SUBSTITUTING FOR CFCs USED AS CLEANING AGENTS: (1) PARTICLE REMOVAL

Freon-TF is no better a cleaning fluid than deionized water for removing either organic (polystyrene latex spheres) or inorganic (glass beads) particles from silicon wafer surfaces, as shown by comparative measurements of cleaning efficiencies in both an ultrasonic bath and a hydrodynamic cleaning apparatus. A primary reason for this lackluster performance is the higher van der Waals’ forces of adhesion for particles on silicon when immersed in Freon-TF. In the ultrasonic bath this difference in van der Waals’ adhesion plus the lower cavitation intensity of Freon-TF causes particle removal efficiency to be lower in Freon-TF than in deionized water. In the hydrodynamic cleaning apparatus, the higher drag forces exerted on the particles by the Freon-TF compensate for the higher van der Waals’ adhesion so that the particle cleaning efficiencies in Freon-TF and deionized water are about the same. Particle removal efficiency is only one minor criterion by which chlorofluorocarbon (CFC) replacements will be selected. Safety and nonparticulate contaminant removal will often be more important. A first task of the Surface Cleaning Technology Consortium (SCTC), a multiclient cooperative research program now starting up at the Research Triangle Institute, is to provide guidelines for selecting suitable CFC substitutes based on the full spectrum of performance criteria.