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SIMS Theory: Sample Charging
The SIMS primary ion beam, secondary ions, and secondary electrons produce a net electric current at the sample surface. If the sample material conducts, the current flows through the sample into the instrument. However, insulating samples undergo charge buildup. Sample charging diffuses the primary beam and diverts it from the analytical area, often eliminating the secondary ion signal entirely. Sample charging also changes the energy distribution of the secondary ions, which effects their transmission and detection by the mass spectrometer. When the sample is a thin dielectric on a conducting substrate, a strong electric field develops. Mobile ions such as sodium and lithium migrate in the electric field and depth profiles no longer reflect the original compositions of the layers. Several techniques are available to manage sample charging, and they are often used in combination.
Electron Bombardment
Electrons compensate for positive charge buildup that results from positive primary ions and/or negative secondary ions and electrons. Low energy electron beams work better because higher energies produce more than one secondary electron for every incoming electron. Low energy electron beams are more easily implemented in quadrupole SIMS instruments, making quadrupoles the system of choice for insulating materials. In contrast to quadrupoles, magnetic sector instruments maintain the sample at high positive potential for positive ion spectroscopy, making it difficult to bring in a low energy electron beam. High energy electron beams, though less effective, are widely used.
Adjacent Conductors
Conducting grids placed over the sample reduce the effects of charging on ion optics and bring a source of electrons near to positively charged areas of the sample. When struck by a primary ion, the conductors emit secondary electrons that migrate to the charging area. Similarly, samples are often coated with conducting materials such as gold or carbon. Before starting the analysis, the coating must be sputtered away, but only in the analytical area.
Negative Primary Ion Beams
The most common negative primary ion beam is O, available from the same duoplasmatron sources that more commonly produce O2+. Primary O beams find wide use for insulating geological samples.
Automatic Voltage Offset
A continuously variable voltage offset can be applied to the accelerating voltage for samples that are only slightly charging. Automatic voltage offset procedures (called autovolt) are often incorporated into instrument control software. After every cycle in a depth profile analysis, the software invokes an energy distribution measurement and adjusts the voltage offset as needed to keep the peak of the distribution constant.
