Development of Fine Pitch Backside Redistribution Layer (BRDL) Process in Fan Out Panel Level Packaging (FOPLP)

Fan out panel level package (FOPLP) is cost-effective to produce more packages than fan out wafer level package (FOWLP). Nowadays, the patterning complexity and density of redistribution layer (RDL) has increased to cope with the high I/O needs for heterogeneous integration. It is required for FOPLP to implement pattern backside redistribution layer (BRDL) because of package-on-package (POP) for achieving higher levels of performance without increasing in size. However, there is a challenge to realize fine pattern using conventional fabrication method since precisely controlled etching technique is required. We have constructed FOPLP of its own to implement structural characteristics. Especially, BRDL is patterned by photolithography using Dry Film photoresist (DFR), which has the disadvantage of weak adhesion with insulating layer, typically ABF and electroless Cu as seed metal. Due to the weakening of the DFR adhesion, a pattern is fabricated with Cu foot when electroplated Cu is carried out. In BRDL, when electroless Cu is removed to fabricate a pattern, resulting in a pattern delamination and Cu residue defect at a specific point in time, which makes narrow wet etching process margin. In order to improve the wet etch process margin for realizing fine patterns in BRDL, annealing evaluation was conducted. Etching selectivity of electroless Cu and electroplated Cu was verified to determine the etching process window. These investigations have shown that the characteristics of wet etch and fine pitch patterns with high yield have been fabricated, which confirmed wide wet etching process window.