Effect of CU and Si in Aluminum on Stress Change and on TiAl₃ Formation in Al Alloy/Ti Bilayer Films During Annealing

Abstract

Interconnect metallizations used in advanced integrated circuits typically use an Al-alloy sputterdeposited onto a Ti barrier layer. The Ti and Al react above ~ 400°C to form TiAl₃, which affects the stress evolution of the metal stack during thermal cycling. This paper describes results of thin film experiments performed on Ti/Al-alloy bilayer films. Two Al alloys were studied: Al-I%Cu and Al-0.5%Cu-1%Si. The rate of TiAl₃ formation at 430°C was determined for both alloys and used to relate TiAl₃ formation to the stress evolution of the film stacks during thermal cycling. The dominant effect of the TiAl₃ intermetallic formation on stress arises from a change in the stress-temperature behavior of the film stack, due to a change in the yield behavior, effective modulus, and thermal expansion coefficient of the stack. The presence of Si in the Al-alloy markedly reduces both the rate of TiAl₃ formation and the resulting change in composite stress.