Ionization applications and pitfalls for charge neutralization in substrate handling

The substrates used in a variety of applications including LCD manufacturing, circuit board assembly, hybrid circuit handling and ribbon cable manufacture are relatively thick insulating materials with isolated conductive traces on at least one surface. Substrates are transported though a factory over a series of driven rollers. Both rollers and substrates may acquire charge. It is to be expected that damage to the metal structures on these substrates can be damaged by the fields that are caused by the charge in exactly the same way as is the case for photolithographic reticles used in semiconductor manufacturing.¹ Ionizers are often placed directly over substrates along the manufacturing line. This is a good strategy for eliminating fields that extend long distances (Gt 1 mm) from the substrate and thus reducing contamination. This investigation revealed that charge deposited on the bottom substrate surface with ionizers over the top surface damage will result in opposite polarities of electrostatic charge deposited the two surfaces (called bipolar charge), which could result in a discharge between the conducting structures. To simulate the electrostatic environment of a factory, a quartz substrate (150 mm times 150 mm times 3.1 mm) with isolating conductors (2 mm long separated by 1.5 mum) was employed. A voltage of 7500 V was applied to a 1 cm times 1 cm electrode directly under the substrate, and the topside metal structures were damaged at a voltage of 7500 V. With ionization present, the breakdown voltage was found to be only 4000 V. Thus, ionization on the top surface without other implementation of bottom side charge elimination can result in substrate damage. It was also shown that if an ionizer is placed on the side opposite to the charge, the ionizer will cause an equal and opposite charge to be deoposited on the surface opposite to the original charge. One of the consequences of this so called