The electron density in an EUV-generated plasma in hydrogen

The high energy photons (92 eV) of EUV radiation are partially absorbed by the low-pressure (0.1–10 Pa) transparent background gas (e.g. H2 and He) in the lithography machine. Since the typical ionization energy of the gas is about 15 eV, the absorbed photons will ionize the gas. Therefore a plasma is generated everywhere the EUV beam travels. If this plasma is in the vicinity of a surface, a plasma sheath is formed, which generates strong electric fields. These strong fields accelerate ions up to high velocities towards delicate surfaces, e.g. multilayer mirrors. If these ions gain enough energy, they may pose a potential threat to the expensive mirrors. Therefore, the characterization of the extreme ultra-violet induced plasma is essential. In this research the electron density is measured in an EUV-generated plasma in argon and hydrogen with microwave cavity resonance spectroscopy (MCRS). The resonant frequency depends on the permittivity of the medium inside the cavity. The EUV beam creates free electrons, which change the permittivity and thus the resonant frequency shifts. If we measure this shift, we can determine the electron density. The electron density is measured as a function of argon/hydrogen pressure and the repetition frequency of the source.