U.S. patent application number 10/153903 was filed with the patent office on 2002-09-26 for article produced by acoustic cavitation in a liquid insonification medium.
Invention is credited to Madanshetty, Sameer I..
Application Number | 20020134402 10/153903 |
Document ID | / |
Family ID | 23940208 |
Filed Date | 2002-09-26 |
United States Patent
Application |
20020134402 |
Kind Code |
A1 |
Madanshetty, Sameer I. |
September 26, 2002 |
Article produced by acoustic cavitation in a liquid insonification
medium
Abstract
An article produced by acoustic cavitation by controlling
cavitation events in a liquid insonification medium utilizing a
waveform to excite a transducer with a series of tone bursts having
medium recovery intervals between respective bursts so that the
medium repeatedly recovers from cavitation events between bursts.
Articles include a device made from a semiconductor wafer that is
cleaned or otherwise processed by acoustic cavitation, a surface
having been de-coated by acoustic cavitation, a chemical compound
made from a chemical substance reacted by acoustic cavitation, and
recycled paper made from inked paper de-inked by cavitation.
Cavitation events are generated using a transducer and a waveform
generator, e.g., square wave tone bursts, to excite the transducer
with a signal controlled in frequency, burst repetition rate,
duty-cycle and/or amplitude, e.g., utilizing bursts having a
frequency between 500 KHz and 10 MHz, and a duty cycle between 0.1%
and 70%.
Inventors: |
Madanshetty, Sameer I.;
(Manhattan, KS) |
Correspondence
Address: |
Lawrence Harbin
McIntyre Harbin & King
Suite 330
One Massachusetts Avenue, N.W.
Washington
DC
20001
US
|
Family ID: |
23940208 |
Appl. No.: |
10/153903 |
Filed: |
May 24, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10153903 |
May 24, 2002 |
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09488574 |
Jan 21, 2000 |
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6395096 |
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Current U.S.
Class: |
134/1 ;
134/1.3 |
Current CPC
Class: |
G10K 15/043 20130101;
B06B 2201/71 20130101; Y10S 134/902 20130101; B06B 1/0215 20130101;
B06B 1/0603 20130101; B06B 1/067 20130101; G10K 11/006 20130101;
G10K 11/08 20130101; B08B 3/12 20130101 |
Class at
Publication: |
134/1 ;
134/1.3 |
International
Class: |
B08B 003/12 |
Claims
1. An article produced by acoustic cavitation comprising the steps
of: providing a transducer, providing a liquid insonification
medium in which cavitation is induced, coupling the transducer with
the article through a continuum of the liquid insonification
medium, and energizing the transducer with a tone burst waveform
having recovery intervals between respective bursts to produce a
signal that co-acts with the transducer and the medium to generate
on or about the article an acoustic field having multiple high
frequency components and multiple lower frequency components
whereby to perform an event relative to the article.
2. The article produced according to claim 1, further comprising:
providing an impedance-modifying layer on the transducer to
compensate for variance in acoustic properties between material of
the transducer and the liquid insonification medium whereby to
improve energy transfer between the transducer and the medium.
3. The article produced according to claim 2, wherein the
impedance-modifying layer has a thickness of about one-quarter
wavelength of a component of the acoustic field generated by the
transducer.
4. The article produced according to claim 2, wherein the
impedance-modifying layer comprises a polymeric material.
5. The article produced according to claim 1, wherein the
transducer is made from one of a lead titanate zirconate
piezoelectric ceramic material and a lithium niobate material.
6. The article produced according to claim 1, further including
providing a non-fluid chamber on a side of the transducer opposite
the medium.
7. The article produced according to claim 1, further comprising:
controlling an extent of cavitation events generated by the
transducer in accordance with a desired rate of work performed.
8. The article produced according to claim 1, wherein the article
includes a surface and the energizing step includes directing
cavitation events upon a coating of the surface via a liquid
coupling between the transducer and the surface in order to de-coat
the coating on the surface.
9. The article produced according to claim 8, wherein the liquid
coupling is formed by a streaming jet providing a liquid
communicating path between the transducer and the surface.
10. The article produced according to claim 1, wherein the
insonification medium comprises clean water, the article comprises
a device made from a semiconductor wafer, and the energizing step
includes directing cavitation events in the vicinity of the
semiconductor wafer via liquid coupling between the transducer and
the wafer in order to clean the wafer by evicting particulates
therefrom.
11. The article produced according to claim 1, wherein said article
comprises a paper substrate having fused ink thereon and said
energizing step includes directing cavitation events upon the paper
substrate via an acoustic coupling between the transducer and the
substrate in order to de-ink the substrate.
12. The article produced according to claim 1, wherein the article
comprises a chemical compound produced by reacting a chemical
substance, and the energizing step includes directing cavitation
events towards the substance in order to stimulate a liquid-based
chemical reaction.
13. The article produced according to claim 1, wherein the article
comprises a surface and the energizing step includes directing
cavitation events upon the surface via a liquid coupling between
the transducer and the surface in order to remove a coating adhered
to the surface.
14. An article produced by a method of acoustic cavitation
comprising: providing an air-backed resonant mode transducer,
providing a liquid insonification medium in which cavitation is
induced, coupling the transducer and the article through a
continuum of the liquid insonification medium, energizing the
transducer in a thickness direction with a tone burst waveform
having recovery intervals between respective bursts of a frequency
between 500 KHz and 10 MHz to produce a waveform that co-acts with
the transducer and the medium to generate on or about the article
an acoustic field having multiple high frequency components and
multiple lower frequency components, and controlling a duty cycle
of the tone burst waveform between a range of 0.1% and 70%.
15. The article produced according to the method of claim 14,
wherein the method further comprises providing an
impedance-matching layer on the resonant mode transducer.
16. The article produced according to the method of claim 15,
wherein the transducer comprises one of a polymeric material, a
lithium niobate material, and a titanate zirconate material.
17. The article produced according to the method of claim 15,
wherein the bursts comprise a series of in-harmonic waves.
18. An article produced by an acoustic cavitation method that
comprises: providing a resonant mode transducer, providing a liquid
insonification medium in which cavitation is induced, coupling the
transducer and the article through a continuum of the liquid
insonification medium, and energizing the transducer in a thickness
direction with an in-harmonic tone burst signal having recovery
intervals between respective bursts that allow the medium to
recover from cavitation events whereby to produce a waveform that
co-acts with the transducer and the medium to generate on or about
the article an acoustic field having multiple high frequency
components and multiple lower frequency components.
19. The article produced according to claim 18, wherein the
insonification medium comprises clean water, the article comprises
a device made from a semiconductor wafer, and the energizing step
includes directing cavitation events in the vicinity of the
semiconductor wafer via liquid coupling between the transducer and
the wafer in order to clean the wafer.
20. The article produced according to claim 18, wherein the article
comprises paper made from a paper substrate having fused ink
thereon and the energizing step includes directing cavitation
events upon the paper substrate via an acoustic coupling between
the transducer and the substrate in order to de-ink the
substrate.
