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98 I-CONNECT007 MAGAZINE I MARCH 2026 methodologies for silicon-to-silicon and silicon-to- silicon wafer or glass-base panel interposers: • Reflow solder • Hybrid bonding The interposer assembly sequence begins with placing the chiplet die elements face-down onto the interposer panel's upper surface. The electrical interface between the die and interposer may use a tacky-flux dip-and-place process and a reflow- solder process to complete the joining of the die elements to the silicon-based interposer surface. Following the die-joining process, a non-conduc- tive polymer under-fill material is commonly ap- plied to provide physical reinforcement. Reflow solder processing uses a high-temper- ature tin alloy-based solder composition that is plated or deposited onto the copper terminal surfaces. Flux is applied to opposing surfaces, and die elements are aligned and sequentially stacked onto each other. Reflow solder processing com- pletes the joining process, followed by flux residue removal and polymer underfill. Hybrid bonding, on the other hand, is a solderless joining technol- ogy that differs a great deal from the solder joining methods (Figure 3). Hybrid-bond interconnect is described as a heterogeneous or homogeneous direct-bond interconnect technology that enables vertical join- ing of semiconductor die-on-die, die-on-wafer, and even wafer-on-wafer without the use of solder or other additive conductive materials between the attached die surfaces. This technology is increas- ingly being utilized for vertically joining a wide range of semiconductor devices, such as sensors, memory, and logic die elements. The primary advantage of hybrid-bond process- ing is the reduction of terminal pitch and spacing, enabling faster transmission speeds and lower power consumption. The following will compare two successful hybrid joining process variations: fusion bond and oxide bond. Fusion-Bond (Thermo-compression Bonding) The fusion-bond joining process is a two-stage procedure that begins with precise alignment and pre-bonding of the die elements and the inter- poser surface at room temperature. In preparation for joining silicon die elements, or the die directly onto the wafer platforms, a layer of tin (Sn) is first applied to the exposed copper terminal features. Following pre-bond, the die elements are exposed to an annealing process that includes high tem- perature and pressure. When the stacked die or wafers are heated to approximately 400ºC, the tin alloy completely diffuses into the opposing copper land features to form a stable Cu-Sn-Cu (Cu 3 Sn) intermetallic at the land-to-land interface. The cross-section view and SEM image of the resulting bond is furnished in Figure 4. Oxide-Bonding and Direct Bonding Interconnects (DBI) In preparation, all surfaces to be joined are pla- narized and processed by chemical mechanical polishing (CMP). During the process, the copper bond lands are slightly recessed below the sur- rounding oxide dielectric surface. When the two polished surfaces are brought together (at room temperature), an oxide-oxide bond is formed. D ES I G N E R 'S N OT E B O O K Figure 3: Comparing reflow solder to hybrid-bond joined die elements. Figure 4: Post process view of the Cu-Sn-Cu fusion-bond joining process.