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Focus improvement with NIR absorbing underlayer attenuating substructure reflectivity

Process dependent focus leveling errors occur in photolithography when there is unpredicted reflectivity originating from multilayer structures on the fully integrated process wafer. The typical wavelength used in optical focus sensors is in the near infrared (NIR) range which is highly transparent to most dielectric materials. Consequently, the reflected light from underlying structures perturbs the accuracy of the leveling signal reflected from resist surface. To alleviate this issue, air-gauge focus sensors have been used to measure the wafer surface topography for an in-situ calibration to correct the focus leveling error. Using an air-gauge sensor is a slow process and a throughput detractor. Therefore, an NIR-absorbing underlayer has been developed for easy insertion into existing resist coating processes. It has been demonstrated that the air-gauge sensor can be turned off without showing any degradation in leveling data or litho performance on back end of line (BEOL) integrated wafers.