SIMS Theory: Mass Interferences

Mass interferences occur whenever another ion has the same nominal mass as the analyte ion. Such interferences are called isobaric. During the analysis of iron in silicon for example, 28Si2+ interferes because it has the same mass (m/z 56) as 56Fe+. Oxides are common interferences since oxygen-metal bonds are particularly stable. Thus, 40CaO+ can also interfere with 56Fe+ measurements. Low intensity hydrides of many elements appear one mass unit higher than the elements themselves. A good example is silicon-30 hydride (30SiH+) which interferes with trace phosphorous analysis. Primary ions often combine with sample elements to produce interferences. For example 133Cs32S2- is isobaric with Au- (m/z 197) during the measurement of gold in pyrite (FeS2).

High Mass Resolution

Although analyte/interference pairs have the same nominal masses, their exact masses differ by a fraction of a mass unit. The exact mass minus the nominal masses is called the mass defect. Mass defects arise from differences in the nuclear binding energies that hold the protons and neutrons together in the nucleus. Mass defects vary from +0.0078 for hydrogen to 0.1 for elements in the middle of the periodic table to +0.051 for uranium.

The curve of mass defects gives atomic ions higher masses than molecular interferences at relatively low masses and the opposite at higher masses. The following mass spectrum in the m/z 32 region shows separation of 32S and 16O2

Mass spectrometers with sufficient mass resolution can separate atomic ions from molecular ion interferences. Mass resolution is usually specified in terms of m/delta m where m is the nominal mass of the two ions and delta m is their difference. For example, ⁵⁶Fe and ²⁸Si2 (m/z 55.9349 and 55.9539) require m/delta m 5,600 for separation while Au and 133Cs32S2 (m/z 196.9666 and 196.8496) require m/delta m 1700. These two isobaric pairs illustrate the tendency for atomic ions to have lower masses than molecular interferences at relatively low masses and the opposite at higher masses.

Each different kind of SIMS mass analyzers has a range of possible mass resolutions. For example, a (well tuned) double focusing magnetic sector instrument can have mass resolution in the range m/delta m 500 to 10,000. Loss of secondary ion intensity accompanies operation at the high end of the mass resolution range.

Minor Isotopes

Minor isotopes can often resolve interference problems. However, minor isotope secondary ion intensities are lower (by the isotope ratio), detection limits are correspondingly higher (worse), and the RSF values must be adjusted (up). For example, measurement of iron using the 54Fe isotope avoids the interference of 28Si2 with 56Fe, but the 54Fe signal intensity is lower than 56Fe by the 54Fe/56Fe isotope ratio (0.0645). Natural abundance isotope ratios cannot always be used for these adjustments because some processes, such as ion implantation of a specific isotope, alter the ratios.

Elemental Interferences

In a few cases, an isotope of one element has the same nominal mass as an isotope of another. Their separation requires ultra-high mass resolution, beyond the capability of any commercial SIMS instrument. For example, 104Ru and 104Pd would interfere with each other and require m/delta m ~75,000 to separate. Fortunately, non-interfering isotopes are available for most elemental interferences.

Voltage Offset

Sputtering produces secondary ions with a distribution of energies. The voltage offset technique uses the energy analyzer of a mass spectrometer to select secondary ions from the high range of translational energies. The offset is simply a reduction in acceleration voltage. The energy analyzer deflects lower energy ions by a larger angle. A physical barrier (the inner jaw of the energy slit) intercepts the low energy ions. Ions that started with higher translational energies pass through to the mass analyzer. Voltage offset thus discriminates against molecular interferences relative to atomic species. The operator selects the energy cutoff point. A 30 V offset would eliminate essentially all of the di and triatomic ions from a typical distribution of energies. There is an analytical trade-off for this reduction of isobaric interferences. Most of the monatomic ions are also eliminated. A 30 V offset typically reduces the monatomic ion intensity by a factor of ten. The analyst must decide whether this reduction of the analyte signal is tolerable. Reduction of analyte signal intensity can usually be compensated by changing other experimental parameters, higher primary beam current or wider spectrometer slits, for example. The following figure shows the analysis of arsenic in silicon, both with and without voltage offset. A small interfering signal from 28Si30SiOH falls at m/z 75. Note the improved detection limits with 15 volt offset.

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