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Supercritical carbon dioxide. Process and advantages in wafer cleaning.
Silicon wafers are base plates for electronic components and devices. To ensure proper conductivity and function, these small circular washers must be polished to a surface roughness of only a few nanometers and cleaned multiple times during processing.
The silicon is cleaned using supercritical carbon dioxide (SCCO2). This means that the carbon dioxide is in a liquid phase above its supercritical temperature and its supercritical pressure. The fluid features excellent solvent properties in its supercritical phase compared to a wide variety of nonpolar impurities such as fats and oils. In addition, SCCO2 has a low viscosity and surface tension, which improves its ability to penetrate gaps.
The wafers are cleaned in a so-called "wafer cleaning cabinet” in a clean room. The liquid carbon dioxide is in a tank and compressed to 200 bar using a diaphragm pump. To increase the cleaning intensity, some chemicals are added to the compressed CO2 while it is still liquid. A heat exchanger brings the CO2 (mixture) into its supercritical form. Then, the CO2 is injected into the pressure chamber for cleaning the wafers. The supercritical status of the CO2 is reached at a temperature of more than 31 °C and a pressure of at least 74 bar.
The wafers are rinsed with pure CO2 at the end of the cleaning process to ensure that no residual substances remain on the wafers. After cleaning, pressure is lowered causing the CO2 to sublimate immediately so that all components are completely dry after cleaning.
LEWA diaphragm pumps. Solution for the cleaning with supercritical CO2.
With our custom-designed diaphragm metering pumps, we offer an optimal solution for wafer cleaning applications:
It must be noted that the carbon dioxide has to be in its liquid state to be pumped. Due to its compressibility, it is not practical to pump it in its gaseous state.