Theoretical calculation of the acoustic force on a patterned silicon wafer during megasonic cleaning

We have calculated, theoretically, the acoustic pressure field around a linear pattern on a silicon wafer immersed in water subjected to a megasonic beam. The method of calculation is based on a Green's function formalism. The acoustic force applied on the pattern by the pressure field is determined as a function of frequency and the angle the incident megasonic beam makes with the wafer surface. The calculation is applied to two types of features that may be encountered in megasonic cleaning of integrated circuits prior to packaging, namely a micron-size silicon ridge and a metal wire (tens to hundreds of microns in diameter) bonded on a silicon substrate. The calculated acoustic shear stress is found to be orders of magnitude smaller than the shear strength of the features.