Aggressive pitch requirements for line/space pattern devices require the usage of extreme off-axis illumination schemes
to enhance the resolution of the exposure tools. These illumination schemes stress the quality of the optics because of the
anisotropy of the optical paths through the lens. Moreover, the marginalities on the patterning are dramatically enhanced
if two or more illumination modes are requested in the lithography process.
The effects on overlay between double exposure layers exposed with different illumination settings, with one being an
extreme illumination setting, will be discussed and two approaches will be addressed to compensate the resulting overlay
The first approach optimizes the lens setup by means of a dedicated scanner option to minimize the lens effects on
overlay and reduce the distortion for each layer: in this case the simulation time and the impact on other imaging
parameters will be carefully evaluated.
The second methodology corrects the induced misalignment by a high order modelling compensation. This approach
requests the insertion of a suitable set of overlay measurement targets into the product frame to appropriately fit the
distortion matching of the two layers.