Measuring Sonics, Part 2
Barbara Kanegsberg, Ed Kanegsberg, January 2003
Ultrasonics and megasonics have become mainstays for the removal of contaminants
from critical product. As we discussed last month, ultrasonics metrics is in
its infancy, with the most widely used test for ultrasonic functionality still
observation of the visual erosion of aluminum foil. (The test, which is qualitative,
is dependent on technique and foil thickness. It is important to choose standard-weight
foil; heavy-duty foil is unlikely to erode to form the classic orange-peel
pattern.) Another technique, which provides an indicator of ultrasonic cavitation
and frequency, involves immersing a tube filled with a suspension of fine particles
in liquid. If the tube is placed in an ultrasonic tank with active cavitation,
the slurry will partition into bands; the distance between the bands is inversely
proportional to the frequency.
Quantitative ultrasonic and megasonic (u/m) probes have been proposed
and evaluated with varying success. In the past, widespread adoption
of metrics has been
hampered by instability or unexplained variability of readings, lack of standardization
in measuring units, and an unclear understanding or documentation of the
relevance of measurement to process performance. Two techniques that
show promise to
for characterizing u/m sonics are emerging: a u/m meter which provides a
general measure of sound related energy, and a second, which we will
discuss next month,
based on sonoluminescence, a specific phenomenon associated with the collapsing
cavitation bubble.
The quantitative metric probe, which provides a general indication of
energy level, is being adopted for process control. While its principle
of operation
is proprietary, some overall characteristics can be noted. The stainless
steel or quartz probe may be thought of as a sort of super hydrophone in
that it
detects both the imploding solution bubbles and the sound waves produced
by the transducer.
The sonic meter provides a general measure of implosion forces, including
sound. Both cavitation and acoustic forces are detected, and the signature
of the
two forces are separated. It can be used over the entire range of u/m
frequencies; energy is read as watts per sq in of sensor surface.
In addition to being quantifiable, the technique has the advantage over
aluminum foil erosion in that it can be used for frequencies ranging
from ultrasonic
to megasonics. The stable signal has allowed it to be adopted for general
quality control/process monitoring. The meter provides an indication
of how u/m energy
varies within a given tank, both spatially and temporally.
y´he meter has a range of potential uses, including monitoring lysis
of cells and echolocation of dolphins.
Currently, its main applications are mapping relative energy levels in
the u/m tank and monitoring tank life/tank
failure. In one adaptation for use in wafer fabrication, the probe
detector has been configured to the shape of a wafer, which can then be
fixtured along
with the product: this allows product performance in a given system.
Removal of particles, for example, can then be profiled and related to
energy level.
While the technique allows quantifiable monitoring of tank performance,
one study indicated that the meter did not correlate with the
classic aluminum foil corrosion test, where corrosion was measured
gravimetrically.1
However,
the probe appears to be useful for monitoring system performance;
so the group decided to use both the energy meter and aluminum
foil approaches.
There are additional related techniques on the horizon. The current
metric probe does not allow comparison of different energy frequencies.
A second
generation version will allow the user to measure both acoustic
energy (the amplitude
of the ultrasound) and cavitation energy.
We have described a general technique to quantify u/m energy.
Next month, we will discuss a u/m metrics probe based on sonoluminescence,
a new
technique that shows promise of further refining our understanding
of system performance.
References:
1 J. M. Kolyer, A.A. Passchier, and L. Lau, “New Wrinkles in Evaluating Ultrasonic Tanks,” Precision Cleaning magazine, May/June, 2000.2 Y. Wu, C. Franklin, M. Bran, and B. Fraser, "Acoustic Property Characterization of a Single Wafer Megasonic Cleaner,” Presentation and Proceedings, Electrochemical Society, Honolulu, HI, October, 1999 The authors appreciate information and comments of Lawrence Azar of PPB.
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.
