Abrasive slurries are used to remove material and even out any irregular topography on the surface of silicon wafers. This is to prepare the wafer surface for the formation of additional circuit elements. The micro topography of the wafer is related to the size distribution of the slurry used, since size affects the uniformity of the dielectric film processed later on1. The surface charge on the slurry particles determine the stability of the slurries, therefore zeta potential can also affect the polishing process. In this study, two kinds of silica based slurries were characterized by light scattering techniques, in order to study their size, distribution and surface charge properties.
The slurry particles were measured using the Zetasizer Nano, which is capable of dynamic, static and electrophoretic light scattering measurements.
The slurry samples were kindly supplied by Cabot Microelectronics Corporation, Taiwan. Both sample A and sample B are silica based particles. Sample A is dispersed in an ammonia salt solution with a pH around 2.5 - 4, and sample B is dispersed in KOH aqueous solution with pH = 10. The zeta potential of both types of dispersion was measured at various pH values using an MPT-2 autotitrator.
In figure 1, the size of the particles in sample A is plotted as a function of the particle concentration. It can be observed that in the dilute regime (c < 0.04%wt), the size of the particle is found to be constant. While in the concentrated regime, the size tends to decrease with increasing concentration. Measurements of sample A at a concentration of 8% w/v at different cell positions showed that the size obtained is independent of distance from the cell wall. This suggests that the decrease in size with concentration is not due to multiple scattering effects but rather electrostatic interactions between particles increasing the diffusion speed2. A concentration effect is also observed for the polydispersity index (PDI), where the PDI increases with the concentration. Understanding this effect is very important especially for the measurement of charged particles with an extended electric double layer, where the interaction may occur over a long range. If the effect of concentration is ignored, only an apparent size and polydispersity are measured.
Figure 1: the Z-average diameter (upper figure) and PDI (lower figure) of the sample A measured at various concentrations
Sample A was centrifuged in order to reduce particle concentration with respect to centrifugation time. In figure 2, the size measured by DLS is plotted as a function of the centrifugation duration. The plots show that with increasing centrifugation duration, both the particle size and the polydispersity index decrease.
Figure 2: the Z-average diameter of the Sample A and polydispersity index (inset) measured with various centrifugation duration
The measured particle size drops from 56 nm down to 47 nm. This is because during the centrifugation process, a size gradient is formed. Larger particles sediment more rapidly in the cuvette. The centrifugation has a separation effect for this polydisperse particle system; it deposits the heavier components, which also leads to a decreasing PDI with increasing centrifugation time.
Zeta Potential Measurements
The zeta potential of the slurry sample was measured as a function of the centrifugation duration. These samples were measured by dynamic light scattering and show a decreasing size with time (figure 2). However from figure 3, the zeta potential of the particle appears to be independent of the centrifugation time. Thus, the zeta potential of the slurry particles is insensitive to particle size.
Figure 3: the zeta potential of sample A as a function of centrifugation time
There is a chemical equilibrium between the surface of the silica particle and the surrounding aqueous medium3. The particles have the tendency to obtain/lose a proton depending on the environmental pH, which can influence the zeta potential. The zeta potential of both sample A and sample B were measured at various pH values with the Zetasizer Nano combined with an MPT-2 autotitrator. DLS measurements were performed at the same time to see the influence of the pH on particle size.
Figure 4 presents the autotitration measurement results on sample A. The titration started from pH=2.5 and ended at pH=9.5. KOH solution was added to adjust the pH. It can be observed that at low pH, sample A has a positive charge on the surface, while with increasing pH the zeta potential decreases. At pH=6.03, the dispersion reaches an isoelectric point, where the net charge on the surface is zero. Further increasing the pH causes a change in the sign of the zeta potential from positive to negative. The size of the particle at each pH value was also measured on the Zetasizer Nano. It can be seen that around the isoelectric point, the size of the dispersion is significantly larger.
Figure 4: The size and zeta potential of sample A at different pH
HCl was added to sample B. It can be observed from figure 5 that the zeta potential of sample B is always negative in a pH range 3 to 9.5. With decreasing pH, the magnitude of the zeta potential decreases. The size of sample B dramatically increases when pH<6.5, corresponding to a zeta potential of less than 34 mV, presumably due to insufficient electrostatic repulsive forces between the particles to prevent aggregation.
Figure 5: the size and zeta potential of sample B at different pH
Effect of Concentration on Size and Zeta Potential
The results show that the hydrodynamic diameter is only a constant under dilute conditions where the particles undergo unrestricted Brownian motion. At a higher concentration, where there are particle interactions, the observed size is smaller. The interaction between the particles is a very important but often an omitted factor in the DLS measurement, since it changes the diffusion speed of the particles. At low salt content, the electrostatic double layer of the charged particle is extended. This enables the particles to interact at a long distance, even in a very dilute dispersion. One way to avoid this kind of interaction is to add a moderate amount of salt into the solution, which can effectively screen the surface charge.
Centrifugation removed the largest particles, reducing the apparent size. However, the zeta potential of the particles showed almost no effect due to centrifugation. There are two factors involved in this. One is that the zeta potential is relatively insensitive to the particle size but determined by the surface charge density. Another important point is that centrifugation does not change the environmental conditions such as pH and ionic strength, which have strong effects on the zeta potential.
Effect of pH on the Zeta Potential and Size
pH affects zeta potential by modifying the surface charge2. Unlike particle size, zeta potential is a relative concept, and it is meaningless to talk about the zeta potential without mentioning the measurement conditions like pH and ionic strength.
The zeta potential is an indicator of the stability of the dispersion. In general systems tends to be stable if the amplitude of the zeta potential on a particle is larger than an absolute value of ±30mV. This rule can be observed from the autotitration measurements on both sample A and sample B. From figures 5 and 6, we can see that the size of particle is constant at high zeta potential, while the size of the particles becomes significantly larger when the zeta potential is smaller than -30mV due to low electrostatic repulsive force i.e zeta potential. Understanding this is very important for the slurry, because the size, distribution and homogeneity of the particles directly affects the chemical mechanical polishing quality.
For DLS measurements, the measured size and the polydispersity of the charged particles depends on the dispersion concentration. In order to measure the real size of the particle, the long range electrostatic interaction between charged particles which influences their diffusion speed should be taken into account. Centrifugation changes the concentration and also the size distribution of a polydispersed system by selectively removing the larger components. This makes both the size and the polydispersity index smaller. The zeta potential was, however, found to be independent of centrifugation time.
The zeta potential of the slurry samples can be measured with the Zetasizer Nano in the disposable capillary cell up to a concentration of around 16% w/v. Centrifugation does not change the chemical environment, like pH and salt content and therefore might be a good approach to measuring the zeta potential of samples at high concentration.
Use of the high concentration cell accessory for the Zetasizer Nano is expected to enable measurements at higher concentrations.
Knowledge of the optimal pH range may benefit both the production and the application of the CMP slurries.
- Jea-Gun P, Takeo K, Ungyu P, Nanotopography Impact in Shallow Trench Isolation Chemical Mechanical Polishing-Dependence on Slurry Characteristics. Journal of Rare Earths, 2004;z2
- W. R. Bowen, A. Mongruel, Colloid and surface A, 138, 161-172 , 1998
- Iler, R K, The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; Wiley: New York, 1979