HRTEM and X-ray diffraction analysis of Au wire bonding interface in microelectronics packaging

Abstract

Interfacial microstructures of thermosonic Au wire bonding to an Al pad of die were investigated firstly by high-resolution transmission electron microscopy (HRTEM) and X-ray micro-diffractometer. The equal-thickness interference structures were observed by HRTEM due to diffusion and reaction activated by ultrasonic and thermal at the Au/Al bond interface. And X-ray diffraction results showed that three different interplanar crystal spacings (‘d’ value) of the interfacial microstructures were 2.2257 Å, 2.2645 Å, and 2.1806 Å respectively from the high intensity of diffraction to the low intensity of diffraction. These indicated that the intermetallic phase AlAu₂ formed within a very short time. It would be helpful to further research wire bonding technology.

Introduction

The wire bonding technology is widely used in microelectronics packaging interconnection. As most of the failure results from the bond interface, the interface features have always been concerned [1], [2], [3]. The long-term thermal reliabilities of an aluminum wire wedge bonding on aluminum and Au/Ni/Cu pads were detected by using energy dispersive X-ray (EDX) testing results of Scanning Electron Microscopy (SEM), and the interfacial intermetallic compounds (IMC) generated at bond interface after aging in air at 200 °C for 240 h [4]. Generally, a gold (Au) wire is bonded to an aluminum (Al) pad of die by using a ball bonder. Actually, with the improving of wire bonding technology, Au ball wire bonding is finished only within 5–15 ms (ms), so, the IMC of bonding interface may be a very thin layer with thickness of several tens of nanometers, then the IMC can’t be observed by many packaging engineers with optical microscopy and SEM [5].

Recently, the reactant of thermosonic Flip Chip bonding was identified as the intermetallic phase (Au₄Al) by using scanning transmission electron microscopy (STEM) by Li et al. [6], [7]. EDX testing results only shown their composition of Au–Al, while lattice structure of IMC wasn’t proven, and the same composition may form different lattice structures. IMC of ultrasonic Al–Si wire wedge bonding to a Au/Ni/Cu pad was reported as Al₃Au₈ by using transmission electron microscopy (TEM) by Geiβler [8], Karpel Adi [9]and Ji [10]et al. However, none of the above intermetallic phases is firmly identified on atomic scale concerning no matter ball or wedge bond interface, and is still debated [10], [11]. As we all know, X-ray diffraction spectra of lattice structure is like one’s fingerprint, and can give more precise microstructure parameters than TEM.

So, in our study, the interfacial characteristics of Au wire ball bonding to an Al pad of die were investigated firstly by using the high-resolution transmission electron microscopy (HRTEM) and the X-ray diffractometer. Based on captured images and data, microstructure of the Au–Al interface layer was discussed.