Link between silica-metal agglomeration and high porosity ultra-low k scratch formation during Chemical Mechanical Polishing

Sub-surface hydrophilisation and an increase in effective k is known as damage to a highly porous ultra-low k (ULK) film when removing a metal layer from this ULK film by Chemical Mechanical Polishing (CMP). The degree of ULK hydrophilisation is linked to the amount and depth of scratches formed on the dielectric during polishing. To be able to trace key factors that could lead to a higher resistance against scratches, we looked into the root cause of scratch formation and how it relates to the type of overlaying metal film that we remove during the CMP step. Our work shows that the ULK scratch formation during CMP is linked to the formation of large metal-containing particles in the slurry on the pad. We were able to evidence and separate those particles and characterized them as silica-metal agglomerates. Those "harder" agglomerates are more detrimental to the ULK surface - i.e. more and deeper scratches are formed - when clearing Ta-based metal layers from the ULK film.