Environmental protection has attracted more and more attention, especially the increase in haze weather. Purification engineering is one of the environmental protection measures. How to use purification engineering to do a good job in environmental protection? Let’s talk about the control in purification engineering.
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, people’s 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 increase of 1 degree will cause a linear expansion of 0.24um, so there must be a constant temperature of ±0.1 degrees, 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.
Excessive humidity creates more problems. When the relative humidity exceeds 55%, condensation will form on the cooling water pipe wall. 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 adhering to 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 that of an adjacent space with a low cleanliness level.
3. The doors between connected clean rooms should be opened to rooms with higher cleanliness levels.
The maintenance of pressure difference depends on the amount of fresh air, which must be able to compensate for the amount of air leaking from the gaps under this pressure difference. Therefore, the physical meaning of pressure difference is the resistance of the leaked (or infiltrated) air volume when it passes through various gaps in the clean room.
