Open issues in mask technology as EUV enters high volume manufacturing

After years of optimistic projections and false starts, 2019 is finally the year that EUV will enter volume production. Mask shop investment in EUV-capable equipment, including writing, inspection, metrology, repair, review and cleaning tools as well as related infrastructure for storage, transportation, and pellicle support has been substantial. However, in both mask shops and wafer fabs, key questions remain unanswered even as high volume manufacturing (HVM) begins in the fab. We will highlight several of these questions that still need to be answered to develop comprehensive, end-to-end strategies for mask inspection, use, and qualification strategies. In particular, we will show how uncertainty over pellicle technology options and timing cascade into a series of questions related to reticle qualification flows throughout the lifetime of a mask. Additional uncertainty comes from the lack of data on reticle contamination mechanisms during use in high-power EUV exposure tools. Concerns over hydrocarbon deposition and reaction with intense EUV photons as well as with the out-of-band DUV present in the system, will require the development of careful monitoring and re-qualification plans. Reticle requalification cycles will be gated not just by the number of wafers exposed, but by the number of times a reticle is loaded and unloaded from the scanner and how long it sits in storage between cycles. We anticipate that a combination of wafer-based and reticle-based inspection will be required to fully ensure reticle quality, especially if a pellicle solution is adopted which does not allow 193nm based inspection. These tradeoffs and uncertainties will be discussed in the context of a full, mask blank to wafer fab reticle qualification strategy for EUV volume manufacturing.