Particle adhesion and removal mechanisms during brush scrubber cleaning

Abstract

Brush scrubbers are among the most commonly used instruments for wafer-cleaning applications nowadays. However, the removal mechanisms of nanosized particles are far from clear, especially because no direct experimental data are available to backup theoretical models in the literature. This study combines a theoretical approach based on a force analysis with an experimental study of the removal of nanosized slurry particles. In the theoretical part, all forces affecting the adhesion and the removal of particles are evaluated to determine which are dominant in two extreme removal mechanisms: lifting and rolling. In the experimental part, the removal efficiency of 34nm SiO2 particles is investigated by using the haze approach. Based on a study of the aging of contaminated wafers, conditions are selected where no chemical bonds are formed between a particle and a substrate. Force analysis and experimental observations both show that nanosized particles cannot be lifted directly by a brush. Instead, rolling should be the main particle-removal mechanism. The average fluid film thickness between brush and wafer surface is determined based on power measurements of the brush motor and a friction analysis, indicating that the system is in a hydrodynamic lubrication regime across a wafer in average. In this frame, results also show that the hydrodynamic drag force is the dominant removal force for nanosized particles.