Principle

SIMS uses charged primary ions focused at a sample to sputter off atoms and molecules characteristic of the outermost atomic layers of the material. The primary ion dose can be carefully controlled so that the damage to the surface is negligible and the technique is essentially considered as non-destructive (so called static SIMS). Larger numbers of ions per unit area can also be used to sputter into the bulk of the sample (so called dynamic SIMS). This mode of operation allows chemical depth profiles to be obtained.

In the SIMS process in our instrument primary gallium ions knock out secondary ions and molecules from the surface atomic layers. These secondary ions are detected as a mass-to-charge ratio by the quadrupole mass spectrometer. As well as secondary ions, this sputtering process generates large numbers electrons, which can be used to produce a physical image of the sample surface similar to that obtained in an SEM.

In non-conducting samples the sputtering process causes the sample to charge, which can severely affect the sensitivity of the analytical technique. To eliminate this problem the MiniSIMS instrument is fitted with a charge compensation gun, which indirectly floods the sample with low energy electrons.

SIMS spectra can be acquired in a point and shoot manner. Multiple static points, small areas and large areas can be scanned from chemical or physical (SEM) images highlighting the areas of interest. These typically only take 30s to acquire. Chemical and molecular maps can also be rapidly (about 30s each) acquired to determine local inhomogeneities in surface chemistry.

Information Available from SIMS

Main features of the laboratory's SIMS capability

• Analysis of elements and isotopes atomic numbers Z=2 to Z=103 as well as molecules and fragments from 2-300amu).
• 300amu quadrupole mass spectrometer for atomic and molecular ion detection in both positive and negative ion modes.
• Focused gallium primary ion source with an ultimate instrument lateral resolution of 5μm.
• The analytical field of view can be zoomed in from 4.5x4.5mm by 20x to 225x225μm.
• Secondary electron detector for SEM of conductors and semiconductors to allow rapid determination of the area of interest for SIMS.
• Electron source for charge compensation of insulators.
• Instrument capable of large area, small area and point spectroscopy. These areas can be chosen from the SEM image, any chemical image or from an optical image before sample loading.
• Rapid molecular and atomic ion chemical imaging (typically 30s / map).
• Extremely fast sample entry system into a high vacuum (HV) chamber. It has a high sample throughput compared to other types of SIMS instruments, ideal for quality control roles or getting good sampling statistics.
• Basic chemical depth profiling is possible down to depth of ~100nm.
• The instrument takes solid samples up to 10mm diameter (12.5mm possible in exceptional cases) and 4mm thick. Larger samples can be cut carefully down to size in the laboratory.

Our site expert on SIMS is Dr Stephen Jenkins.

For more information about the Millbrook MiniSIMS instrument and it applications look at the manufactures web site at http://www.minisims.com/applications.htm

Typical Applications

• Routine metal component batch release - protection against surface contamination in a furnace heat treatment process.
• Checks for weak boundary organic layers affecting paint or adhesive adhesion.
• Improvement of chemical cleaning or etching processes.
• Drying stain root cause investigations.
• Metallic component emissivity process rejects due to ineffective thermal blackening.
• Sputtered layer process characterisation and checks of target end of life.
• Surface segregation of heater coating contaminants.
• Corrosion failures due to electrochemical action.
• Use of tracer elemental isotopes.