Cu-Al IMC micro structure study in Cu wire bonding with TEM

Copper (Cu) wire bonding is getting more common as interconnections on aluminium (Al) bond pad metallization in microelectronics due to lower cost compared to gold wire bonding. The Cu-Al intermetallic compounds (IMC) growth in Cu ball bonds has been investigated by many researches but the understanding of the IMC phases is still incomplete and the impact to bond reliability know-how is limited. This paper studies the Cu-Al IMC using TEM on 5N Cu wire bonded over the AlSiCu pad under 280°C bond temperature which was subjected to high temperature aging of 175°C at 0, 168h, 500h and lOOOh respectively. Conventional mechanical cross section along the ball bond was prepared for subsequent Focus Ion Beam (FIB) lamella preparation at the ball center and ball edge; followed by detail Scanning Transmission Electron Microscope (STEM) inspection to reveal the micrograph of the Cu-Al IMC and Energy Dispersive X-ray (EDX) quantitative analysis using atomic percentage to determine the IMC phases. The result of STEM and Energy Dispersive X-ray EDX confirmed the reported IMC phases CuAl₂(θ), CuAl(η₂), Cu₄Al₃(ζ₂), Cu₃Al₂(δ) and Cu₉Al₄(γ₁). However, new IMC phase Cu₃Al (α₂) which were rarely reported in Cu wire bonding had been discovered in this study. Generally, the IMC can be grouped as Cu-rich and Al-rich IMC, and there is small separation developed between it at low temperature aging. As the temperature aging getting higher, the separation had increased and became bigger between Cu-rich and Al-rich IMC. In many cases, separation between Cu and Cu-rich IMC or even within Cu-rich IMC became more dominant at high aging. The separation had created reliability risk to copper wire bonding even though the IMC thickness is only around 1 μm.