Shear Thickening in Colloidal Silica Chemical Mechanical

SHEAR THICKENING IN COLLOIDAL SILICA

CHEMICAL MECHANICAL POLISHING

SLURRIES

Anastasia Krasovsky

A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of

Mines in partial fulfillment of the requirements for the degree of Master of Science (Chemical

Engineering).

Golden, Colorado

Date ______

Signed: ______Anastasia Krasovsky

Signed: ______Dr. Matthew W. Liberatore Thesis Advisor

Golden, Colorado

Date ______

Signed: ______Dr. David W. M. Marr Professor and Head Department of Chemical and Biological Engineering

ABSTRACT

Chemical mechanical polishing (CMP) is used by the semiconductor manufacturing industry to polish materials under high shear for the fabrication of microelectronic devices such as computer chips. Under these high shear conditions, slurry particles can form agglomerates causing defects, which cost the semiconductor industry billions of dollars annually.

The shear thickening behavior of colloidal silica CMP slurries (28-38 wt%) under high shear was studied using a rotating rheometer with parallel-plate geometry. The colloidal silica slurries showed continuous thickening and irreversible behavior at high shear rates (>10,000 s-1).

Changing the silica concentration, adding monovalent chloride salts (NaCl, KCl, CsCl, and

LiCl), adjusting the pH (pH 4 to pH 10.5), and mixing two particle sizes (d = 20 nm and 100 nm) within the slurry altered the thickening behavior of the slurries. Shear thickening behavior can be eliminated with certain large to small particle ratios.

iii

TABLE OF CONTENTS

ABSTRACT…………………………………………………………………………………….. iii

LIST OF FIGURES……………………………………………………………………………... vi

LIST OF TABLES…………………………………………………………………………...... viii

LIST OF SYMBOLS……………………………………………………………………………. ix

LIST OF ABBREVIATIONS……………………………………………………………………. x

CHAPTER 1 BACKGROUND………………………………………………………………… 1

1.1 Chemical Mechanical Polishing…………………………………………………. 1

1.2 High Shear Rheology…………………………………………………………….. 5

1.2.1 Common Techniques and Geometries…………………………………… 5

1.2.2 Parallel-Plate High Shear Rheology……………………………………... 6

1.3 Chemical Mechanical Polishing Slurries………………………………………… 8

1.4 Stability of Colloidal Silica……………………………………………………... 10

CHAPTER 2 SHEAR THICKENING OF CHEMICAL MECHANICAL POLISHING SLURRIES UNDER HIGH SHEAR WITH ADDED ELECTROLYTE……… 14

2.1 Abstract…………………………………………………………………………. 14

2.2 Introduction……………………………………………………………………... 14

2.3 Experimental Methods………………………………………………………...... 16

2.4 Effect of Electrolyte on Shear Thickening…………………………………….... 17

2.5 Varying Silica Concentration Under Constant Salt Concentration……………... 22

2.6 Conclusions……………………………………………………………………... 24

CHAPTER 3 PARTICLE SIZE AND pH IMPACT ON SHEAR THICKENING OF COLLOIDAL SILICA CMP SLURRIES……………………………………… 26

iv 3.1 Abstract…………………………………………………………………………. 26

3.2 Introduction……………………………………………………………………... 26

3.3 Experimental Methods………………………………………………………….. 28

3.4 Shear Thickening Behavior Examined in a Range of pH Values………………. 29

3.5 Mixtures of Small and Large Colloidal Silica Particle Slurries………………… 33

3.6 Conclusions……………………………………………………………………... 36

CHAPTER 4 CONCLUSIONS AND RECOMMENDATIONS……………………………... 38

4.1 Conclusions……………………………………………………………………... 38

4.2 Recommendations………………………………………………………………. 39

REFERENCES CITED…………………………………………………………………………. 40

LIST OF FIGURES

Figure 1.1 Schematic of the CMP process, where Ωp and Ωc are the rotational speeds of the platen and wafer carrier, respectively. Adapted from Gokale and Moudgil [1]………………………………………………………………………. 2

Figure 1.2 Top (a), angled (b), and side view (c) of a concentric ring patterned polyurethane polishing pad used for experimentation…………………………… 2

Figure 1.3 Cross-section schematic of the damascene processing method for the sequential deposition of the: (a) dielectric, (b) diffusion barrier, and (c) interconnecting Cu layers. (d) Extra Cu must be removed via CMP in order to create a planarized surface for depositing the next layer of interconnects. Adapted from Matijevic and Babu [2].…………………………… 4

Figure 1.4 Schematic of a parallel-plate rheometer…………………………………………. 7

Figure 1.5 Transmission electron microscope (TEM) image of colloidal silica…………….. 9

Figure 1.6 Total interaction energy as predicted by DLVO theory versus particle separation distance between two hard spheres………………………………….. 11

Figure 2.1 Steady shear rate ramp (filled symbols) and reduction (open symbols) for 30wt% silica slurry with an added electrolyte concentration of 0.17 M KCl (black circles), 0.17 M NaCl (blue diamonds), 0.17 M CsCl (green triangles), 0.17 M LiCl (red squares), and no salt added (orange diamonds)……………… 18

Figure 2.2 Viscosity of small silica particles with a weight percent of 30% with an added electrolyte concentration of 0.17 M during shear rate ramp and reduction………………………………………………………………………... 19

Figure 2.3 Schematic of the adsorption of Cs+, K+, Na+, and Li+ onto a hydrated silica surface. Small and more hydrated counterions (Na+ and Li+) penetrate deeper into the hydration layer of silica than larger, weakly hydrated cations (Cs+ and K+). This figure is an adaption of a graphic published in Colic et al. [3]…………………………………………………………………... 22

Figure 2.4 Steady shear rate ramp (filled symbols) and reduction (open symbols) for a 28 wt% (green diamonds), 30 wt% (orange circles), 33 wt% (red diamonds), 36 wt% (black squares), and 38 wt% (blue triangles) silica slurry with an NaCl concentration of 0.17 M………………………………………………….. 23

vi Figure 2.5 Viscosity of small silica particles at 28 wt%, 30 wt%, 33 wt%, 36 wt%, and 38 wt% with an added sodium chloride concentration of 0.17 M during shear rate ramp and reduction…………………………………………………... 23

Figure 3.1 Steady shear rate ramp (filled symbols) and reduction (open symbols) for a 33 wt% small particle silica slurry pH adjusted with NaOH or HCl to pH 4 (black squares), pH 7 (blue diamonds), pH 9 (orange triangles), pH 10 (green circles), and pH 10.5 (red triangles)…………………………………….. 29

Figure 3.2 Steady shear rate ramp (filled symbols) and reduction (open symbols) for a 33 wt% large particle silica slurry pH adjusted with NaOH or HCl to pH 4 (black squares), pH 7 (blue diamonds), pH 8 (orange triangles), pH 10 (green circles), and pH 10.5 (red triangles)…………………………………….. 30

Figure 3.3 Viscosity of small silica particles (33 wt%) pH adjusted with NaOH or HCl to pH values of 4, 7, 9, 10, and 10.5 during shear rate ramp and reduction……. 30

Figure 3.4 Viscosity of large silica particles (33 wt%) pH adjusted with NaOH or HCl to pH values of 4, 7, 8, 10, and 10.5 during shear rate ramp and reduction……. 31

