40 μm Ag/Au Flip-Chip Joints by Solid-State Bonding at 200°C

Abstract

AbstractÀIn this research, 40 mm silver/gold (Ag/Au) composite flip-chip interconnect joints between silicon (Si) chips and copper (Cu) substrates were demonstrated. The bonding was achieved by a solid-state process at a low temperature of 2008C for 5 min with the pressure applied at 250À400 psi (1.7À2.7 MPa), corresponding to 0.22À0.35 g of force per joint. To begin with, an array of 50 3 50 30 mm Ag/10 mm Au columns with 40 mm in diameter and 100 mm in pitch was fabricated by photolithographic and electroplating processes on silicon (Si) chips which were first coated with chromium (Cr) and Au films. The columns on the chip were then bonded to a Cu substrate by solid-state bonding. Cross-sectional scanning electron microscopy (SEM) images show that the exposed Ag/Au columns were well bonded to the Cu substrate. No joint breakage was observed despite the large coefficient of thermal expansion (CTE) mismatch between Si and Cu. A pull test was conducted. The breaking force and fracture strength are 6.5À7.3 kg and 2,940À3,310 psi (20.2À22.8 MPa), respectively. The breaking force is 2.53 of the criterion specified in MIL-STD-883E. Fracture modes were examined. Three modes were classified by fracture interfaces as Si-glue, Si/Cr/Au/Ag, and Au-Cu bonding interface. Of all joints evaluated, 27% of them break on the Au-Cu substrate bonding interface. Accordingly, the bonding interface is least likely to break among interfaces of the joint structure.

Direct metal to metal bonding method for heterogenous 3D integration

“…Cu commonly used for CMOS and Au commonly used in MEMS can be solid-state bonded together without additional material. A study has demonstrated for Cu-Au bonding at 200 °C for 5 min where Au was deposited onto columns to prevent oxidation of Cu[54]. However, the Cu bonding interface (unprotected surface) must be cleaned before bonding to remove copper oxides which will cause the bonding to fail.2.2.8 Cu-Sn Solid-State BondingCu-Sn solid-state bonding was performed in studies achieving lowbonding temperature from 150 to 200 °C [55] [56].…”