Application of an Angular Exposure System to Fabricate True-Chip-Size Packages for SAW Devices

Surface Acoustic Wave (SAW) filters are key components of mobile phones. In SAW components mechanical waves propagate on the surface of the chip, so the package must provide a cavity. Solid materials on the chip would inhibit the propagation of the surface waves. Reduction of size and cost, improved reliability and electrical performance are the main trends of SAW component evolution. Therefore, advanced cavity package technologies are required for SAW components. The Die Size SAW Package (DSSP) developed by EPCOS is a true chip size wafer level package that meets these requirements. The cavity package of the DSSP causes a significant topography. As a consequence the following rerouting process has to deal with the photolithographical challenge in terms of conformal resist coating and resist exposure. While conformal coating can be achieved by a spray process the exposure of patterns at sidewalls of topographic structures needs a customized solution. Exposure at a selectable angle instead of standard perpendicular exposure will be discussed in this paper based on EPCOS' DSSP technology.