The effect of interactions between water and polishing pads on chemical mechanical polishing removal rates

The elastic properties of polishing pads critically affect polishing results during chemical mechanical polishing (CMP) of integrated circuit substrates. The ability of water to plasticize polishing pads and the resulting effects on pad performance were investigated. Water is hypothesized to penetrate the surface of the polishing pads, disrupting hydrogen bonds between adjacent polymer molecules within the pads and altering the pad elastic modulus. Infrared spectra obtained using an attenuated total reflection technique were used to confirm the effect of water on the polishing pad structure. Contact with deionized water reduced the pad elastic modulus due to the formation of a hypothesized soft layer on the pad surface. Removal of the soft layer from the pad, as occurs during conditioning, increased observed CMP removal rates. Experiments on a commercial dual-axis polisher using a noncommercial process showed decreased oxide removal rates as the duration of the pad exposure to water increased. A pad asperity-wafer contact model was developed. This model indicates that if the surface of the pad asperities is softened by the penetration of water compared to the bulk pad, then the asperities will deform to a greater degree under the applied load than in the absence of the softening. As a result, the effective pad-wafer contact area for a given applied load will increase and the average pad-wafer separation will decrease, causing the applied load to be carried by a larger number of asperity-wafer contacts and reducing the local load on the wafer surface. The reduced local loads will contribute to reduced polishing rates, consistent with experimental observation. The maximum depth of penetration of water into the pad surface was estimated to be ∼20 μm.