Wafer-Level-Packaging

Topic

LTCC wafer with cavities and integrated gold vias for anodic bonding process
LTCC wafer with gold vias (above), close-up view of a polished LTCC surface (below)

MEMS for high reliable applications need a hermetically sealed single package with a ceramic body (also see section MEMS packaging). Major drawbacks of this technology are single MEMS die processing and package dimensions mostly larger than the MEMS dies.

For this reason a new packaging process with ceramic multilayer interposers on Wafer-Level was developed in cooperation with Fraunhofer ENAS. Thereby a silicon wafer is bonded, isolated, hermetically sealed and rewired simultaneously with a ceramic wafer in one process step. Specific LTCC-multilayer ceramics were developed which have an excellent thermo-mechanical adaption to silicon. Gold vias in every single LTCC green sheet realize the rewiring within the ceramic body. These sheets are stacked, laminated and sintered to a multilayer ceramic wafer. Printing processes on the green sheets do the integration of passive elements in the ceramic. After sintering the ceramic multilayer surface is polished to allow for the anodic bonding process.

Afterwards an anodic bonding process bonds the functionalized ceramic wafer with the silicon wafer at 300° C. The electrical interconnection are realized by  thermocompression bonding of through silicon vias (Pt or Cu material) in the silicon wafer in combination with gold conductor pads on the ceramic wafer of the metal layers during the anodic bonding process. The sensory structures on top of the sense wafer could be hermetically sealed by an additional structured LTCC cap wafer on wafer level.

By means of this developed bonding process high reliable bonds with shearing strength of > 114 N/mm² of the joints and a high yield (> 90 %) can be generated between the silicon and ceramic wafer.

Functional bond of silicon wafer (MEMS) and ceramic wafer