A study of the mechanisms for ESD damage to reticles

Reticles were exposed to the fringing field from an electrode biased to a high voltage. The reticles in the study included special reticles intended to benchmark the ESD hazard of a photobay and production reticles of a variety of feature sizes. It was found that without any electrical contact between the reticle and the electrode, reticle damage could be done. A wide bandwidth transient-EMI sensing antenna revealed that the reticle sparked when a voltage as low as 2000 V was applied to the electrode. The tests showed that the ESD threshold of reticles with smaller feature sizes was lower than for reticles with larger feature sizes. Reticles were scanned under a microscope for reticle damage. It was found that when the voltage was ramped to 17 kV and returned to zero, damage to the reticle was observed. When a voltage of 7.5 kV was applied once, no damage was observed but when it was applied 100 times, reticle damage was observed. This study confirms that ESD damage is done to a reticle by charged objects in the vicinity of the reticle in contrast with the prevailing belief that reticle damage is done only by charged reticles. The study also showed that reticles can be sufficiently damaged to cause printing errors due to the accumulated damage caused by repeated low level exposure to the fringing field of a charged object in the vicinity of the reticle.