Why Strictly Control Humidity in Cleanrooms

Relative humidity is a commonly used environmental control condition during cleanroom operations. Typical relative humidity targets in semiconductor cleanrooms are controlled in the approximate range of 30 to 50%, with tolerances within a narrow range of ±1%, such as in photolithography areas—or even more in deep ultraviolet processing (DUV) areas. Small - while elsewhere it can be relaxed to within ±5%. Relative humidity has a series of factors that may degrade the overall performance of a clean room, including: 1. bacterial growth; 2. the range in which the staff feels comfortable at room temperature; 3. the occurrence of static charges; 4. metal corrosion; 5. water vapor condensation; 6. Degradation of lithography; 7. Water absorption. Over the years, controlling the air humidity within the specified range has obliged us to bear the capital and operating costs. But why is it worth spending so much money on controlling relative humidity in a pharma clean room? Bacteria and other biological contamination (mold, viruses, fungi, mites) can thrive in environments with relative humidity above 60%. Some flora can grow when the relative humidity exceeds 30%. The effects of bacteria and respiratory infections are minimized when the relative humidity is in the range of 40% to 60%. Relative humidity in the range of 40% to 60% is also a moderate range for human comfort. High humidity can make people feel stuffy, while humidity below 30% can make people feel dry, chapped skin, respiratory discomfort and emotional discomfort. High humidity actually reduces static charge build-up on cleanroom surfaces—a desired outcome. Lower humidity is more suitable for charge accumulation and a potentially damaging source of electrostatic discharge. Static charges begin to dissipate rapidly when the relative humidity exceeds 50%, but they can persist on insulators or ungrounded surfaces for long periods of time when the relative humidity is less than 30%. Relative humidity between 35% and 40% is comfortable, and semiconductor cleanrooms typically use additional controls to limit the build-up of static charges. The rate of many chemical reactions, including corrosion processes, will increase with increasing relative humidity. All surfaces exposed to the air surrounding the cleanroom are quickly covered with at least a monolayer of water. When these surfaces are composed of thin metal coatings that can react with water, high humidity can speed up the reaction. Fortunately, some metals, such as aluminum, can form a protective oxide layer with water and prevent further oxidation reactions; but the other case, such as copper oxide, is not protective, so in the In high humidity environments, copper surfaces are more susceptible to corrosion. In an environment with high relative humidity, capillary forces in the form of concentrated water form bonds between the particles and the surface, which can increase the adhesion of the particles to the siliceous surface. This effect -- Kelvin's concentration -- is not significant when the relative humidity is less than 50 percent, but becomes the dominant force for particle-to-particle adhesion when the relative humidity is around 70 percent. By far the most pressing need for moderate control in semiconductor cleanrooms is photoresist sensitivity. Due to the extremely sensitive nature of photoresist to relative humidity, its control range of relative humidity is the most stringent. In fact, both relative humidity and temperature are critical for photoresist stability and precise dimensional control. Even under constant temperature conditions, the viscosity of the photoresist will drop rapidly as the relative humidity rises. Of course, changing the viscosity changes the thickness of the protective film formed by the fixed component coating. Referring to two cities, one test confirmed that a 3% variation in relative humidity would change the thickness of protection by 59.2A. In addition, in high relative humidity environments, photoresist swelling is aggravated after bake cycles due to moisture absorption. Photoresist adhesion can likewise be negatively affected by higher relative humidity; lower relative humidity (about 30%) makes photoresist adhesion easier even without the need for polymeric modifiers such as hexamethyldisiloxane Azane (HMDS).

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