The ultrasonic wedge/wedge bonding process investigated using in situ real-time amplitudes from laser vibrometer and integrated force sensor

The ultrasonic transversal force transmitted to a chip during ultrasonic bonding is derived from measurements of the vibration amplitude at the tool tip and the die edge. To proof the derivation, the transversal force is measured as well by means of a microsensor, which is sensitive to the stress field in the silicon die. The force measured by the microsensor is further referred to as “y-force” To Al-metalized test pads with the integrated microsensors, AlSi1 wire of 25μm diameter was bonded using a wedge/wedge autobonder. Measurements of the vibration amplitudes and the y-force during bonding were conducted for nine different bonding parameter settings of force and ultrasound (us) amplitude. They confirm a theory for the friction cleaning phase as it was described earlier and will be partially presented here. Compared to earlier measurements of Au-ball-bonds, the results largely show the same behavior and imply that us wedge bonding and thermosonic ball bonding are similar processes. Furthermore, the data approves former interpretations of the bonding process starting with a stiction phase. A clear break off point was found in all pad amplitude measurements, which is followed by a friction plateau that implicates the need of a minimum friction cleaning power.The discussion made in this paper is interesting for a bond process control system. The transversal force reflects the important stages of the bond process and contains the information to suit as a control signal. But it is impractical to measure the transversal force in situ under the wedge in industrial production, where chip, substrate and bonding table create a complex setup with a high geometric variety. An indirect measurement of the transversal force via the tool tip amplitude opens up new possibilities for gaining an efficient control variable, because the geometry and the properties of the bonding machine are well defined. As a first step it is shown by correlating vibration measurements with microsensor signals, that the tool tip amplitude measured by laser vibrometer contains all of the necessary information needed to control the bond process. From that point, process integrable measurement systems – which are cheaper, more handy and more fail safe than the laser vibrometer – might be developed.