Analysis of room-temperature bonded compliant bump with ultrasonic bonding

Room temperature microjoining of Au or Cu bumps in the air ambient has been achieved by bonding cone-shaped microbumps with ultrasonic application. This technology has been applied to fabrication of near infrared (NIR) image sensor of q-VGA (quarter video graphic array) resolution, where InGaAs/InP photosensor array is joined with CMOS read out in the pixel level and, therefore, low temperature bonding is strongly required to address problems caused by mismatch in thermal expansion of the two materials. In this work, we investigate bonding mechanism of the cone shaped microbump using bumps made of Au. Die shear tests shows that shear strength of the bonded chips is proportional to the bonded contact area of the bump and that room temperature bonding gives sufficient bonding strength for applications while bonding at elevated temperature results in higher bonding strength. Analysis of change in bump height shows that “softening” of Au bump takes place under the application of ultrasonic vibration. Transmission electron microscopy shows that crystal grains at the bonded interface transform to small crystallites.