For decades it has been possible to produce optical coatings on plane surfaces of almost any size. It is new however to produce coated surfaces with very small dimensions, down to the micrometer range (pixels). Through the photolithographic micro-patterning technology it has become possible to cover partial areas of a substrate with an optical coating.

 

The micro-sectioning of optical layers is similar to the lithographic methods used in semiconductor industries: At first, a photoresist is applied to the whole area. Then, using an appropriate mask, it is exposed and developed in all places where the layers shall be deposited on the substrate. During the following coating process, all areas that shall not be covered with optical layers remain protected by the photoresist. After the coating process, it will be removed, and the surface of the substrate can be sectioned and coated again, with differently shaped patterns and an altered spectral characteristic of the layer system.

 

The thin-film design itself remains unrestricted by the micro-pattern technology. Receptive areas on photodiode-wafers, for example, are coated with an optical filter, while the respective bond pads are except from it. Several micro-patterned layers on one surface form arrays of interference filters. The lateral measurements of the coated areas are definable within a range from centimeters to micrometers. Both the dimensional accuracy and the positioning accuracy have a maximum tolerance of only a few micrometers.

 

In addition to the production of patterned filter arrays, the micro-sectioning is used to realize the transition area from the uncoated substrate to the layer system, or from a layer system to another area within only a few micrometers. As a major application, this method allows the design of long pass filters as order sorting filters to suppress higher diffractive orders in miniature spectrometers.