Lee M. Goldman

Recent advances in spinel optical ceramic

New military requirements have reinvigorated the need for transparent magnesium aluminate (MgAl2O4) spinel. Surmet has developed a process that yields high quality transparent spinel at production scale. Several issues related to the extreme requirements of processing ultrafine spinel powders are described. Transmission data is presented for a significant dataset of parts made by this process. More recently, the process has been expanded to include a capability for producing domes for the Joint Common Missile program. Domes at nominal 6” and 7” diameter have been successfully fabricated. Despite early challenges related to the forming portion of the process, a repeatable capability for these domes has been demonstrated. Several challenges remain in spinel processing in order to support additional military requirements. In particular, the strength of the material needs further improvement. Also, improvements in optical quality with regard to inclusions are needed.

Recent advances in ALON optical ceramic

Aluminum Oxynitride (ALONTM Optical Ceramic) is a transparent ceramic material which combines transparency from the UV to the MWIR with excellent mechanical properties. ALON’s optical and mechanical properties are isotropic by virtue of its cubic crystalline structure. Consequently, ALON is transparent in its polycrystalline form and can be made by conventional powder processing techniques. This combination of properties and manufacturability make ALON suitable for a range of applications from IR windows, domes and lenses to transparent armor. The technology for producing transparent ALON was developed at Raytheon and has been transferred to Surmet Corporation where it is currently in production. Surmet is currently selling ALON into a number of military (e.g., windows and domes) and commercial (e.g., supermarket scanner windows) applications. The capability to manufacture large ALON windows for both sensor window and armor applications is in place. ALON windows up to 20x30 inches have been fabricated. In addition, the capability to shape and polish these large and curved windows is being developed and demonstrated at Surmet. Complex shapes, both hyper-hemispherical and conformal, are also under development and will be described.

Atomic layer epitaxy of InP using trimethylindium and tertiarybutylphosphine

Abstract Atomic layer epitaxy (ALE) growth of InP was investigated using trimethylindium (TMI) and tertiarybutylphosphine (TBP) in a horizontal atmospheric reactor. The studied growth parameters were exposure times, TMI and TBP fluxes, and growth temperature. Self-limiting ALE growth of InP was achieved at a growth temperature of 340°C. Substantial increases in the TMI flux and higher growth temperatures exceeding 340°C resulted in growth rates exceeding 1 monolayer per cycle. Uniform ALE InP layers were verified by cross-sectional transmission electron micrography and sputtered Auger profiling. The application of thin ALE InP layers (15 A) on GaAs surfaces was investigated using X-ray photoelectron spectroscopy, 77 K photoluminescence, and 300 K photoreflectance. The absence of arsenic oxide and an increase in the photoluminescence intensity by a factor of 2 were observed after InP passivation.

ALON optical ceramic transparencies for window, dome, and transparent armor applications

Surmet continues to invest in and expand its manufacturing capability for ALON® Optical Ceramic, as the market demand for this material increases. The biggest demand and opportunity continues to be in the area of transparent armor, however, the market for sensor domes and windows, made from ALON, continues to grow at an impressive rate as well. ALON® Transparent Armors unsurpassed ballistic performance, combined with the robustness of ALONs manufacturing process and reproducibly high material quality make ALON the leading candidate for many future armor systems. Recent results for ALON armor windows will be presented. Advances being made in Surmets production capability to support the very large quantities of material required by the transparent armor market also benefit the sensor market. Improvements in quality, quantity and manufacturability of ALON material, combined with improvements being made in optical quality, ensure a robust supply of high quality material for high volume window and dome applications. Recent advancement in ALON® window and dome blanks, as well as in optical fabrication will be presented.

Recent advances in aluminum oxynitride (ALON) optical ceramic

Aluminum Oxynitride or ALON optical ceramic is transparent material, developed and patented by Raytheon, which is very similar to sapphire, being comprised mostly of Al2O3 with a small amount of additional nitrogen. This nitrogen addition has the effect of producing a cubic material whose optical and mechanical properties are isotropic. Importantly, this means that it can be produced by powder processing methods, which are scalable to larger sizes, and at lower prices than can be achieved by the single crystal growth techniques that are used to grow sapphire. Furthermore, its isotropic properties make it much easier to grind and polish than sapphire. Recently, the interest in ALON optical ceramic has grown substantially following impressive results in ballistic testing. Ballistic laminates, containing ALON layers, have demonstrated protection against armor piercing rounds, at half the areal density and thickness of conventional ballistic laminates. ALON plates as large as 14x20in are being produced, under Air Force funding, for evaluation as IR windows and transparent armor, using conventional powder processing techniques. The production processes themselves are now being scaled to produce large pieces and large quantities of ALON optical ceramic.

