Chapter 40.2.2 - ESD Controls in Cleanroom Environments - Problems Resulting From Charge in Cleanrooms

40.2.2Electrostatic Damage

Electrostatic attraction and its effect on contamination control is not the only negative effect of static charge. In addition, static charge is responsible for electrostatic damage due to the discharge from one object to another. Such a discharge can cause physical damage to a product that experiences the discharge owing to either electrical overstress (voltage punch-through) or damage caused by the energy deposit by the discharge (spark). Electrostatic discharge (ESD) is particularly important in photolithography. Damage to reticles is caused by either charge on the reticle or by induced discharges caused by electric fields from other charged objects in the proximate environment.

The rapid uncontrolled transfer of static charge from one object to another is referred to as ESD. One aspect of such a discharge is that the source and destination of the electrical current are extremely localized (~μm). This means that the energy dissipated in the two objects that participate in the discharge are heated extremely locally. When such a discharge takes place between two electrical conductors, the speed of the transfer is typically in the nanosecond regime. The nominal capacitance of nominal objects in a fab (~tens of cm) are on the order of 10⁻¹¹ to 10⁻¹⁰ F. Thus, if they are conductors, they have RC time constants <1 ns. So even including inductive effects, metal-to-metal discharges are extremely fast.

ESD damage can be caused by tribocharging of the victim object followed by a discharge to ground. In this case, the object becomes charged by handling and then, when it is moved toward a socket or is approached by an end effector, the discharge occurs. The other effect that can lead to damage is called the field induced model.⁷ When a victim object (typically a die, wafer, or reticle) is subjected to an electric field, a potential difference is created between the victim and an adjacent ground. This damage mechanism is common in back-end processing. In this case, the insulating package becomes charged from handling and produces an electric field. This field drives a discharge from the die in the package to a ground pin on the test socket. See Fig. 40.4.

In another example of field induced model (FIM) damage, a reticle can experience a discharge from one chrome structure to an adjacent one when the reticle is subjected to an external electric field. This commonly happens when the reticle is stored in an insulating reticle cassette that becomes charged. Most often the charge is located immediately beneath the handle, which becomes charged when the operator lifts the cassette and inadvertently rubs his fingers over the cassette structure. Such discharges can be very serious, because the manufacturing process can experience a precipitous yield

drop in spite of the fact that the tiny discharge across the reticle cannot be sensed by the operator. See Fig. 40.5 for such FIM damage.

Most recently, semiconductor wafers with the smallest feature sizes (≤0.090 μm) have exhibited occasional gate oxide punch-through due to processing steps that are known to cause high charging (i.e., cleaning and certain wet-processing steps).

40.2.2Electrostatic Damage

Electrostatic attraction and its effect on contamination control is not the only negative effect of static charge. In addition, static charge is responsible for electrostatic damage due to the discharge from one object to another. Such a discharge can cause physical damage to a product that experiences the discharge owing to either electrical overstress (voltage punch-through) or damage caused by the energy deposit by the discharge (spark). Electrostatic discharge (ESD) is particularly important in photolithography. Damage to reticles is caused by either charge on the reticle or by induced discharges caused by electric fields from other charged objects in the proximate environment.

The rapid uncontrolled transfer of static charge from one object to another is referred to as ESD. One aspect of such a discharge is that the source and destination of the electrical current are extremely localized (~μm). This means that the energy dissipated in the two objects that participate in the discharge are heated extremely locally. When such a discharge takes place between two electrical conductors, the speed of the transfer is typically in the nanosecond regime. The nominal capacitance of nominal objects ...