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.