Reticle protection is our business
Microtome Precision 2004. CONFIDENTIAL 35
u1. CD degradation occurs at the lowest levels of induced voltage.
There is a threshold below which metal migration does not take place -
the level of this is not known but it is very low by ESD standards. It is also
not known whether this migration process will continue indefinitely or
reach a point of equilibrium, perhaps related to the chrome film thickness.
u2. Gap bridging occurs at medium levels of induced voltage.
Once a gap is bridged currents flow back and forth continuously when the
fields around the reticle change, causing localised Joule heating. If the
currents are low this heating progressively oxidises the surrounding
chrome lines creating “chrome foam”. If the currents are high enough the
heating can vaporise the bridge completely.
There are three physically different forms of damage that take place
depending on the amount of reticle exposure to electric fields:
u3. Air Discharge occurs at higher levels of induced voltage.
The lowest-energy discharges land on the upper surface of the chrome
and so can damage the anti-reflective coating. Higher-energy discharges
vaporise metal from the line edges and cause “mouse bites” (these are
the most commonly known features of ESD damage).