Since 1990 thin film optical coatings have taken a prominent role in the development of highly efficient solar power concentrators for future space applications. During the initial development of this coating technology, the Boeing High Technology Center explored various ways of protecting ENTECH's DC93-500 silicone Fresnel lenses from the harsh space environment. ENTECH's mini-dome lenses focused solar energy onto small high-efficiency solar cells for generating electrical power. To protect the silicone lenses from solar UV darkening, one early approach involved a cerium-doped glass cover cemented over the lens. Unfortunately, during launch simulation shock testing the glass lens covers cracked. We next explored the deposition of a UV blocking thin film coating directly to the silicone lens surface. This was a problem of immense proportions analogous to pouring concrete on to the surface of a reservoir filled with "Jell-O." Differential in coefficient of thermal expansion between the DC93-500 silicone and the deposited dielectric optical coating had to be balanced with intrinsic stress of the optical coating materials. Ion Beam Optics' work has culminated, some fifteen years later, in the current coating technology that is being incorporated in the Stretched Lens Array SquareRigger (SLASR). SLASR is designed to replace classic flat panel solar arrays with a lighter, lower cost, and more efficient (30%) concentrator arrays for future space applications. This paper will describe the coating technology and show its performance and benefits for SLASR space power systems. Results from both ground tests and space flight tests will be presented.
Energetic process development in the production of optical coatings has progressed significantly over the last two decades, permitting the practitioner of thin film coating depositions a wide choice of deposition parameters. Primarily, a series of important advances has occurred in the nearly ubiquitous use of Ion Assisted Deposition (IAD) for the production of high performance optical coatings. Progressing from the rudimentary use of ionized gas technology for pre-cleaning substrates, to the advanced IAD produced telecom filters (DWDM), energetic processes now play a vital role in most optical coating production. The advances in IAD technology culminate in the development of stable and durable thin films for a wide variety of stringent spectral specifications from the UV to the Far IR. The technical progression from IAD use in either sputtered or physical vapor deposition (PVD) processes to the development of Ion-Assisted Filtered Cathodic Arc Deposition (IFCAD) technology for applications from temperature-sensitive optics to future space coatings will be discussed.
An innovative Ion-Assisted Filtered Cathodic Arc Deposition (IFCAD) system has been developed for low temperature production of thin-film coatings. The IFCAD system employs electro-magnetic and mechanical filtering techniques to remove unwanted macroparticles and neutral atoms from the plasma stream. Therefore, only ions within a defined energy range arrive at the substrate surface, depositing thin-films with excellent mechanical and optical properties. Ion- Assisted-Deposition is coupled with Filtered Cathodic Arc technology to enhance and modify the arc deposited thin- films. Using an advanced computer controlled plasma beam scanning system, high quality, large area, uniform IFCAD multi-layer film structures are attained. Amorphous Diamond- Like-Carbon films (up to 85% sp3 bonded carbon; and micro- hardness greater than 50 GPa) have been deposited in multi- layer thin-film combinations with other IFCAD source materials (such as: Al2O3) for optical and tribological applications. Rutile TiO2 (refractive index of 2.8 at 500 nm) has been deposited with this technology for advanced optical filter applications. The new IFCAD technology has been included in development programs, such as: plastic and glass lens coatings for optical systems; wear resistant coatings on various metal substrates, ultra smooth, durable, surface hydrophobic coatings for aircraft windows; EUV coatings for space instrumentation; transparent conductive coatings; and UV protective coatings for solar cell concentrator plastic Fresnel lens elements for space power.
The variety of optical components produced using this ion- assisted-deposition (IAD) technique, spans a wide ken of applications: immersed beamsplitters; band pass and edge filters; laser filters; IR windows; durable anti-reflection filters, fiber optics, and enhanced metals. Each product type has a specific set of constraints that establishes the design and production strategy: substrate type, material indices, intrinsic stress, temperature, distribution, uniformity, thickness monitoring, spectral performance specification, and environmental requirements. The end-Hall IAD processes were performed using an ion-beam current of approximately one Ampere over a range of 40 to 120eV, providing a uniform ion current density impinging on a large substrate area. Compared to films deposited using either conventional physical vapor deposition, or standard gridded ion sources, this process produces optically stable films with abroad ion-beam that is well suited for volume manufacturing of complex optical coatings.
A wide variety of thin-film materials (TiO2, SiO2, Al2O3, HfO2, Ta2O5, ITO, Y2O3, CeO2, Si3N4, ZrO2, Au, Ag, and MgF2) were deposited using conventional electron beam and thermal evaporation techniques with a concomitant bombardment of energetic oxygen ions from a gridless end-Hall ion source. The oxygen-ion-assisted deposition was performed with an ion-beam current of approximately one Ampere over a range of 40 to 120 eV, providing a uniform ion current density (.3 to .5 mA/cm2) impinging on a large substrate area (5024 cm2). Compared to film deposited using standard gridded ion sources this process produces optically equivalent materials with a broad ion beam which is well suited for volume manufacturing.
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