ABLATION AND SORPTIVE REMOVAL OF FILMS AND PARTICLES FROM SURFACES USING CARBON DIOXIDE

摘要

The use of carbon dioxide in its various states: supercritical (SC-CO_2), liquid (L-CO_2) or pellet form (snow-CO_2) for cleaning the surfaces of precision metallic/optical components and semiconductor electronic devices is well documented in the literature, but an understanding of the fundamental mechanisms responsible for carbon dioxide-based cleaning processes is lacking, Although carbon dioxide is an excellent solvent for removing non-polar contaminants from a variety of surfaces, other CO_2-based cleaning processes are based on mechanical or morphological-induced changes in the interfacial region. The extremely low surface tension of CO_2 is a favorable property in terms of its rapid and complete removal from the substrate after cleaning has been affected, while this characteristic of CO_2 also accounts for its negligible effect on the morphology of the substrate, as utilized in critical point drying. However, this is counter intuitive to what is achieved when using SC-CO_2 or L-CO_2 for surface cleaning or alteration. Mechanical ablation of particulates and other contaminants on surfaces with CO_2 is governed by a complex array of factors including van der Waals and electrostatic interactions, drag and lift forces, and phase changes that occur when the CO_2 molecule impacts on the surface. Ultimately particle removal from surfaces is dependent on the viscosity and density of SC-CO_2, two factors which impact on the hydrodynamics of the boundary layer at the fluid-surface interface. Theoretical calculations show that the boundary layer is a parabolic function of the CO_2 velocity, exhibiting only a weak dependence on temperature; and it is these factors which determine the size of the particle that can be removed with SC-CO_2. By contrast, CO_2-based cleaning of surface films exhibiting low or negligible solubility in SC-CO_2 requires a different mechanistic or theoretical treatment as well as experimental confirmation. Application of adhesion science principles using such parameters as Hamaker constants or two-dimensional solubility parameters (δ) can be used to qualitatively explain the ability of SC-CO_2 or snow-CO_2 to remove some surface films, For polymeric films, such as those that occur on semiconductor devices, there is a complex array of factors which can account for the observed effect of SC-CO_2 "cleaning". Interaction between the supercritical fluid and polymeric film can result in sorption of the fluid by the polymer, resulting in swelling or crystallization, SC-CO_2 can also alter the molecular weight distribution of the polymeric film via extraction of specific oligomers, induce depolymerization, or increase the molecular weight of the polymer by a pressure-induced polymerization reaction.