Lothar Bauch

Development and integration of a new metal structuring process for 256 MDRAMs

The reduction of chip size by using the stitched word line architecture for the 256 M DRAM design demands a maximum metal sheet resistance of 140 mOhm/sq. To meet this requirement a metal thickness of 380 nm is necessary. Our investigations showed that the lithographic as well as the process window of the metal etch process are significantly widened by using a hardmask for the metal etch. The hardmask deposition doesnt have any detrimental effect on the metal film. Both processes, the mask open etch and the metal etch, can be used to adjust the final metal dimension. The key for saving a high metal short yield is the clean after mask open etch. Because of the absence of resist during the metal etch, the formation of polymer rails along the metal lines is suppressed and the post metal etch clean is no longer necessary. The new metal etch process was checked within volume production of the 64 M DRAM without any yield loss. In addition, several reliability investigations dont show any problems. This hardmask metal etch process has, because of the low resist thickness, a large potential for further shrinking.