Copper has been used to replace conventional aluminium interconnection to
improve the performance of deep submicron integrated circuits. This study
used the saturated interfacial phenomena found in thermosonic ball bonding
of gold wire onto aluminium pad to investigate thermosonic ball bonding of
gold wire onto copper pad. The effects of preheat temperatures and
ultrasonic powers on the bonding force were investigated by using a
thermosonic bonding machine and a shear tester. This work shows that under
proper preheat temperatures, the bonding force of thermosonic wire bonding can
be explained based on interfacial microcontact phenomena such as energy
intensity, interfacial temperature and real contact area. It is clearly
shown that as the energy intensity is increased, the shear force increases,
reaches a maximum, and then decreases. After saturation, i.e. the
establishment of maximum atomic bonding, any type of additional energy input will
damage the bonding, decreasing the shear force. If the preheat temperature
is not within the proper range, the interfacial saturation phenomenon does
not exist. For a preload of 0.5 N and a welding time of 15 ms in thermosonic
wire bonding of gold wire onto copper pads, a maximum shear force of about
0.33 N is found where the interfacial energy intensity equals 1.8×106 J m-2for preheat temperatures of 150°C and
170°C. Moreover, the corresponding optimal ultrasonic power is about
110 units.