Measuring Sonics, Part 1
Barbara Kanegsberg, Ed Kanegsberg, December 2002
Liquids used in critical surface preparation are carefully characterized and
monitored to establish and maintain chemical composition as well as to minimize
particulates and dissolved contaminants. Appropriate chemical and physical
properties are selected to optimize the process; most are well understood.
While adding sound waves to a liquid results in invaluable attributes, system
properties (liquid plus ultrasonics or megasonics) may not be adequately understood
or monitored. A system with specific parameters is chosen based on past performance.
Do we have the equivalent of an optimally-tuned sports-car? We really don’t
know.
Both ultrasonics and megasonics involve the interaction of sound with
liquid to produce activity such as compressible cavitation bubbles
and acoustic streaming.
While often treated as separate techniques, they actually represent a continuum
ranging from y´elatively low frequencies (e.g., 25 kHz for heavy-duty
ultrasonics) to very high frequencies (e.g., 1000 kHz is clearly megasonics). There
are distinctions. Megasonics, for example, is a line-of-sight technique and
in general produces less
surface damage. (As used here, the term “sonics” will
refer both to ultrasonics and megasonics unless otherwise specified.)
Liquids vary markedly in their response to a given set of sonic parameters.
The challenge is to compare and monitor system behavior. Certain solvents
are relatively unresponsive to sonic energy; some aqueous products become
more
active than anticipated. In response, equipment suppliers offer a dizzying
array of options and features such as frequency, power, sweep, on-off
cycle, and transducer placement.
Accurate, reproducible sonic metrics are needed to enhance contamination
control, optimize surface preparation, achieve appropriate quality
control, and compare
system options. It would be helpful to monitor power distribution within
a tank including impact of the acoustic field and various aspects of
cavitation. It would be useful to spatially map the activity within
a given tank, track
changes over time, and compare performance among tanks of a single
design or
among tanks from several manufacturers. Inconsistent sonics effectively
means that, despite initial analysis and study, we are using poorly
controlled solutions.
Unfortunately, quantifiable sonic metrics for surface preparation is
in its infancy. For lower frequencies, the classic
test is based on aluminum foil.
Because it is sensitive to erosion and chemical incompatibility,
foil acts
as an early warning signal of ultrasonic problems, a bit like a canary
in a mine shaft. A thin sample of commercial foil is submerged in
the liquid for
a consistent time, then inspected for the presence of a characteristic,
distinctive,
slightly dimpled orange-peel pattern. The absence of this pattern
indicates lack of ultrasonic activity. Visible erosion is a sign of undesirably
high activity. Gravimetric determinations may also be performed where
weight
loss is presumed due to erosion of the surface (and perhaps removal
of trace contaminants)
and is taken as an indication of activity of the system of liquid
plus
sonic activity.
The foil test is rapid and inexpensive. For emergency troubleshooting,
it is not unknown to enlist foil from the cafeteria. Although
the method is
often
thought of as unsophisticated, the foil approach does have value
as an indicator of cavitation activity. However, the method is
qualitative, subjective, and
dependent on sample preparation. There are indications that slightly
wrinkled foil may be more sensitive to erosion than very smooth
foil. In addition,
the technique becomes less applicable at higher frequencies;
in fact, the orange-peel
pattern is actually assumed by some megasonics proponents to
indicate the unsuitability of ultrasonics for delicate components.
Witness or test samples provide an indirect indication of sonics
metrics. Where there is suspicion that fixtures have dampened
activity, the
product can be
replaced with aluminum foil to see if the characteristic pattern
is achieved. As a more sophisticated extension, a witness sample
may be
processed
with the component, then analyzed to establish surface quality
(absence of erosion,
removal of contamination). This approach has been used in wafer
fabrication to establish megasonic quality and uniformity.
Next month: progress in the development of quantifiable ultra-and
megasonic probes.
Reference:1 J. M. Kolyer, A.A. Passchier, and L. Lau, “New Wrinkles in Evaluating Ultrasonic Tanks,” Precision Cleaning magazine, May/June, 2000.
Acknowledgements:
The authors appreciate information and comments from L.
Azar of PPB; M. Beck of ProSys, and G. Ferrell of SEZ.
Contact Barbara Kanegsberg and Ed Kanegsberg at BFK Solutions;16924 Livorno Dr., Pacific Palisades, CA 90272,310-459 3614; barbara@bfksolutions.com;
