Systematic approach on illustrating the challenges represented by optical bidirectional measurements using rigorous simulations

摘要

Optical microscopy is widely used for the characterization of micro- and nanostructures in the field of unidirectional andbidirectional dimensional metrology. Despite the general high recognition in the metrological community, the inherentdifficulties which are bound to optical bidirectional measurements using commercial vision-based metrology tools are notsufficiently investigated, yet, and require additional insight, which we intend to provide here. We demonstrate the need forsophisticated analysis methods to find a threshold value which locates the correct physical edge position within themicroscopical image. The common assumption for the threshold to be at 50% of the intensity level of the edge signal is inessentially any imaging configuration wrong and leads to large systematic measurements errors. For example, the correctthreshold values for transmission light microscopy using high NA objectives on chrome on quartz photomasks, are within15% and 35% of the intensity level in the simulated images. For other measurement configurations the threshold variationcan be even much larger. Since the correct threshold values depend on the illumination and imaging parameters of theimaging system as well as on the geometrical and optical parameters of the measurement object, we showcase a selectionof them and their respective influence on the determination of the threshold values. Rigorous simulations are the keyfeature for this analysis since they require all the relevant parameters to be included in the simulation of a microscopicalimage which enables the correct threshold determination and to extract the correct bidirectional quantities out of the opticalimages.