Article

Structural Assessment of (Sub-)Monolayer Coatings in Device Processing at High Spatial Resolving Power by TOF-SIMS Tandem MS Imaging

Surface Analysis Spotlight: TOF-SIMS

by Jacob Schmidt Staff Scientist

Surface characterization is essential for the advancement of electronic and heterogeneous devices, yet confidently assigning spectral features remains a persistent challenge. This difficulty often stems from a lack of comprehensive background data and the absence of reliable reference spectra, particularly when dealing with complex or novel materials. The work presented here demonstrates how tandem mass spectrometry, when used in conjunction with time-of-flight secondary ion mass spectrometry (TOF-SIMS), serves as a powerful analytical approach.

One example of using tandem MS involves the investigation of a polymer-coated surface that experienced a blooming event—an unexpected surface accumulation of material that can affect performance and appearance. To identify the compound responsible, TOF-SIMS analysis revealed a prominent ion at m/z 284. Assignment of the peak at m/z 284 using standard chemical formula matching algorithms is challenging due to the large number of potential formulae. By employing tandem MS, researchers were able to fragment the ion at m/z 284 and analyze its product ions. Furthermore, the spatial distribution of the product ions can be monitored, as shown in Figure 1. The distributions of two fragment ions in particular showed complementary patterns, indicating the presence of two separate compounds sharing the same precursor mass. The ability to distinguish these species using tandem MS was critical for accurately identifying the bloom components and understanding their origin within the polymer system.

Figure 1 –MS¹ and MS² spectra and images of a polymer coated surface. On the bottom, standard TOF-SIMS analysis accurately identifies low molecular weight ions. On the top, tandem MS is used to identify two separate components both with a precursor mass of 284 m/z.

In a second example, TOF-SIMS tandem MS imaging was used to confirm the presence and determine the structure of metal-organic ligands incorporating Au, Pd, or Pt atoms bound to bipyridine or triphenylphosphine groups within patterned monolayer films. Metal loading was performed after immobilization of the monolayer, and the patterned regions were clearly distinguished by their characteristic atomic metal ion signals. Several precursor ions observed in the MS1 spectra fell within the expected m/z range for the target metal-ligand complexes, suggesting possible candidates. Tandem MS imaging enabled both structural elucidation of the complexed bipyridine and triphenylphosphine ligands and visualization of their two-dimensional spatial distributions. Additionally, the technique allowed for confirmation of the metal identities through their distinct natural isotope patterns, further supporting the successful formation of the desired metal-organic structures.

Figure 2 – Tandem MS spectrum of a selected region of interest of the bipyridine patterned monolayer film. Isotopic analysis was used to confirm the presence of Pt and Cl within the metal-ligand complex.

Gain deeper insights into TOF-SIMS tandem MS imaging by joining PHI at the North American SIMS conference, held June 8-12th, on the campus of California State University Northridge in Northridge California, where Dr. Jacob Schmidt will present this information at the poster session.