Technology Advances
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TECHNOLOGY ADVANCES
Supercritical Carbon Dioxide Technology Offers Ecological and Cost-Effective Way to Clean Chips Four main methods are available to produce a specific chip—layering, patterning, doping, and heat treatment. In patterning and doping, a polymeric coating is applied to the surface of a silicon wafer, and the coating is patterned using photolithography, in which a high-intensity light exposes a pattern into the polymeric photoresist. The photoresist will either become a shorter chained (weaker) polymer or a cross-linked (stronger) polymer in the exposed region, depending on the type of resist. The weaker polymer is chemically dissolved. The remaining photoresist is then removed by using radio-frequency plasmas in an “ashing” step, followed by a wet-chemistry solvent rinse with aggressive acids and/or corrosives or organic chemicals. However, since feature sizes of 130 nm will be making entry into the market by the fourth quarter of 2001, and smaller dimensions are on the way with higher aspect ratios (10:1, in some cases), traditional wet chemistry stripping will become less successful at cleaning the wafers. Surface tension and capillary forces of the liquids will not allow penetration into the deep, high-aspect-ratio features on the chip (if wetting is not obtained). While chip-making is referred to as a “clean industry,” on an average day of operations at a wafer-fabrication plant, an estimated four million gallons of wastewater and thousands of gallons of corrosive hazardous materials, such as hydrochloric and sulfuric acids, are produced during the cleaning process. An innovative technology called SCORR (supercritical carbon dioxide resist removal) has been developed at Los Alamos National Laboratory (LANL) as a cleaner approach to photoresist removal. By pressurizing and heating liquid carbon dioxide above its critical pressure and temperature (~7450 kPa and 32°C) it becomes a supercritical fluid. The supercritical carbon dioxide (scCO2) exhibits gaslike viscosity and liquidlike density, with diffusivity between that of a gas and a liquid, but with no surface tension. It replaces the hazardous chemicals used to strip the photoresist from the silicon wafer. In addition to the solvent replacement, millions of gallons of ultrapure water used to rinse away the solvents are no longer needed. When the pressure and temperature are lowered, the scCO 2 returns to the gas phase, leaving the wafer completely dry and with no residue. This
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eliminates the need for an isopropyl alcohol drying step that follows the conventional ultrapure water-rinse step. By itself, scCO2 is ineffective at photoresist removal, so it is generally combined with small amounts of other solvents. The co-solvents used in the SCORR process (such as propylene carbonate) are much safer than the typical methyl ethyl ketones (MEKs) used currently in industry. The defining step of the SCORR process involves swelling of the polymeric photoresist by the carbon dioxide. After swelling, scCO2 creates stresses at the wafer-resist interfa
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