Particles on Surfaces 8: Detection, Adhesion and Removal pages:77-128
8th International Symposium on Particles on Surfaces location:Providence, RI date:24-26 June 2002
In this work the deposition of submicrometer-sized particles from a carry-over layer onto a substrate being rinsed is investigated. The particles used are 80 nm spherical colloidal silica particles,the substrates are thermal oxide, chemical oxide on silicon and silicon nitride. This paper focusses only on stagnant solutions. Two mechanisms are proposed by which particles can deposit: (i) adsorption during immersion and (ii) evaporative deposition during drying. The latter is caused by the evaporation of liquid, leaving the particles on the surface as a drying residue. In a first series of tests the particle deposition after immersion in a stagnant particle suspension is determined. When the net particle-substrate interaction is strongly repulsive at the pH of this suspension, only evaporative deposition takes place. In the opposite case, i.e., when the particle-substrate interaction is strongly attractive, diffusion limited adsorption takes place. By comparing the experimental data with theoretical models for both extreme conditions we can determine the pH regions where attraction or repulsion between the particles and the surface is dominant. In a second series of tests, the immersion in a particle suspension is followed by a stagnant rinsing step. In that case particle adsorption can also take place during immersion in the rinsing liquid. If the pH values of the particle suspension and rinsing liquid are different, the pH in the vicinity of the substrate (inside the carry-over layer) changes during the rinsing step from the pH of the particle suspension until the pH of the rinsing liquid is obtained. This process is denoted as the pH shock, and an analytical model to calculate it is derived in this paper. Using the relation between particle-substrate interactions and pH (obtained in the first series of tests) together with the calculation of the pH shock, a model is developed to predict particle adsorption during stagnant rinsing. This is done for each substrate separately. The resulting models are experimentally verified by performing tests where a particle-adsorption step at very low pH (0.2) is followed by a rinsing step in neutral water. For all three substrates, a satisfactory agreement between experimental results and theoretical models is observed.