Film thickness dependent microstructural changes of thick copper metallizations upon thermal fatigue
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Megan J. Cordill Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben 8700, Austria
Daniel Kiener Department of Materials Physics, Montanuniversität Leoben, Leoben 8700, Austria (Received 14 December 2016; accepted 1 May 2017)
With increasing performance requirements in power electronics, the necessity has emerged to investigate the thermo-mechanical behavior of thick Cu metallizations ($5 lm). Cu films on rigid substrates in the range of 5–20 lm were thermally cycled between 170 and 400 °C by a fast laser device. Compared to the initial microstructures, a texture transition toward the {100} out-of-plane orientation with increasing film thickness was observed during thermo-mechanical cycling, along with an abnormal grain growth in the {100}-oriented grains and a gradual development of substructures in a crystallographic arrangement. Compared to the well-studied thin Cu film counterparts (#5 lm), the surface damage showed a 1/hf dependency. Transition from an orientation independent (hf 5 5 lm) to an orientation specific thermo-mechanical fatigue damage (hf 5 10, 20 lm) was observed following a higher damager tolerance in {100} oriented grains.
I. INTRODUCTION
Since the large scale production of integrated circuits in the 1960s, thin film materials for microelectronic devices have become a major topic in the modern materials science community. The ongoing performance increase with the number of transistors and switching frequency coupled with the miniaturization of semiconductor components require sophisticated deposition processes and excellent film material properties.1 In many silicon-based microelectronic devices, increasing power densities lead to significant temperature rises up to ;400 °C within the microsecond regime during switching operations.2 This results in large thermal stresses due to different coefficients of thermal expansion (CTE) of the materials, where plastic deformation can occur if the yield stress of the metallization is reached.3 Such a repetitive dynamic temperature rise leads to a thermomechanical damage of the metallization in the form of severe surface roughening and voiding. These features can result in local excess temperatures leading to a thermal runaway and local melting of silicon.2 To overcome these problems, one key concept of metallization development is to increase the film thickness (hf), which leads to a greater specific heat capacity and longer operational lifetime. In a previous study, it has Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.199
been shown that increasing the copper metallization thickness to 20 lm will result in the largest relative benefit gain such as the maximum peak temperature during a power pulse.4 Copper, used in modern power semiconductor metallization schemes,5 exhibits very interesting structural properties. Besides its excellent electrical and thermal properties, copper has a very low stacking fault energy (SFE) resulting i
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