Novel methodology for postexposure bake calibration and optimization based on electrical linewidth measurement and process metamodeling

Abstract

By combining electrical linewidth measurements and neural-network ͑NN͒ process metamodeling, lithography simulators can be calibrated in an efficient way. In this work we present a novel methodology for characterizing postexposure bake using a very large experimental data set, so that the calibrated model can be used as a truly predictive tool. The adoption of a special test reticle mask allowed us to collect more than 700 000 critical dimensions CDs from 24 silicon wafers for a matrix of postexposure bake ͑PEB͒ time, and temperature conditions. The lithographic patterns included isolated, semidense and dense lines for structures of 0.25, 0.20, 0.175, and 0.15 m nominal size replicated across the exposure field and across the wafer. As a result of this particular metrology, each measured CD was associated with both topological ͑position on the wafer and position within the field͒ and process information ͑exposure dose, PEB time, and temperature͒. Database management techniques were implemented in order to extract and analyze such a massive data set. Process metamodeling ͑PMM͒ was used for the calibration of a PEB model describing the joint effect of photoacid diffusion and photoacid loss, coupled with a deprotection reaction. PMM creates a NN model of the PEB original model ͑a ''model of a model''͒ so that the diffusion coefficient, the acid loss, and the deprotection rates can be estimated by inversion of the NN mapping. The comparisons between experimental and simulated data show excellent agreement that is maintained across the entire process space.

Physical modeling of deprotection-induced thickness loss

“…Current models for PEB and development do not take into consideration this shrinkage when simulating resist profiles. At present, workers at AMD [2] are developing a new methodology for characterizing PEB. Adding shrinkage characterization to the understanding of bake and development will significantly improve our understanding of lithography resist processing.…”