Ion fluxes in EUV-induced plasma and their applications for optical components tests

In modern Extreme Ultra-violet (EUV) lithography machines, sensitive optical components, like multi-layer mirrors and photomasks, may be affected by plasma interactions. The new 13.5 nm research and testing system, EUV-Beam-Line 2 (EBL2), designed to provide accelerated tests for next generation lithography, is used to investigate EUV-induced plasma phenomena. Ions, radicals and secondary electrons emitted from the plasma may influence the lifetime and the performance of contact-sensitive components of the lithography optics such as multi-layer mirrors, photomasks and pellicles. First systematic measurements of ion fluxes produced in an EUV-induced hydrogen plasma are reported, with operating conditions including 5 and 20 Pa gas pressure, 3 kHz EUV pulse repetition rate and 4.2 W total EUV beam power produced in a 10-15 ns EUV pulse. Space- and time-resolved distributions of ion fluxes and ion energies were measured using a retarding-field ion energy analyzer mounted next to the EUV beam. Typical ion energies were in the range of 1-8 eV and typical ion fluxes were in the range of 2-8·10¹⁷ ions·m^-2s^-1. The obtained ion fluxes are applied in a photomask lifetime test to understand the material effects after an EUV exposure.