unwanted feature and is usually removed by a second exposure. Application A benefit of using phase-shift masks in lithography is the reduced sensitivity to variations of feature sizes on the mask itself. This is most commonly used in alternating phase-shift masks, where the linewidth becomes less and less sensitive to the chrome width on the mask, as the chrome width decreases. In fact, even with no chrome the phase edge can still print, as noted above. Some cases of attenuated phase-shifting masks also demonstrate the same benefit (see left). As phase-shift masks are applied to printing smaller and smaller features, it becomes more and more important to model them accurately using rigorous simulation software, such as Panoramic Technology or Sigma-C. It becomes especially important as the mask topography starts to play an important role in scattering the light, and the light itself starts to propagate at larger angles. The performance of phase-shift masks can also be previewed with the use of aerial image microscopes. Defect inspection remains a critical aspect of phase-shift mask technology, as the set of printable mask defects has expanded to include those with phase effects in addition to conventional transmission effects. Attenuated phase shift masks have been in use in production since the 90 nm node. References Further reading Lithography (microfabrication)