Ag-Sn Fluxless Wafer Bonding Technology

Wafer bonding technology is important for most MEMS devices´ packaging, especially for RF-MEMS devices. Different materials systems, such as Au-Sn, Au-In, have been developed for wafer bonding. A new bonding system, using Ag-Sn, is investigated in this paper. Comparing to well developed Au-Sn bonding (typically bonding temperature of 280degC ), Ag-Sn would provide a potentially lower temperature, lower cost wafer-level bonding method. Both cap wafer and bottom wafer were sputtered with thin film Ti/Ni. Ti is the adhesion layer, and Ni is the wettability and barrier layer. Sn/Au was evaporated on Ti/Ni for cap wafer. The cap wafer was patterned by lift-off process. Ag was sputtered on Ti/Ni for bottom wafer. The thickness of Sn and Ag was designed based on the composition ratio of Ag₃Sn in the bonding layer. Thin Au film could protect Sn from oxidation. Afterwards the cap wafer and substrate wafer were brought into contact and sent into the bonding machine. Bonding process was performed in a nitrogen environment. Different bonding parameters such as bonding force, bonding temperature, dwell time are studied. During bonding, the Sn melted when the temperature reached the melting point of Sn and the liquid Sn broke up the barrier layer of the intermetallic compound of Au and Sn. Then the liquid Sn wetted and dissolved the Ag layer to form the intermetallic compound Ag₃Sn. The growth of such an interfacial Ag₃Sn intermetallic compound is a liquid-solid diffusion process. The main compositions of the bonding interface are pure Ag and Ag₃Sn intermetallic compound, which was investigated by SEM and ED AX. Shear strength of the as-bonded structure and those after reliability tests were measured by a shear force tester. The reliability tests include pressure cooker test (PCT) and thermal cycling test (TCT). It has been concluded that the Ag- Sn bonding with delicately designed bonding process one of the potential methods for wafer- bonding.