As the semiconductor industry moves to 300 mm, it's being challenged to create processes that
are more uniform and repeatable. Non-contact acoustic cleaning could be a key to the puzzle.
Cleaning Charts a Course to the Big Time
By Mark Beck
Non-contact acoustic cleaning – megasonics – is becoming a more readily recognized technology now that
semiconductor manufacturers are converting to 300 mm processing and are ready to meet the more
stringent requirements of single-wafer cleaning.
This high-frequency acoustic cleaning is widely known as the method for assisting many current wet
cleaning and processing steps because the process parameters can be more precisely controlled and
cavitation-induced damage to patterned layers can be eliminated.
Parameters that affect megasonic processing include frequency, sonic intensity, chemistry, process
time and temperature. The most important physical parameters, however, are turning out to be the
cavitation energy, acoustic boundary layer and acoustic micro-streaming induced forces.
How it works
In megasonic cleaning, a piezoelectric crystal array transducer converts alternating electrical
energy into mechanical energy using the piezoelectric effect whereby certain materials change
dimension when an electrical charge is applied. A ceramic piezoelectric crystal is excited by
high-frequency AC voltage, between 500 and 2,000 kHz, causing the ceramic material to rapidly
change dimension or vibrate.
Figure 1: Megasonic cleaning uses the piezoelectric effect to produce acoustic waves that move
through the cleaning liquid.
These vibrations are transmitted by the resonant masses of the transducer and directed into the
liquid, producing acoustic waves in the cleaning fluid. Acoustic cavitation produced by pressure
variations in the sound waves moving through the liquid and the effects of acoustic streaming
cause particles to be removed from the material being cleaned (see Figure 1).
Acoustic cavitation is the generation and action of cavities, or bubbles, in a liquid. Acoustic
waves move through a liquid and produce variations in the liquid's pressure. When the liquid pressure
momentarily drops below the vapor pressure during the low-pressure portion of the acoustic wave,
small evacuated areas, or cavities, are formed. These cavities quickly become filled with gas (a
foreign contaminant such as dissolved oxygen or air) and/or vapor (a gaseous form of the surrounding
liquid). These tiny bubbles are set in motion by the acoustic wave.
Figure 2: Plot of bubble radius versus frequency.
The tiny bubbles can expand and contract in the liquid. Bubble expansion is caused by reducing the
ambient pressure in the liquid. The bubbles can become large enough to be seen by the unaided eye.
The bubbles may contain gas or vapor or a mixture of both. If the bubbles contain gas, their expansion
can be caused by rectified diffusion.
Rectified diffusion is the diffusion of dissolved gas from the liquid into the bubble, and vice versa,
with the pressure oscillations resulting in a net diffusion into the bubble. This net inward diffusion
occurs because the bubble surface area increases during inward diffusion and decreases during outward
diffusion; a higher surface area leads to more diffusion.
Figure 3: Table of physical parameters affecting the cavitation threshold.
If the ambient liquid is not saturated with gas, then rectified diffusion must compete with ordinary diffusion
from the bubble to the liquid. In that case, the sound pressure amplitude must exceed a certain value for the
bubbles to significantly increase in size.
The pressure oscillations that created the bubbles can also cause them to expand and contract. If the
pressure variation is great enough to reduce the local liquid pressure down to or below the vapor pressure
in the negative parts of the acoustic cycle moving through the liquid, any minute cavities or bubbles that
are present will grow larger. If the range of the pressure variation is increased to produce zero and then
negative pressures locally in the liquid, then bubble growth is increased. Gas from the liquid diffuses
into a bubble during expansion, and leaves the bubble during contraction.
Figure 4: Comparison of boundary layer in ultrasonic and megasonic cleaning.
When the bubble reaches a size that can no longer be sustained by its surface tension, the bubble will
expand and then collapse, or implode, which is an important action of the cavitation phenomenon. The
bubble action of cavitation has sufficient energy to overcome particle adhesion forces and to dislodge
particles attached to substrates in the stream of bubbles. Imploding cavitation bubbles generate shock
waves that dislodge particles from substrate surfaces. Cavitation breaks down the molecular force by
which a particle is held to surface either by direct impact from bubble implosion or by the fatiguing
action caused by repeated bombardment.
Cavitation implosion force varies with the size and contents of the bubble. Larger bubbles are unstable
and implode with larger force; smaller bubbles are stable and collapse with less force. Vapor collapses
more quickly, resulting in larger implosion force, whereas gas cushions and slows the collapse,
resulting in a smaller implosion force.
Figure 5: Table of physical parameters affecting the acoustic boundary layer.
