(Invited) Fan-out and Cu- Compatible Dielectric Processing of Low Temperature Polyimides for Next-Generation 3D Advanced IC Back End Applications

Abstract

Fan-out (FO) wafer and panel level processing in advanced backend applications requires the curing of RDL (Redistribution Layers) or interconnect dielectrics at lower temperatures in order to manage stress and relevant warpage for multi-level or 3D processing. In addition, the need to transition to all-Cu interconnect has resulted in a temperature limit of 230°C for large numbers of end products. Most of these products employ the advanced packaging capabilities of heterogeneous and 3D integration, 3D stacking and system-in-package (SIP) technology, whereby a variety of chiplets and other components are stacked, delivering high compute density and high performance at low cost. Multi-layer RDLs are used to connect these dissimilar chips on organic, glass, silicon or fan-out substrates. Consequently, process technology for polyimide, photo imageable dielectric (PID) and photo sensitive polyimide (PSPI) must now enable a faster cure at lower temperatures to achieve lower stress, while also delivering better film properties.In this talk, the physical, mechanical, thermal, and electrical properties of different types of polyimide materials will be discussed as a function of different process parameters, under atmospheric and vacuum process conditions. The talk references a study in which a variety of low temperature RDL polyimides such as HD-7110, Asahi BL-301 and Fuji-LTC9320 were processed at a temperature of less than 230°C, and then compared to conventional polyimide HD-4100. Vacuum cure even at lower temperatures resulted in higher thermal stability, lower outgassing, and better thermal properties. In electrical characterization, vacuum cure showed better dielectric strength as well as a lower dissipation factor. These results are consistent with the assumption that cure under vacuum reduces the amount of volatile and volatilizable material remaining in the film after the cure process.The use of sub-atmospheric curing or annealing technology at low temperature improved both the quality and the performance of the cured polyimide films for RDL applications. These results are relevant to the multi-level metallization process of Fan Out Wafer Level Packaging (FOWLP) and Fan Out Panel Level Packaging (FOPLP), and therefore to 3D stack integration.