Formation of solid-solution Cu-to-Cu joints using Ga solder and Pt under bump metallurgy for three-dimensional integrated circuits

Three-dimensional (3D) integrated circuits (ICs) are the most important packaging technology for next-generation semiconductors. Cu-to-Cu throughsilicon via interconnections with micro-bumps are key components in the fabrication of 3D ICs. However, significant reliability concerns have been raised due to the formation of brittle intermetallic compounds in the entire 3D IC joints. This study proposes a Ga-based Cu-to-Cu bonding technology with Pt under bump metallurgy (UBM). A systematic analysis of reactive wetting between Ga solders and polycrystalline, single-crystalline, and Ptcoated Cu substrates was conducted. Pt UBM as a wetting layer was identified to be a key component for Ga-based Cu-to-Cu bonding. Pt-coated Cu substrates were bonded using Ga solders with various Ga-to-Pt ratios (n) at 300℃. When n ≥ 4, the Cu/Pt/Ga/Pt/Cu interface evolves to Cu/facecentered cubic (fcc)/γ₁-Cu₉Ga₄/fcc/Cu, Cu/fcc/γ₁-Cu₉Ga₄ + Ga₇Pt₃/fcc/Cu, and finally Cu/fcc + Ga₇Pt₃/Cu structures. The desired ductile solid solution joint formed with discrete Ga₇Pt₃ precipitates. When n ≤ 1, a Cu/Ga₇Pt₃/Cu joint formed without Cu actively participating in the reactions. The reaction mechanism and microstructure evolution were elaborated with the aid of CALPHAD thermodynamic modeling.[Figure not available: see fulltext.]