3D Conformal Antireflective Coatings by ALD
Antireflective (AR) coatings based on the interference of the reflections at the interface of alternating thin films with low and high refractive indices require precise thickness control. Conventional physical vapor deposition techniques usually produce a non-uniform thickness distribution on strongly curved substrates which severely affects the optical function. We demonstrate the suitability of atomic layer deposition (ALD) to achieve high AR performance even on steeply curved substrates. ALD is based on cyclic self-limiting surface reactions. The thickness of each layer is determined by the number of ALD cycles regardless of the substrate’s shape.
An ALD Al2O3/ TiO2/SiO2-multilayer system has been applied to a fused silica half-ball lens to reduce the reflectance to Rav < 0.3 % in the wavelength range of 390 nm to 750 nm. Excellent agreement of all measured spectra along the lens surface and the design is demonstrated.
Furthermore, single layer AR coatings consisting of nanoporous SiO2 have been applied. These layers have been realized by the deposition of Al2O3:SiO2 composite materials, where the alumina component was removed by subsequent wet chemical etching. We achieved a conformal AR with Rav < 0.1 % in the wavelength range of 600 nm to 700 nm on an aspheric B270 lens.
Atomic layer deposition is a promising technology for coating thin optical films on complex shaped components, such as convex and concave lenses, cylinders, ball lenses, in tubes or other substrates which are difficult to functionalize precisely with conventional coating technologies.
Authors: Kristin Pfeiffer, Lilit Ghazaryan, Ulrike Schulz, Adriana Szeghalmi