Impact of acoustical reflections on megasonic cleaning performance

Electrical measurements have shown a direct impact of reflection of acoustic waves back into a transducer. Impedance measurements illustrate in specific cases the existence of multiple resonance peaks when reflected acoustic waves are present. Current and voltage measurements have confirmed this result. From these results, one can already conclude that acoustic reflections have a large impact on the operation of a transducer. Furthermore, it is shown that for megasonic cleaning tools with a face-to-face configuration of transducer and wafer, a precise control over the distance (control over the reflections) between the transducer and wafer is very important. Particle Removal Efficiency (PRE) measurements immediately show a major dependence on the position of the wafer. The PRE dependence is directly linked to the forward power consumed by the transducer, which is largely influenced by the position of the wafer or, in other words, by the reflection of acoustic waves.