Topographic Phase Control For Overlay Measurement
Metrology tools and methods are provided, which estimate the effect of topographic phases corresponding to different diffraction orders, which result from light scattering on periodic targets, and adjust the measurement conditions to improve measurement accuracy. In imaging, overlay error magnification may be reduced by choosing appropriate measurement conditions based on analysis of contrast function behavior, changing illumination conditions (reducing spectrum width and illumination NA), using polarizing targets and/or optical systems, using multiple defocusing positions etc. On-the-fly calibration of measurement results may be carried out in imaging or scatterometry using additional measurements or additional target cells.
1 . An optical system having a first detection focus location of a collection path in a metrology tool, the optical system comprising at least two beam splitting elements along the collection path which are positioned to provide at least two corresponding additional focus locations having different collection path lengths than the first detection focus location.
2 . A metrology tool comprising the optical system of claim 1 , further comprising an on-the-fly calibration module configured to provide a per-layer grab centering.
3 . The optical system of claim 1 , further comprising a reticle at a field plane thereof, configured as a reference for target images at the focus locations.
4 . The optical system of claim 1 , further comprising a mirror, wherein the two beam splitting elements and the mirror are configured to provide the first detection focus location and the two corresponding additional focus locations.
5 . The optical system of claim 1 , wherein the optical system provides three images having equal power and corresponding to the first detection focus location and the two corresponding additional focus locations.
6 . The optical system of claim 5 , wherein the three images are detected by a detector without any mechanical drift.