21. The article produced according to claim 18, wherein the article
comprises paper made from paper pulp including fused ink thereon
and the energizing step includes directing cavitation events
towards the pulp via an acoustic coupling between the transducer
and the pulp in order to de-ink the pulp.
22. The article produced according to claim 18, further comprising:
providing an impedance-modifying layer on the transducer to
compensate for variance in acoustic properties between material of
said transducer and the liquid insonification medium whereby to
improve energy transfer between the transducer and the medium.
23. The article produced according to claim 22, wherein the
impedance-modifying layer has a thickness of about one-quarter
wavelength of a component of the acoustic field generated by the
transducer.
24. The article produced according to claim 22, wherein the
impedance-modifying layer comprises a polymeric material.
25. The article produced according to claim 18, wherein the
transducer comprises one of a lead titanate zirconate piezoelectric
ceramic material and a lithium niobate material.
26. The article produced according to claim 18, further including
providing a non-fluid chamber on a side of the transducer opposite
the medium.
27. The article produced according to claim 18, further comprising:
controlling an extent of cavitation events generated by the
transducer in accordance with a desired rate of work performed.
28. The article produced according to claim 27, further comprising
controlling a duty cycle of the tone burst signal between 0.1% and
about 70%.
29. The article produced according to claim 27, further comprising
providing a frequency of said tone burst between 500 KHz and 10
MHz.
Description
CROSS-REFERENCE TO RELATED PATENTS AND PATENT APPLICATIONS
[0001] This invention is a continuation of and claims the benefit
of commonly-owned, co-pending U.S. application Ser. No. 09/488,574
filed Jan. 21, 2000 in the name of the inventor hereof and entitled
"Single Transducer ACIM Method and Apparatus" (U.S. Pat. No.
6,395,096 with amended title "Method of Producing Acoustic
Cavitation in a Liquid Insonification Medium).
[0002] The inventor hereof has also been issued U.S. Pat. Nos.
5,594,165 and 5,681,396, which disclose microcavitation for
submicron particle detection and removal.
BACKGROUND OF THE INVENTION
[0003] This invention relates to methods of acoustic cavitation,
but more specifically, to an article of manufacture produced by
acoustic cavitation. In particular, the invention relates to
performing a task by subjecting the article or component thereof to
microcavitation events generated in a liquid insonification
medium.
[0004] Acoustic microcavitation, which is the inducement of micron
or sub-micron size bubbles in a liquid or fluid medium that survive
a few microseconds or less, is to be contrasted with ultrasonic,
megasonic, and cyrogenic aerosol cleaning methods.
[0005] Microcavitation has been used on a limited scale or
conceived for use in microparticle or sub-nanometer particle
detection in ultrapure liquids, submicron particle eviction from
silicon wafers, deinking of recyclable paper, paint removal,
surgical procedures, destructive and non-destruction testing and
measuring, thin film processing applications, etc.
[0006] Previously, at least two transducers were required to
initiate and maintain cavitation. Prior ACIM was induced using a
low frequency, high intensity primary acoustic field and a higher
frequency, low intensity coaxing acoustic field. To effect ACIM,
the two fields were substantially simultaneously directed at a site
of a workpiece or object. It was crucial that at least part of the
high frequency acoustic waves in the fluid medium pass the desired
ACIM site precisely when the tensile part of the low frequency
waves was present at the site. In this arrangement, it sometimes
became unwieldy to articulate two transducers of different
frequencies to achieve the desired ACIM zone, stationary or moving,
where the different acoustic fields were to be synchronized and
collocated.
[0007] Therefore, a need has arisen to simplify ACIM apparatuses
and techniques to make them more practical to apply to the various
applications identified herein.
[0008] In general, cavitation is the formation of cavities or
bubbles in a liquid where the ensuing bubble dynamics and energy
concentration result in implosive collapse of bubbles that achieve
unique and surprising results. In the design of mechanical systems,
cavitation has known destructive effects and therefore, was
avoided. Cavitation remains enigmatic today as it was when Lord
Rayleigh first investigated cavitational erosion of propellers
almost a century ago. Cavitation is a mature subject and an
encylopedic collection of information on acoustic cavitation is
compiled in "Acoustic Bubble" by Tim Leighton (1997). Hydrodynamic
cavitation is discussed in "Cavitation and Multiphase Flow
Phenomena" by Frederick Hammitt (1980). Whether induced
acoustically or associated with hydrodynamic flows, the mechanics
and effects of cavitation are essentially the same. Acoustic
cavitation has was also exhaustively reviewed by Flynn (1964),
Neppiras (1979), Apfel (1981) and Prosperetti (1986).
[0009] Consider, for example, a free bubble in the path of a sound
wave. In response to the sound wave, the bubble expands and
contracts, and the energy mechanically stored during expansion is
released in a concentrated manner during implosive collapse of the
bubble. Should the bubble grow to about two and a half times its
nominal or equilibrium size during negative excursions of acoustic
pressure, then during the following positive half cycle of
pressure, its speed of collapse could become supersonic
(Lauterborn, 1969) thereby releasing excess energy that
catastrophically explodes the bubble. Such almost single cycle
violent events are called transient or inertial cavitation, and may
explain the energetic manifestations of cavitation which, among
other things, are useful for surface erosion or particle
eviction.
[0010] Unlike dramatic bubble growth within a single acoustic cycle
seen in transient or inertial cavitation, there exists a more
gradual process, termed rectified diffusion. Under favorable
conditions, a small bubble exposed to a continuous sound wave tends
to grow in size if rectified diffusion is dominant. According to
Henry's law, for a gas soluble in liquid, the equilibrium
concentration of dissolved gas in the liquid is directly
proportional to the partial pressure of the gas above the liquid
surface, the constant of proportionality being a function of
temperature. When the bubble expands, the pressure extant at the
bubble's interior falls and gas diffuses into the bubble from the
surrounding liquid. When the bubble contracts, the pressure in the
interior increases and the gas diffuses into the solution of the
surrounding liquid. The area available for diffusion, however, is
larger in the expansion mode than in the contraction mode.
Consequently, there is a net diffusion of the gas into the bubble
from the surrounding liquid over a complete cycle, which causes
bubble growth due to rectified diffusion.
[0011] However, a bubble can grow only up to a critical size--to a
resonance radius determined by the frequency of the impressed sound
wave. For small amplitude oscillations, a bubble acts like a simple
linear oscillator of mass equal to the virtual mass of a pulsating
sphere, which is three times the mass of displaced fluid. Stiffness
is primarily given by the internal pressure of the bubble times the
ratio of specific heats. Surface tension effects are, however,
significant for small bubbles. Following Minnaert (1933) and
ignoring surface tension, there is a simple relation for the
resonance radius of air bubbles in water:
(Resonance radius in .mu.m).times.(insonification frequency in
MHz)=3.2
[0012] This relation is valid within 5% even for a bubble radius of
about 10 .mu.m. Bubble response becomes increasingly vigorous at
the resonance radius, and is limited by damping mechanisms in the
bubble environment--e.g., viscous damping, acoustic radiation
damping, and thermal damping. A post-resonance bubble may exhibit
nonlinear modes of oscillations, or become transient if the applied
acoustic pressure amplitude is adequately high.
[0013] The above discussion presupposes the presence of a free
bubble in the path of a sound wave. Free bubbles, however, do not
last long in a body of water. Larger ones are rapidly removed due
to buoyancy and the smaller ones dissolve even in nearly saturated
water. While a 10 .mu.m air bubble rises in water at a terminal
speed of 300 .mu.m/s, it can survive for about five seconds before
dissolving completely. Dissolution is driven essentially by the
excess pressure inside the bubble due to the surface tension.
[0014] It is very difficult to cavitate clean liquids (Greenspan
and Tschiegg, 1967). A pure liquid purged of particulate impurities
and stored in a perfectly smooth container can attain its
theoretical tensile strength before undergoing cavitation or
fracture. Under ideal conditions, water can be as strong as
aluminum. The tensile strength of water based on the homogeneous
nucleation theory exceeds 1000 bars. In cavitation studies, tensile
strength is often quoted in terms of negative pressures, and
cavitation threshold is understood as the pressure amplitude at
which the first occurrence of cavitation is detected. Observed
strengths (thresholds) in practice, however, are very much lower,
rarely exceeding a few bars for reasonably clean liquids. This is
because there exist gas pockets within the liquid which provide the
necessary seeding for cavitation to occur at lower pressures.
[0015] A gas or cavitation site is often stabilized in a crevice
(Harvey et al., 1944), either in a container wall or on a
fluid-borne particle. Incomplete wetting traps gas at the root of a
sharp crevice, stabilizing it against dissolution. Unlike a free
bubble, though, surface tension in this case acts on a meniscus
which is concave towards the liquid. Over-pressuring the liquid for
sufficient duration prior to insonification can force the meniscus
further into the crevice thereby causing full wetting of the
crevice, which then gives rise to increased cavitation
thresholds.
[0016] Until recently most acoustically generated, cavitation
employed for cleaning applications, primarily used standing waves
generated in a bath of liquid in which objects to be cleaned were
immersed. In such ultrasonic cleaners, acoustic frequencies used
were typically between 20 kHz to 100 kHz. Some implementations used
propagating pulse trains instead of standing waves to improve
cleaning efficiency, to minimize hot spot damage, and to reduce
power consumption. Even so, when these applications were extended
to semiconductor applications, cavitation was deemed detrimental to
the delicate wafer surfaces, which spawned the use of megasonic
cleaning to avoid cavitation (e.g. U.S. Pat. No. 4,854,337 to
Bunkenburg et al., 1989; U.S. Pat. No. 4,979,994 to Dussault et
al., 1990; U.S. Pat. No. to 5,247,954 to Grant et al., 1993; and
U.S. Pat. No. 5,355,048 to Estes, 1994) thus teaching the use of
frequencies in the range of high kilohertz or low megahertz
(typically 1 MHz).
[0017] Such high frequencies were used because it was believed that
cavitation does not occur at higher frequencies. Quoting from the
recent book edited by Takeshi Hattori (1998) titled, "Ultraclean
Surface Processing of Silicon Wafers--Secrets of VLSI
Manufacturing;" "[w]hen the oscillation frequency is 1 MHz or
above, cavitation no longer occurs." It is precisely the supposed
inability of generating cavitation at low megahertz frequencies
that such high frequency acoustics were used in diagnostic
ultrasound for medical imaging and fetal monitoring. As a further
precaution to preclude bubble growth that may occur due to
continuous wave insonification, diagnostic instruments deployed
short pulses at low duty cycles, e.g., 1%, which incidentally also
facilitates the pulse echo method of information collection
essential for their function. Therefore, prior systems rely on
using high frequency tone burst acoustics, such as 1 MHz, when the
explicit objective is to avoid the occurrence of cavitation.
[0018] Microcavitation, i.e., the inducement of micron or
sub-micron size bubbles in a liquid or fluid medium that survive a
few microseconds or less, occurs if the pressure amplitude in the
acoustic beam is significantly greater than a threshold value, and
if appropriate cavitation nuclei are present. In the absence of
cavitation nuclei, water-like liquids cannot be fractured or
cavitated by pressure amplitudes of less than 1000 bars peak
negative, the threshold for homogeneous nucleation of water at
standard temperature and pressure (STP), which corresponds to an
atomic or molecular size vacancy or cavity in the liquid bulk
caused by thermal, stochastic density fluctuations. Stronger
tensile pressures are needed to cavitate smaller bubbles or
cavitation nuclei. A 60-atmosphere peak negative pressure wave, for
example, might cavitate a 50-nanometer bubble nucleus.
[0019] Planar piezoelectric transducers cannot generate very high
pressure amplitudes with moderate power inputs. With increased
power, however, cavitation might occur on the surface of the
transducer crystal itself which will cause destruction of the
crystal. By using focused transducers, however, it is possible to
achieve additional pressure amplification by virtue of the focusing
action at a particular site. Even so, high intensity acoustic waves
invariably become non-linear because of inherent properties of the
propagation medium. The nonlinearity in shape manifests an enhanced
compressive peak and reduced tensile peak of the wave pulse.
Cavitation at a nucleation site cannot occur if the tensile part of
the wave is not stronger than the threshold value. If the nonlinear
pulse is reflected at a pressure release boundary, then phase
reversal takes place and the compressive peak reflects as a tensile
peak and vice versa.
[0020] Using reflected nonlinear waves, it becomes easier to bring
about cavitation because now a stronger tensile peak is available.
U.S. Pat. No. 5,523,058 to Umemura et al. obviates the need for
using suitable reflecting structures to achieve enhance tensile
peaks by using two resonant transducers--one driven at a
fundamental frequency and the second driven at a second harmonic
frequency, and then superposing them in proper phase relation
between the fundamental driving frequency pulse wave and its second
harmonic wave to obtain a resultant pulse with enhanced tensile
peak and weakened compressive peak. This method of generation, like
other methods of cavitation in the past, also relies on the
availability of appropriate cavitation nuclei in the insonified
medium. Without the presence of appropriate nuclei the tensile peak
is ineffective in causing cavitation.
[0021] Although Umemura teaches that "the efficiency of cavitation
generation depends on the relative phase relation between a
fundamental wave and a second harmonic" wave and he is able to
access smaller bubble sizes (half the resonant bubble size
corresponding to the fundamental frequency), he still relies on the
availability of appropriate bubbles or bubble bearing crevice
structures in the liquid host to initiate cavitation.
[0022] Further, Umemura does not use too high frequencies at which
cavitation ordinarily does not occur. It is known in the art that
transducers generating high pressure amplitudes at high frequencies
are technologically unfeasible (high frequency resonant crystals
are necessarily thin and cannot support stresses needed for
generating high pressures), and yet to generate cavitation at high
acoustic frequencies, the pressure amplitudes necessary are
excessive.
