Low Pressure Solid-State Bonding Using Silver Preforms for High Power Device Packaging

Silver (Ag) has been emerging as an attractive die-attach material for high power devices because of its highest thermal conductivity among metals and high melting stability. The most well-known silver die-attach technique is to sinter micro-or nano-silver pastes. The challenging issues of sintered Ag joints are pores in the joint and migration of unfriendly species such as chlorine ions through these pores. In this paper, a novel Ag die-attach technique using foils is reported. The foils are fabricated in house using many runs of cold rolling and subsequent annealing. Annealing is needed to establish favorable microstructure. X-ray diffraction (XRD) is carried out to reveal the crystallographic information. Solid-state bonding is conducted in 0.1 torr vacuum at 300 °C assisted by low applied pressure (1,000 psi). This pressure is several orders of magnitude lower than what used in conventional thermal compression bonding. The Si/Ag/Cu structure, where Ag is the foil, is bonded in one step to achieve two bonding interfaces. Prior to bonding, Si is metallized with thin Cr and Au layers. Cross section SEM images show that there are no large voids and cracks in the interfacial regions. The Ag region is a dense pure silver layer without any foreign substances. Regardless of significant coefficient of thermal expansion (CTE) mismatch between silicon and copper, the bonded samples do not crack after cooling down to room temperature. This indicates that the ductile Ag layer is able to manage the stress produced by the CTE mismatch. The new Ag die-attach method produces joints of lowest possible thermal resistance and extremely high operation temperature. It should be very valuable to high power and high temperature electronics and photonics.