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SOLID–LIQUID interdiffusion (SLID) wafer-level bonding is an attractive processing technique for MEMS packaging and encapsulation, especially for devices which may operate at high temperatures. The bonding principle is based on rapid intermetallic compound (IMC) formation between two metal components; a low-melting component (In, Sn) and a high-melting component (Cu, Ag, Au).[1–5] The bonding is performed at a temperature above the melting point of the low-melting component. During the bonding process, the high-melting component diffuses into the low-melting component and reacts to form solid IMCs with higher melting point. The final bond will thereafter consist of IMCs and any excess highmelting component metal. This enables the final bond to tolerate various applications that require operation at, or exposure to, high temperatures (HT). For applications that require operating at high temperatures such as in automotive, aerospace, and petroleum industry, long-term reliability is critical due to the decline of material properties at high temperatures.[6–9] It is important that the materials used in a bond-line have high-melting point and a high mechanical integrity at high temperatures without rapid diffusion of materials. To avoid the unwanted diffusion, it is strongly beneficial to THI-THUY LUU, Researcher, NILS HOIVIK, KNUT E. AASMUNDTVEIT, and KAIYING WANG, Professors, are with the Department of Micro and Nanosystems Technology (IMST), Buskerud and Vestfold University College, Raveien 215, 3184 Borre, Norway. Contact e-mail: [email protected] ASTRID-SOFIE B. VARDØY, Research Scientist, is with the SINTEF ICT Instrumentation Department, Forskningsveien 1, 0373 Oslo, Norway. Manuscript submitted March 8, 2015. Article published online July 28, 2015 5266—VOLUME 46A, NOVEMBER 2015

ensure thermodynamically stable bond-lines. Cu-Sn SLID wafer-level bonding is a promising technology for these applications, as it enables low-cost metallization, high mechanical strength[2,10–12], and high-temperature stability. According to the Cu-Sn phase diagram, the final Cu/Cu3Sn/Cu bond-line is thermodynamically stable, and the Cu3Sn phase is solid up to 949 K (676 C). Reliability testing of Cu-Sn SLID wafer-level bonding in previous works has shown stable mechanical strength and reliable electrical properties during thermal cycling, high-temperature storage, and vibration at high-temperature tests.[11,13–18] Although these tests were performed to prove the technology for high-temperature applications, the shear strength of the tested samples was all carried out at room temperature. Mechanical integrity at temperature surpassing the melting temperature of Sn has long been predicted, but experimental verification is scarce. In this study, the mechanical integrity of Cu-Sn SLID bonding was investigated at high temperature. The initial Cu-Sn thicknesses and the bonding process were designed to ensure a thermodynamically stable Cu/Cu3Sn/Cu bond-line. The mechanical integrity of Cu-Sn bonded samples (shear strength and fractography) was in