First Principle-based State Estimator for Photolithography Control

In DUV photolithography, mask patterns and processes are increasing in complexity, while IC critical dimensions continue to shrink at a rapid pace. As a result, the proportional variability of the gate CD will increase to unacceptable levels unless a more sophisticated means of advanced process control is introduced. The proposed process control framework exploits scatterometry, which provides rapid, in-line, full-profile metrology. The major obstacle to implementing scatterometry in a process control setting is profile inversion—deriving estimated input conditions from the measured profile. In this work, profile inversion is accomplished by extending standard library-based scatterometry to include a first principlebased simulator (PROLITH). This lithography process simulator is used to create a collection of inputconditions-to-profile pairs; in turn, these profiles are used to generate simulated diffraction responses, resulting in a library that links the simulated diffraction response to the corresponding process input conditions. Finally, the empirically measured diffraction response is matched to a simulated diffraction response in this library, allowing for estimation of actual process conditions. Preliminary, simulation-only results suggest that the framework has the potential to be successful even with limited library size, particularly if approximate values of some input conditions are provided by independent sensors.