Seeing is believing and new techniques have the potential to light the way to rapid, comprehensive surface characterization.
Look at the contamination
BFK Solutions LLC
Pacific Palisades, Calif. Manufacturing decisions are made based not only on the situation of the moment, but also on visions of the future. We will periodically look into the future and have picked the year 2020 as a focal point. Why 2020? The year 2020 is just a bit more than six years away, so specula-
tions can be fairly realistic.
The term “20/20” is commonly also associated with visual acuity.
Visual observation by the unaided eye is the first line of defense
against contamination. The eye, while a marvelous optical instrument, frequently is not enough. However, detection of contamination is more difficult as the size of detrimental contamination
becomes smaller. Microscopes of increasing complexity and optical power can push the limit of detection. However, until recently,
it has been difficult to observe objects that are smaller than the
wavelength of the light used as the probe.
One way to push the envelope of resolution is to use shorter
and shorter wavelengths. This is why scanning electron microscopes (SEM) combined with energy dispersive X-ray (EDX)
detection has become a staple of contamination forensics. There
are drawbacks. SEM requires the sample to be in a vacuum; therefore, the true picture of surface contamination may be altered.
High energy electrons or the X-rays they induce can damage
substrates, and the X-rays are less sensitive to lower atomic number materials. While elemental constituents can be identified, and
while carbon can be detected, the technique cannot be used for
characterization or identification of organic contaminants.
A new generation
During the past fifteen years or so, a number of advances in optical imaging have intrigued researchers, especially those working
with biological systems. Some techniques using fluorescence are
pushing the envelope on diffraction limited size resolution, to distinguish objects with dimensions smaller than the wavelength of
the probe light.1 If an object does not have fluorescent properties,
dyes or labels need to be added to provide the visibility.
One technique that does not require labels, coherent anti-Stokes Raman scattering (CARS), shows particular promise
for contamination forensics. Dr. Eric Potma of the University
of California, Irvine, is an enthusiastic proponent of CARS.
Spend even a little time in his lab, and his enthusiasm becomes
infectious. You begin to envision ways that CARS could provide a pathway to rapid, comprehensive surface characterization. The technique shows promise for commercialization. In
the near future, it might be used in manufacturing.
Potma demonstrated and explained some of the advantages
of the CARS techniques. Most significantly, it combines microscopy (the ability to see small objects) with spectrometry (the
ability to distinguish the chemical constituency) using two different wavelength lasers, typically near-infrared. Moreover, it can
accomplish the task quickly, requires little or no sample preparation, does not need labeling, is not conducted under vacuum
and, for most applications, is non-destructive. It has been used
for in vitro and even for in vivo observations. It is useful for
imaging the surface and near-surface and for detecting what
might generally be considered to be subtle changes. For example, CARS has been used to assess the effect of hair treatment
ingredients on the structure of the human hair. 2
Raman and FTIR
Raman scattering spectroscopy is similar to Fourier transform
infrared (FTIR) spectroscopy in that both techniques are sensitive to the vibratory characteristics of atoms in a molecule
and the characteristics of those vibrations can help “
fingerprint” the molecular compound. In contrast with FTIR, 3 an
energy absorptive process, Raman scattering is not absorptive.
Rather, the light wavelengths are slightly shifted due to interaction with the vibrating atoms in the target molecule.
The two techniques are complementary. FTIR requires a
change in dipole moment in the molecule, such as in vibrations
of C-O, N-O, and O-H bonds. Raman requires a change in
polarizability, such as in vibrations of C-C, C=C, and C-H bonds.
Many molecular bonds can be detected by Raman, but not FTIR
and vice versa. For example, samples in water are difficult to
observe via FTIR because the very strong IR absorption by water
saturates the detector, but they are easy to study via Raman.
Raman scattering was observed by Sir Chandrasekhara
Venkata Rāman in 1928, so why has it taken Raman microscopy 80-plus years to develop? The reason is that there had
to be development and melding of technologies. Microscopy
involves focusing light on a small spot, collecting a scattering
signal, then moving the focused spot to perform a raster scan.
If ordinary light (like from an incandescent bulb) is used, the
signal of most utility in Raman scattering is very weak, and
the scattered photons are randomly phased, so it takes a considerable amount of time to collect a usable signal. Use lasers
to induce Raman scattering, and the effect is very different.
With lasers, the photons constituting the signal are in phase
and they interfere constructively. This increases the sensitivity