Megasonic Cleaning to Remove Nano-Dimensional Contaminants from Wafer Surfaces: An Analytical Study

Abstract

Megasonic cleaning traditionally refers to use of acoustic fields in the 800 kHz 1 MHz range to remove contaminants adhered to surfaces immersed in liquid media. However, even fields driven by frequencies in the > 400 kHz regime exhibit virtually all characteristics of conventional megasonics. These include: unidirectional pumped flow of liquid (acoustic streaming) normal to the transducer, at velocities that scale as square of frequency; and, a near-absence of cavitational phenomena associated with ultrasonic cleaning. For the latter reason, megasonic cleaning is preferred over ultrasonics when attempting to remove contaminants from delicate, fragile, erodible or feature-rich surfaces. Silicon wafer cleaning in semiconductor manufacturing, integrated circuit cleaning, and printed circuit board cleaning have utilized megasonics (with appropriate chemistry) for several decades. The megasonic frequency offers the additional benefit of a very thin boundary layer over the immersed surface, which effectively exposes even sub-micron and nanodimensional particles to the flow of the cleaning liquid.