New Life for Copper Plating Baths

One of the main challenges in copper electroplating today is precise management of the "additives" used to control parameters such as how quickly the copper fills, and the planarity of the copper surface at the end of the plating step. Also known as accelerators, brighteners, levelers and suppressors, additives are organic materials added to the basic copper electrolyte solution. Over the past few technology generations, the industry has seen an increase in the number of additives used to better control the bath, and a decrease in the actual amount of each additive — usually measured in the ppm or ppb range (See "Progress in Copper: A Look Ahead ").

In addition to being difficult to measure, the additives are difficult to control because they are consumed over time, even if the plating solution is not in use. They also break down unevenly, making it difficult to maintain a steady-state solution. It's necessary, therefore, to monitor them constantly. "The organic additives are critical," said Peter Bratin of ECI Technology (East Rutherford, N.J.). "The brighteners are generally sulfur-bearing polymers. The suppressors are generally glycols — polyethylene polypropylene — or different molecular weights. The levelers are nitrogen-bearing materials."

ECI manufactures a cyclic voltimetric stripping (CVS) analysis called the Quali-Line that measures the concentration of the additives, as well as the acid, Cu and chloride in the bath. "It's an electroanalytical method. We plate the copper on the platinum electrode and then dissolve it back into solution," Bratin said. "We monitor how the additives affect the deposition. We're not looking for the analytical concentration of these species, we're looking for the activity because they affect each other's performance. The pure analytical concentration is not always a good indicator of what's going on in a solution. It's the effective concentration."

By removing TOC from copper electroplating solutions, it’s possible to reuse the solution, while improving wafer uniformity. (Source: Mykrolis)

A new approach developed by Bedford, Mass.-based Mykrolis Corp. (pronounced Micro-liss, the company was formerly the Microelectronics Division of Millipore Corp.) is to treat that waste by removing the additives, returning it to a state such that it can be reused in the plating bath. "What we've done is come up with a process that takes that slipstream and basically burns off the entire organics that are left over, getting you back to the base electrolyte, which is sulfuric acid, copper sulfate and water," explained Christopher Wargo, program manager of Mykrolis' New Business Development Group. "Knowing then that it's completely additive-free, they just add a specific amount of additive back in and plumb it back into the bath. It's essentially a closed-loop process."

In addition to being a good "green" initiative, Mykrolis believes the new process can actually improve wafer uniformity and is presently in beta tests at customer sites to collect the uniformity data. The process uses strong oxidizing agents to complete the breakdown of organic impurities. The company said tests using this oxidation technique were performed on chemistries having total organic content (TOC) levels ranging from 50 to 4000 ppm. Batch test results illustrate that the TOC levels have been reduced by at least 90-99% for all three chemistries (Figure). The reduction in TOC is governed by the organic makeup of the electroplating bath, the type and amount of oxidizer, and the amount of time the solution is contacted with each oxidizer in question. Wargo said Mykrolis has proven that it can effectively burn 99.99% of the organic additives out of there. That's not a trivial thing, since organic additives run the full spectrum, from high molecular weight to low molecular weight.

Along with the organic impurities, up to 90% of the chloride is removed during the oxidation process. Using patented ion removal technology, ion exchange filters will selectively remove any trivalent (or higher valence) cations from solution. Suspended particles will be removed via commercially available acid-resistant filters.

For additional information on wafer processing, go to www.semiconductor.net/wafer