N242-074 TITLE: Infrared Window/Dome Refurbishment and Repair

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Advanced Materials; Hypersonics; Sustainment

OBJECTIVE: Design and develop methods to refurbish and/or repair infrared (IR) sensor or missile seeker system windows and domes that have been damaged through their operational environments to their pristine optical and physical/mechanical condition.

DESCRIPTION: Over the course of the last 50 years, the Military Services have increasingly relied on sensors, trackers, and seeker systems operating in the IR spectrum. Windows and domes for such systems, exposed to rain, sand, salt spray, contaminants, and other degraders in their intended operational environments, typically erode with the resulting surface damage degrading optical quality and limiting their serviceable lifetime. Consequences include degraded sensor system performance and significant yearly investment for replacement.

Environmental damage to IR windows and domes may include optical coating full or partial delamination, pitting and/or gouging, both shallow and deep scratching, wide-area abrasion, and smudging from contaminants typical of operational environments. Coating remnants may be uneven, as dielectric coatings are sometimes applied over a sparse metallic mesh on the window/dome surface. Additionally, coating remnants on damaged windows and domes may contain trace amounts of hazardous materials (e.g., heavy metals such as cadmium and chalcogenides). To date, no approach has satisfactorily demonstrated removal or repair of damaged surface layers in single or poly-crystalline (e.g., sapphire, spinel, Silicon (Si) or Germanium (Ge)) optical windows or domes, to include maintenance of the original optical quality (i.e., transmission, absorption, and wavefront error) of the pre-damaged material. Past limited attempts to fill pits or provide spot repairs have resulted in optical quality degradation and limitations due to mismatches in indices of refraction, stress, or thermal expansion.

The integrated circuit and solar cell industries, however, routinely cut and polish single and poly-crystalline window materials such as Si, Ge, and gallium arsenide (GaAs) from boules via slicing and chemical-mechanical processing (CMP) to a level of surface quality, with the absence of defects and underlying strain/stress, that far exceeds current requirements for IR windows and domes. Surface finish, as measured via the bi-directional reflectance distribution function (BRDF), for instance, routinely approaches 1 x 10E-7 sr-1 without any further processing or treatment. It is postulated that damaged optical windows and domes made of these or other single-boule grown crystalline materials could be restored in a multi-step process that includes removal of the damaged surface layers, CMP or other processing to restore a pristine surface with undamaged underlayer, and epitaxial, chemical vapor deposition (CVD), or other deposition mechanisms to "grow" a new top layer to the optical window/dome using the same material and crystalline structure as the original substrate. The result would be a window/dome of a single optical material, eliminating prior barriers to window/dome repair, such as thermal mismatch, refractive index mismatch, mechanical stress, and sub-surface defects. In the case of single-crystal sapphire, use of the same material, deposited in the same crystallographic orientation, would also eliminate impacts to design and performance due to single-crystal sapphire’s inherent birefringence.

Further processing of the restored window/dome blank would be limited to final polishing/shaping and surface coating, with no changes required to polishing methods, coating materials, or coating design currently employed in the window/dome production process.

Innovative sources and methods are sought for the repair/refurbishment of sapphire, Ge, and Si IR windows and domes that have experienced damage as described above to the strength (i.e., Young's modulus, Poisson's ratio, Knoop hardness), shape (including original thickness), material (sapphire, Ge, or Si, depending on the substrate), crystallographic orientation, and optical quality (i.e., absorptivity, transmissivity, refractive index) of a pre-damaged, pre-coated (i.e., no anti-reflective coating), pre-polished window or dome blank, with the project goals of a final per-unit refurbishment cost not to exceed $30,000 and 3 months for flat sapphire windows, to 10 in. (25.40 cm) across, and for hemispheric Ge domes to 9 in. (22.86 cm) in diameter. The notional approach described above serves only as an example; providers are free to explore approaches that may or may not be similar. All proposed methods, however, must explicitly address the challenges of thermal and mechanical stress, possible separation of the repair layer and understructure, and impacts to optical performance, birefringence, and current processing/polishing techniques and coating designs.

