Diffraction-based overlay metrology using polarization-resolved dark-field digital holographic microscopy

Background: Accurate pattern positioning precision between two layers in a chip
stack, known as an overlay (OV), is crucial in semiconductor device manufacturing.
To facilitate the ongoing trend in the semiconductor industry to fabricate integrated
circuits with smaller feature sizes, new dark-field OV metrology imaging systems are
being investigated. A large challenge of dark-field microscopy is to correct the non-
uniform intensity profile of the illumination beam.
Aim: We present a polarization-resolved dark-field digital holographic microscope
to measure OV from the polarization response of diffraction-based OV (DBO) tar-
gets. It will be shown that polarization-resolved imaging of OV targets enables OV
metrology that is robust against target position offsets and less sensitive to inhomo-
geneities in the intensity of the beam profile.
Approach: We retrieve the polarization response of the DBO target by simultane-
ously measuring the interference signals from two orthogonally polarized reference
beams along with the image signal. The OV is retrieved from the polarization
response of the þ 1st and −1st diffraction order.
Results: We experimentally demonstrate the OV-dependent first-order polarization
response of a DBO target where the retrieved OV values remain independent of the
target’s position.
Conclusions: We present a polarization-resolved dark-field microscope for OV
retrieval that remains robust for varying intensity profiles of the illumination beam.