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A New Photosensitive Dielectric Material for High-Density RDL with Ultra-Small Photo-Vias and High Reliability
Keywords: dielectric materials, Ultra-Small Photo-Vias, reliability
Abstract This paper presents an advanced ultra-thin photosensitive dielectric Material (PDM) newly developed with high resolution, low CTE and low residual stress for next-generation high-density redistribution layer (RDM) for 2.5D interposer and high-density fan-out package applications. For high-density RDL, photosensitive dielectric materials need to have high resolution and low CTE to achieve high package reliability. Demand for higher data bandwidth between the multiple dies drives ultra-high density packaging technologies, such as 2.5D interposers and high-density fan-out wafer level packages (FO-WLP). Such high- density packages require conductor copper wiring below 5 m and micro vias below 10 m to accommodate high-density RDLs. Recent advancements in semi-additive processes (SAP) offer ultra-smooth interfaces to enable conductor wiring as small as 2 m line and space. However, current micro-via formation processes using CO2 or Nd-YAG lasers drilling do not provide micro-vias below 10 m. Therefore, micro-via formation by photolithography using photosensitive dielectric materials is getting high interest in the applications. In the material formulation, we employed nano-size fillers in order to reduce the CTE of the material without resolution degradation. Currently, we can form 5um dry film structure as minimum thickness. We successfully demonstrated the minimum micro-via diameter of 3um in the 5um thickness. The CTE of the material is 30-35ppm /K. Curing temperature of the PDM is 1800C x 60 min. which is lower than most of the advanced dielectric materials. Low-temperature curing process results in low stress. We have calculated residual stress in the cured PDM from a test result of warpage measurement on 4 inch wafer. As another benefit of the PDM material in curing process, the PDM can be cured in air oven. Most of advanced photo dielectric materials need to cure in N2 oven due to prevent an oxidation of the material. We have demonstrated copper traces of 2um lines and spaced on the PDM by using semi- additive process (SAP) with sputtered Ti/Cu seed layer. Thanks to the low CTE and low residual stress due to the low- temperature curing, it passed thermal cycle test (TCT) with daisy chain structure which has 400 vias in the structure. Also we tested BHAST (Bias- Highly Accelerated-Stress Test, 1350C, 85% R.H.) with line and space of 2um / 2um. As the test result, the PDM passed 300-hour insulation reliability test as well. The PDM can be developed by two different types of developers after photo exposure. One is 0.8-1.0wt% Na2CO3 which is commonly used in PCB and IC package substrate industry. This aqueous Na2CO3 developer makes the fabrication process environment- friendly and cost-effective. The other one is 2.38% TMAH which is commonly used in front-end process of semiconductor IC industry and wafer level packaging. This developer avoids the contamination of metal ions such as Na+ in Na2CO3. Thus, the PDM can be applied for both high- density package substrates by conventional package processes, silicon interposers and wafer level fan-out processes. It can be concluded that the newly developed PDM is a promising dielectric material for highly reliable high-density redistribution layer (RDL) for 2.5D interposers and fan-out wafer level package applications.
Daichi OKamoto,
Sandy Springs, Georgia
United States

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