Time-resolved ion energy distribution functions in the afterglow of an EUV-induced plasma

Since the introduction of extreme ultraviolet (EUV) lithography (EUVL), the inevitable presence of EUV-induced plasmas inside the lithography tools impacts the operation of EUV optical components. EUV-induced plasmas are created everywhere in the optical path due to the ionizing interaction between the high energy (92 eV) EUV photons and the tools' background gas, which typically is hydrogen gas at a pressure of 1-10 Pa. From a physical point of view, the main impact of the plasma is due to the presence of ions that imping the plasma-facing surfaces. Experimental research into the fluence and energy distribution functions (IEDFs) of ions from EUV-induced plasmas has been limited to time-averaged measurements. In this Letter, we present time-resolved measurements of IEDFs for H+, H-2(+), and H-3(+) ions from an EUV-induced plasma in pure hydrogen gas. To this end, an electrostatic quadrupole plasma (EQP) analyzer has been used. The measurements pinpointed momentary fluxes up to three orders of magnitude higher than earlier reported average ion fluxes. In addition, the mean ion energy was unexpectedly found to remain elevated up to 50 mu s after the gas had been irradiated with EUV photons. Also, it was shown that the EQP detects H-2(+) ions on time scales much larger than expected. The presented results are valuable not only for the understanding of elementary processes regarding EUV-induced plasmas interacting with surfaces but also for simulating and predicting the impact of EUV-induced plasma on the lifetime and stability of optical components in EUVL.