Quantifying low fluence ion implants in diamond-like carbon film by secondary ion mass spectrometry by understanding matrix effects

Abstract

Minor and trace elements in diamond-like carbon (DLC) are difficult to quantify using SIMS analysis because minor elemental and structural variations can result in major matrix effects even across individual, cm-sized samples. While this material is most commonly used for tribological coatings where minor element composition is not of critical importance, it is being increasingly used in electronic devices. However, it is a unique application that spurred this work: anhydrous, tetrahedrally-coordinated DLC (ta-C) was used as a solar wind (SW) collector material in the Genesis solar-wind sample return mission (NASA Discovery 5). So, for ∼15 years, we have been working on attaining accurate and precise measurement of minor and trace elements in the Genesis DLC using SIMS to achieve our mission goals. Specifically, we have learned to deal with relevant matrix effects in our samples, ion implants into ta-C. Our unknown element for quantification is SW Mg, a low-dose (1.67 × 10¹² at cm⁻²; ∼6 μg g⁻¹ ²⁴Mg), low-energy (∼24 keV average energy) implant; our standard is a high-dose (∼1 × 10¹⁴ at cm⁻² of both ²⁵Mg, ²⁶Mg) 75 keV laboratory implant for which the absolute ²⁶Mg/²⁵Mg ratio had been measured to account for variable instrumental mass fractionation. Analyses were performed using O₂⁺ primary ions having both a low impact energy and a current density of ∼2 × 10¹⁴ ions per cm². Although our unknown was solar wind, the method is applicable to many situations where minor elements in DLC need to be quantified. Recommendations are presented for modifying this data-reduction technique for other SIMS conditions.