Fundamental studies on silicon dioxide chemical mechanical polishing

Chemical Mechanical Polishing (CMP) has lately been adopted on a large scale by the semiconductor industry for planarizing and patterning metal and dielectric films. Additionally, CMP has been used for hundreds of years for optical polishing. Still, several aspects of this process remain poorly understood. In this study, some results on CMP of SiO₂ are presented with a view to characterizing the effects of abrasive properties and slurry chemistry on the polishing process. Additionally, some results from a novel in-situ friction force measuring instrument are also presented.; The friction force results showed the effect of several parameters such as surface roughness, solution pH and ionic strength on wafer-pad interactions. Additionally, monitoring the friction as a function of velocity showed that the transition from boundary lubrication to full fluid-film lubrication depends on the roughness (conditioning) of the polishing pad.; The parameters investigated in the polishing experiments include abrasive size and concentration. From the experimental results, it was found that an optimum concentration exists for each abrasive size, which shifts to lower values and becomes narrower as particle size increases. From calculations, this was attributed to a decreased ability of the large particles to chemically modify the surface of the SiO₂ films. The smaller particles, having a much larger surface area, are able to better adsorb dissolution and abrasion products at high concentrations, thus leading to high removal rates under those conditions.; Studies on the effect of slurry ionic strength showed that the ability of a metal ion to shield the surface charge on the surfaces interacting during polishing is what determined removal rate. This was due to the reduced electrostatic repulsion between the surfaces, which resulted in better contact and thus higher polishing rates. These results were corroborated by the earlier friction force measurements.; Finally, the influence of particle density on polishing was shown, with denser alumina particles being able to polish SiO₂ much more effectively. Some preliminary results on polishing with different abrasives as a function of slurry pH indicate that the material properties of the abrasives seem to change around their Iso-electric Points (IEP), resulting in almost no polishing, and severe particle contamination on the SiO₂ surface.