[0023] In attempting to clean effectively throughout a cleaning
tank, Honda (U.S. Pat. No. 5,137,580, 1992) uses at the bottom of
the tank a Langvin type resonator with two resonating segments, and
drives them alternately at the two resonance frequencies for
periods of up to several milliseconds, which are adequate to setup
standing wave fields in the liquid. At the lower frequency, a
standing wave field causes large bubble cavitation to populate at
pressure antinodes to form bubble bands at specific levels in the
tank. At higher resonance frequency, Honda supposes that these
bubbles cavitate and collapse to cause some measure of cleaning,
but more importantly, because the standing wave pattern is broken,
the previously structured bubble bands move upwards due to buoyancy
and radiation forces to bring about some cleaning.
[0024] Honda suggests that these large bubbles will break down at
higher frequencies and fill the tank with smaller bubbles. In
actuality, the higher frequency waves merely reflect off the larger
bubbles. A given frequency cannot significantly affect larger
bubbles not corresponding to the characteristic resonance size.
When the low frequency is again switched on, these small bubbles
nucleate large bubble cavitation whose fragments will serve a next
sweep by the higher frequency. Most cleaning is expected to be done
by the large bubble cavitation effervescing throughout the extent
of the tank. Honda does not explicitly state the frequencies he is
using but the Langevin sandwich type transducer and the kind and
scale of cavitation he mentions leads one to believe that he must
be using acoustics in the low kilohertz range, between 20 kHz to 60
kHz.
[0025] If Honda were to use only one frequency, he would obtain a
banded structure in the tank, and once the bubbles are setup in
their locations, no significant cavitation would be sustained and
no further cleaning effect would ensue due to occurrence of bubble
effervescence. While Honda also teaches farming effectively
available bubble fields for cavitation between two frequencies,
Murry, before Honda taught how to cultivate bubble fields starting
from the smallest of bubbles that he suggests are available in the
liquid. Murry (U.S. Pat. No. 3,614,069, 1971) in his patent
"Multifrequency Ultrasonic Method and Apparatus for Improved
Cavitation, Emulsification and Mixing" teaches that operating on
the assumption there will always be some very small bubbles in the
bulk medium, insonification starts with using continuous wave
insonification of a very high frequency corresponding to which the
supposed pre-existing small bubbles are resonant. Near resonant
bubbles exposed to continuous acoustic stimulus will respond by
growing due to rectified diffusion. To continue this bubble growth
they will have to be insonated by progressively decreasing the
drive frequency. This downshifting insonification is achieved by
using broadband transducers, not resonant transducers.
[0026] As the bubbles grow by downshifted continuous wave
insonification, Murry applies a low frequency intense field to
cavitate these bubbles. He upshifts or upconverts this low
frequency to high intensity field so as to capture and
cavitationally collapse any slightly smaller bubbles that may
exist, as not all bubbles grow uniformly and simultaneously to a
given size. Murry, operating on the assumption that very small
bubbles exist in the liquid, concentrates on cultivating
appropriate size bubbles by continuous wave insonification. Such
bubbles are gas-filled as a result of rectified diffusion, they are
not vacuous or nearly empty. Implosion of gas-filled bubbles is
less energetic because the collapse is cushioned by the cavity
contents.
[0027] Starting from a few tiny seed bubbles whose existence is
assumed, Murry cultivates bubble fields with bubbles progressively
growing over time in response to frequency downshifted
insonification, and then violently collapsing them by applying low
frequency high intensity acoustic field, the latter being
subsequently upshifted in frequency to harvest all possible bubbles
for cavitation. He uses two broad-band transducers to facilitate
frequency shifting, and even interchanges the roles of the bubble
grower and bubble exploder transducers for appropriate cycling and
sustaining cavitation throughout the extent of the bulk being
processed for emulsification or mixing.
[0028] In summary the prior art teaches that a perfectly clean
liquid absent of bubbles or bubble-like structures cannot be easily
cavitated. To bring about cavitation in ultra clean hosts,
especially at high frequencies, is almost impossible primarily
because the acoustic drivers, the piezoelectric transducers used to
generate cavitation cannot be made to generate high pressure
amplitude sound waves at high frequencies. It is possible to a
limited extent to generate high tensile pulses, but only with
reduced compressive pulses if one drives the transducer in both
fundamental and second harmonic excitation in precise phase
relationship.
[0029] To achieve this, one must use two transducers. In resonant
mode excitation, the transducer can only be driven at odd harmonics
of the fundamental frequency. Even if one is able to obtain high
pressure amplitude at high frequency, one needs to assume that a
population of small bubbles always exist in a liquid, then
insonifying the liquid medium with continuous acoustic waves of
appropriately high frequency, frequency specific to excite
resonance in the bubbles, can grow the bubbles to a larger size
through rectified diffusion, whence subsequent insonification by a
lower frequency of sufficient intensity one can bring about
cavitation. Being gas-filled these long-lived bubbles cannot
sufficiently implode to create high energy density points in the
medium, and are thus ineffective to bring about the effects of ACIM
described herein.
[0030] It is known in physics of liquids that free bubbles in a
liquid are unstable and do not survive for any significant duration
after their creation. Larger bubbles rise and escape out of the
liquid because of buoyancy, while smaller bubbles dissolve due to
surface tension forces which are dominant for small bubbles. Any
bubble-like structure that survives in liquid has to be anchored in
a crevice like feature in a solid, e.g., a wall or liquid-borne
particle. Not all liquid borne particles are capable of supporting
such partially wetted crevices, particularly, smooth spherical
particles cannot harbor such gas-filled cavities.
[0031] Apart from the inventor's own work, the teachings of the
entire prior art appears to rely on cavitation as a chance
dominated phenomenon. In addition, it is not taught or suggested in
the prior art how to create cavitation nuclei when none exist a
priori, and then to control such cavitation after onset.
[0032] Therefore, to achieve useful applications provided by the
present invention in a practical and convenient manner, prior
systems and methods do not take into account: (i) how to activate
or nucleate a cavitation event from a particle, regardless of
whether or not it has a gas bearing crevice, (ii) how to
acoustically activate or nucleate cavitation amongst particles,
however, small they may be, or whatever be their composition or
surface morphology, (iii) consideration of the number of times a
cavitation event ensues in relation to a given or created gas
bearing crevice and/or point phase boundary, or (iv) attaining
vacuous cavitation to the maximum extent possible rather than
gaseous cavitation.
[0033] In vacuous cavitation the cavity is nearly empty. Only
transiently (or inertially) generated cavitation involves vacuous
cavities. Cavitation generated by continuous waves is gaseous
cavitation. Only vacuous cavitation can be imploded, unimpeded,
unto a point, and hence, only vacuous cavitation can culminate in
high energy density at points. To be able to implement items (i)
through (iv) implies that cavitation is being constructively
controlled in all phases--inception, evolution and intensity.
