In the previous two columns we discussed the importance of characterizing and comparing ultrasonic and megasonic (u/m) systems, including influence of critical variables: the tank, chemistry, and temperature. In addition to aluminum foil erosion and partition of a slurry, a metric probe which quantifies energy was reviewed.
A second relatively new probe measures a specific u/m attribute, sonoluminscence (SL), a phenomenon associated with a collapsing cavitation bubble. In SL, the energy generated heats the gas in the bubble to incandescent temperatures, and there is a brief flash of light. Flashes are associated with extremely high temperatures, as high as 10,000°C, the equivalent of having a collapsing bubble “living on the sun.” Luminescence of a different kind can also be observed by the addition of luminol, a di-cyclic oxygenated amine also used to enhance the visibility of minute amounts of blood found at crime scenes. Addition of luminol to an operating tank greatly enhances the visualization of cavitation, including patterns of greater or lesser intensity. However, with this chemiluminescent process, it is then necessary to deal with a tank containing luminol.
The SL probe, introduced initially for megasonics applications but being extended to ultrasonics as well, detects SL in a more controlled, quantifiable manner. The probe consists of a small, enclosed cell with a thin tantalum “acoustic window.” The window material and dimensions were chosen for chemical inertness and to maximize transparency to the acoustic field yet minimize the likelihood of erosion of the window (as can happen with aluminum foil). The cell is designed to allow the liquid chemistry in the tank to enter through a serpentine route designed to eliminate extraneous photons, sort of a very miniaturized walk-in photographic darkroom. SL is viewed through a photomultiplier tube (PMT), with transmission via an optical fiber. Use of the optical fiber with the PMT provides sensitivity, maximizes robustness, and allows sufficient miniaturization of the cell that it can be fixtured between wafers, to monitor behavior of the chemistry immediately proximal to the product.
Preliminary experiments with the SL probe are intriguing. In one study, the power was increased step-wise over time. A point of diminishing returns was reached. That is, there was a maximum SL, followed by decreasing detected photons with increasing power. It would be interesting to repeat the study with several different chemistries and to reverse the process, going from higher to lower power, to see if there is an inherent “fatigue” in SL after a certain amount of time. In another study, the probe was used to map a megasonic tank; and variability (including ineffective “cold spots”) consistent with empirical observations of performance was observed.
Of course, factors in addition to those directly associated with SL may be associated with overall performance of the system. Even in terms of cavitation, there may be various types of implosion, all influenced by specific variables of the system including u/m variables, the liquid, and the temperature of operation. Acoustic microstreaming may be influential in removal of certain categories of contamination. Further, not only the force of implosion but also the shape (including implosion asymmetry) may influence interaction of the liquid with the substrate. As additional studies are conducted, it will be instructive to see where contamination removal correlates with the metric; where this metric either tracks performance of a given tank spatially or temporally; and to what extent system designs can be compared.
In recent columns we have discussed aluminum foil erosion, partition of a fine slurry, a metric probe which indicates overall u/m energy, and a probe which specifically measures sonoluminescence. Which technique is best? This may be like asking whether ënfrared spectroscopy is better than gas chromatography. Probably all of them will have value, depending on the application. What is most encouraging is that there is at long last, promise of quantifying and characterizing u/m systems.
For Further Reading
G. W. Ferrell and L.A. Crum, “A novel cavitation probe design and some preliminary measurements of its application to megasonic cleaning,” J. Acoust. Soc. Am. 112: September, 2002