Temperature dependent transition of intragranular plastic to intergranular brittle failure in electrodeposited Cu micro-tensile samples A. Wimmer a , M. Smolka b , W. Heinz a , T. Detzel c , W. Robl d , C. Motz e , V. Eyert f , E. Wimmer f , F. Jahnel g , R. Treichler g , G. Dehm h,n a Kompetenzzentrum Automobil- und Industrie-Elektronik GmbH, A-9524 Villach, Austria b Institute of Sensor and Actuator Systems, Vienna University of Technology, A-1040 Vienna, Austria c Infineon Technologies Austria AG, A-9500 Villach, Austria d Infineon Technologies Germany AG, D-93049 Regensburg, Germany e Chair Experimental Methods of Material Science, University of Saarland, D-66123 Saarbrücken, Germany f Materials Design SARL, F-92120 Montrouge, France g Siemens AG, Otto Hahn Ring 6, D-81739 München, Germany h Max-Planck-Institut für Eisenforschung GmbH, D-40237 Düsseldorf, Germany article info Article history: Received 27 June 2014 Received in revised form 2 September 2014 Accepted 5 September 2014 Available online 16 September 2014 Keywords: Micromechanics Mechanical characterization Microanalysis Grain boundaries Fracture Plasticity abstract Smaller grain sizes are known to improve the strength and ductility of metals by the Hall–Petch effect. Consequently, metallic thin films and structures which must sustain mechanical loads in service are deposited under processing conditions that lead to a fine grain size. In this study, we reveal that at temperatures as low as 473 K the failure mode of 99.99 at% pure electro-deposited Cu can change from ductile intragranular to brittle intergranular fracture. The embrittlement is accompanied by a decrease in strength and elongation to fracture. Chemical analyses indicate that the embrittlement is caused by impurities detected at grain boundaries. In situ micromechanical experiments in the scanning electron microscope and atomistic simulations are performed to study the underlying mechanisms. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Cu is widely used as a metallization material in microelectronic devices, light emitting diodes, and micro-electromechanical sys- tems. While its excellent electrical and thermal conductivity are the main characteristics of the Cu metallization, its functionality is also strongly bound to its mechanical performance. Thus, it is of paramount interest to measure the stress–strain response of Cu thin film materials and small scale structures at different tem- peratures. The main method to study the mechanical response of thin films is the wafer curvature approach where the thermal expansion mismatch between the film and substrate is utilized to induce strain in the film [1,2]. This technique mimics the thermo- mechanical exposure occurring in devices due to Joule heating and/or external temperature fluctuations but also complicates an interpretation of the underlying deformation mechanisms; this is because temperature and stress are always coupled and the strain is typically limited to less than 1% [3]. The other most frequently used approach for mechanical testing of thin films is nanoindenta- tion, which provides hardness and (reduced) Young's modulus values. It is not capable, however, of providing full stress–strain curves and strain to failure [4,5]. In the last decade, several miniaturized mechanical testing methods have been developed, which permits one to perform quantitative measurements on small scale compression [6], tension [7], and bending [8] samples. Recently, Smolka et al. put the approach of miniaturization forward by implementing a resistance heating system into a miniaturized test rig which can perform tension tests on micron- sized samples inside a scanning electron microscope (SEM) at temperatures up to 673 K with a load resolution of 10 mN [9]. Thus, with this method, the mechanical properties of freestanding thin films can be measured and analyzed without any influence of the thermal expansion mismatch – and consequently thermal stress – between the metallic thin film and the substrate at variable temperatures with respect to their microstructure. In this study we will focus on electrodeposited Cu films, which have, dependent on the additive system and resulting impurity level, a purity of more than 99.99 at% and a grain size of several Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A http://dx.doi.org/10.1016/j.msea.2014.09.029 0921-5093/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: dehm@mpie.de (G. Dehm). Materials Science & Engineering A 618 (2014) 398–405