Electromigration reliability test and RF characterization of Cu-Sn-In µbumps for heterogeneous integration

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School of Electrical Engineering | Master's thesis
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2025-07-31

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Smart Systems Integrated Solutions

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Joint International Master in Smart Systems Integrated Solutions

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en

Pages

72

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Abstract

The relentless demand for increased performance and functional density in modern electronics has propelled the semiconductor industry from traditional transistor scaling toward heterogeneous integration (HI) and advanced packaging. This new paradigm of integrating multiple components—fabricated from different materials, processes, or nodes—into a single package is central to the industry’s technology roadmap. Central to this paradigm are high-density micro-scale interconnects, which are critical for achieving high bandwidth, low latency, and high IO density. Low-Temperature Solid-Liquid Interdiffusion (LT-SLID) microbumps (µbumps) composed of a Cu-Sn-In ternary system have emerged as a compelling solution, offering low bonding temperature (≈150°C) while forming robust, high-melting-point (>600°C) intermetallic compounds (IMCs). However, a critical knowledge gap exists regarding the long-term electromigration (EM) reliability and high-frequency electrical performance of these µbumps, and addressing this gap is essential for their adoption. This thesis presents a comprehensive investigation into both the EM reliability and high-frequency performance of Cu-Sn-In LT-SLID µbumps. To assess reliability, fabricated test structures were subjected to accelerated EM stress tests at high current densities (2 to 3.2 × 105 A/cm²) and an elevated temperature (150°C). Two samples withstood over 500 and 600 hours of EM stress. Their resistance increased only marginally, from 6.87 Ω to 7.13 Ω and 6.32 Ω to 7.49 Ω, suggesting a high resistance to EM-induced failure. Crucially, non-destructive IR imaging and destructive cross-sectional analysis using Scanning Electron Microscopy (SEM) were performed to correlate electrical performance with microstructural integrity. For the high-frequency characterization, another set of devices embedding Cu-Sn-In µbumps within Coplanar Waveguides (CPWs) was designed, simulated using Ansys HFSS, and fabricated. The S-parameters of these structures were measured using a Vector Network Analyzer (VNA) over a frequency range of 40 kHz to 30 GHz. The results revealed excellent performance, yielding an insertion loss and return loss of less than ≈1 dB and ≈15 dB respectively.

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Supervisor

Tiwary, Nikhilendu

Keywords

Heterogeneous Integration (HI), electromigration (EM), Cu-Sn-In µbumps, Solid-Liquid Interdiffusion (SLID) bonding, LT-SLID, RF characterization, advanced packaging, reliability testing, 3D-IC / 2.5D packaging

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https://urn.fi/URN:NBN:fi:aalto-202508196363

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[dipl] Sähkötekniikan korkeakoulu / ELEC