As the feature sizes on silicon integrated-circuit (IC) chips become smaller (65 nm feature sizes are in production and 45 nm sizes are coming soon), precisely overlaying the different lithographic levels in IC-chip fabrication becomes ever more difficult. For example, the period of grating-based overlay targets (used for aligning layers to one another) becomes so small that the gratings only reflect zero-order specular light, making them useless for their purpose.
Now, Richard Silver and his colleagues at the National Institute of Standards and Technology (NIST; Gaithersburg, MD), K T Consulting (Antioch, CA), and International Sematech (Austin, TX) have developed grating overlay targets based on feature geometries much denser than the Rayleigh criterion for resolution, but that according to simulations are visible to optical systems using light at a 546 nm wavelength when overlaid. In a vernier fashion, two gratings with slightly different (190 and 200 nm) subwavelength pitches produce a much lower-frequency grating, which is easily detected in visible light. In addition, the targets magnify the effect of small offsets-for example, a 2 nm actual offset appears to be a 40 nm offset. Chipmakers are already working with NIST on implementing the technology. Contact Richard Silver at [email protected].