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Processing and Performance of AL Polymer Films for Wafer Level Packaging
Keywords: Polymer Dielectrics, Repassivation, Stress Buffer Layer
For bumping and wafer level packaging (WLP) applications, BCB and polyimide are the predominant spin-on dielectric materials used for repassivation, redistribution, interlayer dielectric, and stress buffer layers. Each of these materials has their respective strengths and weaknesses. BCB has exceptionally low shrinkage on cure and moisture absorption, low curing temperature, and a low dielectric constant, but is a mechanically brittle material which limits its application in bump-on-polymer applications. Polyimides are superior mechanically to BCB and are utilized more in bump-on-polymer structures, but suffer from much higher shrinkage on cure and moisture absorption (which can lead to blistering if not carefully processed), have higher dielectric constants, and often have much higher cure temperatures. Asahi Glass Co. has previously presented data on their AL spin-on polymer dielectric (1), which is formulated to combine low shrinkage, low moisture absorption, low dielectric constant, and excellent mechanical and stress buffering properties with a cure temperature at or below 200C. RTI has been working with AGC, developing and evaluating the AL polymer properties and processing. In this presentation we discuss the general process flow and photo properties of two formulations of AL polymer designed to cover different coating thickness ranges including spin speed curves, resolution plots (as a function of exposure dose, exposure gap, etc), and curing dynamics. We also present data on the planarization capability of AL polymer compared to BCB and polyimide. The adhesion of AL polymer has also been evaluated on typical surface materials such as Si, SiO2, SiN, Cu, Al, and Au using bump shear of polymer bumps and through modified edge liftoff test (m-ELT). Data on bump shear of eutectic SnPb solder bumps fabricated on AL polymer and comparisons to bump shear on BCB and polyimide is currently underway and expected to be presented as well. (1) Eriguchi, et al, “Low Temperature Curable Photosensitive Polymer Dielectric for Wafer Level Packaging Applications”, proceedings of 2007 IMAPS International Conference and Exhibition on Device Packaging, Scottsdale, AZ, March 19-22, 2007.
Alan Huffman, Research Engineer
RTI International
Research Triangle Park, NC

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