Because of the lack of volatile copper compounds, copper could not be patterned by the previous techniques of photoresist masking and plasma etching that had been used with great success with aluminum. The inability to plasma etch copper called for a drastic rethinking of the metal patterning process and the result of this rethinking was a process referred to as an additive patterning or a "Damascene" or "dual-Damascene" process. In this process, the underlying silicon oxide insulating layer is patterned with open trenches where the conductor should be. A thick coating of copper that significantly overfills the trenches is deposited on the insulator, and chemical-mechanical planarization (CMP; also called chemical-mechanical polishing) is used to remove the copper to the level of the top of the insulating layer. Copper sunken within the trenches of the insulating layer is not removed and becomes the patterned conductor. Damascene processes generally form and fill a single feature with copper per Damascene stage. Dual-Damascene processes generally form and fill two features with copper at once, e.g., a trench overlying a via may both be filled with a single copper deposition using dual-Damascene. http://en.wikipedia.org/wiki/Copper-based_chips --

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