Scanned Mask Imaging Ablative DPSS UV Laser Process for 2μm L/S RDL
Authors: Myles David , Milne David , Shephard Jonathan D 
Published: October 2015

Laser embedding conductors within a dielectric offers numerous advantages in fabricating redistribution layers (RDLs) for chip packages. Ablation of features down to 2μm L/S gives more routing space per layer and addresses the technology gap between semiconductor and PCB technologies. Microvias are made in the same process step as the circuitry, facilitating near padless vias further increasing the routing space available per layer. For a given package, this reduces the layer count and conductor path length required reducing the height profile of the package and improving signal integrity. Embedding the conductor can also improve its adhesion to the substrate and improve the co-planarity of subsequent layers in the build up. It also removes the need for the wet photochemistry associated with lithographic techniques. This presentation analyses the results of a novel UV, diode pumped solid state (DPSS), ablative mask imaging laser system for cost effective, high volume, 3D structuring of organic dielectrics.
Two methods are widely used to micro-structure materials by laser: mask projection and direct write. Excimer lasers are typically used in mask projection systems, where their high pulse energy and low coherence make them well suited to imaging. These systems can achieve the required ablation quality with 2–3μm line width and space, however excimer lasers have a high capital cost and require regular and costly maintenance when compared with DPSS lasers. The high beam quality and lower pulse energy of DPSS lasers makes them better suited to a direct write approach. A galvanometer scan head used in conjunction with an f-theta scan lens can be used to scan a focused beam across a substrate. Since the pattern is defined by a CAD file, these systems are very flexible and thus appropriate for low volume prototyping. However, complicated control systems are required to accurately control the ablated depth, and constraints in the circuit design are imposed by the direct write approach. Also, because each feature is marked sequentially, the process time is proportional to the pattern complexity, which makes these tools prohibitively slow for high volume manufacture of the high density RDLs required in the next generation of device packages.
This presentation outlines a scanned mask imaging system, wherein a low maintenance, cost efficient, frequency tripled, nanosecond, multimode UV solid state laser is used to illuminate a binary reticle. The multimode beam has an approximately Gaussian beam profile which is homogenised to form a square, flat top profile. A galvanometer scan head is used to raster scan the binary reticle. The reticle is subsequently imaged onto the substrate by a projection lens. Ablation of various features down to 2μm L/S in a variety of low K organic dielectrics is demonstrated. Accurate registration of pads with vias down to 5μm diameter highlights the feasibility of the process for high density RDLs and micro-vias for organic interposers. The process can achieve an ablation quality comparable to that of an excimer laser system, but with the advantage of significant cost saving and ease of maintenance in an industrial environment.