[0034] To the inventor's knowledge, the entire prior art concerns
itself with cavitation as chance dominated phenomenon, and does not
teach how to manage cavitation in a practical and efficient way to
perform a useful purpose, except in the inventor's two recent U.S.
Pat. No. 5,681,396 (1997) and U.S. Pat. No. 5,594,165 (1997), which
deal with acoustic coaxing methods for constructive control of the
cavitation phenomenon using confocal transducers.
[0035] ACIM methods described herein, on the other hand, employ a
single transducer to more effectively control the onset, evolution
and intensity of microcavitation. Generating ACIM with a single
transducer enables expanded utility including, improved deinking of
paper (e.g., removal of bonded, laser printed Xerox ink, i.e.,
toner-based ink compositions), practical depainting of surfaces
(including selective removal of layers in a multi-layered painted
surface (primer and/or top coat)), thin film strength testing and
surface preparation prior to thin film deposition; semiconductor
wafer cleaning; improved microparticle detection in clean liquids;
improved particle removal for precision cleaning of delicate
surfaces; and better particle size control in the preparation of
nanometer particles like gold sols. In addition, improved ACIM
methods and apparatuses of the present invention may be used to
erode metallic surfaces, help shatter kidney stones, accelerate
chemical reactions and even lead to light production, i.e.,
sonoluminescence.
SUMMARY OF THE INVENTION
[0036] In a first embodiment, the invention comprises an article
produced by acoustic cavitation by providing a transducer, which is
preferably air-backed and operated in resonance mode; providing a
liquid insonification medium, e.g., clean water, in which
cavitation is induced; coupling the transducer with the article
through a continuum of the liquid insonification medium; and
energizing the transducer with a tone burst waveform having
recovery intervals between respective bursts to produce a signal
that co-acts with the transducer and the medium to generate on or
about the article an acoustic field having multiple high frequency
components and multiple lower frequency components whereby to
perform an event relative to the article.
[0037] In another embodiment, the invention comprises an article
produced by providing a resonant mode transducer; providing a
liquid insonification medium in which cavitation is induced;
coupling the transducer and the article through a continuum of the
liquid insonification medium; and energizing the transducer in a
thickness direction with an in-harmonic tone burst signal having
recovery intervals between respective bursts that allow the medium
to recover from cavitation events whereby to produce a waveform
that co-acts with the transducer and the medium to generate on or
about the article an acoustic field having multiple high frequency
components and multiple lower frequency components.
[0038] Other features and aspects of the invention include, but are
not limited to, controlling or varying the waveform source in
waveform shape, frequency, duty cycle, tone burst repetition rate,
amplitude or other parameters. The invention, though, is pointed
out with particularity by the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1(a) shows an exemplary ACIM apparatus including a
waveform source, a transducer, a liquid or fluid medium, a delivery
mechanism, and a workpiece or surface subjected to ACIM in
accordance with the present invention.
[0040] FIG. 1(b) shows a prior art ACIM apparatus comprising a pair
of confocal high frequency and low frequency transducers to produce
an ACIM field.
[0041] FIG. 2 illustrates dynamics of gas cap formation on a
particle subjected to ACIM acoustics that initiates
microcavitation.
[0042] FIG. 3 depicts an exemplary tone burst waveform applied to
the ceramic transducer of FIG. 1(a) for generating acoustic coaxing
fields.
[0043] FIG. 4(a) is a two-dimensional illustration of a ceramic
transducer useful for generating acoustic coaxing fields according
to the present invention.
[0044] FIG. 4(b) is a perspective view of an elongated transducer
module in accordance with the present invention.
[0045] FIG. 5 depicts an exemplary apparatus for subjecting a
workpiece or surface to ACIM fields according to the present
invention.
[0046] FIG. 6 depicts an exemplary apparatus for deinking paper or
other planer substrates using ACIM methods according to the present
invention.
[0047] FIG. 7 illustrates an exemplary ACIM method carried out by
the present invention.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0048] Acoustic coaxing induced microcavitation (ACIM) methods and
apparatuses described herein may be used to control microcavitation
at point solid boundaries of an object or workpiece to perform work
on the object; examine free bubbles in a fluid or liquid for
testing or measuring; induce or assist a chemical reaction; or
perform other scientific, industrial, or medical tasks. ACIM tools
may be constructed to perform abrasion, cutting, drilling, or other
action with respect to a variety of organic and inorganic
materials, including tissue and bone.
[0049] Controlled ACIM enables one to control the onset, evolution,
and intensity of acoustic microcavitation stemming from the
creation of new nuclei or the presence of available cavitation
nuclei in a liquid medium, such as de-ionized or tap water. Such
nuclei may come from free bubbles or from liquid-borne solid
particulates with crevice-like features that stabilize significant
gas pockets. Suspended particulates may include sub-micron
polystyrene particles (e.g., 0.984 micrometer mean diameter),
silica, dust, etc. that enhances the presence of cavitation nuclei
for enhanced cavitation. Ordinarily only a small fraction of the
particles present in a host medium (liquid, fluid, gel, or other
acoustic propagation medium) is capable of harboring stabilized gas
pockets that serve as potential cavitation nuclei. Smooth spherical
particles as illustrated in FIG. 2, for example, do not easily
nucleate cavitation because they have no significant crevices to
support gas pockets. Microcavitation, therefore, is a chance
dominated process primarily dictated by the presence of
adventitious motes. ACIM, however, can cultivate cavitation nuclei
where none existed before.
[0050] FIG. 1(b) shows a prior art system where microcavitation was
brought about by the coordinated confluence of beams (i.e.,
alignment in space and time) of two separate acoustic fields 10 and
12 deployed substantially simultaneously in space and time at a
desired ACIM site 14. Acoustic field 12 was produced by a focused,
e.g., sectioned spherical or parabolic, piezoelectric transducer 16
of low frequency and high intensity, and the second acoustic field
10 was produced by a second transducer 18 operating at high
frequency but low intensity. Low frequency transducer 16 produced
an acoustic field of about one megahertz and high frequency
transducer 18 produced an acoustic field of about thirty megahertz.
Each transducer was operated by applying a sinusoidal driving
voltage at its fundamental resonance frequency, in tone bursts of
low duty cycle, and by directing their respective acoustic fields
upon the surface of a workpeice at site 14. This arrangement
provided only limited utility due to limitations on the size of the
ACIM site 14, and complexity and physical constraints of the
transducers 16 and 18, e.g., requirement of spatial and temporal
coincidence as well as alignment of acoustic fields, limited
latitude control of ACIM, and other limitations.
[0051] FIG. 1(a) depicts one arrangement of the present invention
where a single transducer 20 efficiently produces coaxing high and
low frequency acoustic fields at a site 22. The exemplary ACIM
apparatus of FIG. 1(a) comprises a tone burst generator 40, an RF
amplifier 42, an optional oscilloscope 44, transducer support 46, a
fluid chamber 45 in which the support 46 and transducer 20 are
immersed, and a fixture 48 for supporting a workpiece at coaxing
site 22.
