CMP slurry

Chemical mechanical polishing (CMP) is an integral part of any silicon SEMI fab. Integrated circuits made using lithography and thin film deposition invariably employ CMP to achieve the desired planarity of the substrate and the deposited layers. CMP slurries typically consist of a nano-sized abrasive powder dispersed in a chemically reactive solution. While chemical etching softens the material, the mechanical abrasion removes the material, thus flattening the topographic features and making the surface planar. Only chemical etching is isotropic and does not flatten the surface topography, while mechanical abrasion flattens the surface but introduces surface defects. A properly designed CMP process can achieve planarity without introducing surface defects.

The size distribution of the abrasive particles is the critical design parameter in a CMP slurry, affecting key metrics such as the rate of material removal and surface defects. Another important parameter is the dispersion/aggregation of particles in the slurry. Agglomerated particles behave like oversized particles, resulting in surface damage during the polishing process. 

The typical size range of CMP abrasive particles is 10-250 nanometres. Several particle sizing techniques are capable of measuring particle size in this range with varying accuracy and precision. Laser diffraction, dynamic light scattering, and small-angle X-ray scattering are the key techniques for delivering high accuracy and precision in this size range.

Aggregation may cause oversize aggregates of up to 10 microns typically in the ppm range in numbers. Slurry stability against particle aggregation can be determined using zeta potential. The presence of oversized particles can be detected using light scattering or imaging techniques.