Evaluation of Void Formation Mechanism in Cu Thin Films; Separation of The Effect of Electron Wind Force and Stress
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0968-V05-07
Evaluation of Void Formation Mechanism in Cu Thin Films; Separation of The Effect of Electron Wind Force and Stress Yousuke Fujii1, Masanori Tsutsumi2, Junya Inoue1, and Toshihiko Koseki1 1 Department of Materials Engneering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo, 113-8656, Japan 2 Fujitsu Co.Ltd., 50 Fuchigami, Tokyo, 197-0833, Japan
ABSTRACT Electromigration (EM) and stress-induced voiding (SIV) in thin films of Cu and Cu alloys were studied separately. The characterization of EM was evaluated in a modified interconnect structure where the effect of current density can be observed in the absence of stress disturbance. SIV, on the other hand, is examined in detail by applying a uniform stress to Cu films without electric current. Results of EM tests clearly showed that void and hillock formations were concentrated mainly in the region of high current density gradient without stress disturbance. SIV test performed under the high stress of 1GPa showed that voids were observed to form at grain boundaries and grain boundary triple junctions. Under the high stress, SIV was shown to be generated in the absence of stress gradient. Also, the effect of alloying to EM and SIV was investigated in detail. INTRODUCTION EM and SIV are the serious reliability concerns in today’s Cu Dual-Damascene Interconnects (Cu-DDI) because of the downward scaling of feature sizes in large scale interconnects[1]. However, most of studies on EM and SIV in Cu-DDI have been conducted on the multilevel interconnects structure and focused only on estimating the MTTF (Mean Time To Failure) [2-5] of EM and SIV because of the importance of Cu-DDI reliability for practical use. The atomic flux produced during EM can be the mixed product of EM and SIV, which complicates the understanding of the mechanisms [6-8]. In addition, it is very difficult to deal with the stress distribution in Cu-DDI due to its complicated structure. To clarify the mechanisms of EM and SIV, they have to be separately analyzed in a simpler way. In this study, we conducted an experimental study of the EM behavior of Cu and Cu alloys thin films using a simple interconnect structure where the effect of current density and its gradient is investigated in the absence of stress and thermal disturbances. Also, we conducted experimental study of SIV behavior using the same Cu and Cu alloys thin films as EM test to evaluate only the effect of applied uniform uniaxial stress, and evaluated the void formation.
EXPERIMENTAL Film preparation Cu and Cu alloy films (Cu-0.9at%Sn, Cu-3.7at%Ag) were deposited up to a thickness of 100nm on the substrates using electron beam physical vapor deposition (EB-PVD) method. In some test samples for EM test, 10nm-thick Ta film was deposited on the Cu film as a capping layer in order to suppress vacancy sinks and sources on the free surface of Cu films, which was made in the same batch after the deposition of Cu films without breaking a vacuum. For all samples for SIV test, 10nm-thick Ta film was deposited on the Cu film as a ca
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