Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analyzes mass fragments of elements and molecules dislodged from the surface by an ion beam. This technique is often selected because it is very surface sensitive (~15Å) and can detect organic molecule fragments to the ppm level. TOF-SIMS can detect hydrogen and lithium and is definitive for the presence of silicone.
Because of its good sensitivity to many species, the spectra obtained from TOF-SIMS can be difficult to interpret. Although similar surfaces can be compared, TOF-SIMS should not be considered quantitative unless careful standards are available. The technique gets its name because a time-of-flight mass analyzer is used in the technology. TOF-SIMS is not considered a destructive technique.
TOF-SIMS Capabilities Include
- 15Å analysis depth
- 2000Å spatial resolution
- Molecular identification of organic molecule fragments
- Detection to ppm level for most elements and surface species
- SIMS maps to show lateral distribution of surface species
- Depth profiling
Material issues analyzed with TOF-SIMS include polymer surface segregation, surface cleanliness and silicone analysis.
Blooming in Polymers
A PET film had subtle discoloration that was rejected due to a phenomenon described as “blooming.” This is a subtle visual defect that is often characterized by surface segregation of low-level additives. TOF-SIMS can identify complex organic additives that are segregated to the surface in these types of failures. The spectra demonstrate surface segregated trisarylphosphite, a common stabilizer, was present in the visually substandard area.
Numerous peaks from the additive were present in the range of mass analysis from 1-700, including those at 647 (the molecular weight of the molecule) and at 632 (647-15 amu from the fragmented loss of a -CH3 group).
Bond Pad Failure
TOF-SIMS was used to investigate the cause of bond pad failure. This technique was chosen because SIMS is definitive for silicone. TOF-SIMS maps of elemental aluminum and silicon and silicone (i.e. polydimethylsiloxane) show the distribution of the contaminant on the surface. Siloxane is mapped distinctly from elemental silicon.