Novel zero side-etch process for <1μm package redistribution layers

Higher bandwidth interconnections are critical for More-than-Moore systems. This requires higher IO-density package redistribution layers (RDL) for faster chip-to-chip communication. Scaling down RDL critical dimensions is thus an essential research component for advanced packaging. There are numerous challenges pertaining to the further miniaturization of package RDL. Copper seed-layer etching is one of the most important steps in traditional semi-additive processing (SAP). It is very critical to accurately control the copper etch rate for achieving fine-line redistribution layers (RDL). However, due to the isotropic nature of the chemical etchants, it has proven to be the main bottleneck in developing fine line RDL using SAP. Using the traditional wet etchants, over-etching of copper seed-layer results in large reductions in the dimensions of RDL traces. There are reports in literature on using dry etching for copper seed removal with tight process control. However, dry etching does not provide the throughput and cost advantage that comes with wet processing. This paper demonstrates a novel zero side-etch RDL patterning process using the traditional wet SAP. A titanium barrier is coated on the sidewalls of copper RDL traces. Traditional wet etching is then performed to remove the copper seed-layer. Over-etching can be performed to ensure complete removal of seed-layer. The barrier protects the copper traces from the etching effect of chemicals thus preventing the changes in the RDL linewidth and spacing. After seed-layer removal, the titanium barrier can be completely removed from the RDL traces using a wet process. This process enables the use of SAP for achieving RDL critical dimensions lower than 1 μm.