What is TOF-SIMS
Barbara Kanegsberg & Ed Kanegsberg, December 2003
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.
Next month: Mars and beyond.
1 Geosling and Koran, Chapter 3.2, “Contamination Control and Analytical Techniques,” in Handbook for Critical Cleaning, Kanegsberg & Kanegsberg ed., CRC Press, (2001).2 “Time-of-Flight Secondary Ion Mass Spectrometry (ToF SIMS),” Amy Walker’s Research Group At Washington University, http://wunmr.wustl.edu/~walker/ToF_SIMS.html3 “What is Time-of-Flight Secondary Ion Mass Spectrometry? (TOF SIMS)?” Surface Science Western Analytical, http://www.uwo.ca/ssw/services/tofsims.html
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical cleaning, precision cleaning, surface preparation, and contamination control. They are the editors of “Handbook for Critical Cleaning,” CRC Press. Contact them at BFK Solutions LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.