PHASE I: Design and demonstrate feasibility of novel approach(es) to repair/refurbish single-boule-grown IR optical windows and domes that have surface damage characterized by pitting, scratches, abrasions, oil-based and salt spray contamination, and fragmented/delaminated surface coatings and/or coating remnants. First demonstrations will include optical grade flat single-crystal sapphire substrates of 0.75 in. (1.9 cm) diameter or larger, with no fundamental physical barrier to later applications of similar approaches to dome or ogive shapes, or other common boule-grown crystalline IR window material systems listed in the references. Selected methods and materials must have no intrinsic limitations to scaling to sizes of 100 square in. (254 square cm) (flat sapphire window) or 10 in. (25.4 cm) in diameter (hemispherical Ge dome). The Phase I effort will include selection of measurement and assessment techniques to evaluate the repaired window internal structure, stress/strain, refractive index, mechanical strength, and optical quality, as well as development of prototype plans to be implemented under Phase II.

PHASE II: Optimize processes developed under Phase I and demonstrate restoration of a scratched, eroded, partially-coated 5-in. (12.7 cm) (minimum) diameter, 0.25 in. (.635 cm) thick sapphire flat to the optical quality (i.e., absorptivity, transmissivity, lack of surface/subsurface defects), strength (i.e., Young's modulus, Poisson's ratio, Knoop hardness), and thickness of a pristine, unpolished, uncoated 0.25 in. (.635 cm) thick sapphire window blank, with nothing to preclude extension of the technology to larger sizes and to Ge dome materials systems, at a per-unit cost below $30,000. Process may be demonstrated on either government-furnished damaged single-crystal sapphire window pieces, or a supplier-produced surrogate made with at least one dielectric layer deposited over an uneven or partial metallic deposition layer on a single-crystal sapphire substrate.

PHASE III DUAL USE APPLICATIONS: Demonstrate the repair/refurbishment of up to 8 damaged optical windows or domes provided as government furnished equipment (GFE), at a per unit repair cost below $30,000, and time to repair below 3 months. GFE units will be 0.25 in. (.635 cm) thick boule-grown Ge (to 9 in. [22.86 cm] diameter) or Si (to 4 in. [10.16 cm] diameter) hemispherical domes or 0.25 in. (.635 cm) thick single crystal sapphire flats to 100 square in. (254 square cm) in size, with damage that may include surface pitting, scratching, abrasion, contamination/smudging, and full or partial delamination of metallic micro-mesh and/or multilayer dielectric surface coatings. Repair must be to the full original substrate thickness, allowing for additional material removal during a subsequent GFE polishing step (i.e., substrate will maintain 0.25 in. [.635 cm] thickness after polishing), with material hardness, optical quality, index of refraction, and internal stress commensurate with that of a single uniformly-boule-grown flat or dome of the same substrate material. Repaired/refurbished items will be delivered to the U.S. Government for further testing.

Sapphire windows are routinely used in grocery store check-out lines as a durable optical quality material through which laser scanners may read barcodes over long durations, without fear of degradation or damage. Being able to repair/refurbish such windows could have a marked impact on the grocery store infrastructure suppliers. Of greater impact, the ability to repair optical-grade windows will have a tremendous effect on the cost and availability of laboratory-grade sensors, cameras, and laser optics.

REFERENCES:

1. Harris, D. C. "Materials for infrared windows and domes: properties and performance (Vol. 158)." SPIE press, 1999. https://worldcat.org/title/1027372720
2. Rogatto, W. D. "The infrared and electro-optical systems handbook (Vol. 3)." Society of Photo-Optical Instrumentation Engineers, 1993. https://www.worldcat.org/search?q=The+Infrared+and+Electro-Optical+Systems+Handbook%2C+Volume+3%3A+Electro-Optical+Components
3. Biddut, A. Q.; Zhang, L. C.; Ali, Y. M. and Liu, Z. "Damage-free polishing of monocrystalline silicon wafers without chemical additives." Scripta Materialia, 59(11), 2008, pp. 1178-1181. https://www.precision-manufacturing.unsw.edu.au/sites/pm/files/uploads/Publications/Cutting_Drilling_Polishing/damage-free_polishing_of_monocrystalline_silicon.pdf
4. Hetherington, D. L.; Stein, D. J.; Benecke, J. D. and Hester, P. J. "Polysilicon chemical-mechanical polishing process characterization using a non-contact capacitance probe technique." AIP Conference Proceedings, Vol. 550, No. 1, January 2001, pp. 416-420. American Institute of Physics. https://doi.org/10.1063/1.1354435
5. Pandey, K. and Pandey, P. M. "Chemically assisted polishing of monocrystalline silicon wafer Si (100) by DDMAF." Procedia engineering, 184, 2017, pp. 178-184. https://doi.org/10.1016/j.proeng.2017.04.083

KEYWORDS: infrared windows; infrared domes; IR windows; IR domes; infrared sensors; IR sensors; missile seekers; missile warning; optical window

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