Figure 3.5 Steady shear rate ramp (filled symbols) and reduction (open symbols) for mixtures of small (d = 20 nm) and large (d = 100 nm) silica particles at a constant overall volume percent (23%) and a pH value of 9…………………… 33

Figure 3.6 Viscosity of mixtures of small (d = 20 nm) and large (d = 100 nm) silica particles during shear rate ramp and reduction at a constant volume percent (23%) and a pH value of 9……………………………………………………… 34

Figure 3.7 Schematic representation of how small particle spheres fit within the spaces between large particle spheres (a) and a TEM image of a mixture or small and large particle spheres (b)………………………………………….. 36

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LIST OF TABLES

Table 1.1 Applied shear rates during CMP slurry processing [4]…………………………... 1

Table 2.1 Critical shear rate for small silica particle slurries with a weight percent of 30% and an added electrolyte concentration of 0.17 M……………………... 21

Table 3.1 Conductivity and zeta potential values for small and large particles at 33 wt% for a range of pH values……………………………………………….. 32

Table 3.2 Conductivity and zeta potential values for mixtures of ratios to small and large particles at a constant volume percent (23%) and a pH value of 9……….. 35

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LIST OF SYMBOLS

Density………………………………………………………………………………………….... ρ

Measured torque……………………………………………………………………………….... M

Particle radius…………………………………………………………………………………...... a

Rheometer gap height…………………………………………………………………………… H

Rheometer plate radius…………………………………………………………………………... R

Rotational speed…………………………………………………………………………………. Ω

Salt concentration………………………………………………………………………………... C

Shear rate………………………………………………………………………………………… �

Shear stress……………………………………………………………………………………….. τ

Surface tension…………………………………………………………………………………... Γ

Viscosity…………………………………………………………………………………………. η

LIST OF ABBREVIATIONS

Chemical Mechanical Polishing……………………………………………………………... CMP

Cabot Microelectronics Corporation………………………………………………………… CMC

Derjaguin-Landau-Verwey-Overbeek theory……………………………………………… DLVO

Dynamic Light Scattering…………………………………………………………………..... DLS

Electrical Double Layer……………………………………………………………………… EDL

Transmission Electron Microscopy………………………………………………………….. TEM

CHAPTER 1

BACKGROUND

The following techniques and materials were used to study the high shear rheology of chemical mechanical polishing slurries and to examine changes to the slurry structure under shear. This chapter provides background information on chemical mechanical polishing (CMP), high shear rheology, CMP slurries, and colloidal silica stability.

1.1 Chemical Mechanical Polishing

Chemical mechanical polishing (CMP) is used in the semiconductor industry to ensure that the thickness of the dielectric materials is uniform and that interconnects between the multilayer chips are reliable [5]. In the semiconductor processing industry, CMP has become the primary technique for the planarization of integrated circuits [6]. The goal of CMP is provide a uniform, level surface on the semiconducting material that is free of defects, such as scratches or pits. In a commercial process, shear rates can exceed 1,000,000 s-1 [4]. This shear rate is significantly greater than any other shear rates the slurry is exposed (Table 1.1). The hypothesis is that slurry particles form large agglomerates. These shear-induced agglomerates increase the viscosity of the slurry leading to surface defects on the semiconductor wafers during the CMP process.

Table 1.1: Applied shear rates during CMP slurry processing [4].

Processing Step Shear Rate Range (s-1) Settling <0.1 Pumping 1 – 100 Filtering 1 – 100 Mixing 50 – 2,000 CMP >100,000

1 During the CMP process (Figure 1.1), liquid slurry is deposited onto a polishing pad. A polishing pad is typically made of a porous, flexible polymer material, such as polyurethane or urethane-coated polyester felt [7]. The pad properties (hardness, porosity, fillers, and surface morphology) affect the quality of the CMP process. A harder and rougher pad will provide higher removal rates, but an extremely hard pad will scratch the wafer surface. The polishing pads have porous cells on the surface that absorb the slurry (Figure 1.2).

Figure 1.1: Schematic of the CMP process, where Ωp and Ωc are the rotational speeds of the platen and wafer carrier, respectively. Adapted from Gokale and Moudgil [1].

Figure 1.2: Top (a), angled (b), and side view (c) of a concentric ring patterned polyurethane polishing pad used for experimentation [8].

2 It is desirable to prolong the lifetime of the polishing pad but pad degradation can occur from a blockage of the slurry channels or from pad wear over time. Degradation of the pad reduces the removal rate during the polishing process [9]. The polishing pad has to be conditioned before it is used for polishing to help prevent pad degradation. Conditioning is achieved through a grinding process where a rotating diamond plated wheel is positioned on top of the pad [10]. The coarse surface of the conditioner continuously removes layers of the polishing pad, which exposes fresh pad surface. Pad conditioning prolongs the lifetime of the polishing pad and increases the peak removal rate and product throughput.

When the polishing pad is conditioned and fully saturated, a semiconductor wafer is placed in a carrier face down. The wafer is pressed down near the pad at a set pressure (~ 1 to 10 psi) depending on the type of polishing performed [11]. The polishing pad is mounted on a rigid base plate that is rotated about its center axis. During polishing, the pad and wafer are both rotated which applies large shear stresses to the wafer causing them to slip out of the wafer carrier. A retaining ring is used to hold the wafer in place so that it does not slip out [12]. The relative velocity, with respect to the polishing pad, is the same at every point on the wafer when the rotational speed of the platen and the carrier are equal. This promotes uniform material removal across the entire wafer surface [13].

Slurry is pumped onto the pad near the center and centripetal force spreads the slurry over the pads surface. The composition of the slurry contains abrasives and chemical reagents that vary depending on the material that is being planarized. Alumina, ceria, and silica particles with diameters between 50 to 200 nm are typically used as abrasives in CMP slurries [10,14,15].

A more detailed discussion on CMP slurries can be found in Section 2.3.

Figure 1.3: Cross-section schematic of the damascene processing method for the sequential deposition of the: (a) dielectric, (b) diffusion barrier, and (c) interconnecting Cu layers. (d) Extra Cu must be removed via CMP in order to create a planarized surface for depositing the next layer of interconnects. Adapted from Matijevic and Babu [2].

During the planarization process, heterogeneous layers containing insulating dielectric materials and conducting metallic materials are continually exposed. A multi-stage procedure known as damascene or dual damascene is used to add the dielectric and metallic layers onto the chip [6,14,15,16]. During this process, a dielectric material is deposited and trenches are etched into this layer (Figure 1.3a). A diffusion barrier layer is deposited to prevent the diffusion of copper through the dielectric layers (Figure 1.3b). Copper is then deposited by electroplating

(Figure 1.3c). A chemical mechanical polishing process is used to remove the excess copper,

4 leaving the planarized wiring (Figure 1.3d). Devices are fabricated on silicon wafers and they have to be electrically interconnected before they are functional [2,17,18].

The current generation of silicon chips contains a billion or more electrical elements.

Interconnecting such large numbers of electrical elements in a very small area involves a complex, multilayer structure. Current devices require at least eight metal layers to create the necessary wiring structures [2].

