14 � February 2009 Controlled Environments � www.cemag.us

Cavitation inUltrasonic Cleaningand Cell Disruption

It is important to quantify the cavitation energy inall applications ranging from ultrasonic cleaners to

cell disruptors. A cavitation meter measures the energyintensity and frequency within ultrasonic and megasonic cleaners,including probes with side-mounted sensors that can be placed within themegasonic jet streams and single wafer cleaners, as well as thoseresembling a beaker to quantify the energy emanating from the ultrasonichorns used for cell disruption and homogenizing.

Lawrence Azar

Ultrasonic and megasonic cleaners are used invaried applications to clean substrate surfaces. Ina typical assembly, a cleaning system includes atank that holds a fluid medium such as an aque-ous solution, which generally includes additivessuch as surfactants and detergents that enhancethe cleaning performance of the system. Lately,more distinctive means of delivery have been uti-lized, particularly for single wafer applications. Inone example, megasonics is diverted into a streamof fluid that impacts the substrate surface. Inanother, megasonics imparts directly on a film offluid no more than a few millimeters thick on thewafer surface. Ultrasonics can also be deliveredvia an ultrasonic horn, which is a popular methodnot for cleaning but for cell disruption, emulsifica-tion, and homogenizing of biological matter. Inboth cleaning and cell disruption applications, it isthe phenomenon of cavitation that drives theactions.

FUNDAMENTALS OF CAVITATIONThe term “ultrasonic” represents sonic waves hav-ing a wave frequency above approximately 20kHz and includes both the traditional ultrasoniccleaning spectrum which extends in frequencyfrom approximately 20 kHz to 500 kHz, and themore recently used megasonic cleaning spectrumwhich extends in frequency from about 0.5 MHzto about 5 MHz. The device used for cell disrup-tion has traditionally been the ultrasonic horn.This device works at a fixed frequency, normallybetween 20 and 50 kHz, and is designed to beresonant in the longitudinal mode of vibration.

In a typical ultrasonic cleaner, a transducermounted on the bottom generates high frequencyvibrations in the cleaning tank in response to anelectrical signal input. Once generated, the trans-ducer vibrations propagate through the fluidmedium in the cleaning tank until they reach thesubstrate to be cleaned. Cavitation bubbles are

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formed and grow when a liquid is put in significantstate of tension. Liquids, though unable to supportshear stresses, can support compressive stresses, andfor short periods, tensile stresses.2 The acoustic pres-sure wave undergoes a compression and rarefactioncycle, and the pressure in the liquid becomes a nega-tive during the rarefaction portion of the cycle. Whenthe negative pressure falls below the vapor pressure ofthe fluid medium, the ultrasonic wave can cause voidsor cavitation bubbles to form in the fluid medium.

Coleman et al showed a remarkable photograph(Figure 1) of a bubble collapsing near a boundary.3

Once the cavitation is generated, a cavitation bubblemay undergo two different kinds of radial oscillations.One may oscillate nonlinearly during many cycles ofthe acoustic wave, termed “stable cavitation.” The othermay grow rapidly and collapse (i.e. implode) violentlyin one or two acoustic cycles, termed “transient cavita-tion.”4 During bubble implosion, surrounding fluidquickly flows to fill the void created by the collapsingbubble. This flow results in an intense shock wavewhich is uniquely suited to substrate surface cleaning.Specifically, bubble implosions that occur near or at thesubstrate surface will generate shock waves that candislodge contaminants and other soils from the sub-strate surface. When the bubble collapses, pressure upto 20,000 psi and a “high local temperature, possibly inthe order of 5,000K,”5 are achieved.

In almost all cleaning applications, it is important tocontrol the cavitation energy. When an insufficientamount of cavitation energy is provided, undesirablylong process times may be required to obtain a desiredlevel of cleaning, or in some cases, a desired level ofsurface cleaning may not be achievable. On the otherhand, excessive cavitation energy near a substrate having delicate surfaces or components can cause sub-strate damage. The levels of cavitation energy are alsocritical in assuring complete and rapid cell disruption.The presence of solid impurities and dissolved gasdetermines the threshold of cavitation. Many use tapwater, which varies widely in solid and gas content. Asimple way of ensuring more uniform results is to use

distilled water and then degass it. The liquid can nowbe engassed by bubbling the desired gas through it,which will ensure optimal cavitation.2

The bubble dynamics in the acoustic field is describedby the well-known Rayleigh-Plesset equation as:

Where R is the radius (m) of cavitation bubble at anytime, µ is the viscosity of the liquid medium (Ns/m2), �is the surface tension (N/m), Pi is the pressure insidethe bubble (N/m2) and P� is the pressure in the liquidfar from the bubble (N/m2).6 Studies have shown thathigh density low viscosity, and middle range surfacetensions and vapor pressure are the ideal conditions formost intense cavitation.7 There are significant tempera-ture effects on these properties, and cavitation itself willbe dramatically affected with increasing temperature.

