Over the last decade, cluster ion beams displays its capability for depth profiling organic materials and biological specimens. Compared with monatomic ion beams, cluster ion beams possess non-linear enhancement of sputter yield, minimum damage accumulation and generate high mass fragments during sputtering. These properties allow the successful secondary ion mass spectrometry (SIMS) analysis of soft materials beyond the static limit. While the intensity of molecular ions of high mass is still low, enhancing the intensity of these secondary ions while preserve samples in their original state is the key to high sensitivity molecular depth profiles. In this work, bulk poly(ethylene terephthalate) (PET) was used as modeling materials and was analyzed using a time-of-flight (ToF) SIMS with a pulsed Bi3++ primary ion. 10 kV Ar2500+ gas cluster ion beam (GCIB) and low kinetic energy (50-500 eV) oxygen ion beam (O2+) were employed to co-sputtering for depth profile and examining the effect of beam parameters to the yield of positive secondary ions. The result of depth profile showed that the characteristic signal of PET declines steadily during GCIB sputtering. In order words, damage accumulation was observed. On the other hand, the secondary ion intensity can reach a steady state and was enhanced during GCIB-O2+ cosputtering. Since the secondary ions were generated by a fixed Bi3++ bombardment and was independent to the sputtering beam parameter, this enhancement of intensity is attributed to the enhanced ionization yield. This enhanced ionization yield is attributed to the oxidation of molecules and the formation of hydroxyl group that serve as a proton donator to particles emitted from the surface. The enhanced ion intensity also masked the damage to the chemical structure hence steady signal intensity is obtained. However, as O2+ is known to alter chemical structure and damage accumulation is apparent beyond the static limit, high fluence leads to the suppression of molecular ion intensity. Nevertheless, with optimized Ar2500+ and O2+ ratio that leads to enhanced ionization yield while the damage is masked, improved depth profile is obtained. The results indicated that optimized GCIB-O2+ cosputtering could provide improved depth profiles from soft materials in terms of sensitivity and preserve the chemical structural in the remaining surface.

Enhancement of molecular-ion intensity by Ar2500+ -O2+ cosputtering for depth profiling of soft materials in secondary ion mass spectrometry

249th National Meeting (2015)

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