Advances in process overlay: alignment solutions for future technology nodes

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.