Sustainable Recycling and Improved Stability of Ceria Slurry with Polyacrylic Acid for Enhanced Glass Polishing Performance

Enhancing dispersion stability and recyclability of ceria slurry with polyacrylic acid for improved glass polishing performance

Ceria (CeO2) was demonstrated to have the highest polishing rate of glass than the other ceramic oxides by Cook [1] in 1990. Since then CeO2 particle was increasingly applied as the abrasive in chemical mechanical planarization (CMP) of the SiO2 and Si3N4 based dielectric materials. At present, the share of the CeO2 slurry has grown to the second largest part in the CMP slurry market just next to the silica slurry. However, the price of precursor of CeO2 synthesis, as one of the rare earth materials, rose significantly as long as the export quota was restricted for the political policy by the rare earth material exporting countries [2]. This rise in price would have direct influence on the cost of ownership and the profit margin of the semiconductor manufacturing companies. Therefore, an alternative is required to totally or partially replace the CeO2 slurry in the present dielectric CMP application. ZrO2 and TiO2 had the second and third high polishing rates in the Cook’s report. However, the extremely high mechanical hardness of ZrO2 prohibited its potential application in the dielectric CMP due to the concern of producing scratches. TiO2 has the moderate mechanical hardness and applied in large number of applications including food, cosmetic, catalyst, and paint [3]. Therefore, the annual production capacity and the price would be more stable than that of the rate earth materials. For this reason, we investigated the SiO2 CMP using the TiO2particle in the study. Figure 1 shows the Transmission Electron Microscopy of the TiO2 particle used in the study. We used commercially available TiO2(P25, Degussa, Germany) nanoparticles. Degussa P25 consists of two crystal forms of rutile and anatase, and is widely used in photocatalytic degradation studies, because of its chemical stability, ready availability, reproducibility, and activity as a catalyst in oxidation processes [4]. Figure 2 indicates the polishing rate of SiO2 film in the TiO2 slurry as a function of the added polyacrylic acid (PAA) concentration. The polishing rate was below 200 Å/min in the TiO2 slurry. However, it increased suddenly approximately to 700 Å/min as the PAA concentration increased to 0.025 wt%. Surprisingly, the polishing rate abruptly dropped below 100 Å/min as the PAA concentration increased by 0.005 wt% to 0.03 wt%. The dispersion stability of the TiO2slurry was good as long as the PAA concentration was over 0.02 wt%. Therefore, the poor dispersion stability can explain the low polishing rate at the PAA concentration that was lower than that of the critical point. However, the dispersion stability was failed to explain the abrupt decrease in the polishing rate over 0.03 wt% of PAA. Figure 3 shows the viscosity of the TiO2 slurry as a function of the PAA concentration. Each slurry shows the shear thining behavior as the sear rate increases due to the structural rearrange of the adsorbed PAA on the TiO2 particle. The magnitude of the viscosity shows a totally inverse trend as that of the polishing rate as a function of PAA concentration. The adsorbed PAA on the TiO2particle probably undergoes a structural change as the PAA concentration over the critical concentration of 0.025 wt%. Therefore, the abruptly decreased polishing rate would be resulted from this structural change. The TiO2 slurry showed a moderate polishing rate of SiO2 with PAA only in a very narrow concentration range. Therefore, we need more investigation to understand the CMP mechanism of the TiO2 slurry and find out a more effective additive to increase the polishing rate of SiO2 in order to satisfy at least the same polishing performance as the CeO2 slurry.

