Synchrotron X-ray Micro-diffraction Analysis on Microstructure Evolution in Sn under Electromigration
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Synchrotron X-ray Micro-diffraction Analysis on Microstructure Evolution in Sn under Electromigration Albert T. Wu1,2, N. Tamura2, J. R. Lloyd3, C. R. Kao4 and K. N. Tu1 1
Dep. of Materials Science and Engineering, UCLA, Los Angeles, CA 90095 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 3 T. J. Watson Research Center, IBM, Yorktown Heights, NY 10598 4 Chemical & Materials Engineering, National Central University, Chungli, Taiwan 2
ABSTRACT White Sn (β-Sn) thin film stripe shows a voltage drop about 10% when subjected to electromigration testing. Since β-Sn has anisotropic crystal structure, it possesses different resistivity along a-, b- and c axis. The direction of the axes determines the resistance in each grain. Under electromigration, low resistance grain tends to grow in the expense of the neighboring high resistance grains. The changes of grain orientation in the Sn stripe before and after electromigration was studied by synchrotron x-ray microdiffraction (~1µm diameter) to achieve grain-by-grain analysis. Grain growth involves grain boundary migration and rotation of neighboring high resistance grains. A model different from normal grain growth is proposed to describe the condition and mechanism of microstructural evolution under electromigration.
INTRODUCTION Since the dimension of the interconnects are in the trend of the range of sub-micron or nano size, the current density can reach up to 106 A/cm2. It causes mass transport in the line at the device operation temperature of 100°C and leads to void formation at the cathode and extrusion at the anode, which is the phenomenon called electromigration. It is the most persistent and serious reliability failures in thin film integrated circuits. As device miniaturization demands smaller and smaller interconnects, the current density goes up, so does the probability of circuit failure induced by electromigration. It is a subject which has demanded and attracted much attention. Moreover, in solder joints, the traditional eutectic Sn-Pb solder is going to be replaced by Pb-free solder, in which the composition is mainly pure Sn, due to the environmental concern. The electromigration behavior on pure Sn is fundamental and crucial for the future understanding and application of Pb-free solder [1,2,3]. Lloyd observed a voltage drop while studying the electromigration behavior in Sn thin film strip [4]. For the same amount of current, the voltage drop implies that the resistance drops in the line. It is legitimate to think that it’s the anisotropic property of tin that causes the phenomenon. White tin (β-Sn) has a body center tetragonal structure with lattice parameter of a=b=5.83Å, and c=3.18 Å. The resistivity along a, b is 13.25µm-cm and c is 20.27 µm-cm, respectively. Due to this anisotropic property, it is reasonable to think that the microstructure evolves while passing current through the strip. The high resistance grain might rotate and align themselves in the direction of the low resistance grains in order to min
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