Thermo-Mechanical Reliability of 3D-integrated Microstructures in Stacked Silicon
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0970-Y02-04
Thermo-Mechanical Reliability of 3D-Integrated Microstructures in Stacked Silicon Bernhard Wunderle1, R. Mrossko2, O. Wittler1, E. Kaulfersch2, P. Ramm1, B. Michel1, and H. Reichl3 1 Dept. Micro Materials Center, Fraunhofer IZM, Gustav-Meyer-Allee 25, Berlin, D-13355, Germany 2 AMIC, Berlin, Germany 3 Technical University Berlin, Berlin, Germany ABSTRACT This paper investigates the thermo-mechanical reliability of inter-chip-vias (ICV) for 3D chip stacking after processing and under external thermal loads relevant for the envisaged field of application (mobile, automotive) by Finite Element simulation. First the materials are characterised by nano-indentation to determine elasto-plastic data. Finite Element simulations are used to reproduce these data and to extract local material properties like E-modulus and yield stress. Accumulated plastic strain is used as failure indicator under periodic thermal loading of an ICV. Geometrical, material and process-related parameters are varied to obtain first design guidelines for this new technology. The locations of stress and strain accumulation are given. INTRODUCTION Stacked silicon die allow an unparalleled integration density using low-cost back-end processes for maximum device functionality. Thereby, inter-chip vias (ICV) assure short electrical interconnections for high frequency response and ultrafast devices [1, 2]. The technology therefore employed uses inter-chip vias and solid-liquid inter diffusion processes for vertical interconnect formation between thinned silicon dies (ICV-SLID) using e.g. a CuSn alloy [3]. Thereby, thin metallic and intermetallic layers are created with dimensions in the micrometer range. On integrating microelectronic systems with these new technologies, material combinations and dimensions it is of great importance to assure the function of these systems and its individual constituents. So it is necessary to analyse and evaluate systematically their reliability under given boundary conditions in order generate lifetime-models and design guidelines for lifetime prediction. These models need to reflect the physics behind the failure mechanisms and which has to be reproduced consistently by experiment and simulation [4]. This paper investigates the stresses and strains of such structures after processing and then under external thermal loads relevant for the envisaged field of application (mobile, automotive). As this work considers a new technology under development in a running project, this is the first publication of a systematic approach drawing upon both simulation and experimental methods. We begin with an investigation using Finite Element (FE) simulations. First the materials are characterised, as materials may behave differently in their mechanical properties in smaller dimensions of a just a few microns and show a process-dependence. This characterisation is a challenge, as no standard methods can be applied due to the impossibility of fabricating standard test specimens. So methods like nano-indentation to determine
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