1.2 High Shear Rheology

Rheology is the study of the flow and deformation of matter. Rheological studies measure material properties such as viscosity and yield stress. To imitate the high shear nature of a typical

CMP process, shear rates of greater than 10,000 s-1 must be studied. A general overview of high shear rheological techniques and its limitation are presented here.

1.2.1 Common Techniques and Geometries

High shear rheological measurements include cylindrical Couette flow with small gaps

[19], flow between parallel plates [20], and capillary rheometers [21]. The capillary rheometer is most commonly used for high shear rheological measurements. Capillary rheometers function by measuring the pressure drop as the solution flows through a capillary. The flow rate through the channel and the diameter determines the shear rate [22,23].

Although capillary rheometers are convenient for measuring high shear rates, they have many limitations:

1. Caking of particles along instrument walls disturbs measurement accuracy

2. The reversibility of shear thickening in the sample cannot be examined

3. The centripetal motion of the polishing process is not imitated.

As a result, capillary rheometry was not used in this study.

5 1.2.2 Parallel-Plate High Shear Rheology

A conventional rheometer with a parallel-plate geometry attachment is another technique used at high shear rates [21,24]. Parallel-plate rheometry is the preferred technique when working with small sample quantities. In the parallel-plate geometry (Figure 1.4), the bottom plate is stationary and the top plate rotates at a constant angular velocity Ω, which creates a velocity gradient through the thickness of the sample. The following assumptions can be made:

1. Steady, laminar, isothermal, and incompressible flow

2. vθ(r,z) only; vr=vz=0

3. Cylindrical liquid boundary

4. Negligible body forces.

2 2 From these assumptions, the equation of motion can be reduced to ∂ v /∂z =0 [25]. Integrating θ this equation with respect to z shows a linear velocity profile

�! = � � � + �(�) (1.1)

where A(r) and B(r) are functions of the plate radius. Applying the boundary conditions, vθ=0 at

z=0 and vθ=rΩ at z=H, to Equation 1.1 provides an expression for vθ using known parameters:

!!! � = (1.2) ! !

The shear rate (�) is described as the velocity gradient in the axial direction z (∂vθ/∂z). At the edge of the plate (r=R), the assumption of unidirectional flow is most valid because the linear velocity is the greatest. Using this assumption, the shear rate is:

6 !! � = (1.3) ! !

Newton’s law of viscosity (� = �!"�) can be used to determine an expression for viscosity (η) where τzθ is the shear stress. An expression for the shear stress needs to be derived before viscosity can be determined.

Figure 1.4: Schematic of a parallel-plate rheometer [8].

In a conventional rotating rheometer, the total measured torque (M) required to turn the upper plate is related to the shear stress and the following expression is determined [24]:

! ! !" (!) �!" = ! 3 + (1.4) !!! ! !"(!!)

For a Newtonian fluid, � ln (�) � ln (�!) = 1 so the shear stress will be given by [26]:

!! � = � = (1.5) !" ! !!!

where τa is the apparent shear stress. The fluid’s viscosity can be calculated from the torque

7 measurement when the equations for shear stress and shear rate are entered into Newton’s law.

!!! � = (1.6) !!!!

The parallel-plate geometry is a useful tool for obtaining viscosity data at high shear rates

(>10,000 s-1). According to Equation 1.3, increasing the plate radius, increasing the rotational speed Ω, or decreasing the gap height H can increase the shear rate. There is a threshold to how fast the instrument can revolve due to the mechanical limitations of the rheometer motor.

Operating at small gap heights allows for working at higher shear rates while keeping manageable rotational speeds.

1.3 Chemical Mechanical Polishing Slurries

Slurries are used in the chemical mechanical polishing process as abrasive agents to remove excess material. These slurries must be designed in a way that is compatible with the metal while also providing the most efficient polishing process. Slurry can be composed of silica, alumina, or ceria particles, ranging in size form 50 nm to 150 nm, that are used as an abrasive.

Depending on the material that is being polished, slurries can contain 0.1% to 10% solid content by weight [27]. Chemical components such as oxidizing agents, complexing agents, corrosion inhibitors, and pH buffers are used in slurry formulation [28]. Oxidizing agents typically include hydrogen peroxide, ferric nitrate, or potassium iodate. Surfactants and biocide are often added to aid in the stability of the slurry.

Slurries used for the planarization of dielectric films such as silicon dioxide or fluorinated silicon dioxide (FSG) are typically composed of silica particles at an alkaline pH value. The pH of the slurries is adjusted with potassium hydroxide to pH 10-11. The higher pH values provide greater polishing rates but silica particles begin to dissolve at pH values greater than 11.5.

8 Removal rates for this process are typically around 2500 Å/min. The surfactant, CnTAB, is used to prevent agglomeration of the silica particles [11,27]. These slurries often contain a biocide to prevent the growth of bacteria during storage.

Copper is used as an interconnecting material on the semiconductor chips as described by the damascene process (Section 1.1). Copper CMP slurries are composed of lower silica content

(2-4% by weight) and are at a slightly acidic pH of 4-6. The slurries contain an oxidant, hydrogen peroxide, a copper complexing agent, and a corrosion inhibitor (benzotriazole or

BTA). The copper removal rates are between 500-6000 Å/min, which can be tuned by varying the oxidant, complexant, and inhibitor concentrations [27, 29].

Figure 1.5: Transmission electron microscope (TEM) image of colloidal silica.

For this study, we will examine concentrated (∼33-42 wt%) colloidal silica CMP slurries provided by Cabot Microelectronics Corporation (CMC). Colloidal silica particles are prepared through the hydrolysis and condensation of alkoxysilanes in a mixture of alcohol, water, and ammonia. Since water and alkoxysilanes are immiscible, a homogenizing agent such as an alcohol is necessary. Base is used as a catalyst for the hydrolysis and condensation reactions

[30]. Silica spheres with radii between 10 to 500 nm can be formed through this process [31].

Colloidal silica particles of two different diameters, 20 nm and 100 nm, were used in this study.

9 These particles are dispersed in an aqueous solution and are electrostatically stabilized at a pH value of 9 through the addition of a hydroxide base. The stability and rheology of the colloidal silica particles used for this study will be discussed further in Chapter 2.

1.4 Stability of Colloidal Silica

Stability of colloidal silica is a concern when it is used in the chemical mechanical polishing process. The addition of electrolytes, adjustment of pH, change of silica weight fraction, and the ratio of small to large diameter particles are all factors that affect the stability of silica. The effect that these factors have on colloidal silica stability will be discussed in detail in

Chapter 2.

Colloidal suspension stability has been described by the Derjaguin–Landau–Verwey–

Overbeek (DVLO) theory and founded on the assumption that the forces between two surfaces in a liquid can be considered to be the sum of the attractive force (FA) and repulsive force (FR) contributions (Figure 1.7) [32,33]:

�!"#$ = �! + �! (1.7)

The attractive forces are London–van der Waals forces. The electrical double layer forces are considered to the repulsive force. These are due to the electromagnetic effects of the molecules and the overlapping of the electrical double layers of two neighboring particles. If the two spherical particles have an even charge, the van der Waals interaction force are as follows

[33,34]:

!! ! � = !"! (1.8) ! !"! where A131 is the Hamaker constant, a is the particle radius, and h is the interparticle separation distance. The electrostatic repulsive force is expressed as [33,34]:

! ! !!! !!!! �! = 32��!�! tanh exp (−�ℎ) (1.9) !! !!!!

