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Abstract

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a versatile analytical method widely used in the field of surface science and thin films. Although it gives elemental, molecular, and isotopic information, it has a very low detection limit and high lateral resolution, is fast and works with all the vacuum-compatible samples, and can be used for the analysis of the topmost few atomic monolayers, depth profiling, and imaging, it also has some limitations. One of the limitations is the matrix effect which is the nonlinear effect of the substrate containing the compound of interest on the intensity of the secondary ion signals from this compound. The effect is strong enough to significantly influence detection limits, prevent quantitative analysis, and even cause problems in differentiation between similar compounds.
Different approaches to matrix effect reduction were developed because of its significance, and in this study, we studied gas-flooding. The introduction of the reactive gases in the analytical chamber during the SIMS analysis notably changes the ionization yields of secondary ions and causes the formation of cluster secondary ions. However, most of the previous research was done with the O2 while the use of other reactive gases was almost never considered. In the first part of our study, we studied the influence of the H2, C2H2, CO, and O2 flooding during depth profiling of different thin metal, metal oxide, and alloy layers. The application of the H2, C2H2, and CO is a novelty in the field of SIMS studies and was introduced by our research group. The largest improvements compared to the ultra-high vacuum (UHV) were observed when analyses were done in the H2 atmosphere. Differentiation of layers became easier, and their identification unambiguous, interfaces were sharper, and depth resolution improved. There was even no decrease in the sputter rate which can be observed in the cases of all the other gases. An improved analysis was possible via measurements of negative metal (Mn-), metal hydride (MnHm-), and metal oxide (MnOm-) cluster secondary ions. H2 flooding adequately optimized ToF-SIMS depth profiling in a way that this approach can be applied for routine analyses.
We further investigated processes occurring during sputtering in UHV and H2 atmosphere. Atomic force microscopy (AFM) measurements of surface topography confirmed that some of the improvements in the field of SIMS depth profiling are caused by the reduced surface roughening observed in the H2 atmosphere. Namely, the continuous removal of material caused by the prolonged bombardment with primary ions, that is, the process taking place during depth profiling, causes the initially smooth surface to develop a higher degree of surface roughness. This process is in many cases reduced if H2 flooding is applied instead of the UHV environment. We also observed that samples with initially rough surfaces can become smoother after sputtering with the 1 keV Cs+ ion beam.
In the final study, we compared the quantification capabilities of the SIMS method to measure the chemical composition in UHV, O2, and H2 atmospheres. Pure metals and different alloys containing these metals were included in the analysis. The results indicate that the UHV environment offers the worst conditions for potential quantification. Flooding with O2 improved results significantly, but deviations of measured intensities of SIMS signals from true values of chemical composition were still non-optimal. Improvements achieved with the H2 atmosphere provided additional optimization especially when analyzing transition metals, which in our study were Ti, Cr, Fe, Co, and Ni. Analyses of these transition metals in H2 showed deviations from the true atomic ratio in alloys of only 46% at maximum. O2 atmosphere and UHV environment gave deviations of 66 and 228%, respectively.
Our findings indicate that gas adsorbs to the surface and forms a new matrix, which reduces the differences between initial chemical environments and electronic structures of the surface. The quantitative aspects of the SIMS method can be due to a new matrix at the surface improved, especially with the help of H2 flooding.