Abstract
Multi-chip integrated Fan-Out packages and high I/O CSPs demands for higher routing density on wafer level. Due to that, the classical mask aligner lithography and photosensitive thin-film polymers used for BEOL reach its limits and new technologies and materials are necessary to generate lines and space down to two μm. These multi-metal layers set also higher demands on the mechanical properties of the materials.
This paper presents a new excimer laser dual damascene process for ultra-fine routing for BEOL. Various materials like low cure temperature polyimide, BCB and 15-μm thick dry-film ABF material are structured by using an excimer laser stepper with a reticle mask to realize feature size below four μm with a high throughput.
Micro-vias with a diameter below five μm are realized with high aspect ratio, which overcome the photolithographic limitations of the common used photosensitive thin-film polymers. The laser structuring allows to use innovative dielectric materials for WLP with optimized mechanical and electrical parameters for example inorganic filled polymers like dry-film ABF materials, which do not have to be photosensitive. The ablations depth per laser pulse and the cross-section of the ablated structures in dependence of the ablation parameters was investigated. The depth of embedded lines was set by number of pulses aside of integrated micro-vias. The lines and micro-vias were metallized with copper by galvanic process and the following CMP step removes the copper outside the ablated structures. The CMP removes only the copper and the metal of the seed-layer, which has the functions of an adhesion and barrier layer, stays intact. The under-etching of the conventional wet etch seed layer removal is a major problem for the fine line structures realized by the Laser Dual Damascene process. Due to that, the removal of the seed layer (usually titan) was investigated and it could be shown, that this layer can be removed by the excimer laser system. The stepper like system allows a sub-micron alignment accuracy with no need of a capture pad of the embedded lines. Test structures have been designed and fabricated with lines and spaces below 10 μm to demonstrate the dense multi-layer routing capability where the excellent reliability can be proven by air to air thermal cycling (from −55°C up to 125°C), current leakage and electro migration test.