Optimizing flatness

Coatings for special applications

Stress in thin films may cause a slightly deformed surface, concave if the stress is tensile, convex if it is compressive. The easiest way to avoid deformation is to use thicker substrates but often this is not possible.

LASEROPTIK has approved coating processes in place that help to reduce stress in the coating and to avoid unwanted (a)spherical effects for substrates with extraordinary flatness requirements.

Additionally, the stress can be reduced significantly by optimizing the coating design. Taking the Stoney equation into account, the number and thickness of the layers can be adapted to the size and elastic properties of the substrate. But this may lead to a higher total film thickness and reduced spectral performance. For optimizing coating processes LASEROPTIK has a dedicated machine with an optional in-situ interfero­meter.

If it is not possible to achieve the desired  flatness with the chosen coating technique or design there are two further options but some more effort is required: A specially designed coating on the rear surface can compensate the stress of the main coating. This rear side coating can also be optimized for extra features like a high transmission pass band.

The most extensive way for stress compen­sation is to use pre-curved substrates. They ha­ve to be manufactured with a curvature adap­ted exactly to the computed stress of the coat­­ing, so standard substrates cannot be used anymore.