An increased ion content in ambient air can reduce the electrostatic charge on objects in the environment. In fields where electrostatic discharge poses serious problems, such as the semiconductor chip fabrication field, the use of air ionizers is common. An air ionizer typically includes sharply pointed electrodes, to which high voltages are applied. Gas molecules near the electrodes, especially near the sharply pointed tips, become ionized when they either gain or lose electrons. Because the ions take on the charge of the nearest electrode, and like charges repel, they are repelled from that electrode. In typical air ionizers, an air current is introduced to the device in order to carry the ions away from the electrodes to a "target region" where an increased ion content is desired.
Ions in the air are attracted to objects carrying an opposite charge. When an ion comes in contact with an oppositely charged object, it exchanges one or more electrons with the object, lessening or eliminating the charge on the object, which makes electrostatic discharge less likely. Excess electrostatic charges on objects may also attract dust and other particulate contaminants. By reducing or eliminating excess electrostatic charge, ions in the air can reduce contamination of objects in the environment.
When ionized air is used to control electrostatic changes on objects in the environment, increased levels of both positive and negative ions are necessary. The stray electrostatic charges which build up on the objects to be protected can be of either polarity. If increased levels of ions of both polarities are present in the region of an object, ions which have a charge opposite that of the object are attracted to the object, which tends to neutralize the charge on the object. To the degree that the total charge of positive ions in a region is the same as the total charge of negative ions, the region is said to be "balanced." If the ion content in the region of an object is unbalanced, the more predominant ions may actually impart a charge to otherwise uncharged objects. For this reason, it is important that air ionizers which are used to control electrostatic charges produce a balanced number of positive and negative ions, and that the balance is present in the target region.
Several methods are used to make the ion content of the target region more balanced. U.S. Pat. No. 5,055,963 to Leslie W. Partridge, which is incorporated by reference herein in its entirety, discloses some methods for balancing the ion content of the target region. One method is to minimize the exposed surface area of the grounded components of the ionizer, and to position such grounded components in the ionizer such that they are, to the extent possible, equally distant from each electrode. This reduces the tendency of ions from one electrode to be attracted to ground, allowing more ions of the opposite polarity to reach the target region.
Another technique is to place electrodes of opposite polarity near each other to minimize the differences between the paths of ions from either electrode to objects in the target region. Such differences can result in an increased number of ions of one polarity in some parts of the target region. This technique is limited because locating electrodes of opposite polarity near each other increases the number of ions which simply move between the two electrodes, decreasing the number of ions which end up in the target region. Thus, locating electrodes near each other increases ion balance, but negatively affects the overall ion content level.
To help ensure that the numbers of ions produced by the electrodes are balanced, the high voltage supply, which is connected to the electrodes, can be isolated from ground. This allows the high voltage supply, and the electrodes, to acquire a Direct Current (D.C.) bias which acts to reduce any unbalance in the ions produced. When a molecule of one of the gases constituting air becomes positively ionized at a positive electrode, it loses at least one electron to the positive electrode, imparting a negative charge, equal in magnitude to the positive charge acquired by the molecule, to the entire high voltage supply. When a negative ion is produced at a negative electrode, at least one electron is removed from the electrode, imparting a positive charge to the high voltage supply. If the total charge of all positive ions produced is equal in magnitude to the total charge of all negative ions produced, the effect of these charges on the high voltage supply will cancel out, and no D.C. bias will be acquired. If more ions of positive polarity are produced, however, the high voltage supply will gain a D.C. bias of negative polarity. This D.C. bias causes more negative ions to be produced, until balance in the number of ions of each polarity has again been achieved. The same mechanism acts to produce more positive ions when too few have been produced.
While these efforts at balancing the ions in the target region are largely successful, some imbalances in the target region may still occur when these methods are used. Because conventional air ionizers use an airflow past the electrodes to carry the ions to the target region, conventional ionizers are open on at least one side other than the side facing the target region. This allows ions, which may move much faster than the air flow, to interact with charged and grounded objects located outside the ionizer and the target region. Such interaction diminishes the number of ions in the target region, and may act to unbalance the ion production.
In the case of an ionizer which is to be used very near a target region, or near grounded objects outside of the target region, the ion content in the target region may be unbalanced. When objects in the target region are nearer the electrodes, it is more likely that there will be asymmetrical coupling, leading to unbalanced ion content in the target region. Also, other grounded objects near the ionizer have a tendency to draw away many of the ions, leaving fewer in the target region. If the coupling with these objects is asymmetrical, then the ion content will likely be further unbalanced. What is needed is an air ionizer which delivers a balanced distribution of ions to a nearby target region in an environment which may contain grounded objects near the ionizer in a direction other than that of the target region.