|M.Sc Student||Naveh Royi|
|Subject||Solder-Based Bonding for MEMS Vacuum Packaging|
|Department||Department of Mechanical Engineering||Supervisor||Professor Eyal Zussman|
Many MEMS devices contain fragile dynamic structures, such as membranes, vibrating beams, optical and electrical components, etc. Those structures are influenced by their environment, which must be controlled in order to maintain high reliability for long periods of time. In order to minimize damage to the device during handling, it is recommended that its hermetic environment is realized at the time of its production, i.e., at wafer level.
This paper reports the development of a novel vacuum bonding technique for MEMS applications. The method relies on a multiple-chip fluxless solder-based joining technique, forming a seal. Key process steps are: (1) creation of small-scale (small Bond number) beads of solder (deposited onto a compatible metallization (UBM) on a chip by electroplating); (2) reflow of the solder; (3) creation of bumps on the other chip, to serve as stoppers; (4) pre-bonding of the chips; and (5) solder reflow in a controlled ambience (vacuum), using a designated oven. We selected as a solder a eutectic tin lead solder (Sn63/Pb37), of a 183°C melting temperature. We show both analytically and experimentally that a molten bead with parallel contact lines is stable when the ratio between the solder’s height and width is less than ½. This is consistent with Sonin's theory (J. Fluid Mechanics, 343, 1997). The final bonding process was performed on a fixture with a vertical load of 10 N in a vacuum chamber at an air pressure of one milli Torr, in order to achieve vacuum encapsulation of silicon dies. The bonding temperature was slightly higher than the melting temperature of the solder, in order to assure the reflow. Gross leak tests with isopropanol as well as fine leak tests with a Helium-based leak detector, confirmed the hermetic demands of the Mil-STD-883D. Shear test results showed that the joint was sufficiently strong. Microscopic (SEM) views showed the fully reflown solder as well as the condition of the sealed cavities, which were fine and uniform. This novel method is especially suitable for vacuum bonding wafers, which contain MEMS and other micro devices, of low temperature. Consequently, sealed, encapsulated and released wafers can be diced without damaging the MEMS devices at the wafer level.