Wafer Level Solid Liquid Interdiffusion Bonding: Formation and Evolution of Microstructures
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https://doi.org/10.1007/s11664-020-08530-y 2020 The Author(s)
TMS2020 MICROELECTRONIC PACKAGING, INTERCONNECT, AND PB-FREE SOLDER
Wafer Level Solid Liquid Interdiffusion Bonding: Formation and Evolution of Microstructures V. VUORINEN ,1,2 H. DONG,1 G. ROSS,1 J. HOTCHKISS,1 J. KAAOS,1 ¨ CKEL1 and M. PAULASTO-KRO 1.—Department of Electrical Engineering and Automation, School of Electrical Engineering, Aalto University, PO Box 13340, 00076 Aalto, Finland. 2.—e-mail: [email protected]
Wafer-level solid liquid interdiffusion (SLID) bonding, also known as transient liquid-phase bonding, is becoming an increasingly attractive method for industrial usage since it can provide simultaneous formation of electrical interconnections and hermetic encapsulation for microelectromechanical systems. Additionally, SLID is utilized in die-attach bonding for electronic power components. In order to ensure the functionality and reliability of the devices, a fundamental understanding of the formation and evolution of interconnection microstructures, as well as global and local stresses, is of utmost importance. In this work a low-temperature Cu-In-Sn based SLID bonding process is presented. It was discovered that by introducing In to the traditional Cu-Sn metallurgy as an additional alloying element, it is possible to significantly decrease the bonding temperature. Decreasing the bonding temperature results in lower CTE induced global residual stresses. However, there are still several open issues to be studied regarding the effects of dissolved In on the physical properties of the Cu-Sn intermetallics. Additionally, partially metastable microstructures were observed in bonded samples that did not significantly evolve during thermal annealing. This indicates the Cu-In-Sn SLID bond microstructure is extremely stable. Key words: Low-temperature SLID bonding, TLP bonding, Cu-In-Sn system, reliability
INTRODUCTION Functional structures used in microelectromechanical systems (MEMS/MOEMS) must be electrically connected and hermetically encapsulated. Wafer-level bonding though 3D-integration enables multi-sensor fusion with logic in a vertical highspeed package. Solid liquid interdiffusion (SLID) bonding, also known as transient liquid-phase (TLP) bonding, is becoming an increasingly attractive method for wafer level packaging of MEMS/ MOEMS devices.1–5 SLID bonding offers several technological advantages for the development of
(Received July 18, 2020; accepted September 25, 2020)
reliable wafer-level packaging. These advantages include lower bonding temperature by utilizing a low melting point metal that ultimately results in a bond with a much higher re-melting temperature, large reduction of the required bonding footprint, integrated hermeticity and, integration of vertical electrical interconnects using through silicon vias (TSVs). The low (T < 350C) bonding temperatures enable packaging of temperature sensitive materials, and minimize residual stresses when bonding materials with different thermomechanical properties, such as t
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