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 in an imaging metrology tool, the optical system comprising an adjustable reference path having a reference signal integrated in a collection path thereof, wherein the reference path is configured to provide an adjustable phase of the reference signal and the optical system is configured to adjust the phase of the reference signal to modify a topographic phase of an imaging metrology target to be an integer multiple of π
2 . The optical system of claim 1 , wherein the adjustable reference path includes a Linnik interferometer with a reference objective identical to an objective of the imaging metrology optical system and an adjustable mirror.
3 . The optical system of claim 2 , further comprising an illumination path and a beam splitter, wherein the illumination path and the collection path are associated with the objective via the beam splitter.
4 . The optical system of claim 1 , further comprising a neutral density filter and/or mirror configured to control amplitude of the reference signal.
5 . An optical system in an imaging overlay metrology tool, the optical system comprising an illumination path, a main objective, and a collection path, wherein the optical system further comprises a reference path having a reference signal with controllable amplitude and phase which are configured to minimize a difference of topographic phases between zeroth and first order diffraction signals from at least two target layers.
6 . The optical system of claim 5 , wherein the reference path is incorporated in the optical system in a Linnik interferometer configuration.
7 . The optical system of claim 5 , wherein the minimization is carried out according to Equation 12.