Microstructure Evolution During Electric Current Induced Thermomechanical Fatigue of Interconnects
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Microstructure Evolution during Electric Current Induced Thermomechanical Fatigue of Interconnects Robert R. Keller, Roy H. Geiss, Yi-wen Cheng, and David T. Read Materials Reliability Division, National Institute of Standards and Technology, 325 Broadway, Boulder, CO 80305, U. S. A. ABSTRACT We demonstrate the evolution of microstructure and deformation associated with the use of electrical methods for evaluating mechanical reliability of patterned interconnects on rigid substrates. Thermomechanical fatigue in aluminum and copper interconnects was induced by means of low frequency (100 Hz), high density (> 10 MA/cm2) alternating currents, which caused cyclic Joule heating and associated thermal expansion strains between the metal lines and oxidized silicon substrate. The failure mechanism involved formation of localized plasticity, which caused topography changes on the free surfaces of the metal, leading to open circuit eventually taking place by melting at a region of severely reduced cross-sectional area. Both aluminum and copper responded to power cycling by deforming in a manner highly dependent upon variations in grain size and orientation. Isolated patches of damage appeared early within individual grains or clusters of grains, as determined by a quasi in situ scanning electron microscopy and automated electron backscatter diffraction measurement. With increased cycling, the extent of damage became more severe and widespread. We document some examples of the types of damage that mechanically confined interconnects exhibited when subjected to thousands of thermal cycles, including growth and re-orientation of grains in a systematic manner. We observed in the case of Al-1Si certain grains increasing by nearly an order of magnitude in size, and reorienting by greater than 30°. The suitability of electrical methods for accelerated testing of mechanical reliability is also discussed.
INTRODUCTION We describe the evolution of microstructure and deformation associated with thermomechanical fatigue of patterned interconnects induced by low frequency, high density alternating currents. This study is part of an ongoing program to develop electrical test methods for measuring mechanical response of thin films and interconnects on silicon substrates. The intent of pursuing such methods is to circumvent difficulties of implementing methods such as microtensile testing of free-standing films [1] or nanoindentation of films on substrates [2]. Although these methods are well developed, non-trivial preparation of specimens or interpretation of data can render them cumbersome for routine measurement of properties such as strength or fatigue lifetime. This is a particularly important issue for measurement of small structures such as sub-100 nm interconnects, from which microtensile specimens may be difficult to fabricate, or accurate positioning of an indentation tip may be difficult to achieve. Reliability issues associated with thermomechanical fatigue center on failures that take place after accumulation of cyclic
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