Novel Temporary Adhesive Materials for RDL-First Fan-Out Wafer-Level Packaging

Recently, fan-out wafer-level packaging (FOWLP) has become a mainstream technology to meet increased demand for larger scale high-density packages. FOWLP provides multi-chip functionality, with a smaller footprint and thinner package, resulting in higher input/output (I/O) along with improved thermal and electrical performance. There are two principal approaches to manufacture FOWLP: chip-first (face up or face down) and chip-last (redistribution layer [RDL]-first). The chip-first process starts with placing dies on a substrate and then overmolding the substrate with an epoxy mold compound (EMC) material to form a reconstituted substrate, after which the RDL will be built on the EMC for interconnect. Chip-first approach is typically used for low I/O die, where the RDL is minimal and yields are very high. However, for high-value die (large I/O), the RDL-first process is preferred, so that the RDL structure can first be electrically tested or visually inspected, to avoid placing good die on bad sites. During the RDL-first process, a laser release material is coated on a carrier, then multi-layer fine RDLs are built up on top of the release layer. After die attachment and molding, the release layer is separated by exposure to a UV laser that passes through the carrier. Laser release materials will play an important role in FOWLP technology. The challenges of laser release material design include: addressing process conditions (e.g. coating uniformity), obtaining desired mechanical and chemical properties such as glass transition temperature (Tg), coefficient of thermal expansion (CTE), thermal stability, and chemical resistance. These materials must also demonstrate good adhesion to the substrate as well as the ability to successfully undergo laser ablation. Currently, in the temporary bonding and debonding (TB/DB) processes, most laser release materials cannot be bonded at low temperatures, and therefore they need to be used in combination with a bonding material. Although this simplifies the material design, it needs additional process steps and results in lower throughput. This work describes the use of a series of newly developed photosensitive-polymer-based temporary adhesive materials. Due to the low-Tg properties of these novel materials, they can be bonded or laminated on different substrates at moderately low temperatures. These materials can be thermally or UV cured to obtain the desired mechanical stability during subsequent downstream high-temperature and -vacuum processes, and possess good chemical resistance necessary to pass extreme backside processing. Moreover, they can be debonded via laser ablation due to their high sensitivity to UV laser. In this paper, we will present in detail how these novel temporary adhesive materials can achieve reliable performance for use in advanced package technology.