Au-Sn SLID Bonding: A Reliable HT Interconnect and Die Attach Technology TORLEIF ANDRE ´ TOLLEFSEN, ANDREAS LARSSON, MAAIKE MARGRETE VISSER TAKLO, ANTONIA NEELS, XAVIER MAEDER, KRISTIN HØYDALSVIK, DAG W. BREIBY, and KNUT AASMUNDTVEIT Au-Sn solid–liquid interdiffusion (SLID) bonding is an established reliable high temperature (HT) die attach and interconnect technology. This article presents the life cycle of an optimized HT Au-Sn SLID bond, from fabrication, via thermal treatment, to mechanical rupture. The layered structure of a strong and uniform virgin bond was identified by X-ray diffraction to be Au/f (Au 0.85 Sn 0.15 )/Au. During HT exposure, it was transformed to Au/b (Au 1.8 Sn 0.2 )/Au. After HT exposure, the die shear strength was reduced by 50 pct, from 14 Pa to 70 MPa, which is still remarkably high. Fractographic studies revealed a change in fracture mode; it was changed from a combination of adhesive Au/Ni and cohesive SiC fracture to a cohesive b-phase fracture. Design rules for high quality Au-Sn SLID bonds are given. DOI: 10.1007/s11663-012-9789-1 Ó The Minerals, Metals & Materials Society and ASM International 2013 I. INTRODUCTION A. Background HIGH temperature (HT) environments offer great challenges for electronic systems. In applications like automotive, aerospace, and drilling and well interven- tion systems, the electronic components are often exposed for temperatures above 500 K (~200 °C). The number of commercially available HT wide band gap semiconductors is rapidly increasing. [1] Silicon carbide (SiC) and gallium nitride (GaN) are commonly consid- ered as the semiconductors of choice for HT applica- tions. [2] SiC has a wide band gap, a high breakdown field strength, a high thermal conductivity, and an operating junction temperature of up to 850 K (~600 °C). [2–5] However, lack of qualified HT packaging technologies limits the market growth. [2,6–8] The range of HT die attach and interconnect techniques is restricted. [6,9,10] Alternatives include sintered nanopar- ticle Ag bonds, [11,12] liquid-based solder bonds, [13] com- posite solder bonds, [14] bismuth-based solder bonds, [15] Au-Au thermo-compression bonds, [16] and solid–liquid interdiffusion (SLID) bonds. [17,18] The latter, also called transient liquid phase (TLP) bonding, [19,20] isothermal solidification, [21] or off-eutectic bonding, [22] has proven to be an excellent candidate. [22–25] It utilizes a binary system with one low and one high melting point metal. Examples of SLID systems include Ag-In, [17] Ag-Sn, [26] Au-In, [17,27] Au-Sn, [22–25,27–33] and Cu-Sn. [34–37] In the present work, the focus is on the Au-Sn system. B. Processing A combination of solid-state and liquid-state diffusion takes place during SLID bonding. [33] First, the bonding surfaces are brought into contact and heated to a temperature above the melting point of the low melting point metal, quickly creating new intermetallic com- pounds (IMCs) by liquid-state diffusion. Second, if the temperature is kept high enough, solid-state diffusion will continue until a uniform bonding layer is obtained. [27] The solidification is isothermal, and the final joint has a higher melting point than the processing temperature. Since liquid-state diffusion is approximately three orders of magnitude faster than solid-state diffusion, [27] the latter step will take longer to complete. Complete wetting of the bonding surfaces is required to create a high quality SLID bond. This can be difficult to attain for a Au-Sn alloy. [28] The main challenge is associated with oxidation of the Sn and Au-Sn surfaces, preventing bonding. [28] Methods to achieve complete wetting include scrubbing or static pressure combined with H 2 ,N 2 , or vacuum environment during forma- tion. [38–42] A superb Au-Sn SLID bond has, e.g., been achieved by applying a small clamp force in combina- tion with low vacuum (10 kPa). [25] An optimization, generally minimization, of bonding time and temperature is required to make Au-Sn SLID suitable for industrialized manufacturing. Several TORLEIF ANDRE ´ TOLLEFSEN, Ph.D. Student, is with the SINTEF ICT Instrumentation, 0373 Oslo, Norway, and also with the Institute for Micro and Nanosystems Technology, Vestfold University College, 3184 Borre, Norway. Contact e-mail: torleif.tollefsen@ sintef.no ANDREAS LARSSON, Senior Scientist, and MAAIKE MARGRETE VISSER TAKLO, Research Manager, are with the SINTEF ICT Instrumentation. ANTONIA NEELS, Section Head, and XAVIER MAEDER, Post Doc, are with the XRD Application Lab & Microscopy, Microsystems Technology Division, CSEM Centre Suisse d’Electronique et de Microtechnique SA, 2002 Neuchaˆ tel, Switzerland. KRISTIN HØYDALSVIK, Post Doc, and DAG W. BREIBY, Associate Professor, are with the Department of Physics, Norwegian University of Science and Technology, 7491 Trondheim, Norway. KNUT AASMUNDTVEIT, Associate Professor, is with the Institute for Micro and Nanosystems Technology, Vestfold University College. Manuscript submitted December 2, 2012. Article published online January 12, 2013. 406—VOLUME 44B, APRIL 2013 METALLURGICAL AND MATERIALS TRANSACTIONS B