Liquid, supercritical, gaseous: Cleaning of silicon wafers with supercritical carbon dioxide
Electronic components such as integrated circuits (ICs) are produced from silicon wafers. They are the basic components of all electronic devices, including laptops, tablets, smartphones, cameras, and LCD televisions.
Cleaning plays an essential role in the production of silicon wafers to ensure the proper conductivity and function of produced integrated circuits (ICs). The production of these wafers is a highly complex process, which takes several weeks. The cleaning processes are important for a high-quality result. Traditionally, the wafers are cleaned using an ultrasonic bath with solvent/ water. However, this process is very complex, harmful to the environment and not always perfect.
"Many renowned manufacturers in the semiconductor industry have completely converted to the carbon dioxide cleaning process with diaphragm pumps.” says Joachim Bund, Head of Sales Division Process Industry & Downstream at LEWA. Cleaning with supercritical CO2 offers several advantages for the semiconductor industry.
For this purpose, LEWA offers cleanroom-compatible metering pumps with PTFE sandwich diaphragms, which are especially well-suited for this process. The diaphragm pump is installed in a "wafer cleaning cabinet" where the liquid CO2 is compressed to at least 75 bar. In the next step, the carbon dioxide is heated to at least 35°C by a heat exchanger, thereby reaching its supercritical state, SCCO2. In this state, it has excellent solvent properties against non-polar impurities, and can penetrate the smallest gaps in the wafer thanks to its low viscosity. After cleaning, the wafer is flushed with pure CO2 to ensure that no residue remains on it. The pressure is then lowerd, which causes the sublimation of the CO2. As a result, the wafer is dried and free of solvents.
Since heat is generated during compression in the pump head, cooling is particularly important for liquefied gases. “Once the liquid is near its vapor pressure, it transitions into the gaseous state. This may cause cavitation during the suction stroke, which makes it necessary to use an additional cooling jacket for the pump head." says Bund.
In order to be able to configure the appropriate unit for each cleaning task, the material selection of wetted parts can also be individually adapted.
Source: LEWA GmbH