Using Rheology to Characterize Colloidal Silica CMP Slurries

Colloidal silica slurries are utilized in CMP, the process of chemical mechanical polishing. It is employed to make silicon wafers for circuitry perfectly smooth, this is a crucial process in the semiconductor industry.

Throughout the polishing process, it is important to characterize the rheological behavior of the slurry at a high shear rate as there is a large amount of shear applied to the CMP slurry. The slurries can be filtered to enhance their properties, this article describes how the filtration impacts the viscosity of slurries, and how FluidicamRHEO is used as a precise method to measure the viscosity of the slurries.

Reminder of the Technique

FluidicamRHEO employs a co-flow microfluidic principle to calculate viscosity. The sample and a reference solution are introduced simultaneously into the microfluidic channel (normally 2.2 mm X 150 μm) with flow rates which are controlled.

This results in a laminar flow in which the interface location between reference and sample relates to the viscosity ratio and flow rates.

FLUIDICAM measuring principle.

Figure 1: FluidicamRHEO measuring principle.

Method

By utilizing FluidicamRHEO, a series of CMP slurries were examined. First of all, S1, the stock solution, was assessed, then diluted to acquire D1, which was also tested. Next, the diluted slurry was filtered for 5 minutes and 24 hours, then re-tested. These two solutions are labeled F5 and F24.

To simulate the polishing process, applied shear rates were between 5000-75000s-1. All of the tests were carried out with a 50 μm chip utilizing a reference solution of water with dye. Each sample was measured at 25 °C twice.

Viscosity Measurements of CMP Slurries

Figure 2, the graph below, shows the two flow curves for each sample, S1, D1, F5 and F24.

Visocisty of CMP Slurries at 25 °C.

Figure 2. Visocisty of CMP Slurries at 25 °C.

This graph shows that there is a decrease in viscosity when S1 is diluted to obtain D1. Shear thinning behavior is observed for both, with a little more shear thinning for S1. When D1 was filtered for 5 minutes to attain F5 there is a noticeable increase in viscosity.

The slurry filtered for 24 hours, F24, has a viscosity which is even higher. Nonetheless, after filtration both solutions show decreased shear thinning behavior.

Comparative Study: Particle Size Distribution

As observed in image a), slurry solutions contain a mixture of small and large particles.

Granulometry measurements were taken to establish the particle sizes in the solutions, in order to understand why the viscosity of the solutions increases after filtration. The information for the smallest (<0.6 μm) and largest particles (>2.0 μm) in the solution is displayed in table 1.

Table 1: Particle size measurements using granulometry.

Particle size D1
(non-filtered)
F5
(5 min-filtration)
F24
(24 h-filtration)
<0.6 μm 2673676 2551440 2522058
>2.0 μm 24579 3321 3093

After filtration, it is clear that the amount of large particles in the solution decreases dramatically, around 78% fewer than D1. The number of small particles only decreases slightly, around 6% less than D1.

It is determined that the increase in viscosity is due to the small particles when this information is paired with the FluidicamRHEO data. After the solution is filtered the large/small particle ratio is altered because the big particles are removed, this results in a higher viscosity being seen for the filtered samples.

The non-filtered samples’ shear thinning behavior is a result of the large particles, because the shear thinning decreases after filtration. FluidicamRHEO can be used to establish the viscosity of the solution and to give an indication of the size of the particles which are present in the solution.

Conclusion

FluidicamRHEO offers a fast and simple technique to assess the dispersion of the particles in CMP slurries because of the relationship that exists between viscosity and particle size. It provides crucial information to help formulators tailor to their individual requirements. Measurements are done quickly (in a couple of minutes) and accurately with no re-sampling required between different temperatures.

This information has been sourced, reviewed and adapted from materials provided by Formulaction.

For more information on this source, please visit Formulaction.

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