Reflection acoustic microscopy for thick specimens

Abstract

This paper presents a theoretical modeling and experimental verification on the V(Z) characteristics of a three-media system in small numerical aperture reflection acoustic microscopy. The three-media system consists of either water-specimen-water or water-specimen-defect and is used as a model for a one-dimensional discontinuity or defect in a thick specimen. The following factors are treated in detail: acoustic attenuations in water and specimen, aberrations in specimen, and nonuniformity of lens pupil function. Measurements were carried out using a 150-MHz reflection scanning acoustic microscope and thick specimens such as silicon wafers and aluminum and steel plates. The findings are the following: (1) The acoustic attenuation in water causes the V(z) curve to deviate significantly from the ideal sinc function. Also, an absolute maximum in the V(z) curve is created when the focus is brought upon the second boundary of a thick but moderately attenuating specimen (e.g., silicon wafer). (2) The acoustic attenuations in water and in specimen degrade the system sensitivity. (3) The nonuniformity of the generalized pupil function smoothes out the periodic nature of the previously mentioned sinc function. (4) The aberrations in the specimen result in an increased axial movement of the specimen for a given plane to be focused. (5) The acoustic lenses with the aperture angle between 7° and longitudinal critical angle are desirable for imaging the defects in most commonly used solid materials, especially for those located at a few acoustic wavelengths beneath the host medium. Utilization of the model for defect characterization is also illustrated through measurement of the V(Z) curve and recording of the acoustic images of a solder bond which is located at 20 mil beneath the silicon wafer of a thick flip-chip microelectronic module. Finally, it is also suggested that the model can be generalized to treat multiple-media systems involving more than three media and an acoustic lens with large numerical aperture.