Comparison of carbon cleaning mechanisms in EUV-induced and SWD plasmas in H2 and He environments

Cleaning of contamination of optical surfaces by amorphous carbon (a-C) is highly relevant for extreme ultraviolet (EUV) lithography. We have studied the mechanisms for a-C removal from a Si surface. By comparing a-C removal in a surface wave discharge (SWD) plasma and an EUV-induced plasma, the cleaning mechanisms of two different gas environments (hydrogen and helium) were determined. The C-atom removal per incident ion was estimated for different sample bias voltages and ion fluxes. It was found that H2 plasmas generally had higher cleaning rates than He plasmas: up to seven times higher for more negatively biased samples in the EUV induced plasma. Moreover, for H2, EUV induced plasma was found to be 2-3 times more efficient at removing carbon than the SWD plasma. It was observed that the carbon removal under exposure to He in both SWD and EUV induced plasmas is due to physical sputtering by He+ ions. In H2, on the other hand, the increase in carbon removal rates, compared to He plasmas, is due to chemical sputtering. The proposed explanation is strengthened by the observation that lower sample temperatures—allowing longer residence times for adsorbed hydrogen species—results in higher cleaning rates. This is a new C cleaning mechanism for EUV-induced plasma, which we call “EUV-reactive ion sputtering.”