Chapter 40.6 - ESD Controls in Cleanroom Environments - Air Ionization for Static Charge Control

Air is a good insulator, so any static charge that develops on a clean surface of a good insulator will remain on that surface indefinitely. Air ionizers are used to decrease the electrical resistivity of the air through the addition of ions that act as charge carriers. Air consists primarily of nitrogen, oxygen, and carbon dioxide, with water vapor and trace gases. Air ions are gas molecules in the air that have either lost or gained an electron. Air ionizers are used to introduce air ionization into the room. Ions naturally occurr in the air, especially after a lightning storm, but in a cleanroom the HEPA or ultra low penetrating air (ULPA) filters the ions from the air. Because air ions are naturally occurring items, they have no negative effects on the health of operators.

Positive and negative air ions are present in normal outside air, where they are the result of radioactive decay of materials in the soil (e.g., uranium) and gases in the air (e.g., radon). But most air ions are stripped out of cleanroom air by high efficiency air filtration. This makes the air in the cleanroom very insulative and encourages the generation of static charge. Air ionizers restore and increase the level of air ions in the filtered cleanroom air. When the ionized air comes in contact with a charged insulating surface, the charged surface attracts air ions of the opposite polarity. As a result, the static charge on the insulator is neutralized. Air ions of both polarities are required for neutralization, because both polarities of static charge are created in the cleanroom. This is shown in Fig. 40.8.

There are three methods that are commonly used to make air ions for charge control. These methods are corona, photoelectric, and radioisotope ionizers. While the ions created by each are identical, each method generally has its own application. The tradeoffs involve issues of cleanliness, ion distribution requirements, shielding issues, voltage balance (precision) level, and government regulations. Each type is discussed in the following sections.

Air is a good insulator, so any static charge that develops on a clean surface of a good insulator will remain on that surface indefinitely. Air ionizers are used to decrease the electrical resistivity of the air through the addition of ions that act as charge carriers. Air consists primarily of nitrogen, oxygen, and carbon dioxide, with water vapor and trace gases. Air ions are gas molecules in the air that have either lost or gained an electron. Air ionizers are used to introduce air ionization into the room. Ions naturally occurr in the air, especially after a lightning storm, but in a cleanroom the HEPA or ultra low penetrating air (ULPA) filters the ions from the air. Because air ions are naturally occurring items, they have no negative effects on the health of operators.

Positive and negative air ions are present in normal outside air, where they are the result of radioactive decay of materials in the soil (e.g., uranium) and gases in the air (e.g., radon). But most air ions are stripped out of cleanroom air by high efficiency air filtration. This makes the air in the cleanroom very insulative and encourages the generation of static charge. Air ionizers restore and increase the level of air ions...