Through-package-via hole formation, metallization and characterization for ultra-thin 3D glass interposer packages
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here is an increasing demand for higher bandwidth (BW) between logic and memory ICs for future smart mobile systems. Such high BW are proposed to be achieved using 3D interposers that have ultra-small through-package-via (TPVs) interconnections to connect the logic device on one side of the interposer to the memory on the other side. The current approach is primarily based on organic or silicon interposers. However, organic interposers face several challenges due to their poor dimensional stability, and coefﬁcient of thermal expansion (CTE) mismatch to silicon ICs. Silicon interposers made with back-end-of-line (BEOL) wafer processes can achieve the required wiring and I/O density, but are not cost effective, and in addition exhibit higher electrical loss due to the semiconducting nature of the Si substrate. In this research, ultra-thin 3D Glass Interposers are studied as a superior alternative to organic and silicon interposers. The fundamental focus of this research is to achieve ultra-small TPVs in thin glass with dimensions similar to that of through-silicon-vias (TSVs) in silicon. The objective of this research is to study and demonstrate ultra-small pitch (30µm) TPV hole formation (10µm diameter), metallization and electrical characterization in ultra-thin (30µm) glass substrates. To meet these objectives, this study focusses on four main research tasks: a) electrical modeling and design of ultra-small TPVs in glass, b) small diameter TPV hole formation with minimum defects, c) copper metallization of TPVs with reliable adhesion, and d) electrical characterization of TPVs. This research reports the first demonstration of ultra-small TPVs (10-15µm in diameter) in ultra-thin glass interposer substrates (30µm). A thin-glass handling method is developed using polymer surface layers to achieve defect-free handling of glass even at thicknesses as low as 30µm. Several TPV formation methods are explored including excimer laser ablation using 193nm (ArF) lasers to form TPVs with smallest diameter and pitch. A brief study on the through-put capabilities of these excimer lasers is also discussed. The fundamental approach to TPV metallization involves a semi-additive-plating process (SAP) using electroless and electrolytic copper deposition techniques. The resulting side-wall surfaces of TPVs after metallization are analyzed through SEM imaging of TPV cross-sections, and are further characterized using nano-indentation tests. Additionally, thermo-mechanical reliability tests and failure analysis are performed to study the reliability of TPVs that are metallized with Cu. This research culminates in design, fabrication and electrical characterization of small pitch TPVs in ultra-thin glass interposers (30µm).