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Tech Brief: Elements of Electroplating

August 13, 2018

Electroplating is a common manufacturing process that applies a thin layer of one metal onto another. The U.S. penny, for example, has been made of zinc with a thin, electroplated coating of copper since 1982. Jewelry and flatware are also frequently electroplated to improve visual appearance or provide wear and corrosion resistance. Today, electroplating is widely performed in the electronics industry to deposit conducting metals used in printed circuit boards, connectors, and most recently in semiconductor interconnects.

Throughout the chipmaking process, layers of dielectric (insulating) and metal (conducting) materials are deposited. Depending on the type of material and structure being made, different physical or chemical techniques are employed. Electroplating is used to create the copper interconnects and vias that link components together in an integrated circuit. Copper deposited by electroplating has lower resistivity and better fill characteristics than other deposition methods such as physical vapor deposition.

Before we look at the chipmaking case, let’s first look at a basic electrolytic cell that could be used to coat a brass key with a layer of pure copper – an experiment you might have done in chemistry class. The key and a piece of copper are both connected to a power supply, which is typically a battery. Without touching each other, the key and copper piece are submerged in a bath that is electrically conductive, completing the circuit. As electric current flows, ions dissolve from the copper source and deposit on the key. Eventually, the key will be completely coated with a layer of copper.

The same electroplating principles apply to semiconductor processing. A silicon wafer and a copper source are placed in a plating bath, which typically contains copper sulfate and sulfuric acid. When a current is applied, copper ions deposit on the wafer. The amount of copper deposited on the wafer is directly controlled by the current flow, which supplies electrons needed for the cupric ion reduction reaction. Parameters such as temperature of the bath, the rate of solution flow, and chemical composition of the plating solution control the properties of the copper that is deposited on the wafer.

Interconnects

It’s a bit more complicated when creating interconnect structures. Copper contamination in the interlevel dielectric layers can be catastrophic. To prevent this contamination, a diffusion barrier layer (Ta/TaN) is the first step in the process flow. Since copper electroplating processes typically cannot nucleate on these highly resistive diffusion barriers, a very thin copper seed layer is deposited on the barrier using a physical vapor deposition method. Copper electroplating is then used to form a Cu film of the desired thickness. Fully filling the deep and narrow interconnect trenches presents additional challenges. When the process is not well-controlled, voids or seams can form and will degrade the electrical functionality and reliability of the chip as well as the yield of the chip fabrication process.

Bottom-Up Fill

Void-free fill can be achieved when copper deposits in the trenches from the bottom up. The “superfill” process involves adding chemicals to the bath that accelerate copper deposition at the trench bottom and suppress plating on the field and the sidewalls. A third additive called a leveler minimizes the undesirable copper bumps that form once the feature is filled. When chosen correctly and used in appropriate concentrations, these additives enable high quality metal fill.

Packaging Applications

Electroplating of copper and other metals is used for a number of advanced wafer level packaging (WLP) applications, such as forming conductive bumps, pillars, and redistribution layers, and for filling through-silicon vias (TSVs). While similar to copper fill for back-end-of-line (BEOL) interconnects, electroplating for WLP and TSV is at a much larger scale. This often requires long deposition times, even at high plating rates, and multi-step processing. High productivity, high uniformity systems will likely continue to increase in demand as WLP applications continue to grow.

Successful electroplating, whether for BEOL interconnects or packaging applications, depends on finding the right additives for bottom-up fill, maintaining a stable bath composition, and minimizing the impurity level in the plated metal. New barrier/seed materials and fill metal, as well as changes in feature size and aspect ratio, present further process challenges. When performed correctly, electroplating produces low resistivity, void-free, high reliability structures.

Jon Reid was a consultant on this article.

 

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