15μm Silver flip-chip technology with solid-state bonding

Formation of pure silver (Ag) flip-chip interconnect of silicon (Si) chips on copper (Cu) substrates is reported. In this research, we first demonstrate that Ag joints of 40μm in diameter can be fabricated between Si chips and Cu substrates, and later successfully push the dimension of the Ag joints down to 15μm in diameter. For 40μm Ag joints, arrays of 50×50 Ag columns, and for 15μm Ag joints, arrays of 125×125 Ag columns, are fabricated on 2-inch Si wafers which are first coated with chromium (Cr)/gold (Au) dual layers. The Si wafers are diced into 6mm×6mm chips, each having 50×50 or 125×125 Ag columns. The Si chip with plated Ag columns is directly bonded to Cu substrate at 260°C in 80 millitorr vacuum to inhibit oxidation. The static pressure for 40μm Ag bonding can be achieved as low as 680 psi, corresponding to a load of 0.021oz per column. The pressure for 15μm Ag bonding is 800 psi at initial try, but we believe that it can be further reduced. During bonding, the Ag columns deform and conform to the Cu substrate. They are well bonded to the Cu. No molten phase is involved in the bonding process. The joints consist of pure Ag only. The ductile Ag joints are able to accommodate the thermal expansion mismatch between Si and Cu. It is well known that in nearly all soldering processes used in electronic industries, intermetallic compound (IMC) formation is essential to make a solder joint. In the pure Ag interconnect, no IMCs exist. Thus, reliability issues associated with IMCs are eliminated. Compared to tin-based lead-free solders, pure Ag joints have superior electrical and thermal properties.