10 In this equation, �! is the permittivity of the vacuum, �! is the dielectric constant of the medium, kb is the Boltzmann’s constant, T is the absolute temperature, e is the electron charge, and z is the ion valency. The surface potential, �!, is often approximated as the zeta potential and κ is the reciprocal Debye layer thickness, which is defined as follows [35]:

!/! !∗!"!!!!!! ! � = ! (1.10) !!!!!!! where NA is Avagadro’s number and C is the salt concentration in the suspension. The electrostatic force is more sensitive to variations in the salt concentration and pH than the van der

Waals forces.

Figure 1.6: Total interaction energy as predicted by DLVO theory versus particle separation distance between two hard spheres [8].

While the DLVO theory is a good model for colloidal interactions and stability, it has several limitations at small separation distances and high electrolyte concentrations. There are stability concerns at small separation distances where the attractive energy dominates. If particles collide with large enough kinetic energies to overcome the energy barrier, the particles will agglomerate and stability is lost. The deviations from the DLVO theory have been attributed to

11 surface roughness and the discreteness of the charge [36,37]. At high electrolyte concentrations, the DLVO theory can be inaccurate when compared to experimental data since there is ion exchange occurring on the surface, which causes instability and agglomeration in colloidal silica dispersions [38]. When ion exchange is occurring, the silica surface charge changes from strongly hydrated silanol groups to having both exchanged and unexchanged sites.

In an aqueous environment, the surface of the silica particles is covered in silanol groups

(SiOH or SiO-). It is believed that a hydration force causes the abnormal stability of colloidal silica. At the isoelectric point, pH 2, silica has an equal amount of associated SiOH and dissociated SiO- surface groups, which creates a neutral net surface charge. With this arrangement, it has been proposed that one water or alcohol molecule can bond to two silanolic hydrogens at one time. This creates a strongly hydrated surface, causing colloidal silica to behave as a lyophilic colloid [38, 39].

Under highly alkaline conditions (pH > 10), silica dissolution occurs as it undergoes a

- - deprotonation reaction (SiOH + OH ↔ SiO + H2O). The dissolution rate increases with increasing pH values under highly alkaline conditions. When electrolytes are added to highly alkaline dispersions, the silica surface will be composed of hydrated cations [32,37,40,41,42].

The addition of salt to the slurry can reduce electrostatic repulsions and cause irreversible agglomeration of particles through shear-induced bridging.

We seek to obtain an understanding of the high shear rheological response of CMP slurries and gain insight into the changes the slurry undergoes during the simulated polishing process. The hypothesis that will be studied is that shear thickening of CMP slurries leads to defect formation during the polishing process. High shear rheological behavior (>100,000 s-1) will be studied under varying conditions. Chapter 2 will focus on the change in shear thickening

12 behavior with the addition of electrolyte. Chapter 3 will address the adjustment of the pH of the slurries through the addition of NaOH or HCl to reach desired pH values. This chapter will also focus on mixtures of varying small to large particle ratio slurries to determine the effect particle size has on the slurries. The shear thickening behavior was studied under a range of conditions to determine what influence this has on the viscosity of the slurry and how this can be connected to defect formation during the polishing process.

CHAPTER 2

SHEAR THICKENING OF CHEMICAL MECHANICAL POLISHING SLURRIES UNDER

HIGH SHEAR WITH ADDED ELECTROLYTE

High shear rheology was employed to study the shear thickening of colloidal silica slurries. Adding electrolytes alters the thickening behavior of the slurries. This chapter will focus on the effect that the addition of electrolytes has on the thickening of silica slurries.

2.1 Abstract

We examined how the addition of various salts (KCl, NaCl, LiCl, and CsCl) influenced the shear thickening behavior of the slurry. Slurries with KCl, NaCl, and CsCl displayed shear thickening behavior while slurries with LiCl showed no thickening behavior. While the ion hydration theory (less hydrated ions cause slurry to thicken at lower shear rates), applied for fumed silica slurries, the same trend does not apply to colloidal silica. Shear thickening was also examined while holding the salt concentration constant at 0.17 M and varying the silica concentration. Sodium chloride added slurries with silica concentrations of 30 wt%, 33 wt%, 36 wt%, and 38 wt% showed irreversible shear thickening while a silica concentration of 28 wt% did not display thickening. The thickening dependence on electrolyte concentration implies the existence of additional structure forces that are not predicted by the Derjaguin-Landau-Verwey-

Overbeek colloidal stability theory.

2.2 Introduction

High shear rheology is typically done using a capillary rheometer. A capillary rheometer measures the pressure drop as the solution flows through a capillary [22,23]. While a capillary rheometer can be useful, it has many limitations for the purpose of this study such as the

14 reversibility of shear thickening cannot be studied and the polishing process is not imitated using this method. A parallel-plate rheometer was used for this study. In the parallel-plate geometry, the bottom plate is stationary and the top plate rotates at a constant angular velocity, which creates a velocity gradient through the thickness of the sample [21,24]. This geometry allows for working at small gap heights, which correlates to higher shear rates, while keeping manageable rotational speeds.

Shear thickening is exhibited by a suspension if the particle volume fraction is high and if the system has mainly repulsive interactions [43]. Several theories have been proposed to explain shear thickening. Hoffman [44] suggested that shear thickening is caused by order-to-disorder transitions. For this case, the structure at rest is ordered by an applied flow. The structure forms into layers, which helps avoid collisions so the viscosity will be lower than that of a disordered fluid. The increased collisions in the disordered state cause an increase in viscosity. Boersma et al. [45] concluded that the flow instability is controlled by a balance between the forces of repulsion between particles and the hydrodynamic forces in concentrated colloidal dispersions.

Bender and Wagner [43] suggested that cluster formation was the cause of shear thickening instead of order-to-disorder transitions.

Shear thickening is a reversible phenomenon by definition and any increases in viscosity are abandoned when the shear rate is decreased to its initial value. Shear thickening is caused by the formation of jamming clusters bound together by hydrodynamic lubrication forces, known as

“hydroclusters.” The clusters are composed of groups of particles that are formed when shear forces bring the particles almost into contact with each other. Lubrication forces cause the observed increase in viscosity and hard-sphere suspensions thicken without ordering

[43,46,47,48]. Irreversible shear thickening can be caused by shear-induced bridging where the

15 polymer bridge will be adsorbed on two or more particles simultaneously [49]. Shear induced gels are another example of irreversible shear thickening. Gelation occurs if a connected network is formed by particle aggregation [50]. There has been little data published on shear-induced flocs or gels without the use of polymer.

The stability of colloidal silica and its tendency to aggregate varies depending on the pH of the dispersion and on the addition of electrolytes. The DLVO theory is often used to describe the stability of silica but it has limitations. At high electrolyte concentrations, there is ion exchange occurring on the surface, which can lead to instability and aggregation [38]. The addition of salt to a suspension reduces the Debye length of the solution, which is related to the ionic strength, and decreases the zeta potential of the particles [51]. The addition of electrolytes to the colloidal silica slurries affects the aggregation of the dispersions.

Silica obeys the Hofmeister series prediction for “hydration stabilization”. The

Hofmeister series arranges cations from least hydrated to most hydrated (Cs+ < K+ < Na+ < Li+).