[0052] Use of a single transducer 20 facilitates control of the
onset, evolution, and intensity of ACIM events to achieve more
useful industrial, scientific and medical applications. A key
factor required in acoustic coaxing is that the frequency (MHz)
pressure (peak negative bars) product is maintained above a certain
value (typically greater than 5 MHz-bars). ACIM is achieved by
driving transducer 20 not only with "sinusoidal" signal but also
with a complex waveform, such as square wave tone bursts (FIG. 3)
where duty-cycled controlled bursts have Fourier components that
produce a combination of acoustic fields which, when converged at
ACIM site 22, produce substantially the same or similar effect as
multiple acoustic fields generated by prior art confocal
transducers 16 and 18 of FIG. 1(b).
[0053] Fluid chamber 45 may comprise a tank, reservoir, channel,
conduit, nozzle, or other confine which couples the acoustic field
with an object, workpiece, tissue, or surface and which confines
the liquid medium about the transducer and ACIM site 22. In
transducer support 46, the liquid medium fills the space between
the transducer 20 and the ACIM site 22. Liquid may be confined to
the container, it could be made available as a liquid jet medium
between transducer 20 and ACIM site 22, it may be contained by a
sponge or other liquid retaining structure, or it may be a gel or
any other medium that can undergo phase changes involving liquid
(or liquid like) phase and gas phase (bubble like or vacuum). In
some cases, to minimize the gel or liquid volume the ACIM
transducer is coupled to the workpiece with an acoustic horn and
using the liquid or the gel in the small gap between the horn and
the workpiece surface.
[0054] Still referring to FIG. 1(a), generator 40 produces, for
example, one megahertz square wave tone bursts (i.e., 10 .mu.s
pulse width) with a burst repetition rate of about one kilohertz.
This differs from sinusoidal waves previously used. The duty cycle
of the generator output may be controlled between about 0.1% to
50%. Higher duty cycles, e.g., up to 80 to 90%, can be used but may
cause the transducer to overheat and lose its efficiency and
transducing capacity. Lower duty cycles, on the other hand, improve
cooling of the transducer. Conventional cooling systems, such an
circulating the fluid medium through a heat exchanger, can also be
used with ACIM. As indicated above, it is preferable to induce
transient cavitation with newly formed bubbles in order to obtain a
more intense and vigorous release of energy during implosion. Lower
duty cycles enable recovery periods for bubble formation between
burst repetitions. At duty cycles beyond 50%, cavitation tends to
result also from rectified diffusion, e.g., gaseous, which results
in lower intensity upon collapse of the bubbles due to the bubble's
ingestion of gases from the surrounding medium, thus diminishing
the overall intensity of energy release. For cleaning, particle
eviction, or destructively removing particles from a surface, it
has been found that only a few tone bursts are needed, which means
that deinking of paper (substrate or bulk form), for example, will
occur in a few milliseconds or less, or a fraction of a
millisecond. Accordingly, most ACIM applications can be
accomplished using very low duty cycles.
[0055] Thus, the intensity of cavitation at site 22 may be
controlled the burst repetition rate and duty cycle of generator
40, and/or by controlling the gain of amplifier 42.
[0056] Square wave tone bursts produced by generator 40, for
example, when Fourier-decomposed, yield harmonics with amplitudes
decreasing inversely as the harmonic frequency increases. So a
transducer driven with square wave tone bursts will contain odd
harmonics (half wavelength thick acoustic transducers suppress even
harmonics) with precisely reducing amplitudes while maintaining the
frequency-pressure product uniform. Instead of using a square wave,
other waveforms or harmonics may be selected and/or combined to
achieve constructive control of cavitation. Thus, coaxing becomes
more efficient, permitting one to use a single transducer to induce
cavitation.
[0057] Instead of square wave tone bursts, microcavitation may be
induced by generator 40 producing triangular waves for driving the
transducer. This will produce all odd harmonics with a 1/N.sup.2
amplitude dependence. For efficient coaxing, 1/N dependence is
appropriate, however, a coaxing effect can occur at any non-zero
amplitude dependence of high frequencies. To attain coaxing
effects, the transducer produces a range of acoustic intensity at
the focal point, or in the effective coaxing zone, up to about ten
kilowatts/cm.sup.2 at lower frequencies to about several hundred
watts/cm.sup.2 at higher frequencies. Depending on the application,
one may choose the required harmonics and amplitudes to be used. At
high intensity, it is also possible to achieve coaxing effect
merely by using fundamental excitation frequencies, e.g., a
sinusoidal driving voltage.
[0058] To aid understanding of the invention, FIG. 2 shows bubble
dynamics. Weak high frequency planar waves of about thirty
megahertz and a pressure amplitude of 0.5 bar create very high
accelerations (6.5.times.10.sup.5 g units) of particle 34 during
the passage of sound wave 32 in the medium 30. As known, air is 830
times lighter than water--strong density contrast with respect to
water host. At high acceleration in a reduced pressure environment,
the tensile environment expropriates dissolved gas or vapor from
the liquid onto the solid particle resulting in cavitation
nuclei.
[0059] The onset of cavitation may be enhanced by adding particles
to the medium 30, such as 0.984 .mu.m, 0.481 .mu.m, and 0.245 .mu.m
diameter smooth polystyrene latex particles; 0.784 .mu.m silica
particles variously sintered; and tap water with its natural
particulate content--varying the dissolved air content of the host
water. Varying the number density of particles present and
different acoustic duty cycle settings invariably can achieve
reduced microcavitation thresholds. Moreover, coaxing induced
cavitation activity at or near threshold intensity is directly
proportional to the particle number density in the test cell.
[0060] Strong density contrast combined with high acceleration
enhances kinetic buoyancy effects, which further encourage
formation of gas caps 36, 37 of diameter d on the oscillating
particle 34. Unlike a free bubble in a liquid, the gas cap
structure is provided by an isotropic tensile environment 30
surrounding the entrained particle 34, and not by the pressure of
the cavity contents. In fact, the gas caps 36, 37 will be mostly
vacuous, and only provide a necessary discontinuity that develops
opposition between the surface tension forces anchoring along the
contact perimeter and the tensile forces trying to pull the caps
off. The particle 34 is much smaller compared to the acoustic
wavelength and therefore experiences a uniform pressure over its
extent--maximum particle size of 1 .mu.m, wavelength in water a 1
MHz and 30 MHz are 1500 .mu.m and 50 .mu.m, respectively, and the
particle 34 is fully entrained in the host fluid.
[0061] For cavitation to occur, the negative pressure p in the
tensile environment of the low frequency cavitation field should
overcome the surface tension .sigma. (where surface tension
force=.sigma..pi.d) acting on the contact perimeter of the gas cap
36, 37 is represented by:
.sigma..pi.d=p(.pi.d.sup.2/4).
