Exploring Particle Size Analysis of Silicas

Silicon oxides such as SiO₂ are the most abundant component of the earth’s crust. They can be found in nature in crystalline form and are industrially manufactured in many forms, including colloidal silica, fumed silica, precipitated silica, and silica gels.

Image Credit: AlexLMX/Shutterstock.com

Table 1 exhibits a variety of silica processes and particle sizes.

Colloidal silica dispersions are dense (2.1 – 2.3 g/cm³), shapeless particles with an alkaline pH of 9–11 and a viscosity similar to water. Particle sizes are between 5 – 150 nm with widths from narrow to broad and PDI values from 0.008 – 0.350.¹ Smaller size dispersions may need to be stabilized.

Controlling the particle size in silica suspensions is essential for customizing their properties to meet the specific demands of diverse applications and industries, driven by various factors such as:

• Uniformity and Stability: Obtaining consistent particle size distribution gives the suspension stability and uniformity. It maintains a uniform mix by preventing the suspension from settling and agglomeration. This is an important factor in the performance and attributes of the finished product.
• Rheological Properties: Smaller particles tend to increase viscosity, which can have an impact on flow characteristics. Controlling particle size thus helps when it comes to managing viscosity, which is key in applications like coatings and printing inks.
• Surface Area and Reactivity: Smaller particles yield a greater surface area, amplifying reactivity and proving advantageous for catalysis, adsorption, and diverse chemical processes.
• Processability: While smaller particles may enhance properties, they can also complicate manufacturing processes due to the increased surface area and potential difficulties in handling.

Silicas find extensive use across various industrial applications, such as desiccants, binders, papermaking, catalysts, and as abrasives in CMP slurries, where Entegris boasts significant expertise.

Additionally, Entegris utilizes silica suspensions for filter retention testing. Notably, Entegris holds a unique position, possessing expertise in formulating silica CMP slurries, conducting particle size testing, and designing and manufacturing the particle size analyzers employed, including the Nicomp and AccuSizer product lines.

Table 1. Colloidal Silicas. Source: Entegris

Laboratory Particle Size Measurements

Mean Size and Zeta Potential by DLS

Particle size and surface charge (the zeta potential) are key components affecting dispersion stability. The chemistry and physical attributes need to be controlled during the continuous and dispersed phases to create a stable dispersion.

Achieving stable dispersions necessitates precise control over the chemistry and physical characteristics of both the continuous and dispersed phases. Optimization of the continuous phase chemistry involves adjusting surfactant selection and concentration, modifying salt concentration, controlling pH levels, or a combination of these factors.

Enhancing stability in the dispersed phase can be achieved through methods such as applying a polymer coating to the surface (steric stabilization), increasing surface charge (electrostatic stabilization), or employing a combination of both approaches.

Analyzing the mean size and zeta potential of silica suspensions¹ can be accomplished with dynamic light scattering (DLS). The size distribution and zeta potential of two different colloidal silicas measured on the Nicomp DLS system are shown in Figure 1. The zeta potential of the narrow distribution silica was -18.4 mV and -22.1 mV for the wide distribution silica.

Figure 1. Nicomp DLS mean size and zeta potential of colloidal silicas. Image Credit: Entegris

Figure 2 exhibits four Nicomp results for a mix of two colloidal silicas with mean diameters near 8 and 50 nm. The results highlight the Nicomp algorithm’s ability to successfully split a bi-modal silica mixture.

Figure 2. Bimodal Nicomp result. Image Credit: Entegris

Large Particle Size & Count by SPOS

A small concentration of large particles (tails) in silica suspensions can reduce stability, change optical and mechanical attributes, and create surface coarseness. CMP slurries with large particle counts (often defined as particles/mL > 1 micron) can create imperfections on the silicon wafers and reduce semiconductor performance.

The tails of silica suspensions are commonly measured utilizing single particle optical sizing (SPOS), which is the operational principle behind the Entegris AccuSizer product line.²  This range encompasses the AccuSizer AD, AccuSizer APS, and AccuSizer FX Nanosystems.

 The selection of the instrument depends on the size range of interest and the design of dilution fluidics.

