Biomolecular & Analytical Mass Spectrometry

Depth profiling: ultra low angle microtomy

Molecular depth profiling by ion beam erosion still deals with some bottlenecks such as the depth dependence of the SI yield, sputter induced topography, anisotropic erosion, limited depth range and extremely low sputter rate for some materials. To avoid these problems, a complementary method is developed.

The combination of ultra low angle microtomy (ULAM) with TOF-S-SIMS surface analysis of the section allows to characterise the molecular 3D distribution over a larger depth range or to investigate buried interfaces.

Example: UV inkjet single dots

Single inkjet dots with a diameter of 100 µm and height of 22 µm, jetted on a PET substrate with primer for better adhesion, have been used as a test system for the methodology. Samples were cut at an angle of 0,5° using a rotary microtome equipped with tungsten carbide knife. Images of the single dots are taken with Bi₃⁺ in “burst alignment mode” with a beam spot of 300 nm.

The figure below (bottom left) shows the molecular distribution of the pigment at a depth of ± 3 µm. It is clear that the pigment is surface-enriched at the top of the inkjet dot. At a depth of ± 20 µm the pigment image (figure below, bottom right) reveals a central depletion zone, which can be caused by a substrate-driven redistribution. Problems such as topography development and depth dependence of the SI yield with direct sputter beam depth profiling are avoided by the ULAM-S-SIMS methodology.