Proceedings Article | 21 March 2008
KEYWORDS: Lithium, Plasma, Dielectrophoresis, Sputter deposition, Helium, Extreme ultraviolet, Ions, EUV optics, Chemical species, Liquids
The selection of extreme ultraviolet (EUV) light as the next generation lithography light source as been a
natural progression of the development in the semiconductor industry based on past history of chip size and the
projection of where chip sizes will go. The sources of EUV light convert about 1-4% of net deposited plasma energy
into photons between 13-14 nm, the remaining energy generates out of band radiation and the production of highly
energetic ions and neutrals in the dense hot plasma that move outwards in all directions. A fraction of these ions strike
electrode surfaces, injection nozzles, and the vacuum chamber producing low-energy sputtered atoms termed debris.
There are limitations to various mitigation schemes and some debris will still reach the collector optics. As such, the
mechanism and ability of Li removal from the collector like optics needs to be studied and modeled to be able to provide
a predictive value for lifetime of the optics. This removal can be accomplished with the presence of a secondary helium
plasma which can selectively sputter lithium from the collector optic surface while providing high EUV photon
transmission and have the advantage of being a nondestructive in situ cleaning method for the collector optics. This will
allow development of long lasting collector optics and operating regimes in addition to expanding the knowledge base
about lithium transport and interaction.
First, a lithium magnetron source was developed to provide low energy lithium debris like that which is present
in EUV sources. This magnetron was plasma was characterized through the use Langmuir probe analysis to yield a
mapping of the temperature and density of the plasma, in addition to the ionization fraction. From here, a secondary
helium plasma source was developed, employed, and studied in the same manner to also provide information on the
electron density, temperature, and ionization fraction so as to accurately model and measure the deposition flux of
lithium and sputter flux of helium on the sample surface. The model correlated well to experimental observations. The
development of a model of simultaneous lithium deposition and evaporation in the presence of a secondary plasma held
below the lithium sputtering threshold was well correlated to experimental observations.
Finally, the simultaneous process of deposition, evaporation, and sputtering of lithium was modeled and
corroborated with experimental observations such that no net deposition on the collector optic was measured. This lends
the model to being available model was developed that can be used for predictive scenarios involving deposition,
evaporation, and sputtering are undertaken.
This work, while relevant for the commercial EUVL community, is relevant in physics for applications
involving plasma physics, plasma material interactions, magnetron sputtering, solid metal sputtering, liquid metal
sputtering, liquid metal evaporation, soft x-ray optics, and modeling of these interactions.
