Modelling of wire bonding Cu-Al intermetallic formation growth towards interfacial stress

Abstract

Quality requirement in the semiconductor industry is getting more stringent due to the application of semiconductor components that are widely used in automotive. Furthermore different materials combination is being introduced to semiconductor packages to improve their package performance and reduce cost. The new materials combination can increase the packages functionality adversely mismatch of different materials properties induced higher interconnection and package stress. With such scenario, quality of interconnects in the semiconductor products need more robust in order to withstand more severe condition and longer life. In this study the interfacial stress of Copper (Cu) Aluminium (Al) intermetallic compound (IMC) structure during high thermal stress condition is focused. Cu wire has been widely accepted as a main-stream interconnects to replace Au wire. Whereas Al bond pad is always the mostly used material for silicon semiconductor bond pad. According to Arrhenius equation, intermetallic growth is proportional to temperature. The Cu-Al IMC is complex due to the nanoscale and multiple phases exist. The Cu-Al IMC layer formed are normally in nanometer thickness, whereas Au-Al IMC is more thicker in micrometer. Analysis performed on Cu-Al IMC required more sophisticated and advanced tools. This is a big challenge for Cu-Al IMC studies. Thus the formation homogenous of Cu-Al IMC is more difficult to form compare to gold (Au) and Al IMC. Therefore using finite elements analysis (FEA) simulations of 3D models of various Cu-Al IMC compounds with different thickness is conducted to understand the stress and strain distribution at IMC layer. The numerical simulation is linear in nature and is based on linear isotropic material properties. Hence the equivalent stress is linear against increment of temperature. The modeling is using thermal-mechanical structure as the loading system. The effect of different IMC compound material properties is examined. From the numerical analysis once the IMC formed between Cu-Al interconnect the interfacial stress increased 22%. The stress reduced ∼3% when IMC growth thicker. The equivalent stress saturated when IMC growth more than 50% of the total Al bond pad thickness. Comparing different Cu-Al IMC compound Cu9Al4 showed the most dominant impact towards the interfacial stress.