Recent Developments in Low Temperature Wafer Level Metal Bonding for Heterogenous Integration

Abstract

The low temperature metal bonding processes (<200°C) are attractive methods for novel applications in 3D packaging with simultaneous formation of electrical interconnections and hermetic encapsulation for microelectromechanical systems.Metal wafer bonding is a generic category of processes consisting of various sub-categories, each one defined by the different principles governing the process. One can differentiate between eutectic wafer bonding (a eutectic bonding layer is formed during process by liquid-solid interdiffusion), intermetallic bonding (an intermetallic alloy is formed as bonding layer during the process by solid-liquid interdiffusion, process known also as Solid Liquid Intermetallic Diffusion – SLID or Transient Liquid Phase – TLP), and metal thermo-compression (TC) wafer bonding.The bonding temperature range for the two types of processes showing liquid interface is relatively narrow (<10-20°C) while for metal TC the bonding temperature range is quite broad (~200°). This important parameter is differentiating the focus in studying the processes: for the low temperature eutectic and SLID/TLP the focus is primarily on the development of new materials systems and secondary on process optimization, while for metal TC the focus is equally shared between metal layers optimization (e.g. fabrication method, oxide management) and process optimization.The SLID bonding process studies were focusing on the Cu–In–Sn, Au–In and Bi–In material systems. It was demonstrated that electrical interconnections can be fabricated at bonding temperatures as low as 150°C by taking advantage of the Cu–In–Sn ternary system. The low processing temperatures allow for bonding of temperature sensitive materials and of materials with dissimilar thermo-mechanical properties by thermally induced stress management. Microstructural characterization confirmed the high bonding quality with low amount of defects in interconnections composed of a layered Cu / intermetallic / Cu structure, with a single-phase intermetallic Cu6(Sn,In)5 phase. Moreover, the mechanical testing shows that the bonds have tensile strength above 30 MPa.Au–In SLID bonding can be performed at temperatures slightly higher than the In melting temperature of 156°C. Successful bonds were demonstrated at 180°C. The resulting intermetallic bonding layer has a remelting temperature >450°C and a high shear strength above 30 MPa both at room temperature and 300°C.Another system studied was Au–In–Bi, utilizing the low eutectic point of In–Bi (72oC). Bonds at temperatures of 90 °C were proven. However, in order to consume the various In–Bi intermetallics with low melting temperatures (T< 110 oC), a bonding temperature higher than 110 oC should be used for a reliable result. We have demonstrated such SLID bonds at 115 oC, with a bond line consisting of Au / Au–In IMCs / Au, with Bi inclusions. All these materials have melting temperatures > 271 oC, with the bonding layer being stable at 450 oC if Bi does not form continuous structures.The study of metal TC bonding is reporting recent developments on the optimization of Cu-Cu and Al-Al wafer bonding. To decrease the bonding temperature of Cu-Cu TC bonding, Cu layers fabrication and oxide management are essential. By using certain surface pre-treatment methods such as plasma and self-assembled monolayer (SAM), Cu-Cu low temperature bonding was investigated. Various plasma concentrations were used to investigate the influence of plasma composition on bond strength.Further investigations were performed on the impact of different morphological Cu-properties and their effects on the bonding yield. The comparison of the interdiffusion behavior of layers deposited using different methods (e.g. physical vapor deposition - PVD Cu vs. electroplated - ECD Cu), as well as different surface preparation methods is being reported here. Different surface treatments like in-situ and ex-situ oxide management, as well as pre-bond CMP processes were tested in combination with the respective deposition methods. By combining the most effective sample and process parameters, the necessary bonding temperature for a successful Cu-Cu TC bonding could be reduced to temperatures of 175°C. The successful interdiffusion behavior at such low temperatures was confirmed by cross section TEM measurements.Apart from Cu, Aluminum is widely used material, due to its good electrical conductivity and compatibility with CMOS processes. Aluminum has a very stable native oxide and therefore high temperature and pressure are necessary to generate a successful Al-Al bond at wafer-level. The low temperature Al-Al TC bonding using a thin Palladium passivation layer at 350°C is reported here. The thermo-compression bonding was performed on both blank and patterned wafers.This work is reviewing recent achievements in the reduction of metal wafer bonding process temperature to less than 200°C based on the mechanical, structural and electrical characterization of the metal bonding layers.