EXPERIMENT

Mass Resolution and Mass Accuracy

Figure 1: Dirty aluminium stub

Recently, the mass resolution, and hence also the mass accuracy, has been improved to better than 2000 (M/ΔM). The spectrum above is simply taken from a dirty aluminium stub and shows organic and inorganic contaminants. Labels are shown with tick marks at their expected exact mass. It is very clear which measurement peak belongs to each species.

Data from an older instrument, demonstrating various applications, is shown below.

Figure 2: Silicon wafer contaminated with copper, iron and chromium

The above mass spectrum shows a narrow mass range from mass 50 to mass 71 for a silicon wafer with surface contamination of copper and iron and chromium at levels of ~2 x 10¹² atoms/cm² (one thousandth of a monolayer coverage). The mass resolution (M/ΔM) is >1000, permitting separation of metals from hydrocarbons at the same nominal mass. The mass accuracy permits extra levels of confidence when assigning peaks. For instance, the peak at mass 62.94 is a single peak, and corresponds to ⁶³Cu, whose exact mass is indeed 62.94. The peak at mass 65 is a doublet, with the main contribution from a mass at 64.95. This is the ⁶⁵Cu isotope, exact mass 64.93. Typically the mass accuracy is ≥20 milli mass units. At mass 55, there is a single peak with a mass ‘excess’ of 0.06 mass units, and this is C₄H₇, exact mass 55.055. One mass unit higher, the main peak is at 55.94, corresponding to ⁵⁶Fe, exact mass 55.935. At mass 52 there is another doublet, one peak at mass 51.94 and another at 52.04. These correspond to ⁵²Cr (mass 51.94) and C₄H₄ (mass 52.03).

Sensitivity

The SurfaceSeer is recording >26,000 counts at Cu in a five minute acquisition for a known surface concentration of 2 x 10¹² atoms/cm², with a detection limit of ~2 x 10⁹ atoms/cm².

Mass Range

Although the system does not have a post-accelerated detector, the system is capable of measuring out past 1000 m/z (provided ions are created by the SIMS process). Examples are shown below:

Figure 3: Caesium iodide clusters in positive SIMS

Figure 4: Molybdenum oxide clusters in negative SIMS (MoO₃)₃

Figure 5: Full mass range spectrum of crystal violet, with the M-Cl⁺ peak at mass 373

Insulator Analysis

SIMS analysis of insulating samples could not be easier. All types of insulating samples can be analysed. Low energy electrons are pulsed onto the sample every TOF cycle, preventing charge build-up.

Figure 6: Positive SIMS spectrum from double-sided Scotch tape

This tape is particularly clean and free from siloxane contaminants. Note the presence of Lithium on the surface (mass 6 and 7 in the correct isotopic ratio)

Figure 7: Positive SIMS spectrum from generic double-sided tape

By contrast, this generic double-sided tape shows the classic signs of siloxane surface contamination: higher than usual peaks at 28, 43, 73 and 147.

Figure 8: Positive SIMS spectrum from generic double-sided tape

If we zoom in to mass 28, we see that it is a split peak; the lower mass peak is silicon 28, and the higher mass peak is C₂H₄. Detection of atomic silicon, along with the other characteristic peaks from PDMS, confirms the siloxane identification. The message is to avoid low-cost generic double-sided tape products, and use Scotch brand products that are very clean and suitable for mounting samples in SIMS.

Figure 9: Negative SIMS spectrum from generic double-sided tape

The negative SIMS spectrum also shows characteristic peaks due to siloxane at 28 (Si), 59 (CH₃SiO), 60 (SiO₂), 149 (CH₃)₃Si-O-SiO₂ and 165.

In the next example we see mass spectral data taken from an unprinted paper (blue trace), and the same paper with an ink print (red trace). The unprinted paper has a characteristic peak at mass 39.96 due to calcium, which on paper surfaces is normally due to kaolin loading of the paper (extremely white paper has a high kaolin loading). Once the paper has been printed, the paper is covered over, and so the Ca peak disappears. By contrast the hydrocarbon peaks increase due to presence of the organic-based ink.

Figure 10: Positive SIMS spectrum from paper

Finally a couple of relatively pure polymer examples

Figure 11: Positive SIMS spectrum from PET

Positive SIMS spectrum from PET (polyethylene terephthalate). Characteristic peaks observed at 104/105, 149 and 191/193.

Figure 12: Positive SIMS spectrum from PTFE (logarithmic scale)

Positive SIMS spectrum from PTFE tape, showing characteristic ions through to mass 531. The peaks are all assignable to various C_xF_y combinations. Note that to accommodate the large dynamic range, a logarithmic scale has been used.