM>, <NUM> on inner side <NUM>, <NUM> or can be applied to an outer surface of one or more intermediate layers (e.g., intermediate layer <NUM>) applied to inner side <NUM>, <NUM>. Conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can be applied to fully cover inner side <NUM>, <NUM> with a substantially uniform thickness, thereby providing substantially uniform EMI protection over all portions of optical element <NUM>, <NUM>. Application of conductive and SWIR-MWIR transparent coating <NUM>, <NUM> on inner side <NUM>, <NUM> may be desirable to protect conductive and SWIR-MWIR transparent coating <NUM>, <NUM> from environmental damage.

Conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can be a conductive zinc oxide material comprising zinc oxide doped with a conductive material. Ideally, conductive and SWIR-MWIR transparent coating <NUM>, <NUM> exhibits the same or substantially similar optical properties of substrate <NUM>, <NUM> such that there is little to no loss in the transmission of optical information. Zinc oxide is a SWIR-MWIR transparent material. Zinc oxide can be applied directly to a surface of an oxide-based ceramic substrate <NUM>, <NUM> without the addition of an intermediate adhesion or bond coat.

The conductive dopant can be uniformly dispersed within the zinc oxide material to provide a conductive shield sufficient to substantially block radio frequency electromagnetic radiation from reaching an EO/IR sensor. An amount of the conductive dopant in conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can be selected to provide a required degree of EMI protection while maintaining necessary SWIR-MWIR transmittance through conductive and SWIR-MWIR transparent coating <NUM>, <NUM>. For example, in some embodiments, conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can include up to about <NUM> wt. % dopant, or up to about <NUM> wt. % dopant, or between about <NUM> wt. % and <NUM> wt.

The conductive dopant can comprise an element having an oxidation state of +<NUM> to contribute an electron to the conduction band of zinc oxide, making SWIR-MWIR transparent coating <NUM>, <NUM> electrically conductive. For example, in one embodiment, the conductive dopant can be aluminum oxide. In other embodiments, the conductive dopant can be an oxide of gallium or indium. In yet other embodiments, the conductive dopant can be a rare earth oxide, such as yttria or gadolinia. In yet other embodiments, other rare earth elements having a +<NUM> oxidation state can be used to dope zinc oxide. Generally, any element having a +<NUM> oxidation state can be used to dope zinc oxide provided the element is not too large to replace zinc in the zinc oxide lattice. Although oxides can be preferrable for ease of manufacturing, dopants can alternatively include elemental materials, such as elemental aluminum, or other elements having three valence electrons.

Conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can be deposited by RF magnetron sputtering. RF magnetron sputtering is a conventional technique used to deposit thin film oxides. A sputtering device includes a chamber with one or more targets, an RF power supply, and substrate <NUM>, <NUM>. In one embodiment, the target can include a desired conductive and SWIR-MWIR transparent coating composition, for example, an aluminum oxide-doped zinc oxide, having a defined amount of dopant. For example, the target can include a source of zinc oxide doped with <NUM> wt. % aluminum oxide to form conductive and SWIR-MWIR transparent coating <NUM>, <NUM> having up to <NUM> wt. % aluminum oxide. During operation, the species from the target can be sputtered onto a rotating substrate <NUM>, <NUM> using one or more RF power sources. Certain atoms sputter more efficiently than others. As such, it may be desirable to use a target having a different weight percent of dopant than the desired weight percent dopant in the resulting conductive and SWIR-MWIR transparent coating <NUM>, <NUM>. For example, a target having <NUM> wt. % dopant may produce a coating having less than <NUM> wt.

In other embodiments, a first target can include a source of zinc oxide and a second target can include a source of dopant, e.g., elemental aluminum. During operation, the species from each of the first and second targets can be co-sputtered onto the rotating substrate <NUM>, <NUM> using one or more RF power sources. The compositional ratio of the individual target material species into the resulting conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can be controlled through control of the RF power sources. Conductive and SWIR-MWIR transparent coating material composition, crystalline structure, grain size, and coating thickness can be optimized by controlling target composition and operational parameters, including deposition temperature, pressure, power, and deposition time.

Other methods of depositing conductive and SWIR-MWIR transparent coating <NUM>, <NUM>, including physical and chemical deposition techniques known in the art are within the scope of the present disclosure. For example, in one or more embodiments, zinc oxide can be deposited by RF magnetron sputtering or other methods known in the art and can be doped by ion implantation to form conductive and SWIR-MWIR transparent coating <NUM>, <NUM>. For example, aluminum, gallium, or indium ions may be implanted in the zinc oxide coating. In some embodiments, doping ions can be present in a partial thickness of conductive and SWIR-MWIR transparent coating <NUM>, <NUM>.

Conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can be deposited in a single layer or multiple layers depending on a desired thickness and deposition technique used. In accordance with one or more embodiments, conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can have a thickness of up to about <NUM> micron, or between about <NUM> micron and about <NUM> micron. Generally, increased thickness of conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can provide increased electrical conductivity resultant of an increased amount of dopant in conductive and SWIR-MWIR transparent coating <NUM>, <NUM>, but can also result in reduced optical transmission.

Conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can have an electrical conductivity of about <NUM> to <NUM> ohm/square or resistivity of about <NUM>-<NUM> ohm-cm. Conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can provide optical transmittance over the SWIR spectrum and a portion of the MWIR spectrum for application with targeted wavelengths in a range of about <NUM> to about <NUM>). A desired electrical conductivity can be provided by increasing or decreasing the amount of doping ions in conductive and SWIR-MWIR transparent coating <NUM>, <NUM>. An optimal optical transmittance of conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can be achieved by carefully controlling deposition parameters and annealing temperature.

Intermediate layer <NUM> can optionally be provided to an outer surface of substrate <NUM>, <NUM> on outer side <NUM>, <NUM>. Intermediate layer <NUM> can be disposed between substrate <NUM>, <NUM> and conductive and SWIR-MWIR transparent coating <NUM>, <NUM> to provide a transition layer between materials having different refractive indices and/or can be provided to improve adhesion between substrate <NUM>, <NUM> and conductive and SWIR-MWIR transparent coating <NUM>, <NUM>. Intermediate layer <NUM> can provide a shared element between adjoining conductive and SWIR-MWIR transparent coating <NUM>, <NUM> and substrate <NUM>, <NUM> to promote adhesion between the adjoining materials. For example, an intermediate layer <NUM> comprising yttria can share the common element oxygen with an NCOC substrate <NUM>, <NUM> and zinc oxide of conductive and SWIR-MWIR transparent coating <NUM>, <NUM> to promote adhesion between the NCOC and zinc oxide. In some embodiments, conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can have a refractive index substantially similar to a refractive index of substrate <NUM>, <NUM>. In such embodiments, an intermediate layer <NUM> may be unnecessary. In some embodiments, conductive and SWIR-MWIR transparent coating <NUM>, <NUM> can have a different refractive index than substrate <NUM>, <NUM> and optical impedance matching through application of intermediate layer <NUM> may be necessary to provide a smooth transition between the two materials. Intermediate layer <NUM> can be an oxide material having a refractive index that is between the refractive indices of conductive and SWIR-MWIR transparent coating <NUM>, <NUM> and substrate <NUM>, <NUM>, and thereby can minimize optical losses by reflection at the material interfaces. A thickness of intermediate layer <NUM> can vary depending on the difference in refractive indices of conductive and SWIR-MWIR transparent coating <NUM>, <NUM> and substrate <NUM>, <NUM>. In some embodiments, one or more intermediate layers <NUM> can be provided. The chemical composition of intermediate layer <NUM> can be selected using conventional modeling software as known in the art.

Anti-reflective coating <NUM> can optionally be provided on inner side <NUM>, <NUM> of substrate <NUM>, <NUM> directly on an inner surface of substrate <NUM>, <NUM> or outward of conductive and SWIR-MWIR transparent coating <NUM>, <NUM> if coating <NUM>, <NUM> is applied on inner side <NUM>, <NUM>. Anti-reflective coating <NUM> can be used to increase optical transmittance and can further aid in protecting an EO/IR sensor. Anti-reflective coating <NUM> can be an oxide coating as known in the art and typically used in optical devices. Anti-reflective coating <NUM> can be deposited by RF magnetron sputtering or other deposition techniques known in the art.

Claim 1:
A shortwave to midwave infrared SWIR-MWIR optical window comprising:
a substrate (<NUM>, <NUM>) formed from a nanocomposite optical ceramic material; and
a coating (<NUM>, <NUM>) disposed on the substrate (<NUM>, <NUM>) to provide electromagnetic interference EMI protection, wherein the coating (<NUM>, <NUM>) is electrically conductive and SWIR-MWIR transparent and wherein the coating (<NUM>, <NUM>) comprises a doped zinc oxide material,
characterized in that the nanocomposite optical ceramic material is a multi-phase composite material formed of a mixture of two or more ceramic phases that are mutually insoluble, thereby forming a multi-phase grain structure having distinct phase separation between two or more constituents.