Ionizers are the heart of static control program in clean room environment.
Many insulative materials such as plastic, Teflon, ceramic and glass are widely used as part of the process materials.
Static charges build on these insulative materials when there is a contact and separation with other materials during the processing activities. Unlike the conductive material, static charges on the surface of the insulative materials cannot be drained away. There just stay there.
Charged insulative material can induce opposite charges on a neighbouring ESD sensitive part, which is placed within its electric field. Damage to the part is done when a person touch it in the presence of the electric field. Some ESD sensitive items, for example, reticles are damaged when present in the electric field.
ESD sensitive parts, for example, semiconductor wafer, that has static charges on its surface would attract particulate from the air and contaminate the wafer.
There are a few ways to remove or neutralize static charges on the insulative material.
Using an antistatic solution that is compatible to the environment (clean room compatible when used in clean room environment) or ionizers are the two common methods currently available to minimize static charges on the insulative materials.
Ionizers provide permanent solution when it comes to combat static charges on the insulative materials.  Neutralization of static charges occurs when ions produced by the ionizers are attracted to the opposite polarity of static charges on the material surfaces.
There are two common types of ionizers that are widely used. There are corona air ionizer and alpha nuclear ionizer.
Corona air ionizers themselves are divided into three types: AC corona ionizers, Steady state DC corona ionizers and pulsed DC corona ionizers.
Corona air ionizers apply high voltage to the sharp emitter points. The strong electric field would accelerate the free electrons that are present in the atmosphere to the positive emitter points. The fast moving electrons collide with the gas molecules and dislodge the weak bonded electrons from the molecules, giving rise to positive ions, which are then repelled, from the positive emitter points.
At the negative emitter points, the free electrons in the atmosphere are pushed away from the emitter points. These fast moving electrons collide with gas molecules and dislodge electrons from the molecules. These dislodged electrons are in the atmosphere for not long before they are captured by other neutral gas molecules and give rise to a flood of negative ions.
Corona air ionizers are not suitable for all applications. Depending on the type of emitter point material used, the ionizers tend to introduce particulate contaminants to the air. The ions balance for corona ionizers would never be zero.
For that reasons, alpha ionizers are the answer. Alpha ionizers are normally used in a volatile environment and in an environment where very sensitive ESD sensitive items are handled which require balanced ionizers.
Alpha ionizers use Polonium 210 radioactive source, which emit alpha particles.
The alpha particle collides with a gas molecule and dislodges an electron. The free electron is then captured by neighbouring neutral gas molecule and becomes a negative ion. A balance number of positive and negative ions are produced in the alpha nuclear ionization process.
There is a local government regulatory rules that need to be followed when alpha ionizers are used. Periodic annual renewal of the alpha nuclear source is also required to ensure proper functionality of the ionizers.
A Charge Plate Monitor (CPM) is commonly used to check the effectiveness of an ionizer.
Monitoring the balance ions (offset voltage) and discharge decay time would be able to tell whether the ionizer performs its intended function.
 

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This entry was posted on Tuesday, January 23rd, 2007 at 16:11 and is filed under ESD workstation. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.