Characterization of Temporary Extrusion Failures in Quarter-Micron Copper Interconnects
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Characterization of Temporary Extrusion Failures in Quarter-Micron Copper Interconnects Yan Zhang1, Junho Choy2, Glenn H. Chapman1 and Karen L. Kavanagh2 1 School of Engineering, 2Department of Physics, Simon Fraser University, Burnaby, BC, V5H 3H3, Canada. ABSTRACT We report an unusual circuit failure mode induced by short-lived extrusions observed during DC and AC electromigration (EM) tests of quarter-micron damascene copper interconnects. This novel “soft” failure mode consists of extrusions forming, then self-dissolving before the traditional permanent void or extrusion failure. These failures shorten the lifetime significantly and bring new challenges to reliability tests. Two self-dissolution mechanisms under DC test conditions are discussed and extrusion shape evolution is modeled assuming both capillary and electron wind forces are present. Our model confirms that the electrical stress will accelerate the shape evolution process. INTRODUCTION Copper (Cu) interconnects have replaced aluminum in 0.13 m technology and beyond due to its lower resistivity and higher reliability to electromigration [1] [2]. In Cu metallization, damascene electroplating is implemented with Chemical Mechanical Polishing (CMP) because it is difficult to dry-etch Cu [3]. This transition in materials and processes brings new interconnect failure mechanisms. Due to the poor adhesion between Cu and dielectrics and the inherent defects induced by CMP, instead of grain boundaries, the interface along the Cu/capping layer becomes the major atomic diffusion pathway [4] [5]. Besides the traditional void failure, extrusion failures are widely detected in Cu damascene interconnects [6]. It is normal to observe interfacial delamination along the capping layer at the Cu/Inter Metal Dielectrics (IMD) interface, due to mechanical stress induced by EM and thermal stress from accelerated reliability tests. Cu atoms are able to extrude into cracks to short neighboring metal lines [7]. Our EM tests on 0.13 m technology, Cu dual damascene interconnects also support this scenario. However, they also reveal a novel “soft” failure mode due to temporary extrusions. Extrusion can form and self-dissolve multiple times under both DC and 0 offset AC conditions and this phenomenon brings new challenges to EM reliability tests. EXPERIMENTS A planar and via-free serpentine line with 26 turns was employed as the test structure, figure 1 (a). The Cu line is 1800 m in length, 0.25 m in width, and is isolated from parallel Cu monitor lines by IMD 0.15 m wide. The major part of the monitoring lines is wider than 3.9 m and all metal lines are 0.37 m in thickness. The grain texture in electrodeposited Cu is expected to be (111) [8], however, we have not confirmed this in our samples. EM is reported not to depend on the Cu grain texture, as most voids and hillocks form along the top surface of Cu lines [9]. A thin layer SiNx (on top) and TaNx (on all other sides) improve the adhesion between electroplated Cu and Inter Layer Dielectrics (ILD) and IMD, respectively
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