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

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/cm(2) 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 μm 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.

A parametric review of sonochemistry: Control and augmentation of sonochemical activity in aqueous solutions

“…That is, if P ON T is short enough a bubble may dissolve completely during P OFF T before the next P ON T pulse, or subsequent P ON T pulses may not cause remaining bubbles to become active [117,176,179]. Conversely, a longer P ON T time can result in the bubbles becoming too large to dissolve to an active size during P OFF T. This may cause bubbles to become inactive degas bubbles primarily via coalescence [176,181]. A P ON T can be chosen such that more bubbles become active during successive pulses and there can be a substantial increase in radical production [178].…”

Influence of surface tension on cavitation noise spectra and particle removal efficiency in high frequency ultrasound fields

“…A reduction of r is expected to enhance the growth of bubbles also under acoustic cavitation processes, when the diffusion constant is not altered. The bubble dissolution time, in the absence of any acoustic field, can be estimated from below equation 4,11,27 …”

“…III D. It has been clearly demonstrated that a very close correlation exist between bubble dissolution times and the off-time phase of a pulsed acoustic field. 11 The optimum off-time, corresponding to a maximum bubble activity, is determined by bubbles which dissolve (during the pulse off time) to or below the resonance size. At the next pulse-on time, these partially dissolved bubbles may be reactivated and brought back to resonance size.…”

“…For higher dissolved O 2 values, bubbles will experience a reduced concentration gradient (between the excess partial pressure in the bubble and the surrounding liquid) and, as a consequence, they will take longer to dissolve. 11 In an extreme case, bubbles will not dissolve anymore due to insufficient Laplace pressure, which limits the effective gas concentration or the allowed reduction in surface tension. Experimental results reflect this behavior: the optimal off-time is represented by the broad peak, whose center (in ms) increases for 90%, 100% 105% O 2 saturation levels, respectively.…”

“…At the same time, controlling the bubble size is crucial, since their radius is an important parameter in all the effect cited above. Among other techniques, the use of pulsed acoustic fields to gain control over the resonance bubble size and recycle active bubbles, 11 and the use of traveling waves to promote the transport of cleaning bubbles directly to the surface of interest, 12 have been studied in detail. Recently, high resolution chronoamperometry measurement in a megasonic field showed a higher cavitation activity when Triton X-100 was added to the liquid.…”