Knut E. Aasmundtveit

Au-Sn SLID Bonding—Properties and Possibilities

Au-Sn solid–liquid interdiffusion (SLID) bonding is a novel and promising interconnect technology for high-temperature applications. This article gives a review over previously published work on Au-Sn SLID bonding. An overview of the crystal phases and the thermomechanical properties of the Au-Sn phases relevant for Au-Sn SLID bonding is given. A summary of the bonding conditions used during Au-Sn SLID bonding is presented together with results from reliability tests. Additional challenges, possibilities, and recommendations for how a reliable high-temperature Au-Sn SLID bonding should be constructed are also discussed.

Wafer-Level Cu/Sn to Cu/Sn SLID-Bonded Interconnects With Increased Strength

Wafer level Cu-Sn solid liquid interdiffusion (SLID) bonding of interconnects was achieved by bonding two-layered Cu/Sn structures to each other. The bonded interconnects were investigated by mechanical, electrical and microscopic techniques. The Cu-Sn SLID interconnects were created by wafer-level bonding at 260°C. The bonded interconnects show shear strength of 45 MPa and a resistance of the order 100 mΩ . A major advantage of the Cu/Sn to Cu/Sn bonding scenario is to avoid the dynamic wetting of molten Sn to Cu, and simply replace with a liquid to liquid integration. Furthermore, the Sn overflow problem in a Cu/Sn SLID system was successfully addressed by designing a margin of 15 μm at the Cu pads to tolerate any Sn spreading. The uniformity requirement for electroplated Cu-Sn layers, which is crucial for achieving successful wafer-level bonding, is discussed. This wafer-level Cu-Sn SLID bonding process is a promising technique for 3-D assembly and packaging.

High Temperature Interconnect and Die Attach Technology: Au–Sn SLID Bonding

Au-Sn solid-liquid interdiffusion (SLID) bonding is a novel and promising interconnect and die attach technology for high temperature (HT) applications. In combination with silicon carbide (SiC), Au-Sn SLID has the potential to be a key technology for the next generation of HT electronic devices. However, limited knowledge about Au-Sn SLID bonding for HT applications is a major restriction to fully realizing the HT potential of SiC devices. Two different processing techniques-electroplating of Au/Sn layers and sandwiching of eutectic Au-Sn preform between electroplated Au layers-have been studied in a simplified metallization system. The latter process was further investigated in two different Cu/Si3N4/Cu/Ni-P/Au-Sn/Ni/Ni2Si/SiC systems (different Au-layer thickness). Die shear tests and cross-sections have been performed on as-bonded, thermally cycled, and thermally aged samples to characterize the bonding properties associated with the different processing techniques, metallization schemes, and environmental stress tests. A uniform Au-rich bond interface was produced (the ζ phase with a melting point of 522°C). The importance of excess Au on both substrate and chip side in the final bond is demonstrated. It is shown that Au-Sn SLID can absorb thermo-mechanical stresses induced by large coefficient of thermal expansion mismatches (up to 12 ppm/K) in a packaging system during HT thermal cycling. The bonding strength of Au-Sn SLID is shown to be superb, exceeding 78 MPa. However, after HT thermal ageing, the ζ phase was first converted into the more Au-rich β phase. This created physical contact between the Sn and Ni atoms, resulting in brittle NixSny phases, reducing the bond strength. Density functional theory calculations have been performed to demonstrate that the formation of NixSny in preference to the Au-rich Au-Sn phases is energetically favorable.

Intermetallic Compound Formation Mechanisms for Cu-Sn Solid–Liquid Interdiffusion Bonding

Cu-Sn solid–liquid interdiffusion (SLID) bonding is an evolving technique for wafer-level packaging which features robust, fine pitch and high temperature tolerance. The mechanisms of Cu-Sn SLID bonding for wafer-level bonding and three-dimensional (3-D) packaging applications have been studied by analyzing the microstructure evolution of Cu-Sn intermetallic compounds (IMCs) at elevated temperature up to 400°C. The bonding time required to achieve a single IMC phase (Cu3Sn) in the final interconnects was estimated according to the parabolic growth law with consideration of defect-induced deviation. The effect of predominantly Cu metal grain size on the Cu-Sn interdiffusion rate is discussed. The temperature versus time profile (ramp rate) is critical to control the morphology of scallops in the IMC. A low temperature ramp rate before reaching the bonding temperature is believed to be favorable in a SLID wafer-level bonding process.

Fluxless wafer-level Cu-Sn bonding for micro- and nanosystems packaging

For wafers with integrated and released sensitive micro- and nanosystems a fluxless wafer-level hermetic packaging solution is required. By using a 1.5 µm thick Sn layer as oxidation barrier for 5.0 µm thick Cu bond frames, the surface does not require pre-cleaning or use of any flux agent prior to, or during Cu-Sn bonding. With a tailored temperature and pressure bonding profile, the amount of Sn squeeze-out is reduced. Both for Cu-Sn bonds performed with new and aged electroplated films the measured shear strength is above 30 MPa. Further temperature cycling of bonded dies does not result in any reduction in bonding yield or shear strength.

