The surface cleaning of EUV optic material by plasma exposure experiment (SCOPE) has been designed to provide fundamental data on Li and Sn interaction with various sample materials, including grazing incidence optic materials. SCOPE is a multifunction tool capable of studying ion implantation and diffusion and sputtering yields as a function of incident angle, energy, and sample temperature through the use of a stable Li+ ion gun capable of current densities of 15-50 mA/cm2 with a working distance of 15-40 mm for incident energies of ranging from 100-3000 eV and spot sizes can be adjusted from 50-1000 nm. The ion beam is characterized through the use of an integrated Faraday cup. Sputtering yields and energy distribution of sputtering particles can also be measured through the use of an electrically isolated water cooled quartz crystal microbalance (QCM). SCOPE is also capable of studying cleaning of grazing incidence optical samples through the use of an electron beam evaporator to deposit Li and Sn thin films on samples that undergo plasma exposure. Thin film deposition is monitored through the use of a QCM and a definitive study of that plasma can be accomplished. SCOPE was built on an accelerated time frame to provide a way to study the interaction of energetic ions with grazing incidence optic materials and plasma interaction with these optics so as to develop a viable mitigation scheme necessary for the continued development of EUV lithography.
One of the critical technological challenges of implementation of EUV lithography with 13.5 nm photons is the degradation of the collector optic material from the particle emission that results from the plasma generation of EUV photons at 13.5 nm from either a dense plasma pinch or a laser plasma pinch. Xe was initially used to produce this pinch, but the need to reach higher source power levels facilitated the need to move alternative fuels of Li and Sn because of their higher conversion efficiency.
During the plasma pinch, energetic ions and sputtered neutrals are produced and move in all directions. A fraction of these particles will then interact with the collector optics. Low energy atoms from sputtered materials or condensable fuels can deposit on the collector mirror optics while higher energy ions and charge-exchanged neutral atoms can sputter, implant, and roughen the optics. These various interactions will lead to a reduction in reflectivity of the condenser optic and reduce the overall efficiency of the EUV light source.
Particle production can be limited to a certain extent, but it is unreasonable to expect this production to go to zero. As such, mitigation schemes and cleaning protocols of the grazing incidence optic materials must be developed so as to prolong the life of the optics as much as possible to make high volume manufacturing economically viable.
Therefore, understanding the various particle interactions with the grazing incidence optic material is crucial to the development of optic cleaning protocols. Thus, a new facility, the surface cleaning of EUV optics by plasma exposure (SCOPE) at the University of Illinois has been constructed for this purpose. SCOPE is currently designed to study the interaction of energetic ions from an ion gun with grazing incidence optic material at various ion energies, incident angle, and temperature of the optic material. SCOPE is also setup to study low energy atomic interaction of material from an electron beam evaporator source with grazing incidence optic material at various incident angle and temperature of the optic material. Lastly, the SCOPE facility is capable of studying plasma interaction with the optic material alone or in combination with the ion gun and electron evaporator source. This work describes the experiment, its major components as well as present relevant data on tests of the individual components of SCOPE.
SCOPE consists of several systems that work together to ultimately investigate and measure fundamental particle interactions with the grazing incidence optic material that is necessary for the development of optical cleaning protocols. These integrated systems inside a vacuum chamber include a programmable heated sample holder, an ion gun (IG) with Faraday cup, an electron beam evaporation system (E-VAP), a quartz crystal microbalance (QCM), a helicon plasma source, a RF compensated Langmuir probe, and a residual gas analyzer (RGA).