Last month we discussed Surface Acoustic Wave (SAW) detectors for monitoring Airborne Molecular Contamination (AMC). We indicated that contaminants on the sensor chip can be identified using Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS).
In both static TOF-SIMS and dynamic SIMS, the sample surface is placed under vacuum and bombarded by energetic ions, causing a secondary cascade of positive and negative secondary ions from the surface. The pattern of secondary ions can be used to identify surface molecules. SIMS is useful for identification of contamination and for studies of surface quality.
Static or TOF-SIMS
In TOF-SIMS, the time-of-flight mass spectrometer characterizes masses relative to arrival time at a detector, with lighter masses arriving most quickly.1 TOF-SIMS is relatively non-invasive. The impact of pulses of ions (commonly 69Ga+ but also Cs+, Ar+, and O2+) on the surface, is analogous to that of sand on a windshield during a drive through the desert on a calm, clear day. A few grains of sand hit and pit the windshield but don’t appreciably change its transparency. Because TOF-SIMS often preserves organic fragments, it can be used to provide a chemical map of the surface, with sub-micron resolution.2 Static SIMS characterizes the “surface of the surface”(roughly one to three monolayers) with a sensitivity down to ppm levels. Mass spectra up to 10,000 atomic mass units (amu) can be obtained.3
Because the technique can be used to identify inorganic materials as well as high molecular weight organic polymers and because the location and pattern of surface components can be determined, SIMS is used in an array of applications ranging from imaging of biological materials to identifying contamination in painted surfaces. For example, monolayer films of specific high molecular weight lipids have been imaged and characterized. In another application, siloxanes were definitively identified as a contaminant in a crater defect on an automotive paint surface.
Dynamic SIMS
Dynamic SIMS is a comparatively destructive technique. The much higher number of bombarding ions is analogous to driving through a sandstorm; in a short time the windshield becomes pitted. The intense ion flux allows depth profiling, with a single non-pulsed primary beam, to depths of several microns typically with greater elemental sensitivity. Since only a few elements can be monitored at a time, prior knowledge of the species of interest is needed, usually obtained a priori via a mass spectrum. The maximum mass range is typically ~ 250 amu.
Depth Profiling with TOF-SIMS
Due to the pulsed nature of the technique, a short duration destructive ion beam can be interlaced with the primary beam in many instruments. This allows ultra-shallow depth profiling of thin films (< few nm to ~ 1 micron), far less than that attainable with dynamic SIMS but with superb depth resolution.
Provisos
Static and dynamic SIMS are sophisticated techniques requiring complex instrumentation, thorough understanding of the sample to be analyzed, thoughtful design of the experiment and careful interpretation of results. There are limitations to SIMS. For example, loosely-bound contaminants that interfere with the product or process could be lost through volatilization during the evacuation process. In addition, because the secondary ion yield is substrate-dependent, matrix-specific reference standards are required for quantification. Further, although TOF-SIMS is relatively non-invasive, since every ionic bombardment modifies the surface, the actual product(s) used for analysis could be altered.
Extreme care in sample handling and sample shipment is a must. Because TOF-SIMS may detect spurious contaminants, it is very desirable to also run control samples. In this case, a control would consist of an uncontaminated sample handled the same as the test sample.
The authors thank James Francis from the Surface Science Western Laboratory at the University of Western Ontario for his helpful comments and contributions.
References
- Geosling and Koran, Chapter 3.2, “Contamination Control and Analytical Techniques,” in Handbook for Critical Cleaning, Kanegsberg & Kanegsberg ed., CRC Press, (2001).
- “Time-of-Flight Secondary Ion Mass Spectrometry (ToF SIMS),” Amy Walker’s Research Group At Washington University, http://wunmr.wustl.edu/~walker/ToF_SIMS.html
- “What is Time-of-Flight Secondary Ion Mass Spectrometry? (TOF SIMS)?” Surface Science Western Analytical, http://www.uwo.ca/ssw/services/tofsims.html