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Temperature and air pressure control in clean rooms for purification engineering
2024-11-26
1. Temperature and humidity control in clean rooms
The temperature and humidity of clean spaces are mainly determined according to process requirements, but under the condition of meeting process requirements, human comfort should be taken into account. With the improvement of air cleanliness requirements, there is a trend that the process requirements for temperature and humidity are becoming more and more stringent.
The specific process requirements for temperature will be listed later, but as a general principle, as the processing accuracy becomes more and more refined, the requirements for temperature fluctuation range are getting smaller and smaller. For example, in the photolithography exposure process of large-scale integrated circuit production, the difference in thermal expansion coefficients between glass and silicon wafers as mask materials is required to be smaller and smaller.
For a silicon wafer with a diameter of 100 um, a temperature rise of 1 degree causes a linear expansion of 0.24 um, so a constant temperature of ±0.1 degrees must be maintained, and the humidity value is generally required to be low, because after people sweat, the product will be contaminated, especially in semiconductor workshops that are afraid of sodium. Such workshops should not exceed 25 degrees.
There are more problems caused by excessive humidity. When the relative humidity exceeds 55%, condensation will occur on the wall of the cooling water pipe. If it occurs in precision devices or circuits, it will cause various accidents. Rust is easy to form when the relative humidity is 50%. In addition, when the humidity is too high, the dust on the surface of the silicon wafer will be chemically adsorbed on the surface by water molecules in the air, making it difficult to remove.
The higher the relative humidity, the harder it is to remove the adhesion. However, when the relative humidity is lower than 30%, the particles are also easily adsorbed on the surface due to the effect of electrostatic force, and a large number of semiconductor devices are prone to breakdown. The optimal temperature range for silicon wafer production is 35-45%.
2. Air pressure regulations in clean rooms
For most clean spaces, in order to prevent external pollution from invading, it is necessary to keep the internal pressure (static pressure) higher than the external pressure (static pressure). The maintenance of pressure difference should generally comply with the following principles:
1. The pressure of clean space should be higher than that of non-clean space.
2. The pressure of a space with a high cleanliness level should be higher than the pressure of an adjacent space with a low cleanliness level.
3. The doors between connected clean rooms should be opened to rooms with a high cleanliness level.
The maintenance of pressure difference depends on the amount of fresh air, which should be able to compensate for the amount of air leaking from the gap under this pressure difference. Therefore, the physical meaning of pressure difference is the resistance of leakage (or infiltration) air through various gaps in the clean room.
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