The AccuSizer AD and APS incorporate single and two-stage dilution fluidics, respectively, alongside the LE400 sensor, offering a dynamic range of 0.5-400 µm. Notably, the AccuSizer APS stands out as the most automated dilution system capable of accurately diluting samples up to a ratio of 1 million to one.

Figure 3 depicts the large particle tail results for three distinct silica CMP slurries above, with the reproducibility of two individual measurements showcased below.

Figure 3. AccuSizer APS silica CMP LPC results. Image Credit: Entegris

AccuSizer FX Nano Results

Newer and cleaner silica CMP slurries can have limited particles/mL greater than 0.5 µm, so measuring at smaller particle sizes is required. The AccuSizer A9000 FX Nano offers measurement capabilities down to 0.15 µm, or 150 nm. While this primarily covers the tail of the distribution, certain samples may contain "working particles" within this size range.

This instrument integrates the LE400 sensor with the FX Nano, providing a broad dynamic range from 0.15 µm to 20+ µm. Due to this wide dynamic range, the AccuSizer FX Nano requires measuring a sample in three distinct measurement ranges (FX Nano high gain, FX Nano low gain, and LE400), which are then combined to generate the final result.

The AccuSizer A9000 FX Nano results for a colloidal silica CMP slurry are illustrated in Figure 4.

The upper portion displays the differential distribution, with particle counts depicted on the X axis using a linear scale, while the lower portion employs a log scale on the X axis. The log scale offers a more effective visual representation of the larger particle tail.

Figure 4. AccuSizer FX Nano silica CMP LPC results. Image Credit: Entegris

Entegris is a leading supplier of CMP filter solutions for the semiconductor industry. Particle retention tests for these filters are performed using the AccuSizer FX Nano because of its extended dynamic range.

Figure 5 exhibits filter retention results from 0.15 – 1+ µm for several conditions after 5 (solid lines) and 15 minutes (dashed lines) of recirculation

Figure 5. AccuSizer FX Nano filter retention data. Image Credit: Entegris

In-Process Particle Size Measurements

Mean size by Mini DLS System

The Mini DLS system³ is an adaptable and refined solution to an extensive list of nanoparticle manufacturing methods, including milling, homogenizing, or microfluidizers.

A pressurized stream of suspension product is connected to the Mini DLS system. Subsequently, this sample undergoes automatic dilution to attain an optimal light scattering intensity for measurement purposes. The particle size distribution is then determined, and the system undergoes automatic flushing and cleaning before the measurement sequence is repeated.

Two silica suspension samples were analyzed using the Mini DLS system mimicking in-process monitoring: Ludox TM 50 and a silica CMP slurry. The results are displayed in Figure 6.

Figure 6. In-process Mini DLS results of silica samples. Image Credit: Entegris

Large Particle Size and Count by AccuSizer Mini

AccuSizer Mini systems are used in semiconductor fabrication (fab) slurry delivery systems to provide continuous monitoring of LPC concentrations. These Mini systems combine a choice of sensor (LE400, FX, and FX Nano) with various dilution fluidics matched to specific CMP slurries.

The Mini LE (LE400 sensor) can be utilized for standard silica-based slurries and the Mini FX Nano for extremely clean, colloidal silica slurries. Results from the Mini LE and Mini FX Nano for silica CMP slurries are detailed in Figure 7.

Figure 7. Mini LE (above & middle) and Mini FX Nano silica CMP results. Image Credit: Entegris

Conclusions

Entegris provides a wide range of particle size and count solutions for businesses manufacturing or using silica suspensions. The mean size can be analyzed in the lab using the Nicomp system and in-process using the Mini DLS system. The large particle count (tail) is best analyzed using the AccuSizer lab system or Mini in-process analyzer.

Entegris’ expertise in manufacturing and testing instrumentation, CMP slurries, and filters positions the company as the ideal partner for any customer manufacturing silica suspensions.

References and Further Reading

1. Entegris application note Mean Particle Size and Zeta Potential Analysis of CMP Slurries
2. Entegris application note Detecting Tails in CMP Slurries
3. Entegris Mini DLS data sheet

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

For more information on this source, please visit Entegris.