Chapter 40.6.2 - ESD Controls in Cleanroom Environments - Air Ionization for Static Charge Control

40.6.2 Photoelectric Ionization

The photoelectric ionizer uses extreme UV to soft x-ray photons to create ions. When the energy of a photon exceeds the ionization potential of the medium it traverses, the photon can create an ion by collision with an atom. This is called the photoelectric effect.¹⁷ The most common method of photoelectric ionization involves the use of very soft x-rays. These are x-rays in the range of 5 to 10 keV as contrasted with medical or dental x-rays, which are in the range of 80 to 175 keV. The lower energy makes them suitable for use as an ion source, because the air is rather opaque to photons in this energy range. A soft x-ray source has a ~100 percent efficiency for creating an ion within about 1.5 m for each photon it emits. While neither the licensing (in the United States by the FDA) nor the safety requirements are very difficult to manage, photoelectric ionization represents extra steps that are necessary for the use of this technology. This type of ionizer is not common in semiconductor processing due to the high cost of the product. It is, however, a very efficient method for generating and delivering ions in a mini-environment.

40.6.3 Alpha Ionization

The alpha ionizer method involves the use of ionizing radiation to make ions. While several forms of ionizing radiation sources are available, only α (alpha) sources are used for static control. The other forms of radiation source have a much longer range and thus require shielding in impractical amounts. Most commonly, Polonium²¹⁰ is employed. It produces α particles with a range of only 3.8 cm in air and 0.02 mm in aluminum¹⁸ so that virtually none of the α particles travel far enough to represent a health hazard or create a shielding requirement. The other aspect of Po²¹⁰ that makes it an ideal choice is that it decays to Pb²⁰⁶, which is a naturally occurring stable isotope. Unlike Po²¹⁰, many other sources decay into isotopes that may be active as well. Alpha ionizers can be used in high temperature (~100°C) and in vacuum applications.

40.6.2 Photoelectric Ionization

The photoelectric ionizer uses extreme UV to soft x-ray photons to create ions. When the energy of a photon exceeds the ionization potential of the medium it traverses, the photon can create an ion by collision with an atom. This is called the photoelectric effect.¹⁷ The most common method of photoelectric ionization involves the use of very soft x-rays. These are x-rays in the range of 5 to 10 keV as contrasted with medical or dental x-rays, which are in the range of 80 to 175 keV. The lower energy makes them suitable for use as an ion source, because the air is rather opaque to photons in this energy range. A soft x-ray source has a ~100 percent efficiency for creating an ion within about 1.5 m for each photon it emits. While neither the licensing (in the United States by the FDA) nor the safety requirements are very difficult to manage, photoelectric ionization represents extra steps that are necessary for the use of this technology. This type...