For extreme ultraviolet lithography (EUVL) to become a high volume manufacturing technology for integrated
circuit manufacturing, the cleanliness of the system, especially the photomask, is of high importance. For EUV
photomasks, which cannot be protected from contamination by the use of a pellicle, an effective and quick
cleaning technology needs to be ready in order to maintain wafer throughput. There are challenges to extend
current wet cleaning technologies to meet the future needs for damage-free and high efficiency mask cleaning.
Accordingly, a unique process for cleaning particulates from surfaces, specifically photomasks as well as wafers,
has been evaluated at the University of Illinois Urbana-Champaign. The removal technique utilizes a high density
plasma source as well as pulsed substrate biases to provide for removal. Helium is used as the primary gas in the
plasma, which under ionization, provides for a large density of helium metastable atoms present in the plasma.
These metastable helium atoms have on the order of 20 eV of energy which can transfer to particles on the
substrate to be cleaned. When the substrate is under a small flux of ion bombardment, these bonds then remain
broken and it is theorized that this allows the particles to be volatilized for their subsequent removal. 100 %
particle removal efficiency has been obtained for 30 nm, 80 nm, and 200 nm polystyrene latex particles. In
addition, removal rate has been correlated with helium metastable population density determined by optical