Electrostatic Protection Consultancy Service
The nature of reticle electrostatic damage has changed.

ESD damage in reticles used to be infrequent and easily detected. Now, due to the constantly shrinking feature separations on reticles and improvements in static control in reticle handling environments, reticle electrostatic damage has become more subtle.

It has changed to a continuously operating degradation mechanism called EFM (Electric Field induced Migration)

Microtome first identified and characterized this reticle damage mechanism in 2003. We later quantified it through experimental research at the University of Colorado at Colorado Springs.

A  subsequent independent investigation of reticle degradation by IBM has confirmed the extremely low threshold for chrome migration that our study had indicated. EFM can be caused by an electric field that is more than 100x  weaker than that which will cause ESD.

EFM is a continuously operating and cumulative reticle degradation process that can change the critical dimensions of the reticle pattern by more than 6nm per second.

Like 193nm haze, EFM damage builds up over time and gradually degrades the lithography process window. However, unlike haze, the degradation cannot be reversed by simply cleaning the reticle. It is permanent.

EFM is caused by a high local field strength at the edge of a reticle feature. A reticle acts as an electric field amplifier so the internal field within the reticle can easily be 1000x stronger than the field measured externally.

This makes it far more important to control the electric field around reticles than would be indicated by considerations of ESD risk alone, but this requires more than simply putting in place more comprehensive ESD prevention measures. Doing that can actually make things worse.

For example, equipotential bonding - which is universally employed to control ESD risk - actually increases the risk of damage in a field-sensitive object llike a reticle.

Such is the poor state of understanding of this problem within the semiconductor industry and ESD prevention community today.
Schematic representation of electrostatic risk during reticle handling
The schematic diagram above indicates how a reticle in normal use may experience electrostatic stress that is capable of causing EFM, without ever suffering ESD damage. So the absence of ESD does not indicate the absence of electrostatic risk. And as stated before, widely used grounding practices that are designed to minimise the risk of ESD can increase the electrostatic stress a field-sensitive object sustains.

Static dissipative or even "conductive" plastic reticle pods can transmit fast field transients of the kind illustrated above to the reticle inside the pod. In fact, static dissipative materials generate these transients and convert external field changes into transient events inside the pod, effectively doubling the stress the reticle receives.

Properly understanding the risks and knowing how to minimise them without inadvertently making the situation worse, such as with inappropriate grounding or the inappropriate use of static dissipative plastic, is essential if reticle electrostatic damage is to be avoided.

We know that getting the basics of electrostatic protection wrong has cost one of the most advanced fabs in the world millions of dollars in lost output - we have seen it for ourselves. This was despite adopting industry best practice and having regular audits by a professional ESD consultancy firm. Can you take the risk that this serious problem is not fully understood in your company or by your ESD consultant?

This is why our expertise in this highly specialised area is so valuable to you - we have the necessary knowledge and experience in depth and we can draw your attention to some potentially costly misunderstandings about the protection of extremely electrostatic sensitive (EES) items.

So, whether your interest is simply in updating the education of your reticle handling staff to include awareness about EFM, or in having a facility-wide reticle safety check to help identify areas of your operation that might expose your reticles and other extremely electrostatic sensitive items to invisible electrostatic risk, we are able to help you.

Please contact us for further details

References

Rider, G. C., Kalkur, T. S., "Experimental quantification of reticle electrostatic damage below the threshold for ESD," Proceedings of SPIE Advanced Lithography Vol. 6922-73 (2008)

Rider, G. C., "Electric Field Induced Chrome Migration In Reticles" Proceedings of SPIE Photomasks Vol 7122-14 (2008).

Sebald, T., Rider, G. C.," High sensitivity electric field monitoring system for control of field-induced CD degradation in reticles (EFM)", Proceedings of SPIE Photomask and NGL Mask Technology XVI, Vol 7379-60 (2009)

SEMI E163-0212 - "Guide for the Handling of Reticles and Other Extremely Electrostatic Sensitive (EES) Items Within Specially Designated Areas" (link)

International Technology Roadmap for Semiconductors 2011. (link)

All our technical papers can now be downloaded from ResearchGate