The intensity and effect of cavitation on materials being cleaned is related to the type of acoustic
cavitation produced. Two types of acoustic cavitation have been identified: transient cavitation and
stable cavitation. Transient acoustic cavitation is produced by ultrasonic cleaning frequencies,
between 20 and 350 kHz, which transform low-energy-density sound waves into high-energy-density
collapsing bubbles. In transient cavitation, the mostly vapor-filled bubbles exist for only a few
acoustic cycles, followed by a rapid and violent collapse. This type of cavitation is likely to produce
violent events in the acoustic field, such as radiation of light (sonoluminescence) and shock waves.
The level of violence produced is believed to be dependent on the maximum size of transient bubbles
that is related to the acoustic frequency. Because transient cavitation concentrates energy into very
small volumes and tends to produce very high local temperatures and pressure, it can cause surface
erosion and damage to sensitive substrates.
Bubble size decreases as acoustic frequency increases, and the smaller the maximum bubble size, the less
violent the cavitation produced. The high frequencies used in megasonic cleaning, from 500 to 2,000 kHz,
produce controlled acoustic cavitation that is characterized by mostly small, gas-filled cavities. Unlike
the violent implosion associated with vapor-filled cavities in transient cavitation, controlled cavitation
bubbles exhibit less violent collapse, producing lower temperatures and pressure. As a result, megasonic
cleaning substantially minimizes surface erosion and damage to substrates being cleaned. Stable cavitation
produces light in the visible range (violet), while the light produced by transient cavitation is primarily
in the UV range (with a peak at 270-290 nm) (see Figure 2).
Cavitation does not occur until a specific threshold is reached. The cavitation threshold is defined as
the minimum pressure amplitude required to induce cavitation.
The cavitation threshold has been found to increase with increasing hydrostatic pressure (under most
conditions) and to decrease with increasing surface tension, with increasing temperature and with an
increasing number of solid contaminants. A reduction in the number of hydrophobic ions (such as C1-
and F-) will also decrease cavitation threshold because these ions collect at bubble surfaces and
prevent cavitation bubbles from dissolving.
A lower cavitation threshold enables cavitation to occur more readily. This suggests that cavitation
could be mitigated under the following conditions: low surface tension, high hydrostatic pressure,
low temperature and the presence of as few solid surfaces and contaminants as possible (see Figure 3).
During megasonic cleaning, the cleaning solution flows swiftly past the substrate being cleaned,
forcing chemistry into contact with contaminant particles, removing them from the surface and
carrying them away. On a microscopic scale, during acoustic cleaning, fluid friction at the
surface of the substrate being cleaned causes a thin layer of solution to move more slowly than
the bulk solution.
Figure 6: Table of physical parameters affecting the acoustic streaming velocity.
This layer of slow-moving fluid at the surface is called the boundary layer. The boundary layer effectively
shields the substrate surface from fresh chemistry and shields contaminant particles from the removal forces
of the bulk fluid (see Figure 4).
Megasonic cleaning has proven especially effective at removing submicron particles in part because it reduces
the boundary layer. The higher frequencies of megasonic cleaning reduce the boundary layer to less than 0.5
micron, compared to the boundary layer of 2.5 microns produced by ultrasonic cleaning frequencies. The primary
affect of acoustic streaming is the bulk fluid motion of the cleaning solution. The thickness of the boundary
layer decreases as the velocity of bulk fluid motion increases.
Figure 7: Ultrasonic vs. megasonic cleaning frequencies and particle sizes.
Reduction of the boundary layer yields several benefits. It allows fresh chemistry to come closer to the substrate
and come into contact with smaller particles. This higher chemistry refresh rate results in faster cleaning.
Boundary layer reduction increases the effectiveness of the acoustic streaming removal forces by allowing the
cleaning solution to rush past the substrate closer to the substrate's surface, forcing chemistry onto particles,
removing them from the surface and carrying them away.
The small, controlled cavitation bubbles generated by megasonics are able to remove contaminants within the thinner
boundary layer. This effect is especially important in removing small particles and accessing small surface features.
Reducing the boundary layer results in increased removal of submicron particles, particles that were previously
protected by the boundary layer, as well as increased particle removal overall (see Figure 5).
Acoustic streaming is considered another primary particle removal mechanism of megasonic cleaning. Acoustic streaming
is a time-independent fluid motion generated by a sound field. This motion is caused by the loss of acoustic momentum
by attenuation or absorption of a sound beam. Acoustic streaming enhances particle dissolution and the transport of
detached particles away from surfaces, thereby decreasing particle re-deposition.
