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Polymer residue formation in vias caused by plasma etching of underlying titanium-rich films

Thick polymer residues were observed in vias after plasma etching (in a CF4/CHF3/N2 atmosphere) and subsequent photoresist strip. It was determined that these hardened residues were caused by the etching of underlying titanium-rich films (such as titanium, titanium nitride, titanium tungsten, and titanium silicide) during the via over etch. Polymer residues were not seen when the underlying film was aluminum or tungsten. In addition, hardened polymer formation was not observed when the total amount of titanium-rich metal being etched was decreased below a certain threshold value. The results of this work indicated that when relatively large amounts of titanium- rich films were etched using standard CF4/CHF3/N2 oxide etch chemistries by, thick polymer residues formed in the vias. Because the residues were metal-rich, they were insensitive to changes in the CF4/CHF3 ratio. It was determined that the most straightforward way of eliminating the residues was by using an amine based photoresist strip solvent (EKC 265). However, it was found that the EKC 265 must be properly rinsed in order to achieve acceptable via chain resistance.