Next Generation Panel-Scale RDL with Ultra Small Photo Vias and Ultra-Fine Embedded Trenches for Low Cost 2.5D Interposers and High Density Fan-Out WLPs

This paper presents a novel, high density, and large panel compatible thin film redistribution layer (RDL) process, with 2 μm diameter microvias and 2 μm fine line and space Cu traces on a thin glass panel using a photo-lithographic embedded trench approach. Newly developed thin dry film photosensitive polymer dielectrics were used for the fabrication of photo-vias and photo-trenches by conventional i-line 365 nm UV lithography in this work. Smallest feature dimensions of 1.5 μm trace widths and spaces were achieved by using liquid photosensitive polymer dielectrics. Such a high density, panel scale RDL is applicable to both high performance 2.5D interposers and high-density fan-out packages. The photo embedded trench process retains the cost advantages of the conventional semi-additive process (SAP) while overcoming its scaling challenges. The photo embedded trenches integrated with photovias using a thin, dry film photodielectric have higher resolution, precise via registration, and lower cost compared to the existing embedded trench methods. A dual-layer photo trench process was proposed and developed to address the challenges of copper plating uniformity in vias, pads and trenches. A test coupon consisting of six area array test structures with via diameters ranging from 2 to 5 μm at 20 μm pitch and lines and spaces of 2.5, 3, and 5 μm were designed and demonstrated on a 100 μm thin panel glass substrate. The 5 μm wide features was designed to capture vias within the trenches for a padless multi-layer RDL structure, with via diameters close to the line width enabled by the improved dimensional stability of glass compared to organic laminates. A new type of configuration, via-in-trench, with higher routing capacity than dog-bone or via-in-pad structures, was proposed and demonstrated on thin glass panels based on the photo embedded trench technology. Routing structures of line-space-via-trench (L, S, Via, T) of 3, 3, 3, 5 and 2.5, 2.5, 2, 2.5 μm on thin glass substrates were demonstrated without the use of via capture pads.