[0062] FIG. 3 shows the signal output of exemplary tone burst
generator 40 (FIG. 1(a)), which is a square wave tone burst 38 of
about 1 MHz (1/C) and a duty cycle (A/B) of about 1%. In the
exemplary waveform, A=10 .mu.s, B=1 millisecond, and C=1 .mu.s. The
value of these parameters may be widely varied without departing
from the spirit of the invention, the objective being to excite a
single transducer 62 with a waveform comprising harmonics that
cause the transducer to produce an ACIM region. Generator 40 is
capable of generating a bi-polar square wave with or without an
adjustable baseline bias. Waveforms having other shapes, e.g.,
triangular, or a combination of waveforms of various shapes may be
used provided they effect ACIM field generation by transducer 62.
The frequency of the square wave during the "on time" may range
between 500 kHz and 10 MHz (more or less), with one megahertz being
generally used for ACIM. The waveform has a maximum open circuit
voltage amplitude swing of one volt rms (peak-to-peak) before being
suitably attenuated and applied to a 50-ohm input impedance of
broadband (bandwidth typically between 500 kHz and 100 MHz), linear
power amplifier 42, which has a typical maximum available gain of
55 to 60 dB. It is particularly useful if the waveform generator 40
has the capability to generate arbitrary waveforms of any desired
shapes that have high frequency harmonics. Very high frequency
components are diminished and/or damped due to inherent material
properties of the ACIM transducer. Harmonic frequencies up to about
100 MHz are usable in coaxing effects over short ranges of fluid
paths.
[0063] To induce microcavitation, the waveform 38 produced by
generator 40 need not be symmetrical over the time axis 39 of the
waveform shown in FIG. 3. The primary waveform should be
convertible tone bursts of various duty cycles ranging from about
0.1% to 50%, or even continuous for short duration of intermittent
schedule. Waveform generator 40 may be controlled in frequency
and/or duty cycle and the amplifier 42 control the amplitude of the
tone bursts applied to transducer 20 in order to control the onset
and evolution of induced cavitation.
[0064] As indicated, coaxing is a mechanism of inducing a phase
change as it expropriates a miniscule amount of dissolved gas from
the liquid onto a liquid-borne solid particle, however small the
particle may be. The gas phase inoculated on the particle is
independent of the surface morphology or the material attributes of
the solid phase. The gas phase may also include local vaporization
of the host liquid itself. This nucleation of the gas seed on the
particle can occur even when there are no preexisting crevice
trapped gas sites or any bubble precursors present. It is almost a
homogeneous nucleation induced or promoted by a local tensile
environment and high acceleration field. The frequency-pressure
product produced in medium 30 (FIG. 2) which determines the wave
associated acceleration must be high enough to ensure around a
million-g units of acceleration in the vicinity of a particle. This
means the harmonic content and strengths of the waveform produced
by generator 40 and amplifier 42 cause transducer 20 to emit
acoustic fields such that the frequency-pressure product is almost
a uniform value exceeding 5 bar-Mhz in a relatively clean liquid
environment.
[0065] The duty cycle of waveform 38 should not be 100% because one
wants to induce fresh nucleation sites. If the duty cycle is 100%
or insonification is of longer duration or continuous, then once
the nucleation site is formed it will grow by rectified diffusion,
the bubbles will be gassy, and the transient or inertial cavitation
is diminished. For energetic ACIM effects, generator 40 should be
controlled to induce transient cavitation events involving vacuous
or empty cavities imploding. ACIM also will not be effectively
brought about by spike pulses of generator 40 because one wants the
leading wave pulses to initiate the nucleation ab initio and the
following pulses or waves in the tone burst to modulate the
inertial cavitation implosion.
[0066] High surface tension increases the energy intensity of point
ACIM events, and high viscosity of medium 30 (FIG. 2) slows the
rate of deposition of energy. The net strength of the implosion is
not so much determined by the compressive peak of the driving tone
burst 38, but by the converging induced momentum in the medium 30
surrounding the expanding cavities 36 and 37, i.e., the return of
the stored inertial effect. Cavitation activity does not occur at
all intensities of insonification. There is always a threshold
intensity for a given environment below which cavitation does not
occur. With ACIM it is possible to lower the threshold or even
increase it, and also the cavitation activity above threshold can
be controlled.
[0067] Medium 30 should be clean i.e. particle free (except the
particles inducing the ACIM event) at least in the region 22 where
ACIM events are expected. Medium 30 also should be slightly
undersaturated at ambient (STP) conditions, and free of any
chemicals or surfactants that compromise the surface tension
properties. The use of tap water or de-ionized water as medium 30
has produced good ACIM effects.
[0068] FIG. 4(a) shows an exemplary transducer module useful for
implementing the methods and apparatuses of the present invention.
The transducer module is a low loss, high efficiency, impedance
matched, air-backed or re-reflection backed high Q (resonance mode
operation) transducer. It has resonant peaks at all odd harmonics
of the fundamental frequency. The module comprises a parabolic,
spherical section, or hemispherical ceramic transducer 62, a
similarly shaped impedance matching layer 64, and air-backed
resonator or housing 60 communicating with a rear surface of
ceramic transducer 62, and an electrode 66 and conductor 68 that
energize the ceramic transducer 62. The transducer module is a
custom-design which uses LTZ-1 (Lead Titanate Zirconate) shaped
(focused spherical or other shaped segment) piezoelectric ceramic
available from Transducer Products, Inc. LTZ-1 is a trade name for
the transmitter application PZT. Driven by a square wave,
transducer 62 naturally generates all odd harmonics with a 1/N
amplitude dependence. One may use quartz, lithium niobate, a
composite material, or any high frequency acoustic device.
[0069] Instead of being parabolic or spherically shaped, transducer
62 may take on other shapes that preferably focus to concentrate
the acoustic field at a site upon the surface of an object. With
certain high intensity acoustic fields, or in the presence of a
medium having low cavitation threshold, there would be no need to
focus or concentrate acoustic energy thereby permitting the
transducer to have any suitable shape, e.g., flat surface, to
induce microcavitation. A spherical section transducer 62 having a
6" radius, for example, may produce an ACIM area of about 40-50
square millimeters and a "depth-of-field" of about 2.0 to 5.0 mm
where cavitation is most prominent, e.g., at or above its -6 dB
nominal intensity level. At sufficiently high ACIM driving
intensity, the cavitation threshold may be exceeded in regions of
insonification other than the focal region of transducer 62 thereby
achieving ACIM effects in larger volumes of the medium. The size
and location of cavitation is defined by transducer geometry,
whereas the duty cycle and amplifier gain define the intensity of
cavitation at that location. Surprisingly, the focal length may be
relatively long since medium 30, e.g., water, transmits acoustic
energy fairly well. In addition, the transducer module may be
elongated or formed as a cylindrical section, as illustrated in
FIG. 4(b), to establish a linear acoustic field, which is more
useful for cleaning planar substrates (e.g., a sheet of paper or a
painted panel) in a sweeping action. In FIG. 4(b), the elongated
transducer module comprises an air-backed chamber 60, a ceramic
transducer 62, an impedance-matching layer 64, and an electrode 66
for energizing the transducer 62. A spherical or cylindrical,
elongated module produces a linear ACIM field along an axis in
front of the module at a distance equal to a radius r or focal
length of the transducer. When deployed in a cleaning device, the
linear field is swept across the surface of a substrate (e.g.,
paper or wafer) by moving either the transducer or the substrate.
