Electroless Plating Scale-up for Microelectronics Application

Introduction

Autocatalytic plating systems are notoriously unstable and difficult to scale-up in volumes that are fit for large-scale manufacturing. From a chemical engineering perspective, they are essentially chemical reactors and all principles of mass transfer, heat transfer and chemical reaction engineering apply. In contrast to electrolytic systems that pump electrons to the cell by means of an outside current source, electroless systems contain in-situ reducing agents, such as dimethyl amino borane, and stabilizer compounds that eliminate the need for complex masking processes found in electrolytic baths. The overall system is simplified through the elimination of masking processes (photolithography) and stripping processes. The bath formulations are, however, much more complex and drive unique design and process control requirements from an engineering perspective. In addition, preparation steps such as oxide removal and seeding of a catalyst are required.

In this case study, challenges associated with such electroless processes are summarized and solutions are presented.

Challenges

Although there are many electrolytic plating formulations sold and used for microelectronics applications, 100% autocatalytic systems have not been widely used due to the following factors :

Cost - such electroless systems tend to be highly unstable and baths are not easily replenished, if replenished at all. Waste management costs are significantly higher than those of electrolytic systems. When compared to such conventional electrolytic processes, these negative cost factors tend to overshadow the positive factors associated to the overall process simplification. In order to be economically viable, a stable bath is a must.

Stability - Electroless systems are very sensitive to tank design, purity of the base materials, contamination and process control techniques. They have a tendency to produce extraneous plating conditions that translate into product yield loss. The plate-out can be sufficiently pronounced to completely de-stabilize the chemistry to a point where the bath must be dumped, translating into excessive and unsustainable operating costs.

Process Control - In order to have manufacturability, a simple control and replenishment scheme is required. If too complex, opportunities for errors render the process as not having manufacturability.

Expert Solution Provided

As a process engineer in a large microelectronics firm, I supervised the design, the installation and the startup of such systems. The plating process was part of a large-scale project to build and assemble microelectronics modules. An electroless Nickel-Boron line and an electroless Gold plating line were qualified to run 24 hours/day and 7 days per week. Tank volume was up to 300 liters, which is the largest scale-up of such systems ever qualified.

A tank design was implemented that maximizes bath stability. Key design parameters include :

• Tank geometry or aspect ratio
• Heat exchanger design that minimizes hot spots
• Temperature measurements and control to eliminate temperature spikes
• Volume turnover and recirculation scheme to optimize bath uniformity
• Unique rinsing tank design to control bath cross-contamination and residues at the parts surface

A system to control the chemistry of etching, seeding and plating solutions was implemented in order to maintain the optimum ratio of key components (reducing agent, stabilizer, metal salt). Simple techniques such as electrochemical potential probing, auto-titration and XRF thickness measurements were used and SPC charts were implemented for early trend detection. Online systems were installed and chemicals were automatically fed to the bath. Adjustments were kept small to eliminate any shocking effect. Yield data from downstream operations was back-correlated to the key process parameters. An offline lab was built-up and chemical technicians trained.

Yield and reliability tests were performed as part of an overall technology qualification. The system was qualified and released to the manufacturing team.

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