Forming electrically conducting vias, contacts, and conductors of copper or other metals becomes increasingly challenging as feature sizes are reduced. Techniques for forming such metal features include physical vapor deposition (PVD), chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD) and electrochemical deposition (also referred to as electroplating or electrodeposition).
The general nature of the electroplating process is well known. The wafer is immersed in an electrolytic bath containing metal ions and is biased as the cathode in an electric circuit. With the solution biased positively, the metal ions become current carriers, which flow towards and are deposited on the exposed surfaces of the wafer. Electroplating is particularly well suited for the formation of small embedded damascene metal features due to the ability to readily control the growth of the electroplated film for bottom-up filling, and due to the superior electrical conductivity characteristics of the electroplated film. However, there are also several obstacles, which need to be overcome to fully realize these advantages.
One challenge facing damascene processing techniques is the difficulty of the growth of the metal film within recessed features without forming voids. Voids increase the resistance of the conductor. Also, trapped electroplating solution in sealed voids may corrode the metal. This may lead to degraded device performance or device failure in extreme cases.