Spherical polymer particles in isotropic conductive adhesives a study on rheology and mechanical aspects

Isotropic conductive adhesive (ICA) filled with metal coated polymer spheres has been studied as a novel approach to increase the flexibility, and hence the reliability of the adhesive compared to traditional metal filled ICAs. In this paper, we have investigated the rheological properties of the novel ICA to evaluate its applicability in practical use. The current work also involves the investigation of the mechanical properties including shear strength of the novel ICA. Spherical polymer particles (SPP) of sizes Ø6 µm and Ø30 µm were investigated in the present study. The results show minor differences in the rheological properties and the adhesion strength for adhesives filled with particles in different sizes. Filling SPP into the adhesive matrix increases the viscosity of the system monotonically and continuously, in excellent accordance with model systems previously reported in the literature. Furthermore, the novel ICA exhibits high mechanical shear strength, being comparable to the traditional solder joint technology and twice higher than the traditional metal filled ICA.

Au-Sn fluxless SLID bonding: Effect of bonding temperature for stability at high temperature, above 400 °C

Fluxless SLID (Solid-Liquid InterDiffusion) bonding based on Au and Sn is presented, using two different processes, and bonding temperatures in the range 300–350 °C. The decomposition of the bond was tested by applying shear force while heating the samples. No bond delamination was observed for temperatures up to 350–400 °C, with 95 % of the tested samples surviving 400 °C without bond delamination. This is more than 100 °C higher than the melting temperature of the commonly used eutectic Au-Sn bond (80 wt% Au, melting at 278 °C). The Au-Sn system is particularly interesting since it is oxidation resistant, allowing fluxless bonding. With the SLID process, the metal system is applicable for true high-temperature applications.

Surface evolution and bonding properties of electroplated Au/Sn/Au

Sandwich-structural multilayer films Au (4.0 mum)/Sn (2.0 mum)/Au (x mum) with x = 0.1, 0.3 have been electroplated on metalized silicon wafers. Interdiffusion of thin film Au in metallic Sn at room temperature is observed by light microscope on fresh electroplated multilayer surface. For the multilayer films with Au thickness x = 0.1, 0.3 mum, the Sn surface concentration on surface are roughly increased from 0 to 20(at)% after 1 and 4 days aging, respectively. The sandwich structure is bonded to wafers with x mum layer of electroplated Au. The bonding experiments are performed at 280degC, in atmosphere without flux. Scanning Electron Microscope (SEM) cross-section images show the formation of gold-tin alloy in bonded region and shear testing results indicate that the bonding strength reaches more than 11 MPa.

High-temperature shear strength of solid-liquid interdiffusion (SLID) bonding: Cu-Sn, Au-Sn and Au-In

Solid-Liquid Interdiffusion (SLID) bonding is a promising bonding technique, particularly for high-temperature applications. Based on intermetallics as the bonding medium, the bonds are stable at temperatures far above the processing temperature which is in the range of normal solder temperatures. This work confirms experimentally this high-temperature stability through shear strength testing as function of temperature (room temperature to 300 °C) for three different SLID systems: Cu-Sn, Au-Sn and Au-In. All three systems remain solid within the tested temperature range, as expected, but they show remarkably different temperature dependence of mechanical strength: Au-Sn SLID bonds show strongly decreasing shear strength with temperature (but at 300 °C it is still well above the MIL-STD requirement); Cu-Sn SLID bonds show only small changes; whereas Au-In SLID bonds show increased shear strength at 300 °C, accompanied with a change in fracture mode from brittle to ductile. All three behaviours can be explained from the phase diagrams with the actual phases in use.

Die Shear Testing of a Novel Isotropic Conductive Adhesive—Epoxy Filled With Metal-Coated Polymer Spheres

Isotropic conductive adhesives (ICAs) filled with metal-coated polymer spheres (MPS) are introduced to improve the mechanical reliability compared with conventional ICAs filled with silver (Ag) flakes. This paper deals with the die shear performance of an MPS-based ICA; an epoxy filled with 45 vol% of Ø30 μm Ag-coated monodisperse polymer spheres. The curing kinetics of the ICA is also studied. The MPS-based ICA is compared with two ICAs filled with Ag flakes: an in-house prepared ICA (with the same epoxy matrix as the MPS-based ICA) and a commercially available ICA. Both ICAs with Ag flakes have a lower particle volume fraction than the MPS-based ICA. Loading 45 vol% of MPS into the epoxy matrix has no significant effect on the curing kinetics and the glass transition temperature (Tg) of the matrix. The MPS-based ICA has 140% higher die shear strength than the in-house prepared ICA with Ag flakes, and 23% higher die shear strength than the commercial ICA with Ag flakes. The MPS-based ICA also has higher shear strain at failure, and less scatter in shear strength between repeated tests. Hence, the MPS-based ICA has a good potential for demanding applications; it has better die shear performance and higher Tg than the commercial ICA with Ag flakes.