Characterization of Wafer-Level Au-In-Bonded Samples at Elevated Temperatures
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INTRODUCTION
SOLID liquid interdiffusion (SLID) wafer-level bonding is a promising technology for system integration that allows applications at temperatures higher than the actual bonding temperature. It enables the use of low-cost metallization, flux-free bonding, fine-pitch micro-interconnects, encapsulation, and probably the most important: repeated stacking/integration without re-melting the previously bonded materials.[1] The bonding technique is based on the rapid formation of intermetallic compounds (IMCs) between two metal components, one being a low-melting and the second a high-melting component. The bonding temperature is usually kept above the melting point of the low-melting component. The layer thicknesses are designed such that the low-melting metal is completely consumed in the THI THUY LUU, PhD Candidate, NILS HOIVIK, KAIYING WANG, and KNUT E. AASMUNDTVEIT, 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. VARDBY, Research Scientist, is with the SINTEF ICT Instrumentation Department, Forskningsveien 1, 0373 Oslo, Norway. Manuscript submitted June 13, 2014. Article published online March 31, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
process, leaving a final bond-line of IMCs with a higher re-melting temperature than the initial bonding temperature. Traditionally, the layer thicknesses are designed with excess high-melting point metal, resulting in remaining layers of this material after bonding. For applications that require operation at, or exposure to, high temperatures the main requirements to the bonding material are that the bond-line has a highmelting point and that the shear strength remains high at elevated temperatures. In order to achieve high shear strength at elevated temperatures, it is important to minimize the thermally induced stresses in the bond stack; this is typically done by reducing the bonding temperature, having a low CTE mismatch between the utilized materials, and by having a good thermal conductivity. In this study, we have investigated the high-temperature integrity of Au-In SLID wafer-level bonded samples. Compared to the better known SLID systems Cu-Sn and Au-Sn which are assembled at 523 K to 573 K (250 C to 300 C) and 573 K to 623 K (300 C to 350 C),[1–4] Au-In can be assembled using a lower bonding temperature 453 K (180 C).[5,6] This is of interest for many applications since a lower bonding temperature may reduce thermal stress in the package, and also facilitate bonding of temperature sensitive devices.[7] Furthermore, with a properly designed VOLUME 46A, JUNE 2015—2637
bond-line of Au-In, the final bond has demonstrated excellent reliability during thermal cycling and hightemperature storage/aging tests,[5,6,8,9] as well as longterm hermeticity.[10] Au-In wafer-level bonded samples have also shown bond-integrity up to 742 K (469 C) by applying a load to a test specimen that was isothermally heated.[11,12]
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