Thermosonic wire bonding of gold wire onto copper pad using the saturated interfacial phenomena

Abstract

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-2 for preheat temperatures of 150°C and 170°C. Moreover, the corresponding optimal ultrasonic power is about 110 units.

A brief review of selected aspects of the materials science of ball bonding

“…Although Harman's early studies show that friction does not play a role in the process of ultrasonic and thermosonic welding, the work of Mayer et al [4][5][6] introduces the concept of 'friction power' in the bonding process and Jeng et al [42][43][44] also consider frictional energy to play a key role in the bonding process. This is contrary to the accepted view that friction only serves to scrub away surface debris and contamination that may obstruct the bonding process while ultrasound welds together the same or dissimilar metals by creating high levels of dislocations and point defects that facilitate interfacial mixing.…”

Oxidation of copper pads and its influence on the quality of Au/Cu bonds during thermosonic wire bonding process

“…The thermal energy is an important driving force for atomic inter-diffusion between the gold ball and copper pad during thermosonic bonding [23]. Therefore, lower bondability and insufficient bonding strength at lower bonding temperatures are mainly attributed to insufficient thermal energy applied during thermosonic bonding.…”

Development of a thermosonic wire-bonding process for gold wire bonding to copper pads using argon shielding

“…A larger bonded area generally ensures a higher ball-shear force. 6 However, too high an ultrasonic energy causes severe deformation and even rupture of the gold ball, as depicted in Fig. 11c.…”

“…Bondability and reliability are also significantly deteriorated. 6 To accommodate the widely used TS bonding process for the packaging of chips with copper interconnects, the copper pad must be prevented from oxidizing during TS bonding to maintain superior bonding quality and reliability.…”