Strongly hydrated ions (Li+ and Na+) are less likely to adsorb to silica’s surface while less hydrated ions (Cs+ and K+) adsorb more easily to silica’s surface [40,52,53,54]. Slurry particle stability can be weakened through the addition of electrolytes. Our objective is to examine the effect of electrostatic and hydration forces on the thickening behavior of concentrated (28-38 wt%) colloidal silica CMP slurries under high shear (>100,000 s-1). Chloride salts (NaCl, KCl,

CsCl, and LiCl) were added to the slurries to adjust the interparticle repulsions.

2.3 Experimental Methods

For this study, colloidal silica slurry (Cabot Microelectronics Corporation, Aurora, IL) was used. We examined colloidal silica particle slurries (20 nm in diameter) that were adjusted with NaOH to a pH value of 9. The supplied slurry had a silica concentration of 33.7 wt%. Silica

16 slurries were diluted with salt solution to reach silica concentrations of 28 wt%, 30 wt%, and 33 wt%. To obtain silica concentrations of 36 wt% and 38 wt%, the silica slurry was dried in an oven at 80°C for about 2 hours. The dried particles were then dissolved in water and adjusted with salt solution to obtain the desired concentrations. Slurries were sonicated for 2 hours to ensure thorough mixing of silica particles in salt solution. Analytical grade (≥99.0% purity) LiCl

(Fisher Scientific, Fair Lawn, NJ), KCl (Mallinckrodt Chemicals, Phillipsburg, NJ), NaCl

(Mallinckrodt Chemicals), and CsCl (Johnson Matthey Electronics, Ward Hill, MA) solutions were added. After dilution, slurries were stored under ambient conditions for 24 hours before beginning rheological measurements to guarantee that the ion adsorption onto the surface of silica had reached equilibrium.

Rheological measurements were performed using TA Instruments’ AR-G2 rheometer

(New Castle, DE) with a parallel-plate geometry. The top, rotating plate, is 60 mm in diameter and is made of stainless steel. The gap height used was 30 μm, which allowed the study of shear rates up to 300,000 s-1. All measurements were done at 20°C, with a temperature control of

±0.1°C provided by a Peltier plate.

Shear thickening behavior was examined using a steady shear rate ramp procedure. Shear rates ranging form 1,000 s-1 to 270,000 s-1 were studied. At higher shear rates, data resolution was increased to determine where thickening begins. A shear rate reduction step immediately followed the shear rate ramp step and tested the irreversibility of shear thickening in the colloidal silica slurries.

2.4 Results - Effect of electrolyte on shear thickening

The rheology of colloidal silica slurries with an added concentration of electrolyte was examined. Initially, viscosity was measured as a function of shear rate at a constant silica weight

17 percent (30%) with electrolytes (NaCl, KCl, CsCl, and LiCl) added at a concentration of 0.17 M

(Figure 2.1). NaCl, KCl, and CsCl added slurries showed an earlier onset of shear thickening than the slurry without added salt, however the LiCl slurry showed no thickening behavior. The degree of shear thickening (Figure 2.2) summarizes the viscosity during the shear rate reduction step is greater than the viscosity during the shear rate ramp step. The salt added slurries presented a greater degree of thickening as compared to the no salt added slurry. All the salt added slurries showed thickening behavior with the exception of the LiCl added slurry.

Figure 2.1: Steady shear rate ramp (filled symbols) and reduction (open symbols) for 30wt% silica slurry with an added electrolyte concentration of 0.17 M KCl (black circles), 0.17 M NaCl (blue diamonds), 0.17 M CsCl (green triangles), 0.17 M LiCl (red squares), and no salt added (orange diamonds).

Figure 2.2: Viscosity of small silica particles with a weight percent of 30% with an added electrolyte concentration of 0.17 M during shear rate ramp and reduction.

Colloidal silica and fumed silica shear thickening behavior has been studied previously with the addition of electrolyte. Fumed silica slurries undergo irreversible, discontinuous shear thickening behavior due to shear-induced jamming [55]. At high shear rates, particles are driven together and they become coupled, either by hydrodynamic lubrication or through an irreversible agglomeration process. Colloidal silica slurries do not show discontinuous thickening behavior.

The thickening behavior of the spherical silica particles is continuous, which may be connected to hydroclusters. Particles organize into clusters due to hydrodynamic lubrication forces [56].

Both colloidal and fumed silica slurries showed irreversible thickening behavior. After thickening has begun, the slurries continue to thicken during the descending shear rate ramp. The hydrocluster mechanism does not explain the irreversibility of thickening adequately since thickening is reversible under that mechanism. The irreversible shear thickening displayed by both slurries silica slurries is more likely due to a clustering process [55,56].

19 Shear thickening behavior in colloidal silica systems has been studied previously. Bender and Wagner [43] studied colloidal silica dispersions that have larger particles sizes (d = 160nm,

330 nm, and 400nm) at high volume fractions (0.64), which showed shear thickening behavior.

These dispersions showed thickening behavior at lower shear rates (and the highest tested shear rate was 100 s-1). Shear thickening occurs in the examined dispersions when hydrodynamic shear forces overcome Brownian repulsive interactions in hard-sphere suspensions [43]. Maranzano and Wagner [56] studied colloidal silica dispersions that had an added concentration of nitric acid, which was used to reassociate any remaining unreacted silanol groups. Shear thickening was reversible and only observed at higher particle concentrations. Boersma et al. [45] considered the shear thickening behavior of colloidal silica in glycerol and water at high volume fractions (0.50-0.60). They worked at low shear rates and determined that the critical shear rate was around 1-2 s-1 for a volume fraction of 0.48. Allen and Matijević [38] examined the effect of electrolytes on colloidal silica stability. They had studied at what salt concentrations and pH values, coagulation of the dispersions would be observed. Past work on the shear thickening behavior of colloidal silica dispersions included the use of large diameter particles, high solids concentrations, or the addition of other compounds such as glycerol to see shear thickening behavior. Our work focuses on smaller sized particles, lower solids concentrations, and higher shear rates to observe the shear thickening in the colloidal silica dispersions.

The destabilization of silica by electrolytes occurs through an ion exchange mechanism

(SiOH + Mn+ ↔ SiOM(n-1)+ + H+ where Mn+ is an ion of charge n+). With cation exchange, the silica surface changes from consisting of strongly hydrated silanol groups to one where both exchanged and unexchanged sites exist. The cations undergo a significant increase in the amount of ion exchange as the pH is increased above 6 [57]. Since the test slurries were at pH 9, well

20 above pH 6, the cations are strongly exchanged and the silica slurry coagulates more readily. The dehydration of the silica surface that supplements ion exchange is proposed to increase coagulation. Silica dehydration occurs as cations are exchanged on the silanol surface and silica loses a site for hydrogen bonding. As more molecules of strongly adsorbed water molecules are removed from the surface, an ion undergoes exchange [38]. This result is consistent with the results for the Na+, K+, and Cs+ ions which all show an onset of thickening (Table 2.1) at lower shear rates and a greater increase in viscosity corresponding to a greater degree of shear thickening than the no salt added slurry. The onset of thickening corresponds to the critical shear rate, which was defined to occur at a 15% increase in viscosity.

Table 2.1: Critical shear rate for small silica particle slurries with a weight percent of 30% and an added electrolyte concentration of 0.17 M.

