Static dissipative single reticle SMIF pods were developed almost twenty years ago in response to significant electrostatic damage that was being caused by single reticle pods made with insulating plastic.
This was at a time when little was known about the mechanism of reticle electrostatic damage and few fabs were using static prevention technologies in their photo bay.
Almost as soon as static dissipative pods became available, research showed that they were not fully protective against electric fields. But they offered a reduction in ESD damage at a time when reticle ESD was becoming a huge problem.
They are now the de facto standard in most modern semiconductor fabs.
BUT - in 2003 Microtome discovered EFM - a pernicious reticle damage mechanism that takes place when an electric field more than 100x weaker than would be needed to cause ESD passes through a reticle.
EFM continuously degrades a reticle, with even tiny amounts of electric field exposure capable of causing cumulative degradation.
NO AMOUNT OF ELECTRIC FIELD EXPOSURE
SHOULD BE CONSIDERED SAFE FOR A RETICLE!
Static dissipative or "conductive" plastic single reticle SMIF pods do not keep reticles safe from electric field exposure. In fact, they actually increase the EFM risk faced by a reticle because they convert static fields into transient fields, which are far more hazardous to a reticle.
The information presented here shows that far from being protective for a reticle, static dissipative plastic single reticle pods - especially those in which the reticle is grounded through the support points - are probably the WORST environment one could create for reticle handling and storage.
By contrast, Microtome's E-Pod is capable of completely
shielding a reticle from transient and high frequency fields that can cause continuous reticle degradation.
Figure 1. Schematics of how a reticle reacts to electric field
|The above simplified graphic shows what happens when an electric field (red line) passes through a reticle. As the field strength increases (a), charge is displaced within the reticle as shown by the blue trace. Charge moves until its displacement has created an equal and opposite electric field to the one being applied externally. At the point when the electric field within the reticle is cancelled out, charge displacement stops. There is no further electrostatic stress in the reticle and no further electrostatic damage takes place - even though the external electric field is still present. EFM takes place when charge is moving within the reticle, and ceases when the reticle has established a new equilibrium. This is illustrated by the constant amount of damage in the green trace after the initial application of the field.
Clearly a static dissipative single reticle SMIF pod is not protective against EFM - it actually increases the risk of EFM by doubling the electrostatic stresses that reach the reticle inside it. But are "conductive" plastics any better?
When the external electric field is removed as in the latter part of (b), the displaced charge within the reticle returns to its original location. As it does there will be further degradation of the reticle through EFM, as shown by the second step in the green trace. So, applying an electric field to a reticle and then removing it again causes two electrostatic stresses, once as the field through the reticle increases and again as it decreases to zero. Every change in the field conditions - including the orientation of the field as well as its strength - causes further charge displacement and with it further risk of induced EFM damage.
A reticle reacts almost instantaneously to an applied field. So if a very short duration transient field passes through a reticle, the charge displacement and the damage caused will be as shown in (c). A short transient reaching a reticle will cause just as much damage as would be caused by a constant electric field. This is where the response of static dissipative plastic to electric field becomes positively dangerous for a reticle.
a) Field applied to a static dissipative reticle pod
b) Field transmitted to the reticle inside the pod
Figure 2. Measurements of field transmitted to a reticle inside a static dissipative single reticle pod. (Levit, 2001)
These two measurements in fig.2 were taken to assess the transmission of electric field by a static dissipative reticle pod. It can be seen that the applied field with a profile as in fig.1b is converted by the pod into two transients of opposite polarity inside the pod. The damage caused by each transient would be as in fig.1c. The dissipative pod therefore INCREASES the risk of EFM degradation by converting constant electric fields outside the pod into short-duration transients that penetrate the reticle inside the pod. Every change to the field outside the pod (as in fig1a) is transmitted to the reticle as a transient field "spike", carrying with it double the risk of EFM degradation.
The following recording shows multiple transient fields reaching a reticle inside a standard static dissipative single reticle pod during normal handling in a semiconductor factory which is equipped with normal ESD countermeasures. Every one of these field transients is capable of causing EFM in the reticle.
Figure 3. Electric field data recorded inside a static dissipative reticle pod during normal handling, kindly provided by Estion of Germany.
Similar measurements taken from these "conductive plastic" pods record no transient fields, suggesting that they are fully protective against field penetration. BUT THIS IS A FALSE CONCLUSION! The sensor reticle used for these measurements is incapable of recording the very fast transients that such pods transmit to the reticle. Helmholz and Lering in 2006 found that fields capable of inducing ESD in a test reticle can penetrate even the most conductive plastic SMIF pod available, made with carbon nanotube loaded PEEK. If ESD could be induced by the transient fields penetrating the pod (which could not be detected electronically due to their extremely short duration) then EFM would certainly take place inside such a pod.
The Microtome E-Pod has been tested for field penetration using the same sensor reticle as used to produce fig 3. This is the result:
Figure 4. Field penetration test of Microtome E-Pod. Note the flat trace while the sensor reticle is inside the all-metal pod, but the unstable trace when the sensor is being returned to its docking station inside a "conductive" plastic single reticle pod (carbon nanotube loaded PEEK).
it is recognised that the sensor reticle producing this trace is incapable of detecting very fast field transients, it is known that solid aluminum provides full Faraday Cage protection so the flat trace when the reticle is inside the E-Pod really is indicative of a lack of field penetration. The unstable trace when the reticle was transferred into the PEEK pod is indicative that the electrostatic environment inside that pod was not stable - electric field was certainly reaching the reticle from outside the pod, and a normal reticle would probably experience EFM degradation under these circumstances.
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