BCB-based dry film low k permanent polymer with sub 4 µm vias for advanced WLP and FO-WLP applications
There is a strong demand to increase the routing density of the RDL to match the requirements for future microelectronic systems which are mainly miniaturization and performance. Photo-resists for structuring the metallization or acting as a mold for electroplating are common for very fine lines and spaces due to the developments in the front-end processing. For example chemical amplified Photo-resists are now moving in the back-end and wafer level packaging process. The results are mainly governed by the performance of the equipment i.e. the photo-tool. This is different for the permanent dielectric polymer material. The major difference in photo-resists and dielectric photo-polymer are the different functions of the material systems. Photo-resists are only temporary masks for subsequent process steps like etching and plating. This is different for the photo-polymers which are a permanent part of the future systems. In this paper a new technology is discussed which uses a laser scanning ablation process and BCB-Based Dry Film low k Permanent Polymer. Laser ablation of polymers is in principle not a new technology. Low speed and high cost was the major barrier. But the combination of a scanning technology together with quartz masks has opened this technology to overcome the limitation of the current photo-polymer process. The new technology is described in detail and the results of structuring BCB-Based Films down to less than 4 mm via diameter in a 15 mm thick film has been shown. The via side wall can be controlled by the fluence of the laser pulse. Test structures have been designed and fabricated to demonstrate the excellent electrical resistivity of the vias using a two-layer metallization process.