Another factor that affects the size of the cavitationbubbles and the corresponding cavitation energy is thefrequency of the ultrasonic wave. Specifically, at higherwave frequencies there is less time for the bubble togrow. The result is smaller bubbles and a correspon-ding reduction in cavitation energy. Low frequencyultrasound has superior particle removal efficiencies(PRE) for large particles, and that high frequency ultra-sound is best suited for submicron particles.8

Figure 1: Photograph of liquid jetformation during cavitation bubblecollapse.3

Figure 2: Numerical simulation of a megasonic transducer. (Image courtesy of PPB Megasonics)

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Relative Pressure Field above the Centerline of a Square Megasonic Transducer

Transducer Width (mm)

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m T

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r (m

m)

Rd2R

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Pi � P�� 2�

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dt2 2 dt � R R dt( ) ( )][

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16 � February 2009 Controlled Environments � www.cemag.us

Another factor that affects cavitation energy is theintensity of the ultrasonic wave (i.e. wave amplitude)produced by the transducers. In greater detail, higherwave intensities cause each point along the wave tooscillate over a larger pressure range (between rarefac-tion and compression), which in turn, produces largercavitation bubbles and larger cavitation energy. Thus,there is a direct correlation between the intensity ofthe ultrasonic wave, the pressure range that the fluidmedium oscillates between, and cavitation energy.

There also exists a constructive/destructive patterngenerated by the transducer(s) in the bath. Depend-ing on the location within the bath, you may haveultrasound arriving in phase, generating constructiveinterference, and in another location, it will be out ofphase, generating destructive interference. Tank man-ufactures change the pattern by sweeping thefrequencies, thereby improving the uniformity withinthe bath. As a consequence, portions of a substrate thatare located at different locations within the bath willexperience different levels of cavitation energy. It hasbeen somewhat challenging to uniformly clean a sub-strate. Figure 2 shows a numerical simulation of a 1MHz megasonic bath with a square plate transducer onthe bottom. The constructive/destructive pattern is evi-dent, even though only one transducer is shown. Theenergy is directed primarily over the transducer becausethe overall dimension of the transducer is much largerthan the wavelength of the ultrasound.

In additional to cavitation, there is another effectfrom ultrasound, acoustic streaming, which occurs

when the momentum absorbed from the acoustic fieldmanifests itself as a flow of the liquid in the directionof the sound field. There is a second type of streamingassociated that occurs near small obstacles placedwithin a sound field, called “microstreaming,” generat-ed by the oscillations of an acoustically driven bubble.1

This can lead to additional biology effects andenhanced cleaning effects. The microstreaming associ-ated with bubble motion can be very significant inbiological systems. Suspended cells or macromoleculesare carried in streaming orbits and may be broughtmomentarily into the boundary layer near a bubbleonce during each traverse of an orbit. When in thisboundary layer, they may be distorted or fragmentedby the high shearing stresses.2

CAVITATION METERSUltrasonic cavitation can produce the cavitation noisespectrum including harmonics, subharmonics, and con-tinuous of the driving frequency, and the relativeintensity of ultrasonic cavitation can be acquired byanalyzing the cavitation noise.4 PPB Megasonics (LakeOswego, OR) has developed a method to analyze thecavitation fields and measure the cavitation intensity byseparating the acoustic energy intensity at a particularlocation in the ultrasonic fields into two components:the energy intensity due to the ultrasonic itself and thecavitation activity. The cavitation meter then outputs theRMS of the cavitation energy intensity (units of wattsper square inch) and the frequency of the ultrasound.These meters are not hydrophones, which typically fil-ter out the higher frequency cavitation signatures andinherently spatially average compression and rarefac-tion resulting in misleading data. The cavitationsignature is superimposed on the acoustic measure, buthas no spatial average component. The acoustic signa-

Figure 3: Mapping of a megasonic bath using the cavitationmeter. (Image courtesy of Sebastian Barth)

Chart 1: Cavitation meter readings versus applied generatorpower. (Chart courtesy of J.M. Kolyer of Boeing)

Meter Reading vs. Percent Applied Power

0

10

20

30

40

50

60

70

80

0 20 40 60 80 100

Percent Applied Power

Met

er R

eadi

ng (W

/in2 )

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ture is removed, compression and rarefaction, leavingbehind only the cavitation implosion forces.

