Contact lithography defect reduction and monitoring for the 90 nm node

Technology advances within lithography are placing greater demands on defect management. Focus-exposure process windows have shrunk as a result of the introduction of sub-wavelength lithography. Thus, an efficient monitoring of yield detractors, as well as clear knowledge of defectivity at the process window boundaries are required at the critical steps in the process. Patterning of contacts and vias for the 90nm technology node at ST Microelectronics Crolles, France was identified as a yield-limiting step, due to out-of-specification densities of single missing or undersized contacts. In order to track these elusive, yet killer defects, a Photo Cell Monitor methodology was initiated based on a brightfield inspection tool with selectable Narrow Band UV illumination. Automated defect classification allowed the impact of resist batch changes or hardware interventions on the scanner on this particular class of defects to be quantified. With the goal of optimising the process for minimum defectivity, the effects of coating, develop, rinse and bake processes were investigated, as well as the effect of focus and exposure.