-1 Slurry �!"#$#%&' (s ) No Salt 250,000 CsCl 200,000 NaCl 63,100 KCl 63,100 LiCl N/A

When LiCl is added to the silica slurry, no shear thickening is observed. No thickening may be attributed to the lubricating shell, also known as the hydration layer. When only water molecules surround the silica particles, at least one layer of water molecules adsorb to the silica surface [58]. When electrolytes are added, there is a prevalent hydration layer that counterions penetrate into at different depths (Figure 2.3). Larger, less hydrated ions such as Cs+ and K+ have a thinner and less effective lubricating shell. Smaller, more hydrated cations such as Li+ and Na+ have a thicker and more effective lubricating shell composed of water [58,59].

Figure 2.3: Schematic of the adsorption of Cs+, K+, Na+, and Li+ onto a hydrated silica surface. Small and more hydrated counterions (Na+ and Li+) penetrate deeper into the hydration layer of silica than larger, weakly hydrated cations (Cs+ and K+). This figure is an adaption of a graphic published in Colic et al. [3].

When ions have a thinner lubricating shell, it is easier for silica particles to overcome the hydration layer and form agglomerates. With smaller ions, the large lubrication shell inhibits silica particles from coming together and forming agglomerates. The interaction between silica surface plates separated by thin layers of electrolyte solutions was studied by Peschel et al. [60] to represent the silica-electrolyte system. It was determined that Li+ is less adsorbed on the silica surface through the charge regulation model. The strength of the hydration force decreases with increasing degree of hydration of the counterion [60,61].

2.5 Results - Varying silica concentration under constant salt concentration

Shear thickening was also examined while holding the salt concentration constant at 0.17

M and varying the silica concentration. Sodium chloride added slurries with silica concentrations of 30 wt%, 33 wt%, 36 wt%, and 38 wt% showed irreversible shear thickening while a silica concentration of 28 wt% did not display thickening (Figure 2.4). The viscosity of the 36 wt% and

38 wt% salt added slurries were lower than the other slurries which showed increases in viscosity with increasing silica concentration. The degree of shear thickening increased with increasing silica concentration (Figure 2.5). All slurries display thickening behavior with the exception of

28 wt% silica slurry.

Figure 2.4: Steady shear rate ramp (filled symbols) and reduction (open symbols) for a 28 wt% (green diamonds), 30 wt% (orange circles), 33 wt% (red diamonds), 36 wt% (black squares), and 38 wt% (blue triangles) silica slurry with an NaCl concentration of 0.17 M.

Figure 2.5: Viscosity of small silica particles at 28 wt%, 30 wt%, 33 wt%, 36 wt%, and 38 wt% with an added sodium chloride concentration of 0.17 M during shear rate ramp and reduction.

23 The degree of shear thickening is expected to increase with increasing silica concentration [62]. Since a greater number of silica particles are in solution, particles will create more aggregates leading to a larger increase in viscosity during the shear rate reduction step. At

28 wt% silica with 0.17 M NaCl added, the particles exhibit Newtonian behavior. At this concentration, a large concentration of SiO- groups on the silica surface exist along with a layer of Na+ counterions, which form an electrical double layer, EDL [62]. The EDL produces strong interparticle repulsions causing particles to not aggregate within the tested shear rates.

The viscosity decreases between the 33 wt% and 36 wt% silica salt added slurry, which may be connected to the structure of silica at higher concentrations. At greater silica concentrations, particles are more likely to arrange themselves into an ordered structure.

Matsuoka et al. [63] studied the structure of colloidal silica dispersions using small-angle x-ray scattering (SAXS) and De Kruif et al. [64] studied the structure using small-angle neutron scattering (SANS). They determined that a nonrandom, regular distribution of particles is observed at higher silica concentrations (40-50 vol%) for dispersions with no salt added. Silica particles will have a lower viscosity if the structure is layered since it is easier for particles to slide past each other [65].

2.6 Conclusions

Under high shear rates (>10,000 s−1), colloidal silica CMP slurries continuously and irreversibly thicken. The shear-induced thickening of these slurries can be adjusted through the addition of chloride salts (KCl, NaCl, CsCl, and LiCl). The onset of shear thickening occurred earlier for KCl, NaCl, and CsCl added slurries while LiCl added slurries showed no thickening behavior. The slurries follow the ion hydration theory (less hydrated ions cause slurry to thicken at lower shear rates) except slurry with added CsCl, which thickened at a higher shear rate.

24 Small, more hydrated cations (Na+ and Li+) penetrate deeper into the hydration layer of silica than larger, less hydrated cations (Cs+ and K+). The limitation of the reach of poorly hydrated cations into silica’s hydration layer increases the repulsive forces.

When the silica concentration is changed while holding the salt concentration constant, the thickening behavior changes depending on the concentration. Silica concentrations of 30 wt%, 33 wt%, 36 wt%, and 38 wt% showed irreversible shear thickening while a silica concentration of 28 wt% did not display thickening. The viscosity of the 36 wt% and 38 wt% salt added slurries were lower than the other slurries which showed increases in viscosity with increasing silica concentration. The electrical double layer produces strong interparticle repulsions affecting the aggregation of the tested slurries along with the structure of silica particles in the dispersion.

The irreversible thickening behavior exhibited by the colloidal silica slurries is a combination of silica surface hydration, repulsive forces, and hydrodynamic forces. Adding salts to the slurry reduces the electrostatic and hydration repulsions affecting the shear thickening behavior and stability of silica. The combination of forces and the structure of the silica particles in the slurry alter the shear thickening behavior of the colloidal silica slurries.

CHAPTER 3

PARTICLE SIZE AND pH IMPACT ON SHEAR THICKENING OF COLLOIDAL SILICA

CMP SLURRIES

Colloidal silica slurries exhibit continuous shear thickening behavior under high shear conditions. The pH of the slurries and the silica particle size were varied to determine the effect on the viscosity and thickening behavior. This chapter focuses on the impact that pH and particle size has on the rheology of the colloidal silica slurry.

3.1 Abstract

The shear thickening behavior of colloidal silica slurries was studied to determine the effect of altering the pH of the slurry and the silica particle size. Small and large colloidal silica particle slurry was studied from pH values of 4 to 10.5. Both small and large particle slurries show continuous shear thickening behavior until pH 10 while no thickening behavior is observed at pH 10.5, which is due to the electrostatic repulsion forces and the stability of silica at high pH values. The shear thickening behavior was also studied for mixtures of different ratios of large and small colloidal silica particles. The pure small particle dispersion displays a greater degree of shear thickening and a larger viscosity than the pure large particle dispersion. The structure of silica and the hydrodynamic and electrostatic repulsion forces affect the shear thickening behavior of the slurries under the conditions described.

3.2 Introduction

26 “hydroclusters.” The clusters are composed of groups of particles that are formed when shear forces bring the particles almost into contact with each other. Lubrication forces cause the observed increase in viscosity and hard-sphere suspensions thicken without ordering

[43,46,47,48]. Irreversible shear thickening can be caused by shear-induced bridging where the polymer bridge will be adsorbed on two or more particles simultaneously [49]. Shear induced gels are another example of irreversible shear thickening. Gelation occurs if a connected network is formed by particle aggregation [50]. Few publications examine shear-induced flocs or gels without the use of polymer, which will be studied here.

