Particle Crystallinity and Shapes

In addition to bulk density, particle crystallinity is another physical property that can be related to its hardness and potential effectiveness in providing the mechanical force in CMP. For example, alpha alumina abrasive has a higher hardness, thereby producing higher removal rate in comparison to its gamma crystalline counterpart [80]. As expected, the number of defects is also higher for alpha alumina due to the inherent hardness [81]. 

Silica can exist in many crystalline forms such as quartz, cristobalite, and tridymite. Fumed silica on the contrary tends to be amorphous, which could be attributed to the fabrication process of the abrasive. The amorphous nature is probably caused by the rapid cooling employed in the process [83]. Colloidal silica, which is usually synthesized via wet chemical methods, is highly amorphous as well. In addition, colloidal silica particles are usually spherical and highly hydrated in nature, which makes them far less likely to cause scratches on metal substrate surface. 

Two major classifications for diamond crystals are mono- or single crystalline and polycrystalline. The monocrystalline diamond particles tend to have more uniform surfaces and sharp edges. The abrasiveness of the monocrystalline diamond is mainly governed by its particle size. In the case of polycrystalline diamond, it is sometimes determined by the packing arrangement and the interaction of these single crystals that are related to friability [84]. Friability is defined as the readiness of a substance to crumble and form fine particles or fibers under the application of external pressures. 

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