Semiconductor industry has an increasing demand for improvement of the total lithographic overlay performance. To
improve the level of on-product overlay control the number of alignment measurements increases. Since more mask
levels will be integrated, more alignment marks need to be printed when using direct-alignment (also called layer-to-layer
alignment). Accordingly, the alignment mark size needs to become smaller, to fit all marks into the scribelane. For
an in-direct alignment scheme, e.g. a scheme that aligns to another layer than the layer to which overlay is being
measured, the number of needed alignment marks can be reduced.
Simultaneously there is a requirement to reduce the size of alignment mark sub-segmentations without compromising the
alignment and overlay performance. Smaller features within alignment marks can prevent processing issues like erosion,
dishing and contamination. However, when the sub-segmentation size within an alignment mark becomes comparable to
the critical dimension, and thus smaller than the alignment-illuminating wavelength, polarization effects might start to
occur. Polarization effects are a challenge for optical alignment systems to maintain mark detectability. Nevertheless,
this paper shows how to actually utilize those effects in order to obtain enhanced alignment and overlay performance to
support future technology nodes.
Finally, another challenge to be met for new semiconductor product technologies is the ability to align through semi-opaque
materials, like for instance new hard-mask materials. Enhancement of alignment signal strength can be reached
by adapting to new alignment marks that generate a higher alignment signal. This paper provides a description of an
integral alignment solution that meets with these emerging customer application requirements. Complying with these
requirements will significantly enhance the flexibility in production strategies while maintaining or improving the
alignment and overlay performance. This paper describes the methodology for optimization of the alignment strategy.