It follows that the geometry, physical dimensions, configurations,
or other parameters of transducer 62 may be varied to meet the
requirements of any scientific, medical, or industrial
application.
[0070] An impedance matching layer is often used to improve
transmission and coupling of acoustic energy to the medium. In an
optional embodiment of the invention, impedance matching layer 64,
if used, may comprise a variety of materials whose property are
dictated by the tensile strengths and damping characteristics of
the transducer and the insonification medium 30. In one embodiment,
polyurethane or other polymeric material is employed. The thickness
of the layer 64 is chosen to be about one-quarter wavelength of the
acoustic field generated in the medium in order to reduce
reflection losses, which serves to optimize the exchange acoustic
energy between transducer 62 and insonification medium 30.
[0071] FIG. 5 illustrates another embodiment of an ACIM cleaning or
surface treatment system where cavitation region 50 about the
surface of a workpiece 54 lies beneath a transducer module 53.
Transducer module 53, as previously described in connection with
FIG. 4, is disposed within a nozzle 52 which includes a jet 55
through which liquid medium 56 flows when pumped or otherwise
transferred to chamber 57 via conduit 59 from the reservoir of tank
51 or external source. Medium 56 need not be recirculated through
conduit 59, as shown in FIG. 5 or elsewhere in this description,
but may instead be continuously fed from a fresh external source.
The system of FIG. 5 provides acoustic coupling between the
transducer and the ACIM region 50. The flow of fluid 56 maintains
an acoustic communication path between chamber 57 and ACIM region
50 so that the ACIM field generated by the transducer module 53 in
the chamber 57 is directed through the jet 55 onto the surface of
workpiece 54. Workpiece 54 may, for example, be a semiconductor
wafer or other substrate immersed within a fluid medium 56 of
deionized water, for example. A holder or platen 58 supports the
substrate so that its surface lies in the effective ACIM region 50.
In a practical application, platen 58 may spin, oscillate, or
laterally move the workpiece 54 across region 50 to expose the
entire surface to the ACIM region. Alternatively, the module 52 may
be swept across the surface of workpiece 54.
[0072] Again, the nozzle 52 may take on a variety of geometries.
The distance between nozzle 52 and workpiece 54 may range from a
few millimeters to several centimeters or more, as desired,
depending on the application so long as acoustic coupling is
maintained.
[0073] FIG. 6 shows an ACIM arrangement useful for deinking paper.
In operation, a sheet of paper 60 is de-inked as it is pulled
through an insonification medium 65 by roller pairs 62, 63 across
an ACIM field 61 generated by a transducer module 64 disposed above
the paper 60. Instead of being located above the paper, module 64
may be located beneath the paper 60 or at any angular orientation
with respect to the paper. Multiple transducer modules 64 may be
deployed and/or the depth of field of such transducer modules may
be staggered in order to provide serial, in-line stations for more
effective de-inking or cleaning. Alternatively, in case of pulp
deinking, for example, the pulp can be pumped through a pipe or
conduit having ACIM transducers disposed on the internal wall
thereof. This permits bulk processing of large quantities of pulp.
Slurries other than pulp may be similarly processed using ACIM.
[0074] FIG. 7 illustrates a method of acoustic coaxing induced
microcavitation. The method comprises providing a transducer module
76 that generates an ACIM field 77 using a single transducer 78,
providing a waveform generator 70 that produces tone bursts having
a given or controllable duty cycle and frequency, amplifying the
output of the waveform generator 70 by an amplifier 72, optionally
switching the output of the amplifier 72 using a on-off
(transmit-receive) switch 74 before supplying the same to the
transducer module 76, directing the transducer to an ACIM region 79
on an object, and providing acoustic coupling through an acoustic
transmission medium between the transducer 78 and ACIM region 79
during microcavitation. When the transmit switch is on, the ACIM
field is activated whereas, when the receive switch is on, a
detector (e.g., passive detector 19 (FIG. 1(b) or even the ACIM
transducer itself could act as a detector in receive mode), is
activated to receive echoes. The transmit/receive switch operates
mutually exclusively.
[0075] The method may be modified by sweeping the ACIM field across
the object by moving the object or the transducer, providing square
wave tone bursts of about one 1 MHz with a burst repetition
frequency of about 1 KHz, varying or controlling the duty cycle of
the tone bursts, varying or controlling the gain of the amplifier
72, enhancing the acoustic coupling medium with cavitation nuclei,
providing a transducer having an elongated shape for producing a
linear ACIM region, providing an array of ACIM transducers, and/or
providing an ACIM transducer to perform a useful operation
including, but not limited to, abrasion, cutting, drilling,
lapping, polishing, machining, inducing a chemical reaction,
measuring and testing, surface or thin film treatment, paint
removal, deinking, surface erosion, wafer cleaning, surgery,
submicron particle detection or eviction, or any other useful
purpose. Multiple transducers may also be provided in communication
with a common or multiple insonification chambers. Based on the
description of the apparatus set forth above, various other
apparatuses may be constructed to carry out the stated methods.
[0076] The invention is also useful for thin film analysis. Binding
or adhesion strength, for example, can be easily determined by
observing the time required to remove a patch of the film by ACIM
of a given intensity. A plot of film residence time versus
insonification pressure amplitude has an inverse reverse
relationship which can be used to determine binding or adhesion
strength of the film to the substrate, or to determine substrate
erosion strength--the extrapolated intersection with the pressure
axis directly corresponds with the spontaneous removal time of the
thin film, and hence determines the binding or adhesion
strength.
[0077] In view of the above teachings, it is apparent that
single-transducer ACIM methods and apparatuses, as a core
technology, may be used to perform various useful scientific,
medical and industrial tasks with respect to an object, a
substance, or as an investigatory tool for measuring and testing.
Methods and the design of apparatuses for carrying out the methods
can be configured or constructed to match the specific needs
encountered in which microcavitation is used, constructively or
destructively. The single-transducer teachings set forth above
provide distinct advantages over dual low and high frequency
transducers for ACIM in terms of the design or construction of a
nozzle, platen, transducer shape, work tool and/or transducer head
design. Multiple single-transducer modules may be ganged together
or arrayed in a common or in separate fluid reservoirs. Various
waveforms or combination of waveforms having high and low frequency
components, other than those described herein, can be applied to
the single-transducer to effect ACIM in addition to square or
triangular wave tone bursts. Various materials for impedance
matching layers may be deployed. Thus, the description of the
illustrative embodiments should not be considered limiting of the
invention. By the appended claims, it is the intent to include all
such modifications and adaptation that may come to those skilled in
the art based on the teachings herein.
* * * * *