Investigation of molecular weight effects of polystyrene in ToF-SIMS using C60 and Au primary ion beams

In secondary ion mass spectrometry, polyatomic primary ion sources are known to enhance yields from many surfaces including polymers. In order to understand the fundamental causes for these increases, the enhancement as a function of material type and molecular weight needs to be delineated. In this article, we report results from a systematic investigation of polymeric films of polystyrene (PS) with varying molecular weights to examine the influence of the primary ion beam on the secondary ion yields in time of flight secondary ion mass spectrometry (ToF-SIMS). The masses of the polymers investigated ranged from 1000 to 20,000 Da, or from about n = 10 to 200 where n indicates the number of polymeric units in a polymer chain. The polymers had a narrow molecular weight range (PDI < 1.07). The multilayer polymeric films (10-30 nm) characterized by AFM were prepared by spin-casting onto silicon substrates and were analyzed using Au⁺ and C₆₀⁺ primary ion beams. The analysis with the two beams provided a useful comparison between atomic and polyatomic primary ion sources. Information gathered from this study provides insight into the role of molecular weight on the observed yield enhancement from polyatomic ion sources.