Abrasive particle effect

In this test for transparent plastics, the loss of optical effects is measured when a specimen is exposed to the action of a special abrading wheel. In one type of test the amount of material lost by a specimen is determined when the specimen is exposed to falling abrasive particles or to the action of an abrasive belt. In another test, the loss of gloss due to the dropping of loose abrasive on the specimen is measured. The results produced by the different tests may be of value for research and development work when it is desired to improve a material with respect to one of the test methods. The variables that enter into tests of this type are... 

In 1999, Luo and Domfeld [110] proposed that there are two typical contact modes in the CMP process, i.e., the hydro-dynamical contact mode and the solid-solid contact mode [110]. When the down pressure applied on the wafer surface is small and the relative velocity of the wafer is large, a thin fluid film with micro-scale thickness will be formed between the wafer and pad surface. The size of the abrasive particles is much smaller than the thickness of the slurry film, and therefore a lot of abrasive particles are inactive. Almost all material removals are due to three-body abrasion. When the down pressure applied on the wafer surface is large and the relative velocity of the wafer is small, the wafer and pad asperity contact each other and both two-body and three-body abrasion occurs, as is described as solid-solid contact mode in Fig. 44 [110]. In the two-body abrasion, the abrasive particles embedded in the pad asperities move to remove materials. Almost all effective material removals happen due to these abrasions. However, the abrasives not embedded in the pad are either inactive or act in three-body abrasion. Compared with the two-body abrasion happening in the wafer-pad contact area, the material removed by three-body abrasion is negligible. 

As can be seen from the above discussions, the process parameters affect the tribology at the interface. In the following sections, the effect of various pad characteristics, slurry compositions, and abrasive particle characteristics on the tribology of CMP is elucidated. 

Qualitatively, since the contact area increases linearly with applied pressure, the effective pressure is constant for a given pad. Soft, compliant pads have a larger contact area and lower effective pressure whereas hard, stiff pads have a smaller contact area and higher effective pressure. Thus soft pads push abrasive particles against the wafer over a larger area but with less force than hard pads do. 

Dispersion stability is critically important for all CMP slurries. For example, particle-particle aggregation may significantly increase the mean particle size of a slurry and result in surface defects such as scratches and delamination. Severe aggregation can also accelerate the settling of slurry, which can drastically alter the physical and chemical characteristics of a slurry. Therefore, a proper dispersion of metal oxide or abrasive particles in an aqueous environment is fundamentally important for CMP applications [58]. One of the most effective and commonly used methods for the stabilization of a colloidal dispersion involves the use of surfactant. 

A wide variety of materials have been implemented as abrasive particles in CMP processes. They include alumina, silica, ceria, zirconia, titania, and diamond. The effectiveness and suitability of these particles in CMP with particular applications are greatly influenced by their bulk properties (density, hardness, particle size, crystallinity etc.) and the surface properties (surface area, isoelectric electric point (lEP), OH content, etc.). This section will focus on the evaluation of alumina, silica, diamond, and ceria as the major abrasives used for the CMP of metals. 

When polishing silicon dioxide films, it is often observed that on a per abrasive particle basis, ceria polishes planar surfaces significantly more effectively than silica [15]. For example, as shown in Fig. 13.5, the polishing rate for planar silicon dioxide is higher with slurry containing 0.5 wt% of ceria than that for silica-based slurries containing 13 wt% of silica. 

Interactions between the ceria abrasives and the oxide surface have been investigated using both the chemical and the instrumental approaches. Suphantharida and Osseo-Asare [27] used zeta-potential measurements, silicate adsorption, and polishing experiment to investigate the role of ceria abrasives-Si02 surface interaction. T o determine the effect of pH on the surface charges, the zeta potentials of abrasive particles were measured (Fig. 13.21). The points of zero charge (pzc) or isoelectric point is at pH 6.0 for ceria and pH 1.5 for silica. These values are consistent with those reported by others [28,29]. 

EFFECT OF ABRASIVE PARTICLE SIZE ON REMOVAL RATE AND DEFECTIVITY... 

From this study it was observed that the secondary particle size has a significant effect on the number of scratches. The scratch count rapidly increases from approximately 10 to 100 counts if the mean particle size is greater than 340 nm. Therefore, this suggests that the main cause of the scratch formation is possibly the uncontrolled large secondary abrasive particles. 

Compared to silica, ceiia particles polish the planar surfaces more effectively. It has been shown that some ceria slurries exhibit the reverse-Prestonian behavior on certain polishing platforms and not for silica-based slurries. Ceria-based slurries often contain additives that are specifically designed to alter the surface charge on the abrasive particle and/or wafer surface, whereas silica slurries generally do not contain additives other than dispersion stabilizers and pH-adjusting agents. 

Copper CMP process is a very delicate balance between selective copper removal at the protruded area and targeted copper surface protection at the recessed area. To enhance the removal, an oxidizer and a complexing agent are commonly used in the copper CMP slurry in addition to the mechanical force provided by the abrasive particles and the pad. To protect the copper in the recessed area or avoid isotropic dissolution of copper, a corrosion inhibitor such as benzotriazole (BTA) is usually added to the slurry. BTA is very effective in protecting the copper surface from the corrosive attack. On the... 

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