Low Temperature Curable PI/PBO for Advanced Packaging

Photo-definable polyimides (PI) and polybenzoxazoles (PBO) have been widely used as dielectrics for re-distribution layers in wafer level chip size packages (WL-CSP). These materials can simplify the manufacturing process and ensure high reliability owing to their good mechanical properties and high thermal stability. For next generation electronic components fabricated by utilizing advanced packaging technologies such as 3D-stacking using TSV, package-on-package, fan-out WL-CSP etc., the most important requirements for dielectric materials are high lithographic performance, high adhesion to Cu RDL, high chemical resistance and low temperature curability. In this paper, we will report on our novel low temperature (<200C) curable PBO and PI. A novel alkaline positive tone PBO was developed by re-designing key components of the formulation to enhance lithographic performance, Cu adhesion and chemical resistance. It was found that the new PBO material showed higher lithographic performance than conventional PBOs due to its high dissolution contrast and which resulted in a resolution of 2micron (L/S) with a 7μm cured thickness and 3micron (L/S) with a 15micron cured thickness, respectively. This material also produced strong Cu adhesion and high chemical resistance at curing temperatures <200C with no delamination from the Cu RDL being observed after a 168hr Pressure Cooker Test (PCT). Furthermore, the new formulation showed high TCT resistance due to its high elongation below 0C. In addition, a novel solvent negative tone PI was also developed by incorporating a cross-linker to accelerate low temperature curability as well a photo-initiator to improve lithographic properties. As a result, the novel PI when cured at 175C for 1hr showed high Cu adhesion after 168hr PCT as well as high film properties. The new PI also showed excellent lithographic properties with a resolution of 6micron (L/S). Furthermore, the low temperature curable PI and PBO materials were used as dielectrics to fabricate WL-CSPs for both chip and board level reliability testing. The test results indicated that both the novel PBO and PI showed excellent reliability after thermal cycling (TCT) due to the significant improvements made to Cu adhesion and chemical resistance. These materials are expected to be promising for next generation WLP applications. Details are described in the presentation.