A novel ToF-SIMS operation mode for improved accuracy and lateral resolution of oxygen isotope measurements on oxides

Abstract

Oxygen isotope exchange with subsequent time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a highly valuable tool for determining oxygen diffusion coefficients in oxides. Since ToF-SIMS analysis enables an elemental and chemical mapping, it can also be used to visualize oxygen exchange-active zones by determining the local oxygen isotopic fraction. However, measuring accurate isotopic fractions can be a challenging analytical task owing to secondary ion interaction and signal saturation, particularly when dealing with high secondary ion intensities as commonly found when analyzing oxygen ions from oxides. It is shown that in many cases the calculated ¹⁸O⁻ fraction erroneously shifts to higher values and can lead to systematic errors in the determination of diffusion coefficients. A novel ToF-SIMS operation mode, called “Collimated Burst Alignment” (CBA) mode, is therefore introduced to enable a more accurate determination of oxygen isotopic fractions with an optimized lateral resolution of sub-100 nm. Both improvements are rendered possible by a modified beam guidance in the primary ion gun. This modification reduces detector dead time effects and ion interactions to a minimum and secondary ion intensities can be obtained more accurately. The result of this optimization is demonstrated in measurements of the natural isotope abundance of several different oxides including SrTiO₃ and Sr-doped LaCoO₃.