This paper proposes a new highly sensitive scatterometry based Probe-Pattern Grating Focus Monitor. The high sensitivity is achieved by placing transparent lines spaced at the strong focus spillover distance of around 0.6λ/NA from the centerline of a 90 degree phase-shifted probe line that functions as an interferometer detector. The monitor translates the focus error into the probe line trench depth, which can be measured by scatterometry techniques. Simulations of optical imaging, resist development and Optical Digital Profilometry measurements are used to evaluate the expected practical performance. A linear model is developed to estimate focus error based on the measured probe trench depth. The results indicate that the ODP measurement from a single wafer focus setting can detect both the defocus direction and the defocus distance to well under 0.1 Rayleigh unit of defocus.
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