Concerning the influence of pattern symmetry on CD-SEM local overlay measurements for double patterning of complex shapes

We have developed a new local overlay measurement technique on actual device patterns using critical dimension scanning electron microscope (CD-SEM), which can be applied to 2D device structures such as an SRAM contact hole array or more complex shapes. CD-SEM overlay measurement can provide additional local overlay information at the site of device patterns, complementary to the conventional optical overlay data. The methodology includes the use of symmetrically arranged patterns to cancel out many process effects and reduce measurement uncertainty. The developed methodology was applied to local overlay measurement of double patterning contact hole layers of leading edge devices. Local overlay distribution was successfully captured on device structures on different length scale, and the result shows the possibility of assessing process induced shift on device structures and collecting denser sampling for better intra-chip overlay control. The measurement uncertainty of CD-SEM overlay metrology was assessed by comparing with conventional optical overlay metrology for 1D and 2D structures. Very good correlation was confirmed between SEM and optical overlay metrology with net residual error of ~1.1nm. Measurement variation associated with pattern roughness was analyzed for 1D structure, and identified as one of major variation sources for CD-SEM overlay metrology.