Sputtering, SIMS matrix effect

Let us begin by looking at the parameters that make SIMS measurements difficult to quantify. First, the secondary ion yield (defined as the ratio of the number of secondary ions sputtered from the surface of a solid sample to the number of primary ions incident upon the specimen) varies over four orders of magnitude from element to element. Second, the yield of each ion is affected by the composition of the matrix. This is the well known SIMS "matrix effect . Third, instrumental effects and ion collection/detection efficiencies can vary from instrument to instrument and specimen to specimen. 

Quantification in shallow profiles is another issue for quantitative analysis with SIMS. Matrix effects and sputtering rate changes are necessary considerations in quantification of shallow profiles to obtain an accurate profile with concentration and depth scales [14]. Comparison to high resolution Rutherford backscattering (HRBS) data helps in evaluating shallow SIMS profiles [36]. 

In SNMS, sputtered neutrals are post-ionized before they enter the mass spectrometer. In contrast to SIMS, SNMS does not suffer from the matrix effects associated with the ionization probability of sputtered particles. Here, the sensitivity for a cer-... 

In contrast to SIMS, in SNMS - where the evaporation and ionization processes are decoupled -the matrix effects are significantly lower, because the composition of sputtered and post-ionized neutrals corresponds more closely to the composition in the solid sample (compared to the sputtered secondary ions in SIMS), which means the RSCs of elements vary by about one order of magnitude. Consequently, a semi-quantitative analysis by SNMS can also be carried out if no suitable matrix matched CRM is available. This is relevant for thin film analysis, especially for the determination of elemental concentration profiles in depth, for studying the stoichiometric composition of thin films and interdiffusion effects. 

Quantitative depth profiling using polyatomic MCs+ and MCs2+ ions instead of atomic ions M= ions is well established in surface analysis using SIMS. The MCs+ technique, which reduces matrix effects significantly, was proposed by Gao in 1988.100 The formation of MCs+ has been explained by the recombination of sputtered neutral atoms (M) with... 

Figure 1.15 shows the lateral and depth resolution achievable with the three mass spectrometric techniques described in this section. As can be seen, the depth resolution obtained with the GD techniques is similar to that with dynamic SIMS (with the additional advantage of less matrix effects in the GD sources). However, the lateral resolution obtained with SIMS is much better because the primary ion beam in SIMS is highly focused whereas in a GD the limitations in the source design make it necessary to sputter a sample area with a diameter of 14 mm. On the other hand, the depth resolution obtained with techniques based on lasers is not yet as good as with SIMS or GDs. 

Other techniques are based on sample sputtering followed by mass spectrometry of the vaporized products, including secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Beam sizes are in the few to tens of pm range. Elemental sensitivities for SIMS are highly variable depending on ion yield, and quantification can be difficult because of matrix effects in the ion production process. SIMS and LA-ICP-MS have very high sensitivities for some elements and low sensitivity for others. These and other microanalytical techniques used in earth science research are described in Potts et al. (1995). 

Neutral atom yields are high, especially for low energy incident ions. Since the ionisation probability of secondary particles obtained directly from the sputtering process is low, the yield of secondary ions can be enhanced by post-ionisation. The matrix effects occurring in SIMS are expected to be considerably smaller in SNMS and therefore quantification of the spectra to determine elemental composition may be much more accurate. 

Reduction of the matrix effects because ionization of neutrals in vacuum limits the interaction between constituents in the analyzed microvolume. Residual matrix effects are due to the sputtering step but remain orders of magnitude smaller than those for directly emitted secondary ions. Hence, quantification in SALI is by far less demanding with respect to calibration standards than SIMS. 

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