Leveraging sub-EO dose assessment methodology to improve EUV lithography cluster dose performance

摘要

The effective dose delivered by an EUV lithography cluster is composite function of the dose provided by the scanner EUV radiation source and illuminator, the reflectance of the EUV mask, the transmission of the scanner projection optics and the bake conditions experienced by the EUV sensitive imaging resist on the wafer processing track. Open frame test wafer exposures and the sub-EO analysis technique described at SPIE2018 have been adopted to characterize and monitor the impact of the factors above on the effective dose stability and uniformity. Wafer exposure sequences and layouts, and the details of the analysis methodology were customized to study adverse dose factors in each of the areas described above. We will describe the wafer processing details, experimental plans and analysis strategies that have enabled us to monitor, modulate and ultimately improve the overall lithography dose performance. Periodic sub-EO monitors have been employed to characterize and enhance medium and long term scanner dose stability. Controlled exposure sequences have revealed the accretion of dose reducing contamination on reflective EUV masks. Subsequent analysis identified this as carbon, which can accumulate via radiation assisted deposition under low dose conditions and we have explored potential mitigation. Modification of the sub-EO experimental conditions have enabled assessment of the long range stray light or flare dosage that results from radiation scattered in the scanner projection optics. The Sub-EO methodology was also adapted to probe process delay effects and this led to the definition of dose stability enhancing cluster protocols.