Shoot ions out of harm's way
09/01/2002
Bring in the air ionization equipment when your first line of defense needs a little backup
by Ryan Cliche
You may have instituted first-line de fense techniques such as wrist straps, ground bond wires and have replaced insulating materials with conductive materials in your ongoing battle with electrostatic discharge (ESD), electromagnetic interference (EMI) and electrostatic attraction (ESA). But when these tired and true methods fall short, it might be time to bring in the cavalry.
Choose your weapon
Air ionization is critical in neutralizing electrostatic charge and there is a wide range of ionization equipment to choose from when you're ready to take the leap.
Nozzles and guns, for example, use filtered, dried-compressed air ionized at the point of use. These types of ionizers are mainly used where cleaning off debris and neutralizing static charges are at issue. Some applications include preparation of optical components for coating, swarf removal from molded and machined plastic housings and cleaning components prior to applying a finish.
Bar-type ionizers can be made in various lengths and can be used in applications from point-of-use to bars that are built into a laminar flow hood, allowing total flexibility.
Bars may be configured to operate in the pulse DC mode to provide rapid static discharge where ion balance is not critical, or they can be configured in the straight DC mode to provide ion balance with the trade-off of a slightly longer discharge time.
Ionizing air blowers are available in both the "tabletop" and overhead varieties. They are both similar products, using a built-in fan to blow the ionized air to the work area. Overhead ionizing air blowers are typically installed to provide general coverage for the workbench. Some units (equipped with specially selected clean fans) are used within cleanrooms to protect sensitive magnetoresistive (MR) heads during fabrication, assembly and test.
Whole-room ionization systems typically consist of modules mounted on the ceiling of a room. They provide general coverage where ESA or EMI are a concern.
Performance Measurement
On the road to neutralizing your particular ESD problem, proper and reliable performance measurement must be conducted.
Charge plate monitor (CPM) readings are the standard method used for evaluating ionization equipment.
"Take a charge plate monitor, place it under the ionizer and then adjust the ionizer to give CPM readings that are within the spec for that cleanroom," says Larry Levit, chief scientist at Ion Systems Inc. (Berkeley, CA). After some length of time, depending on the environment of the cleanroom, you need to check the CPM reading once again to make sure the equipment is functioning properly.
Electronic ionizers can go out of balance because the points erode and actually release material (particles) into the cleanroom. Particulation from the points has been a problem for years, but ionization equipment manufacturers have been working with different types of metals that could potentially eliminate the production of particulate.
Tom Caffarella, director of Q.A. and engineering, NRD LLC (Grand Island, NY), says NRD's point-of-use (alpha) ionizers don't go out of calibration, out of balance or produce particles.
Is discharge time an issue? Sometimes yes, sometimes no. Levit uses ceiling emitters in semiconductor applications to examine this question. "Ceiling emitters are used primarily with stationary objects; and stationary objects, since they're not going anywhere, eventually will get discharged," he says. In this instance, discharge time is not a major issue. "For example, if you had a window that was highly charged, became discharged in a couple minutes and stayed discharged, then that would be fine."
However, in other instances, discharge time is an issue. "Let's contrast that [the stationary object] example with a wafer that is moving through a minienvironment, and it is only passing through the ionization for 10 to 20 seconds," continues Levit. "In this case, since the product is moving rapidly and only sees the ionization for a short time, discharge time is a major concern.".
For updated listings of ionization equipment manufacturers, point your browser to the Product Directory at cleanrooms.com to search by product or company name. Vendors can update their listings for free at anytime.