ALD Technologies at Fraunhofer IOF

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

Highly curved lenses for ALD coating.

Highly curved lenses for ALD coating.

Reflectance of an ALD-multilayer AR coated half-ball lens.

Reflectance of an ALD-multilayer AR coated half-ball lens.

Reflectance of a nanoporous SiO2 single-layer AR coated aspheric lens.

Reflectance of a nanoporous SiO2 single-layer AR coated aspheric lens.

Atomic Layer Deposition of Antireflection Coatings

Low-loss SiO2 coating on fused silica substrate.

Low-loss SiO2 coating on fused silica substrate.

Measured transmittance of a double-sided broadband antireflection coating on two NSF8 substrates.

Measured transmittance of a double-sided broadband antireflection coating on two NSF8 substrates.

The deposition of conformal coatings with a precise thickness is crucial for the production of high quality optical thin film stacks. Conventional techniques such as physical vapor deposition (PVD) are suitable for the deposition of thin films on plane surfaces. However, with PVD a non-uniform thickness is generated on curved surfaces, leading to distortions of the optical function of thin film stacks.

We are establishing atomic layer deposition (ALD) as an alternate method of meeting the high requirements on the thickness uniformity of the single layers and also on highly curved lenses and to control the thickness of optical film stacks without in situ monitoring.

The main advantage of ALD is its capability to deposit conformal coatings on structured surfaces with high aspect ratios. The film thickness is defined exactly by the number of ALD cycles. The deposited layers have a high lateral uniformity and low roughness.

With the developed ALD processes for Al2O3, TiO2, HfO2, and SiO2, we achieve a high reproducibility and a linear growth rate that is necessary for precise thickness control. Our results show that atomic layer deposited films exhibit only little absorption losses down to a spectral range of 200 nm (SiO2, Al2O3), 260 nm (HfO2), and 400 nm (TiO2). Laser calorimetric measurements (λ=1064 nm) of 300 nm films on quartz substrates revealed low absorption values of only 5.4 ppm (TiO2), 3.3 ppm (Al2O3), und 3.9 ppm (SiO2), whereas the substrate itself has losses of about 2.3 ppm. Using the materials SiO2 and HfO2, we demonstrated a broadband antireflection coating for the spectral range of 390 nm to 1100 nm. The thicknesses of the single layers are just controlled by the number of ALD cycles. In further experiments other optical elements such as dichroic mirrors and narrow bandpass filters will be realized.

 

Authors: Kristin Pfeiffer, Svetlana Shestaeva, Astrid Bingel, Peter Munzert, Adriana Szeghalmi