To develop a novel method for the preparation of thiolated polyacrylic acid nanoparticles via ionic gelation. In a first step nanoparticles were generated by ionotropic gelation of polyacrylic acid (PAA) of three different molecular weights (100, 240 and 450 kDa) and various cations including Ca2+, Mg2+, Zn2+, Al3+ and Fe3+. Via in vitro characterization of the particles (particle size, size distribution and zeta potential) the optimal preparation conditions were established. Taking into consideration, that thiolated polyacrylic acid (PAA-Cys) displays higher mucoadhesive and permeation enhancing properties than unmodified PAA, PAA-Cys nanoparticles were produced in the same manner with Ca2+, as the most promising results concerning particle size and stability of particles could be achieved with this ionic crosslinker. The nanoparticles were stabilized via the formation of inter- and intrachain disulfide bonds within these particles due to oxidation with H2O2. Ca2+ was removed proximately by the addition of EDTA and exhaustive dialysis. Using the preparation method described above PAA-Cys nanoparticles of a mean diameter of about 220 nm (PAA(100)-Cys), 250 nm (PAA(240)-Cys) and 295 nm (PAA(450)-Cys) can be generated. In comparison to PAA nanoparticles ionically crosslinked with Ca2+, the removal of the crosslinker Ca2+ from PAA-Cys particles led to a nearly three-fold decrease in the zeta potential, from about -7 up to -20 mV. Apart from this advantage, covalently crosslinked PAA-Cys nanoparticles were more firm as they remained stable when incubated in hydrochloride solution, whereas ionically crosslinked particles dissolved at pH lower than 5. This novel nanoparticulate delivery system seems to be a promising vehicle for the administration of therapeutic proteins, genes and antigens via mucosal membranes.

Interpolyelectrolyte complexes or polyplexes can be seen as interesting alternatives in the purpose of active ingredients encapsulation. Working on polymethylmethacrylate derivatives with special focus on controlled oral drug delivery, the influence of charged polyelectrolytes (polyacrylic acid, polyethylenimine, and amino‐dextran) and noncharged ones (polyvinyl alcohol, dextran 40, and Pluronic F68) has been investigated on the precipitation of two pH‐sensitive Eudragit polymers, namely, L100 and E100. Moreover, the possibility of preparing polyplexes involving the two polymethylmethacrylate derivatives with different charged and noncharged secondary polyelectrolytes has been studied. The obtained dispersions have been characterized in terms of mean particle size, size distribution, zeta potential, and morphology. Direct precipitation of Eudragit L100 by medium acidification in a batch process and in the presence of polyethylenimine allowed the production of particles with a narrow size distribution. The mean size was around 200 nm. In this case, the zeta potential was found to be +45 mV at pH = 7 in 1mM aqueous NaCl solution, and the produced suspension was stable in time since no aggregation and sedimentation have been observed. A precipitation pH of 8.16 allows us to suggest the preparation of a polyplex based on Eudragit L100 and polyethylenimine. In contrary, polyvinyl alcohol has shown ability to induce an increase in particle mean size whereas other polyelectrolytes showed no significant effect. Moreover, it was observed that polyethylenimine and polyacrylic acid solutions were able to directly induce Eudragit E100 precipitation whereas amino‐dextran and noncharged polyelectrolytes showed no effect on its precipitation and on particle size distribution.

Chemical mechanical polishing (CMP) slurries formulated of cerium oxide nanoparticles (CNPs) and polyacrylic acid (PAA) are widely used in shallow trench isolation (STI) application. The critical parameter in slurry formulation is the amount of PAA which determines the extent of its adsorption on CNPs and the excess amount. The adsorption of PAA on CNPs results in a softer surface and prevents defects such as scratches on the wafer surface. The excess PAA selectively adsorbs on silicon nitride, slowing down its etching as compared to silicon dioxide.1 Therefore, the understanding of PAA adsorption on CNPs in a slurry formulation is critical to predict the CMP slurry characteristics. In previous studies the adsorption of PAA on CNPs has been widely studied;2 however there is still a lack of understanding between adsorption behavior of PAA and the interfacial properties of CNPs. Here, we have systematically investigated the PAA adsorption on ceria slurries and correlating it to the CNPs interfacial properties. Three different commercial ceria slurries were obtained having similar hydrodynamic size, but different surface area as determined using BET. The catalytic properties of CNPs is assessed from the ratio of Ce3+/Ce4+ on its surface, determined using XPS.3 The surface functionalization of CNPs is evaluated from FTIR and NMR depicting three different types of organic moieties resulting in different zeta potentials. Lattice strain and grain size are another important parameter which signifies the internal energy of CNPs. These parameters were calculated using Williamson-Hall plots obtained from the XRD curve. The adsorption isotherm of PAA on CNPs is determined by measuring the carbon content in the supernatant separated from the CNPs using centrifugation. The carbon content is measured using TOC analyzer and the amount of PAA adsorbed on CNPs is calculated by mass balance equation. 1. Ronay, M., Selective polishing with slurries containing polyelectrolytes. Google Patents: 2005. 2. (a) Saraf, S.; Neal, C. J.; Das, S.; Barkam, S.; McCormack, R.; Seal, S., Understanding the Adsorption Interface of Polyelectrolyte Coating on Redox Active Nanoparticles Using Soft Particle Electrokinetics and Its Biological Activity. ACS applied materials & interfaces 2014, 6 (8), 5472-5482; (b) Kim, S.; So, J.-H.; Lee, D.-J.; Yang, S.-M., Adsorption behavior of anionic polyelectrolyte for chemical mechanical polishing (CMP). Journal of colloid and interface science 2008, 319 (1), 48-52. 3. Pirmohamed, T.; Dowding, J. M.; Singh, S.; Wasserman, B.; Heckert, E.; Karakoti, A. S.; King, J. E.; Seal, S.; Self, W. T., Nanoceria exhibit redox state-dependent catalase mimetic activity. Chemical communications 2010, 46 (16), 2736-2738.

