Humidity control is one of the important conditions, dust-free workshop production must have relative humidity is dust-free workshop, a commonly used in the process of clean room operation environment control condition. Typical of semiconductor dust-free workshop, clean room control the relative humidity in the target about 30 to 50%, within the scope of the permissible error in plus or minus 1% of narrow range, such as photolithography area or in the far uv processing (DUV) or less in other places can be relaxed to + / - 5% range.
In recent years, keep the air handling process in the scope of these regulations, we must take the capital and operating costs. But why is worth so much money in dust-free workshop, clean room control relative humidity? The rule is simple! Because of the relative humidity has a series of factors that might make clean room overall performance drops, including:
1, bacterial growth;
2, staff feel comfortable room temperature range;
3, a static charge;
4, metal corrosion;
5, water vapor condensation;
6, lithography degradation;
7, water imbibition.
Bacteria and other biological pollution (mold, viruses, fungi, mites) in more than 60% relative humidity environment can be active to breed. Some bacteria in the relative humidity can increase more than 30%. In relative humidity between 40% to 60% range, can make the bacteria and the influence of respiratory tract infection to a minimum.
The range of 40% to 60% relative humidity is also moderate scope of human feel comfortable. Humidity is too high can make people feel quite stuffy, while lower than 30%, can let a person feel dry, the skin is broken, and respiratory discomfort and emotional upset.
High humidity actually reduces the electrostatic charges accumulated on the surface of the dust-free workshop, clean room ─ ─ this is the result of all hope. Low humidity fit in with the charge accumulation and become a potential source of destructive electrostatic discharge. When more than 50% relative humidity, electrostatic charges began to dissipate quickly, but when the relative humidity is less than 30%, they can be in the insulator or not on the surface of the earth to continue to exist for a long time.
Between 35% and 40% relative humidity can be used as a satisfactory compromise, semiconductor dust-free workshop, clean room generally use extra control device to limit the accumulation of static charges.
A lot of chemical reaction speed, including the corrosion process, as the increasing of relative humidity and speed up. All exposed in the air around the dust-free workshop, clean room surfaces were soon covered with at least a monomolecular layer of water. When the surface is composed of thin metal coating can react with water, high humidity can make response speed. Fortunately, some metals such as aluminum, can form a layer of protection type oxide with water, and to prevent further oxidation reaction; But another case is, such as copper oxide, is not capable of protection, therefore, in high humidity environment, the copper surface are more susceptible to corrosion.
when the relative humidity is around 70%, has become the main power in the adhesion between particles.
So far, the most urgent in semiconductor dust-free workshop, clean room need to properly control the sensitivity of the photoresist. Due to the nature of photoresist on relative humidity sensitive, it is the requirement of the control range of relative humidity of the strictest standards.
In fact, relative humidity and temperature for the stability of the photoresist and accurate size control is essential. Even under the condition of constant temperature, viscosity of photoresist will drops rapidly with the rise of relative humidity. Change the viscosity, of course, will change the thickness of protective film formed by a fixed component coating. Refer to the two cities, A test proof, relative humidity of 3% of the variation will make the protection thickness change 59.2 A (sic).
In addition, in high relative humidity conditions, due to the absorption of moisture, make the baking cycle after the photoresist expansion. Photoresist adhesion can also be affected by the negative impact of higher relative humidity; Low relative humidity (about 30%) make it easier to resist adhesion, don't even need to polymer modified agent, such as six methyl two silazane (HMDS).
To sum up in the semiconductor dust-free workshop, clean room control relative humidity is not random. However, as the change of time, review the best common reasons and basis of generally accepted practice.
