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 ýelatively 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.
The authors appreciate information and comments from L. Azar of PPB; M. Beck of ProSys, and G. Ferrell of SEZ.