Quality assessment of adherent and fusion-welded joints in silicon wafers by nonlinear ultrasound

Abstract

The interest in fusion-welded silicon wafers has increased recently because of their excellent electrical and mechanical properties. Typical applications are in acceleration and pressure sensors. The interface of room temperature bonded wafers contains a physically adsorbed water layer. Bond temperatures higher than 373 K cause a reduction of the adsorbed water by diffusion and thus an increase in bond strength. If electronical components are protected by surrounding silicon wafers, the joining temperature must not be higher than 673 K. The bond strength of the interface essentially defines the reliability of the components. Therefore, a nondestructive method to evaluate the bond quality is required. Binding forces are nonlinear and cause a nonlinear modulation of transmitted or reflected ultrasonic waves. Inhomogeneities in bond strength of an interface can be imaged by the local ultrasonic amplitudes of the insonified frequency and of its higher harmonics measured interferometrically in transmission. A quantitative evaluation of the image data yields information about the variations in bond strength and the size of delaminations and of weakly bonded areas. This paper presents experimental and theoretical investigations of the interface of bonded silicon wafers exploiting the nonlinear transmission of ultrasound. The data are compared to destructive tests.