Characterization of stable and transient cavitation in megasonically irradiated aqueous solutions

• Identified conditions in the range of 1–3 MHz to achieve damage-free cleaning.
• Characterized cavitation using microelectrode and hydrophone measurements.
• OH concentration measured using fluorescence spectroscopy.
• Transient cavitation dominant at 1 MHz and stable cavitation prevalent at 3 MHz.

Megasonic cleaning is routinely used for removal of particulate contaminants from various surfaces in integrated circuit industry. One of the drawbacks of megasonic cleaning is that although it can achieve good particle removal efficiencies at high power densities, it also causes feature damage. The current paradigm is that damage is primarily caused by transient cavitation whereas cleaning is affected by streaming and stable cavitation. In order to develop a damage-free and effective megasonic cleaning process, it is essential to understand the acoustic bubble behavior and identify conditions that generate significant stable cavitation without any transient cavitation. In the current work, microelectrode based chronoamperometry, pressure measurements using a hydrophone and fluorescence spectroscopy studies were conducted under different acoustic frequencies (1–3 MHz) and power densities (2–8 W/cm2) to fundamentally investigate the type of cavitation produced under these conditions and also establish a correlation to the generation of hydroxyl radicals for characterization of transient cavitation.

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