Fabrication of fine patterned structure for high-density Fan-Out Wafer Level Package using dry etching technology

Currently, the miniaturization technology of wiring in Fan-Out Wafer Level Packaging (FO-WLP) is needful to multi chip module system fabrication using the redistribution layer (RDL) wiring technologies. In addition, FanOut Package on Package (FO-PoP), in which a memory package is mounted on an application processor (AP) package, is adopted to suppress signal loss with the distance of long signal writing. In this paper, we report the dry etching technique for high-density FO-PoP The first is the fabrication of polyimide (PI) vertical via to form in RDL. In the current mass production, the technique of photo-lithography and descum is widely used for the fabrication of PI via. The shape of typical via is the diameter of approximately 10um and aspect ratio (A.R) of approximately 1.5. To increase the writing density, it is necessary not only to reduce the L/S but also to form fine via. However, it would be difficult to form fine via with high A.R using the existing technology. Thus, dry etching technology is recommended as the method to fabricate fine PI via with vertical shape, and high A.R. As the result, a PI vertical via with A.R of 4 and a diameter of 3um is formed. The profile of PI vertical via can be demonstrate by dry etching. Secondary, we introduce the dry desmear technique. Currently, the technology of laser dill via in epoxy mold compound (EMC) is focused as micro via formation and cost reduction technology. The laser drill via is manufactured through the steps of laser drill, desmear, seed layer formation, and plating. These vias are formed in the substrate of EMC by femtosecond-laser. Then, the shape of these vias is the dimeter of 40um, the depth of 200um and A.R of 4. On other hands, the side wall of laser drill via is rough under the effect of SiO2 filler in EMC. Before the formation of the seed layer, it is necessary desmear technique with smoothing of via side wall. Our experiment shows the estimated average roughness (Ra) by laser microscope is decreased from 0.57um to 0.23um. By the desmear treatment, the consecutive Cu/Ti layer can be form on side-wall of laser via.