Low-Cost MEMS Packaging Includes Protective Cavities
John Baliga, Associate Editor -- Semiconductor International, 2/1/2001
Among the challenges associated with packaging MEMS devices is providing the appropriate mechanical environment while performing all the necessary packaging functions. For accelerometers, the moment arm must be allowed to move freely, but it must also be protected from the outside environment. There are ways to make a hermetic package with a cavity, but the challenge is to do so inexpensively with minimal amounts of packaging material. Some interesting approaches have been presented recently. Optical ETC (Huntsville, Ala.) and Auburn University, supported by the U.S. Air Force, have produced a novel flip-chip approach to packaging accelerometers.1 The final packaged unit was a ceramic cube, metalized on five sides, with accelerometers on three sides. Flip-chip attachment was chosen over wire bonding to meet high-g acceleration requirements. Wire bonds could deform in use, changing their electrical characteristics, and the bonds could possibly pull loose. Gold-wire bump bonding with underfill was found to meet the requirements.
The ceramic substrate has an etched cavity surrounded by bond pads (Fig. 1). The MEMS die, with gold bumps, is bonded to the substrate using thermocompression. The heights of the bond and the cavity are chosen so that capillary action is strong enough around the edges to pull the underfill material in, but too weak in the cavity area for the underfill to go into the cavity. The resulting bond has sufficient strength, and it provides a degree of environmental protection for the cavity area. The bonded die can be encapsulated in epoxy for increased strength and environmental protection.
Researchers at IMEC (Leuven, Belgium) have developed an indent-reflow-sealing (IRS) method for packaging accelerometers and micro relays.2 This method places one die on top of another so that a cavity exists between them.
1. This bump-bond method developed for the U.S. Air Force takes advantage of the difference in capillary force to keep underfill out of the cavity. (Source: Optical ETC) |
2. In the IRS method, the two die are prebonded, leaving an opening in the solder wall (a). In the reflow oven, the cavity can be evacuated and refilled as desired (b). Reflow seals the cavity (c). (Source: IMEC) |
The cavity is formed by a wall of solder enclosing the desired area (Fig. 2). One small section of that wall is shorter than the rest, or indented. Initially, the capping die is attached at a temperature below the melting point of the solder, so that it sticks well enough to stay in place. The indentation in the solder wall allows for the evacuation of the cavity and, if desired, refilling with a specific gas. When the solder is reflowed, the solder levels out, sealing the opening.
REFERENCES
1. D. Strembicke, A. Werkheiser, R. Dean, R. W. Johnson, "A Novel Flip-Chip Packaging Method for 3-Axis MEMS Accelerometers," IMAPS Advanced Technology Workshop, Packaging and Integration of MEMS & Related Microsystems, Nov. 10-12, 2000.
2. H. Tilmans, M. Van de Peer, E. Beyne, "Zero-Level Packaging for MEMS or MST Devices: The IRS Method," Advancing Microelectronics, November/December 2000.
For additional information on assembly and packaging, go to www.semiconductor.net/assembly
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