TEM interfacial characteristics of thermosonic gold wire bonding on aluminium metallization

Abstract

A native alumina thin film existing on aluminium and its evolution in thermosonic gold wire bonding were characterized using high resolution transmission electron microscopy (HR TEM). It has been found that the partly fracture of this alumina film by the ultrasonic vibration allows the interdiffusion at the Au/Al interface, such that the intermetallic compounds (IMCs) are formed. The IMCs are identified as Au 4 Al and AuAl 2 with a thickness of approximately 300 nm. With a higher ultrasonic energy, IMCs are formed in more contact areas, resulting in a stronger bond. Based on these experimental results, a mechanism of ultrasonic gold wire bonding is proposed, which helps to understand the correlation among ultrasonic energy, interfacial structure and bonding strength.

Some thoughts on bondability and strength of gold wire bonding

“…18 HRTEM image of the layered-diffusion phenomenon at the bond interface [28] Fig. 19 Shear force and shear strength versus bonding power [30] new methods that could shed further light on the thermosonic Au wire bonding mechanism. In addition, the suggested new approaches: examining the effect of individual phase and surroundings on the strengthening produced by the Au-Al intermetallic compound; combining FEA based on friction and wire bonding parameters and correlating TEM results with results obtained from other techniques should enable a more detailed understanding of the bondability and strength of thermosonic Au wire bonds.…”

“…Thermosonic wire bonding produces ultrasonic vibration that locally and physically breaks down the native aluminium oxide layer and enables direct Au and Al contact under certain bonding pressure, as indicated by Xu et al [30,31]. Xu et al [31] noted that the dominant intermetallics created during the wire bonding process are Au 4 Al and AuAl 2 .…”

“…Consequently, this produces numerous diffusion pathways via grain boundaries and dislocations that have lower activation energy. The formation of the Au and Al intermetallic compounds could significantly improve the bond strength [30,31]. Figure 19 shows the shear force and shear strength versus bonding power graphs obtained by Xu et al [30] to describe the influence of ultrasonic power on bonding strength.…”

“…Several researchers have used TEM to examine the wire bonding mechanism in more detail [27][28][29][30]. Karpel et al [27] indicated that during the bonding stage, ultrasonic and thermal activation can result in the formation of a liquid state Fig.…”

“…Some of the methods, namely footprint morphology, intermetallic coverage, microstructural and frictional bonding analyses, have concluded that solid-state diffusion is the main mechanism of thermosonic Au wire bonding [1,[3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. However, a new possible bonding mechanism (liquid-state diffusion) has been proposed based on TEM examinations [27][28][29][30]. Each method for analysing the bonding mechanisms of thermosonic Au wire bonding has its own advantages and disadvantages, but further improvement is still possible.…”