Improvement of the gas cluster ion beam-(GCIB)-based molecular secondary ion mass spectroscopy (SIMS) depth profile with O2+ cosputtering

Abstract

Over the last decade, cluster ion beams have displayed their capability to analyze organic materials and biological specimens. Compared with atomic ion beams, cluster ion beams non-linearly enhance the sputter yield, suppress damage accumulation and generate high mass fragments during sputtering. These properties allow successful Secondary Ion Mass Spectroscopy (SIMS) analysis of soft materials beyond the static limit. Because the intensity of high mass molecular ions is intrinsically low, enhancing the intensity of these secondary ions while preserving the sample in its original state is the key to highly sensitive molecular depth profiles. In this work, bulk poly(ethylene terephthalate) (PET) was used as a model material and analyzed using Time-of-Flight SIMS (ToF-SIMS) with a pulsed Bi₃²⁺ primary ion. The optimized hardware of a 10 kV Ar₂₅₀₀⁺ Gas Cluster Ion Beam (GCIB) with a low kinetic energy (200–500 V) oxygen ion (O₂⁺) as a cosputter beam was employed for generating depth profiles and for examining the effect of beam parameters. The results were then quantitatively analyzed using an established erosion model. It was found that the ion intensity of the PET monomer ([M + H]⁺) and its large molecular fragment ([M − C₂H₄O + H]⁺) steadily declined during single GCIB sputtering, with distortion of the distribution information. However, under an optimized GCIB-O₂⁺ cosputter, the secondary ion intensity quickly reached a steady state and retained >95% intensity with respect to the pristine surface, although the damage cross-section was larger than that of single GCIB sputtering. This improvement was due to the oxidation of molecules and the formation of –OH groups that serve as proton donors to particles emitted from the surface. As a result, the ionization yield was enhanced and damage to the chemical structure was masked. Although O₂⁺ is known to alter the chemical structure and cause damage accumulation, the concurrently used GCIB could sufficiently remove the surface layer and allow the damage to be masked by the enhanced ionization yield when the ion-solid interaction volume was kept shallow with a low O₂⁺ energy. This low O₂⁺ energy (200 V) cosputtering also produced a smoother surface than a single GCIB. Because the oxidized species were produced by O₂⁺ and removed by GCIB simultaneously, a sufficiently high O₂⁺ current density was required to produce adequate enhancements. Therefore, it was found that 10 kV with 2 × 10⁻⁶ A per cm² Ar₂₅₀₀⁺ and 200 V with 3.2 × 10⁻⁴ A per cm² O₂⁺ produced the best profile.