Spatial ALD lends itself perfectly for optical stacks for multi-layer optical band-pass filters. Because ALD layers are build up atom by atom, the level of control over layer thickness is very high and is determined by the number of deposition cycles. Exactly this is needed for multi-layer interference filters.
The following shows a practical example demonstrating the level of agreement between the theoretical optical performance of an ideal band-pass filter and the actual product made by ALD technology.
A three layer coating consisting of Al2O3 and TiO2 was deposited on PET foil by a R2R ALD tool with the goal of achieving maximum reflectance at 550 nm. The ALD process is based on the use of TiCl4+H2O and TMA+H2O as process gasses at a deposition temperature of 100°C.
The result was a very uniform reflective coating with maximum reflectance at 550 nm. As can be seen in the graph below, there was a near perfect fit with calculated theoretical model.