Low Temperature 0-Level Hermetic Packaging for MEMS based on Solder and Polymer Bonding

In order to be compatible with a broad range of MEMS processes, IMEC developed sealing methods with a low temperature budget (T less than 250°C).

A two substrate approach is used: a wafer with the MEMS devices processed, and a capping wafer, in which a cavity can be etched. Depending on the need, the capping wafer is thinned to about 100 micron to reduce the total thickness of the 0-level package. The MEMS and its capping are assembled using a die-to-wafer flip-chip or a wafer-to-wafer temperature controlled bonder.

Three sealing methods will be presented in detail: two based on low temperature solder, and one based on polymer bonding. The first two methods rely on eutectic soldering (SnPb on Ni/Au and In on Au). The third method uses photosensitive BenzoCycloButene (BCB) as a sealing material.

For all sealing methods high shear strengths (> 20 MPa) have been obtained. Different tests have been used to evaluate the hermeticity of the 0-level packages: gross leak and fine leak tests, through-hole leak testing and in situ pressure measurement using MEMS microbolometers. In addition to microbolometers, MEMS RF switches have also been packaged.