Cobalt silicide re-deposition during batch spray strip process resulting in open contacts

Functional fails due to open contacts were determined to be the main cause of systematic yield loss. Electrical analysis correlated the yield loss to single bit fails (SBIT) in the SRAM cell. Physical analysis on several SBIT fails showed blocked contacts due to titanium (Ti) defects in the silicide loop. Several partition experiments in the silicide loop pointed to the batch spray strip process as the main contributor to Ti defects. This paper describes the methodology used to isolate the open contacts to defects in the cobalt silicide (CoSi) loop starting at Co sputter and ending with Co anneal. It identifies the sources of Ti defects and determines the role of the batch spray strip process in forming these defects and blocking the contacts. Corrective actions put in place to reduce Ti, including the elimination of bare Si surfaces from the spray processor during Co strip.