Use of Supercritical Fluids in the Semiconductor Industry
Georgia Guenther - Arizona State University, Candidate, Masters of Science in Technology (SSA Journal Volume 14 Number 1 - Spring 2000 pp. 27 - 34 )
The purpose of this paper is to discuss the environmental, health and safety benefits of using supercritical fluids in the semiconductor industry. Supercritical fluid use is an idea whose time has come. As industries, particularly the semiconductor industry, continue to produce toxic byproducts and wastes, worker health and safety is paramount and there is a great need to substitute the usual array of chemicals, where appropriate, in terms of environmental friendliness. A supercritical fluid is created through phase change of a material (e.g., from liquid to gas). The critical point is defined by both a critical pressure and a critical temperature. When a substance is taken beyond the critical point, defined as being in the supercritical fluid region. At this point, the phase boundary between the gas and liquid states disappears and the fluid exhibits properties of both a gas and a liquid, which leads to excellent solubilities for many compounds. Supercritical fluids have been used since 1985 and were originally developed as a substitute for methylene chloride in the process of decaffeination of coffee beans. They are employed in several food and pharmaceutical processes, as well as dry cleaning industries and soil remediation. Carbon dioxide (CO2) is commonly used as a supercritical fluid. Its gaseous properties allow it to fill a container and under pressure readily penetrates into cracks and pores. Dissolving substances and carrying them away is the result of its liquid properties. By increasing its density (which can be done over a wide range) carbon dioxide becomes more liquid like and enhancing its solvent properties. Thus, the high density fluid is able to carry off the solute molecules. By varying the density the solvent power can be manipulated which makes it an attractive candidate for use as a solvent. Because of increasing regulatory and environmental pressures on hydrocarbons and ozone depleting emissions, supercritical fluids are gaining much respect as replacements for organic solvents in industry. For those industrial applications using solvents and corrosives, the process of water treatment, both prior to and after the industrial process, can be expensive and problematic. To comply with the Montreal Protocol and to stay competitive in the global market, the US requires economical replacements for ozone depleters, toxics and carcinogenics. The semiconductor industry has a reputation as a "clean industry". This image can be further enhance by utilization of supercritical fluids in its processes. Supercritical water is already being used in a commercial process to oxidize organic compounds, and in the destruction of organic wastes. This paper will define supercritical fluids and explain their delivery and uses. It will demonstrate the representation of states of materials in phase diagrams and other charts. It will discuss the options for solvents of choice, as well as considerations of availability, cost and feasibility using previous and current research in the field of supercritical fluid use in the semiconductor industry, and in similar industries where there might be similar applications.