Acoustic streaming velocity is a function of energy intensity, geometry, energy absorption, liquid density and
viscosity and sound speed in the liquid. Streaming velocity has been found to increase linearly with acoustic
intensity (power). Velocity also increases linearly with frequency. Streaming velocity also decreases with distance
from the source, due to attenuation.
Figure 8: Single wafer megasonic concept: vertical orientation.
Acoustic streaming comprises several important effects: 1) bulk motion of the liquid, 2) microstreaming and 3)
streaming inside the boundary layer.
The primary effect of acoustic streaming is bulk motion of the liquid, the strong localized flow of cleaning
solution. The shear force of the bulk liquid motion is the primary particle removal agent. In a closed tank,
forces due to sound pressure variation create this bulk fluid motion that carries particles away from the
substrate once the molecular attraction of the particle to the surface is broken and the particle is dislodged.
Bulk fluid motion increases linearly with acoustic intensity. The bulk fluid motion shear force combines with
the other effects of acoustic streaming to increase particle removal.
Figure 9: Single-wafer megasonic concept: horizontal orientation - knife.
A second effect of acoustic streaming is microstreaming. Microstreaming, also known as Eckart streaming, occurs
near oscillating bubbles or any compressible substance in the liquid. Microstreaming occurs at the substrate
surface, outside the boundary layer, due to the action of bubbles as acoustic lenses that focus sound power in
the immediate vicinity of the bubble.
This is a powerful type of streaming in which the bubbles scatter sound waves and generate remarkably swift
currents in localized regions. The currents are most pronounced near bubbles that are undergoing volume resonance
and are located along solid boundaries. Microstreaming aids in dislodging particles and contributes to megasonic cleaning.
Figure 10: Single-wafer megasonic concept: Busnaina design.
Most of the flow induced by acoustic streaming occurs in the bulk liquid outside the boundary layer. However,
there is a third effect of acoustic streaming-Schlichting streaming-which is associated with cavitation collapse
and is believed to assist in the removal of small particles and their transport away from surfaces. Schlichting
streaming occurs outside the boundary layer and is characterized by very high local velocity and vortex (rotational) motion.
The vortices are of a scale much smaller than the wavelength. Schlichting streaming results from interactions
with a solid boundary. Steady viscous stresses are exerted on the boundaries where this type of rotational
motion occurs, and these stresses may contribute significantly to removal of surface layers.
The combined effects of acoustic streaming produced in megasonic cleaning may slide, roll or lift a particle
from its initial position on a substrate, depending on the size and shape of the particle, as well as the
nature of the hydrodynamic force being applied. Acoustic streaming, both inside and outside the boundary layer,
enhances cleaning and other chemical reactions. The strong currents and small boundary layer thickness that
result from acoustic streaming aid particle transport significantly (see Figure 6).
Ultrasonic cleaning frequencies, between 20 and 350 kHz, produce transient acoustic cavitation. Megasonic
cleaning operates at much higher frequencies, from 500 to 2,000 kHz, which produce controlled acoustic
cavitation. Controlled cavitation is characterized by stable bubbles that are relatively permanent, can
exist for many acoustic cycles, and do not cause damage to substrate surfaces because the cavitation radii
is much smaller at higher frequencies and has less energy upon collapse. Thus, megasonic controlled acoustic
cavitation is best suited for sensitive substrate surfaces that cannot withstand the heat and pressure of
In addition, ultrasonics simultaneously cleans all surfaces of a submerged object. This means that
ultrasonic cleaning subjects areas of the substrate that do not need to be cleaned to the previously
described effects of transient acoustic cavitation. Megasonics accomplishes line-of-sight cleaning; it
affects only those surfaces of the object that are in the path of the acoustic wave.
The mechanical effects of both ultrasonic and megasonic cleaning can be helpful in speeding particle
dissolution and in displacing particles. Ultrasonics and megasonics have also been demonstrated to speed
or enhance the effect of many chemical reactions. In addition, residual cleaning chemicals can be removed
quickly and completely by either ultrasonic or megasonic rinsing. However, there are applications for
which megasonic cleaning clearly would be favored.
The effects of ultrasonics and megasonics on substrate surfaces and particle removal results provide the
basis for identifying the best applications for each process. Ultrasonic cleaning is most appropriate for
strong, heat-tolerant substrate materials requiring multi-surface cleaning. Ultrasonics is also well
suited for the removal and/or dissolution of large particles from chemically tolerant substrates.
Megasonics is most appropriate for heat- or chemical-sensitive substrates that cannot withstand the heat
and pressure of transient cavitation and for applications requiring line-of-sight-dependent cleaning.
