NDLI: Ag-Sn Fluxless Wafer Bonding Technology

Content Provider	IEEE Xplore Digital Library
Author	XiaoGang Ji Jian Cai YoonChul Sohn Qian Wang Woonbae Kim
Copyright Year	2006
Description	Author affiliation: Tsinghua Univ., Beijing (XiaoGang Ji; Jian Cai)
Abstract	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 $Ag_{3}Sn$ 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 $Ag_{3}Sn.$ The growth of such an interfacial $Ag_{3}Sn$ intermetallic compound is a liquid-solid diffusion process. The main compositions of the bonding interface are pure Ag and $Ag_{3}Sn$ 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.
Starting Page	1
Ending Page	6
File Size	2616510
Page Count	6
File Format	PDF
ISBN	9781424408337
DOI	10.1109/EMAP.2006.4430616
Language	English
Publisher	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Publisher Date	2006-12-11
Publisher Place	China
Access Restriction	Subscribed
Rights Holder	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Subject Keyword	Wafer bonding Tin Testing Temperature Bonding forces Intermetallic Gold Bonding processes Force measurement Microelectromechanical devices inter-diffusion Ag3Sn bonding
Content Type	Text
Resource Type	Article

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