Patterning of copper for multilevel metallization: reactive ion etching and chemical-mechanical polishing

Copper has been suggested as a desirable alternative for aluminum in future multilevel-metal ULSI circuits. The use of copper as an interconnect material will require, as one of the critical steps, the development of a highly accurate and manufacturable patterning method. Two possible approaches to this problem are the subtractive patterning of the metal by reactive ion etching and producing inlaid metal patterns by chemical-mechanical polishing. A general overview of issues involved with these two methods is presented. Recent work in the dry etching of copper is then reviewed, with particular reference to the use of halogen-based plasmas. The rather low volatility of the resulting etch products generally requires a substrate temperature above 200°C, but submicron-resolution patterning is feasible with a high-temperature-stable mask. Various approaches to lowering the etching temperature are examined. Additionally, recent work on the chemical-mechanical polishing of copper is discussed, and examples of the fabrication of inlaid metal structures are given. In conclusion, a brief comparison of the two patterning methods is made, with a view on their future application.