The critical role of the metal / porous low-k interface in post direct CMP defectivity generation and resulting ULK surface and bulk hydrophilisation

Surface hydrophilisation of pristine low-k (ULK) is known as a CMP-induced damage mechanism [1]. This phenomenon already enhanced by several factors (e.g. mechanical polishing action, solid content in the slurry, pH of the slurry solution, presence of organic residues, etc ...) extends to bulk hydrophilisation when polishing metal/ULK systems. The degree of bulk hydrophilisation depends on the nature of the selected metal/low-k combination, the metal being either a hard mask (for low damage patterning purposes) or a Cu diffusion barrier. The phenomenon is more or less pronounced depending on the nature of the overlaying metal film (Ta>TaN>Ti>TiN). It also correlates with the post CMP defects generation and more specifically with the presence of scratches with depths ranging from ~178nm down to ~6nm as measured with a 0.19μm tip depending on the metallic layer. These scratches can be reduced in number and depth by overpolishing leading thereby to reduced hydrophilicity. Besides selecting properly the overlaying metal film, UV curing the ULK for mechanical properties improvement and/or engineering the metal/ULK interface by inserting an ultra-thin dielectric layer with higher mechanical properties to prevent the metal from contacting the low-k surface significantly limits the direct CMP-induced bulk hydrophylisation.