Considerations for CPM Measurements of Fast Switching Ionizers
Larry Levit (1), Alvin Lau (1), Bruce Williams (2), Ronald Slaby, (2) Joshua Y.H. Yoo (3)
(1) MKS Ion Systems, 1750 North Loop Road Alameda, CA 94500 USA tel.: 510 217 0600 fax: 510 217 0484 email: LBL_Scientific@SBCGlobal.net
(2) TREK, Inc. 11601 Maple Ridge Road Medina, NY 14103 USA tel.: 585 798 3140 fax: 585 798 5033 email: firstname.lastname@example.org, email@example.com
(3) MKS Korea Co. Ltd.1st Floor DK Plaza I, 375-1 Geumgok-dong, Bundang-gu Seongnam-city, Kyonggo-do Korea 463-805 tel.: 82-31 717 9244 email: Joshua_Yoo@MKSinst.com
Charged Plate Monitors (CPM) are commonly used to calibrate and monitor ionizer performance. When the CPM was developed, it did an excellent job of characterizing ionizer performance for an ionizer used in a 150 mm wafer fab as well as many other applications, including DC applications. Today, there are a number of ionizers operating at higher frequencies, including pulsed AC and piezoelectric ionizers. In these cases, the underlying physics is in a new regime and conventional measurements do not exactly apply. Details of the CPM wave shape and the application must be considered when using such ionizers. These effects need to be accounted for in standards that apply to CPM use, when measuring ionizer performance.
Some manufacturing disciplines have found that low cost AC ionizers can be a viable component of a static control program. However, when the disk drive industry moved to manufacturing of ultra-sensitive MR heads, these ionizers proved to be more damaging to heads than helpful, owing to their capability to produce excessive hidden voltage swings on the devices in which the ionizer was meant to protect. The voltage swing was not recognized (hidden) for some time because ionizer performance was measured with a Charged Plate Monitor (CPM), which couldn’t respond quickly enough to display the 50-60 Hz voltage variation caused by an AC ionizer[i].
The flat panel display (FPD) industry uses pulsed AC ionizers in many applications. These ionizers are effective in eliminating static charge but if they are calibrated using a conventional CPM some erroneous conclusions can be drawn. In some cases, the size and handling, generally by large robots, of the product is very different than in the disk drive industry. Therefore the currents and fields experienced by the product are different than those experienced by a manually handled disk head.
Modeling an Ionizer with a CPM
A corona ionizer can be an AC or a DC device. This means that high voltage applied to the emitter points is either left on continuously (DC) or switched (Pulsed). A CPM responds to the currents and the fields that are applied to it. These readings are dependent upon the bandwidth of the signal emitted from the ionizer.
The elements of the circuit connecting the ionizer and the CPM are shown below for a DC ionizer, case  see Figure 1. In this case, the capacitance is the CPM is 20 pF, the ionizer is represented by a battery, the discharge time is related to the RC time constant of the circuit (DT=Ln (10)*RC ) and the offset voltage is given by the ionizer (battery) voltage and the resistive divider ratio.
Figure 1 Equivalent Circuit for a DC Ionizer.
While this equivalent circuit has proven accurate for the case of steady state DC, other factors come into play for ionizers which are AC switched.
As shown in Figure 2, the capacitance between the ionizer and the CPM must also be considered. Its effect is to couple the edges of the ionizer voltage signal into the plate or the product. This signal, the displacement current is a component of what the CPM will see. To complicate matters, the capacitance of the product and the plate is likely to be very different and thus will not be modeled well by the CPM.