As the semiconductor device is shrinking and gets complicated, the chemical mechanical polishing (CMP) process has become important. Various types of abrasive particles are used as CMP slurry, and among them, ceria slurry is in the spotlight due to its high material removal rate (MRR) and selectivity on oxide CMP process. It is known that the ceria abrasives form Ce-O-Si chemical bonds with oxide film, which makes it possible to achieve a high CMP performance.To further advance the performance of ceria slurry, we conducted a series of researches on ceria. A study was conducted on scanning mobility particle sizer (SMPS) measurement to accurately evaluate particle size distribution along with the basic polishing mechanism of monodisperse ceria nanoparticles. In addition, an optimization study was conducted considering the CMP performance according to the abrasive concentration of ceria slurry and the influence of ceria nanoparticles remaining on the wafer surface before the post-CMP cleaning process. Furthermore, research on synthesis method and surface modification of ceria was also conducted to improve the polishing performance. Ce3+ ions on the surface of ceria form Ce-O-Si chemical bond with the silicon dioxide film. Therefore, an important key point is to increase the concentration of Ce3+ ions to improve polishing performance. To achieve high Ce3+ ion concentration, ceria nanoparticles were synthesized through a hydrothermal method, and this performance was improved through lanthanide doping. On the other hand, methods to enhance Ce3+ ion concentration by inducing a reduction reaction on the surface of ceria nanoparticles were also proposed. First, the polishing performance was improved by adding trace metals (FeCl2, CrCl2) to induce a reduction reaction of ceria nanoparticles. Next, the reduction reaction of ceria was induced by applying an eco-friendly method which did not add chemical agents. Ultraviolet (UV) irradiation and hydrogen-gas reduction methods showed sufficient reduction reaction to improve the polishing performance of ceria slurry, and maintained physical properties without causing CMP defects such as poor surface roughness and micro-scratches.As with previous polishing studies, post-CMP cleaning is also important to decease defects. While ceria nanoparticles exhibit great polishing performance for silicon dioxide due to their ability to form chemical bonds, however, they also have the disadvantage of being difficult to remove during the post-CMP cleaning process. To efficiently remove ceria nanoparticles from the wafer surface during the post-CMP cleaning process, various methods have been studied. First, the buff cleaning process was studied in which a deionized water-based cleaning solution is injected instead of slurry after the CMP process. Since the buff cleaning process must simultaneously minimize damage of wafer surface and maximize cleaning performance, optimization research on the properties of the physical factors and cleaning solution is important. For an environmentally friendly cleaning method, we proposed controlling the viscosity of deionized water or using gas-dissolved water. It has been proven that physical factors of cleaning solutions (viscosity, micro bubbles, etc.) can be effective in removing ceria nanoparticles. Especially, hydrogen gas in gas-dissolved water induced a reduction reaction, breaking Ce-O-Si bonds and improving the cleaning performance. Limited to the bulk ceria CMP process, we also studied the improvement of cleaning performance of remained ceria nanoparticles using the tangential flow filtration (TFF) method before CMP process. We focused that small contaminant particles are difficult to remove during the cleaning process, and through the TFF method, small abrasive particles in the slurry are filtered in advance while maintaining the polishing performance.

