Thermomechanical Stresses in Copper Films at Elevated Temperature

• Published in: Journal of microelectronics and electronic packaging Vol. 7; no. 2; pp. 99 - 104
• Main Authors: LEDERER, Martin, ZARBAKHSH, Javad, RUI HUANG, DETZEL, Thomas, WEISS, Brigitte
• Format: Journal Article
• Language: English
• Published: Washington, DC International Microelectronics and Packaging Society 01.04.2010
• Subjects: Applied sciences; Electronics; Exact sciences and technology; Materials; Creep; Thermal cycle; Temperature effect; Copper films; Plastic deformation; High temperature; Modeling; plasticity modeling; Metallic thin films; Thick film; Plasticity; Silicon; Copper; Wafer; Thermomechanical stress; wafer curvature; Temperature dependence; Annealing; temperature cycle; Durability; Thermal expansion; Electronic component; Reliability; Yield stress; Room temperature
• ISSN: 1551-4897
• DOI: 10.4071/1551-4897-7.2.99

Thermomechanical stresses in metallic films are a root cause for material fatigue which limits the lifetime of electronic devices. Since the yield stress of metals is temperature dependent, plastic deformations during thermal cycling are increased at elevated temperature. This effect reduces the reliability of electronic parts. In order to investigate this problem, a 20 μm thick copper film was deposited on a silicon wafer. After annealing at 400°C, the sample was exposed to thermal cycles in the temperature range between room temperature and 600°C. The different values for the coefficient of thermal expansion of copper and silicon lead to a curvature of the sample. The wafer curvature was measured by a multilaser beam method. On the basis of the experimental results, a new theoretical model was developed that describes the stress evolution in the film during thermal cycling. In this investigation, the relation between wafer curvature and film stress is calculated by analogy to a model by Freund which is an improvement to the well-known Stoney formula. In addition to the elastic response, the new model considers plasticity of the copper film as well as temperature dependence of creep. It is demonstrated that the model can describe the experiment well and thus thermomechanical stress in copper films.