Effects of Pulse Duration and Polarity on the Electromigration Behavior of Copper Interconnects under Pulsed Current Str
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1079-N05-07
Effects of Pulse Duration and Polarity on the Electromigration Behavior of Copper Interconnects under Pulsed Current Stress Meng Keong Lim1,2, Chee Lip Gan1, Yong Chiang Ee2, Chee Mang Ng2, Bei Chao Zhang2, and Juan Boon Tan2 1 School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, 50 Nanyang Avenue, Singapore, 639798, Singapore 2 Chartered Semiconductor Manufacturing Ltd, 60 Woodlands Industrial Park D, Street 2, Singapore, 738406, Singapore ABSTRACT Direct current (D.C.) is usually employed to characterize the electromigration reliability of interconnects. However, D.C. characterization techniques might not reflect the actual reliability of interconnects that are carrying pulsed D.C. or A.C. (alternating current) signals during operation. This study investigates the effects of unipolar and bipolar pulsed currents on the electromigration lifetime of copper (Cu) interconnects. A series of long period pulsed current (i.e. 2, 16, 32 and 48 hours) were applied to Cu interconnects. Lifetime enhancement is observed when the half-period of pulsed current is shorter than the median-time-to-failure (t50) of D.C. stressed samples. Relatively minor increase in resistance occurring in-between pulses for unipolar pulsed current stressed samples, and occurrence of damage healing in bipolar pulsed current stressed samples are reasons attributed for the observed enhanced lifetime. We obtained longer t50 when the period of the pulsed current is shorter. INTRODUCTION Electromigration, a persistent reliability issue that affects both aluminum (Al) interconnects in the past and advanced copper (Cu) interconnects in present integrated circuits, is usually characterized by applying direct current (D.C.) when conducting reliability studies [1-3]. However, most interconnects carry time varying electrical signals during operation conditions. A mismatch between the actual operation conditions versus the accelerated testing conditions applied in reliability studies is therefore present, and the failure behaviors extrapolated from these accelerated testing conditions might be different from those of operation conditions. Limited electromigration reliability studies that were conducted under pulsed D.C. or A.C. (alternating current) conditions observed that Al interconnects exhibit longer electromigration lifetime as compared to D.C. conditions [4,5]. Although Cu interconnects are expected to exhibit enhanced lifetime under pulsed D.C. or A.C. conditions too, however, substantial work in this area is either lacking or limited at this moment. If the electromigration behavior of Cu interconnects carrying time varying current is well characterized, then the information acquired may be applied to relax the progressively stringent current density design rules and interconnect technology challenges beyond the 22 nm technology node [6]. This paper intents to look at the effects of unipolar and bipolar pulsed currents on the electromigration lifetime and behavior of Cu/low-κ interconnects through their
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