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Title: Enhancement of cavitation activity and particle removal with pulsed high frequency ultrasound and supersaturation
Authors: Hauptmann, Marc ×
Frederickx, Floris
Struyf, Herbert
Mertens, Paul
Heyns, Marc
De Gendt, Stefan
Glorieux, Christ
Brems, Steven #
Issue Date: 2013
Publisher: Butterworth Heinemann
Series Title: Ultrasonics Sonochemistry vol:20 issue:1 pages:69-76
Abstract: Megasonic cleaning as applied in leading edge semiconductor device manufacturing strongly relies on the
phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude
of interdependent effects, the amount of cavitation activity in the cleaning liquid strongly depends on the
sonication conditions. It is shown that cavitation activity as measured by means of ultraharmonic cavitation
noise can be significantly enhanced when pulsed sonication is applied to a gas supersaturated
liquid under traveling wave conditions. It is demonstrated that this enhancement coincides with a dramatic
increase in particle removal and is therefore of great interest for megasonic cleaning applications.
It is demonstrated that the optimal pulse parameters are determined by the dissolution time of the active
bubbles, whereas the amount of cavitation activity depends on the ratio between pulse-off and pulse-on
time as well as the applied acoustic power. The optimal pulse-off time is independent of the corresponding
pulse-on time but increases significantly with increasing gas concentration. We show that on the
other hand, supersaturation is needed to enable acoustic cavitation at aforementioned conditions, but
has to be kept below values, for which active bubbles cannot dissolve anymore and are therefore lost during
subsequent pulses. For the applicable range of gas contents between 100% and 130% saturation, the
optimal pulse-off time reaches values between 150 and 340 ms, respectively. Full particle removal of
78 nm-diameter silica particles at a power density of 0.67 W/cm2 is obtained for the optimal pulse-off
times. The optimal pulse-off time values are derived from the dissolution time of bubbles with a radius
of 3.3 lm and verified experimentally. The bubble radius used in the calculations corresponds to the linear
resonance size in a 928 kHz sound field, which demonstrates that the recycling of active bubbles is
the main enhancement mechanism. The optimal choice of the pulsing conditions however is constrained
by the trade-off between the effective sonication time and the desire to have a sufficient amount of active
bubbles at lower powers, which might be necessary if very delicate structures have to be cleaned.
ISSN: 1350-4177
Publication status: published
KU Leuven publication type: IT
Appears in Collections:Molecular Design and Synthesis
Surface and Interface Engineered Materials
Acoustics and Thermal Physics Section
× corresponding author
# (joint) last author

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