Use of life-cycle inventory as a screening tool for environmental performance: Supercritical carbon dioxide in the semiconductor industry

Supercritical carbon dioxide (scCO ₂) is attractive as a green solvent because it has environmental benefits and attractive physical properties for many applications. In addition, removal of supercritical conditions after use can allow perfect separation and recycle of the solvent. In this work, we investigate using scCO ₂ as a rinsing agent to replace ultrapure water in semiconductor manufacturing. By conducting a gate-to-gate life-cycle inventory (LCI) of scCO ₂ usage, we begin to evaluate the overall environmental impact the process may incur. After conducting the LCI of a particular scCO ₂ rinsing process we find the process requires 1 kg of new CO ₂ and 0.27 kg of new propylene carbonate per wafer cleaning cycle because of the need for startup, purging, and separation processes. The total pumping requirements are 88 kj/cycle, whereas cooling requirements are 249 kj/cycle and require cooling to 10° C. Heating requirements are 616 kj/cycle and require reaching temperatures of 65° C for cleaning or 25.5° C for separations. Refrigeration cycle requirements add an additional 298 kj/cycle to the energy requirements so the total energy inputs to the process, including the refrigeration, are 1002 kj/cycle. These material and energy needs are much lower than the requirements of the ultrapure water technologies currently in use for wafer rinsing. Companies now have baseline data to evaluate whether this new technology will be suitable for their needs. Finally, throughout any LCI, there are many key assumptions and barriers to obtaining high quality data, which the chemical and environmental engineering communities should begin addressing as more life-cycle inventories and assessments are completed.