First Demonstration of Fine Line RDL Yield Enhancement using an Innovative Ozone Treatment Process for Panel Fan-out and Interposers
Authors: Gupta Atul , Snyder Eric , Gottschalke Christiane , Wenzel Kevin , Gunn James , Lu Hao , Suzuki Yuya , Sundaram Venky , Tummala Rao 
Published: September 2017

As front end transistor scaling by Moore's law faces economic and technical challenges, interconnect scaling by advanced packaging technologies has started to add value at system level for a variety of electronics applications including consumer, high performance computing and automotive. The focus on yield improvement at every node that has enabled transistor scaling is now becoming a very critical need for high volume manufacturing of advanced packaging technologies such as 2.5D interposers and high density fanout [1]. This paper describes the first demonstration of a novel atmospheric approach based on ozone as an alternative to vacuum-based plasma treatment for photoresist cleaning to enhance the re-distribution layer (RDL) yields in advanced semi-additive process (SAP) processes. The ozone process is applicable to wafers as well as large panels, and is suited for small feature sizes down to 1um that are required for interposers and future fan-out packages. Ozone process provides an environmentally friendly solution eliminating the need for hazardous chemicals used in wet cleaning processes and has the potential to significantly increase throughput and reduce process cost compared to plasma processing by eliminating the need for vacuum chambers. The goal of this research is to demonstrate the effectiveness and benefits of the atmospheric dry ozone process developed using an MKS Instruments ozone delivery system for electrolytic copper plating yield improvement and dry film resist (DFR) residue cleaning, for 1-10um RDL critical dimensions (CD). The paper will describe the process fundamentals and the manufacturing tools, and discuss the characterization by contact angle measurements to confirm wettability of the plating surfaces, as well as demonstrate improvement of fine line RDL plating quality.

The ever-increasing requirement for higher computing power in both high power applications and low power hand held or wearable devices is driving the need for higher signal bandwidth connections between logic and memory chips enabled through advances in the packaging world. The wiring density and I/O pitch are scaled down to achieve high bandwidth interconnections on a package with limited routing space. Photolithography, electrolytic copper plating, and copper seed layer etch are three crucial defining the yield and minimum feature size of RDL in the SAP flow. With the feature size scaling down to 3 μm, the quality of the metallized copper structure is crucial for high performance applications. Traditionally, wet chemical cleans are used for improving copper plating yields. The wet chemical process uses hazardous chemicals such as trichloroethylene (TCE) that are not environmentally friendly. These processes may be substituted with an oxygen plasma treatment that can clean organic residues in DFR trenches and improve wettability of the seed layers prior to plating as well as DFR residues after copper plating and DFR stripping. However, such plasma treatment approach requires a vacuum chamber which limits its throughput and cost effectiveness for high volume manufacturing.

This paper proposes a higher throughput alternative solution 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 demonstrate the applicability of the ozone treatment to 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. The effectiveness of both ozone and oxygen plasma treatments were qualified against a control sample with no treatment. The 7 μm thick DFR was laminated on the copper seed layers, then patterned with a projection lithography tool, and a minimum feature size of 3 μm was resolved. After photolithography, the substrates were subjected to ozone or plasma treatments. The 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. Copper plating quality was then compared between samples. Both the ozone and plasma treatments resulted in excellent copper metallization quality due to the creation of a hydrophilic surface. The effectiveness of the ozone treatment at 50 deg C was confirmed, thus minimizing any impact on DFR stripping. The ozone treatment was also applied to clean the DFR residues after resist stripping and our results confirmed that the ozone process removed any remaining photoresist residues from the copper surface.

In conclusion, this paper proposed and demonstrated high throughput, atmospheric pressure ozone treatment as an innovative alternative to plasma treatment for cleaning the surfaces prior to electrolytic copper plating, as well as for photoresist residue removal after resist strip. The results show yield improvement of plated RDL and DFR residue cleaning. The ozone process does not use any hazardous chemicals or gases and also does not require any vacuum steps, which makes it environmental friendly and high throughput, and offers a promising approach for fine line RDL for interposers and fan-out packages in meeting the semiconductor industry roadmap needs.