Changing the pH of the colloidal silica slurries will change the shear thickening behavior of the dispersions. In the range of pH 3-5, the rate of aggregation increases with pH. The stability of the dispersion decreases with increasing pH due to the increased concentration of hydroxyl ions. Above pH 6, the rate of aggregation decreases with increasing pH due to an increasing charge on the particles. In the range of pH 8-10, the dispersions are stable with very slow aggregation of the particles since particles can no longer come together close enough to form interparticle siloxane bridges [39,66].

The viscosity of colloidal dispersions is influenced by particle size. In dispersions with particles of more than one size, smaller particles can fit into spaces between larger particles, resulting in more efficient packing. This packing structure can also act as a lubricant for large particles. The non-Newtonian behavior of the dispersions is caused by a combination of viscous forces, Brownian motion of the particles, and colloidal forces of interaction between the particles. Viscous forces will increase with increasing shear rate and particle size while colloidal forces decrease with increasing particle size and shear rate [67,68]. Therefore, the viscosity will vary depending on the particle size distribution.

27 3.3 Experimental Methods

For this study, small and large colloidal silica particle slurry (Cabot Microelectronics

Corporation, Aurora, IL) was used. We examined colloidal silica particle slurries that were adjusted with NaOH to a pH value of 9. The supplied slurry had a silica concentration of 33.7 wt% for the small particle slurry (d=20 nm) and a silica concentration of 42.2 wt% for the large particle slurry (d=100 nm). For pH adjustments, NaOH was used to increase pH and HCl was used to decrease pH along with the dilution of slurries to a silica concentration of 33 wt%.

Mixtures of varying ratios of small to large particle slurries were prepared on a volume basis.

Zeta potential measurements were conducted using Brookhaven Instruments’ (Holtsville,

NY) ZetaPALS. The ZetaPALS instrument uses light scattering to determine the electrophoretic mobility of charged, colloidal suspensions with an accuracy of ± 2%. The samples had to be diluted, using a solution of the same pH value as the samples being tested, to a particle concentration of 0.01 vol% or less. Conductivity measurements were performed using the Fisher

Scientific accumet Basic AB30 Conductivity Meter with a measurement accuracy of ± 0.5%.

Rheological measurements were performed using TA Instruments’ AR-G2 rheometer

±0.1°C provided by a Peltier plate.

Shear thickening behavior was examined using a steady shear rate ramp procedure. Shear rates ranging form 1,000 s-1 to 200,000 s-1 were studied. At higher shear rates, data resolution was increased to determine where thickening begins. A shear rate reduction step immediately follows the shear rate ramp step to test the reversibility of shear thickening.

28 3.4 Results - Shear thickening behavior examined in a range of pH values

Small and large colloidal silica particle slurry was studied from pH values of 4 to 10.5

(Figure 3.1 and Figure 3.2). Small and large particle slurries show continuous shear thickening behavior until pH 10 while no thickening behavior is observed at pH 10.5 (Figure 3.3 and Figure

3.4). The viscosity for the small particle slurry increases with increasing pH until the value reaches pH 10 where the viscosity decreases with increasing pH. For the large particle slurry, viscosity increases with increasing pH until the value reaches pH 8 where the viscosity declines with increasing pH.

Figure 3.1: Steady shear rate ramp (filled symbols) and reduction (open symbols) for a 33 wt% small particle silica slurry pH adjusted with NaOH or HCl to pH 4 (black squares), pH 7 (blue diamonds), pH 9 (orange triangles), pH 10 (green circles), and pH 10.5 (red triangles).

Figure 3.2: Steady shear rate ramp (filled symbols) and reduction (open symbols) for a 33 wt% large particle silica slurry pH adjusted with NaOH or HCl to pH 4 (black squares), pH 7 (blue diamonds), pH 8 (orange triangles), pH 10 (green circles), and pH 10.5 (red triangles).

Figure 3.3: Viscosity of small silica particles (33 wt%) pH adjusted with NaOH or HCl to pH values of 4, 7, 9, 10, and 10.5 during shear rate ramp and reduction.

Figure 3.4: Viscosity of large silica particles (33 wt%) pH adjusted with NaOH or HCl to pH values of 4, 7, 8, 10, and 10.5 during shear rate ramp and reduction.

The zeta potential (ζ) examines the electrical double layer forces between particles in suspension, as both the potential and charge density are dependent on pH. The measured zeta potentials range from -38 mV to -49 mV for large colloidal silica particles dispersions and from

-30 mV to -41 mV for small particle colloidal silica dispersions, across a pH range of 4 to 10.5

(Table 3.1). Large particle slurries have more negative zeta potentials than small particle slurries, corresponding to a greater stability. Conductivity measurements were also performed along with the zeta potential measurements (Table 3.1). For large particle slurries, conductivity values ranged from 190 µS/cm to 360 µS/cm and from 8,000 µS/cm to 10,000 µS/cm for small particle slurries. Small particle slurries show greater conductivity than large particles due to their greater concentration of surface hydroxyl groups. Since small particles will have a greater surface area, more hydroxyl groups can attach to the surface and the concentration of hydroxyl groups is proportional to conductivity [69,70].

31 Table 3.1: Conductivity and zeta potential values for small and large particles at 33 wt% for a range of pH values.

Conductivity Zeta Potential pH (µS/cm) (mV) Large Particles 4 270 -48 5 290 -48 6 290 -46 7 260 -44 8 240 -49 9 190 -49 10 340 -38 10.5 360 -38 Small Particles 4 10,000 -34 7 9,900 -31 9 9,800 -30 10 8,300 -40 10.5 8,000 -41

Colloidal silica dispersions typically show stability from pH 4 to pH 10. At pH > 9, the ionic strength is a lot greater than at lower pH values. With a greater ionic strength, little electrostatic repulsion exists to limit the close approach of the two colloidal silica particles leading to faster coagulation [39]. For both small and large particles, the degree of shear thickening is greatest for pH 10 corresponding to the quicker coagulation of the particles. The stability of silica solutions decreases around pH 10.5 where silica begins to dissolve [37,66].

Both slurries showed no thickening at pH 10.5, which is likely an effect of the dissolution of silica. The dissolution rates of silica strongly increase at higher pH values so particles will dissolve and particle sizes will decrease leading to no formation of aggregates [37].

For all tested pH values, small particle slurries show higher viscosities as compared to the large particle slurries at the same volume fraction and both slurries show a viscosity decrease.

The viscosity decreases for large particle slurries at pH 8 and for small particle slurries at pH 10.

This difference is due to electrostatic repulsions between the particles [68]. The large particles

32 can move more freely at pH 8 so the viscosity decreases but small particles do not move more freely until pH 10.

3.5 Results - Mixtures of small and large colloidal silica particle slurries

The shear thickening behavior changes noticeably by mixing different ratios of large and small particles (Figure 3.5). The pure small particle dispersion displays a greater degree of shear thickening and a larger viscosity than the pure large particle dispersion (Figure 3.6). Mixtures with a majority of large particles (25S-75L and 40S-60L) demonstrated little or no shear thickening. The greatest amount of hysteresis was seen for the 75S-25L mixture.

