Innovative Excimer Laser Dual Damascene Process for Ultra-Fine Line Multi-layer Routing with 10 µm Pitch Micro-Vias for Wafer Level and Panel Level Packaging

The demands of higher routing density on wafer level are driven by multi-chip integrated fan-out packages and high I/O CSPs. New technologies and materials are necessary to generate lines and spaces down to 2 µm. Multi-metal layers are necessary for the higher wiring effort on panel level packaging (PLP) for example to contact dies which are embedded together. This places higher demands on the mechanical properties of the materials that are used for the redistribution layer. This paper presents a new excimer laser-enabled dual damascene process for ultra-fine routing for BEOL which was developed in a joint project with SUSS MicroTec. In the project, various materials e.g. low temperature cure polyimide, BCB and dry-film ABF materials were structured by using an excimer laser stepper with a reticle mask to pattern feature sizes below 4 µm with a high throughput. Micro-vias with a diameter below 5 µm were ablated with an aspect ratios up to 4 which is exceeding the photolithographic resolution limits of the established photosensitive thin-film polymers. The laser structuring allows the usage of innovative dielectric materials for WLPs/PLPs with optimized mechanical and electrical parameters, for example polymers with inorganic fillers like dry-film ABF material. Also, the ablations depth per laser pulse was investigated. The ablated lines and micro-vias were metallized by applying a galvanic process and CMP. The stepper-like system allows a sub-micron alignment accuracy with no need of a capture pad in the redistribution layers. Test multi-layer structures have been designed and fabricated with lines and spaces below 10 µm to demonstrate the dense routing capability with an excellent reliability which was verified by air-to-air thermal cycling (from -55 °C up to 125 °C).