Topographic Phase Control For Overlay Measurement

Abstract

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.

What is claimed is: 
     
         1 . An optical system in a metrology tool comprising:
 a first detection focus location of a collection path;   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;   an objective lens;   a reticle at a field plane thereof configured as a reference for target images at the focus locations, wherein the reticle is disposed in the collection path between the objective lens and the beam splitting elements;   three detectors that capture images at different axial displacements, wherein the detectors capture images simultaneously; and   a processor configured to provide per-layer grab centering during image grabbing around a center of a sum of scatterings from areas of a plurality of gratings in each layer of a wafer using the optical system.   
     
     
         2 . 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. 
     
     
         3 . The optical system of  claim 2 , wherein the first detection focus location and the two corresponding additional focus locations are at different locations relative to the detection plane. 
     
     
         4 . 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. 
     
     
         5 . The optical system of  claim 1 , wherein separate images of the first detection focus location or the two corresponding additional focus locations are generated, and wherein the separate images are configured to determine displacement of target layers imaged using the optical system. 
     
     
         6 . The optical system of  claim 1 , wherein the reticle is a reference center for determination of a displacement of target layers imaged using the optical system. 
     
     
         7 . The optical system of  claim 1 , wherein the optical system is configured to operate at a spectral range Δλ≤10 nm, at an illumination numerical aperture NA≤0.1, and at a focus position that corresponds to zero error overlay magnification according to a dependency between an overlay error magnification and a level of defocusing. 
     
     
         8 . The optical system of  claim 1 , wherein the processor is further configured to estimate an inaccuracy magnification factor of images and to determine a best contrast position by identifying a sign change of an inaccuracy magnification factor with respect to focus positions.