Ultra-Low Temperature FOWLP Process for the Embedding of Low Thermal Budget Sensors and Components Using SU-8 as Dielectric

Fan-Out Wafer Level Packaging (FOWLP) has recently seen a tremendous growth in a broad span of application in telecommunications, automotive and other markets. Its versatility allows its continuous development to accommodate more and more types of components. In light of expanding the technology to include new family of sensors such as MEMS/NEMS, Bio-chips with Microfluidics, magneto-resistive devices and Micro-batteries, the upper limit temperature of FOWLP processing has to be challenged. FOWLP, namely the WLFO technology of NANIUM, based on Infineon/Intel eWLB technology, is a packaging technology based on wafer reconstitution and RDL processes. The “LTC - low temperature cure” dielectrics used today, are cured at > 200°C, while solder ball soldering peaks ~260°C. Despite being considered low temperature, this limit is still not compatible and considered high to a wide range of the new family of sensors targeted to integrate in WLFO. In this work we describe the strategy to lower overall process temperature to <;160°C, using as demonstrator the embedding of a magneto-resistive sensor, which degrades at temperature above that limit. The challenges consist of finding legitimate alternative materials and processes while maintaining all the critical characteristics. Changes made focused on replacing both structural (such as dielectrics and solder ball alloy) and manufacturing support materials (e.g., support tapes used in molding process), by suitable low temperature substitutes. In terms of process flow, the higher thermal budget processes were identified, and then studies were carried out to find low temperature options and flow variants. The dielectrics currently used for redistribution layer construction are the major boundaries for low temperature processing. The selection of SU-8 aimed to overcome this limitation as it cures as low as 150°C. While SU-8 widely used in the fabrication of MEMS, as well as BIO-MEMS/microfluidics due to biocompatibility, in this work it has been tested unprecedentedly as a structural dielectric for the redistribution layers on 300mm fan-out wafers. Its good thermal and chemical resistance and also mechanical and rheological properties, in a wider range of available thicknesses, makes it suitable to be used both as structural material and dielectric. Main concerns during the evaluation of SU-8 as a new WLFO dielectric focused on processability; adhesion to multi-material substrate and metals (Cu, Al, Au,...); adhesion between layers of very different thicknesses; and overall reliability. For the first time, a set of conditions has been defined that allows processing SU-8 on WLFO. On the process side, compression molding and mold de-taping was demonstrated at temperatures <;150°C using new supporting tapes and process settings. Regarding solder balls, low temperature solder alloy solutions, such as InBi, were successfully demonstrated on WLFO at a reflow profile of <;150°C peak in both ball drop and stencil printing. The development of ultra-low temperature WLFO process is a crucial achievement for NANIUM, solving one of the largest barriers between this technology and the sensors domain, enabling innovative solutions and new WLFO application opportunities.