In-situ Microscopic Study of Cu Intragranular Electromigration
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0907-MM13-04.1
In-situ Microscopic Study of Cu Intragranular Electromigration K. C. Chen, C. N. Liao, W. W. Wu and L. J. Chen Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu Taiwan 30013, R.O.C. ABSTRACT Electromigration (EM) in unpassivated copper lines at room temperature has been investigated in ultra-high vacuum by in-situ transmission electron microscopy (TEM). The electric current induced atomic migration in a (211)-oriented Cu grain has been successfully recorded in real-time video. The atomic image of the (211) grain was found to vanish directionally when applying an electric current density of 2 × 106 A/cm2 through the Cu line. The results suggested that the combination of {111} planes and directions to be the easiest EM path in crystalline copper. By performing selective area diffraction (SAD) analysis on a single Cu grain with (111) crystal orientation, some unusual electron diffraction patterns appeared after passing an electric current through the Cu line. It is believed that the EM-induced Cu twinning may be held responsible for the unique diffraction patterns. INTRODUCTION Electromigration [1] (EM) is certainly the most serious and persistent reliability problem in microelectronic technology. Copper is utilized as an interconnection material in ultra-large scale integrated circuits devices because of its lower electrical resistivity and higher resistance to EM than aluminum. The EM-induced atomic flux can cause mass accumulation and depletion at flux divergences sites along the metal line. Mass depletion will lead to nucleation and growth of voids, and hence open circuits eventually. The EM characteristics of interconnections are closely related to metallization microstructure which is a function of deposition methods, barrier materials and process conditions [2,3]. It has been reported that Cu lines with improved (111) texture may have better EM performance due to minimize high angle grain boundaries that are subjected to faster atomic diffusion. However, the details regarding the microstructure-dependent Cu EM behavior in atomic scale have not been studied thoroughly. In order to accelerate and observe EM-induced failures within a reasonable time frame, the EM-related reliability tests were performed at elevated temperature, e.g. 300 0C. Nevertheless, the normal operating temperature of microelectronic devices is approximately 100 0C. It is likely that the Cu EM mechanisms deduced at elevated temperatures may not be the same as those at low temperatures [1]. To clarify the issue we need a method of detecting the minute EM-induced microstructure evolution and/or the corresponding property change of the stressed Cu lines at low temperatures. By in-situ transmission electron microscopy (TEM) very tiny microstructure change induced by electric current can be recorded in real time [4,5]. It also provides the feasibility of investigating the EM-induced failures in the very early stage. Numerous in-situ TEM experiments have been carried out for Al metallizations [4,
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