Atomic Layer Deposition
Atomic Layer Deposition (ALD) is a special Chemical Vapour Deposition (CVD) technique for innovative and custom-tailored optical applications with extreme film conformality and thickness control at the sub-nano level.
ALD bases on a sequential, self-limiting, layer-by-layer deposition. A solid film is formed by chemical reactions between the gas phase of precursor molecules taking place on the substrate surface, which is typically preheated (150-350 °C) to thermally initiate this reaction.
The film forms sequentially by only one monolayer per cycle. This provides an extreme deposition conformality and thickness control but increases the coating time. The growth is naturally pinhole free and the packing density is close to the bulk material.
Another special feature of ALD is the possibility to coat nearly any 3D-optic, even with a high aspect ratio or strongly curved surfaces, e.g. special prisms, hemispheres or tubes. All surfaces can be coated at once or can also be protected if needed.
As there is no need to add energy (like using an additional ion source) to achieve a better density, chemical processes such as ALD typically show low internal stress.
Several different reaction chambers are available to support substrate sizes from a few millimetres to 300 mm in all dimensions and can easily be exchanged.
The most common coating materials (SiO2, Al2O3, TiO2, Ta2O5…) are available while pure metals or fluorides are still challenging and require some additional development and optimization.