Enhancement of cavitation activity and particle removal with pulsed high frequency ultrasound and supersaturation

► Cavitation activity and particle removal can be significantly enhanced under pulsed sonication and gas supersaturation. ► The optimal pulse parameters are determined by the bubble dissolution time, the amount of activity by the duty cycle. ► We show theoretically and experimentally that the optimal pulse parameters depend on the gas concentration. ► We show theoretically and experimentally that a critical window for the dissolved gas concentration exists. 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 340ms, respectively. Full particle removal of 78nm-diameter silica particles at a power density of 0.67W/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 3.3μm and verified experimentally. The bubble radius used in the calculations corresponds to the linear resonance size in a 928kHz sound field, which demonstrates that the recycling of active bubbles is the main enhancement mechanism. The optimal choice of the pulsing conditions h