This paper reports a detailed study of several hydrogen-based plasma cleans prior to plasma-enhanced chemical vapor deposition of silicon carbonitride cap films, and it finds a tradeoff between improved electromigration and increased copper resistivity. Previously proposed mechanisms do not explain this tradeoff, and we propose an alternative mechanism for the cap/copper interface modification. Electromigration is improved by forming a copper silicide interfacial layer, but the copper resistivity is also increased by silicon diffusion into the copper from the cap/copper interface. Hydrogen-based plasmas generate silicon by reacting with the silicon nitride seasoning layer on the chamber surfaces and transporting the silicon to the copper surface. The transport of silicon can be prevented by adding nitrogen to the plasma or removing the seasoning layer. (C) 2007 The Electrochemical Society.