Scatterometry-based overlay metrology

Abstract

Scatterometry provides a new, vibration-tolerant technique of overlay metrology. Gauge repeatability and reproducibility is improved by an order of magnitude over imaging-based overlay metrology. To measure the overlay of patterned layers A and B by scatterometry, one line grating is placed in layer A and another in layer B. The two gratings overlap when they are viewed in the direction that is normal to the wafer. The line gratings in layers a and b are of equal pitch and their lines are parallel. In one method, overlay is measured by fitting the optical properties of the target with spectra calculated using a model of the target and rigorous coupled wave analysis. A faster and simpler method obtains overlay by applying a linear estimator to a difference of spectra. Optical properties of targets were measured by a normal incidence spectroscopic reflectometer. Test wafers representing three overlay applications were fabricated: contact mask to shallow-trench, first metal mask to contact, and gate-mask to shallow-trench. Overlay measured by scatterometry agree with imaging-based measurements and offsets intentionally written to the reticle.