Sputtering Yields for Mixtures of Organic Materials Using Argon Gas Cluster Ions

The sputtering yield volumes of binary mixtures of Irganox 1010 with either Irganox 1098 or Fmoc-pentafluoro-l-phenylalanine (FMOC) have been measured for 5 keV Ar₂₀₀₀⁺ ions incident at 45° to the surface normal. The sputtering yields are determined from the doses to sputter through various compositions of 100 nm thick, intimately mixed, layers. Because of matrix effects, the profiles for secondary ions are distorted, and profile shifts in depth of 15 nm are observed leading to errors above 20% in the deduced sputtering yield. Secondary ions are selected to avoid this. The sputtering yield volumes for the mixtures are shown to be lower than those deduced from a linear interpolation from the pure materials. This is shown to be consistent with a simple model involving the changing energy absorbed for the sputtering of intimate mixtures. Evidence to support this comes from the secondary ion data for pairs of the different molecules. Both binary mixtures behave similarly, but matrix effects are stronger for the Irganox 1010/FMOC system

Ambient Surface Mass Spectrometry Using Plasma-Assisted Desorption Ionization: Effects and Optimization of Analytical Parameters for Signal Intensities of Molecules and Polymers

Results are presented on the optimization and characterization of a plasma-assisted desorption ionization (PADI) source for ambient mass spectrometry. It is found that by optimizing the geometry we can increase ion intensities for valine and by tuning the plasma power we can also select a more fragmented or less fragmented spectrum. The temperature of the surface rises linearly with plasma power: at 19 W it is 71 °C and at 28 W it is 126 °C. To understand if the changes in signal intensity are related to thermal desorption, experiments using a temperature-controlled sample stage and low plasma power settings were conducted. These show markedly different signal intensities to experiments of equivalent surface temperature but higher plasma power, proving that the mechanisms of ionization and desorption are more complicated than just thermal processes. Four different polymers, poly­(methyl methacrylate) (PMMA), poly­(ethylene terephthalate) (PET), poly­(lactic acid) (PLA), and poly­(tetrafluoroethylene) (PTFE), are analyzed using PADI. Mass spectra are obtained from all the polymers in the negative ion mode and from PMMA and PLA in the positive ion mode. For each polymer, characteristic ions are identified showing the ability to identify materials. The ions are formed from bond cleavage with O and CH₂ as common adducts. Ions were detected up to <i>m</i>/<i>z</i> 1200 for PTFE

Improving Secondary Ion Mass Spectrometry C₆₀^<i>n</i>+ Sputter Depth Profiling of Challenging Polymers with Nitric Oxide Gas Dosing

Organic depth profiling using secondary ion mass spectrometry (SIMS) provides valuable information about the three-dimensional distribution of organic molecules. However, for a range of materials, commonly used cluster ion beams such as C₆₀^<i>n</i>+ do not yield useful depth profiles. A promising solution to this problem is offered by the use of nitric oxide (NO) gas dosing during sputtering to reduce molecular cross-linking. In this study a C₆₀²⁺ ion beam is used to depth profile a polystyrene film. By systematically varying NO pressure and sample temperature, we evaluate their combined effect on organic depth profiling. Profiles are also acquired from a multilayered polystyrene and polyvinylpyrrolidone film and from a polystyrene/polymethylmethacrylate bilayer, in the former case by using an optimized set of conditions for C₆₀²⁺ and, for comparison, an Ar₂₀₀₀⁺ ion beam. Our results show a dramatic improvement for depth profiling with C₆₀²⁺ using NO at pressures above 10^–6 mbar and sample temperatures below −75 °C. For the multilayered polymer film, the depth profile acquired using C₆₀²⁺ exhibits high signal stability with the exception of an initial signal loss transient and thus allows for successful chemical identification of each of the six layers. The results demonstrate that NO dosing can significantly improve SIMS depth profiling analysis for certain organic materials that are difficult to analyze with C₆₀^<i>n</i>+ sputtering using conventional approaches/conditions. While the analytical capability is not as good as large gas cluster ion beams, NO dosing comprises a useful low-cost alternative for instruments equipped with C₆₀^<i>n</i>+ sputtering