Scanning acoustic microscopy: Resolution reduction due to attenuation of acoustic signal in materials

Abstract

The selection of appropriate parameters is essential to ensure an optimum pixel density for detecting critical defect size. For scanning acoustic microscopy (SAM), frequency is one of the key factors that can have significant impact on the spatial resolution of the system. Attenuation of acoustic signal due to coupling fluid has been reported to degrade the resolution significantly, especially at high frequency (>150MHz). However, the resolution reduction due to attenuation in materials has not yet been extensively studied. In this work, we investigate the attenuation of acoustic signal in different materials as a function of frequency, i.e. 15MHz to 75MHz. Results analysed by Fast-Fourier Transform (FFT) confirmed that moulding compound (MC) tends to attenuate the high frequency component of the acoustic signal, and therefore, significantly degrade the achievable spatial resolution of SAM tool. Conversely, a comparatively small frequency downshifting has been observed in copper lead frame and die attach. Results also show the important of taking this factor into consideration during experiment planning stage, in order to prevent from misinterpretation of SAM results, i.e. critical defect size is undetectable due to resolution reduction.