Detection and Characterization of Plasma-Generated Stannane: Influence of Surface Composition on Species Formation

In extreme ultraviolet (EUV) light sources for lithography, tin droplets are used as target material and evaporate during plasma generation, leading to tin deposition on surrounding surfaces. A widely used method to remove surface-bound tin involves exposure to hydrogen plasma, resulting in the formation of stannane (SnH₄). As a volatile compound, stannane is efficiently pumped out of the system; however, it may also decompose upon contact with surfaces, potentially leading to secondary tin contamination. Despite its relevance in this field, the pathways of stannane formation and decomposition remain only partially understood. Owing to the near absence of mass spectrometric data, comprehensive reference measurements had to be established as part of this study. Mass spectrometry proves to be a suitable technique for the analysis and characterization of tin hydrides, as previously outlined. In order to better understand the etching mechanism of elemental tin exposed to a hydrogen plasma, a laboratory-scale experiment was performed to identify and characterize both neutral and ionic tin hydride compounds. All ten natural isotopes of tin, along with tin hydride fragments, are exhibited by the experimental mass spectra. These species are observed as both native ions, which are formed directly in the plasma, and secondary products, which are generated through electron ionization within the ion source. The relative distribution of the fragments is calculated from the isotopically superimposed mass signals using the custom analysis program RASP. The results of the experimental analysis demonstrate the notable presence of oxygen-containing species in the plasma-generated stannanes. It is of particular significance that our findings underscore the importance of surface reactions in the formation of these oxygenated species, offering valuable insights into the complexities of plasma-surface interactions.