The aluminum foil test is a rudimentary methodsometimes used to evaluate ultrasonic baths. JohnKolyer from Boeing has written articles on this topic,including one that compared the use of the foil test ascompared to a cavitation meter. He states in the arti-cle: “A Meter That Works” that “…the UltrasonicEnergy Meter will take over the task of monitoringtank performance.”9 His data showed a dramaticimprovement in accuracy and time to evaluate a bathusing the cavitation meter. His article also charts howthe energy measure of the unit changes with varyingpower settings from a standard 40 kHz bath, whichare shown in Chart 1. It is clear that the measurementsare directly proportional to the energy present withinthe bath. Figure 3, provided by a customer, shows adetailed mapping of a megasonic bath at mid-depthusing the cavitation probe. The constructive/destruc-tive patterns are quite evident, as are the subsequentpeaks and valleys present within the bath.

The probes for ultrasonic cleaning applicationshave been specially designed to isolate any resonantaffects by utilizing a chemically-resistant polymer tohouse a sensor mounted on an acoustically matchedquartz lens. All-Quartz probes are used for megasonicapplications, and quartz probes with side-mountedsensors, shown in Figure 4, have been developed formeasurements within megasonic nozzle streams. Anewly released “beaker” style probe, shown in Figure5, has been introduced to quantify the energy emanat-ing from ultrasonic horns used for cell disruption.

The cavitation meter has proven to be an invalu-able metrology tool to quantify the energy within theultrasonic and megasonic cleaners and those emanat-ing from ultrasonic horns. Using the cavitation meteras a process control tool will improve both the yieldand throughput of the cleaning and cell disruptor

operations. NIST Traceable calibration certificates areavailable.

In early 2009, PPB Megasonics is introducingpatented multi-sensor probes that provide real-timeenergy distribution profiles across wafer or other sub-strate surfaces being cleaned, vital in minimizingdamage but also assuring effective cleaning. An earlyprototype is already being used by a leading mega-sonic manufacturer on a single-wafer cleaner. The firstgeneration model will include up to 64 channels thatare displayed simultaneously.

References

1. T.G. Leighton, The Acoustic Bubble, Academic Press, London, UK, 1994.

2. F. Ronald Young, Cavitation, Imperial College Press, London, UK, 1999.

3. A.J. Coleman, J.E. Sauders, L.A. Crum, and M. Dyson., Ultrasound inMed. Biol., 12, 69, 1987.

4. Zhafeng Liang, Guanping Zhou, Shuyu Lin, Yihui Zhang and Hongli Yang,Study of low-frequency ultrasonic cavitation fields based on spectralanalysis technique., Ultrasonics, 44, 1, 2006.

5. A. Henglein and M.J. Gutierrez., J. Phys. Chem. 97, 158, 1993.

6. Parag Kanthale, Muthupandian Ashokkumar and Franz Grieser, Sonolu-minescence, sonochemistry and bubble dynamics, UltrasonicsSonochemistry., 15, 2, 2008

7. Charlie Simpson, Bearing the Load., CleanTech, 2, 2, 2002.

8. Remuka Rodrigo and Timothy Piazza, Intelligent Ultrasonics for the DiskDrive Industry., A2C2, 3, 8, 2000.

9. J. M. Kolyer, A.A. Passchier, and L. Lau, “New Wrinkles in Evaluating Ul-trasonic Tanks,” Precision Cleaning magazine, May/June, 2000.

Lawrence Azar is the president and owner of PPB Megasonics(Portland, OR), established in 1996. He received his Bachelor of Sci-ence in engineering from University of California at Berkeley, anda Master of Science in engineering from the Massachusetts Instituteof Technology (MIT). His research at MIT focused in the nondestruc-tive evaluation (NDE) of materials, focusing on ultrasonic testing,and published a number of articles in this field. He has two patentsrelating to ultrasonic cleaning, and is a member of the AcousticalSociety of America, IEEE and SEMI. Lawrence can be reached atazar@megasonics.com or visit the Web site at www.megasonics.com

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Figure 4: Cavitation probe for megasonic jet streams.(Image courtesy of PPB Megasonics)

Figure 5: Cavitation probe for ultrasonic horns. (Image courtesy of PPB Megasonics)

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