An increase in the removal selectivity between silicon oxide and silicon nitride was attempted by adding organic additives to a ceria slurry for the application of shallow trench isolation (STI) chemical mechanical planarization (CMP). The protection behavior of poly(acrylic acid) (PAA) and the acceleration behavior of RE-610 in a ceria slurry were studied. PAA served as a protector of the silicon nitride due to the change in zeta potential. RE-610 worked as a hydration accelerator of the silicon oxide. When the two additives were added to the ceria slurry, the removal selectivity increased to 31:1. Moreover, PAA improved the stability of the ceria slurry.

Influence of anionic polyelectrolyte addition on ceria dispersion behavior for quartz chemical mechanical polishing

The present study aims to develop a pH thermosensitive nanocarriers as a drug delivery system to better controll drug release. Nanoparticles was developed by the combination of smart polymers, chitosan and poly(acrylic acid) were chosen as biodegradable vectors to encapsulate and transport the drug. The used method was based on the polymerization of acrylic acid using reticulated chitosan as a template. Analysis of particle size, Zeta potential, and size distribution revealed that most of the resulting nanoparticles had an average diameter less than 100nm, with a high Zeta potentiel about -29.7 mV and a narrow size distribution. In addition, the developed system showed an encapsulation efficiency around 97%. In vitro release test was achieved using different buffer solutions with pH equal to 1.2, 3.6, 4.2, 4.8, 6.8 and 7.4. The release profiles showed that nanoparticles provide drug protection at different pH values. They responded at pH = 3.6 and provided sustained controlled release of up to 62.62% over 8 hours. The results reveal that the prepared nanoparticles can be used as drug delivery carriers. They can improve therapeutic efficiency of the drugs used in the treatment of inflamed tissues where the pH is around 3.6 as in the Crohn disease.

Mucus permeating thiomer nanoparticles

The effects of cationic and anionic surfactants and polyelectrolytes, as well as their binary mixtures, on the electrokinetic potential and the degree of aggregation/sizes of particle flocs in bentonite suspensions have been studied. The amounts of the above reagents, compositions and ratios of positive and negative charges in the mixtures, charge densities of the polyelectrolytes, and the order of the introduction of the components into systems have been varied. It has been shown that the addition of cationic surfactants or polyelectrolytes and mixtures thereof causes a substantial reduction of the negative ζ-potentials of the particles and reversal of the sign of their charge. The addition of anionic surfactants or polyelectrolytes leads to a twofold increase in the ζ-potential as a result of adsorption of negatively charged surfactant ions/polymer segments on negatively charged clay particles via non-Coulomb forces. In the presence of mixtures of anionic and cationic polyelectrolytes, irrespective of their composition and the order of the introduction of the components into a suspension, the particles acquire a negative ζ-potential, which is characteristic of particles that adsorb an anionic polyelectrolyte alone. Synergistic flocculating action is observed for mixtures of weakly and moderately charged anionic and cationic polyelectrolytes. This effect takes place even for mixtures with a manifold excess of negatively charged segments relative to positively charged ones. The observed regularities have been explained by the structural peculiarities of mixed adsorption layers of polyelectrolytes, i.e., the coexistence of a thin layer of adsorbed cationic polymer chains characterized by a large number of contacts with the surface and an extended layer of an anionic polyelectrolyte (long loops and tails of macromolecules), in which the cationic polymer layer is “drowned.” The electrokinetic potential and the ability of the particles to aggregation (via the formation of polymer bridges) in mixed solutions of anionic and cationic polyelectrolytes are primarily determined by the adsorption value of the anionic polymer.

