Using Light Scattering to Characterize SiO2 Chemical Mechanical Planarization Slurries

Chemical mechanical planarization (CMP) slurries are typically colloidal dispersions of silicon dioxide (SiO2) in an acid or base solution. They are integral to the semiconductor industry, emerging as the primary method for removing surface irregularities on silicon wafers. Critical parameters influencing the performance of these slurries are particle size and size distribution. Particles that are too large for a particular application can, for example, cause defects and scratches, ruining high-value material. This article demonstrates the use of a powerful light scattering instrument, the Zetasizer Nano (Malvern Instruments, Worcestershire, U.K.), for the characterization of CMP slurry particles without dilution. The method provides valuable insight into the fundamental relationships between slurry particle behavior and factors such as zeta potential, pH, and concentration.

CMP slurries

The use of abrasive CMP slurries has increased rapidly in recent years. Today, CMP is indispensable when fabricating multiple-layer semiconductors, providing the necessary ultrasmooth finish for additional circuitry. The microtopography of a silicon wafer is related to the particle size distribution of the slurry used in its manufacture, since particle size affects the uniformity of subsequent dielectric film processing.1

Accurate and precise control of the size and distribution of particles is essential to ensure maximum effectiveness of CMP slurries. Measuring zeta potential provides a key indicator, since the surface charge of slurry particles is a major determinant of slurry stability, which influences the polishing process. It is also necessary to monitor slurries for the presence of oversized particles. Even in minute concentrations, large particles can cause scratching during polishing, which results in significant damage to the wafer. Such irregularities in particle size can occur through contamination or agglomeration of the slurry, either during manufacture or storage.

Characterization using light scattering

Of all the commercially available particle characterization techniques, light scattering has many advantages for CMP analysis. The data presented in this paper demonstrate the use of the Zetasizer Nano, which enables dynamic, static, and electrophoretic light scattering measurement.

Materials and method

Two silica-based slurries were characterized to examine their size, distribution, and surface charge properties. The Zetasizer Nano can analyze the samples without dilution, saving time and avoiding any unintended changes to environmental conditions that could affect the particle size. It enables the highly accurate measurement of three important parameters: particle size, zeta potential, and molecular weight.

The slurry samples used were supplied by Cabot Microelectronics Corp. in Taiwan, a leading supplier of sophisticated polishing compounds for semiconductors. Both samples, labeled A and B, contain silica-based particles. Sample A is dispersed in an ammonia salt solution with a pH of around 2.5–4.0; sample B is dispersed in KOH aqueous solution at pH 10.

An MPT-2 autotitrator (Malvern Instruments) was used together with the Zetasizer Nano to measure changes in zeta potential over a pH range. The autotitrator provides a sample preparation station that automates the changes in sample conditions between measurements.

Results

Particle size

Figure 1 - The Z-average diameter (upper figure) and PDI (lower figure) of sample A measured at various concentrations.

In Figure 1, the particle size of sample A is plotted as a function of particle concentration. The graph shows that in the dilute range (concentration [c] <0.04 wt%), particle size is constant. Above this value, particle size tends to decrease as concentration increases. To separate out the effects of particle interactions and an interfering effect for light scattering methods called multiple scattering, the Zetasizer Nano makes it possible to measure the size at different depths within the sample. Measurements at a concentration of 8% w/v at different cell positions showed that particle size data obtained are independent of the distance from the cell wall. This suggests that the decrease in size with concentration is not due to multiple scattering effects, but can be attributed to electrostatic interactions between particles, which increase the diffusion speed.2

A concentration effect is also demonstrated when measuring the polydispersity index (PDI), which is shown to increase with higher concentrations. This effect is important, especially for measuring charged particles with an extended electric double layer, where the interaction may occur over a long range. The data show that concentration effects cannot be ignored for accurate determination of both particle size and PDI.

Figure 2 - The Z-average diameter of sample A and polydispersity index (inset) as a function of centrifugation duration.

The slurry samples were centrifuged, and Figure 2 shows the particle size of the supernatant of sample A measured by dynamic light scattering (DLS), plotted as a function of centrifugation time. It illustrates that with increasing centrifugation time, both particle size and PDI decrease. Measured particle size decreases from 56 nm to 47 nm, due to the formation of a size gradient during centrifugation: larger particles sediment more rapidly in the cuvette. Centrifugation causes separation in this polydisperse system, with the deposition of heavier components, leading to a decreasing PDI.