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 speculations can be fairly realistic.
Look at the contamination
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 by several orders of magnitude. A raster scan that could take a week using classic Raman techniques can be done in a few minutes with CARS. In addition, because the light is not absorbed, the higher intensities from a laser do not heat and damage the sample and can be used to penetrate below the surface to study sub-surface behavior.
Looking to the future
Frequently, as in SEM/EDX or gas chromatography/mass spectrometry (GCMS), several analytical techniques can be incorporated into the same instrument to boost its utility. Potma combines the CARS technique with other techniques such as pump-probe, second harmonic generation (SHG), and fluorescence. Pump-probe involves perturbing the sample with one laser pulse (the pump) followed in a very short time by a pulse of a different wavelength (the probe). By varying the time between pump and probe, typically a few femtoseconds (1 fs = 10-15 s), it is possible to characterize both the surface and the near surface on many materials. For example, pump-probe can differentiate between lapis lazuli obtained from Afghanistan, Chile, and synthetically produced.4 The implications are that this technique, especially if coupled with CARS, can be used to non-destructively profile coatings and the interface of coating and surface.
Potma explains that CARS is valuable because it allows inspection “with the least amount of perturbation to the sample—no harsh treatment or staining protocols.” He adds that the contrast achieved with CARS represents “an enormous step forward. It clearly widens the current landscape of scientific investigation.”5 CARS and associated techniques have the potential to add powerful tools to assist scientific researchers as well as manufacturers of critical product.
Potma concludes, “Other applications are waiting to be discovered. It is inevitable that CARS will continue to have a major impact in the biological and material sciences. It’s the way forward.”5
Acknowledgements: We thank Dr. Alex Small, California Polytechnic University, Pomona, for introducing us to the topic of a new generation of optical microscopy, as well as for suggesting Dr. Eric Potma as a resource.
Barbara Kanegsberg and Ed Kanegsberg (the Cleaning Lady and the Rocket Scientist) are experienced consultants and educators in critical and precision cleaning, surface preparation, and contamination control. Their diverse projects include medical device manufacturing, microelectronics, optics, and firstname.lastname@example.org
This article appeared in the October 2013 issue of Controlled Environments.