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Why does the FAB clean room have to control humidity?
Humidity is a common environmental control condition in the operation of cleanrooms. The target value of relative humidity in the semiconductor clean room is controlled to be in the range of 30 to 50%, allowing the error to be within a narrow range of ±1%, such as a photolithographic area - or even smaller in the far ultraviolet processing (DUV) area. - In other places, you can relax to within ±5%.
Because relative humidity has a number of factors that may contribute to the overall performance of the clean room, including:
● bacterial growth;
● The range of comfort that the staff feels at room temperature;
● Static charge appears;
● metal corrosion;
● Water vapor condensation;
● degradation of lithography;
● Water absorption.
Bacteria and other biological contaminants (mold, viruses, fungi, mites) can actively multiply in environments with relative humidity above 60%. Some flora can grow when the relative humidity exceeds 30%. When the relative humidity is between 40% and 60%, the effects of bacteria and respiratory infections can be minimized.
Relative humidity in the range of 40% to 60% is also a modest range in which humans feel comfortable. Excessive humidity can make people feel depressed, while humidity below 30% can make people feel dry, chapped, respiratory discomfort and emotional discomfort.
High humidity actually reduces the accumulation of static charge on the surface of the clean room - this is the desired result. Lower humidity is more suitable for charge accumulation and a potentially damaging source of electrostatic discharge. When the relative humidity exceeds 50%, the static charge begins to dissipate rapidly, but when the relative humidity is less than 30%, they can persist for a long time on the insulator or the ungrounded surface.
Relative humidity between 35% and 40% can be a satisfactory compromise, and semiconductor cleanrooms typically use additional controls to limit the accumulation of static charge.
The speed of many chemical reactions, including the corrosion process, will increase as the relative humidity increases. All surfaces exposed to the air surrounding the clean room are quickly covered with at least one monolayer of water. When these surfaces are composed of a thin metal coating that can react with water, high humidity can accelerate the reaction. Fortunately, some metals, such as aluminum, can form a protective oxide with water and prevent further oxidation reactions; but another case, such as copper oxide, is not protective, so In high humidity environments, copper surfaces are more susceptible to corrosion.
In addition, in a high relative humidity environment, the photoresist is expanded and aggravated after the baking cycle due to the absorption of moisture. Photoresist adhesion can also be negatively affected by higher relative humidity; lower relative humidity (about 30%) makes photoresist adhesion easier, even without the need for a polymeric modifier.
Controlling relative humidity in a semiconductor clean room is not arbitrary. However, as time changes, it is best to review the reasons and foundations of common, generally accepted practices.
Humidity may not be particularly noticeable for our human comfort, but it often has a great impact on the production process, especially where humidity is high, and humidity is often the worst control, which is why In the temperature and humidity control of the clean room, humidity is preferred.
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