Model Based Laser Focus Scanning - The path towards improved lateral accuracy

摘要

In principal, optical measurement methods suffer from physical limits related to finite wavelengths and diffraction. Inlaser focus scanning, vertical resolutions below 1 nm can be achieved while the lateral accuracy is more or less restrictedby the diameter of the focused laser beam, i.e. values of half the wavelength can be reached in the best case. We presenta model based approach having the potential to show a way out of this limitation. It is based on the rigorous modeling ofthe complete measurement device including sophisticated ray tracing in combination with Maxwell based modeling ofthe sample diffraction and scattering providing a simulated signal for an assumed sample profile. Furthermore, thesample profile is parametrized on the basis of a priori information. The simulated signal is then iteratively compared withthe measured signal while updating the floating parameters of the model in order to improve the match between the twosignals. Eventually, the improved sample profile obtained in this way is considered to represent the real sample profile assoon as a certain goodness of fit is achieved. On the other hand, the profile model has to be changed in case there is nosatisfying fit. In this way, the lateral accuracy can be increased considerably. Edge detection errors below a few tensnanometer or even below 10 nm become possible while measuring with visible light. This is demonstrated by firstcomparisons of modeled and measured signals and validation by alternative metrology techniques.