News from the M in CMP - Viscosity of CMP Slurries, a Constant?

It is well known and accepted that the viscosity of CMP-slurries has an effect on polishing results. Even though the literature on rheology recognizes that viscosity is not always constant and the slurry can show non-Newtonian behavior or even dilatant effects, all calculations have been performed with constant viscosity. However, the “real” viscosity of a CMP-slurry during polishing can change significantly with shear rate. The typical equipment for viscosity measurement is based on a rotating cylinder or a plate. But even with a plate system it is only possible to reach a shear rate range up to 50,000 1/sec. A calculation of the shear rate between the wafer and the polishing pad is based on a relative velocity of 1 m/sec and a distance between the wafer and the pad of 20 μm; this correlates to a shear rate of 50,000 1/sec. If parts of the polishing pad come closer to the wafer or especially closer to the edge of structures on the wafer (for example 1 μm), the shear rate will increase locally to 1,000,000 1/sec. When the shear rate is high enough, viscosity depends mostly on hydrodynamic factors like viscosity of continuos phase, solids content, particle size, particle size distribution and shape of the particles. The shape of fumed metal oxides is controlled during the synthesis in the flame process. But the slurry-making process is also responsible for particle size distribution, shape of the particle and the high shear rate viscosity of the CMP-slurry. The high shear rate viscosity of different silica slurries in dependence from BET-surface area, used milling energy, concentration and preparation direction was measured in this investigation.