What is Megasonics
Megasonics are emerging as an increasingly important, widely
accepted cleaning method for contamination-sensitive products.
A growing number of manufacturers in the integrated circuit,
hard drive, raw silicon, mask, flat panel display, and other
industries affected by contamination are turning to megasonic
cleaning to help meet stringent cleanliness requirements.
Megasonic cleaning uses the piezoelectric effect to enable
removal of submicron particles from substrates. A ceramic piezoelectric crystal is excited by high-frequency AC voltage, causing it to vibrate.
This vibration generates an acoustic wave that is transmitted
through a cleaning fluid, producing controlled cavitation.
As the wave passes across the surface of an object, it causes
particles to be removed from the material being cleaned. The
technology was originally developed by the U.S. Navy as an
element in anti-submarine warfare.
How does it work?
Megasonics work by generating controlled acoustic cavitation in the cleaning fluid. Acoustic cavitation is produced by
the pressure variations in sound waves moving through a liquid.
Cavitation, the formation and activity of bubbles (or cavities),
is believed to be an important mechanism in the actual particle
removal process, because cavitation has sufficient energy
to overcome particle adhesion forces and cause particles to
be removed. Controlled megasonics cavitation becomes acoustic
streaming, which pushes the particles away so they don't
reattach to the material being cleaned.
The pressure amplitude, or megasonic power, required to achieve
cavitation has been proven to depend on the pulse width, dissolved
gas content in the cleaning fluid, and the power input. Megasonics
cleaning is controlled by varying the power input. Pulsing
the input power provides better control over cavitation than
applying continuous input power.
Exposure time and megasonic power are the most significant
variables affecting megasonics cleaning. As megasonic power
or exposure time increases, particle redeposition decreases.
Pulsed input power (pulsed-wave megasonics) achieves greater acoustic power levels in a cleaning
bath than continuous input power (continuous-wave megasonics)
at the same average input. Typical exposure times are 10 to
Megasonics cleaning compared to ultrasonic cleaning
The difference between ultrasonic cleaning and megasonics
cleaning lies in the frequency that is used to generate the
acoustic waves. Ultrasonic cleaning uses lower frequencies;
it produces random cavitation. Megasonics cleaning uses higher
frequencies at 1000 kHz; it produces controlled cavitation.
An important distinction between the two methods is that
the higher megasonic frequencies do not cause the violent
cavitation effects found with ultrasonic frequencies. This
significantly reduces or eliminates cavitation erosion and
the likelihood of surface damage to the product being cleaned.
Parts that would be damaged by ultrasonic frequencies or cavitation
effects can often be cleaned without damage in a megasonic
bath using the same solution.
With ultrasonics, cavitation occurs throughout the tank,
and all sides of submerged parts are cleaned. With megasonics,
only the side of the part that is facing the transducer(s)
The megasonics cleaning technique is effective for removing
0.15-micron particles from silicon wafers and other products,
without damage. The method is currently being used by manufacturers
of integrated circuits, flat panel displays, and hard disks,
as well as by mask makers and raw silicon suppliers.
Megasonics cleaning may be used with a variety of chemistries.
Although it is used primarily for particle removal, it may
also be used to increase the efficiency of chemical cleaning
with surfactants or detergents. Removal of other contaminants
depends on the solutions in the tank.