Topography in secondary ion mass spectroscopy images

Abstract

Interpretation of time-of-flight secondary ion mass spectroscopy (TOF-SIMS) images from rough samples such as particles, fibers, or biological specimens can be problematic because the images are influenced not only by the sample chemistry but also by topographical features. In this article we have investigated the influence of spherical and cylindrical features on total ion yields, relative ion yields, and feature shape. TOF-SIMS images of Pluronic coated fibers and polystyrene spheres were collected using both triple focusing time and reflectron geometry instruments and a 25keV Ga+ primary ion source. The fibers and spheres were analyzed on both conducting and insulating substrates to assess the importance of field effects. Trends in the images have been explored using principal components analysis and Poisson and multinomial mixture models. The T2 test was employed to assess the statistical significance of results. The results identify three important topographic effects. The size and shape of features can be distorted as a result of the incidence angle of the primary ion beam. Additionally, both the absolute and relative intensities of ion peaks vary as a result of topography. In regions where the primary ion beam impacted the sample at a glancing angle, the relative intensity of molecular fragments characteristic of the Pluronic surfactant was up to three times higher than in regions where the beam impacted the sample at a normal angle. Comparison of results from conducting and insulating samples suggests that changes in the relative ion yields resulted primarily from differences in the incidence angle of the primary ion beam while changes in the total ion yield are influenced by both the incidence angle and distortion of the electric field by the particle. This study documents that topographic features can influence not only the absolute intensity of ion peaks but can also alter peak ratios in a statistically significant manner. In this light, a greater degree of caution is recommended when interpreting TOF-SIMS images from topographically complex samples.

Topography and Field Effects in Secondary Ion Mass Spectrometry – Part I: Conducting Samples

“…However, significant topographic features in the scale of tens or even hundreds of microns are commonly encountered on these samples. This can cause many unwanted artefacts in SIMS spectra and images [1][2][3][4], which significantly hinder data interpretation and quantification. As a result, chemical characterization of surfaces with microscale topography remains a significant challenge due to the lack of systematic and validated measurement methods.…”

Mass spectrometry imaging of mating Tetrahymena show that changes in cell morphology regulate lipid domain formation

“…This variation may reflect signal fluctuations on the basis of both topography and the local chemical matrix. Normalization of the signal of interest to the total ion signal accounts for these factors so that the signal more accurately represents chemical concentration (25)(26)(27). A representative cell pair imaged at 1 h is shown in the first column of Fig.…”

“…Although this might be extremely exciting, these (25)(26)(27) and exhibited by the normalized PC images (Fig. 4Q-T) show that normalization to total counts is a useful approach to use SIMS imaging to investigate concentrations of surface molecules.…”

Topography and field effects in secondary ion mass spectrometry Part II: insulating samples

“…[1][2][3][4][5] Topography in the micrometer scale is common for many industrially relevant samples, including microelectronic or microfluidic devices, medical devices, organic electronic displays and both hair and fibres in the personal care industry. There are many aspects where the analysis of topographic samples is different from that for a flat uniform surface.…”