An Advanced Photosensitive Dielectric Material for High-Density RDL with Ultra-Small Photo-Vias and Ultra-Fine Line/Space in 2.5D Interposers and Fan-Out Packages

In this paper, we introduce an advanced photosensitive dielectric material (PDM) we recently developed to realize a high resolution and low coefficient thermal expansion (CTE) for next-generation high-density re-distribution layer (RDL) for 2.5D interposer and high-density fan-out package applications. For high-density RDL, photosensitive materials need to have (1) a high resolution, (2) high insulation reliability and (3) semi-additive process (SAP) compatibility. We have developed an advanced photosensitive dielectric material witch meets these three requirements. We demonstrate ultra-small via (3 um) by i-line stepper. For SAP compatibility, we have fabricated copper traces of 2um lines and spaces on the PDM by using SAP with sputtered Ti/Cu seed layer. The insulation reliability test was performed at the condition of Bias-Highly Accelerated-Stress Test, 135 deg.C. 85% R.H. on test coupon with 5um thick PDM. As a result, over 300 hours insulation reliability was confirmed. These results mean that the newly-developed PDM is suitable for next generation 2.5D interposer and high-density fan-out package applications. In terms of a package reliability, a low CTE and low process temperature are desired. In order to reduce the CTE of the material and maintain high resolution, nano-size fillers were integrated into the material. As a result, CTE of 30-35 ppm / deg.C was achieved. Curing temperature of the PDM is designed at 180 deg.C which is lower than most of the advanced dielectric materials. These two features contribute to reduce warpage of high density substrates and interposers, so it is expected to be applied to multiple layer application. We also fabricated test coupon with daisy chain structure connected by ultra-small vias and very stable via resistance was confirmed. In conclusion, our newly-developed PDM is a promising dielectric material for highly reliable high-density redistribution layer (RDL) for 2.5D interposers and fan-out package applications.