Repeatability and Intermediate Precision of the Relative-Intensity Scale in Static TOF-SIMS

Static Time-of-Flight Secondary-Ion Mass Spectrometry (TOF-SIMS) is a highly-sensitive surface analysis technique commonly used to identify materials or substances at surfaces and to quantify further these sought components. Quantification is usually achieved through measuring relative peak intensities and by comparing these with appropriate peaks from reference materials. Repeatability and intermediate precision are important for these measurements in order to determine if there are significant variations in the analyses. In this study, two important instrumental contributions to the uncertainty of static SIMS intensity measurements were assessed: (a) the repeatability; and (b) the intermediate precision of positive-ion relative-intensity measurements. With negative ions, the control of the surface potential for insulators is not yet sufficiently good for high repeatability. Validation was done using ISO 23830:2008(R2017) and poly(tetrafluoroethylene) (PTFE) tape as reference material. Emission current of Bi gun was maintained at 0.80±0.01 μA, with pulsed target current measurements between 0.5000 pA to 1.000 pA. Parametric conditions applied were: (a) 500×500 raster area (μm2); (b) random mode; (c) 128×128 raster size (pixel); and (d) 5×1011 dose density (ions/cm2), in order to achieve static mode. The analysis was also assisted with an electron flood gun for charge compensation. Calculated SIMS relative-intensity repeatability were 1.3533%, 1.4584%, 1.8310%, and 0.6112%, for each of the participant, signifying that the method passed the 3% repeatability limit and is fit for purpose. Intralaboratory comparison showed that intermediate precision is also within the acceptable range both for the lighter ionic masses (A1/A2 values within ±2σ) and the heavier ionic masses (A3/A2 values within ±2σ).