Ion fluxes and energies increase with pressure in EUV-induced H2 plasmas

The next-generation lithography tools currently use Extreme Ultraviolet (EUV) radiation to create even smaller features on computer chips. Wherever the light travels, the high energy photons (92 eV) induce a plasma in the low pressure background gas by photoionization. Industries have realized that these plasmas are of significant importance with respect to machine lifetime, because impacting ions affect exposed surfaces. The mass resolved ion energy distribution function (IEDF) is therefore one of the main plasma parameters of interest.
In this research an ion mass spectrometer is used to investigate mass resolved IEDFs of ions impacting on a surface in EUV-induced plasmas. A xenon pinch discharge produces EUV radiation, which is focused into a measuring vessel with a low pressure hydrogen environment. In this vessel, photoionization creates free electrons with energies up to 76 eV, which further ionize the background gas by electron impact ionization.
The influence of the pressure on plasma composition and IEDFs has been investigated in the range of 0.1 to 10 Pa. In general the ion fluxes towards the surface increase with pressure. However, above 5 Pa the flux of H2+ is not affected by the increase in pressure, which is explained by the balance between the creation of H2+ and the conversion of H2+ to H3+ due to reaction with background gas molecules (H2). These measurement results will be used to benchmark plasma scaling models and verify numerical simulations.