The Study of Highest Thickness Photo Resist for Cu Post of Fan-Out Wafer Level Packaging

The mobile device’s limited thermal budget hardly allows the use of a high-performance application (AP) at its full speed. However, as artificial intelligence technologies has been rapidly applied for mobile devices, the demand like high-speed and large-capacity signal processing is continuously increasing. Therefore, controlling the heat generation of AP chip is becoming a key factor, and it is necessary to develop a Re-Distribution Layer (RDL) based Fan-Out Package (FOPKG) structure that does not increase the thickness of the package while maximizing the die thickness for heat dissipation. The height of Cu post is getting higher for the production of FOPKG that can apply thick die, and in this study, the world’s thickest photoresist material (>350um thickness) was developed for the production of Cu post (>300um thickness). The effect of light transmittance of the photoresist and the effect of solubility depending on the molecular structure of the main polymer were studied for lithography process of thick photoresist. Based on the understanding of this thick photoresist behavior, the optimal liquid type photoresist composition was developed. Through lithography evaluation based on thick photoresist characteristics, deep-hole via implementation and Cu electro-plating were performed to obtain a yield of Cpk 1.27 in the AP product design applied wafer. Depending on this in-depth understanding and experimentation on thick photoresist, it is possible to build an advanced research foundation of increasing the photoresist thickness and finer Cu post pitch to secure heat dissipation characteristics and improve the degree of freedom of architecture.