Taoufiq Mouhib

Organic secondary ion mass spectrometry: Signal enhancement by water vapor injection

The enhancement of the static secondary ion mass spectrometry (SIMS) signals resulting from the injection, closely to the sample surface, of H2O vapor at relatively high-pressure, was investigated for a set of organic materials. While the ion signals are generally improved with increasing H2O pressure upon 12 keV Ga+ bombardment, a specific enhancement of the protonated ion intensity is clearly demonstrated in each case. For instance, the presence of H2O vapor induces an enhancement by one order of magnitude of the [M+ H]+ static SIMS intensity for the antioxidant Irgafos 168 and a ∼1.5-fold increase for polymers such as poly(vinyl pyrrolidone).

Improving secondary ion mass spectrometry C60 n+ 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(n+) 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 C60(2+) 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 C60(2+) and, for comparison, an Ar2000(+) ion beam. Our results show a dramatic improvement for depth profiling with C60(2+) using NO at pressures above 10(-6) mbar and sample temperatures below -75 °C. For the multilayered polymer film, the depth profile acquired using C60(2+) 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(n+) 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(n+) sputtering.

Influence of the Organic Layer Thickness in (Metal-Assisted) Secondary Ion Mass Spectrometry Using Ga(+) and C(60)(+) Projectiles.

This article investigates the influence of the organic film thickness on the characteristic and molecular ion yields of polystyrene (PS), in combination with two different substrates (Si, Au) or gold condensation (MetA-SIMS), and for atomic (Ga+) and polyatomic (C60+) projectile bombardment. PS oligomer (m/z ∼ 2000 Da) layers were prepared with various thicknesses ranging from 1 up to 45 nm on both substrates. Pristine samples on Si were also metallized by evaporating gold with three different thicknesses (0.5, 2, and 6 nm). Secondary ion mass spectrometry was performed using 12 keV atomic Ga+ and C60+ projectiles. The results show that upon Ga+ bombardment, the yield of the fingerprint fragment C7H7+ increases as the PS coverage increases and reaches its maximum for a thickness that corresponds to a complete monolayer (∼3.5 nm). Beyond the maximum, the yields decrease strongly and become constant for layers thicker than 12 nm. In contrast, upon C60+ bombardment, the C7H7+ yields increase up to the monolayer coverage and they remain constant for higher thicknesses. A strong yield enhancement is confirmed upon Ga+ analysis of gold-metallized layers but yields decrease continuously with the gold coverage for C60+ bombardment. Upon Ga+ bombardment, the maximum PS fingerprint ion yields are obtained using a monolayer spin-coated on gold, whereas for C60+, the best results are obtained with at least one monolayer, irrespective of the substrate and without any other treatment. The different behaviors are tentatively explained by arguments involving the different energy deposition mechanisms of both projectiles.