Study on the mechanical properties of CMP pads

The mechanical properties of chemical-mechanical planarization (CMP) pads are very important to their performance. In this paper, pads are described in terms of their bulk properties and the properties of their asperity layer for applications in copper CMP. In particular, the elastic properties of these layers are studied. Pads were soaked in a copper polishing slurry or water for specified times and changes in their elastic moduli were measured. In general, pad immersion in water reduced the bulk modulus because the, water penetrated the pads and disrupted hydrogen bonds between adjacent polyurethane chains. In addition, it was found that surface grooving affected the pad elastic modulus, especially that of the asperity layer. To evaluate the effect of use in copper CMP on the pad properties, the elastic moduli of the bulk and asperity layers on a pad were measured following pad use in a commercial polisher. The periodic loads during CMP were believed to facilitate the penetration of water into the pads, lowering their bulk elastic moduli. A slight increase in the elastic modulus of the asperity layer was seen as a result of pad use in copper CMP. The inherent variations in modulus measurements for dry pads were also measured.