Achieving Extra-High-Quality Images in ToF-SIMS Molecular Imaging of Insulating Samples Using a Low Current O₂⁺ Auxiliary Beam and Back Side Au Coating

Molecular imaging enabled by ToF-SIMS has become increasingly desirable in many research fields in the last several decades. However, complex charging on highly insulating samples and at phase-separation boundaries may lead to dark edges and boundaries in ion images, presenting a big obstacle to obtaining high quality ion images, even with low energy electron charge compensation. Depositing a thin metal layer on the sample surface or using a combination of a low energy electron beam and a low energy Argon ion beam can be helpful. However, surface metal deposition may lead to serious contamination, and the extra low energy argon ion gun is not available in most ToF-SIMS instruments. In this work, we report that a combination of a low current O₂⁺ ion beam and a low energy electron beam can be used to resolve complex charging conditions and yield extra-high-quality ion images. A sucrose thin film sample was used as a model system to show that molecular imaging with this approach can be very feasible due to minimal beam-induced damage. For highly insulating samples, back side Au coating can be further helpful when combined with a low current O₂⁺ ion beam. A few representative real-world samples were examined to verify the effectiveness of our new method. Additionally, the principle of our new method was also discussed. Since an O₂⁺ ion source is regularly available in most ToF-SIMS instruments, we believe our new method will be widely used in ToF-SIMS imaging and spectral analysis of various insulating samples.