Measuring Sonics, Part 3: Sonoluminescence
Barbara Kanegsberg & Ed Kanegsberg, February 2003
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* The Sound of Light, http://pluto.apl.washington.edu/harlett2/artgwww/acoustic/sound.html
The authors appreciate information and comments of Mark Beck, ProSys and Gary Farrell, SEZ.
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.
