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54 DESIGN007 MAGAZINE I OCTOBER 2019 more uncased die elements (often from mul- tiple sources) within a single package outline, these high-density interposers can significant- ly enhance product performance by enabling much shorter circuit interconnects for critical signal paths. Key issues the designers will need to con- sider when developing the 2.5D interposer is choosing the most suitable base materials for interposer development. The three primary base materials commonly selected for the 2.5D interposer application are epoxy-glass-based organic composites and silicon and glass di- electrics. Designers will also need to learn the basic methodologies and complexities for met- alization and imaging the interconnecting cir- cuit pattern. High-Tg, Low-CTE Organic Base Material The semiconductors function best when they are electrically interconnected to related devices with the shortest path. While many organic dielectric materials have traditionally proved suitable for a broad range of wire-bond package applications, a number of leading sup - pliers have developed a more advanced lam- inate material that closely matches the very low thermal coefficient of expansion (CTE) of the silicon die element through elevated oper- ating temperature as well as meeting the fine- line interconnect challenge for new genera- tions of high-I/O, face-down, mounted semi- conductors. Shinko Electric in Japan, for example, has developed an ultra-low CTE (1.5 ppm/°C) or- ganic substrate material that will provide a sta- ble core layer for buildup, multiple-layer inter- posers (commercial FR-4 laminate has a CTE of ~16 ppm/°C while the silicon-based die el- ement has a CTE of ~3 ppm/°C). Thermal sta- bility of the core material is also a concern. The bismaleimide-triazine (B-T) laminate ma- terial has a rated Tg (glass transition) close to 300°C while the commercial FR-4 Tg is rated at 150–170°C. The B-T laminate material also furnishes a higher elastic modulus at ~40 GPa (megapas- cals) where commercial FR-4 laminate modu- lus is in the range of 10–15 GPa. A real ad- vantage is that organic-based 2.5D interposer fabrication can utilize the existing PCB manu- facturing infrastructure that is already employ- ing direct-imaging laser ablation for via forma- tion, and have the lithographic capability for furnishing fine-line, semi-additive copper cir- cuit processing. Silicon Interposer Base Material A majority of commercial semiconductor manufacturers utilize thin silicon wafers to provide a stable base for integrated circuit pro- cessing. The silicon-based material provides excellent electrical and mechanical properties and is a natural choice for the 2.5D interpos- er because it perfectly matches the CTE of the silicon die element(s) that will be mounted onto its surface. Fabrication of the interposer is commonly performed within the semiconduc- tor foundry environment; however, the pro- cesses for via hole ablation and metalization are very different from the basic semiconduc- tor manufacturing processes (Figure 2). Initially, suppliers utilizing silicon wafers or panels will commonly adopt mass plas- ma ablation technology to first form the via holes and employ a series of copper metaliza- tion processes to provide via filling and enable circuit redistribution layers (RDL) with lines and spaces measured in micrometers (µm). To maximize assembly efficiency, the base mate- rial can be furnished in the traditional 300-mm diameter wafers or in a reconstituted silicon panel format. Current panel variations include 300-mm and 500-mm square panels, but some Key issues the designers will need to consider when developing the 2.5D interposer is choosing the most suitable base materials for interposer development.