The target value of relative humidity in a semiconductor (FAB) clean room

The target value of relative humidity in a semiconductor (FAB) clean room is approximately 30 to 50%, allowing a narrow margin of error of ±1%, such as in the lithography zone – or even less in the far ultraviolet processing (DUV) zone – while elsewhere it can be relaxed to ±5%.
Because relative humidity has a range of factors that can reduce the overall performance of clean rooms, including:
1. Bacterial growth;
2. Room temperature comfort range for staff;
3. Electrostatic charge appears;
4. Metal corrosion;
5. Water vapor condensation;
6. Degradation of lithography;
7. Water absorption.

Bacteria and other biological contaminants (molds, viruses, fungi, mites) can thrive in environments with relative humidity of more than 60%. Some bacterial communities can grow at relative humidity of more than 30%. The company believes that humidity should be controlled in the range of 40% to 60%, which can minimize the impact of bacteria and respiratory infections.

Relative humidity in the range of 40% to 60% is also a moderate range for human comfort. Too much humidity can make people feel stuffy, while humidity below 30% can make people feel dry, chapped skin, respiratory discomfort and emotional unhappiness.

The high humidity actually reduces the accumulation of electrostatic charges on the cleanroom surface – a desired result. Low humidity is ideal for charge accumulation and a potentially damaging source of electrostatic discharge. When the relative humidity exceeds 50%, the electrostatic charges begin to dissipate rapidly, but when the relative humidity is less than 30%, they can persist for a long time on an insulator or an ungrounded surface.

Relative humidity between 35% and 40% can be used as a satisfactory compromise, and semiconductor clean rooms generally use additional controls to limit the accumulation of electrostatic charges.

The speed of many chemical reactions, including corrosion processes, will increase with the increase of relative humidity. All surfaces exposed to the air around the clean room are quick.