Analytical Applications of SIMS

Both positive and negative secondary ions can be generated and analyzed. In practice, positively charged ions are most valuable for examining the elements on the left side of the periodic table, whereas the negative ions are most useful for elements on the right side of the periodic table (nonmetals). 

In both SIMS and ISS, insulating samples can be analyzed if a flood gun or combination of flood gun and ion gun is used to charge the surface and to maintain charge balance. This can be seen in the SIMS spectrum of a glass surface (Fig. 14.40) collected without a flood gun (top spectrum) and with a flood gun (bottom spectrum). It can be seen that the spectrum has a flatter baseline and improved resolution when the flood gun is used. 

The principles of ISS and SIMS are similar, in the method of bombarding the sample, the utilization of high vacuum, and the measurement of ions. In ISS the scattered ion beam is examined and the energies of the ions measured in SIMS the ions ejected from the surface are examined and their mass-to-charge ratio measured. Both techniques are 

Negative SIMS spectrum showing both elements and polyatomic fragments of molecules. [Courtesy of Physical Electronics USA, Inc. Eden Prairie, MN (www.phi.com).] 

The analytical signal obtained in SIMS depends on a number of factors, namely, the abundance of the isotope examined on the surface, the properties of the surface, and the bombarding ion. The relationship is complex calibration curves may or may not be continuous. Certihed calibration standards that are matrix-matched and cover the analytical range are required. For bulk analysis using dynamic SIMS, certified standards can be obtained from sources such as NIST or commercial standards firms for many metals and alloys and for some ceramics and glasses, but such standards are rarely available for surfaces, thin layers, and multilayered materials. The excellent sensitivity of SIMS allows it to distinguish between ppb and ppm concentrations of analyte in solids. SIMS is not used for quantitative analysis at the % level XPS or methods such as XRF are better suited to this purpose. 

The analytical signal obtained in SIMS depends on a number of factors, namely, the abundance of the isotope examined on the surface, the properties of the surface, and the bombarding ion. The relationship is complex calibration curves may or may not be continuous. Certifled calibration 

SIMS is used for quantitative depth profile determinations of trace elanents in solids. These traces can be impurities or deliberately added elements, such as dopants in semiconductors. Accurate depth profiles require uniform bombardment of the analyzed area and the sputter rate in the material must be determined. The sputter rate is usually determined by physical measurement of the crater depth for multilayered materials, each layer may have a unique sputter rate that must be determined. Depth profile standards are required. Government standards agencies like NIST have such standard reference materials available for a limited number of applications. For example, standard reference material (SRM) depth profile standards of phosphorus in silicon, boron in silicon, and arsenic in silicon are available from NIST for calibration of SIMS instruments. P, As, and B are common dopants in the semiconductor industry, and their accurate determination is critical to semiconductor manufacture and quality control. 

---