Ceria particles

Relaxations in the double layers between two interacting particles can retard aggregation rates and cause them to be independent of particle size [101-103]. Discrepancies between theoretical predictions and experimental observations of heterocoagulation between polymer latices, silica particles, and ceria particles [104] have promptetl Mati-jevic and co-workers to propose that the charge on these particles may not be uniformly distributed over the surface [105, 106]. Similar behavior has been seen in the heterocoagulation of cationic and anionic polymer latices [107]. 

Nevertheless, the oxalate coprecipitation method has some problems. For example, this method usually results in rodlike doped ceria particles, which are agglomerations of smaller particles with irregular shapes. Hence, the green density of the compact body is relatively low, so it is difficult to fabricate a dense electrolyte film or membrane. In addition, the poor flow of the rodlike powder makes forming difficult. 

The importance of the support in the gold-catalysed liquid-phase oxidation of n-heptanal at 323 K has been emphasised by use of variously prepared ceria as supports.78 Very small ceria particles, in samples described as either nano- or meso-structured,79 make much better supports in terms both of activity and selectivity, than one prepared conventionally. The most active Au/Ce02 gave 100% conversion in about 3h, and 95% selectivity aromatic aldehydes were also oxidised successfully. 

Silicate ions of varying concentrations were added to ceria dispersions in order to study the electrostatic interaction between ceria particles and silicate ions. It was observed that the zeta potential of ceria becomes less positive or more negative. With an increase in Na2Si03 concentration, the isoelectric point shifts to lower pH values. At high silicate concentration, the zeta potential of ceria particles follows a trend similar to that of silica. This is a clear indication that the silicate ions adsorb onto the ceria particles [30,31]. 

An instrumentation approach was explored by Abiade et al. [32] who investigated ceria-silica interactions using atomic force microscopy (AFM) and SEM. Based on these studies, a model for silica polishing with ceria particles is illustrated in Fig. 13.23. 

The mean particle size is typically represented by the so-called D50, which indicates when the cumulative distribution of particles for a particular particle size reaches 50%. The particle size distribution is typically represented by the difference between D50 and D99. The larger the gap between D50 and D99, the greater the particle size distribution. In a representative study by Chandrasekaran et al. [36], the effects of ceria particle size and size distribution on the removal rate of dielectric materials were carefully elucidated. 

Considering the lEPs of ceria particles, oxide, and nitride surfaces, explain the potential advantages and disadvantages of conducting STI CMP at pH = 4 to 10. 

Chandrasekaran N, Taylor T, Sabde G. Effect of ceria particle-size distribution and pressure interactions in chemo-mechanical polishing (CMP) of Dielectric Materials. Mater Res Soc Symp Proc 2003 767 F3.2. 

This type of defect is very rare. Radioactive contamination is effective at destroying the transistors [11]. Some ceria particles may contain radioactive impurities depending on the source of raw materials. Therefore, it is important to screen for radioactive impurities for slurries that may contain radioactive raw materials. In addition, a radioactive contaminated packaging material may also cause irreversible damage if used to store CMP consumables. 

FIGURE 18.11 Transmission and back scattering plots for ceria-1 STI CMP slurry (ceria particles). 

Ceda-based oxides can be obtained by the decomposition of some compound precursor, such as hydroxide, nitrate, halides, sulfates, carbonates, formates, oxalates, acetates, and citrates.For example, nanosize or porous cerium oxide particles have been prepared at low temperatures by pyrolysis of amorphous citrate," which is prepared by the evaporation of the solvent from the aqueous solution containing cerium nitrate (or oxalate) and citric acid. In the case of mixed oxides, the precursor containing some cations in the same solid salts is prepared. In the same manner of ceria particles, the precursors complexing some cations with citrates are useful to synthsize ceria-zirconia mixed oxides and their derivatives. Also. Ce02-Ln203 solid solutions, where Ln = La. Pr, Sm. Gd. and Tb, have been synthesized from the precursors obtained by the evaporation of nitrate solutions at 353 K in air from an intimate mixture of their respective metal nitrates. The precursors are dried and then heated at 673 K to remove niU ates, followed by calcination at 1073 K for 12h. 

Another process to obtain uniform fine ceria particles is the forced hydrolysis method that is useful for preparation of metal oxides and hydroxides. 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 jim in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fme oxide particles can be prepared from metals, alloys, and intermciallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-mcthoxycthanol at 473-523 K yields ultrafine ceria particles (ca 2 nm). 