Non-cementitious grouts have been tested in Olkiluoto for the sealing of fractures with the small hydraulic apertures. A promising non-cementitious inorganic grout material for sealing the fractures with the apertures less than 0.05 mm is commercial colloidal silica called silica sol. The potential relevance of colloid-mediated radionuclide transport is highly dependent on their stability in different geochemical environments. The objective of this work was to follow stability of silica sol colloids in low salinity Allard and saline OLSO reference groundwater (pH 7–11) and in deionized milliQ water. Stability of silica sol colloids was followed by measuring particle size distribution, zeta potential, colloidal and reactive silica concentrations. The particle size distributions were determined applying the dynamic light scattering (DLS) method and zeta potential based on dynamic electrophoretic mobility. The colloidal silica concentration was calculated from DLS measurements applying a calibration using a standard series of silica sol. Dissolved reactive silica concentration was determined using the molybdate blue (MoO4) method.These results confirmed that the stability of silica colloids dependent significantly on groundwater salinity. In deionized water, particle size distribution and zeta potential was rather stable except the most diluted solution. In low salinity Allard, particle size distribution was rather constant and the mean particle diameter remained less than 100 nm. High negative zeta potential values indicated the existence of stable silica colloids. In saline OLSO, particle size distribution was wide from a nanometer scale to thousands of nanometers. The disappearance of large particles, decrease in colloidal particle concentration and zeta potential near zero suggest flocculation or coagulation. Under prevailing saline groundwater conditions in Olkiluoto silica colloids released from silica sol are expected to be instable but the possible influence of low salinity glacial melt water has to be considered.

The influence of temperature and the effect of the purity of polyacrylic acid (PAA) on its conformation on the zirconium oxide surface were studied. Spectrophotometric measurements, potentiometric titrations, electro-kinetic and viscosity measurements allowed the determination of the amounts of polymer adsorbed, the surface charge and the zeta potential of solid particles in the absence and presence of the polymer together with the thickness of the polymer adsorption layers. All measurements were carried out over the temperature range 15—40°C. The occurrence of a minimum in the adsorption of PAA between 25–30°C was caused by conformational changes of the polymer macromolecules. For non-filtered PAA, a decrease in polymer adsorption and an increase in the thickness of the adsorbed polymer layers were observed. These result from changes in the polydispersity index of filtered PAA and the blocking of some active sites on the oxide surface by inorganic contamination present in the non-filtered PAA samples. The free energy of PAA adsorption was calculated from the zeta potential data. Some parameters characterizing the conformation of PAA chains in the bulk liquid phase and on the ZrO2 surface were determined.

The effect of aging and selective sedimentation of ceria slurry on the slurry characteristics, such as the particle size distribution, the large-particle count, and the dispersion stability, and on the number of defects on the wafer surface during the shallow trench isolation (STI) chemical mechanical planarization (CMP) process was investigated. The aging time and temperature were considered as important variables giving great influence on the slurry characteristics and STI–CMP. By applying aging and selective sedimentation, the number of large particles produced by either strong agglomeration or soft flocculation in ceria slurry was successfully reduced, and the dispersion stability of the ceria particles was also improved. A CMP evaluation demonstrated that the number of defects on the wafer surface was significantly reduced, while maintaining the reasonable oxide removal rate and oxide-nitride selectivity.

To improve the performance of shallow trench isolation chemical–mechanical polishing (STI-CMP) in terms of the removal selectivity of oxide and nitride films and the formation of surface defects, we investigated the effects of the calcination and milling process conditions during ceria slurry synthesis. We have focused on the effects of particle size distribution, the large-particle size, and the dispersion stability in a ceria slurry. We determined the optimum bead size for milling and appropriate calcination temperatures in order to obtain a reasonable particle distribution, with lower numbers of fine primary particles and large, agglomerated particles, in ceria slurry. This was achieved by reducing the quantity of aggregated particles during milling and two-step calcination process generating higher-density particles. These results can be qualitatively explained by abrasive collisions occurring between the milling beads and the decarbonation of cerium carbonate through diffusion during the manufacturing process used for the ceria slurry.