MTL Annual Research Report 2013

CMP Slurry Abrasive Particle Agglomeration Modeling

Formerly we presented results of our theoretical agglomeration model for chemical mechanical planarization (CMP) under the primary motivation of understanding the creation and behavior during the CMP process of the agglomerated slurry abrasive particles, which are known to be a major cause of defectivity, poor consumable utility, and process variation,. In this year’s work, we focused on understanding the outcomes of our fundamental experiments, which observe agglomeration in non-commercial, experimentally created simple slurry mixtures.

Under prior model assumptions, we believed that silica abrasives, the sole focus of our experimental agglomerates, behaved similarly to most other colloidal oxides used as CMP slurry abrasives. However, based on model results paired with preliminary empirical data, we observed that another non-traditional mechanism dominates the creation of silica abrasive particle agglomerates under shear, specifically, the formation of interparticle siloxane bonds. This silica-specific bonding is not accurately described by the traditional surface dissociation model or by the DVLO theory typically used in colloidal modeling of oxide particles.

Therefore, in an effort to gain the physical intuition to describe this type of bonding, its chemical bonding energy, and other kinetic energy terms not included in our model, we reevaluated our systematic study of shear-induced slurry abrasive particle agglomeration for silica particles used in STI CMP. Our experiments examine both the potential chemical (pH, chemical additives, ionic strength) and mechanical drivers (shear, sedimentation) of agglomeration in conditions comparable to that of actual CMP. This work proves, in agreement with new colloidal and interface science literature, that the specific chemical attributes of silica CMP slurry abrasives are the primary drivers of agglomeration, with the secondary being that of the mechanical application of shear forces.  As a result, we have built a both empirically intuitive and theoretically fundamental model for understanding the behavior of these particles in contrast to other oxide particles, as well as their chemical and hydrodynamic properties under the shear caused by CMP.