Optimizing Laser Direct Writing for Fan-Out Packaging

Laser direct exposure is becoming an attractive alternative to high-throughput mask aligners and expensive high-precision steppers for photolithographic processes in packaging. Despite the substantially lower throughput compared to mask-based patterning, laser direct writing has several advantages especially for applications requiring higher resolution and pattern positioning control. One significant application is chip-first Fan-Out packaging. There is a positional accuracy issue with the dies caused by the inaccuracy of the die placer, die shift during embedding, and substrate shrinkage in the RDL (redistribution layer) process. The necessary compensation studies lead to high initial costs for Fan-Out packaging. Through adapted designs (e.g. adaptive patterning by DECA Technologies) combined with maskless laser direct exposure, the need for compensation setups can largely be eliminated, making Fan-Out solutions attractive even for small series production. In this presentation we demonstrate in detail how to measure position deviations on the target and correct the following writing process accordingly. Compensation is done by measuring an array of alignments in two or even three dimensions and applying a nonlinear 2-D position correction to the laser writer's coordinate system. The following sequential writing process allows for local adjustment of substrate warpage and therefore enables the implementation of finer structures. This paper discusses the practical use of laser direct writing in general and tolerances and limits of compensation methods applied to the fan-out process in more detail.