Direct-scatterometry-enabled lithography model calibration

Optical scatterometry is crucial to advanced nodes due to its ability of non-destructively and rapidly retrieving accurate 3D profile information.(1, 2, 3) In recent years, an angle-resolved polarized reflectometry-based scatterometry which can measure critical dimensions, overlay, and focus in single shot has been developed.(4, 6, 20) In principle, a microscope objective collects diffracted light, and pupil images are collected by a detector. For its application of calibrating lithography models, the pupil images are fit to a database pre-characterized usually by rigorous electromagnetic simulation to estimate dimensional parameters of developed resist profiles.(5) The estimated dimensional parameters can then be used for lithography model calibration. In this work, we propose a new method which directly utilizes the pupil images to calibrate lithography models without needing dimensional parameter estimation. To test its feasibility and effectiveness by numerical simulation, a reference lithography process model is first constructed with a set of parameter values complying with ITRS. A to-be-calibrated process model is initialized with a different set of parameter values from those of the reference model. Rigorous electromagnetic simulation is used to obtain the pupil images of the developed resist profiles predicted by both process models. An optimization algorithm iteratively reduces the difference between the pupil images by adjusting the set of parameter values of the to-be-calibrated process model until the pupil image difference satisfies a predefined converging criterion. This method can be used to calibrate both rigorous first-principle models for process and equipment development and monitoring, and fast kernel-based models for full-chip proximity effect simulation and correction. Preliminary studies with both 1D and 2D aperiodic and periodic layouts indicate that when the pupil image difference is minimized, the lithography model can be accurately calibrated.