The SIMS technique
What is SIMS?
Secondary Ion Mass Spectrometry (SIMS) is a highly sensitive surface analysis technique. During SIMS, the sample is bombarded by a beam of primary ions . These primary ions sputter material from the sample. Some of the material sputtered is in the form of charged particles: the secondary ions. The secondary ions are guided into a mass spectrometer where they are analysed.
What kind of information can you get from SIMS?
SIMS is an extremely sensitive surface analysis technique (sensitivities in the ppb range are possible). It is typically used to investigate the distributions of different materials in a sample. SIMS can be used to generate 3 main kinds of information: mass spectra, depth profiles and images. If multiple image planes are acquired as the sample is sputtered, 3D analysis is also possible.
Mass Spectra
Mass Spectra show the signal level usually in counts per second against mass (or more precisely m/z the mass to charge ratio). Peaks in the spectrum indicate the presence of a particular element/molecule.
Imaging SIMS
During imaging SIMS, data acquisition is synchronized with the scanning of the primary beam such that one (or more if the spectrometer is capable of parallel acquisition) mass filtered image is produced.
Depth profiling / 3D analysis
Depth profiling is one of the oldest uses of SIMS and has its origin in the semiconductor industry. During depth profiling, signals are followed as the sample is sputtered creating a profile of intensity versus depth. Depth profiles can be quantified using relative sensitivity factors and profilometry to produce graphs of concentration versus depth.
C. Noel et al, Materials 2019, 12(5), 726
How do you do SIMS?
Dedicated SIMS instruments usually use reactive species for the primary ions (typically Cesium and Oxygen) as these enhance the yields of secondary ions. There are 3 types of mass spectrometer commonly used for SIMS:
Quadropole Mass Spectrometers
These use RF quadropole fields to separate the secondary ions according to their mass to charge ratio.
✓ Simple / compact
X No parallel detection
X Low mass resolution
Time of Flight (TOF) Spectrometers
These separate ions according to the time taken to fly a fixed distance.
✓ High mass resolution
✓ Parallel detection
X Poor duty cycle
Magnetic sector mass spectrometers
These use magnetic fields to separate ions according to their mass to charge ratio.
✓ High mass resolution
✓ DC - no duty cycle
✓ Parallel detection
What is FIB-SIMS?
FIB-SIMS is an add-on technique available for FIB type instruments. Unlike dedicated SIMS instruments which use low brightness Cs or O ion sources, FIB-SIMS takes advantage of the high brightness of the LMIS , GFIS or plasma sources in FIBs to deliver high spacial resolution SIMS. FIB-SIMS add-ons typically use compact versions of one of the spectrometers described above, optimized for the types of application and the range of secondary ions that are produced by the Ga, He ,Ne and Xe beams typically available on FIBs. As these beams do not impart the secondary ion yield enhancements that come from using Cs and O beams, the transmission and duty cycle of the spectrometer become important considerations. The Lion Nano-Systems V500 FIB-SIMS uses a high transmission DC magnetic sector with parallel detection to achieve the highest sensitivity and fastest data acquisition. The V500 is up to 100x more sensitive than a FIB-TOF-SIMS.
