Effect of intermetallic formation on electromigration reliability of TSV-microbump joints in 3D interconnect

Abstract

In this study, electromigration (EM) reliability of TSVmicrobump (µ-bump) joints was investigated. Sn-based µ-bumps with three different schemes of metallization were tested under current stressing at elevated temperatures. EMstressed µ-bumps, together with thermal anneal-only µ-bumps and as-received controls, were cross-sectioned and characterized using scanning electron microscope (SEM), energy dispersed x-ray (EDX) and focused ion beam (FIB). Intermetallic compound (IMC) growth kinetics under EM for the three types of metallization were obtained, and compared with those subjected to thermal annealing only. Results showed good EM performance of the TSV µ-bump joints, indicating that IMC formation plays an important role in improving the EM reliability of µ-bump joints. However, non-EM related voids were observed in the µ-bumps, and the voiding mechanisms were discussed. IntroductionMicro-bumping is one of the key technologies in 3D integration [1]. This technology, together with through-sliconvias (TSVs), forms electrical connections between stacked dies in a 3D integrated circuits. As the I/O counts increase and the pitch size decreases, material selection for the bonding metallurgy and reliability of the fine-pitch interconnect joining have become critical issues in 3D chip stacking. Finepitch silicon-to-silicon stacking approaches include transient liquid phase (TLP) bonding resulting in intermetallic compound (IMC) after joining [2], metal to metal (Cu-Cu) bonding by thermal compression [3], and micro-controlled collapse chip connection (µ-C4) or µ-bumps through mass reflow or thermal compression [4,5].Sn-based µ-bumps are currently used by the microelectronic industry due to its process compatibility with the current assembly technologies. With the joint size significantly smaller than the C4 solder joints and the need to increase the current density, electromigration (EM) has emerged as a critical reliability concern and has drawn considerable interests [6][7][8][9][10]. In Sn-based C4 solder joints, IMC formation resulting from material reactions between Sn and the under-bump metallurgy (UBM) was found to be enhanced under EM and played an important role in controlling the solder reliability [11,12]. For 3D chip stacking interconnection, the solder volume is substantially reduced because of package miniaturization and fine-pitch requirements. The limited supply of Sn induces more extensive IMC formation and can convert the entire bump into IMC upon thermal annealing or under current stressing. The extensive IMC growth in Sn-based µ-bumps gives rise to EM

Thermal cycling effect on intermetallic formation with various surface finish of micro bump interconnect for 3D package

“…In terms of IMC formation and metallization, a symmetric u-joint such as Cu-Sn with Cu finish and Ni-Sn with Ni finish were more predictable than an asymmetric structure joint, as reported by Y.W. Wang et al [8].…”

Development of a stacked WCSP package platform using TSV (Through Silicon Via) technology

“…For the Ni-Sn-Ni case, the entire joint is now converted to Ni 3 Sn 4 . A more detailed investigation of IMC evolution and growth kinetics under thermal annealing and in combination with EM current, can be found in the following paper by the authors [10]. The influence of EM current on IMC growth was found to be negligible since the joints were converted to IMC at t-zero or shortly thereafter due to the test temperature.…”