This paper explains how an electric field and a reticle interact and describes the different kinds of damage that can be caused to a reticle through its exposure to electric field. It is shown why electrostatic reticle damage has changed from ESD damage (which causes yield to suddenly drop precipitously) into a gradual and cumulative form of degradation that is very difficult to diagnose. It is explained why some of the approaches that have been taken to reduce ESD damage in the semiconductor factory, such as equipotential bonding and the use of static dissipative plastics for making reticle pods, actually increase the risk of this cumulative type of electrostatic degradation in reticles. When assessing the risk to reticles and designing an effective protective strategy for reticle handling, it is shown why one must take into account the temporal characteristics of a reticle’s interaction with electric field—including the effect that the reticle’s immediate surroundings will have on that interaction—as well as considering the strength of any electric field in the reticle handling environment. Solutions are presented that would allow the electrostatic risk to reticles to be reduced significantly, without requiring major changes to operating procedures in semiconductor manufacturing facilities.
Reticles can be damaged by electric field as well as by the conductive transfer of charge. As device feature sizes have moved from the micro- into the nano-regime, reticle sensitivity to electric field has been increasing owing to the physics of field induction. Hence, the predominant risk to production reticles today is from exposure to electric field. Measurements of electric field that illustrate the extreme risk faced by today’s production reticles are presented. It is shown that some of the standard methods used for prevention of electrostatic discharge in semiconductor manufacturing, being based on controlling static charge and voltage, do not offer reticles adequate protection against electric field. In some cases, they actually increase the risk of reticle damage. Methodology developed specifically to protect reticles against electric field is required, which is described in SEMI Standard E163. Measurements are also presented showing that static dissipative plastic is not an ideal material to use for the construction of reticle pods as it both generates and transmits transient electric field. An appropriate combination of insulating material and metallic shielding is shown to provide the best electrostatic protection for reticles, with fail-safe protection only being possible if the reticle is fully shielded within a metal Faraday cage.
In recent years a great deal of effort has been expended to try and reduce the reticle ESD damage problem. Methods are almost all based on the standard principles developed for the protection of ESD sensitive electronic devices – but reticles are not the same as electronic devices. Reticles are predominantly damaged by electric field rather than the conductive transfer of static charge, and the physical mechanisms that damage reticles are different from those that damage electronic devices. This paper explains why some of the established methods for ESD prevention are not the best way to protect reticles and in some cases actually increase the risk of reticle damage. Measurements are presented showing that, contrary to the widely held opinion and current practice in semiconductor manufacturing, static dissipative plastic is not the best material to use for the construction of reticle pods. An appropriate combination of insulating material and metallic shielding is shown to provide the best electrostatic protection for reticles.
It has recently been reported  that production reticles are subject to progressive CD degradation during use
and intense study is under way to try and identify the causes of it. One damage mechanism which has already
been identified and quantified  is electric field induced migration of chrome (EFM). This can be caused by
electric fields that are more than 100x weaker than those that cause ESD. Such low level electric fields can be
experienced by a reticle during normal handling and processing steps, as well as coming from external
sources during transportation and storage. The field strength of concern is lower than most electrostatic field
meters are designed to measure and it can be difficult or impossible to measure such fields inside the cramped
environment of equipment.
To measure this risk a new sensor device ("E-Reticle") has been developed having the same materials of
construction and form factor as a standard chrome-on-quartz reticle. It allows the electric field that a reticle
would experience during normal use and handling to be measured and recorded. Results from testing of this
device in a semiconductor production facility are reported, showing that certain processes like reticle washing
are inherently hazardous. It also enables identification of problems with electrostatic protection measures
inside equipment, such as unbalanced ionizers or poor load port grounding. The device is shown to be capable
of recording electric fields in the reticle handling environment that are below the recommended maximum
that is being proposed for the 2009 ITRS guidelines.
Reticles have been found to be susceptible to damage by the Electric Field induced Migration of chrome (EFM) at field
levels >100x lower than those that cause ESD. The experimental quantification data are reviewed briefly and detailed
AFM imagery is presented illustrating the nature of the reticle degradation process. The characteristics of EFM and its
very low onset threshold have significant implications for the protection of advanced reticles so a new reticle handling
methodology is proposed which is designed to minimize the risk of field induced damage.
The damage mechanisms that take place when a reticle is subjected to electrical stress by exposure to an electric field
have been investigated by applying voltage directly to the structures in a special test reticle. Surface current was
recorded at all levels of stress from 1V to 100V. The current/voltage characteristic was polarity dependent and exhibited
increasing non-linearity as the feature spacing was reduced. Atomic Force Microscopy showed that the electrical stress
caused EFM (Electric Field induced Migration of chrome), matching the damage seen in reticles stressed through
induction by an external electric field. No ESD events were recorded, confirming that EFM is independent of ESD and
that it occurs with lower electrical stress. The threshold for EFM was found to be five times lower than the previous
estimate, starting at 1V with 1µm spacing. Damage caused by EFM was shown to be continuous, cumulative and the
rate of CD degradation was measured to be from 3 to 6 nm per second.