Hot-filament CVD diamond coatings for optical applications

The demand for highly scratch resistant transparent coatings is motivated by the industrial sector of consumer goods for example for the protection of displays, touch-screens, watches, and all kinds of optical instruments. In a wide variety of applications, scratch-free and highly wear-resistant surfaces are the enabling factor for improvement of lifetime and thus sustainability of the products. Diamond with its unique properties such as the highest known hardness, low thermal expansion coefficient and chemical resistance (to acids and bases) [1], [2] has outstanding potential for optical applications [3]. The use of thin ultrahard diamond layers with adapted optical properties is expected to enhance the resistance against abrasive wear to a new level. At Fraunhofer IST large-area Hot-Filament Chemical Vapor Deposition (HFCVD) processes have been developed [4], [5] and now utilized for depositing highly uniform and transparent diamond coatings on optical components with sizes of up to 200 mm × 200 mm. The use of single-layer diamond coatings is investigated as well as the integration of diamond films into multilayer film stacks combining anti-reflective properties with mechanical stability (strength). Four-layer anti-reflective coatings (Si3N4/SiO2/Diamond/SiO2) entirely deposited with hot-filament/hot-wire processes and their optical and mechanical performance will be presented.