Modeling, Design and Demonstration of Ultra-short and Ultra-fine Pitch Metastable Cu-Sn Interconnections with High-throughput SLID Assembly Ting-Chia Huang, Vanessa Smet, Satomi Kawamoto # , Venky Sundaram, P. Markondeya Raj, and Rao R. Tummala 3D Systems Packaging Research Center Georgia Institute of Technology 813 Ferst Drive, N.W. Atlanta, GA USA30332 # Namics Corporation, 3993 Nigorikawa, Kita-ku, Niigata City, Niigata Prefecture 950-313, Japan Email: thuang68@gatech.edu / Phone: 678-308-4999 Abstract Advances in high-performance package with high I/O densities, and power modules with escalating current needs are driving the need for a new class of interconnection technologies, with thermal stability, current-carrying capability and pitch scalability beyond that of traditional solders. Solid-liquid interdiffusion (SLID or SoLID) or transient liquid phase (TLP) bonding systems, in which the bonding layer is fully converted to intermetallics, are highly sought after to extend the applicability of solders to pitches below 30μm, and for die-attachment in high-temperature high-power systems. This paper introduces an innovative SLID concept, consisting of isolating a metastable intermetallic phase between barrier layers for a faster conversion to metastable composition than that in traditional SLID. The Cu-Sn system was used for this demonstration with a designed transition to metastable Cu 6 Sn 5 instead of the stable Cu 3 Sn phase, usually targeted. The novel interconnection structure enables assembly within seconds and improved thermomechanical reliability, with all the benefits of SLID bonding such as outstanding thermal stability over 10x reflow and enhanced power handling capability with a current density of 10 5 A/cm 2 . The paper first describes the design and fabrication of the interconnection structure, including the barrier and bonding layers based on diffusion and thermomechanical modeling. Ultra-fast assembly by low-pressure thermocompression bonding was demonstrated on die-attach joints and interconnections at 100μm pitch, followed by extensive reliability characterization, including thermal stability evaluation, electromigration test, and die-shear test. The designed interconnections successfully passed JEDEC standards, qualifying this novel interconnection technology for high-temperature, high-power operations at fine-pitch. 1. Introduction High-performance systems are expected to drive interconnection pitches to 20μm and below in the next decade. The recent split-die trend, where a single large System-on-Chip (SOC) device is broken into two or four smaller dies which are then functionally reconstituted by high- density wiring on the substrate, further reinforces the need for ultra-fine-pitch interconnections. Pitch scaling is accompanied by shrinkage of the interconnections size, which requires a drastic reduction of solder volume in the standard Cu pillar and solder cap technology. In this scope, massive intermetallics will cause serious reliability concerns. Short interconnections with less than 10μm standoff height require low solder volumes in order to prevent bridging at fine-pitch. With standard solder-based interconnections, however, this raises serious reliability concerns, such as inferior power handling, increased stresses at the solder- intermetallic interfaces, and unstable joint compositions. All these including thermal stability, power-handling and minimum risk of bridging at fine-pitch require extending the applicability of solder-based interconnections to pitches below 30μm. The Georgia Tech approach is to extend SLID bonding or TLP bonding forming all-intermetallic interconnections of higher melting point. After the assembly, all the low melting point reactants will be completely consumed and replaced by high melting point resultant materials, which realizes the purpose of low-temperature assembly but high operating temperature capability [1]. SLID bonding has been shown to successfully overcome the limitations of solder and non-solder-based interconnection technologies, and also been highly sought after for die- attachment in power modules with operating temperatures exceeding 200°C, especially for SiC diodes or MOSFETs, or GaN FETs [2]. Standard tin-based die-attach technologies used in Si power modules are qualified for 125 o C normal operations, but were shown to systematically fail beyond 250 o C, even with high-lead content [3,4]. Alternative technologies such as sintering of silver nanopastes have been extensively researched and implemented to some extent in commercial power modules, but the remaining porosity can degrade the electromigration performance [5,6].On the other hand, outstanding thermal stability of Au-Sn SLID has been showed through 2000h high-temperature storage at 175°C, and Cu-Sn SLID successively survived 30,000 power cycles when assembled onto DCB substrates [7,8]. However, further research is needed for new bonding technologies leading to high thermal, mechanical reliability and high throughput, also compatible with both fine-pitch applications and high- temperature die-attachment. Various metallurgical systems have been studied such as Au-In, Ag-Sn, Au-Sn, Ni-Sn and Cu-Sn for SLID bonding. In Grummel’s work, e-beam deposition and sputtering is required to form ultra-thin indium and gold layers, and bonding was performed in vacuum at 200°C for 15minutes [9]. Subsequent phase transformation through multiple intermetallic compositions of Au-In after bonding was shown 978-1-4799-8609-5/15/$31.00 ©2015 IEEE 1377 2015 Electronic Components & Technology Conference