Kohler illumination is the most widely adopted lighting scheme within optical illumination systems. It evolved from Critical Illumination, which images the light source onto the specimen via a lens to illuminate samples transmissive to light.Nevertheless, the helical filament pattern of the lamp will be superimposed onto the specimen. When photographing the sample, it becomes impossible to distinguish whether such texture originates from the specimen itself or the illumination system, as illustrated in the corresponding diagram.

By contrast, the specimen plane in Kohler illumination lies at the spectral plane of the light source. Even if the light source has an uneven intensity distribution, every discrete point on the light source is uniformly projected across the entire reticle plane through the lens, delivering highly homogeneous illumination.

Owing to its prominent advantages, Kohler illumination is standardly equipped on mid-to-high end microscopes with photographic ports as well as photolithography steppers/scanners.The schematic below demonstrates an imaging system of a lithography machine integrated with Kohler illumination, where the projection objective adopts a double telecentric configuration.The numerical aperture (NA) of the illumination section matches the numerical aperture of the projection objective on the reticle (object) side.

For a lithography system with a 4:1 reduction ratio, the numerical aperture on the reticle (object) side is one quarter of that on the wafer (image) side.Take mainstream 193nm immersion lithography tools such as ASML’s NXT series as an example; their image-side numerical aperture reaches 1.35.Accordingly, the object-side NA at the reticle plane equals 0.3375, and the illumination system is configured with the identical NA value of 0.3375, roughly equivalent to an F-number of F1.5 in photography.