Parts that cannot be cleaned with ultrasonics because they are sensitive to the frequency or transient
cavitation effects can often be cleaned with megasonics. Megasonic cleaning is also applicable in the
removal or dissolution of small particles (less than 0.3 micron). For example, this cleaning technique
has been proven effective for removing 0.10-micron particles from silicon wafers and other cavitation-sensitive
products, without causing substrate damage (see Figure 7).
Megasonic cleaning may be used with a variety of chemistries, including water, neutral aqueous solutions,
alkaline aqueous solutions, acidic aqueous solutions, ethyl lactate, alcohol, acetone, N-methyl pyrollidone,
dibasic esters and glycol ethers.
Although megasonic cleaning is used primarily for particle removal, it can also be used to increase the
efficiency of chemical cleaning with surfactants or detergents. Removal of other contaminants depends on
the solution in the tank.
Cleaning chemistries play a significant role in megasonic cleaning, because the chemical composition of
the cleaning solution may affect the zeta potential, or the atomic attraction forces between particles.
The zeta potential is neutral or negative in high pH chemistries. The zeta potential does not change with
up to two orders of magnitude change in the concentration of chemicals.
The addition of megasonic energy to the chemistry substantially enhances particle removal. Chemists have
succeeded in getting very dilute solutions to clean effectively with the addition of megasonics to the cleaning process.
Almost 80 percent of the current manufacturing is already single wafer. The industry is moving to 300
mm and this will challenge processes to be more uniform and repeatable. Cleaning is being required to
be at the point of use, not in central service locations. The process parameters are narrowing with
feature sizes pushing sub-100 nm. While current 200 mm cleaning is predominantly batch, scaling these
batch processes to 300 mm is challenging.
Megasonic has been the standard for featured layer cleaning for some time; damage has recently become
an issue with concentrating megasonic products like nozzles. The need to prevent damage has resulted
in the application of the acoustic energy as close to the point of use as possible. Current single-wafer
products and concepts that meet these needs have been proposed and tested.
Full wafer transducers that closely couple the megasonic energy to the wafer surface as proposed
by Ahmed Busnaina patent application, or the current Verteq Goldfinger single wafer megasonic rod, are
examples of point-of-use applications of acoustic energy. Energy density control allows for reduced power
and no damage. Intensities below 5 W/cm2 have proven to be very effective at removing 0.1 µm particles.
One of the historical drawbacks of megasonic cleaning has been the throughput of the batch systems. The
standard clean has been a two- to three-step process. With the batch systems, process time per tank
can be 10-20 minutes while current single-wafer technology with the acoustic energy delivered to the
precise point of use has resulted in process times in the sub-one-minute range.
Additionally, multiple chemistries can be mixed and the processing time further reduced. With production
facilities like foundries operating a wide mix of products with many split or small lots, single wafer
lends itself to the flexible, quick turn, custom process that manufacturing currently requires (see Figures 8, 9 and 10).
The most important new process parameter driving the development of new megasonic technologies is
the need to provide a more uniform acoustic field in which the substrate is processed. Not all of the
future processes are clean; there are many applications for acoustically assisted chemical processes,
which leads us into the field of Sono-chemistry.
Acoustic energy has proven to assist cleaning. It has also been proven to remove particles in deep trenches
and vias. The oscillations of the bubbles effectively pump and stir the chemistry on the microscopic level
inside the boundary layer near the surface. The danger is that damage can still occur if the uniformity
and intensity of the energy is not precisely controlled. The process is also more repeatable wafer to wafer
if the energy is constant over the whole wafer surface.
The future is Sono-chemistry
The chemical effects of ultrasound are diverse and include dramatic improvements in stoichiometric and
catalytic reactions. In some cases acoustic sound irradiation can increase the reactivity by nearly a million fold.
Ultrasonics has long been used to assist the chemical reactions in baths. Applications include
electroplating where sonic streaming will induce flows that can counteract the depletion of the
diffusion layer between electrode and electrolyte ions, which ordinarily limit the current yield
in plating. Ultrasonics has not been applied to wafer processing to assist chemical processes due
to concerns over damage. Megasonics is currently being applied to many of these processes.
About the author
Mark J. Beck is Chief Executive Officer of ProSys Inc. (Campbell, CA), a company he founded in 1996. He previously
worked for National Semiconductor, Semiconductor Technologies,
and Cypress Semiconductor. He received a BS in mechanical
engineering and manufacturing management from the University
of California, Berkeley and is the co-inventor of tow patents
for megasonic cleaning equipment.