Figure 3.5: Steady shear rate ramp (filled symbols) and reduction (open symbols) for mixtures of small (d = 20 nm) and large (d = 100 nm) silica particles at a constant overall volume percent (23%) and a pH value of 9.

Figure 3.6: Viscosity of mixtures of small (d = 20 nm) and large (d = 100 nm) silica particles during shear rate ramp and reduction at a constant volume percent (23%) and a pH value of 9.

Zeta potential and conductivity values were measured for mixtures of small and large colloidal silica particles (Table 3.2). Zeta potential values for the mixtures range from – 30 mV to – 49 mV. Pure small particle dispersions are less stable than pure large particle dispersions so they will show faster aggregation and greater shear thickening. Conductivity values increase almost linearly from 190 µS/cm for pure large particle dispersions to 9,800 µS/cm for pure small particle dispersions.

The viscosity is greater for the pure small particle dispersion than the large particle dispersion due to the effect of the electrical double layer where ionic clouds around the particle begin to overlap. Smaller particles have a greater surface area as compared to large particles at the same volume fraction so the ionic cloud overlap occurs earlier for smaller particles.

Hydrodynamic forces also influence the flow behavior of the dispersions more at high shear rates

[68]. The result is an increase in viscosity with a decrease in particle size.

34 Table 3.2: Conductivity and zeta potential values for mixtures of ratios to small and large particles at a constant volume percent (23%) and a pH value of 9.

Conductivity Zeta Potential Mixture (µS/cm) (mV) Large 190 -49 10S-90L 160 -42 25S-75L 530 -38 30S-70L 530 -45 40S-60L 660 -40 45S-55L 810 -42 50S-50L 780 -43 60S-40L 1,300 -44 75S-25L 1,900 -46 90S-10L 5,500 -42 Small 9,800 -30

Blends of different sized particles produce dispersions with a lower viscosity than one with only monosized particles [71]. A minimum in the viscosity of mixtures is observed for the

40S-60L mixture so the resistance to flow can be decreased by using a distribution of varying particle sizes, which is likely due to more efficient packing of the spheres, i.e., small spheres fit into the spaces between large spheres (Figure 3.7). The effect of colloidal forces of interaction between the particles on the viscosity of the suspension is strongly dependent on the size of the particles. Dispersions with a larger particle majority have lower viscosities than dispersions with a smaller particle majority [67,68]. The highest degree of shear thickening was observed in the

75S-25L mixture which could be caused by the structure of the particles. More small particles can fit between the large particles, which could lead to a greater degree of shear thickening.

The mixtures are expected to shear thicken at shear rates that are between those that are determined for the pure components [56]. The onset of shear thickening for the large particle dispersion was at 100,000 s-1 and for the small particle dispersion was at 200,000 s-1. All

35 mixtures began thickening at shear rates that were either the same as the pure component dispersions or in between those shear rates.

a) b)

Figure 3.7: Schematic representation of how small particle spheres fit within the spaces between large particle spheres (a) and a TEM image of a mixture or small and large particle spheres (b).

3.6 Conclusions

Small and large colloidal silica particle slurry was studied from pH values of 4 to 10.5 to determine the effect pH has on the shear thickening behavior of colloidal silica CMP slurries.

Small and large particle slurries show continuous shear thickening behavior until pH 10 while no thickening behavior is observed at pH 10.5. Through the measurement of zeta potential and conductivity, it is determined that large particle slurries are more stable than small particle stability. When slurries exhibit greater ionic strength, little electrostatic repulsion occurs to limit the close approach of the two colloidal silica particles, which will cause particles to aggregate faster and is the main force in play when adjusting pH values of the slurry. Dissolution of silica is a concern at higher pH values, which can be linked to the lack of thickening behavior at those values.

The shear thickening behavior changes noticeably by mixing different ratios of large and small particles. From zeta potential and conductivity measurements, it is determined that pure small particle dispersions are less stable than pure large particle dispersions so they will show

36 faster aggregation and greater shear thickening. Dispersions with a larger particle majority have lower viscosities than dispersions with a smaller particle majority. This is the result of more efficient packing of the spheres where small spheres fit into the spaces between large spheres.

The electrical double layer and hydrodynamic forces influence the shear thickening behavior of pH-adjusted slurries. For mixtures of small and large colloidal silica slurries, the colloidal forces of interaction between the particles have the strongest effect on the behavior of these slurries. The viscosity of the suspension is strongly dependent on the size of the particles.

The interplay of these forces determines the shear-induced thickening of the colloidal silica CMP slurries.

CHAPTER 4

CONCLUSIONS AND RECOMMENDATIONS

This chapter outlines the major findings of this work and provides recommendations for future work.

4.1 Conclusions

High shear rheology was performed on colloidal silica CMP slurries using a commercial rheometer with parallel-plate geometry. Shear thickening behavior of the slurries was examined at shear rates >100,000 s-1. The slurries show continuous and irreversible thickening behavior, which was altered by the addition of electrolytes, by pH adjustment of the slurries, and through the mixture of small and large colloidal silica particles.

The addition of electrolytes to the colloidal silica slurries decreases the shear rate at which the onset of thickening occurs. The exception is LiCl added slurries, which showed no thickening behavior in examined shear rate range. Less hydrated ions cause slurries to thicken at lower shear rates with the exception of the CsCl added slurry, which thickened at a higher shear rate. The electrical double layer produces strong interparticle repulsions and the structure of silica particles affects the aggregation of the tested slurries.

Small and large colloidal silica particle slurries were examined under high shear. When these slurries were pH adjusted, they showed continuous shear thickening behavior until pH 10 and no thickening behavior was observed at pH 10.5. At higher pH values, there is concern about the dissolution of silica that would affect the stability of the slurry. The shear thickening behavior of mixtures of small and large colloidal silica particle slurries were also examined and it was determined that the viscosity of the suspension is strongly dependent on the size of the

38 particles. Dispersions with a larger particle majority have lower viscosities than dispersions with a smaller particle majority. The hydrodynamic and colloidal forces of interaction influence the shear thickening behavior of the silica slurries.

4.2 Recommendations

A range of bimodal mixtures can be studied to further examine the effect particle size has on shear thickening behavior. Mixtures of smaller colloidal silica particles (< 30 nm) can be mixed with larger particles (> 300 nm) such as ceria or alumina. Alumina and ceria slurries are also commonly used during chemical mechanical polishing processes. The effect of pH on these mixtures can also be studied to determine the optimal pH for slurry stability and limited particle aggregation. Performing rheological measurements and particle characterization on bimodal mixtures of these particles can provide insight into creating slurries with a desired material removal rate.

The structure changes of the colloidal silica particles during and after thickening should be examined. Rheo-SALS measurements can be performed to monitor the evolution of flow- induced structures linked to shear thickening. The images can be analyzed for the alignment and size of the flow-induced structures. The flow-induced structures can be studied as a function of shear rate and temperature within a range of bimodal colloidal mixtures. The minimum shear rate to observe structure formation can be studied using rheo-scattering measurements and flow- induced structures can be examined. In-situ rheo-SALS can be combined with ex-situ measurements such as dynamic light scattering (DLS) to fully examine the flow-induced structures formed during thickening. These imaging and particle characterization techniques can be used on the bimodal silica-silica mixtures examined previously along with the proposed silica-ceria and silica-alumina systems.

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