What is it about?
Reticles (photomasks) are extremely susceptible to damage by exposure to electric fields. In addition to the all-too-familiar ESD damage that has been the bane of the photomask industry for decades, there are other more subtle and cumulative field-induced damage processes that can cause degradation. These forms of damage can be almost impossible to detect in a semiconductor production environment, even when they are causing printing defects. Current handling practices and the plastic boxes used to store photomasks are not capable of preventing this kind of damage and in some cases they actually increase the risk of damage. All the known risks are described and ways of minimising the risks are defined.
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Why is it important?
When reticle electrostatic damage became a significant problem for the semiconductor industry in the late 1990s, "classical" ESD prevention techniques were adopted to try and reduce the risk. Unfortunately two of the measures that were recommended as protective have been found through experimental research to increase the risk of some kinds of damage rather than reducing it as was the intention. Equipotential bonding (grounding) and static dissipative plastic boxes - which are almost universally adopted worldwide - are shown to put reticles at significant risk of damage by electric fields. Electric field can damage a reticle without the reticle even being touched. Alternative methods are needed to reduce the risk to reticles so that semiconductor production is not jeopardised.
Perspectives
When I first conducted my own research into reticle damage it followed a significant period of research by others in the industry. Their focus had been on field-induced ESD, so the methods recommended to prevent it were "classical" ESD prevention techniques. However, during my research I discovered a new form of reticle electrostatic damage that occurs on exposure to electric fields that are more than 100x weaker than those that might cause ESD damage. Owing to the changing dimensions of reticle features over time as a result of Moore's Law, modern reticles are inherently more susceptible to this form of damage than they are to ESD. Ironically, the methods being used to address ESD risk actually accentuate the risk of this more subtle and cumulative damage mechanism! My objective has therefore been to try and increase awareness of this risk in the semiconductor industry and to recommend the adoption of handling methodology that is specifically designed to address this risk, rather than continuation of the use of ESD prevention methods that actually make the risks worse.
Dr Gavin C Rider
Read the Original
This page is a summary of: How to Protect Reticles from Electrostatic Damage, November 2018, SPIE,
DOI: 10.1117/3.2514864.
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