High-durability infrared transparent coatings

LWIR windows are exposed to harsh conditions during high speed flight. These include high speed rain drop impact, sand abrasion, corrosion, and aerothermodynamic load. With the possible exception of diamond, there are no LWIR transparent window/dome materials which can withstand these various environments. Rain erosion protective (REP) and oxide based abrasion resistant/oxidation resistant durable antireflection (DAR) coatings have been developed for LWIR applications. These coatings have demonstrated a substantial degree of raindrop impact protection (i.e., damage threshold velocities of approximately Mach 1, for 2 mm equivalent waterdrop impacts at normal incidence). The combination of REP + DAR coating have also demonstrated excellent resistance to sand abrasion in simulated flight environments. The high degree of abrasion resistance makes the DAR coatings applicable to ground based systems, using ZnS and ZnSe, windows as well. An additional advantage of the Raytheon REP + DAR combination is that they are transparent from the visible to the LWIR (8 to 12 mm), making them suitable for applications requiring broadband transparency. Furthermore, the DAR coatings have protected ZnS substrates from oxidation at temperatures up to 1000 degree(s)C. The combination of ZnS REP coating and DAR coating are ideally suited for protecting high speed LWIR missiles from rain and sand damage during captive carry, as well as protecting the dome/window from oxidation during high speed flight. Data are presented to demonstrate the rain/sand and oxidation protection provided by these coatings. The REP and DAR coatings have been scaled up to coat windows and domes for far infrared applications.

Motheye Structured Surface Fabrication as Durable Anti-Reflection Treatment on CdZnTe for Space based LWIR Detector Devices

Space based HgCdTe imaging devices, built on CdZnTe substrates, require radiation hardened anti-reflection (AR) treatments in order to withstand the rigors of the space environment. Conventional anti-reflection (AR) coatings provide adequate optical performance but are prone to delamination and degradation due to extreme temperature cycling and irradiation in space. Consequently, there is an intense need for improved AR technology that combines high optical performance with improved durability. Etching physical gradient or motheye structures directly into the CdZnTe eliminates the need to deposit additional layers of different materials onto the substrate, avoiding the possibility of delamination and cross contamination. Motheye AR surfaces, under development at Surmet Corporation, have demonstrated excellent broadband optical performance in the LWIR (7 to14 micron) waveband. Surmets motheye technology involves direct etching of a regular pattern of fine features into the CdZnTe substrate, using standard lithography and dry etching techniques. The results from this ongoing research and development effort are discussed.

Novel methods for shaping thin-foil optics for x-ray astronomy

We report on progress in developing low-cost methods for shaping thin-foil glass x-ray optics. Such optics might serve as substrates for reflection gratings or as foil mirrors in high-throughput missions such as Constellation-X. Novel thermal shaping to lithographically defined pin chucks leads to the desired shape with high accuracy, thereby avoiding the need for replication. To demonstrate this method we have produced 200 micron-thick glass sheets with sub-micron flatness and half power diameter below 10 arc seconds. We also present a process for depositing low-stress metallic coatings that provides high x-ray reflectivity without significant foil distortion.

Development of a repairable IR composite window

Infrared (IR) imaging systems are a critical component of military aircraft operations for navigation, surveillance, and target acquisition. A limiting feature in the readiness and life-cycle costs for such IR systems is the durability of the exposed optical window in the harsh land and sea environments associated with military operations. The Air Force Research Laboratory, Materials Directorate, has been conducting a program to develop a high durability, repairable JR window to both extend the operation life-cycle of the transparencies and to permit the reuse of the optical materials for significant cost reductions. The development effort is focused on a composite JR window composed of a highly JR transparent base material with a removable durable bonded cladding for erosion and abrasion protection. The development activities have included evaluation and selection of the optical materials, assessment of high durability coatings, refinement of the optical adhesive and bonding process, as well as laboratory and flight testing.

Scale up of large ALON windows

Aluminum Oxynitride (ALON® Optical Ceramic) combines broadband transparency with excellent mechanical properties. ALON’s cubic structure means that it is transparent in its polycrystalline form, allowing it to be manufactured by conventional powder processing techniques. Surmet has established a robust manufacturing process, beginning with synthesis of ALON® powder, continuing through forming/heat treatment of blanks, and ending with optical fabrication of ALON® windows. Surmet has made significant progress in our production capability in recent years. Additional scale up of Surmet’s manufacturing capability, for larger sizes and higher quantities, is currently underway. ALON® transparent armor represents the state of the art in protection against armor piercing threats, offering a factor of two in weight and thickness savings over conventional glass laminates. Tiled and monolithic windows have been successfully produced and tested against a range of threats. Large ALON® window are also of interest to a range of visible to Mid-Wave Infra-Red (MWIR) sensor applications. These applications often have stressing imaging requirements which in turn require that these large windows have optical characteristics including excellent homogeneity of index of refraction and very low stress birefringence. Surmet is currently scaling up its production facility to be able to make and deliver ALON® monolithic windows as large as ~19x36-in. Additionally, Surmet has plans to scale up to windows ~3ftx3ft in size in the coming years. Recent results with scale up and characterization of the resulting blanks will be presented.