40μm Ag/Au composite flip-chip interconnect using solid state atomic bonding at 200°C

Abstract

We performed 40μm silver (Ag)/gold (Au) composite flip-chip interconnects joints between silicon (Si) chips and copper (Cu) substrate using solid-state bonding process at 200°C. 50×50 Ag/Au columns with 40μm in diameter and 100μm pitch were fabricated on a chip region by photolithographic and electroplating processes. Then, the Ag/Au columns were bonding to Cu substrate with fresh surface using solid-state at 200°C for five minutes with a static pressure of 1.7~2.7 MPa (250~400psi). The corresponding load for each column is 0.22~0.35 gm. The five minutes is constrained by the equipment. In theory, bonding should occur in seconds. Cross section SEM images show that Ag/Au column is well bonded to Cu substrate with no void or breakage within it. Ag/Au composite joints well manage the shear strain induced by coefficient of thermal expansion (CTE) mismatch. There is no molten phase during the bonding process. Neither flux nor underfill was used. Compared to solder flip-chip joints, this new process has the reduction of electrical resistance of the joints of the same size by a factor of 6. Pull test was conducted. The fracture force and fracture strength are 6.5~7.3kg and 2,940~3,310psi (20.2~22.8MPa), respectively. The fracture force is 2.5× of the criterion in MIL-STD-883E. The SEM/EDX analysis of the fracture interface showed that fracture of bonding interface is least likely to incur in pull test.