An emulsion liquid membrane (ELM) system has been studied for the selective separation of metals. This system is a multiple phase emulsion, water-in-oil-in-water (W/O/W) emulsion. In this system, the metal ions in the external water are moved into the internal water phase, as shown in Fig. 3.4. The property of the ELM system is useful to prepare size-controlled aiKl morphology controlled fine particles such as metals, carbonates/ and oxalates.Rare earth oxalate particles have been prepared using this system, consisting of Span83 (sorbitan sesquioleate) as a surfactant and EHPNA (2-ethyl-hexylphospholic acid mono-2-ethylhexyl ester) as an extractant. In the case of cerium, well-defined and spherical oxalate particles, 20 - 60 nm in size, are obtained. The control of the particle size is feasible by the control of the feed rare earth metal concentration and the size of the internal droplets. Formation of ceria particles are attained by calcination of the oxalate particles at 1073 K, though it brings about some construction of the particles probably caused by carbon dioxide elimination. 

CH4 oxidation has been experienced for ceria supported on a barium hexaaluminate, an heat resistant support. Preparation by a new reverse microemulsion method leads to ceria nanoparticles deposited on support and having a BET area close to 100 mVg after calcination at 1000 0 [72]. Such ultrahigh disperse nanoparticles show exceptional thermal resistance the authors mentioned that ceria particles prepared with a size of 6 nm sinters only to 18 nm after a calcination at 1IOO°C under a water containing atmosphere. Of course excellent activity in methane combustion has been observed. According to their experimental conditions calculated specific activity expressed as mol(CH4).h. m was estimated to 6.4x10 at 500°C whereas Bozo [44J reported a value of 1.5x1 O at the same temperature both values look similar. Thus the difference in methane conversion may be related to BET area only which is spectacularly preserved using the reverse micro-emulsion method for synthesis. 

Ceria particles are considered to be one of the best glass/Si02 polishing abrasives. This is snggested to be because of the reaction between ceria and Si02 film, which resnlts in the formation of a chemical tooth between the silica surface and the ceria particles, and induced localized strain in the glass with particle... 

The solid state displacement reaction method and wet chemical precipitation method were employed for synthesizing the ceria powders, and thus the ceria properties showed different features in several experiments. Figure 15.10 shows the morphology of the ceria particles observed with high-resolution scanning electron microscopy (SEM S900, Hitachi, Japan) and transmission electron microscopy (TEM JEM-2010, JEOL, Japan). In the figure, the ceria particles have a polyhedral shape. Both of the powders have nearly the same size. The primary particle size is approximately 40 mn. However, the difference in crystal shape of the ceria particles was found on TEM analysis. 

The most interesting result is the formation of a transparent colloidal solution of ceria with 2 nm particles. Cerium metal tips with the superficial layers of oxide are allowed to react in 2-methoxyethanol at 250 to 300°C, and removal of coarse ceria particles originating from the superficial layers yields the colloidal solution. Addition of water to the solution does not cause any change except dilution of the color of the solution, but addition of a drop of a solution of any kind of salt immediately causes precipitation of ceria particles. - The reaction mechanism is as follows The solvent slowly dissolves the superficial layers, and when the solvent reaches the metal, rapid reaction occurs, yielding an alkoxide solution. The concentration of the ceria precursor becomes so high that a burst of nucleation occurs, yielding the colloidal solution. The reaction of cerium acetylacetonate in the same solvent yields ceria particles but does not give a colloidal solution. 

The effect of pH on polish rate was explored for both ceria and silica slurries. A 5 wt % ceria slurry (Ferro Electronics Division, Ferro Corporation) was used for all the ceria experiments. Two types of ceria particles, one with an average diameter of 440 run and an isoelectric point (lEP) of 8.5 and another with an average diameter of 140 nm and an lEP of 6.5... 

The effect of temperature of slurry containing ceria particles (average diameter 440 nm and lEP 8.5), on the removal rates of TEOS oxide and silicon nitride was found to be weak as seen in Figure 1. This is similar to the effect of temperature of a silica based slurry on silica pK)lishing in the temperature range of 20-50 °C for the slurry with the pad maintained between 25-30 C [2]. The pad hardness decreases with an increase in temperature [2] and the associated reduction in removal rate may have compensated for any increased chemical removal caused by the increased temperature. 

---