First Demonstration of Photoresist Cleaning for Fine-Line RDL Yield Enhancement by an Innovative Ozone Treatment Process for Panel Fan-Out and Interposers

Abstract

This paper describes for the first time an innovative approach to improve re-distribution layer (RDL) yields in advanced semi-additive processes (SAP). An atmospheric pressure ozone based treatment is proposed as an alternative to oxygen plasma treatment. The ozone treatment process is scalable, being appropriate for process wafers up to large panels, and is suited for small feature sizes down to 1 micron that are required for interposers and future fan-out packages. The ozone process provides an environmentally friendly solution that can also replace wet cleaning processes, eliminating the need for hazardous chemicals that require abatement. The paper demonstrates the potential for two opportunities for integration of ozone treatment steps in the SAP flow for RDL fabrication. These particular steps are identified as 1) ozone treatment after dry film resist (DFR) patterning to improve the electrolytic copper plating yield and 2) removal of DFR residue prior to seed layer etch cleaning. Both these steps resulted in significantly improved RDL yields and demonstrate the feasibility of integrating ozone based cleans in high-yield, high volume cost effective manufacturing of RDL in Advanced Packaging. The paper also proposes ozone treatment as a higher throughput alternative to the plasma treatment process for electrolytic copper plating. Since the ozone gas is generated from oxygen, and reduced to oxygen upon process completion, no hazardous gas is required, or discharged into the atmosphere. To exhibit the scalability of the ozone treatment to both wafer-scale and panel-scale processing, two different types of copper seed layers, physical vapor deposition (PVD) Ti-Cu, and electroless plated copper, were evaluated. Samples were treated with either ozone or oxygen plasma, and the results were compared to a control sample with no treatment. After the photolithography step, in which 7 micron thick DFR laminated on the copper seed layers was resolved to 3 micron feature size, the substrates were subjected to ozone or plasma treatments. The subsequent water contact angle measurements show significant wettability improvement on the surfaces of substrates with copper seed layer, DFR, and DFR mesh patterned on a copper seed layer. Samples were then compared for the quality of the copper plating. Excellent copper metallization quality was achieved in the samples that had been treated with ozone and plasma due to the creation of a hydrophilic surface. An additional benefit emerged in that the ozone treatment was effective at 50 deg C, which minimized any impact on DFR stripping. The ozone treatment was also applied to clean the DFR residue after resist stripping and the results confirmed that the ozone process removed any remaining photoresist residue from the copper surface.