The SIMS Technique
What is FIB-SIMS?
FIB-SIMS is an add-on technique available for FIB instruments. Unlike standalone SIMS instruments which use low brightness Cs, O or cluster ion sources, FIB-SIMS takes advantage of the high brightness of the LMIS , GFIS or plasma sources in FIBs to deliver high spatial resolution SIMS. FIB-SIMS add-ons typically use compact spectrometers, 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.
During FIB-SIMS, the sample is bombarded by ions from the FIB (primary ions), sputtering material from the sample. Some of the material sputtered is ejected in the form of charged particles (secondary ions). The secondary ions are guided into a mass spectrometer where they are analysed.
What kind of information can you get from FIB-SIMS?
SIMS is typically used to investigate the distributions of different materials in a sample. It is one of the most sensitive analysis techniques, with detection limits in the ppb range possible. 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. SIMS has several advantages over X-ray based techniques such as EDS: detection of both light elements and isotopes are 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
Applications of FIB-SIMS
FIB-SIMS is used in a wide range of different application areas including:
Instrumentation for FIB-SIMS
Choosing the right kind of spectrometer makes a big difference to the quality of results achievable in FIB-SIMS. A summary of the three main types of spectrometer used in FIB-SIMS in given below.
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
The Lion Nano-Systems V500 FIB-SIMS uses a unique Thru-Field extraction system with a high solid angle of collection, a high transmission DC magnetic sector spectrometer with an extended focal plane and multi-collection system for parallel detection to achieve the highest sensitivity and fastest data acquisition. The V500 is up to 100x more sensitive than a FIB-TOF-SIMS.