Ultra thick photo resist for FO-WLP

With the current tremendous market growth of mobile devices toward the new era of Internet of Things/Internet of Everything, the requirements for semiconductor packaging structures are getting more challenging. In this situation, FO-WLP (Fan-Out Wafer Level Packaging) has been widely discussed as one of the major solutions in the 3D packaging area. We have been developing negative tone resists (ELPAC THB series) for re-distribution layer (RDL), solder bumps, copper pillars, and micro bumps, which show good lithographic properties, good chemical resistance for various plating chemicals, and good strippability. Based upon these technologies, we are now strongly focusing on new resists for finer RDL and high copper pillars in order to support FO-WLP players to efficiently fabricate their innovative packages. In this paper, we report on the latest ultra-thick photo resist to fabricate high copper pillars over 100um for FO-WLP. The new resist shows excellent coating performance to achieve 100um thickness in a single coat, or over 200um thickness with double coating, on 12 inch wafer. The resist provides good coating uniformity and TTV without bubbles, defects, wrinkles or other errors. On top of that, the new material design enables finer resolution for ultra-thick films, with an aspect ratio of 4 and beyond. We also report high copper pillars fabricated with the new THB resist. It is expected that the new ultra-thick resist will be the best candidate for FO-WLP. We will discuss the new material concept in more detail.