Research paper Accelerated thermo-mechanical fatigue of copper metallizations studied by pulsed laser heating Stefan Wurster a,1,2 , Stephan Bigl a, ⁎ ,1 , Megan J. Cordill b , Daniel Kiener a a Department of Materials Physics, Montanuniversität Leoben, Jahnstrasse 12, 8700 Leoben, Austria b Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstrasse 12, 8700 Leoben, Austria abstract article info Article history: Received 12 April 2016 Accepted 10 August 2016 Available online 20 August 2016 Fatigue is an important reliability issue for microelectronics. In this work, a technique for fast thermal cycling of thin films on substrates is introduced using an infrared laser beam. The advantages of this method are the signif- icantly increased heating and cooling rates compared to conventional slow furnace processes, and the use of readily available small pieces of metallized wafers, which avoid complicated sample preparation. To demonstrate the applicability of the new experimental setup two copper metallization films, differing in their content of addi- tives used for film deposition, on silicon substrates were investigated with respect to microstructural, topograph- ical and electrical changes due to pulsed thermo-mechanical loading. The results, such as grain growth and roughness evolution, are compared to results from specimens which experienced slow infrared furnace cycling. Furthermore, changes in electrical sheet resistance are shown. When analyzing the outcomes from processes of different heating/cooling rates, it can be stated that accelerated laser heating leads to faster changes in these properties which enables fast screening of metallization materials under development. © 2016 Elsevier B.V. All rights reserved. Keywords: Copper metallization Thermo-mechanical fatigue Microstructure Laser heating Electron backscatter diffraction Atomic force microscopy 1. Introduction During switching operations temperatures in semiconductor devices rise substantially. As these devices are composed of a stack of different materials, having differences in elastic (Young's Modulus), plastic (yield strength) and thermal (coefficient of thermal expansion, CTE) properties, stresses evolve. Repeated switching, i.e. repeated cycles of heating and cooling, leads to changes of the film microstructure. The driving force for this phenomenon, besides diffusional processes at ele- vated temperatures, is the existence of stresses within the layered ma- terial. Considering a two material system, such as a thin copper layer on a thick silicon substrate, these stresses originate from differences in the CTEs leading to biaxial compressive or tensile stress states. The CTE for silicon is between ~3.45 ppm/°C for 170 °C and ~3.97 ppm/°C for 400 °C [1], whereas the CTE-values for polycrystalline copper are be- tween ~ 17.9 ppm/°C and ~ 19.3 ppm/°C [2] at the same temperatures. In other words, heating of the specimen leads to compressive stresses in the faster expanding metallization layer, while tensile stress states pre- vail at low temperatures [3–5]. The combined effects of temperature (diffusion) and stresses might be grain growth, surface roughening and texture changes [5,6]. Texture changes can originate from rotations of the crystals or growth of preferentially oriented grains. Thus, micro- structural and topographical changes are very likely to be observed due to thermal cycling. In a progressed state of thermo-mechanical cy- cling, voids can nucleate [5] or cracks might form, thereby degrading the mechanical and electrical properties and limiting the operational life-time. Thus, the investigation of thermo-mechanical fatigue and the integrity of metallization layers is a key issue regarding reliability of semiconductor devices. Copper, being one of the main materials for metallizations in semi- conductor technology due to its outstanding thermal and electrical properties, was chosen as the material to be subjected to thermo-me- chanical cycling. To achieve thicknesses of several micrometers com- monly used for metallization layers, electrochemical deposition was applied for the material used in this study. With the usage of electro-de- posited materials comes the freedom of process parameters and the usage of different additives leading to differently microstructured films having the desired properties. In this publication, changes and similarities in the fatigue behavior of two films containing different amounts of inorganic residuals, originating from the deposition process is one of the topics that will be addressed. For thermal cycling of materials, a variety of experimental proce- dures are available. Usage of the wafer curvature technique offers the possibility to directly determine the evolving stresses [4,7,8]. However, the heating and cooling rates are very low, which lies in the range of Microelectronic Engineering 167 (2017) 110–118 ⁎ Corresponding author. E-mail address: stephan-paul.bigl@stud.unileoben.ac.at (S. Bigl). 1 Both authors contributed equally to this work. 2 Present address: OTTRONIC Technology Laboratory, OTTRONIC GmbH Austria, Villenstrasse 10, 8740 Zeltweg, Austria. http://dx.doi.org/10.1016/j.mee.2016.08.004 0167-9317/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee